WO2002026466A1

WO2002026466A1 - Dimensionally-stable, thin, molded polymeric material at fast cycle time

Info

Publication number: WO2002026466A1
Application number: PCT/US2001/030512
Authority: WO
Inventors: Kevin J. Levesque; David E. Pierick; Levi A. Kishbaugh
Original assignee: Trexel, Inc.
Priority date: 2000-09-29
Filing date: 2001-10-01
Publication date: 2002-04-04
Also published as: WO2002026466A8; AU2001296401A1; WO2002026466A9

Abstract

Polymer molding processes, such as injection-molding processes, include use of a viscosity-reducing supercritical fluid additive resulting in superior dimensional stability of molded articles, optionally at lower temperatures and reduced cycle times. A variety of highly dimensionally-stable parts, such as matrix trays (1300), are described.

Description

Dimensionally-Stable, Thin, Molded Polymeric Material at Fast Cycle Time

Field of the Invention

The present invention relates generally to injection molding of polymeric articles, and more particularly to the injection molding of dimensionally-stable microcellular articles.

Background of the Invention

Polymeric molding is a well-developed field. One broad area of polymeric molding involves introducing a fluid polymeric material into a mold, allowing the polymeric material to assume the interior shape of the mold and to harden therein, as well as removing a resultant polymeric article from the mold. Such techniques are commonly known as injection molding, intrusion molding, and others. Solid polymeric articles, and polymeric foams can be made using these techniques. Polymeric foam articles can be produced by injecting a physical blowing agent into a molten polymeric stream, dispersing the blowing agent in the polymer to form a mixture of blowing agent and polymer, injecting the mixture into a mold having a desired shape, and allowing the mixture to solidify in the mold. A pressure drop in the mixture can cause the cells in the polymer to grow. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes a chemical reaction in the polymer material causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a critical temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. Under some conditions cells can be made to remain isolated in such materials, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results.

Polymeric foam molding is well known. U.S. Patent No. 3,436,446 (Angell) describes a method and apparatus for molding foamed plastic articles with a solid skin by controlling the pressure and temperature of the mold. Microcellular material typically is defined by polymeric foam of very small cell size. Various microcellular material is described in U.S. Patent Nos. 5,158,986 and 4,473,665. These patents describe subjecting a single-phase solution of polymeric material and physical blowing agent to thermodynamic instability required to create sites of nucleation of very high density, followed by controlled cell growth to produce microcellular material.

Microcellular molding techniques are described in the patent literature. U.S. Patent No. 4,473,665 (Martini- Vvedensky) describes a molding system and method for producing microcellular parts using polymeric pellets pre-pressurized with a gaseous blowing agent which are melted in an extruder and extruded into a pressurized mold cavity.

U.S. Patent No. 5,158,986 (Cha et al.) describes a system in which polymeric pellets are introduced into an extruder barrel and melted, a supercritical carbon dioxide blowing agent is mixed with the polymer in the barrel and to form a homogenous solution of blowing agent and polymeric material, thermodynamic instability is induced in the system, thereby creating sites of nucleation in the molten polymeric material, and the nucleated material is extruded into a mold cavity. Nucleation and cell growth occur separately according to the technique; thermally-induced nucleation takes place in the barrel of the extruder, and cell growth takes place in the mold.

International Patent Application No. PCT/US99/26192 of Pierick, et al. filed November 4, 1999 and entitled "Molded Polymeric Material Including Microcellular, Injection-Molded, and Low-Density Polymeric Material", and International Patent Application No. PCT/US98/00773 of Pierick, et al., filed January 16, 1998, published July 23, 1998 (WO 98/31521) and entitled "Injection Molding of Microcellular Material" describe a variety of polymeric molding techniques, systems and molded articles including reciprocating screw systems, accumulator systems, blowing agent injection systems for formation of skin/foam/skin articles, structural foam molded parts, and thin walled parts. Microcellular polymeric articles or non-microcellular polymeric foam articles can be produced having thicknesses, or cross-sectional dimension, of no more than about 0.010 inch, via injection molding. Articles of polymer with melt flow rate of less than about 40 are produced with length-to-thickness ratio at least 250:1.

One problem that can be encountered in the injection molding of polymeric articles, whether solid or foam, is that of dimensional stability. For a variety of reasons, when a molded part is removed from a mold it can change slightly in shape. While many molding processes result in molded articles with relatively good dimensional stability, there is a need for even greater dimensional stability in molded polymeric articles. Summary of the Invention

The present invention provides polymer molding processes resulting in dimensionally-stable parts. In one aspect a series of methods are provided, and in another aspect a series of dimensionally-stable articles are provided. Various methods involve reducing mold cycle time in processes using a supercritical fluid additive as compared to processes using essentially identical conditions but without a supercritical fluid additive. Cycle times can be reduced in this way without compromising dimensional stability and, in preferred embodiments, while improving dimensional stability. In other embodiments cycle time may be reduced or may not be reduced, but is not increased, while dimensional stability is significantly improved.

Articles of the invention include substantially planar free-standing portions, formed in a mold, that exhibit little or no warpage or shrinkage relative to the mold. Molded products are produced that match closely, or precisely, essentially identical solid molded products that are molded under essentially identical conditions but without a blowing agent.

Thin, dimensionally-stable products are produced including those having average part thickness of no more than about 0.3 cm, meeting other dimensionally-stable characteristics of the invention. Specific articles that are advantageously produced according to the invention include matrix trays.

Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

Brief Description of the Drawings

Fig. 1 is a schematic illustration of injection molding apparatus useful for the invention: Fig. 2 is a schematic illustration of alternate injection molding apparatus, including an auxiliary accumulator, useful for the invention;

Fig. 3 schematically illustrates a section of a mold, and a part molded in that section in the context of a definition of dimensionally stable; Fig. 4 also is a schematic illustration of a mold and an article formed therein in the context of the definition of dimensionally stable;

Fig. 5 is an isometric view of a dimensionally-stable molded tray that can be made using techniques of the invention;

Fig. 6 is a bottom plan view of the article of Fig. 5; Fig. 7 is a right side elevational view of two stacked trays of Figs. 5 and 6;

Fig. 8 is a longitudinal cross-sectional view taken along line 8-8 of Fig. 7;

Fig. 9 is a cross-sectional view of one of the trays shown in Fig. 8 with its volumetric outer boundaries shown;

Fig. 10 is an isometric view of a printer chassis molded according to the invention;

Fig. 11 is a bottom plan view of the printer chassis of Fig. 10;

Fig. 12 is a longitudinal cross-sectional view taken along line 12-12 of Fig. 11;

Fig. 13 is a top isometric view of a dimensionally-stable molded matrix tray that can be made using techniques of the invention; Fig. 14 is a bottom isometric view of the matrix tray of Fig. 13;

Fig. 15 is an enlarged fragmentary detail view of a portion of the top of the tray of Fig. 13 as seen along line 15-15;

Fig. 16 is an enlarged fragmentary detail view of a portion of the bottom of the tray of Fig. 14 as seen along line 16-16 of Fig. 14; and Fig. 17 is an enlarged cross-sectional view of stacked (nested) trays of Figs. 13 and 14 taken along line 17-17 of Fig. 13.

Detailed Description of the Invention

Commonly-owned, co-pending international patent publication nos. WO 98/08667, published March 5, 1998 and WO 98/31521, published July 23, 1998, and commonly-owned, copending application filed on even date herewith by Levi Kishbaugh, et al., entitled "Thin Wall Injection Molding" are incorporated herein by reference. The present invention provides systems, methods, and articles in connection with intrusion and injection molding of polymeric material, and other techniques. For example, although injection and intrusion molding are primarily described, the invention can be modified readily by those of ordinary skill in the art for use in other molding methods such as, without limitation, low-pressure molding, co-injection molding, laminar molding, injection compression, and the like.

Techniques described herein can be adjusted by those of ordinary skill in the art to produce a variety of molded polymeric material, including microcellular polymeric material in one set of embodiments. In another set of embodiments, molded foam articles are produced in which at least 70% of the total number of cells in the polymeric portion have a cell size of less than 150 microns. In some embodiments at least 80%, in other cases at least 90%, in other cases at least 95%, and in other cases at least 99% of the total number of cells have a cell size of less than 150 microns. In other embodiments, a molded foam article can be provided in which at least 30% of the total number of cells have a cell size of less than 800 microns, more preferably less than 500 microns, and more preferably less than 200 microns.

The present invention involves, in one aspect, the discovery that improved dimensional stability can be achieved by controlling certain parameters in polymer molding techniques. One such parameter is the controlled addition of a supercritical fluid additive to polymer prior to injection into a mold, which can greatly improve dimensional stability in a resultant molded article.

Addition of a supercritical fluid additive has been found to reduce the viscosity of material injected into the mold and result in lower temperature molding, lower cycle times, reduced injection pressure (and reduced required clamp force). Surprisingly, improved dimensional stability can be achieved in combination with reduced cycle time. Reduced cycle time and dimensional stability have generally been thought to be mutually exclusive by those of ordinary skill in the art. The supercritical fluid additive can be one that results in foaming of polymeric material, and the invention involves, in various aspects, foam molded polymeric articles produced at lower cycle time relative to identical or similar solid polymer molding processes, at the same or better dimensional stability as the solid process; foam molded polymeric articles produced at the same cycle time relative to identical or similar solid polymer molding processes, at better dimensional stability; better products (better dimensional stability) including objects that desirably are flat or have flat sections such as matrix trays for holding computer chips, etc. Reduced injection pressure, achievable according to the invention, results in little or no mold core deflection and more uniform parts (better correspondence between part dimension and interior mold dimension). Even in view of prior art techniques involving supercritical fluid blowing agents, the systems, methods, and articles of the invention are surprisingly advantageous. The invention also involves molding that is carried out at significantly reduced clamp tonnage (without flash - parts are free of plastic outside of the area defined by the mold cavity), saving cost and minimizing complexity.

The various embodiments and aspects of the invention will be better understood from the following definitions. As used herein, "nucleation" defines a process by which a homogeneous, single-phase solution of polymeric material, in which is dissolved molecules of a species that is a gas under ambient conditions, undergoes formations of clusters of molecules of the species that define "nucleation sites", from which cells will grow. That is, "nucleation" means a change from a homogeneous, single-phase solution to a mixture in which sites of aggregation of at least several molecules of blowing agent are formed. Nucleation defines that transitory state when gas, in solution in a polymer melt, comes out of solution to form a suspension of bubbles within the polymer melt. Generally this transition state is forced to occur by changing the solubility of the polymer melt from a state of sufficient solubility to contain a certain quantity of gas in solution to a state of insufficient solubility to contain that same quantity of gas in solution.

Nucleation can be effected by subjecting the homogeneous, single-phase solution to rapid thermodynamic instability, such as rapid temperature change, rapid pressure drop, or both. Rapid pressure drop can be created using a nucleating pathway, defined below. Rapid temperature change can be created using temperature control (heated or cooled extruder portion, mold, etc), a hot glycerin bath, or the like. "Microcellular nucleation", as used herein, means nucleation at a cell density high enough to create microcellular material upon controlled expansion.

A "nucleating agent" is a dispersed agent, such as talc or other filler particles, added to a polymer and able to promote formation of nucleation sites from a single- phase, homogeneous solution. Thus "nucleation sites" do not define locations, within a polymer, at which nucleating agent particles reside. "Nucleated" refers to a state of a fluid polymeric material that had contained a single-phase, homogeneous solution including a dissolved species that is a gas under ambient conditions, following an event (typically thermodynamic instability) leading to the formation of nucleation sites. "Non-nucleated" refers to a state defined by a homogeneous, single-phase solution of polymeric material and dissolved species that is a gas under ambient conditions, absent nucleation sites. A "non-nucleated" material can include nucleating agent such as talc. A "polymeric material/blowing agent mixture" can be a single-phase, non-nucleated solution of at least the two, a nucleated solution of at least the two, or a mixture in which blowing agent cells have grown. "Nucleating pathway" is meant to define a pathway that forms part of microcellular polymeric foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream of the nucleator and at flow rates of greater than about 1 pound polymeric material per hour), the pressure of a single- phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating rapid nucleation. A nucleating pathway defines, optionally with other nucleating pathways, a nucleation or nucleating region of a device of the invention.

"Reinforcing agent", as used herein, refers to auxiliary, essentially solid material constructed and arranged to add dimensional stability, or strength or toughness, to the material. Such agents are typified by fibrous material as described in U.S. Patent Nos. 4,643,940 and 4,426,470. "Reinforcing agent" does not, by definition, necessarily include filler or other additives that are not constructed and arranged to add dimensional stability. Those of ordinary skill in the art can test an additive to determine whether it is a reinforcing agent in connection with a particular material.

For purposes of the present invention, microcellular material is defined as foamed material having an average cell size of less than about 100 microns in diameter, or material of cell density of generally greater than at least about 10⁶ cells per cubic centimeter, or preferably both. "Cell density" is defined as the number of cells per cubic centimeter of original, unexpanded polymeric material. Non-microcellular foams have cell sizes and cell densities outside of these ranges. The void fraction of microcellular material generally varies from 5% to 98%. Supermicrocellular material is defined for purposes of the invention by cell sizes smaller than 1 μm and cell densities greater than 10 cells per cubic centimeter. In preferred embodiments, microcellular material of the invention is produced having average cell size of less than about 50 microns. In some embodiments particularly small cell size is desired, and in these embodiments material of the invention has average cell size of less than about 20 microns, more preferably less than about 10 microns, and more preferably still less than about 5 microns. The microcellular material preferably has a maximum cell size of about 100 microns. In embodiments where particularly small cell size is desired, the material can have maximum cell size of about 50 microns, more preferably about 25 microns, more preferably about 15 microns, more preferably about 8 microns, and more preferably still about 5 microns. A set of embodiments includes all combinations of these noted average cell sizes and maximum cell sizes. For example, one embodiment in this set of embodiments includes microcellular material having an average cell size of less than about 30 microns with a maximum cell size of about 50 microns, and as another example an average cell size of less than about 30 microns with a maximum cell size of about 35 microns, etc. That is, microcellular material designed for a variety of purposes can be produced having a particular combination of average cell size and a maximum cell size preferable for that ' purpose. Control of cell size is described in greater detail below.

In one embodiment, essentially closed-cell microcellular material is produced in accordance with the techniques of the present invention. As used herein, "essentially closed-cell" is meant to define material that, at a thickness of about 100 microns, contains no connected cell pathway through the material.

Referring now to Fig. 1, a molding system 30 is illustrated schematically that can be used to carry out molding according to a variety of embodiments of the invention. Although Fig. 1 (as well as Fig. 2) is similar to figures shown in prior, commonly- owned, published patent applications, differences between this and prior applications will become apparent from the description herein. International Patent Publication WO 98/08667, referenced above, can be consulted for a detailed description of Figs. 1 and 2. System 30 of Fig. 1 includes a barrel 32 having a first, upstream end 34, and a second, downstream end 36 connected to a molding chamber 37. Mounted for rotation within barrel 32 is a screw 38 operably connected, at its upstream end, to a drive motor 40. Although not shown in detail, screw 38 includes feed, transition, gas injection, mixing, and metering sections. Positioned along barrel 32, optionally, are temperature control units 42. Barrel 32 is constructed and arranged to receive a precursor of molded polymeric material, specifically, a precursor of molded polymeric microcellular material. As used herein, "precursor of molded polymeric material" is meant to include all materials that are fluid, or can form a fluid and that subsequently can harden to form a molded polymeric article. Typically, the precursor is defined by thermoplastic polymer pellets, but can include other species. Preferably, a thermoplastic polymer or combination of thermoplastic polymers is used in the invention and is selected from among amorphous, semicrystalline, and crystalline material including polyolefins such as polyethylene and polypropylene, fluoropolymers, cross-linkable polyolefins, polyamides, polyimides, polyesters, polyvinyl chloride, polyaromatics such as styrenic polymers (e.g., polystyrene, ABS), and the like. Thermoplastic elastomers can be used as well, especially metallocene-catalyzed polyethylene. Included as polymers that can be molded in accordance with the invention are those having a melt flow rate of less than about 40, or having a melt flow rate of less than about 10. In one embodiment the precursor can be defined by species that will react to form microcellular polymeric material as described, under a variety of conditions, e.g. thermosetting polymers.

Typically, introduction of the precursor of polymeric material utilizes a standard hopper 44 for containing pelletized polymeric material to be fed into the extruder barrel through orifice 46, although a precursor can be a fluid prepolymeric material injected through an orifice and polymerized within the barrel via, for example, auxiliary polymerization agents. In connection with the present invention, it is important only that a fluid stream of polymeric material be established in the system.

Immediately downstream of downstream end 48 of screw 38 in Fig. 1 is a region 50 which can be a temperature adjustment and control region, auxiliary mixing region, auxiliary pumping region, or the like. In one embodiment, region 50 can be replaced by a second screw in tandem which can include a cooling region. In an embodiment in which screw 38 is a reciprocating screw in an injection molding system, region 50 can define an accumulation region in which a single-phase, non-nucleated solution of polymeric material and a blowing agent is accumulated prior to injection into mold 37. In preferred embodiments a supercritical fluid additive is used in injection molding techniques, and is mixed with polymeric material in polymer processing apparatus such as that described with reference to Fig. 1 prior to injection of the resulting mixture into a mold. The supercritical fluid additive preferably serves also as a blowing agent for forming a molded polymeric foam article, preferably a molded microcellular polymeric article. Thus "supercritical fluid additive" and "blowing agents" are used interchangeably herein, although it is to be understood that in some embodiments of the invention this additive is used in molding processes but solid (non-foam) parts result. Advantages associated with use of a supercritical fluid additive are described more fully below.

Techniques of the invention preferably use a physical supercritical fluid additive (blowing agent), that is, an agent that is a gas under ambient conditions (described more fully below). However, chemical blowing agents can be used and can be formulated with polymeric pellets introduced into hopper 44. Suitable chemical blowing agents include those typically relatively low molecular weight organic compounds that decompose at a critical temperature or another condition achievable in extrusion and release a gas or gases such as nitrogen, carbon dioxide, or carbon monoxide. Examples include azo compounds such as azo dicarbonamide.

As mentioned, in preferred embodiments a physical blowing agent is used. One advantage of embodiments in which a physical blowing agent, rather than a chemical blowing agent, is used is that recyclability of product is maximized. Thus, material of the present invention in this set of embodiments includes residual chemical blowing agent, or reaction by-product of chemical blowing agent, in an amount less than that inherently found in articles blown with 0.1% by weight chemical blowing agent or more, preferably in an amount less than that inherently found in articles blown with 0.05% by weight chemical blowing agent or more. In particularly preferred embodiments, the material is characterized by being essentially free of residual chemical blowing agent or free of reaction by-products of chemical blowing agent. That is, they include less residual chemical blowing agent or by-product that is inherently found in articles blown with any chemical blowing agent. In this embodiment, along barrel 32 of system 30 is at least one port 54 in fluid communication with a source 56 of a physical blowing agent. Any of a wide variety of physical blowing agents known to those of ordinary skill in the art such as, hydrocarbons, chlorofluorocarbons, nitrogen, carbon dioxide, helium, and the like can be used in connection with the invention, or mixtures thereof, and, according to a preferred embodiment, source 56 provides an atmospheric gas, preferably nitrogen or carbon dioxide as a blowing agent. Any of these can be used alone or in combination. As supercritical fluid additive, in one embodiment solely supercritical carbon dioxide, nitrogen, or a combination is used. Supercritical carbon dioxide or nitrogen additive can be introduced into the extruder and made to rapidly form a single-phase solution with the polymeric material either by injecting the additive as a supercritical fluid, or injecting it as a gas or liquid and allowing conditions within the extruder to render it supercritical, in many cases within seconds. Injection of the additive into the extruder in a supercritical state is preferred.

The mixture (preferably a single-phase solution) of supercritical additive and polymeric material formed in this manner has a very low viscosity which advantageously allows lower temperature molding, as well as rapid filling of molds having close tolerances to form very thin molded parts, parts with very high length-to-thickness thickness ratios, parts including thicker distal regions, molding carried out at low clamp force, etc., discussed in greater detail below.

A pressure and metering device 58 typically is provided between blowing agent (or additive) source 56 and has at least one port 54. Device 58 can be used to meter the mass of the blowing agent between 0.01 lbs/hour and 70 lbs/hour, or between 0.04 lbs/hour and 70 lbs/hour, and more preferably between 0.2 lbs/hour and 12 lbs/hour so as to control the amount of the blowing agent in the polymeric stream within the extruder to maintain the blowing agent at a desired level. The amount of supercritical fluid additive in the polymeric stream can be controlled to be at a variety of levels, including between about 0.1% and 25% by weight of the mixture, or between about 1.0% and 25% by weight, or between about 6% and 20% by weight, or between about 8% and 15% by weight, or between about 10% and 12% by weight. In one set of the embodiments one of a selection of maximum supercritical fluid additive levels is selected. In this set of embodiments a supercritical additive is provided in an amount of less than 10%, or less than 5%, 3%, 1%, or even less than 0.5%, 0.05%, or 0.01% by weight. The particular blowing agent used (carbon dioxide, nitrogen, etc.) and the amount of blowing agent used can be selected by those of ordinary skill in the art with benefit of the present disclosure, based upon the polymer, desired viscosity reduction, the density reduction, cell size and physical properties desired. In embodiments where nitrogen is used as blowing agent, blowing agent is present in an amount between 0.1% and 2.5%, preferably between 0.1% and 1.0% in some cases, and where carbon dioxide is used as blowing agent the mass flow of the blowing agent can be between 0.1% and 12% in some cases, between 0.5% and 6.0% in preferred embodiments. Although port 54 can be located at any of a variety of locations along the barrel, according to a preferred embodiment it is located just upstream from a mixing section 60 of the screw and at a location 62 of the screw where the screw includes unbroken flights. A preferred embodiment of the blowing agent port includes multiple ports, e.g. two ports on opposing top and bottom sides of the barrel. In this preferred embodiment, port 54 is located at a region upstream from mixing section 60 of screw 38 (including highly-broken flights) at a distance upstream of the mixing section of no more than about 4 full flights, preferably no more than about 2 full flights, or no more than 1 full flight. Positioned as such, injected blowing agent is very rapidly and evenly mixed into a fluid polymeric stream to quickly produce a single-phase solution of the foamed material precursor and the blowing agent. Port 54, in a preferred embodiment, is a multi-hole port including a plurality of orifices connecting the blowing agent source with the extruder barrel. A plurality of ports can be provided about the extruder barrel at various positions radially and can be in alignment longitudinally with each other. For example, a plurality of ports can be placed at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions about the extruder barrel, each including multiple orifices. In this manner, where each orifice is considered a blowing agent orifice, the invention includes extrusion apparatus having at least about 10, preferably at least about 40, more preferably at least about 100, more preferably at least about 300, more preferably at least about 500, and more preferably still at least about 700 blowing agent orifices in fluid communication with the extruder barrel, fluidly connecting the barrel with a source of blowing agent. Also in preferred embodiments is ah arrangement in which the blowing agent orifice or orifices are positioned along the extruder barrel at a location where, when a preferred screw is mounted in the barrel, the orifice or orifices are adjacent full, unbroken flights 62. In this manner, as the screw rotates, each flight, passes, or "wipes" each orifice periodically. This wiping increases rapid mixing of blowing agent and fluid foamed material precursor by, in one embodiment, essentially rapidly opening and closing each orifice by periodically blocking each orifice, when the flight is large enough relative to the orifice to completely block the orifice when in alignment therewith. The result is a distribution of relatively finely-divided, isolated regions of blowing agent in the fluid polymeric material immediately upon injection and prior to any mixing. In this arrangement, at a standard screw revolution speed of about 30 rpm, each orifice is passed by a flight at a rate of at least about 0.5 passes per second, more preferably at least about 1 pass per second, more preferably at least about 1.5 passes per second, and more preferably still at least about 2 passes per second. In preferred embodiments, orifices are positioned at a distance of from about 15 to about 30 barrel diameters from the beginning of the screw (at upstream end 34).

Downstream of region 50 is a nucleator 66 constructed to include a pressure-drop nucleating pathway 67. As used herein, "nucleating pathway" in the context of rapid pressure drop is meant to define a pathway that forms part of microcellular polymer foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream of the nucleator and at flow rates of greater than about 1 lb polymeric material per hour), the pressure of a single-phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating nucleation. Nucleator 66 can be located in a variety of locations downstream of region 50 and upstream of mold 37. In a preferred embodiment, nucleator 66 is located in direct fluid communication with mold 37, such that the nucleator defines a nozzle connecting the extruder to the molding chamber and the nucleated polymer releasing end 70 defines an orifice of molding chamber 37. According to one set of embodiments, the invention lies in placing a nucleator upstream of a mold. Although not illustrated, another embodiment of nucleator 66 includes a nucleating pathway 67 constructed and arranged to have a variable cross-sectional dimension, that is, a pathway that can be adjusted in cross- section. A variable cross-section nucleating pathway allows the pressure drop rate in a stream of fluid polymeric material passing therethrough to be varied in order to achieve a desired nucleation density. While pathway 67 defines a nucleating pathway, some nucleation also may take place in the mold itself as pressure on the polymeric material drops at a very high rate during filling of the mold.

The system of Fig. 1 illustrates one general embodiment of the present invention in which a single-phase, non-nucleated solution of polymeric material and blowing agent is nucleated, via rapid pressure drop, while being urged into molding chamber 37 via the rotation action of screw 38. This embodiment illustrates an intrusion molding technique and, in this embodiment, only one blowing agent injection port 54 need be utilized. In another embodiment, screw 38 of system 30 is a reciprocating screw and a system defines an injection molding system. In this embodiment screw 38 is mounted for reciprocation within barrel 32, and includes a plurality of blowing agent inlets or injection ports 54, 55, 57, 59, and 61 arranged axially along barrel 32 and each connecting barrel 32 fluidly to pressure and metering device 58 and a blowing agent source 56. Each of injection ports 54, 55, 57, 59, and 61 can include a mechanical shut- off valve 154, 155, 157, 159, and 161 respectively, which allow the flow of blowing agent into extruder barrel 38 to be controlled as a function of axial position of reciprocating screw 38 within the barrel.

The embodiment of the invention involving a reciprocating screw can be used to produce non-microcellular foams or microcellular foam. Where non-microcellular foam is to be produced, the charge that is accumulated in distal region 50 can be a multi-phase mixture including cells of blowing agent in polymeric material, at a relatively low pressure. Injection of such a mixture into mold 37 results in cell growth and production of conventional foam. Where microcellular material is to be produced, a single-phase, non-nucleated solution is accumulated in region 50 and is injected into mold 37 while nucleation takes place.

Although not shown, molding chamber 37 can include vents to allow air within the mold to escape during injection. The vents can be sized to provide sufficient back pressure during injection to control cell growth so that uniform foaming occurs. In another embodiment, a single-phase, non-nucleated solution of polymeric material and blowing agent is nucleated while being introduced into an open mold, then the mold is closed to shape a molded article.

According to another embodiment an injection molding system utilizing a separate accumulator is provided. Referring now to Fig. 2, an injection molding system 31 includes an extruder similar to that illustrated in Fig. 1. The extruder can include a reciprocating screw as in the system of Fig. 1. At least one accumulator 78 is provided for accumulating molten polymeric material prior to injection into molding chamber 37. The extruder includes an outlet 51 fluidly connected to an inlet 79 of the accumulator via a conduit 53 for delivering a mixture, such as a non-nucleated, single-phase solution of polymeric material and blowing agent to the accumulator.

Accumulator 78 includes, within a housing 81, a plunger 83 constructed and arranged to move axially (proximally and distally) within the accumulator housing. The plunger can retract proximally and allow the accumulator to be filled with polymeric material/blowing agent through inlet 79 and then can be urged distally to force the polymeric material/blowing agent mixture into mold 37. When in a retracted position, a charge defined by single-phase solution of molten polymeric material and blowing agent is allowed to accumulate in accumulator 78. When accumulator 78 is full, a system such as, for example, a hydraulically controlled retractable injection cylinder (not shown) forces the accumulated charge through nucleator 66 and the resulting nucleated mixture into molding chamber 37. This arrangement illustrates another embodiment in which a non-nucleated, single-phase solution of polymeric material and blowing agent is nucleated as a result of the process of filling the molding chamber. Alternatively, a pressure drop nucleator can be positioned downstream of region 50 and upstream of accumulator 78, so that nucleated polymeric material is accumulated, rather than non- nucleated material, which then is injected into mold 37.

In another arrangement, a reciprocating screw extruder such as that illustrated in Fig. 1 can be used with system 31 of Fig. 2 so as to successively inject charges of polymeric material and blowing agent (which can remain non-nucleated or can be nucleated while being urged from the extruder into the accumulator) while pressure on plunger 83 remains high enough so that nucleation is prevented within the accumulator (or, if nucleated material is provided in the accumulator cell growth is prevented). When a plurality of charges have been introduced into the accumulator, shut-off valve 64 can be opened and plunger 83 driven distally to transfer the charge within the accumulator into mold 37. This can be advantageous for production of very large parts.

A series of valves, conduits, etc. associated with the arrangement of Fig. 2 is thoroughly described in International Patent Publication No. WO 98/31521, referenced above. The system can be used to make skin/foam/skin structures controllably. The invention involves, in all embodiments, the ability to maintain pressure throughout the system adequate to prevent premature nucleation where nucleation is not desirable (upstream of the nucleator), or cell growth where nucleation has occurred but cell growth is not desired or is desirably controlled.

The invention provides for the production of molded microcellular polymeric articles or molded non-microcellular polymeric foam articles of a shape of a molding chamber, having a void volume of at least about 5%. Preferably, the void volume is at least about 10%, or 15%, 20%, 25%, 30%, 40%, or 50%. The articles of the invention can include the above-noted void volumes in those sections that are of cross-sectional dimensions noted herein.

As mentioned, dimensional stability is an important aspect of the present invention. "Dimensional stability" will be defined below in a variety of ways. Generally, this term defines an article that deviates very little, or not at all, from the shape of the mold in which it was made. This allows for very flat parts, or parts having very flat sections, to be made accurately and reproducibly, which is a very important factor in the manufacture of certain classes of articles.

While not wishing to be bound by any theory, the inventors provide the following explanation for superior dimensional stability achieved in embodiments of the invention using a supercritical fluid additive in the production process. Polymer molecules typically assume a variety of non-linear shapes in their lowest-energy (most-relaxed) state, such as coiled configurations. When such polymers are melted and injected through a relatively small orifice into a mold, their molecules may become elongated, or stretched, as they pass through the orifice and into the mold, and may remain somewhat elongated as the polymer cools and hardens within the mold. That is, polymeric articles molded according to typical known techniques can be defined by a solidified molecular arrangement in which individual molecules are "frozen", not in their lowest energy (most relaxed) state, but in a higher-energy, "stressed" configuration. This can have two detrimental effects. First, if a part so produced is removed from a mold prior to complete cooling and solidification (at a temperature at or too close to its softening temperature or heat deflection temperature), then after removal from the mold the molecules within the article may undergo slight reconfiguration toward their most-relaxed shape, until the article cools sufficiently. This can result in warpage, or other macroscopic change in shape of the article, i.e., deviation from dimensional stability. To reduce or minimize such warpage, articles can be maintained within the mold for longer periods of time ("hold time") to allow them to sufficiently cool and harden. Yet this significantly increases cycle time associated with molding processes, a sigmficant manufacturing disadvantage. Moreover, even with an increase in hold time in a mold (increased cycle time), warpage can result. A second detrimental effect is that even if, in the above scenario, articles are held within a mold for sufficient time to allow sufficient cooling and hardening such that, when removed from the mold, they exhibit a shape very nearly matching the interior shape of the mold, they carry within themselves the stress of higher-energy molecular states, albeit frozen. This means that if these articles are heated, intentionally or accidentally, at a later time to a temperature close to their softening point, the stressed molecules may begin to relax and cause macroscopic warpage of the articles. Most readers will be familiar with this phenomenon as it occurs when a molded polymeric article is left on the dash board of a closed car on a sunny, hot day, causing it to become warped.

Use of a supercritical fluid additive in molding techniques of the present invention can reduce or eliminate warpage in molded articles by significantly reducing the degree of stressed configuration in which molecules are frozen in molding processes. The supercritical fluid additive significantly reduces viscosity in a molten polymer injected through an orifice, which can result in at least two important benefits: better dimensional stability at faster cycle times, and better dimensional stability overall. The first of these benefits is achieved as follows. As viscosity of polymer injected into a mold is reduced, the temperature at which the polymer can be injected into the mold so as to completely fill the mold can be reduced. This means that the temperature at which the resultant part can safely be removed from the mold without loss of dimensional stability can be more quickly reached, reducing overall cycle time. Stated another way, while there may be an inherent trade-off between dimensional stability and cycle time (the more quickly a part is removed from a mold the more likely it may be to warp), the present invention involves taking advantage of the viscosity reduction provided by a supercritical fluid additive to provide the best dimensional stability at the fastest cycle time.

The second of these benefits, better overall stability, results from the fact that when the viscosity of a polymer injected into a mold is minimized via addition of a supercritical fluid additive, the polymer's molecules are freer to relax to their lowest energy configuration within the mold. Thus, when the resultant part is removed from the mold, even if it is still slightly warm and would have the ability to warp slightly, its warpage may be minimized or eliminated because of the relatively relaxed initial state of the molecules. Moreover, the resultant part will be more heat-tolerant; upon exposure to heat at a later point in time, even if it is heated at or near its softening temperature or heat deflection temperature, it may not warp or warp only very slightly because its molecules are already in a relaxed state. Accordingly, one aspect of the invention involves a series of methods of molding polymeric articles at lower cycle times than standard articles, preferably with the same or better dimensional stability or both, or at the same cycle time with better stability. As a comparison, or point of reference, in one embodiment a polymer molding system including an injection unit and a mold, configured to mold solid polymeric articles, is provided. The system is constructed and arranged to deliver blowing-agent-free molten polymeric material from the injection unit into the mold, to solidify the polymeric material in the mold, and to eject a molded polymeric article from the mold. The polymeric article has a void volume of essentially zero because of the blowing-agent-free polymeric material from which it is formed. The system is configured so as to mold solid polymeric articles in a minimum cycle time under a particular combination of set conditions. Those of ordinary skill in the art will understand what is meant by a system constructed to carry out solid polymer molding at a minimum cycle time under set conditions. The set conditions will include, for example, the size of the mold, the interior volume of the mold, the clamp force of the mold, the capacity of the extruder, the heat capacity of the mold, wall thickness, mass, and heat capacity (factors contributing to the ability of the mold to rapidly cool polymeric material within the mold), and the like. The method of the invention involves carrying out molding under the set conditions at a second, faster minimum cycle time no more than 80% as long as the first cycle time. This is accomplished by delivering polymeric material, which can be identical to the polymeric material described above, but mixed with a supercritical fluid additive, from the extruder into the mold and allowing the material to solidify in the mold, and ejecting a molded polymeric article, from the mold in this reduced cycle time. The cycle time can be, in preferred embodiments, no more than 70%, or no more than 60%, or 50%, or even 40% the first cycle time under the set conditions.

This method preferably results in a polymeric article made using a supercritical fluid additive that has at least the same stability as a solid polymer article molded at longer cycle times under the set conditions. This stability can be described in a number of ways, one of which follows. The mold has an interior shape and the polymer molding system is constructed and arranged to eject a solid molded polymeric article (as a comparison, or frame of reference) such that the solid article undergoes warpage/deviation of at least one section from the shape of the mold interior in an amount of at least about 0.4% or shrinkage of at least about 0.2% or both. The method of the invention involves ejecting the molded polymeric article made using a supercritical fluid where this molded article is free of warpage or undergoes warpage including deviation of at least one section from the shape of the mold interior by no more than about 0.3%, or shrinkage of no more than about 0.16%, or both. In another embodiment the solid (comparative) article will include deviation of at least one section from the shape of the mold interior by at least about 0.4%, or shrinkage of at least about 0.2%, or both, and the second molded polymeric article will undergo warpage including deviation of at least one section from the shape of the mold interior by no more than about 0.2%, or shrinkage of no more than about 0.1%, or both. Referring now to Fig. 3, warpage and shrinkage, in this context, is defined. Fig. 3 shows a mold having a section 300 including a flat interior surface 302 of length 1. Produced in the mold is a polymeric article 304 that deviates from the shape of the interior surface of mold section 300. It exhibits both warpage and shrinkage. Warpage is defined as the deviation of surface 306 of article 304 from the shape of interior surface 302 of the mold against which it was molded. Dotted line 308 represents the shape of the interior surface 302 of the mold which, if part 304 had matched exactly the interior of the mold, it would have matched. The warpage of surface 306 is defined as that distance w from which surface 306 deviates from the shape of surface 302. Defined as a percentage, it is the distance w divided by the length 1. Thus, looking at the length of any section of a molded part against that section of a mold against which it was molded, and the deviation of the shape of the molded article along that length divided by the length, defines warpage.

Part 304 also, as shown, exhibits shrinkage. Specifically, the length of the part, 1' is less than the length 1 of the corresponding portion of mold 300. Shrinkage is defined as the amount of reduction of length divided by the original length (1 - 1' /l). That is, viewing any section of a molded article in a particular dimension, as compared to a matching section of the mold in which it was formed, shrinkage is defined as the reduction in that dimension of the molded article as compared to the matching dimension of the mold. Referring again to the description above of the comparison of a polymeric article molded using a supercritical fluid additive versus a solid polymeric article molded from blowing-agent-free material, in another embodiment the system is constructed to eject the solid polymeric article (comparative) such that it undergoes warpage including deviation of at least one section from the shape of the mold interior by at least about w, or shrinkage of at least about s, or both, and the method involves ejecting the second, molded polymeric article formed using a supercritical fluid additive wherein the second article undergoes warpage including deviation of a section, corresponding to the first section of the solid article, from the shape of the mold interior by less than about 80% (w) or shrinkage of less than about 80% (s), or both. In preferred embodiments, the second article undergoes warpage by less than about 70%, 60%, 50%, 40%, 30%, 20%, or 10% w, or shrinkage of less than about 70%, 60%, 50%, 40%, 30%, 20%, or 10% s, or both. Specifically, in this set of embodiments a first, solid molded polymeric article may be formed from the system and undergo warpage by at least about 5%, or shrinkage of at least about 5%, or both, and the second polymeric article undergoes warpage by less than 3%, or shrinkage of less than 3%, or both. This comparative shrinkage and warpage is of identical sections of the first molded polymeric article and the second molded polymeric article. In another set of embodiments the second article, molded using a supercritical fluid additive, may be molded at the same cycle time as that of the first, solid polymeric article, but exhibits better stability. Specifically, the system in this set of embodiments is constructed to form a first, solid article that undergoes warpage in an amount of at least about w, or shrinkage of at least about s, or both. The second polymeric article, formed using a supercritical fluid additive, is formed in a cycle time no greater than the cycle time used to form the solid polymeric article. The second article undergoes warpage of less than about 80% w, or shrinkage of less than about 80% s, or both (with respect to identical sections of the articles. In preferred embodiments warpage and/or shrinkage of less than about 70%, 60% or other lower percentages noted above are observed. The supercritical fluid additive can serve at least two purposes, one being reducing the viscosity of molten polymeric material injected into a mold, and a second being that of a blowing agent, namely, forming a foamed polymeric article, preferably a microcellular article, having a void volume and/or cell density, as described above. In cases where little or no void volume is desired, low levels of supercritical fluid additive can be used. The advantages recognized in accordance with the invention through the use of a viscosity-reducing supercritical fluid additive can be achieved even while forming an article having a void volume of less than about 5%, or less than about 3%, or 1%, or a void volume of essentially 0. That is, the supercritical fluid additive may be present in an amount of less than about 1%, or less than about 0.5%.

Referring again to Fig. 3, preferred second molded polymeric articles undergo shrinkage, relative to mold dimensions, of less than about 0.2%, preferably less than about 0.1%, more preferably less than about 0.02%, more preferably less than about 0.002%. This shrinkage (specifically, lack of shrinkage) is observed well after removal from the mold, for example at least five minutes after ejection of the article from the mold, or even at least about 12, 24, or 48 hours after ejection from the mold. The article also preferably undergoes warpage, relative to mold dimensions, of less than about 0.2%, preferably less than about 0.1%, more preferably less than about 0.02%, more preferably less than about 0.002% even at least five minutes after ejection from the mold, or even at least about 12, 24, or 48 hours after ejection from the mold.

As mentioned, the supercritical fluid additive allows for the injection of a precursor of a molded polymeric article, preferably a molded microcellular polymeric material, into a mold at reduced temperatures. Preferably, injection takes place at a mold temperature of less than about 250°C, preferably less than about 150°C, 75°C, 50°C, or even less than about 10°C.

The invention also provides a system and method to produce foam molded parts with surfaces replicating solid parts. At least a portion of the surface of these parts is free of splay and swirl visible to the naked human eye.

The system of the invention also allows very rapid cycle times of injection molding of polymeric material of void volume of at least about 5% (or higher values noted above). In particular, a cycle time (injecting precursor material, allowing the material to solidify in the molding chamber as a polymeric article, and removing the article from the mold and repeating) can be carried out at cycle time of less than about 3 minutes, more preferably less than about 2 minutes, and more preferably still less than about 1 minute for relatively large parts such as automotive exterior lower door panels and other parts on the order of 3-6 feet in length and width, and of a mass of about 2-20 lbs. For medium-size parts, for example, office system chasses, such as printer chasses, cycle times of less than about 1 minute, more preferably less than about 45 seconds, and more preferably still less than about 30 seconds can be achieved. These medium-sized parts are of typically about Vi lb to about 2 lbs. in weight. For smaller parts, such as matrix trays and/or other parts of weight less than about Vi lb, cycle times of less than about 30 seconds, more preferably less than about 15 seconds, more preferably less than about 10 seconds, and more preferably still less than about 5 seconds can be achieved.

The invention also allows for significantly reduced clamp force in injection molding processes. This aspect of the invention can be described by comparison of an arrangement set up to mold solid polymeric articles, with an arrangement of the invention for molding articles where a supercritical fluid additive is included.

Specifically, a polymer molding system that includes an extruder and a mold constructed and arranged to deliver blowing-agent-free molten polymeric material from the extruder into the mold and to eject a molded polymeric article from the mold having a void volume of essentially zero, will be set up with a minimum mold clamp force. That is, the system will include a clamp force sufficient to keep the mold closed during injection.

The process of the invention allows such an apparatus to operate at a mold clamp force no more than 95% of the clamp force at which the system is held during molding of solid

(blowing-agent-free) material. Preferably, the second mold clamp force (that clamp > ) force required using supercritical fluid additive), is no more than about 85%, or 75%, or

65%, 55%, 45%, or even no more thanabout 35% of the clamp force for the solid material. More specifically, a molded polymeric article can be made that has an average wall thickness of no more than about 0.125 inch, while maintaining a clamp force on the mold of no more than about 3.5 ton/in , or no more than about 3, 1.75, 1.5, or 1 ton/in . According to another aspect of the invention a series of molded products are provided that exhibit reduced warpage and/or shrinkage and exhibit benefits associated with dimensionally-stable products. It is to be understood that articles of the invention can be produced using methods of the invention, thus a series of embodiments of the invention include essentially any combination of method steps and article characteristics of the invention.

In one embodiment a molded polymeric article formed using a supercritical fluid additive, optionally a foam article, is produced that has a substantially planar freestanding portion having a length, a width and a height of no more than about 10% the total of the length and width. The article is one that has been formed in a mold having a substantially planar mold portion corresponding to the substantially planar free-standing portion of the article. The article matches the shape of the mold such that a first line of planarity extending in the substantially planar mold portion is reproduced in the article as a second line of planarity or a curve having a radius of curvature of no less than 200 times the length of the first line of planarity.

With reference to Fig. 3, mold 300 includes a substantially planar mold portion defined by flat wall 302, and article 304 includes a substantially planar free-standing portion defined by surface 306. A first line of planarity in the mold portion, lying along surface 302 in the direction of 1 is reproduced in the article 304 as a second line of planarity or a curve lying along surface 306 in the same direction as the first line lying along surface 302. The second line lying along 306 has a radius of curvature no less than 10 x 1. As will be appreciated from this description, this arrangement requires close matching of planar portions of a mold with planar portions of an article. In preferred embodiments the second line of planarity in the article has a radius of curvature no less than 200 times, preferably no less than 1000 times, preferably no less than 1500 times, and preferably still no less than 2000 times the length of a corresponding line of planarity in the mold. Even flatter articles can be produced that meet these characteristics, including those that have a height no more than about 8% the total of their length and width, or those having a height no more than 5%, 3%, 2%, or 1% the total of their length and width.

Referring to Fig. 4 another example is shown. Fig. 4 shows a cross-sectional view of a mold 400 having a substantially planar mold portion 402. A line of planarity 404 can be drawn through portion 402. An article 406 formed in mold 402, as illustrated, is warped very slightly relative to mold portion 402, yet falling within the definition of dimensionally stable in accordance with the invention. Specifically, surface 408 of the article has a radius of curvature slightly smaller than the radius of curvature of the corresponding inner surface 410 of the mold. Importantly, line of planarity 404 of the mold is reproduced as a curve 412 in the article, where curve 412 has a radius of curvature less than 200 times the length of line 404 within the mold (1"). Curve 412 passes through the precise portions of article 406 that it had passed through when article 406 was within mold 400 and the article corresponded precisely to the interior of mold portion 402. In embodiments where article 406 corresponds precisely to the dimensions of the interior portion 402 of the mold, curve 412 is a line.

The invention also can involve (described with reference to Fig. 4) an article having a surface 408 corresponding to an interior surface 410 of a mold where, if surface 410 is flat, surface 408 is flat or has a radius of curvature no less than 200 times the largest dimension (length) of surface 410. Where surface 410 is curved, the radii of curvature of surfaces 408 and 410 differ by no more than 10%, preferably no more than 5%, preferably no more than 3%, preferably no more than 2%, preferably no more than 1%, preferably no more than 0.5%, and more preferably no more than 0.2%. According to another embodiment, a polymeric article formed using a supercritical fluid additive, optionally a foam article, shows no warpage relative to, or very little warpage relative to an article (e.g., a solid article) produced under essentially identical conditions but without supercritical fluid additive. Specifically, a polymeric foam article is provided that has a substantially planar free-standing portion having a length, a width, and a height, wherein the height is no more than about 10% the total of the length and width (or one of lesser heights described above). The article is formed in a mold. A comparative polymeric article of the same polymeric composition, formed in the same mold under conditions identical to those in which the foam article is formed but without supercritical fluid additive, has a substantially planar free-standing portion corresponding to the substantially planar free-standing portion of the foam article. A first line of planarity extending in the substantially planar free-standing portion of the first article is reproduced in the article made using a supercritical fluid as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity, or no less than 500 times, 1000 times, or other multiple of the first line of planarity described above. As described above with respect to the relationship between surface 408 of article 406 and interior surface 410 of mold 400, in this embodiment a radius of curvature of the second article and the radius of curvature of the solid article differ by no more than 10%, or smaller differences as described above.

Referring together to Figs. 3 and 4, article 304 and 406 need not necessarily have a continuous flat surface or nearly flat surface to fall within the description of a surface that matches the interior surface of a mold, or has a line of planarity or curve matching or closely corresponding to a line of planarity in a mold. For example, surface 306 of article 304 may be discontinuous in that it includes portions that terminate at and therefore help define surface 306, and intervening voids. For example, an injection- molded table with four legs that each ideally terminate in a single plane defines a situation in which the bottoms of the legs define a surface corresponding to a surface within the mold. If the bottoms of the four leg portions of the mold fall precisely within a plane, then the bottoms of the four legs of the molded table will form a plane, or will define a surface having a radius of curvature close to that of the plane as described above.

In another embodiment, a dimensionally-stable article of the invention is an injection molded polymeric article including a substantially planar free-standing portion having a length, a width, and a height, wherein the height is no more than about 10% the total of the length and width (or smaller height as described above), the length and width defining a first area of at least about 25 cm². The article includes four commonly-facing surface points three of which define a plane and the fourth falling no further than 0.2 cm outside of the plane. The four surface points defined therebetween an area at least 75% the first area, or at least 80%, 85%, 90%, 95%, or 100% the first area. That is, the four surface points can each define an extremity, or outside boundary of the substantially planar portion. The first surface area, in other embodiments, is at least 50 cm², or 100 cm², or 200, 300, 400, or greater than 500 cm². The fourth commonly-facing surface point falls no further than 0.05 cm outside of the plane in preferred embodiments, preferably no further than 0.02, or 0.01, or 0.005 cm outside of the plane.

In another embodiment an injection molded polymeric article is produced that includes an average part thickness of no more than about 0.3 cm, the article including at least 4 commonly-facing surface points. Three of the surface points define a plane and the fourth falls no further than 0.1 cm outside of the plane. The at least four surface points define therebetween an area of at least about 25, 50, 75, 100, 200, or 500 cm .

The average part thickness, in other embodiments, may be no more than about 0.2 cm, or 0.1 cm, or 0.05 cm.

Articles of the invention are thermally robust. As described above, they do not include significant internal molecular stress, and when heated do not warp or warp only slightly. Specifically, articles 304 or 406, or other articles of the invention, when heated at a temperature within 5°C of their heat deflection temperature (HDT), reproduce a first line of planarity (with reference to the description of Figs. 3 and 4) in the article prior to heating as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity. Preferred articles will exhibit a second line of planarity or curve having a radius of curvature no less than 500, 1000, 1500, or 2000 times the length of the first line of planarity. Preferably, this dimensional stability is achieved even when the article is heated at its HDT, or at 50 °C, 100 °C, 150 °C, or even 200 °C above its HDT. Nery thin articles can be produced in accordance with the invention that meet dimensional stability characteristics described herein. For example, one article has a maximum thickness of no more than about 0.08", or no more than 0.040", 0.005", 0.004", 0.003", or 0.002". Articles of the invention can include at least one portion having a length-to-thickness ratio of at least about 300:1, 450:1, 600:1, 750:1, 900:1, 1200:1, 1500:1, 1800:1, or 2000:1. Articles with high length-to-thickness ratios, such as at least 300:1 or greater, can exhibit such ratios across a distance of at least about 1" measured in a direction extending away from a gate location on the article, or at least about 2", 5", 10", or 20". Referring now to the remainder of the figures, a variety of molded articles in accordance with the invention will be described, which articles can be made according to any of the methods of the invention.

Fig. 5 is an isometric view of an injection-molded tray 501. The tray typically is of a length on the order of 2-3 feet and a width on the order of 1-2 feet, with an overall height (from the lowest projection to the highest projection) of less than approximately 4", well under 10% the total of the length and width. The article rests on lower points of contact 500, 502, 504, and 506 (506 is not shown in Fig. 5). Ideally, all four of the commonly-facing surface points 500-506 fall precisely within a plane. But where the tray undergoes very slight distortion, yet distortion within the definition of dimensionally stable in accordance with the invention, three of the surface points will define a plane and the fourth will fall no more than 0.1 cm outside of the plane, generally much less. One "substantially planar portion" of the article of Fig. 5 includes the entire article, as the overall article has a height of no more than about 10% the total of the length and width (or less). Where the entire article is considered the substantially planar portion for purposes of the present invention, surface points 500-506 each define an extremity, or outside boundary of the substantially planar portion.

Fig. 6 is a bottom plan view of the tray of Fig. 5, showing downward-facing surfaces 500-506. Surfaces 500-506 define support points, or touchdown points that fall within or closely within a plane. Fig. 7 is a right side elevational view of two stacked trays of Figs. 5 and 6 showing touchdown points 500 and 502 of first tray 501 and 500' and 502' of a second tray 503 stacked upon the first tray. It can be seen that the importance of each of touchdown points 500-506 falling within or nearly within a common plane, and points 500'-506' falling within a second common plane, etc. increases as these trays are stacked. Any lack of dimensional stability/warpage as a tray is removed from the mold or is heated after removal from the mold is exacerbated as trays are stacked one upon the other. As can be seen, a top surface 508 of the lower tray 501 of Fig. 7 mates with a bottom surface 510 of the upper tray 503, and these mating surfaces are not horizontal. Any shrinkage of one tray relative to another can create mismatch between these mating surfaces of adjacent trays, which can be disadvantageous.

Fig. 8 is a longitudinal cross-sectional view taken along line 8-8 of Fig. 7, showing the importance of mating between mating surfaces 506' and 518, and 500' and 512. Fig. 5 also illustrates another way in which the trays fall within the definition of dimensionally stable articles of the invention. In one aspect, section x of each tray defines the substantially planar free-standing portion having a length, width, and height of no more than about 10% the total of the length and width. Specifically, section x of the tray, as illustrated, is simply a flat plastic sheet. The flat sheet defines an area of at least about 25 cm², and includes at least four (the embodiment illustrated includes many more than four; essentially an infinite number) of commonly-facing surface points, three of which define a plane and the fourth falling no further than 0.1 cm outside of the plane, where the four surface points define therebetween an area of at least 75% of the area defined by the length and width. Viewing the article from the perspective of the upward- facing surface of the top tray, where portion x is rectangular, the article includes four surface points that each define the outer extremity of the rectangle that fall within this definition. The article also includes an infinite number of points that meet this definition, so long as the four points are selected at locations so as to define therebetween an area of at least 75% the area of the entire top surface. Portion x of the tray of Fig. 8 also can be considered to be a substantially planar portion such as article 304 of Fig. 3, deviating not at all or only very slightly from the shape of the mold in which it is produced. As mentioned with reference to Fig. 3, surface 306 of article 304 need not be continuous. In this context, an entire tray of Fig. 8 can be considered to fall within article 304 of Fig. 3, where the requirements for dimensionally-stable part as described with reference to Fig. 3 are met. This is shown in Fig. 9 in which tray 501, shown in longitudinal cross-section along line 8-8 of Fig. 7 (same view as Fig. 8) is defined within article 304. That is, the outer boundaries of the tray define a spatial region that matches a corresponding spatial region just as article 304 of Fig. 3 matches, or closely matches the corresponding portion of mold 300.

Referring now to Fig. 10, an isometric view of a computer printer chassis 1000 is shown. Fig. 11 is a bottom plan view of chassis 1000 and Fig. 12 is a longitudinal cross- sectional view taken along line 12-12 of Fig. 11. Chassis 1000 meets the requirements of the dimensionally-stable article in accordance with many embodiments of the invention. As one example, chassis 1000 includes four feet (touchdown points), 1002, 1004, 1006, 1008 upon which it is supported. Chassis 1000 is an example of one embodiment of the invention including an injection molded polymeric article including an average part thickness of no more than about 0.3 cm and at least four commonly-facing surface points (feet 1002-1008) all lying within a plane or three of which define a plane and the fourth falling no further than 0.1 cm outside the plane, the at least four surface points defining therebetween an area of at least 25 cm². The cross-sectional view of Fig. 12 shows thickness t_t and t of less than 0.3 cm, all other portions of the chassis also being of thickness less than 0.3 cm.

Although a printer chassis is discussed exclusively, any office equipment chassis can exemplify a dimensionally-stable article of the invention, such as a computer chassis, a computer stand, or the like.

Fig. 13 is an isometric top view of a matrix tray 1300, for holding computer chips, that defines a dimensionally-stable molded article of the invention, and Fig. 14 is an isometric bottom view of the same matrix tray. Matrix tray 1300, like tray 501, meets the definition of dimensionally-stable article according to a number of definitions herein.

Specifically, matrix tray 1300 is constructed and arranged to carry a plurality of microcircuitry chips and includes a downward-facing section including a support surface 1302. The support surface includes at least four outermost support contact points 1304, 1306, 1308, and 1310 that fall within a plane, or three of which define a plane and a fourth of which deviates from the plane by no more than 0.1 cm or other, smaller dimensions as described above (0.05, 0.02, 0.01, or 0.005 cm). Injection-molded polymeric matrix tray 1300 has dimensional stability sufficient to satisfy standards for use in the integrated circuit fabrication industry. These standards are well-known to those of ordinary skill in the art, and are met even where tray 1300 is a foam polymeric article including any of the void volumes described above, or is produced in a fast cycle time as described above. With reference to Fig. 13, bottom support surface 1302 defines a plane P.

Fig. 15 is an enlarged fragmentary detailed view of the top of tray 1300 as seen along lines 15-15 of Fig. 13. The detail shows a plurality of individual chip holders 1502, 1504, 1506, 1508 including opposing side flanges 1510 and 1512 of holder 1502, opposing flanges 1514 and 1516 of holder 1504, opposing flanges 1518 and 1520 of holder 1506, and opposing flanges 1522 and 1524 of holder 1508. The furthest upward extension (upward-facing surface) of each of the side flanges lies within a plane in preferred embodiments. This will be the case where the supporting surfaces upon which individual chips rest lie in a plane, which is important so that all of the chips supported by the tray lie in a plane. Typically, chips on trays are manipulated by robotics. Where a chip is too high relative to the plane it can be damaged by a robotic actuator. Where it is too low, it may not be picked up when it should be picked up. For these and other reasons, it is very important that such a tray be dimensionally stable. Accordingly, in one embodiment of the invention, any four flanges, each of the four associated with four different chip holders, define four commonly-facing surface points. The surface points, of course, fall within substantially planar free-standing matrix tray 1300 having a length, width, and height no more than about 10% the total of the length and width, the length and width defining a first surface area of at least about 25 cm². The four upward-facing surfaces of the four flanges define therebetween an area of at least 75% of the first area, and fall within a common plane where three define a plane and the fourth falls no further than 0.1 cm outside of the plane, preferably substantially less as described above.

Fig. 16 is an enlarged fragmentary detail view of a portion of the bottom of tray

1300 as seen along lines 16-16 of Fig. 14. Below each chip holder (shown in Fig. 15) is a supporting structure including four corner fingers 1600 which lie in plane P of Fig. 13.

Thus, any of the bottom-facing surfaces of corner fingers 1600 can define four commonly-facing surface points that fall in a plane or three of which define a plane and the fourth falling no further than 0.1 cm outside of the plane, so long as the four corner fingers define therebetween an area of at least 75% of the area defined by the entire matrix tray.

Fig. 17 is an enlarged longitudinal cross-sectional view of nested trays 1300 and

1301 along line 17-17 of Fig. 13, showing bottom-facing supporting surface 1302 and bottom-facing surfaces of corner fingers 1600. Also shown is the nesting structure facilitated by the mating of bottom supporting surface 1302' of tray 1300' with top mating surface 1312 of tray 1300. As can be seen, matrix tray 1300 defines, by its outermost boundaries, (as does the tray as shown in Fig. 9) a volume having a length, width and height of no more than about 10% of its length and width and meeting other requirements associated with that embodiment of the invention. Moreover, the matrix tray includes a section y within the boundaries of which is an essentially flat plastic unit including protrusions extending vertically therefrom to define flanges 1510, 1512, etc., support comer fingers 1600, and other structures. Section y itself is a substantially planar free-standing portion of the overall matrix tray. It includes at least four commonly-facing surface points (flanges, comer fingers, etc.) at least three of which define a plane, the fourth of which falling no further than 0.1 cm outside of the plane where the four surface points define therebetween an area of at least 75% of the area defined by the overall portion y.

A The function and advantage of these and other embodiments of the present invention will be more fully understood from the examples below. The following examples are intended to illustrate the benefits of the present invention, but do not exemplify the full scope of the invention.

Example 1 : Printer Chassis A Milacron 400 ton reciprocating screw injection molding machine was used.

Twenty percent glass-reinforced polyphenylene oxide (PPO; GE Plastics Noryl™ resin) ' was used as the polymer to be molded, and a supercritical fluid additive (nitrogen), also served as the blowing agent to form a molded foam chassis. A system was used as shown schematically in Fig. 1. The printer chassis mold used was a conventional single-cavity mold that operated with two plates and one parting line. It included a valve-gated hot sprue bushing that gated directly into the center of the part. The cavity was fairly complex in design and had a number of slides, core pins, and thin blades that help form the part. The? dimensional requirements for the parts are tight and any warpage would be a concern. The design of the mold was such that it produced a molded part having a nominal wall thickness of 2.5mm. It included a number of bosses and deeply cored sections, which would limit cycle time or could cause dimensional problems using a conventional process. Given the wall thickness of 2.5mm, the part has a flow factor of approximately 150:1, which allowed for a significant weight reduction with an optimized process. Comparative: Molding of Solid Parts without Supercritical Fluid Additive:

Solid parts were produced that weighed 603 grams each. A simple warpage measurement was made with the parts by placing them on a smooth surface and measuring the distance that one comer of the part was out of plane with the other three corners. The solid parts were warped, or out of plane, 1.13mm. The solid parts were used as a baseline for weight and warpage reduction with the supercritical fluid additive process of the invention. Supercritical Fluid Additive Process: Nitrogen was used as a supercritical fluid additive and was varied from 0.05% to 0.10% during various sample runs. Microcellular foamed parts were produced that showed no signs of sink and replicated the mold cavity very precisely. Weight reductions (void volumes) of greater than 5% and on the order of 10% were easily obtained. Higher weight reductions can be achieved with minor process modification.

Cycle time: The mold cycle close time consisted of injection, hold, and cooling times. Solid parts (comparative example, non-supercritical fluid additive) were produced with a cooling time of 15 seconds and a total mold close time of 20.8 seconds. Initial supercritical fluid additive molding times included cooling times of 15 seconds and a total mold closed time of 16.6 seconds. Other runs included cooling times of 10 seconds and a total mold closed time of 11.6 seconds, and were run successfully. Thus, a mold close time reduction of 44% was achieved with the supercritical fluid additive process. Clamp tonnage: When operated using the supercritical fluid additive process of the invention, clamp tonnage was reduced to 200 tons. It could not be lowered any further because 200 tons was the minimum setting on the machine during this run.

Notwithstanding, a 50% reduction in required clamp force was easily achieved, and estimates by the inventors based upon an intimate knowledge of the process show that the clamp tonnage can be set lower, such as on the order of 150 tons. Dimensional stability: The warpage of parts produced according to the supercritical fluid additive process with 15 seconds cooling time was only 0.79mm. When compared to solid parts at 1.13mm, this indicates a 30% reduction in warpage. When operated with only a 10 second cooling time, the supercritical fluid additive parts were only warped 0.97mm, achieving a 14% reduction in warpage relative to solid parts at a significantly faster cycle time. Significant reduction in warpage over a wide range of process conditions was observed. Microcellular polymeric material with an excellent cell structure was obtained over a variety of process conditions.

Example 2: Matrix Tray

LNP PDX-D-95337 carbon fiber-filled polycarbonate was molded in an Engel 150 ton microcellular-capable screw-and-plunger injection molding machine, schematically illustrated in Fig. 2. The mold included a three-plate single-cavity tool with eight pin gate drops to the cavity. Parts were run with a nitrogen supercritical fluid additive in an amount of between 0.11% and 0.24%. As compared to the molding of solid parts (parts without supercritical fluid additive), mold temperature was reduced from 100°F to 50°F, and cure time was reduced from 25 seconds to 15 seconds, all while achieving the same or better dimensional stability in resultant parts (microcellular parts). The cycle can be reduced to 21.5 seconds (39% reduction as compared to non- supercritical fluid additive process), and weight reduction of 11% was observed.

Warpage was reduced from 0.023" for non-supercritical fluid additive parts to 0.006", a 74% reduction in warpage. A uniform microcellular structure, of cells of about 50 microns average diameter, was formed.

Those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the methods and apparatus of the present invention are used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. In the claims the words "including", "carrying", "having", and the like mean, as "comprising", including but not limited to.

What is claimed is:

Claims

1. A method for forming a molded polymeric part comprising: providing a polymer molding system including an extrader and a mold, the system constructed and arranged to deliver blowing-agent-free molten polymeric material from the extruder into the mold, to solidify the polymeric material in the mold, and to eject from the mold a first molded polymeric article having a void volume of essentially zero, all in a first minimum cycle time under set conditions; delivering polymeric material admixed with a supercritical fluid additive from the extruder into the mold, allowing the polymeric material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all in a second minimum cycle time no more than 80% the first cycle time under the set conditions.

2. A method as in claim 1, wherein the second minimum cycle time is no more than 70% the first cycle time under the set conditions.

3. A method as in claim 1 , wherein the second minimum cycle time is no more than 60% the first cycle time under the set conditions.

4. A method as in claim 1 , wherein the second minimum cycle time is no more than 50% the first cycle time under the set conditions.

5. A method as in claim 1, wherein the second minimum cycle time is no more than 40% the first cycle time under the set conditions.

6. A method as in claim 1, wherein the mold has an interior shape and the polymer molding system is constructed and arranged to eject the first molded polymeric article such that the first article undergoes warpage including deviation of at least one section from the shape of the mold interior by at least about w, or shrinkage of at least about s, or both, the method comprising ejecting the second molded polymeric article wherein the second article is free of warpage or undergoes warpage including deviation of at least one section from the shape of the mold interior by no more than w, or shrinkage of no more than s, or both.

7. A method as in claim 6, wherein the polymer molding system is constructed and arranged to eject the first molded polymeric article such that the first article undergoes warpage including deviation of at least one section from the shape of the mold interior by at least about 0.4%, or shrinkage of at least about 0.2%, or both, the method comprising ejecting the second molded polymeric article wherein the second article is free of warpage or undergoes warpage including deviation of at least one section from the shape of the mold interior by no more than 0.4%, or shrinkage of no more than 0.2%, or both.

8. A method as in claim 1 , wherein the mold has an interior shape and the polymer molding system is constructed and arranged to eject the first molded polymeric article such that the first article undergoes warpage including deviation of at least one section from the shape of the mold interior by at least about w, or shrinkage of at least about s, or both, the method comprising ejecting the second molded polymeric article wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 80% w, or shrinkage of less than about 80% s, or both.

9. A method as in claim 8, wherein the polymer molding system is constructed and arranged to eject the first molded polymeric article such that the first article undergoes warpage including deviation of at least one section from the shape of the mold interior by at least about 0.4%, or shrinkage of at least about 0.2%, or both, the method comprising ejecting the second molded polymeric article wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 0.3%, or shrinkage of less than about 0.16%, or both.

10. A method as in claim 1, comprising delivering the polymeric material mixed with the supercritical fluid additive from the extruder into the mold at a mold temperature of less than about 250°C.

11. A method as in claim 1 , comprising delivering the polymeric material mixed with the supercritical fluid additive from the extrader into the mold at a mold temperature of less than about 150°C.

12. A method as in claim 1, comprising delivering the polymeric material mixed with the supercritical fluid additive from the extruder into the mold at a mold temperature of less than about 75°C.

13. A method as in claim 1 , comprising delivering the polymeric material mixed with the supercritical fluid additive from the extrader into the mold at a mold temperature of less than about 50°C.

14. A method as in claim 1 , comprising delivering the polymeric material mixed with the supercritical fluid additive from the extrader into the mold at a mold temperature of less than about 10°C.

15. A method as in claim 1, comprising repeatedly delivering the polymeric material mixed with the supercritical fluid additive into the mold, allowing the material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all at a cycle time of less than about 3 minutes.

16. A method as in claim 1, comprising repeatedly delivering the polymeric material mixed with the supercritical fluid additive into the mold, allowing the material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all at a cycle time of less than about 1 minute.

17. A method as in claim 1, comprising repeatedly delivering the polymeric material mixed with the supercritical fluid additive into the mold, allowing the material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all at a cycle time of less than about 30 seconds.

18. , A method as in claim 1 , comprising repeatedly delivering the polymeric material mixed with the supercritical fluid additive into the mold, allowing the material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all at a cycle time of less than about 10 seconds.

19. A method as in claim 1, wherein the supercritical fluid additive is an atmospheric gas.

20. A method as in claim 1 , wherein the supercritical fluid additive comprises nitrogen.

21. A method as in claim 1, wherein the supercritical fluid additive comprises carbon dioxide.

22. A method as in claim 1 , wherein the supercritical fluid additive comprises helium.

23. A method as in claim 1, wherein the supercritical fluid additive consists of carbon dioxide.

24. A method as in claim 1, comprising introducing a supercritical fluid additive from a source into a flowing stream of polymeric material in the extruder through a plurality of orifices.

25. A method as in claim 8, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 70% w or shrinkage of less than about 70% s, or both.

26. A method as in claim 8, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 60% w or shrinkage of less than about 60% s, or both.

27. A method as in claim 8, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 50% w or shrinkage of less than about 50% s, or both.

28. A method as in claim 8, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 30% w or shrinkage of less than about 30% s, or both.

29. A method as in claim 8, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 10% w or shrinkage of less than about 10% s, or both.

30. A method as in claim 1, wherein the second molded polymeric article has a void volume of less than 5 %.

31. A method as in claim 1 , wherein the second molded polymeric article has a void volume of less than 3%.

32. A method as in claim 1, wherein the second molded polymeric article has a void volume of less than 1%.

33. A method as in claim 1, wherein the second molded polymeric article has a void volume of greater than 5%.

34. A method as in claim 1 , wherein the second molded polymeric article has a void volume of greater than 10%.

35. A method as in claim 1, wherein the second molded polymeric article has a void volume of greater than 15%.

36. A method as in claim 1, wherein the second molded polymeric article has a void volume of greater than 20%.

37. A method as in claim 1, wherein the second molded polymeric article has a void volume of greater than 25%.

38. A method as in claim 1, wherein the second molded polymeric article has a void volume of greater than 50%.

39. A method as in claim 1 , wherein the supercritical fluid additive is present in an amount of less than about 10%.

40. A method as in claim 1 , wherein the supercritical fluid additive is present in an amount of less than about 5%.

41. A method as in claim 1, wherein the supercritical fluid additive is present in an amount of less than about 1%.

42. A method as in claim 1, wherein the supercritical fluid additive is present in an amount of less than about 0.5%.

43. A method as in claim 1, wherein the supercritical fluid additive is present in an amount of less than about 0.05%.

44. A method as in claim 1 , wherein the second molded polymeric article undergoes shrinkage, relative to mold dimensions, of less than 0.2% at at least 48 hours after ejection from the mold.

45. A method as in claim 1 , wherein the second molded polymeric article undergoes shrinkage, relative to mold dimensions, of less than 0.1% at at least 48 hours after ejection from the mold.

46. A method as in claim 1, wherein the second molded polymeric article undergoes shrinkage, relative to mold dimensions, of less than 0.02% at at least 48 hours after ejection from the mold.

47. A method as in claim 1, wherein the second molded polymeric article undergoes shrinkage, relative to mold dimensions, of less than 0.002% at at least 48 hours after ejection from the mold.

48. A method as in claim 1 , wherein the second molded polymeric article undergoes warpage, relative to mold dimensions, of less than 0.2% at at least 48 hours after ejection from the mold.

49. A method as in claim 1, wherein the second molded polymeric article undergoes warpage, relative to mold dimensions, of less than 0.1% at at least 48 hours after ejection from the mold.

50. A method as in claim 1, wherein the second molded polymeric article undergoes warpage, relative to mold dimensions, of less than 0.02% at at least 48 hours after ejection from the mold.

51. A method as in claim 1 , wherein the second molded polymeric article undergoes warpage, relative to mold dimensions, of less than 0.002% at at least 48 hours after ejection from the mold.

52. A method for forming a molded polymeric part comprising: providing a polymer molding system including an extruder and a mold, the system constructed and arranged to deliver blowing-agent-free molten polymeric material from the extrader into the mold, to solidify the polymeric material in the mold, and to eject from the mold a first molded polymeric article having a void volume of essentially zero, all in a first minimum cycle time, such that the first article undergoes warpage including deviation of at least one section from the shape of the mold interior in an amount of at least about w, or shrinkage of at least about s, or both; delivering polymeric material admixed with a supercritical fluid additive from the extruder into the mold, allowing the polymeric material to solidify in the mold, and ejecting a second molded polymeric article from the mold, all in a cycle time no greater than the first cycle time, wherein the second article undergoes warpage including deviation of at least one section from the shape of the mold interior by less than about 80% w, or shrinkage of less than about 80% s, or both.

53. A method as in claim 13, wherein the second molded polymeric article has a void volume of greater than 30%.

54. An injection molded polymeric integrated circuit matrix tray having dimensional stability sufficient to satisfy standards for use in the integrated circuit fabrication industry.

55. An injection molded polymeric foam article including a substantially planar free-standing portion having a length, a width, and a height, wherein the height is no more than about 10% the total of the length and width, formed in a mold having a substantially planar mold portion corresponding to the substantially planar free-standing portion of the article, wherein a first line of planarity extending in the substantially planar mold portion is reproduced in the article as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

56. An article as in claim 55, wherein the curve has a radius of curvature no less than 500 times the length of the first line of planarity.

57. An article as in claim 55, wherein the curve has a radius of curvature no less than 1000 times the length of the first line of planarity.

58. An article as in claim 55, wherein the curve has a radius of curvature no less than 1500 times the length of the first line of planarity.

59. An article as in claim 55, wherein the curve has a radius of curvature no less than 2000 times the length of the first line of planarity.

60. An article as in claim 55, wherein the substantially planar free-standing portion has a height no more than about 8% the total of the length and width.

61. An article as in claim 55, wherein the substantially planar free-standing portion has a height no more than about 5% the total of the length and width.

62. An article as in claim 55, wherein the substantially planar free-standing portion has a height no more than about 3% the total of tlie length and width.

63. An article as in claim 55, wherein the substantially planar free-standing portion has a height no more than about 2% the total of the length and width.

64. An article as in claim 55, wherein the substantially planar free-standing portion has a height no more than about 1% the total of the length and width.

65. An article as in claim 55, including at least one portion having a cross- section of about 0.0075" or less.

66. An article as in claim 55, having a void volume of at least about 5%.

67. An article as in claim 55, having a void volume of at least about 10%.

68. An article as in claim 55, having a void volume of at least about 15%.

69. An article as in claim 55, having a void volume of at least about 20%.

70. An article as in claim 55, having a void volume of at least about 25%.

71. An article as in claim 55, having a void volume of at least about 50%.

72. An article as in claim 55, the article having a maximum thickness of no more than about 0.080".

73. An article as in claim 55, including at least one portion having a length- to-thickness ratio of at least about 300:1.

74. An article as in claim 55, comprising the cells of average size of less than about 100 microns in diameter, and a cell density greater than at least 10⁶ cells per cubic centimeter.

75. An article as in claim 55, comprising the cells of average size of less than about 75 microns in diameter, and a cell density greater than at least 10⁶ cells per cubic centimeter.

76. An article as in claim 55, comprising the cells of average size of less than about 50 microns in diameter, and a cell density greater than at least 10⁶ cells per cubic centimeter.

77. An article as in claim 55, comprising the cells of average size of less than about 30 microns in diameter, and a cell density greater than at least 10 cells per cubic centimeter.

78. An article as in claim 55, comprising the cells of average size of less than about 20 microns in diameter, and a cell density greater than at least 10⁶ cells per cubic centimeter.

79. An article as in claim 55, comprising the cells of average size of less than about 10 microns in diameter, and a cell density greater than at least 10 cells per cubic centimeter.

80. An article as in claim 55, including residual chemical blowing agent, or reaction by-product of chemical blowing agent, in an amount less than that inherently found in articles blown with 0.1% by weight chemical blowing agent or more.

81. An article as in claim 55, including residual chemical blowing agent, or reaction by-product of chemical blowing agent, in an amount less than that inherently found in articles blown with 0.5% by weight chemical blowing agent or more.

82. An article as in claim 55, essentially free of residual chemical blowing agent or reaction by-product of chemical blowing agent.

83. An article as in claim 55, wherein the article has a heat deflection temperature and, when heated at a temperature within 5°C of the heat deflection temperature, reproduces the first line of planarity as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

84. An article as in claim 55, wherein the article has a heat deflection temperature and, when heated at its heat deflection temperature, reproduces the first line of planarity as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

85. An article as in claim 55, wherein the article has a heat deflection temperature and, when heated at a temperature at least 50°C higher than its heat deflection temperature, reproduces the first line of planarity as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

86. An article as in claim 55, wherein the article has a heat deflection temperature and, when heated at a temperature at least 100°C higher than its heat deflection temperature, reproduces the first line of planarity as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

87. An article as in claim 55, wherein the article has a heat deflection temperature and, when heated at a temperature at least 200°C higher than its heat deflection temperature, reproduces the first line of planarity as a second line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

88. An injection molded polymeric foam article including a substantially planar free-standing portion having a length, a width, and a height, wherein the height is no more than about 10% the total of the length and width, formed in a mold, wherein a solid molded polymeric article, formed in the mold under conditions identical to those in which the foam article is formed, has a substantially planar freestanding portion corresponding to the substantially planar free-standing portion of the foam article, wherein a first line of planarity extending in the substantially planar freestanding portion of the solid article is reproduced in the foam article as second a line of planarity or a curve having a radius of curvature no less than 200 times the length of the first line of planarity.

89. An injection molded polymeric article including a substantially planar free-standing portion having a length, a width, and a height, wherein the height is no more than about 10% the total of the length and width, the length and width defining a first area of at least about 25 cm², the article including four commonly-facing surface points three of which define a plane and the fourth falling no further than 0.1 cm outside of the plane, the four surface points defining therebetween an area at least 75% the first area .

90. An article as in claim 88, wherein the four commonly-facing surface points define an outside boundary of the substantially planar portion.

91. An inj ection molded polymeric article including an average part thickness of no more than about 0.3 cm, the article including at least four commonly-facing surface points, three of which define a plane and the fourth falling no further than 0.1 cm outside of the plane, the at least four surface points defining therebetween an area of at least 25 cm².

92. A tray including a downward-facing section including a support surface or surfaces for contact with a supporting structure, wherein the support surface or surfaces include at least four outermost support contact points, three of which define a plane and a fourth of which deviates from the plane by no more than 0.1 cm.

93. A tray as in claim 92, comprising a matrix tray.

94. An office equipment chassis including at least four support contact points for supporting the chassis on a surface, three of which define a plane and a fourth of which deviates from the plane by no more than 0.1 cm.

95. A chassis as in claim 94, comprising a printer chassis.