Patent Publication Number: US-3880790-A

Title: Investment casting wax composition

Description:
United States Patent [1 1 McLaren et al.  
 [ 1 Apr. 29, 1975 INVESTMENT CASTING WAX COMPOSITION [75] Inventors: Robin A. McLaren, Ballwin; Charles A. Alcott, St. Louis, both of MO.; William E. Nasser, Bartlesville,  
 Okla.  
 [73] Assignee: Petrolite Corporation, Wilmington,  
 Del.  
 [22] Filed: May 14, 1973 [21] Appl. No.: 360,044  
 [52] US. Cl. 260/285 A; 106/388 [51] Int. Cl. C08c 11/70 {58] Field of Search 106/38.8; 260/285 A [56] References Cited UNITED STATES PATENTS 3.306,882 2/1967 Pullen ct a1 260/285 A Primary ExaminerMelvyn I. Marquis Attorney, Agent, or FirmSidney B. Ring; Hyman F. Glass [57] 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.  
 10 Claims, No Drawings INVESTMENT CASTING WAX COMPOSITION 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 setsthe wax replica invested with the sandmagnesia 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 l T C H n polymers of -alky1styrenes 1 CH CH G l-1 n&#39; polymers of vinyl alkylbenzenes or copolymers of the above. Also included are copolymers of styrene with a-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 l CCH where R is H or alkyl and at least two of the R groups are alkyl and n is -3.  
  Suitable substituted polystyrenes are further characterized by a Ring and Ball softening point of 50l50 C., preferably 75-l25 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 075% wax and about 050% 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 (5080 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 oz-methylstyrene and vinyl toluene characterized by a molecular weight of 1,000 and a 92 C. softening point (55 parts by weight), paraffin wax of C. melting point (35 parts) and montan wax (10 parts) were heated together at 130 C. till melted and stirred together for about 30 minutes to produce a homogenous blend. The blend was injected at 80 C. and 80 psi into a steel mold to form test bars 5 X /2 X /8 inch.  
 EXAMPLE II A blend of the above copolymer (55 parts), 70 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 C. softening point and 400 molecular weight (55 parts), 70 C. M.P. paraffin (35 parts) and carnauba wax (10 parts) was prepared as in Example I. At 130 C. a homogenous blend was and 0.52% (4 days); stiffness 46 (10 min.) 23 (4 hrs.) produced. The bars obtained after injection showed and 12 (4 days). gross separation of the wax and resin components. Waxes A &amp; B may be replaced by the product of Ex- A comparison of Examples II &amp; III illustrates the ample IV, without the expense of manufacturing new great improvement in blendability of substituted poly- 5 dies, to provide patterns of improved rigidity. styrene (II) relative to unsubstituted polystyrene (III). Since all subsequent examples were prepared and in- WaX shrinkage was meastll&#39;ed y Comparing the jected as in Example I only the compositions and test length of the 5 Inch test bar Wlth the length of the meld results will be given to avoid unnecessary repetition.  
 v y Measurements were made shortly after injection Examples V &amp; VI illustrate the preparation of waxes of (approximately 10 min.), after 4 hrs. and after 3-4 10 very l h i kag and good rigidity. days. Wax stiffness was measured by placing the test bar on 4 inch supports, applying a total weight of I00 EXAMPLE v g. for 10 sec. to the center of the bar and measuring the 1 deflection produced. The weight was applied through a T-bar of cylindr cal cross section attachedto a pene- 15 1 wishingy S yrene mp0 ymer 2 trometer as described in A.S.T.M. D217. Stiffness was Montan wax 10% measured after the same aging times as for shrinkage. shfmkage 000%(10 &#34;&#34;9 010% (4 days) Stiffness 35 (I0 min.), 21 (4 hrs.) l4  
 Table I compares the shrinkage of the compositions (4 days) of Examples I &amp; II with two commercial pattern waxes.  
 Wax A is a widely used commercial product containing substantial amounts of stearic acid. Wax B is a pre- EXAMPLE VI mium commercial product based on chlorinated biphenyl resin. Shrinkage is expressed as a percentage of the Vinyl tolucne/a-methylstyrene copolymer 55% length of the mold cavity. 70 C. M.P. paraffin 35% 25 Candelilla wax 10% Table I Shrinkage 0.00% 10 min.), 0.14% 4 his 0.26% (4 days) shrinkage (670) Stiffness 40 (10 min.), 16 (frat); 10 min. 4 hrs. 34 days y roduct of E p i 8-38 8 2% 8-3: Example VII illustrates a wax of high shrinkage and $632? of Example 1 3 a rigidity comparable to Waxes A and B. Wax B 0.32 0.50 0.55  
 EXAMPLE VII The stiffness of the same four waxes is compared in 35 Table II. The results are expressed in decimillimeters Vinyl tolucne/a-mcthylstyrene copolymer (dmm). A lower number indicates greater stiffness. 70 My, pamfi-m 35% r Carnauba wax 10% Fable 11 85 C. M.P. microcrystalline wax 5% Paracin-l synthetic ester wax 5% I Stiffness 4Q Shrinkage 0.36% (l0 min.), 0.56/r (4 hrszhoagjzg 10 4 days Stiffness 67 (I0 min.), 35 (4 hrs.) 24 (4 days) Product of Example I 37 l7 l0 Product of Example ll 3 1) lg 6 Example VIII illustrates how the present invention may w A wax B 62 30 24 45 be utilized to obtain a flexible wax with low shrinkage.  
 An examination of the data of Tables I &amp; II shows that EXAMPLE the products of Examples I &amp; II are significantly lower in shrinkage and stiffer than the commercial waxes. 0 vinyl tomene/wmethykmem copolymer After 10 min. they are already about as rigid as the if? P sag ontan wax .comrnercial waxes are after hours. After 4 hours they Shrinkage 000% (10 mm) 028% (4 days) are stiffer than the commercial waxes are after several Stiffness 120 (10 min.) 56 (4 days) days. Their improved rigidity improves the handling and stability properties of the wax patterns and their 55 Examples IX &amp; X illustrate pattern waxes containing reduced shrinkage makes it easier for the caster to hold components other h Substituted polystyrene and the desired dimensional toleraheeswaxes. The products of both of these examples have Example IV illustrates the Preparatloh of a Wax improved properties as casting waxes and are comparamatching Waxes A &amp; B in shrinkage characteristics and ble to the products f Examples 1 &amp; IL having improved rigidity. 6O  
 &#39; EXAMPLE Iv EXAMPLE IX A blend containing a-methylstyrene/vinyl toluene copolymer (35%) 70 C j paraffin (45%) ca- Vinyl Toluene/a-methylstyrene copolymer 55% nauba (10%), 90 C. M.P. microcrystalline wax (5%) M p pammn 25% and C. M.P. microcrystalline wax (5%) was prea acid 20% pared and injected as in Example I. The test data were Sh&#39;mkage (112% (10 024% (4 &#39;i1.i$3  
 as follows: Shrinkage .30% 10 min.), 0.46% (4 hrs.) Stiffness 29 (10 min.), 12 4 hrs.), 7 4 days) EXAMPLE X Vinyl toluene/oz-mcthylstyrene copolymer 45% 70C M.P. paraffin 30% lVlontan wax l% 85 C. M.P. microcrystalline wax I00 C. Softening point aliphatic petroleum resin l0% 0.25% min.), 0.40% (4 days) 36 (10 min.), 17 (4 hrs), l0 (4 days) Shrinkage Stiffness The congealing points of all of the above examples were in the range of 5570 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/a-methylstyrene copolymer parts), 70 C. M.P. paraffin parts) and carnauba wax (5 parts) was prepared as in Example I. The temperature of the blend was adjusted to 80 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/a-methylstyrene copolymer (30 parts), 57 C. M.P. paraffin (20 parts), montan wax 10 parts) and beeswax (5 parts) was prepared and filled with parts of a high density polyethylene wax of 700 molecular weight as in Example XI.  
  Examples XIII &amp; 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 C. softening point (35%), 70 C. M.P. paraffin wax (50%) and 80 C. M.P. microcrystalline wax (15%).  
 EXAMPLE XIV The blend, prepared as in Example I, contained poly-a-methylstyrene resin of 700 molecular weight and 85C. softening point (50%) 64 C. M.P. paraffin wax and carnauba wax (10%).  
  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 to C. The other wax is selected from the group containing microcrystalline waxes of about 60 to C. melting point, synthetic hydrocarbon waxes of about 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 &#39;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 2575% 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.  
  &#39;Other chemicals, including minor amounts of other resins and fillers can also be employed when desired.  
 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 l. polymers or copolymers of alpha-alkylstyrenes having repeating units of the formula 2. polymers or copolymers of B-alkylstyrenes having repeating units of the formula 3. polymers or copolymers of vinyl alkylbenzenes having repeating units of the formula 4. copolymers of styrene with a-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 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/oz-methyl styrene copolymer.  
  5. The composition of claim 2 where the substituted polystyrene is a vinyl toluene/oz-methyl styrene copolymer.  
 6. The composition of claim 3 where the substituted polystyrene is a vinyl toluenela-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 050% of other resins, fillers, fatty chemicals and dyes.  
  8. An investment casting wax composition containing 55 parts by weight of a copolymer of a-methylstyrene and vinyl toluene having a molecular weight of 1000 and a 92C. softening point, 35 parts by weight of paraffin wax of 70C. 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 a-mcthylstyrene and vinyl toluene having a molecular weight of 1000 and a 92C. softening point, 35 parts by weight of paraffin of 70C. melting point, and 10 parts by weight of carnauba wax.  
  10. An investment casting wax composition containing 35% of a copolymer of a-methylstyrene and vinyl toluene, of paraffin of 70C. melting point, 10% carnauba wax, 5% microcrystalline wax of 90C. melting point and 5% microcrystalline wax of 85C. melting point.