Anti-Veining Additive for Silica Sand Mold

Abstract

This invention relates generally to a composition for silica sand cores and molds suitable for in the casting of metals. The sand core composition contains a hinder and a uniformly dispersed anti-veining additive. The mixed metal oxides collapse into a visco-plastic state when the foundry mold/core is heated by the molten metal during casting. A change in state of the MMOx from solid to a visco-plastic at the casting high temperatures provides space and lubricity within the foundry shape sufficient to compensate for the thermally-induced physical expansion of the silica grains. Thereby avoiding the mechanical forces which cause cracks and fissures in the mold or core that produce veins and other surface imperfections associated with the high coefficient of thermal expansion of silica sand.

Claims

1. A bonded silica sand mold for use in manufacturing metal castings, the metal castings produced with said bonded silica sand mold are substantially free of surface veins, said mold comprising: an additive of mixed metal oxides.

2. A bonded silica sand mold as claimed in claim 1, wherein said additive mixed metal oxides comprise less than 6.0% by weight of said bonded silica sand mold.

3. A bonded silica sand mold as claimed in claim 1, wherein said additive of mixed metal oxides comprises at least five metals selected from the group consisting of Na, Al, Si, Ca, Mg, K, Fe.

4. A bonded silica sand mold as claimed in claim 1, wherein said additive of mixed metal oxides comprises of at least 40% by weight of Silicon.

5. A bonded silica sand mold as claimed in claim 3, wherein said additive of mixed metal oxides comprises of at least 40% by weight of Silicon.

6. A bonded silica sand mold as claimed in claim 1, wherein said additive of mixed metal oxides combined have a softening temperature of above at least 1,000 F. (540 C.).

8. A bonded silica sand mold as claimed in claim 1, wherein said mold further comprising: an organic binder.

9. A bonded silica sand mold as claimed in claim 1, wherein said additive of mixed metal oxides comprises at least four metals selected from the group consisting of Na, Al, Si, Ca, Mg, K, Fe.

10. A bonded silica sand mold as claimed in claim 1, wherein said additive of mixed metal oxides comprises between 1%-6% by weight of the bonded silica sand mold.

11. A bonded silica sand mold for use to manufacture metal castings as claimed in claim 9, wherein said mold further comprising: an organic binder.

12. A bonded silica sand mold for use in manufacturing metal castings, the metal castings produced with said bonded silica sand mold are substantially free of surface veins, said mold comprising: an additive of mixed metal oxides, mixed metal oxides comprising of at least two metals selected from the group consisting of Na, Al, Si, Ca, Mg, K, Fe; wherein said additive of mixed metal oxides comprises between 1%-6% by weight of the bonded silica sand mold.

13. A bonded silica sand mold as claimed in claim 12, wherein said Ca is at least 9% by weight of the mixed metal oxides.

14. A bonded silica sand mold as claimed in claim 12, wherein said additive of mixed metal oxides combined have a softening temperature of above at least 1,045 F.

15. A bonded silica sand mold as claimed in claim 12, wherein said mold is a phenolic urethane cold box mold.

16. An anti-veining mixed metal oxides additive, said mixed metal oxides comprising of at least four metals selected from the group consisting of Na, Al, Si, Ca, Mg, K, Fe.

17. An anti-veining mixed metal oxides additive as claimed in claim 16, wherein said additive of mixed metal oxides in combination have a softening temperature of above at least 1,045 F.

18. An anti-veining mixed metal oxides additive as claimed in claim 17, wherein said mixed metal oxides comprises of at least four metals selected from the group consisting of Na, Al, Si, Ca, Mg, K, Fe.

19. An anti-veining mixed metal oxides additive as claimed in claim 16, wherein said Na is at least 9% by weight of said mixed metal oxides, and said Si is at least 40% by weight of said mixed metal oxides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a plot of superimposed thermal expansion curves for PUCB bonded silica sand and PUCB bonded silica sand with 4% MMOx anti-veining additive of the present invention,

[0026] FIG. 2 is a plot of a thermal expansion curve of an anti veining MMOx additive of the present invention;

[0027] FIG. 3 is a photograph of step core castings of PUCB bonded silica sand with no additives,

[0028] FIG. 4 is a photograph of step core castings of a PUCB bonded silica sand including 4% by weight a MMOx anti-veining additive of the present invention,

[0029] FIG. 5 is a photograph of step core castings of a PUCB bonded silica sand including an iron oxide additive of 4% by weight.

DETAILED DESCRIPTION OF THE INVENTION

[0030] An additive to foundry sand molding and core aggregates is used to produce sand cores and molds. The additive requires no carbon addition and is unreactive with most foundry sand binders. The additive produces a sand-based foundry molding and core aggregate which resists the formation of some of the defects commonly associated with the production of castings in silica sand-based molding and core aggregates. In particular, the additive improves the surface quality of castings by reducing thermal expansion defects, i.e. vein, in iron, steel, brass and bronze castings. In some instances it may be beneficial for use in aluminum castings, but in general, veins are not nearly as serious a problem in aluminum castings as they are in iron and steel castings.

[0031] The first curvilinear line in FIG. 1 illustrates the coefficient of thermal expansion of a PUCB bonded silica sand. The phenolic urethane binder is the same binder employed throughout the North America foundry industry. PUCB bonded silica sand coefficient of expansion is plotted versus temperature in the graphical curvilinear line of FIG. 1. No additives were included in the bonded silica sand, just silica and a phenolic urethane binder.

[0032] This PUCB bonded silica sand expansion data was obtained by placing a sample of the PUCB bonded silica sand in a holder, placing the holder in a dilatometer, and then heating the bonded silica sand to elevated temperatures. This curvilinear line represents the expansion of conventional quartz sand from 80 F. (27 C.) to about 1950 F. (1065 C.) The silica begins to transform from alpha quartz to beta quartz at about 1100 F. (593 C.), and the transformation to beta quartz is complete at about 1170 F. (632 C.). As the sand is heated to higher temperatures, the beta quartz continues to expand as illustrated.

[0033] The silica sand radically expands as it transforms from alpha to beta quartz. In this region, the sand has a very high coefficient of thermal expansion, with a value of about (0.027 in/in/ F.), a value substantially greater than that of alpha quartz. This high expansion coefficient of beta quartz is a major factor causing molds and cores to split, crack, spall, and otherwise produce surface defects on castings.

[0034] The additive of the present invention may be utilized in conventional silica sand molds and cores including but not limited to PUCB cold box binders. Such mold and core aggregates are usually made from silica sand, with the sand grains being bound together by chemical means. Typically, the mold or core mixture may comprise between about 90% to about 99% of silica sand, less than 2% resin and about 3.0% to about 7% of an anti-veining additive of the present invention.

[0035] In a preferred embodiment of the present invention an anti-veining additive for eliminating expansion defects is to add a visco-plastic mixture of metal oxides (MMOx) to the mold. There are several inexpensive mixed metal oxides commercially available, and the composition of several suitable mixed materials of the present invention is presented in Table I. It is noted that the MMOx compounds have softening temperatures of about 1050 F. (565.5 C.) compared to iron oxide softening at a temperature of about 1780 F. (971 C.).

[0036] In the present invention, an amount of MMOx mixture of sufficient quantity to yield a sufficient amount of strain accommodation within the foundry shape to accept and compensate for the thermally-induced expansion of the silica sand grains is employed. The quantity of the MMOx mixture required is related in significant part to the amount of void volume or space between the silica sand grains.

[0037] When there is an insufficient volumetric quantity of MMOx mixed with the silica grains, very little or no significant anti-veining effect will be achieved. Thermally-induced MMOx plasticity and viscosity decreases combined with expansion pressure from the silica sand grains occurs at the elevated temperatures caused by molten metal.

[0038] The addition of mixed metal oxides to the sand in a quantity of approximately 4% by weight, drastically changes the expansion coefficient and actually causes core shrinkage to occur at the higher temperatures, (see the second curvilinear graphical line in FIG. 1). In other preferred embodiments of the invention the amount of MMOx added into bonded silica sand is approximately within the range of 3%-5% by weight of the total weight of mold material (silica and phenolic urethane binder). In the following table the composition of preferred MMOx additive samples of the instant invention along with their corresponding softening temperature are provided:

TABLE-US-00001 TABLE 1 COMPOSITIONS & SOFTENING TEMPS OF SELECTED MIXED METAL OXIDES Soft- MMOx ening Sample (% Weight) Temp Name Na Al Si Ca O.sub.2 Mg K Fe F. BG 9.7 1.3 47.0 13.8 27.1 1046 CPS 13.02 4.5 59.1 11.46 6 2.9 1.6 1099 GG 9.8 2.7 45.8 12.17 28.83 1.3 1069 SLC 11.31 1.8 58.3 16.2 10.5 0.6 1.1 1051 MCSG 9.98 3.2 46.7 9.88 27.39 2.77 1060 TG 9.6 0.45 48.3 11.5 23.88 2.22 3.98 1074

[0039] The MMOx addition is accomplished by adding a material selected from the group consisting of the Na, Al, Si, Ca, O.sub.2, Mg, K, Fe. Eachof these materials are commercially available and for each MMOx mixture except for MCSG sample in table 1 the amount of Na, Al, Ca, Mg, or K never exceeds 17.0%. As seen in table 1, the amount of Silicon in the MMOx mixtures is between 40%-60%. Silicon is a very affordable and inexpensive additive as are the other materials in the MMOx mixtures: Na, Al, Ca, 02, Mg, K, Fe.

[0040] In accordance with another preferred mixed metal oxide embodiment of the present invention: the amount of sodium (a) in the MMOx additive ranges between about 9.0%-15.0% by weight, the amount of aluminum (Al) in the MMOx additive ranges between 0.3%-5.0% by weight, the amount of silicon (Si) in the MMOx additive ranges between 40.0%-60.0% by weight, the amount of calcium (Ca) in the MMOx additive ranges between 8.0%-20.0% by weight, the amount of oxygen (O.sub.2) in the MMOx additive ranges between 5.0%-30.0% by weight, the amount of magnesium (Mg) in the MMOx additive ranges between 0.0%-3.0% by weight, the amount of potassium (K) in the MMOx additive ranges between 0.0%-2.0% by weight and the amount of iron (Fe) in the MMOx additive ranges between 0.0%-5.0% by weight.

[0041] It is understood by those skilled in the art of oxide formulations that in multi-component oxide systems composition changes or even additions of new oxides can alter the softening temperature. Thus it is clear to those skilled in the art that minor changes in composition and compositions outside the ranges in Table I are within the scope of this patent. The critical feature of using mixed oxides is that the softening point can be controlled to values in the range of 1050 F. (565.5 C.) for the mixed oxides to 1900 F. (1037 C.) for the oxides of iron. Other oxide mixtures may be found with beneficial properties with variations in the ratios of the various oxides.

[0042] In accordance with another preferred present invention embodiment: the amount of Na in the MMOx additive ranges between 10.0%-13.0% by weight, the amount of Al in the MMOx additive ranges between 0.1%-3.0% by weight, the amount of Si in the MMOx additive ranges between 55.0%-61.0% by weight, the amount of Ca in the MMOx additive ranges between 13.0%-19.0% by weight, the amount of O.sub.2 in the MMOx additive ranges between 5.0%-15.0% by weight, the amount of Mg in the MMOx additive ranges between 0.1%-1.0% by weight, the amount of K in the MMOx additive ranges between 0.5%-1.5% and no iron oxide.

[0043] In one preferred SLC embodiment of the invention, see SLC sample in Table 1, the MMOx mixuture comprises approximately of the following: 11% sodium, 0.6% magnesium, 1.8% aluminum, 58% silicon, 1.1% potassium, 16.2% calcium, 0.2% zinc and 10.5% oxygen. The mixture rate of thermal expansion vs temperature for the SLC mixture is plotted in the generally second curvilinear graphical line in FIG. 1. The SLC sample mixture has a softening temperature of 1051 F. (see Table 1). The mixture of bonded silica blended with the SLC additive converts completely to beta quartz at 1083 F. and has a strain of 0.016 in/in/ F., the beta quartz continues to expand until it reaches a temperature of about 1815 F. at a strain of 0.018 in/in/ F. and then the strain starts decreasing until it reaches a value of near zero at 1930 F.

[0044] The addition of the SLC additive caused the expansion of beta quartz to drop to essentially zero at 1930 F. (1055.4 C.). The plasticity of the SLC at temperatures above 1900 F. allows the sand to move slightly under the pressure of the molten metal and close any cracks that might have been formed at lower temperatures. The closed cracks prevent the formation of veins. The mechanical forces induced by thermal expansion of each silica sand grain on silica sand grains adjacent to it within the foundry shape is thereby avoided, and as a result, the cracks and fissures in the foundry shapes that allow metal penetration and cause veining in the cast part are avoided. The plasticity of the molten MMOx prevents spalling of the mold/core surface.

[0045] The SLC sample mixture described above was likewise used in a test trial casting step-cone similar to the silica sand step-cone of FIG. 3, and the silica sand step-cone with iron oxide additive of FIG. 5. FIG. 4 is a photograph of a step-cone casting wherein the sand mold and sand core materials employed were PUCB bonded silica sand with the SLC mixture described immediately above and shown in table 1. This step-cone casting in FIG. 4 is seen to be essentially free of veins and any other objectionable surface defects.

[0046] This data shows that MMOx in the correct composition range provides sufficient lubricity for the sand to collapse on the surface and eliminate surface cracks so long as the pouring temperature of the metal is above 1950 F. (1065.5 C.). Bronze alloys are typically poured about 2200 F. (1200 C.) , gray cast-iron is typically poured about 2450 F. (1343 C., ductile iron is poured about 2550 F(1400 C.), and the steel alloys are typically poured about 2900 F. (1593 C.).

[0047] The plasticity of the SLC mixture at temperatures above its softening temperature of 1050 F. (565.5 C.) provides for the thermal strain accommodation in bonded silica sand molds/cores. The other MMOx additives in Table 1 have relatively similar softening temperatures. As shown in the table, the softening temperatures for each of the sample MMOxs falls within the range of 1045 F.-1075 F. (563-579.4 C.).

[0048] Each of the other five listed MMOx additives (not the FeOxy sample) listed in Table 1 were also tested by similarly blending the samples with phenolic urethane bonded silica sand in a so-called step cone casting as discussed above. Upon inspection, after shakeout and similarly dividing each and every one of the castings in half, no veins were visible in any of the casting formed with the MMOxs listed in Table 1.

[0049] In accordance with an additional preferred MMOx additive of the present invention the MMOx softening temperature ranges between 1,000 F.-1,110 F. (538-593 C.); in another preferred alternative the MMOx softening temperature ranges between 1,025 F.-1,085 F. (552-585 C.) ; and still yet in another preferred embodiment the MMOx softening temperature ranges between 1,040 F.-1,080 F. (560-582 C.).

[0050] In each preferred embodiments disclosed herein the MMOx mixture is distributed throughout the resulting foundry shape in such a manner that the idealized response described above is generally achieved throughout the foundry shape. The MMOx mixture is uniformly distributed throughout the matrix of silica sand sufficient to separate a significant number of silica sand grains so that the plasticity of the MMOx mixture yields adequate space to accept and compensate for the thermal expansion of the bonded silica sand.

[0051] The MMOx is not used in great quantity, in a preferred embodiment of the invention MMOX comprises about 4.0-5.0% of the weight of the total material used in making the bonded silica sand mold/core. In another preferred embodiment MMOX comprises about 3.0-6.0% of the weight of the total material used in making the bonded silica sand mold/core. Further the commercial price of the metals listed in Table 1 are relatively inexpensive so that the overall cost of the MMOx additive is small.

[0052] It is contemplated that the silica sand mold/core of the present invention may alternatively be bonded together with organic binders other than phenolic urethane. Binders such as shell, furan or alkyd may be substituted for the phenolic urethane binder. Still other organic binders for the silica sand mold of the present invention may be employed, including well-known hot box binders, well-known shell mold binders or other resin binders well-known in the industry.

[0053] It is further contemplated that the silica sand mold/core of the present invention may alternatively be bonded together with well-known inorganic binders in the industry, including but not limited to sodium silicate Na.sub.2(SO.sub.2).sub.nO.

[0054] While certain novel features of this invention have been shown and described, it is not intended to be limited to the details above, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the can be made by those skilled in the art without departing in any way from the spirit of the present invention.