Investment casting wax composition

Abstract

This invention relates to investment casting wax compositions containing substituted polystyrenes, as illustrated by backbone substituted and/or ring substituted polystyrenes as illustrated by alpha-methyl styrenes, vinyl toluenes, or copolymers thereof.

Description

Investment casting refers to a method of quantity production of parts, usually having a intricate shape. The parts are normally metal, e.g., brass, steel, zinc or lead and alloys thereof. The advantages of this method, for example, over ordinary sand casting, is that the metal part has an exceptionally good surface, sharp outline, and dimensional accuracy. Another advantage is that a metal part having dimensional accuracy and/or with an intricate shape is obtained without further machining.

In the simplest form of investment casting, a wax replica is made of the part to be reproduced. The wax replica is coated over or invested with a ceramic forming material, e.g., a sand-magnesia mixture. After the investment sets the wax replica invested with the sand-magnesia mixture is heated to melt out the wax and then baked to fuse the sand-magnesia. The wax may be reused. After cooling, the resulting ceramic mold is strong enough to support the molten metal poured into the cavity of the mold. The hot metal is poured into the top vent of the mold; a bottom vent allows for air to escape. After the metal solidifies and cools, the cooled ceramic mold is removed, normally by breaking it away from the metal part. The metal reproduction usually needs no further machining except to trim off the metal that solidified in the vents.

To produce the aforementioned wax replica, a die is required to make repetitions of the wax replica. This die, usually of metal, is machined or cast to match the part to be reproduced with precision. Into the cavity of this die the investment casting wax, as a slush, is injected. After cooling, the solid wax replica is removed from the die. This wax replica has the strength to be handled and invested, and when melted out of the ceramic investment, leaves behind little or nil ash which otherwise would cause imperfections on the surface of the metal reproduction.

Investment casting waxes are also called pattern waxes. This invention provides novel pattern wax compositions, the advantages of which will permit their use in precision casting operations. The compositions of this invention have suitable flow characteristics and are homogenous mixtures; their constituents do not settle out during casting or in any of the steps of the molding process. This invention also provides pattern wax compositions which can be easily removed from metals without damaging them by the removal of mold materials from the inner walls thereof. The constitution of the wax compositions of this invention is such that the components may be recycled and used again and again effecting obvious economies.

High molecular weight polystyrenes of about 10,000 to 100,000 molecular weight such as used for injection molding having been used as pattern materials for investment casting. The injection of such plastics into the die requires higher injection temperatures and pressures and higher clamping pressures than so-called wax pattern materials. These plastics also suffer from the serious disadvantage that removal of the pattern from the ceramic shell without cracking or charring is difficult or impossible. For these reasons such polystyrenes are little used. It has been estimated that 95% of the pattern materials used are wax based and only 5% polystyrene ("Pattern Materials and Their Use in Investment Casting," Report of the B.I.C.T.A. Pattern Making Committee, Edited by E. G. Donaldson (1962)).

Pattern waxes in common use may contain natural or synthetic resins, natural or synthetic waxes and a variety of other materials such as stearic acid. Resins that may be used include rosin, rosin esters, gum damar, modified phenolics, alkyds of low molecular weight, terpene resins, petroleum resins, chlorinated naphthalene, chlorinated biphenyl, etc. Waxes that may be used include beeswax, vegetable waxes such as carnauba and candelilla, mineral waxes such as paraffin wax, microcrystalline wax and montan, and synthetic waxes such as amide waxes, ester waxes, Fisher-Tropsch waxes, castor oil derived waxes, etc.

The properties required of a good pattern wax are described by J. H. W. Booth, Foundry Trade Journal, December 1962 and by D. Mills, B.I.C.T.A. 11th Annual Conference, May 1971. These include melting point, ash content, shrinkage/expansion characteristics, strength, plasticity, viscosity, thermal stability, oxidative stability and surface appearance. Other properties such as resistance to or solubility in acids and bases may be important in certain instances.

Generally speaking low shrinkage is of importance, but in some cases, such as when existing dies are sized to a particular wax and replacement of the dies to accommodate a lower shrinking wax would be very expensive, it is more important to match the shrinkage to the wax in use and improve on other properties.

Polystyrene resins of less than 10,000 molecular weight are available commercially but generally have not found utility in pattern wax blends. The higher molecular weight members of the series have high viscosities and poor compatibility with other common blend components. Very low molecular weight polystyrenes are more compatible but are soft and when blended with waxes, etc., give compositions which are unsuitably plastic. It is one of the requirements of a pattern wax that it be rigid enough to withstand the investment process and to support its own weight in relatively thin sections.

We have now found that blends of substituted polystyrenes with waxes and other materials give compositions which are very suitable for use as pattern materials. The substituted polystyrenes can be polymers of alpha-alkylstyrenes ##SPC1##

or copolymers of the above. Also included are copolymers of styrene with .alpha.-alkyl styrenes or vinyl alkylbenzenes or both, provided that the styrene content does not exceed 50%. In the above formulae R is alkyl preferably lower alkyl and most preferably methyl, ethyl, propyl, butyl, etc., and most preferably methyl. Suitable substituted polystyrenes are also obtained by alkylating polystyrene, for example by the Friedel-Crafts method, provided that at least half the aromatic rings are alkyl substituted. Also included are polymers of polysubstituted styrenes ##SPC2##

where R' is H or alkyl and at least two of the R' groups are alkyl and n is 0-3.

Suitable substituted polystyrenes are further characterized by a Ring and Ball softening point of 50.degree.-150.degree. C., preferably 75.degree.-125.degree. C., and a molecular weight of less than about 8,000, such as less than about 5,000, and preferably less than 3,000 with a minimum of about 200 and preferably a minimum of about 400.

The present invention is a pattern wax composition for investment casting comprising about 25-75% of a substituted polystyrene as defined above, about 0-75% wax and about 0-50% of other resins, fillers, fatty chemicals, dyes, etc.

Suitable waxes are paraffin wax, microcrystalline wax, natural waxes such as beeswax, carnauba, candelilla, montan, etc., synthetic waxes such as amide waxes, ester waxes, Fischer-Tropsch waxes, polyethylene waxes, chemically modified waxes, etc.

Other materials which may be incorporated include terpene resins, rosin and modified rosins, petroleum resins, coumaron/indene resins, ethylene-vinyl acetate resins, fatty acids such as stearic acid and fatty amines such as tallow amine.

A small amount of dye may be incorporated to aid the caster in the visual examination of the pattern for defects. To aid in the casting of thick sections fillers may be incorporated. The filler may be soluble or insoluble in the basic pattern wax blend. Insoluble fillers include inorganic materials such as finely divided silica and organic materials, such as urea, adipic acid, polyethylene glycol, polyacrylic acid, etc. Soluble fillers are incorporated in the blend at a temperature below their melting ponit and remain as discrete particles until the wax is melted from the pattern. They include amide waxes and high melting point hydrocarbon waxes.

The present invention provides pattern materials suitable for injection at relatively low temperatures (50.degree.-80.degree. C.) and pressures (0-500 psi) and meeting the other requirements of a casting wax. By a suitable choice of the amount of substituted polystyrene and of the other components the shrinkage and stiffness of the pattern wax may be readily controlled. A particularly useful embodiment of this invention provides a pattern wax with lower shrinkage and increased stiffness compared to currently used pattern waxes.

Another useful embodiment provides a wax equal in shrinkage to a commercially used material but with improved stiffness. The compositions of this invention provide the great advantage that the wax blender by a suitable choice of formulation is enabled to produce waxes to meet widely varying requirements of different casting situations. In particular the stiffness may be held approximately constant while the shrinkage is varied from high to low or the shrinkage may be held approximately constant while the stiffness is varied.

A further advantage of the present compositions is that they are very stable to heat even in the presence of air. Other pattern waxes contain resins which are thermally unstable or sensitive to oxidation. High temperatures may be maintained for an hour or two during the blending operation and the molten wax is commonly maintained in the injection equipment for hours or even days. It is a common problem to find that thermal or oxidative instability has lead to an increase in the viscosity of the pattern material, requiring a change in the injection conditions which may lead to unsatisfactory patterns. The compositions of this invention do not show a significant increase in viscosity even when exposed to air at elevated temperatures, unless of course an unstable resin included.

Still another advantage of this invention is that the pattern waxes are rigid without being unduly brittle and after injection become stiff and hard rapidly. This enables the caster to reduce the time the wax must remain in the die before removal and the time that the pattern must be stabilized before investment, thus increasing the number of parts that may be produced in a given time.

EXAMPLE I

A copolymer of .alpha.-methylstyrene and vinyl toluene characterized by a molecular weight of 1,000 and a 92.degree. C. softening point (55 parts by weight), paraffin wax of 70.degree. C. melting point (35 parts) and montan wax (10 parts) were heated together at 130.degree. C. till melted and stirred together for about 30 minutes to produce a homogenous blend. The blend was injected at 80.degree. C. and 80 psi into a steel mold to form test bars 5 .times. 1/2 .times. 1/8 inch.

EXAMPLE II

A blend of the above copolymer (55 parts), 70.degree. C. M.P. paraffin (35 parts) and carnauba wax, No. 3 North Country (10 parts) was prepared and injected as in Example I.

EXAMPLE III

A blend of polystyrene of 75.degree. C. softening point and 400 molecular weight (55 parts), 70.degree. C. M.P. paraffin (35 parts) and carnauba wax (10 parts) was prepared as in Example I. At 130.degree. C. a homogenous blend was produced. The bars obtained after injection showed gross separation of the wax and resin components.

A comparison of Examples II & III illustrates the great improvement in blendability of substituted polystyrene (II) relative to unsubstituted polystyrene (III).

Wax shrinkage was measured by comparing the length of the 5 inch test bar with the length of the mold cavity. Measurements were made shortly after injection (approximately 10 min.), after 4 hrs. and after 3-4 days. Wax stiffness was measured by placing the test bar on 4 inch supports, applying a total weight of 100 g. for 10 sec. to the center of the bar and measuring the deflection produced. The weight was applied through a T-bar of cylindrical cross section attached to a penetrometer as described in A.S.T.M. D217. Stiffness was measured after the same aging times as for shrinkage.

Table I compares the shrinkage of the compositions of Examples I & II with two commercial pattern waxes. Wax A is a widely used commercial product containing substantial amounts of stearic acid. Wax B is a premium commercial product based on chlorinated biphenyl resin. Shrinkage is expressed as a percentage of the length of the mold cavity.

Table I ______________________________________ Shrinkage (%) 10 min. 4 hrs. 3-4 days ______________________________________ Product of Example I 0.20 0.32 0.34 Product of Example II 0.20 0.30 0.34 Wax A 0.24 0.44 0.54 Wax B 0.32 0.50 0.55 ______________________________________

The stiffness of the same four waxes is compared in Table II. The results are expressed in decimillimeters (dmm). A lower number indicates greater stiffness.

Table II ______________________________________ Stiffness (dmm) 10 min. 4 hrs. 3-4 days ______________________________________ Product of Example I 37 17 10 Product of Example II 31 15 11 Wax A 70 48 41 Wax B 62 30 24 ______________________________________

An examination of the data of Tables I & II shows that the products of Examples I & II are significantly lower in shrinkage and stiffer than the commercial waxes. After 10 min. they are already about as rigid as the commercial waxes are after 4 hours. After 4 hours they are stiffer than the commercial waxes are after several days. Their improved rigidity improves the handling and stability properties of the wax patterns and their reduced shrinkage makes it easier for the caster to hold the desired dimensional tolerances.

Example IV illustrates the preparation of a wax matching Waxes A & B in shrinkage characteristics and having improved rigidity.

EXAMPLE IV

A blend containing .alpha.-methylstyrene/vinyl toluene copolymer (35%), 70.degree. C. M.P. paraffin (45%), carnauba (10%), 90.degree. C. M.P. microcrystalline wax (5%) and 85.degree. C. M.P. microcrystalline wax (5%) was prepared and injected as in Example I. The test data were as follows: Shrinkage .30% (10 min.), 0.46% (4 hrs.) and 0.52% (4 days); stiffness 46 (10 min.) 23 (4 hrs.) and 12 (4 days).

Waxes A & B may be replaced by the product of Example IV, without the expense of manufacturing new dies, to provide patterns of improved rigidity.

Since all subsequent examples were prepared and injected as in Example I only the compositions and test results will be given to avoid unnecessary repetition.

Examples V & VI illustrate the preparation of waxes of very low shrinkage and good rigidity.

EXAMPLE V

Vinyl toluene/.alpha.-methylstyrene copolymer 55% 57.degree. C. M.P. paraffin 35% Montan wax 10% Shrinkage 0.00% (10 min.) 0.20% (4 days) Stiffness 35 (10 min.), 21 (4 hrs.) 14 (4 days)

EXAMPLE VI

Vinyl toluene/.alpha.-methylstyrene copolymer 55% 70.degree. C. M.P. paraffin 35% Candelilla wax 10% Shrinkage 0.00% (10 min.), 0.14% (4 hrs.), 0.26% (4 days) Stiffness 40 (10 min.), 16 (4 hrs.) 9 (4 days)

Example VII illustrates a wax of high shrinkage and a rigidity comparable to Waxes A and B.

EXAMPLE VII

Vinyl toluene/.alpha.-methylstyrene copolymer 45% 70.degree. C. M.P. paraffin 35% Carnauba wax 10% 85.degree. C. M.P. microcrystalline wax 5% Paracin-1 synthetic ester wax 5% Shrinkage 0.36% (10 min.), 0.56% (4 hrs.), 0.64% (4 days) Stiffness 67 (10 min.), 35 (4 hrs.) 24 (4 days)

Example VIII illustrates how the present invention may be utilized to obtain a flexible wax with low shrinkage.

EXAMPLE VIII

Vinyl toluene/.alpha.-methylstyrene copolymer 55% 50.degree. C. M.P. paraffin 35% Montan wax 10% Shrinkage 0.00% (10 min.), 0.28% (4 days) Stiffness 120 (10 min.) 56 (4 days)

Examples IX & X illustrate pattern waxes containing components other than substituted polystyrene and waxes. The products of both of these examples have improved properties as casting waxes and are comparable to the products of Examples I & II.

EXAMPLE IX

Vinyl Toluene/.alpha.-methylstyrene copolymer 55% 70.degree. C. M.P. paraffin 25% Stearic acid 20% Shrinkage 0.12% (10 min.), 0.24% (4 hrs.), 0.36% (4 days) Stiffness 29 (10 min.), 12 (4 hrs.), 7 (4 days)

EXAMPLE X

Vinyl toluene/.alpha.-methylstyrene copolymer 45% 70.degree. C. M.P. paraffin 30% Montan wax 10% 85.degree. C. M.P. microcrystalline wax 5% 100.degree. C. Softening point aliphatic petroleum resin 10% Shrinkage 0.25% (10 min.), 0.40% (4 days) Stiffness 36 (10 min.), 17 (4 hrs.), 10 (4 days)

The congealing points of all of the above examples were in the range of 55.degree.-70.degree. C. and can therefore be injected at the temperatures commonly used in the investment casting industry. Their other properties such as hardness, appearance, viscosity, toughness, etc., were also satisfactory.

Examples XI and XII illustrate the use of organic fillers.

EXAMPLE XI

A blend containing vinyl toluene/.alpha.-methylstyrene copolymer (25 parts), 70.degree. C. M.P. paraffin (30 parts) and carnauba wax (5 parts) was prepared as in Example I. The temperature of the blend was adjusted to 80.degree. C. and 40 parts of finely divided adipic acid added and the mixture stirred until the adipic acid was evenly dispersed in the base wax. The blend was then cooled with stirring to slightly above the congealing point and then chilled to form a slab.

EXAMPLE XII

A blend containing vinyl toluene/.alpha.-methylstyrene copolymer (30 parts), 57.degree. C. M.P. paraffin (20 parts), montan wax (10 parts) and beeswax (5 parts) was prepared and filled with 35 parts of a high density polyethylene wax of 700 molecular weight as in Example XI.

Examples XIII & XIV illustrate the use of other substituted polystyrene resins.

EXAMPLE XIII

The blend, prepared as in Example I, contained polyvinyl toluene resin of 1600 molecular weight and 105.degree. C. softening point (35%), 70.degree. C. M.P. paraffin wax (50%) and 80.degree. C. M.P. microcrystalline wax (15%).

EXAMPLE XIV

The blend, prepared as in Example I, contained poly-.alpha.-methylstyrene resin of 700 molecular weight and 85.degree.C. softening point (50%) 64.degree. C. M.P. paraffin wax (40%) and carnauba wax (10%).

Further examples are given in Table III.

TABLE III ______________________________________ Substituted Polystyrene Paraffin Other Other Example resin wax wax resin Filler ______________________________________ XV 65 35 XVI 65 25 10 XVII 55 35 10 XVIII 55 25 10 10 XIX 55 10 20 15 XX 45 25 30 XXI 45 25 30 XXII 40 10 20 10 20 XXIII 35 45 20 XXIV 35 25 40 XXV 35 25 35 5 XXVI 30 55 15 XXVII 30 30 10 10 20 XXVIII 25 35 40 XXIX 25 15 10 10 40 XXX 25 30 45 ______________________________________

In Table III the substituted polystyrene resin is selected from polyvinyl toluene, polymethyl styrene and copolymers thereof. The paraffin wax is selected from those paraffin waxes having melting points from about 50.degree. to 75.degree. C. The other wax is selected from the group containing microcrystalline waxes of about 60.degree. to 95.degree. C. melting point, synthetic hydrocarbon waxes of about 120.degree. C. maximum melting point, ester waxes, waxy acids including palmitic acid and stearic acid, waxy amines including tallow amine and hydrogenated tallow amine and waxy amides including oleamide and cocoamide. The other resin is selected from the group containing petroleum resin, terpene resins, diene resins, coal tar resins, chlorinated aromatic resins, rosin and rosin esters. The filler is any suitable organic or inorganic material.

In summary, the casting wax compositions are characterized by the presence of substituted polystyrenes, such as at least about 25% by weight of substituted polystyrene, for example from about 25-75% substituted polystyrene, such as from about 25-65% substituted polystyrene, but preferably about 30-60% substituted polystyrene with an optimum of 35-55%.

The remainder of the investment casting wax composition is a wax or a wax-like material, preferably a hydrocarbon wax such as paraffin wax, with or without other waxes which can be used to replace part of the paraffin wax comprising the following:

microcrystalline wax

montan wax

carnauba wax

candelilla wax

synthetic ester wax

stearic acid, etc.

Other chemicals, including minor amounts of other resins and fillers can also be employed when desired.

Claims

We claim:

1. An investment casting wax composition having low shrinkage and increased stiffness comprising at least about 25% of a substituted polystyrene selected from the group consisting of

1. polymers or copolymers of alpha-alkylstyrenes having repeating units of the formula ##SPC3##

2. polymers or copolymers of .beta.-alkylstyrenes having repeating units of the formula ##SPC4##

3. polymers or copolymers of vinyl alkylbenzenes having repeating units of the formula ##SPC5##

4. copolymers of styrene with .alpha.-alkylstyrenes or vinyl alkylbenzenes or both, wherein the styrene content does not exceed 50%,

5. polymers obtained by alkylating polystyrene wherein at least half the aromatic rings are alkyl substituted, or

6. polymers of polysubstituted styrene having repeating units of the formula ##SPC6##

where R is lower alkyl, R' is H or alkyl and at least two of the R' groups in (6) are alkyl and n is 0 to 3,

said substituted polystyrenes having a Ring and Ball softening point of 50.degree.-150.degree.C. and a molecular weight of about 200-8000,

the remainder of the composition being a wax or wax-like material, with or without other resins and fillers, said composition having a congealing point in the range of 55.degree.-70.degree.C. and low shrinkage and increased stiffness.

2. The composition of claim 1 containing about 25-75% by weight of the substituted polystyrene.

3. The composition of claim 2 containing about 30-60% by weight of the substituted polystyrene.

4. The composition of claim 1 where the substituted polystyrene is a vinyl toluene/.alpha.-methyl styrene copolymer.

5. The composition of claim 2 where the substituted polystyrene is a vinyl toluene/.alpha.-methyl styrene copolymer.

6. The composition of claim 3 where the substituted polystyrene is a vinyl toluene/.alpha.-methyl styrene copolymer.

7. The composition of claim 1 wherein the composition comprises about 25-75% of the substituted polystyrene, up to about 75% of the wax or wax-like material, and about 0-50% of other resins, fillers, fatty chemicals and dyes.

8. An investment casting wax composition containing 55 parts by weight of a copolymer of .alpha.-methylstyrene and vinyl toluene having a molecular weight of 1000 and a 92.degree.C. softening point, 35 parts by weight of paraffin wax of 70.degree.C. melting point, and 10 parts by weight of montan wax.

9. An investment casting wax composition containing 55 parts by weight of a copolymer of .alpha.-methylstyrene and vinyl toluene having a molecular weight of 1000 and a 92.degree.C. softening point, 35 parts by weight of paraffin of 70.degree.C. melting point, and 10 parts by weight of carnauba wax.

10. An investment casting wax composition containing 35% of a copolymer of .alpha.-methylstyrene and vinyl toluene, 45% of paraffin of 70.degree.C. melting point, 10% carnauba wax, 5% microcrystalline wax of 90.degree.C. melting point and 5% microcrystalline wax of 85.degree.C. melting point.