The present invention relates generally to the investment casting art, and more specifically to solid investment mold materials and methods especially useful for casting high melting point metals, such as platinum and the like.
As will be understood by those familiar with the investment casting art, the general procedure for making solid investment molds involves attaching patterns having configurations of the desired metal castings to a runner system to form a set-up or xe2x80x9ctreexe2x80x9d. The patterns and runner system are made of wax, plastic or other expendable material. To form the mold, the set-up or tree consisting of the pattern or patterns attached to the runner system are placed into a flask which is filled with a refractory investment slurry that is allowed to harden in the flask around the tree or set-up to form the mold. After the investment slurry is hardened, the patterns are melted out of the mold by heating in an oven, furnace or autoclave. The mold is then fired to an elevated temperature to remove water and burn off any residual pattern material in the casting cavities.
Conventional investment formulations used for non-ferrous molds are comprised of a binder and a refractory made up of a blend of fine and course particles. A typical refractory usually is wholly or at least in part silica, such as quartz, cristabolite or tridymite. Other refractories such as calcined mullite and pyrophyllite also can be used as part of the refractory.
Gypsum powder (calcium sulfate hemihydrate) is almost universally used as a binder for molds intended for casting gold, silver and other metals and alloys having relatively low melting points. Casting of high melting point metals and alloys, such as platinum and platinum alloys, precludes the use of gypsum bonded investments. Instead, phosphate bonded investments have been and continue to be used to some extent for casting such metals. One type of phosphate investment generally comprises a refractory powder such as silica and a binder formed by reacting phosphate salts or phosphoric acid with a reactive oxide. Typically used phosphate investments generally utilize monoammonium phosphate and magnesium oxide with the latter being present in greater than the stoichiometric quantity required to react with the phosphate salt. Sometimes monomagnesium or monocalcium phosphate have been included to modify the performance of the investment. Illustrative examples of phosphate investments are disclosed in U.S. Pat. Nos. 2,072,212; 2,152,152; 2,209,035; 2,680,890 and 2,928,749.
Phosphate investments of the type described above, while known and available for many years, have not been completely satisfactory for platinum jewelry and efforts have been made by others to provide improved investments for this application. A recent development that has become commercially available consists essentially of silica ground to a relatively fine particle size which is mixed with dilute phosphoric acid. Phosphoric acid investment does not set up in a cementious manner like gypsum bonded investments. Since the investment does not set like a cement, the slurry is poured around the pattern cluster which is mounted on an absorbent surface within a metal flask. The flask is provided with a removable paper extension on its top so that the slurry can be higher than the top of the flask. The mold is then dried further, dewaxed and heated to prepare it for casting. The use of the absorbent surface under the flask permits the mold to dry from both the top and bottom surfaces.
Although the phosphoric acid type investments are capable of producing good castings with smooth surfaces, there are a number of objections to the use of phosphoric acid. It is a hazardous material and requires Hazmat packaging which increases shipping costs. In order to minimize this cost, the acid is usually provided at a high concentration, such as 50 to 85 percent phosphoric acid, which the customer must store, dilute and measure out as needed according to the supplier""s specifications. The use of any additive which must be measured out and added to the mixing water, whether solid or liquid, and whether hazardous or not, requires additional in-plant operations and is a potential for mistakes. The use of a hazardous chemical has the additional disadvantage of adding to in-plant safety concerns. All of these disadvantages are particularly burdensome to users who purchase and use investment materials in small quantities.
It is an object of this invention to provide a phosphate investment material and methods of making phosphate investment molds which do not require the use of phosphoric acid with all of the attendant disadvantages discussed above.
Another object of the invention is to provide a composition and method for making a phosphate investment mold capable of casting high temperature melting metals and alloys, such as platinum and the like, with results which are superior or at least equivalent to those achieved by the use of the phosphoric acid type investments currently available.
According to one aspect of the invention, the foregoing objects and advantages are achieved by a dry investment powder blend comprising phosphate salt, water soluble organic acid and refractory powder.
According to another aspect of the invention, the foregoing objects and advantages are achieved by a method of making an investment mold comprising the steps of mixing refractory powder, phosphate salt, water soluble organic acid and water to form a slurry, investing a disposable pattern with the slurry, allowing the slurry to set to form a mold, and removing the disposable pattern from the mold. The refractory powder, phosphate salt and organic acid can be combined as a dry mixture which is thereafter mixed with water. Alternatively, at least one of the phosphate salt and acid can be added to the water with the refractory powder at the time of preparing the slurry.
The phosphate salt used in the composition and method of this invention is at least one member of the group consisting of phosphate salts of metals and inorganic, non-metallic cations, such as ammonium or boron. More particularly, the phosphate salt used in the composition and method of the invention may be at least one member of the group consisting of (a) monobasic, dibasic and tribasic phosphate salts, and (b) condensed polyphosphate salts. The preferred monobasic phosphate salts have the general formula AH2PO4 and BH4(PO4)2 where A is ammonium or at least one monovalent member of the Group IA of the Periodic Table and B is a divalent metal. The preferred dibasic phosphate salts have the general formula A2HPO4 and BHPO4 wherein A and B are as defined above. The preferred tribasic phosphate salts have the general formula A3PO4, ABPO4, B3(PO4)2 and CPO4, where A and B are as previously defined and C is at least one trivalent element of Group III of the Periodic Table. In certain preferred compositions, B is at least one metal selected from Group IIA of the Periodic Table.
The condensed polyphosphate salt may be at least one member selected from the group consisting of pyrophosphates, long-chain polyphosphates and metaphosphates. All of the poly and metaphosphates are believed to be useful in the practice of this invention. Some suitable sodium metaphosphates are known by specific names such as Graham""s, Maddrell""s or Kurroll""s salts.
Acids suitable for use in this invention are organic acids which melt above room temperature and are water soluble. Illustrative examples include oxalic, citric, adipic, malonic, glutonic, malic, maleic, and succinic acids.
A variety of refractories can be used in the composition and method of the invention, but, generally, natural silica in the form of quartz will be quite satisfactory and is preferred because of its ready availability and low cost. When higher refractoriness or increased inertness is desired or perhaps for other reasons, materials such as alumina, zircon, zirconia, high-alumina aluminum silicates, etc. can be used. Some lower thermal expansion alternatives to quartz tend to slide out of the metal flask on firing, but this can be prevented by crimping or otherwise altering the flasks. Alumina has the unexpected property of permitting water to move out of the investment into the porous supporting media more rapidly and it may be used by itself or as an additive to utilize this effect.
The useful range of ingredients in the compositions of the invention (based on the use of anhydrous salts and acids) is generally:
Phosphate Salt (0.02-4.00%)
Organic Acid (0.01-3.00%)
Refractory Powder-Balance
A more preferred range is:
Phosphate Salt (0.5-1.5%)
Organic Acid (0.25-1.25%)
Refractory Powder-Balance
If any of the hydrated salts or acids are used, the upper limits can be increased to provide the equivalent amounts of anhydrous compounds.
Increasing the amount of organic acid beyond the ranges described is possible, but this increases the cost without commensurate advantages. Increasing the phosphate content similarly increases the cost and may have other disadvantages as well. For example, drying times and total cycle times are increased. Fired strength increases to unnecessary levels and, depending on the circumstances, may reach detrimental levels requiring excessively long shake-out times and/or excessive mechanical action harmful to delicate castings. In addition, increased phosphate content decreases permeability in the fired mold and increases the danger of defective castings.
The general procedure for using the investment material and method of the invention is the same as described above in connection with phosphoric acid type investments. The pattern cluster is placed on a water absorbent surface within the metal flask, and the flask is provided with a removable extension on its top so that the slurry can be poured higher than the top of the flask. Usually, a supematent layer of liquid does not occur with the new investment material of this invention. If it does form, the liquid on the top is decanted and, after further drying, the investment within the extension is cut-off even with the top of the flask. The mold is then dried further, dewaxed and fired to prepare it for casting.
These and other advantages and a fuller understanding of the invention will be had from the following detailed description.
As generally described above, the invention provides an investment material formed from a novel dry investment powder blend containing one or more phosphate salts, one or more water soluble organic acids, and refractory powder. The investment powder is used to make solid investment molds suitable for casting platinum and other high melting temperature metals and alloys.
A considerable number of phosphate salts can be used in the practice of the invention. Generally, they fall into two broad categories: simple salts which can be formed by the partial or complete neutralization of phosphoric acid by a base of the desired cation; and more complex salts where the phosphate ions have undergone condensation reactions to eliminate water and form Pxe2x80x94Oxe2x80x94P linkages (one for each molecule of water eliminated).
The simple salts may be called monobasic, dibasic or tribasic, depending respectively on whether one, two or all three of the hydrogen ions and phosphoric acid were neutralized in their formation. They can be described further as follows:
1. Monobasic phosphate salts having the formulae AH2PO4 or BH4(PO4)2. In these formulae A represents ammonium or a monovalent metal of Group IA of the Periodic Table and B represents a divalent metal of Group IIA of the Periodic Table, or other divalent metal such as zinc, for example.
2. Dibasic phosphate salts having the formulae A2HPO4 or BHPO4, where A and B are defined as above.
3. Tribasic salts of the general formulae A3PO4, ABPO4, B3(PO4)2 or CPO4, where C represents a trivalent element of Group III of the Periodic Table, and A and B are defined as above.
More complex, condensed polyphosphate salts are at least one member selected from the group consisting of pyrophosphates, long-chain polyphosphates and metaphosphates. All of the poly and meta phosphates are believed to be useful. Some suitable sodium metaphosphates are Graham""s, Maddrell""s or Kurroll""s salts.
Acid suitable for use in the invention are organic acids which melt above room temperature and are water soluble. Illustrative examples include oxalic, citric, adipic, malonic, glutonic, malic, maleic, and succinic acids.
A variety of refractories can be used in the new investment material, but generally, natural silica in the form of quartz has been found quite satisfactory and is preferred because of its ready availability and low cost. When higher refractoriness or increased inertness is desired or perhaps for other reasons, such materials as alumina, zircon, zirconia, high-alumina aluminum silicates, etc. can be used. Some of the lower thermal expansion alternatives to quartz tend to slide out of the metal flasks on firing, but this is easily prevented by crimping or otherwise altering the flasks. Alumina has the unexpected property of permitting water to move out of the investment into porous media more rapidly, and it may be used by itself or as an additive to utilize this effect.
The useful range of ingredients in the compositions of the invention (based on the use of anhydrous salts and acids) is generally:
Phosphate Salt (one or more) 0.02-4.00%
Organic Acid (one or more) 0.01-3.00%
Refractory Powder-Balance
A more preferred range is:
Phosphate Salt (one or more) 0.5-1.5%
Organic Acid (one or more) 0.25-1.25%
Refractory Powder-Balance
If any hydrated salts or acids are used, the upper limits expressed above can be increased to provide equivalent amounts of anhydrous compounds. Increasing the acids beyond the ranges indicated is feasible, but increases the cost with little or no commensurate advantages. Increasing the phosphate content similarly increases the cost, but has other disadvantages as well. For example, drying times and total cycle times increase. Fired strength increases to unnecessary levels and, depending upon the circumstances, may reach detrimental levels requiring excessively long shake-out times and/or excessive mechanical action which can be harmful to delicate castings. In addition, decreased permeability in the fired mold increases the danger of mis-run castings.
A paramount objective of the invention is to provide an investment material for high melting metals and alloys which does not require the use of phosphoric acid. Another important objective is to provide such an investment material which can be readily supplied as a powder ready to be used by simply mixing with water. In the event that the second objective is not important to a particular user, the invention contemplates adding the phosphate salts and/or organic acids to the mixing liquid at the time the slurry is prepared. It is also possible to add the salt either to the investment powder or to the mixing liquid with an organic acid in the form of a dilute, aqueous solution.
The large number of phosphate salts which can be used in carrying out the invention provides great flexibility in formulating the investment compositions. Salts containing calcium as the cation, especially CaHPO4 and Ca(H2PO4)2 are readily available and very satisfactory. BaHPO4 produces rapid drying, but is more expensive and is not available from many sources. The use of (NH4)H2PO4 provides the highest refractoriness in the mold by leaving no cation residue after firing. Alkaline metal phosphates, such as sodium and potassium, provide higher fired strength and can be used by themselves or in conjunction with other phosphate salts.
In carrying out the invention, the refractory slurry is invested around a pattern cluster supported in a flask on suitable water absorbent material in order to accomplish drying at the top and the bottom of the flask. The flask has an extension made of paper or the like on its top so that the slurry can be poured higher than the top of the flask. If a supernatant layer of liquid separates on the top, it is decanted and, after further drying, the investment, the paper is cut off even with the top of the flask. The mold is dried further, dewaxed and fired to prepare it for casting.
A single description of set-up and flasking; mixing and pouring; and initial air drying is applicable to all of the following specific examples of the invention and is as follows. The patterns and spruing which constitute the set-up are made of conventional jewelry casting waxes. The assembled set-up is attached at its sprue end directly to the absorbent medium in contrast to conventional jewelry casting where it is generally attached to a plate or base of rubber, wood, cardboard or metal. Suitable porous media used for the absorbent material are common paper towels, but it will be understood that many other items, such as filter paper, kitchen napkins, hand towels, wipers, blotting papers, etc. can be employed.
The paper towels are piled in a stack and the pattern set-up sprue is positioned within the casting flask and both the flask and the casting sprue are wax-welded to the top towel. Sufficient area is provided within the flask circle to permit ready movement of water from the investment slurry into the absorbent stack. Although the number of towels can be varied depending upon particular circumstances, a stack of ten was used in each of the following examples to provide uniformity. A flat piece of corrugated cardboard was used under the stack of towels for handling.
The flask used in each of the examples had a solid wall (no perforations) and no absorbent lining on the inside so as to provide a constant basis for comparing examples. However, it should be understood that drying times can be shortened somewhat by providing an absorbent liner on the inside wall of the flask and by providing perforated flask with their opening sealed by an absorbent paper on the outside. An extension formed by tape was provided on the top of each flask in order to accommodate the expansion of the refractory slurry under vacuum and to provide room for extra investment slurry that was subsequently trimmed off level with the top of the flask at an appropriate time in the drying cycle.
The phosphate salt, organic acid and refractory powder were pre-blended into a dry powder blend which was mixed with water as required. Distilled water was used in the examples. A common mixing procedure was employed with the exception that the mixing time was extended on a particular composition, if needed, in order to achieve desired final fluidity. Other than this variation, investing was similar to normal practice with other types of investment materials now in use.
The metal in the Examples 1-7 was a cobalt alloy having a melting point about 2715xc2x0 F. The metal in Examples 8 and 9 was an 8620 steel alloy cast at a temperature of 3200xc2x0 F. which is approximately the melting point of platinum alloys used for jewelry.