excellent service and quality.
Investment casting wax composition
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.
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 ¦Á-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¡ã-150¡ã C., preferably 75¡ã-125¡ã 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¡ã-80¡ã 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.
