RECYCLABLE BINDERS FOR METAL CASTING MOLDS AND FOR INJECTION MOLDING OF METAL AND CERAMIC PARTS

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application serial no. 60/784,517, filed on March 21 , 2006, entitled "RECYCLABLE BINDERS FOR METAL CASTING MOLDS AND FOR INJECTION MOLDING QF METAL AND CERAMIC PARTS," the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD [0002] The present invention relates to recyclable molds and templates for casting metal and ceramic articles.

BACKGROUND OF THE INVENTION [0003] Metal and ceramic articles are produced by various means, including casting operations. Casting operations involve sand casting, and powder injection molding. Such casting operations are useful in efficient production of articles ranging from simple to relatively complex shapes. Although adequate, current casting operations employ binder materials and extraction media both containing undesirable components, e.g., hydrocarbons and polymers. Manufacturers continue to be challenged in the effort to improve such current operations. For example, many conventional sand/metal binder removal processes provide environmentally undesirable results, including undesirable emissions, by-products, compounds, and wastes.

[0004] Thus, there is a need to lessen toxic products and waste materials without compromising efficiency and performance of mold casting operations to produce more desirable environmental and health issues.

BRIEF SUMMARY OF THE INVENTION [0005] The present invention generally provides a casting mold and methods that eliminate the production of undesirable by-products and waste materials. The present invention employs binder material that is extraction-medium soluble (e.g., carbon dioxide soluble). The binder material is used to bind the base material particles, (e.g., sand particles, metal powder, and ceramic powder), forming the mold or core parts for casting processes, and then to remove and recycle the binder material by dissolution in the extraction medium (e.g., supercritical carbon dioxide). In one example, due to its high solubility in carbon dioxide, a sugar acetate e.g. β-D galactose pentaacetate is selected as a commercially available and inexpensive candidate binding material/compound. During dissolution of the binder material in supercritical carbon dioxide, favorable properties of a supercritical fluid such as low viscosity, high diffusivity, and low surface tension, produce a relatively faster binder dissolution rate than comparable extractions in conventional organic solvents. [0006] In one general example, the present invention provides a recyclable mold for casting metal articles. The recyclable mold and methods for making the mold for casting a metal or ceramic article comprises binding materials that are soluble in carbon dioxide and avoid environmental and health issues. [0007] In one embodiment, the present invention provides a recyclable mold for casting a metal article comprising base material particles and binder material to bind the base material particles together. The base material particles are formed in

a predetermined shape having a cavity in which the metal article is to be cast. The binder material binds the base material particles together to define the predetermined shape. The binder material is soluble in carbon dioxide and comprises a tert-butylated aromatic or a sugar acetate.

[0008] In another example, the present invention provides a method for making a recyclable mold for casting a metal article. The method comprises forming molding material to a predetermined shape. The molding material comprises base material particles to be formed in the predetermined shape having a cavity in which the metal article is to be cast, and binder material binding the base material particles together to define the predetermined shape. The binder material is soluble in carbon dioxide and comprises a tert-butylated aromatic, sugar acetate, or nonfluorous polymer such as polyvinylacetate.

[0009] In another example, the present invention provides a method of making an article from a recyclable mold. The method comprises forming the molding material to the predetermined shape and introducing casting material in the cavity. The method further comprises removing the molding material from the casting material and debinding the molding material to provide separated base material particles and binder material for recycling.

[0010] In yet another example, the present invention provides a method of recycling a mold from a metal casting article. The method comprises contacting the mold with pressurized carbon dioxide and separating the binder material from the base material particles with the pressurized carbon dioxide.

[0011] In still another example, the present invention provides a method for forming a metal or ceramic article by powder injection molding. The method

comprises mixing metallic powder or ceramic powder with binder material to form a molding paste. The metallic powder comprises a metal, e.g., aluminum, magnesium, or titanium, and the ceramic powder comprises ceramic materials e.g. aluminum oxide. The binder material is soluble in carbon dioxide and comprises a tert-butylated aromatic, sugar acetate, or nonfluorous polymer such as polyvinylacetate. In some cases, the binder may also contain components (e.g. a secondary binder) that are left behind when the primary binder component is dissolved in carbon dioxide. The method further includes forming the paste material to a predetermined shape to define a solidified green body, and introducing carbon dioxide to the green body to extract the binder material therefrom. The method further comprises sintering the green body to fuse the metallic particles of the metallic powder together, defining a fused and sintered metal article. [0012] Further objects, features, and advantages of the present invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS [0013] Figure 1 is a perspective view of a recyclable mold for casting an article in accordance with one embodiment of the present invention; [0014] Figure 2 is a cross-sectional view of a conceptual image of base material particles and binder material of the recyclable mold prior to processing with an extraction substance in accordance with one example of the present invention. [0015] Figure 3 is a schematic flow chart of one method for making a recyclable mold in accordance with one example of the present invention.

[0016] Figure 4 is a cross-sectional view of a conceptual image of base material particles and binder material of the recyclable mold during processing with an extraction substance in accordance with one example of the present invention;

[0017] Figure 5 is a schematic flow chart of one method of recycling components of a recyclable mold in accordance with one example of the present invention;

[0018] Figure 6a is a schematic flow chart of one method of recycling a mold from a cast article in accordance with another example of the present invention;

[0019] Figure 6b is a schematic flow chart of one method of separating carbon dioxide from the binder material for recycling in accordance with one example of the present invention;

[0020] Figure 7 is a schematic flow chart of one method for making a metal or ceramic article by injection molding in accordance with another example of the present invention; and

[0021] Figure 8 is a graph of pressure to dissolve 5 wt% of a polymer at 298 K vs. weight average molecular weight depicting comparison cloud points in CO ₂ of additional binder materials.

DETAILED DESCRIPTION OF THE INVENTION [0022] The present invention generally provides a recyclable mold for casting an article. The mold includes binder material that, in combination with an extraction solvent, is extractable from the mold to allow the recyclability of the binder material. [0023] Figure 1 illustrates a recyclable mold 10 for casting an article in accordance with one embodiment of the present invention. As shown, the mold 10 includes base material particles 12 formed in a predetermined shape or

configuration 13 having a cavity 14 from which the article is to be formed. In this embodiment, the mold 10 comprises a binder material 16 that binds the base material particles 12 together to define the predetermined shape. The binder material 16 is soluble in an extraction substance or solvent. Preferably, the binder material comprises a tert-butylated aromatic or a sugar acetate. In another example, the binder material may comprise polyvinyl acetate or polylactic acid. [0024] The recyclable mold 10 is formed by mixing the base material particles

12 with the binder material 16 and forming a mixture in the predetermined configuration 13. Preferably, the base material particles 12 of the recyclable mold 10 include sand. However, other base material particles including inert aggregates such as clay, glass spheres or other inert solid particles may be used without falling beyond the scope or spirit of the present invention. In this embodiment, the tert- butylated aromatic comprises 2,4,6-tri-tert-butylphenol; 1 ,3,5-tri-tert-butylbenzene; 2,6-di-tert-butylphenol; 2,4-di-tert-butylphenol; 2,6-di-tert-butyl benzoquinone; and 3,5-di-tert-butylphenol.

[0025] In another embodiment, the sugar acetate may include any suitable sugar acetate including β-D galactose pentaacetate; α-D galactose pentaacetate, β- D maltose octaacetate, sorbitol hexaacetate, per acetylated cyclic hexa-saccharide, per acetylated cyclic hepta-saccharide, and per acetylated cyclic octa-saccharide. [0026] In this embodiment, the extraction substance is preferably a non-toxic substance such as carbon dioxide. Preferably, the non-toxic substance is a supercritical fluid or a dense gas or a liquid. Above its critical pressure and temperature, the non-toxic substance forms a single phase, termed as supercritical fluid, which exhibits unique physicochemical properties. Supercritical fluids have

offered favorable means to achieve solvating properties which have gas and liquid characteristics without actually changing chemical structure. By proper control of pressure and temperature, a significant range of physicochemical properties (density, diffusivity, dielectric constants, viscosity) can be accessed without passing through a phase boundary, e.g., changing from gas to liquid form. [0027] The supercritical fluid of the present invention is preferably carbon dioxide which may exist as a fluid having properties of both a liquid and a gas above its critical temperature and critical pressure. Carbon dioxide at its supercritical conditions has both a gaseous property, being able to penetrate through many materials and a liquid property, being able to dissolve materials. The supercritical fluid is preferably carbon dioxide, and other suitable gases, e.g. ethane, methane, ethylene, nitrogen, or fluorinated and chlorinated refrigerants, or mixtures of these substances with co-solvents and co-solutes.

[0028] The extraction substance may also include a near critical fluid, e.g., a dense gas or liquid near its critical point.

[0029] The recyclable mold 10 is recycled via a debinding process. In one example of the debinding process, the recyclable mold is removed from the cast article and is contacted with the extraction solvent to solubilize and extract the binder material from the base material particles. The recyclable mold 10 having the base material particles 12 and the binder material 16 is preferably placed in a compartment of a high-pressurized vessel isolatable from the atmosphere. In this embodiment, the base material particles 12 comprise about 50 to 98 weight percent and the binder material 16 comprises about 2 to 50 weight percent of recyclable mold placed in the vessel. The weight ratio of the base material particles to binder

material 16 is preferably at least about 10:1. Then, the compartment is sealed off from the atmosphere. The compartment may be isolated by any conventional means.

[0030] When carbon dioxide is used as the extraction solvent, carbon dioxide is then introduced into the compartment and is pressurized in the vessel to about 500 to 10,000 pounds per square inch gauge (psig). Then, heat is applied to the vessel to heat the vessel to a temperature about 20 to 100 degrees Celsius for between about 2 minutes and 3 hours. These conditions define supercritical fluid, liquid or dense gas conditions of carbon dioxide whereby the binder material 16 dissolves or solubilizes in carbon dioxide. However, other ranges may be used for other extraction substances without falling beyond the scope or spirit of the present invention. Pressurizing and heating the particles with the extraction substance may be accomplished by any conventional means, including continuous processes. [0031] The recyclable mold 10 may further comprise a coating slip 18 disposed on the predetermined shape 13 of the recyclable mold 10. The slip 18 may be comprised of ceramic material and functions to separate the mold and the article to be cast.

[0032] Figure 2 depicts base material particles 12 and binder material 16 of the recyclable mold 10 prior to a debinding process. As shown, the binder material 16 is disposed relatively evenly between the base material particles 12 to provide a rigid structure of the predetermined shape 13. The binder material 16 relatively evenly binds the base material particles 12 together to form the predetermined shape 13 of the recyclable mold 10.

[0033] Figure 3 depicts one method 20 for making a recyclable mold for casting an article in accordance with one example of the present invention. The method comprises forming a molding material to a predetermined shape. As mentioned above, the molding material comprises base material particles to be formed in the predetermined shape having a cavity in which the article is to be cast and the binder material that binds the base material particles together to define the predetermined shape. In this example, the binder material is soluble in an extraction substance and comprises a tert-butylated aromatic. For example, the base material particles may comprise sand, and the binder material may comprise a tert-butylated aromatic, including 2,4,6-tri-tert-butylphenol; 1 ,3,5-tri-tert-butylbenzene; 2,6-di-tert- butylphenol; 2,4-di-tert-butylphenol; 2,6-di-tert-butyl benzoquinone; and 3,5-di-tert- butylphenol.

[0034] The method comprises providing in box 22, the base material particles and the binder material. The method further comprises mixing in box 24, the base material particles and the binding material to define a molding paste of the molding material. The molding paste is capable of taking on a predetermined shape of the mold. In one example, the base material particles are mixed with molten 2,4,6-tri- tert-butylphenol binder material to provide the molding paste. The method further includes forming in box 26, the molding paste to the predetermined shape having the cavity. In one example, the molding paste is disposed in or on a template member having a preformed pattern of the predetermined shape. This may be accomplished by any suitable means, e.g., a simulation software model containing design parameters of the mold and cooperating with an injection molding apparatus to form

the molding paste. The molding paste is disposed on the pattern and cooled to solidification at room temperature to define the recyclable mold for casting an article. [0035] The casting process is then performed in box 28 to produce the article, e.g., a metal article. This may be accomplished by introducing or pouring a casting material, e.g., molten metal, in the cavity of the recyclable mold and cooling the metal to room temperature for solidification. The recyclable mold is then removed from the cast article by any suitable means and sent through a debinding process in box 30. For example, the mold may be cracked or broken away from the article and sent to a debinding process in which the binder material is extracted from the base material particles, e.g., sand, by the extraction substance, e.g., carbon dioxide. The method further comprises recovering the binder material and carbon dioxide in box 31. This may be accomplished by any suitable recovery process. [0036] Figure 4 depicts base material particles 12 and binder material 16 of the recyclable mold during the debinding process. As the extraction substance 32 contacts the components of the recyclable mold, the binder material 16 melts and dissolves from the base material particles 12. Once the binder material 16 is dissolved from the base material particles 12, the base material particles 12 lose adherence to the predetermined shape and may be recycled. The binder material 16 and the extraction substance 32 may be further processed for separation, and the binder material and the extraction substance may both be recycled. [0037] Figure 5 illustrates one method 40 of recycling a mold from a cast article in accordance with another example of the present invention. As shown in Figures 5 and 6a, the method 40 includes forming in box 42, the recyclable mold in a predetermined shape. After a casting process is completed, the method 40 includes

removing in box 44, the recyclable mold from the cast article. This may be accomplished by any suitable means, e.g., by cracking the mold. [0038] The method further comprises contacting in box 46, the mold including the binder material (e.g., tert-butylated aromatic) and the base material particles (e.g., sand) with an extraction substance. In one example, the extraction substance is a non-toxic substance (e.g., high-pressure CO2). In this example, the non-toxic substance is a pressurized or supercritical carbon dioxide having a temperature between 20 and 100 degrees Celsius and a pressure of between 500 and 10,000 pounds per square inch for between about 2 minutes and 3 hours. [0039] The pressurized carbon dioxide dissolves the binder material to cause the sand to lose its adherence to the predetermined shape. This further allows each of the binder material and the sand to be separately recovered, defining a carbon dioxide-binder solution that is separated from the base material particles. The base material particles and the carbon dioxide-binder solution are separated. As shown in Figure 6b, the carbon dioxide-binder solution may then undergo further processing, i.e. depressurizing of CO ₂ , to separate the carbon dioxide from the binder material for recycling as well. The method may be controlled by any suitable means, e.g., software and hardware systems.

[0040] Figure 7 depicts a schematic diagram 70 for a method of casting a metal or ceramic article by injection molding in accordance with another example of the present invention. As shown, the method comprises providing in box 71 fine powdered particles, e.g., metal powder, and binder material mentioned above. In one example, the fine powdered particles may include aluminum or titanium powders and the binder material may comprise a tert-butylated aromatic as mentioned above.

The method further includes mixing in box 72 the components to a relatively homogeneous mixture, defining a molding paste. The molding paste is injection molded in box 74 into a cavity to form a predetermined shape, defining a solidified green-body. This may be accomplished by any suitable injection molding system having a template cavity in which the molding paste is injected and cooled to define the solidified green-body. The green-body may take on any desired or predetermined shape of an article to be produced, such as an automotive part. [0041] As shown, an extraction substance (e.g., supercritical carbon dioxide) is introduced in box 76 and contacted with the green-body to extract the primary binder material therefrom. As the primary binder material is extracted and removed from the green-body in box 78, the fine powdered particles retain the predetermined shape, in the presence of the secondary binder defining a preformed article. The preformed article is then sintered in box 80 at temperatures of at least about 400 degrees Celsius to remove residual binder material. Also during sintering, the powdered particles fuse together, strengthening and maintaining the predetermined shape of the article. The carbon dioxide and binder may then be processed for recycling as shown. In this example, the carbon dioxide and the primary binder are recycled as shown by any suitable means. Furthermore, the method may be controlled by any suitable means, e.g., software and hardware systems, as shown in box 82.

[0042] A number of additional non-fluorous CO ₂ -philic functional groups have been identified. These functional groups can be used as the CO ₂ -phile of various compounds, thereby facilitating the generation of Cθ2-soluble polymers and copolymers. That is, in another example, the binder material may comprise the

following polymers: polyvinyl acetate) (PVAc); poly(1-O-(vinyloxy)ethyl-2,3,4,6-tetra- O-acetyl-β-D-glucopyranoside) (poly(AcGlcVE)); and poly(lactic acid) (PLA) as discussed in greater detail below. Phase behavior results, such as those shown in Figure 8, have shown that each of these polymers is soluble in dense CO ₂ . In one example, such polymers are solids at ambient temperature. Copolymers of each of these polymers with less than 20% of any co-monomer may also be used as binder material.

[0043] As briefly mentioned above, it has been found that polyvinyl acetate)

(PVA) exhibits acceptable solubility in CO _2. It is believed that the interaction is between carbon of CO ₂ which acts as Lewis acid and the oxygen present in the side chain of the polymer which acts as Lewis base and hydrogen bonding (C-H... O) is responsible for high solubility of poly( vinyl acetate) in CO ₂ . Polyvinyl acetate) is a CO ₂ -soluble high molecular weight, non-fluorous polymer. Polyvinyl acetate copolymers with less than 20% of a co-monomer (e.g. 90% vinyl acetate, 10% hydroxyl) may also be used.

[0044] In another example, it has been found that polymers with sugar acetates in the side chains, rather than the backbone, are CO ₂ soluble and may be used as a binder material as well. In one example, poly(1-O-(vinyloxy)ethyl-2, 3,4,6- tetra-O-acetyl-β-D-glucopyranoside) (poly(AcGlcVE) was synthesized. This molecule has peraceylated sugar molecules as a pendent group. Its phase behavior in CO ₂ showed that 5 wt% is soluble in CO ₂ at about 74 MPa. Figure 8 shows the comparison of cloud point in CO ₂ for PVA and PoIy(AcGIcVE). High molecular weight, low melting point polymers with acetylated monosaccharide side chains of

(1-O-(vinyloxy)ethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyr anoside) formed the polymer (poly(AcGlcVE)) in this synthesis.

[0045] Additionally, oligo(lactic acid) and poly(lactic acid) have been shown to be CO ₂ soluble at relatively high pressures and may also be used as a binder material. Figure 8 illustrates data for poly(lactic acid) (PLA). The low molecular weight PLA (about 6000) is a solid that has relatively comparable solubility to (poly(AcGlcVE), and that PLA at this low molecular weight is slightly less CO2 soluble than PVAc.

[0046] While the present invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.