WO2005021188A2

WO2005021188A2 - Compositions and use of sand and powders capable of being heated by microwave or induction energy

Info

Publication number: WO2005021188A2
Application number: PCT/US2004/027341
Authority: WO
Inventors: Kenneth Hillel Peter Harris
Original assignee: Ncc Roads As
Priority date: 2003-08-21
Filing date: 2004-08-23
Publication date: 2005-03-10
Also published as: WO2005021188A3

Abstract

Materials containing a mixture of a binder and a matrix capable of being heated in an electromagnetic field (EMF) and consisting of at least 70 % inorganic substances are used for foundary and other purposes. The matrix materials contain a surface film of a substance that conducts electricity as well as at least 15 % of a transistion element metal in compound form. The transistion element is iron of which at least 50 % is in the divalent state. The main component of the mixture is sand.

Description

COMPOSITIONS AND USE OF SAND AND POWDERS CAPABLE OF BEING HEATED BY MICROWAVE OR INDUCTION ENERGY

BACKGROUND OF THE INVENTION Field of the Invention This invention is directed to compositions in the form of sand or powders for various uses, and more particularly, is directed to compositions and use of sand and powders capable of being heated in an alternating electromagnetic field (EMF), such as by microwaves or induction or radio frequency heating, for foundry and other purposes.

Description of the Related Art A common method of casting metals entails making a model of the item to be cast consisting of a mixture of sand and a binder. It is important that the binder is able to bond the grains of sand strongly enough so that the model resists the stresses of the casting process without deforming or breaking down, and that it quickly achieves this strength. On the other hand, it is also desirable to minimize the proportion of binder in the sand to reduce costs and the evolution of gases that occurs when the binder degrades during casting. The bonded sand molds and cores must also be porous enough to permit the easy egress of gases formed when the molten metal enters the system. Castings are often made in molds consisting of silica sand and a clay/water binder that bonds the sand grains by interfacial tension between the wet clay and sand. Molds made in this way can be satisfactory for small, simple castings, but are too weak for more complex and/or larger parts and for the cores needed to produce hollow castings. These are commonly made with a binder that polymerizes (cures) and whose strength is dependent upon the molecular weight or degree of polymerisation, D_P, of the resulting polymer. The flow characteristics of sand made with such binders deteriorates rapidly once the binder is mixed in, particularly if its content exceeds 2-3 weight percent. While it is possible, in theory, to use any polymerizable binder system for this purpose, the necessity of minimizing cycle times, and the difficulty of heating large mold parts, generally makes it preferable to use systems that cure at room temperature. This limits the range of substances that can be used for this purpose. Curable binder systems consist, almost without exception, of two-part systems, the one being a polymer precursor that can be a monomer, such as a furfural alcohol, or a low molecular weight polymer, such as a phenolic resin, that is converted into a higher polymer by the addition of a second component, the curing agent. This requires the use of specially designed mixers that rapidly and evenly blend a controlled proportion of binder and a curing agent to a much larger quantity of sand. Some binder systems rely on pumping a gaseous curing agent, such as an amine or, occasionally, carbon dioxide or sulphur dioxide, into a mold or core made from a mixture of sand and binder. Both systems can suffer from the build-up of binder on the internal surfaces of the mixer which reduces its efficiency, increases binder consumption and necessitates frequent cleaning. Furthermore, systems that require sand, binder and curing agent to be mixed in a single operation suffer because curing begins at the instant of admixture. The need to maintain sufficient bench time, while ensuring sufficiently rapid curing, thus limits the amount of time available to achieve uniform mixing. Systems that introduce a gaseous curing agent after mixing require that the molds and cores be prepared such that this is possible, with points of entrance and subsequent removal of the gas. After addition of gas, the molds and/or cores may have to be stored for a number of hours to complete the curing process.

Some of these systems suffer, furthermore, from the noxious nature of the curing agent which demands that particular attention be paid to ventilation and removal of such substances from the vented gases. The impracticability of heating molds and cores by normal means to accelerate curing puts a further limitation on the strength that can be achieved with binder systems that are slow to cure to completion at room temperature, such as alkaline phenolic resins and those based upon sodium silicate. Furthermore, where the casting process fails to heat parts of the core or mold to a temperature sufficient to degrade the binder so that it becomes friable, the amount of sand that can usefully be mechanically reclaimed and reused from cores is limited. This can be the case during the casting of aluminum parts and may necessitate the use of expensive reclaim techniques based upon so-called thermal methods, where the residual binder is burnt off in a stream of hot gas. This technique is suitable for binder systems such as phenol-urethanes that yield only gaseous by-products, but is not recommended for those, such as alkaline phenolics, that yield an ash that can react with the sand. Another casting method, the well-known Croning or "shell process" technique, involves the use of a hot metal model to make a sand shell of the core or mold into which the molten metal is poured. This requires that the sand be coated with a dry, fusible thermosetting resin, typically a phenolic resin. The model is inserted into a bed of coated sand, thereby fusing and curing the resin with which it is in contact and producing an accurate impression of the model. This avoids the flow problems that can affect sand bonded with liquid binder, so the Croning technique is often used for intricate parts. However, the Croning process suffers from two main disadvantages: a) The cost of producing models, cores and molds is higher than for most other processes, and b) The size and weight range of castings is limited, typically to below 100kg, since the shells cannot be produced with sufficient strength to withstand the pressure experienced during the casting of larger parts. In recent years, there has been an upsurge of interest in the use of EMF, primarily microwave heating, to assist the drying of coatings applied to the surfaces of molds and cores. Furthermore, EMF is being investigated to accelerate the curing of aqueous binder systems. One advantage of using EMF is that heat is generated uniformly throughout the mass so that all parts share a similar thermal history. EMF is also much faster and cheaper than heating that relies upon conductive or convective methods such as heating in a stream of hot air. However, very many components used by foundries in making sand molds and cores are more or less transparent to EMF in the most- used frequency ranges; neither are they responsive to radio frequency nor induction heating. This includes almost all foundry sand, many commonly used binders such as those based upon isocyanates, and also the mineral components and organic solvents commonly used in the manufacture of coatings for sand molds and cores. This makes it necessary to add a receptor to the system in order that heating can occur. This can be achieved in coatings by the use of a carrier such as water rather than an organic solvent such as ethanol. Similarly, otherwise unreceptive binder systems, such as those based upon phenol-urethanes and furfural alcohol, become receptive upon the addition of a finely powdered EMF absorber, such as iron oxide (Fe₃O₄) or carbon black. While attractive in principle, the above methods suffer from serious deficiencies, the most restrictive being the slowness of the process due to the fact that only a very small proportion of the mass is heated under these circumstances.

In the case of the above water-based coatings, this results in some of the water evaporated by EMF energy being driven into the mold or core itself where it can condense and interfere with the subsequent casting operation. The addition of EMF-absorbers, such as carbon black, to non-aqueous binders may also interfere with the binder's curing properties. Furthermore, the addition of fine particulates to the sand or binder increases the specific surface area of the system, thereby necessitating the addition of thinners and/or the use of additional binder. Finally, since the amount of binder is small compared to the amount of sand (typically 0.8-3.0% by weight of sand), the latter acts as an efficient heat sink absorbing much of the heat generated in the binder, limiting the efficacy and increasing the necessary duration of the EMF treatment. It is difficult to compensate for this heat loss by using a more intensive energy input since this can lead to decomposition of the binder system and a weakening of the core or mold. Thus, the extent by which EMF can reduce curing times for water-based binders, is limited. U.S. patent 6,691,765, patented February 17, 2004 in which I am a co- inventor, the subject matter of which is incorporated herein in its entirety by reference, describes how foundry sand can be manufactured from crushed rock. This method can produce products that can be used as foundry sand and coatings for molds and cores from a variety of minerals, in particular those belonging to the feldspar family such as anorthosite, anorthite, basalt, norite and hyperite. The ability to use such materials in a system that can be heated by EMF methods, particularly microwaves, would be very desirable for the foregoing reasons. Bench time is a measure of the amount of time that a mixture of sand, synthetic binder and an agent used to cure or set the binder remains workable, that is can be shaped and kneaded, before cross-linking renders the whole intractable. The resulting molds are seriously weakened if the mixture continues to be worked once the binder has started to set. The desirability of a rapid set once the mold or core is properly formed while allowing adequate time for manipulation, means that a compromise always has to be made between the production advantage of long bench times and the economic benefit of short cure times. The use of foundry sand that can be easily and quickly heated by EMF would be of major benefit since it allows the binder's curing kinetics to be adapted to bench time requirements in an efficient and controlled manner. It can also permit a reduction to be made in the amount of curing agent used and extend the choice of binders and curing agents to include some that would not otherwise be considered suitable for the purpose. A number of binder systems, such as those based upon alkali-cured phenolic resins and sodium silicate, contain up to 50 weight percent or more of water. This water remains in the mold or core after curing and can cause surface defects in the metal part when it evaporates during casting. The use of foundry sand that can be easily and quickly heated by a high frequency electromagnetic field such as EMF would be of major benefit since it ensures a more complete curing of the binder system giving greater mold strength, while rapidly expelling residual moisture prior to casting. In contrast, a combination of EMF-sensitive binders with conventional (quartz) foundry sand requires considerably more time to achieve a similar effect, since the sand acts as an inert heat sink.

SUMMARY OF THE INVENTION A primary object of this invention is the provision of compositions of sand and powders capable of being heated in an alternating electromagnetic field (EMF), such as by microwaves or induction or radiofrequency heating, and the use of such materials for foundry and other purposes. A further object of this invention is to provide a particular subset of the minerals disclosed in the 6,691,765 U.S. patent characterized by containing a coating of conductive materials such as carbon, or a sufficient proportion of a transition metal element such as iron as FE², to enable them to be heated by EMF methods. A still further object of this invention is the provision of a system comprising sand and powders capable of being heated by an EMF method to manufacture mold coatings, casting shells, molds and cores with new and beneficial characteristics. Another object of this invention is to provide compositions comprising a mixture of a binder and a matrix capable of being heated in an electromagnetic field and consisting of at least 70% of inorganic substances. Yet another object of this invention is to provide a binder system for application as a coating around sand such that the resulting product is dry and free flowing. A further object of this invention is the production of a foundry sand that can be easily and quickly heated by EMF to allow the binder's curing kinetics to be adapted to bench time requirements in an efficient and controlled manner while permitting a reduction to be made in the amount of curing agent used and extending the choice of binders and curing agents to include some that would not otherwise be considered suitable for this purpose. Yet another object of this invention is the production of a foundry sand or the like that can be easily and quickly heated by a high frequency EMF to ensure a more complete curing of the binder system giving greater mold strength, while rapidly expelling residual moisture prior to casting to minimize surface defects in the metal part when it evaporates during casting. A still further object of this invention is the provision of an EMF-sensitive sand that can allow molds, and especially cores, to be made from low cost varieties of common binder systems containing premixed binder and curing agents, with the curing agent being chosen such that it remains inert until heated to ensure effectively unlimited bench time. Upon further study of the specification, additional objects and advantagesnvention will become apparent to those skilled in the art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The use of EMF-sensitive sand can allow molds and, especially, cores, to be made from low cost varieties of common binder systems containing premixed binder and curing agent, the curing agent being chosen such that it remains inert until heated. This ensures effectively unlimited bench time. The following is an example of such a system: Norite 03 sand: 100 parts Dynosol S280® phenolic resin: 2 parts 5% sodium hydroxide solution: 0.3parts Hexamethylene tetramine (HMTA): 0.2 parts

The HMTA acts as a curing agent while the sodium hydroxide solution acts as a thinner and catalyst. The thoroughly mixed sand/binder system is transferred into a mold or core box, which must be transparent to EMF at the point where these enter the system. Where microwaves are used as the EMF source, transparent materials include glass, wood, plywood and glass-fibre reinforced polyester. Where induction methods are used, the mold or core box can also be made from a material that is an excellent conductor of electricity such as aluminum or copper, if necessary traced with cooling elements. Being a poorer conductor, the sand is heated much more rapidly than the metal under such conditions. The whole is then subjected to EMF energy until the sand temperature is sufficient to initiate full breakdown of the HMTA, typically 110-120°C, at which point the mold or core is ready for use and can removed from the box. It is advisable to provide good ventilation during this phase to ensure removal of water vapor and ammonia gas produced during curing. Cores can be used as such, but it is advisable, especially for larger castings, to transfer sand molds into fitted metal boxes that can withstand the thermal and mechanical stresses of casting. The method of the invention also works well with conventional powder glues with thermosetting characteristics, such as those used in the manufacture of oriented strand board. In this case a loading of 1.5 to 3% resin will usually suffice. The method of the invention can also be used to improve and even replace the Croning or shell process, since, as described above, the use of EMF-sensitive sand obviates the need to use costly heated metal models. Furthermore, the approach can produce shells used in the lost foam or lost wax processes as described in more detail in my U.S. provisional application Serial No. 60/496,674, filed August 21 , 2003 and entitled "SYSTEM FOR STABILIZING

SLURRIES USED IN INVESTMENT CASTING OF METALS", the subject matter of which is incorporated herein in its entirety by reference, where a model of the part to be cast is made in wax or expanded polystyrene foam and then dipped in a slurry of the mineral and binder. The binder can then be dried, if necessary, and cured by EMF, if desired at a temperature below that of the melting point of the foam or wax. Surfaces of molds and cores that are to come in contact with molten metal are often coated with a mixture of a finely divided refractory mineral such as alumina, zircon or aluminum silicate and a binder, the whole being suspended in a carrier. This carrier is typically an alcohol, such as ethanol, which is removed by igniting it prior to casting so that it burns away. However a certain amount of alcohol vapor always escapes into the surrounding air during this burn-off. More stringent regulations governing workplace environments have, therefore, led to a demand for coatings that use water as a carrier rather than organic solvents, but as explained above, difficulties can arise when trying to remove the water by radiant, convective or microwave heating. Water-based mold coatings made and dried by EMF according to this invention dry far more rapidly and leave far less residual moisture in the body of the mold or core than is the case when conventional minerals and drying methods are used. Cores and molds used for casting light metals are often difficult to reclaim by mechanical means since the casting temperature can be too low to embrittle the binder system in the center of such cores or molds sufficiently to allow its removal by attrition. To achieve a satisfactorily high reclaim rate with such sand, the foundry must often have recourse to far more expensive thermal methods whereby the residual binder is burnt off in a stream of hot air. This requires a high capital investment and is environmentally undesirable since it leads to increased emissions of greenhouse gas (CO2) and nitrogen oxides. This problem is avoided when a sand is used that is receptive to EMF since the sand can be quickly heated such that the residual binder, especially in the centers of the molds or cores, becomes rapidly embrittled and easily removed by attrition. The small amount of waste product resulting from this process is a fine powder that can beneficially used in the manufacture of asphalt. The use of an EMF-sensitive sand is particularly valuable in cases where the core remains in place after casting because the sand has not been heated sufficiently to degrade the binder. This is often the case when casting complex aluminum parts such as engine blocks. Conventional methods for the removal of such cores entail conveying the castings through large, partly rotating, chambers into which a heated air stream is blown. This eventually heats the embedded core sufficiently to degrade and oxidize the residual binder, allowing the sand to be blown out. This process is time consuming, energy intensive and costly in terms of capital requirement. Using the method of this invention, the core sand can be quickly and efficiently removed by subjecting the part to an electrical induction field, preferably in the presence of oxygen, followed by vibration or an air current to remove the sand so loosened. Being a much poorer conductor of electricity than aluminum, the EMF-sensitive sand is rapidly heated to temperatures sufficient to degrade the binder, while the aluminum part becomes only slightly warm. This is far less energy intensive and requires considerably less capital investment than does a plant of equivalent capacity that uses conventional methods. Thus, the invention relates to a mixture of a binder and a matrix capable of being heated in an electromagnetic field and, preferably, comprising at least 70% inorganic substances. The matrix materials may contain a surface film of a substance such as carbon that conducts electricity and contain at least 15%, and, preferably, between 18 and 40%, of a transition element metal in compound form. Optimally, the transition element metal is iron of which at least 50% is in the divalent state. Preferably, the iron is primarily contained in a compound whose hardness as measured by the Moh scale is at least 5 and melting point greater than 700^°C, such as Fe², in conjunction with titanate (ilmenite), silicate, aluminosilicate, calcium aluminosilicate (norite, staurolite), magnesium aluminosilicate (hyperite) or chromite. The use of these materials in the manufacture of cores and molds for metal casting, including shells used in the lost foam, lost wax and Croning processes, wherein the matrix material has a particle size distribution such that d₉₀/d₁₀ is less than 5, and preferably, less than 3, is advantageous. The binder system is in a liquid that can be cured or set within less 2 minutes upon drying or heating to a temperature greater than 50^°C, but requires more than one hour to achieve the same at room temperature. The average particle size of the materials is less than 150μm and, preferably, less than lOOμm for use in the manufacture of coatings and washes for molds and cores used in metal casting. The binder system may also be a powder that is solid at temperatures below 50^°C, but that, when heated to temperatures above 50^°C, first melts and that, in melted condition, wets the surface of the sand. The recovery and re-use of the foregoing materials is characterized by the spent sand being heated by an EMF to temperatures in excess of 250^°C for at least 30 seconds before being sent to an attrition-based sand reclaim unit. The binder system of the invention, when applied as a coating around sand, results in a product that is dry and free-flowing. The foregoing descriptions should be considered as illustrative only of the principles of the invention. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the preferred embodiments described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims

1. Materials containing a mixture of a binder and a matrix capable of being heated in an electromagnetic field (EMF) consisting of at least 70% inorganic substances.

2. The matrix materials according to claim 1 containing a surface film of a substance that conducts electricity.

3. The matrix materials according to claim 1 containing at least 15% of a transition element metal in compound form.

4. The matrix materials according to claim 3, where the transition element metal is iron of which at least 50% is in a divalent state.

5. The matrix materials according to claim 3 where iron is primarily contained in a compound whose hardness as measured by the Moh scale is at least 5 and melting point greater than 700°C, wherein the iron is Fe² in conjunction with titanate (ilmenite), silicate, aluminosilicate, calcium aluminosilicate, (norite, staurolite), magnesium aluminosilicate (hyperite) or chromite.

6. The materials according to claim 1 used in the manufacture of cores and molds for metal casting, and shells used in the lost foam, lost wax and Croning processes, where the matrix material has a particle size distribution such that d₉₀/d₁₀ is less than 5.

7. The materials according to claim 1 where the binder system is a liquid that can be cured or set within less than 2 minutes upon drying or heating to temperatures greater than 50°C, but requires more than 1 hour to achieve curing at room temperature.

8. The materials according to claim 6 where the binder system is a liquid that can be cured or set within less than 2 minutes upon drying or heating to temperatures greater than 50°C, but requires more than 1 hour to achieve curing at room temperature.

9. The materials according to claim 1 having an average particle size of less than 150um and for use in the manufacture of coatings and washes for molds and cores used in metal casting.

10. The materials according to claim 1 wherein the binder system is a powder that is solid at temperatures below 50 ° C, but when heated to temperatures above 50°C first melt and in melted condition wet the surface of sand.

11. The materials according to claim 6 wherein the binder system is a powder that is solid at temperatures below 50°C, but when heated to temperatures above 50°C first melt and in melted condition wet the surface of sand.

12. The recovery and reuse of materials according to claim 1 characterised by spent sand being heated by an EMF to temperatures in excess of 250°C for at least 30 seconds before being sent to an attrition based sand reclaim unit.

13. The recovery and reuse of materials according to claim 6 characterised by spent sand being heated by an EMF to temperatures in excess of 250°C for at least 30 seconds before being sent to an attrition based sand reclaim unit.

14. The materials according to claim 7, characterised by the binder system being applied as a coating around sand such that the resulting product is dry and free flowing.

15. The materials according to claim 10, characterised by the binder system being applied as a coating around sand such that the resulting product is dry and free flowing.