US3483011A - Fluidized bed coating process and apparatus - Google Patents

Description

wat 5M ATTORNEY R. A.-. cRRAN FLUIDIZED BED COATING PROCESS AND APPARATUS Filed April 29, 1966 United States Patent O 3,483,011 FLUIDIZED BED COATING PROCESS AND APPARATUS Robert A. Curran, Framingham, Mass., assgnor, by

mesne assignments, to Solitron Devices, Incorporated, a corporation of New York Filed Apr. 29, 1966, Ser. No. 546,253 Int. Cl. C23c 1/00; H01b 1/02; B05c 3/00 U.S. Cl. 117-21 5 Claims ABSTRACT OF THE DISCLOSURE A method for producing a uniform coating of a heat fusible material upon the surface of a panel comprising heating said panel unevenly to produce a temperature gradient from one edge to the opposite edge, and while said temperature gradient exists, immersing said heated panel into a fluidized bed of particles of a heat fusible material and withdrawing the coated panel.

This invention relates to the production of coatings and particularly to methods of and apparatus for producing coatings 'by iluidized bed techniques.

In the uidized bed coating processes, an article to be coated is preheated at least to the fusion or melting temperature of a fusible coating material and then imrn'ersed in a iluidized bed of the coating material. The iluidized bed is formed by passing an ascending stream of gas through a body of solid pulverulent coating material. Coatings of dielectric materials, particularly organic plastics, formed by fiuidized bed techniques find utility as supporting substrates for electrical circuitry, particularly, printed circuits. Coatings of both thermoplastic and thermosetting resins may be applied to a relatively large metal panel, that may include openings, to form a support for a pattern of electrical conducting elements comprising a printed circuit and/ or electrical components mounted on the panel. The thickness and uniformity of the dielectric coating on the panel may be critical, particularly in the case of printed wiring boards with component mounting holes with close diameter tolerances and sections which function as male plugs and have close thickness tolerances.

.An object of the present invention is to provide a novel and improved method of producing coatings having accurately controlled and uniform thickness on panels by uidized bed coating techniques.

In the application of coatings to flat panels by fluidized bed processes, the heated panel is usually suspended in a generally vertical plane while being immersed in the iluidized bed so that the lower portion of the panel is introduced first and withdrawn last, With relatively large panels and short immersion times, this means that the lower portion of the panel is immersed in the iluidized bed for a substantially longer period than the upper part of the panel and will result in an uneven coating because the fusible particles begin to melt and adhere to the heated panel immediately as they come into contact with the panel. The larger the panel and hence, the greater the immersion time differential between portions of the panel, the greater the variation in thickness of the coating applied to the panel.

Other objects of the invention are: to provide a method of uidized bed coating in which the panel to be coated is heated differentially to provide a temperature gradient from one edge of the panel to the opposite edge cor- Patented Dec. 9, 1969 responding to the variation in immersion time of the sections of the panel in the lluidized bed; to provide novel and improved means for heating the panel to establish the desired temperature gradient; and to provide means for differentially heating a metal panel with suiiicient rapidity to preclude normalization of the temperature throughout the panel.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involvingy the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of lwhich will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIGURE 1 is a perspective view, partially in section, of heater apparatus embodying the invention;

FIGURE 2 is a somewhat diagrammatic view of the apparatus of FIGUR-E 1; and

FIGURE 3 is a sectional view through a iiuidized bed tank illustrating the method of the invention.

The rate of build-up of a coating on a panel immersed in a iluidized bed is dependent upon a member of factors, particularly the temperature and heat content of the preheated panel, while the thickness of the coating is a function of the rate at which the coating builds up and the length of the period of immersion in the iluidized bed. When a panel is moved, lower edge foremost, downwardly into a yiiuidized bed and then removed, upper edge foremost, in the opposite direction, the lower portion of the panel will experience an immersion period that is longer than the immersion period for the upper portion of the panel. This discrepancy must exist even though the relative proportions of the two extremes of immersion may vary with changes in the speed of introduction and withdrawal, the dwell time within the iluidized bed and the vertical dimension of the panel. For relatively large panels and short dwell times, the difference in immersion periods for upper and lower portions of the panel may be suiicient to cause very substantial variations in the coating thickness and the diameters, after coating, of holes in the panel.

In accordance with the invention, uniform coatings are provided on the surface of a panel by preheating the panel differentially to create a temperature differential in the panel ranging from a low (at least equal to the fusion temperature of the coating material) at the lower edge of the panel to a high at the upper edge of the panel. This temperature dilferential corresponds proportionally to the differences in immersion times for all portions of the panel so that coating bui1dup is relatively slower on the lower portions of the panel and more rapid on the upper portions of the panel. In other words, the rate 0f coating build-up is inversely proportional to the immersion time.

Although supporting panels useful as substrates for printed circuitry may be formed of substantially any material having the requisite physical properties and capable of withstanding the elevated temperatures necessary for iluidized bed coating, metals, particularly aluminum, are preferred. Of course, metals are relatively good conductors of heat so that a metal panel must be heated rapidly in order to produce the requisite temperature gradient and immediately immersed in the fluidized bed while the temperature gradient still exists.

Reference is now'made to the drawings wherein there is illustrated apparatus for performing the method of the invention including a heater for differentially heating fiat panels to produce a temperature gradient in the panels. The heating means of the apparatus comprise a pair of platens and 12 mounted in juxtaposition with one of the platens, designated 12, being movable toward and away from the other platen for clamping a panel between the platens. Movement of platen 12 with respect to platen 10 is effected by mounting the panel on a pair of pivotal arms 14, in turn actuated by suitable means such as a hydraulic piston and cylinder. A typical panel 16 is shown clamped between the platens, is generally rectangular in shape and includes an upper section having an opening in which a hanger 18 may be engaged for suspending and transporting the panel during operations thereon.

Each of platens 10 and 12 is made up of a plurality (five are shown) of modular units designated by the letters a through e. Each modular unit extends from side to side of the platen, includes a separate heating element 20, and is thermally insulated from adjoining modular units. The units are formed of a thermally conductive material, eg., steel, substantially thicker than the panels so as to have a substantially higher heat content than the panels. Heating elements 20 are of the electrical resistance type and each is embedded within a platen module together with a thermostat 22 for controlling the temperature of the particular modular unit. insulating layers 24, formed for example, of polytetrauoroethylene, between adjacent heating modules. Layers 24 are preferably quite thin and function to maintain the temperature differential between adjacent modules. The heating modules of one platen are located in alignment with modules of the opposite platen and the adjacent modules of the two platens are maintained at substantially the same temperature so that the portion of the panel clamped between a pair of modules will be heated to approximately the temperature of those two modules.

In practicing the method of the invention, a panel 16 is clamped between the two platens for a period which is preferably just sufficient to raise the temperature of each portion of the panel to the temperature of the heating modules between which the particular portion of the panel is clamped. The panel is immediately removed from between the heating platens' since allowing it to remain there for a longer period would result in heat transfer from the hotter sections of the panel to the cooler sections of the panel and would eventually result in reduction of the temperature gradient, possibly to the extent of normalizing the temperature throughout the entire panel. In the manner described, the temperature gradient produced is relatively even due to heat conduction within the panel and the provision of a relatively large number of heated platen modules maintained at a temperature differing by comparatively small amounts.

The preheated panel immediately upon withdrawal from between the platens is immersed as shown in FIG. 3, in a uidized bed of the fusible coating material. This uidized bed is contained in a tank 26 having a porous dividing wall 28 near its bottom dividing the tank into a lower chamber 30 into which a gas under pressure is introduced, and an upper chamber 32 containing the finely divided fusible coating material. Porous 4wall 28 permits the gas under pressure to ascend through the powdered coating material causing the latter to behave as a fluid and contact all exposed surfaces of the panel when the latter is immersed in the uidized bed. Panel 16 is suspended in a generally vertical plane during both the preheating and immersion operations so that the lower portion of the panel is introduced into the fluidized bed irst and withdrawn from the uidized bed last, and thus is immersed in the uidized bed for a longer period than the upper portion of the panel. In the coating operation the panel is introduced as rapidly as possible into the fluidized bed, is retained therein during a period of i predetermined duration and then withdrawn as rapidly as possible from the uidized bed. The proportional difference in immersion periods for the upper and lower portie-ns of the panel will, of course, depend upon the size of the panel, the rate at which the panel is moved during introduction and withdrawal and the dwell time -within the fluidized bed. If the panel were heated to a uniform temperature throughout, and this is particularly true of relatively large panels, the thickness of the coating applied to the surfaces of the panel would vary substantially from top to bottom of the panel and since the coating is also applied to the walls of openings in the panel, the finished size of openings, originally of the same size, would als-o vary depending upon the location of the openings relative to the top and bottom of the panel. However, by establishing a temperature gradient in the panel ranging from a temperature at least equal to the fusion temperature of the coating material at the lower edge of the panel to a higher temperature near the upper edge of the panel, the build-up of coating material on each section of the panel proceeds at a rate proportional to the immersion so that coating is of uniform thickness.

As an illustrative example of the method of the invention, a coating of a polyester resin with a thickness of .008 was produced on a l2 X 8 X .030" aluminum panel. The coating thickness and hole diameters were maintained within a tolerance of .0005 for all portions of the panel including tabs or extensions of the panel. The panel was preheated to a minimum temperature of 480 F. to produce a gradient of 60 F. along the 8" dimensions by clamping the panel between a pair of platens comprising 2" modules separated by 1/16" thick insulating layers and maintained at temperatures differing by 20 F. The panel was clamped between the platens for a minimum of 4.8 seconds in order to preheat the panel to the desired temperature. However, it was found that the panel could be retained between the platens for as long as 7 seconds without adversely affecting the temperature gradient, thereby demonstrating suicient tolerance in the preheating time to make mass production feasible. A range of preheating times for a particular panel and coating material is readily determinable by simple experimentation. In the example given, the coating (.008H thick) was produced allowing for a transfer time from platen heaters to the uidized bed of 2 seconds and the coating material employed was a polyester resin in the B stage for which the recommended preheating temperature is 440 F.

Since certain changes may be made in the above process and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1. The method of producing a uniform coating of a heat fusible material upon the surface of a panel comprising:

heating said panel unevenly to produce a temperature gradient therein ranging from a low temperature at one edge of said panel at least equal to the fusion temperature of said material to a substantially higher temperature at the opposite edge of said panel;

while said temperature gradient exists, immersing said heated panel with said one edge foremost into a fluidized bed of finely divided particles of said heat fusible material to cause said particles to melt and adhere to the surface of said panel to form a coating thereon;

retaining said panel immersed in said iiuidized bed for a predetermined period; and

thereafter, withdrawing the coated panel with said opposite edge foremost, from said fluidized bed.

2. The method of claim 1 wherein said panel is suspended vertically with said one edge beneath said opposite 5 edge during immersion in and withdrawal from said uidized bed.

3. The method of claim 1 wherein said panel is formed of metal and is heated and immediately immersed in said iuidized bed with suicient rapidity to insure continuance of said temperature gradient at least during introduction of said panel into said iluidized bed.

4. A method as defined in claim 1 wherein said panel is heated by maintaining sections of juxtaposed platens at different temperatures approximately equal to the temperatures to be produced in corresponding portions of said panel and clamping said panel between said platens.

5. A method as deiined in claim 4 wherein the heat content of said platens is substantially greater than the heat content of said panel.

References Cited UNITED STATES PATENTS 3,218,184 11/1965 Lemelson 117--21 3,282,249 1l/1966 Ramsay 117-21 3,291,631 12/1966 Smith 117-21 FOREIGN PATENTS 639,666 4/ 1962 Canada.

661,766 4/1963 Canada.

WILLIAM D. MARTIN, Primary Examiner RAYMOND M. SPI-EER, Assistant Examiner U.S. C1. X.R. 117-113, 230

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