Description:
BACKGROUND OF THE INVENTION
The present invention relates to materials which retain their shape at room temperature, which can be melted to a flowable condition when raised to an elevated temperature, and which will not shrink appreciably during the transition from the melted stage to the solid stage. Such materials are used in the lost wax process for making metal casting molds, and in related applications, as for example, in investment casting processes. Waxes have been customarily used for such heat flowable materials, and these waxes are customarily called pattern waxes by the art.
In investment casting processes, a pattern wax is molded into the shape of the desired finished metal article. The wax pattern is successively dipped or immersed into a slurry of refractory mold forming materials to coat the wax pattern. The slurry coating is dried, and the dipping process repeated to build up a desired coating or solid mold thickness. The coated pattern is then autoclaved to remove the wax and is then fired to produce a refractory mold having the reverse configuration of the desired metal object. Prior to the time that the wax pattern is dipped into the slurry, a wax sprue is attached to the wax pattern to provide an opening in the refractory mold for receiving the molten metal. The molten metal is poured through the sprue opening and the metal solidified to form the desired article. After the metal is solidified, the mold is broken apart, and any metal projection caused by reason of the sprue opening is removed to provide the finished metal article. The process is capable of achieving great detail in the finished article and is customarily used for making dental inlays, jewelry and intricate machine parts.
No one material has been found which has all of the properties desired of a pattern material. All of the known waxes have a shrinkage during solidification which is greater than desired, and usually well above 5 percent. In general the animal and vegetable waxes, i.e., ester type waxes, are quite soft in their solid state but have desirable plastic flow characteristics. The mineral waxes, i.e., paraffin waxes and microcrystalline waxes have less desirable flow characteristics, but provide a desirably hard solid surface. Pattern waxes, therefore, have sometimes been a blend of mineral and ester type waxes to try to obtain a balance in deficiencies as well as a balance in the desirable properties. To further improve the properties of the waxes, a minor amount of natural or synthetic thermoplastic resins have sometimes been added to increase the hardness and toughness of the material. Also in some instances, an organic filler has been added in an attempt to cut down the shrinkage during cooling. The organic fillers have a melting temperature well above that of the wax and resin materials used, so that they do not change from a liquid to a solid in the molds during the solidification of the waxes. The fillers that have been used heretofore have either been high melting plastics, as for example a methyl polystyrene, or high melting organic polybasic acids.
All of the fillers which have been used heretofore including those mentioned above, contribute undesirable characteristics to the pattern material. Prior art fillers increase the apparent viscosity of the softened material that is injected into the die and thereby decrease the materials ability to flow into small openings in the die cavity. The filled material must be kept agitated when molten in order to prevent the filler from settling out of the molten wax. Further difficulties are encountered in removing filled waxes from the investment molds. Filled waxes flow out of the investment molds with much greater difficulty than do unfilled waxes and have a tendancy towards bridging the outlet opening causing some of the fillers to remain in the investment mold cavity. When polystyrene is used as a filler, the investment molds after removal of the wax are heated to 1,000°F or more to burn out any residual filler. During this operation, some of the polystyrene remains in small mold crevices, expands and produces a cracking of the mold surface. In addition, the polystyrene produces a lot of smoke during burning and is undesirable for this reason. On the other hand, the organic polybasic acids, while water soluble, are corrosive to investment molds made from silica sol, and are therefore noncompatible therewith. Silica sol is a desirable investment mold forming material so that the use of the polybasic acids generally requires the use of more expensive and less desirable mold forming materials. Still other deficiencies exist with each of the above mentioned fillers, as is well understood by those skilled in the art.
An object of the present invention is the provision of a new and improved filled pattern material which has low shrinkage, does not necessitate burning to remove residual filler, and does not include harmful ingredients which will attack silica sol.
A further object of the invention is the provision of a new and improved filled pattern material which has a very low latent heat of solidification, so that solidification of the pattern material in the dies will occur much more quickly.
Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description of the preferred embodiments:
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, a new and improved heat liquefiable pattern material is obtained by incorporating a polyhydric alcohol of tetra substituted methane having alcohol groups of up to and including six carbon atoms with OH groups on at least half of the carbon atoms, into a heat liquefiable material. The heat liquefiable material preferably includes a polymer of cyclic hydrocarbons and substituted cyclic hydrocarbons having a melting point between approximately 70°C and approximately 160°C. Preferred resins are polymers of cyclic alkenes and copolymers thereof, as for example, terpenes and naphthenes which solidify into a hard state such as occurs with rosins, and the petroleum base higher melting naphthenes. Typical higher thermoplastic resins which are used for pattern materials will include terpene phenolics, methyl ester of rosin, hydrogenated rosins, polymerized rosins, rosin derivatives, cold tar derivatives, petroleum derivatives, styrene derivatives, chlorinated polyphenyls, alkyds, polyesters, polyamides, coumarone-indene resin, and diphenyl bis steramide. In some instances, a softer plasticizing resin may be incorporated, as for example, lower molecular weight filler derivatives, lower polyethylene, or polyethylene copolymers such as polyethylene-ethylacetate, ethylene vinyl acetates, polyisobutylenes, cellulose derivatives, and polypropylene derivatives.
While most of the thermoplastic resins given above have slight crystallinity still others do not. The operability of the resin is dependent upon its compatibility or ability to form a solid solution with the wax being used and amorphous thermoplastic resins can also be used. Examples of amorphous resins are rosin and some rosin derivatives, polyisobutylene, polypropylene, polyethylene and ethyl vinyl acetate.
The natural hard resins which are sometimes used include the following: Accroides, Batavia, Damar, Singapore Damar, Batu, Black East India, Congo, Elemi, Kauri, Pale East India, Manila Lopa, Mastic, Run Congo, Run Kauri, Run Manila, Copal, Rosin, terpines and shellac.
In order to soften these thermoplastic resins capable of solidifying into a hard molded shape, waxes of any of the conventional types, as for example, animal and vegetable ester type waxes, petroleum hydrocarbon waxes, and the synthetic waxes are mixed with the resins. The natural waxes will include: Barberry, Bees Wax, Candelilla, Carnauba, Ceresine Esbarto, Japan, Montan, Ouricury, Ozakerite, Spermaceti, Palm Wax, Caster Wax, and fatty acids. The petroleum waxes will include paraffins, petrolatums and microcrystallines. The synthetic waxes will include oxidized microcrystalline, Fisher-Tropsch, derivatives of the naturals, chlorinated paraffins, polyamides, and polyethylene glycols.
It has been found that when the crystalline polyhydric alcohols of a tetra substituted methane as described above are added to mixtures of the thermoplastic hard resins, plasticized with wax, the heat transfer of the mixture is increased. In addition the apparent viscosity of the mixture can be lower than the prior art materials which have been filled with polystyrene or organic acids, such as isophthalic acid, fumaric acid, and adipic acid. All the reasons why the pattern materials of the present invention which include the crystalline polyhydric alcohol of a tetra substituted methane have the improved properties is not known, but it is believed that the polyhydric tetra substituted methane materials only form very weak secondary bonds with the heat liquefiable waxes and resins, and therefore do not increase the apparent viscosity to the extent that the prior art solid fillers have. The fillers of the present invention have a different structure from the cyclic resins and the long chain waxes, and therefore perform differently than do the high melting acids and styrene fillers that have been used heretofore.
The preferred polyhydric alcohol is pentaerythritol as shown by the following example:
EXAMPLE 1
A pattern wax is made by thoroughly homogenizing the following ingredients in the percents by weight given below:
Ingredients Percent ______________________________________ Paraffin wax m.p. 127°F 3% Paraffin wax m.p. 140°F 12% Microcrystalline wax 178°F 11% Polyethylene-vinylacetate 1% copolymer s.p. 204°F Petroleum naphthenes s.p. 210°F 33% (a blend of unsaturated cyclo paraffin hydrocarbons including 5 and 6 membered rings) Pentaerythritol 100-200 mesh 40% ______________________________________
The pattern wax is prepared by melting the waxes together at 250°F and thereafter stirring the molten thermoplastic resin also at 250°F into the molten wax. After the resin and waxes are thoroughly homogenized, the pentaerythritol is blended therein. The material once solidified has a linear shrinkage of only 5 percent.
The filled pattern wax is used by heating, to 250°F with agitation to keep the pentaerythritol dispersed, and injecting the flowable mixture into a metal die having the reverse shape of the desired finished object. The die is cooled and the filled pattern wax was solidified. Thereafter a slurry of powdered ethyl silicate or colloidal silica mixed with powder refractory is prepared and the wax pattern is repeatedly dipped therein and dried between dippings. When an appropriate thickness of slurry has accumulated, the dipped pattern is dried, then autoclaved in steam at 275°-320°F to remove the filled wax. The investment mold so formed is fired at 2,000°F for 2 hours and thereafter molten alloy steel at 2,600°F is poured therein. The investment mold and metal therein is cooled to room temperature, and the mold is broken apart to free the cast steel part thus made. The cast steel part has excellent detail entirely free of surface imperfections caused by remaining wax or filler.
EXAMPLE 2
A filled heat liquefiable pattern material is made from the percentages by weight of ingredients given below:
Carnauba wax m.p. 180°F 10% Microcrystalline wax m.p. 178°F 25% Polymerized terpenes s.p. 250°F 20% Filler (2 hydroxymethyl, 45% 2 methyl, 1-3 propanediol)
This material can be used as a pattern material in the investment casting process described in Example 1. The filler, however, melts at a temperature of 199°C and is therefore acceptable in many instances. Pentaerythritol has a melting point of 260°C and therefore is the most preferred material.
EXAMPLE 3
Another filled heat liquefiable pattern material is made from the percentages by weight of the materials given below:
Spermaceti wax m.p. 120°F 20% Paraffin wax m.p. 140°F 20% Thermoplastic resin 55% (Polymerized Rosin) Filler (2 hydroxymethyl) 5% - 2 nitro 1,3 propanediol
This material can be used as a filled pattern material in the process described in Example 1. The filler of this example has a melting point of 165°C, is very water soluble and therefore is easily removed from investment molds by the use of steam when necessary.
EXAMPLE 4
Another solid filled pattern material is made of the percentages by weight of materials given below:
Polyethylene glycol s.p. 140°F 20% Microcrystalline wax s.p. 180°F 30% Thermoplastic resin 30% (Chlorinated Polyphenyl) Filler (2,2-dinitro-1,3-propanediol) 20%
The filler in the above example is soluble in hot water and can be removed when necessary by water treatment of the investment mold. The filled pattern material although not preferred because of the lower melting point of the filler, nevertheless is acceptable material in many instances.
EXAMPLE 5
Another filled heat liquefiable material is made from the same material as given in Example 1, excepting that 2 methyl, 2-nitro-1,3-propanediol is substituted for the pentaerythritol of Example 1. This filler has a melting point of 147°C, is soluble in hot water and the filled material is acceptable for some uses.
EXAMPLE 6
Another filled pattern material is made using the composition given in Example 1 excepting that 2 amino-2 (hydroxymethyl)-1,2-propanediol is used in place of the pentaerythritol. This material can be used as a filled pattern material for many uses.
The filled pattern materials of the invention will generally include the following ingredients in approximate percentages by weight given below:
Wax 5-60% Thermoplastic resin which 10-60% forms a solid solution with the wax Plasticizer 0-10% Tougheners (metal salts) 0-10% Polyhydric alcohol of tetra 5-45% substituted methane having no more than one substituted nonpolar alkyl carbon atom and having alcohol groups of up to and including six carbon atoms with an OH group on at least half of the carbon atoms of the alcohol groups.
The thermoplastic resins used in the pattern materials are thermoplastics having a narrow melting range, are miscible with the waxes with which they are used, and have the ability to solidify into a definite shape. The waxes that are used serve the function of plasticizing the otherwise hard and sometimes brittle thermoplastic resin.
The following example gives the ingredients in percent by weight for an amorphous filled pattern wax embodying the present invention.
EXAMPLE 7
Microcrystalline wax 5-50% Polymerized rosin 15-40% Ethylene vinyl acetate 0-15% Filler (pentaerythritol 5-45% 100 to 200 mesh)
EXAMPLE 8
Coumarone-indene resins are inexpensive and preferred filled pattern waxes can be made therefrom using the following materials given as broad ranges and as a preferred specific formulation in percent by weight:
Ingredients % Range Preferred % ______________________________________ Wax 5-60 (Hydrogenated Tallow Glyceride) 33 (Carnauba Wax) 17.5 Coumarone-Indene Resin 10-50 14 Other resins 0-10 (Hydrogenated methyl ester 3.5 of rosin) Film Toughener 0-5 (Metal Stearates) 2 Filler 5-45 (Pentaerythritol) 30 ______________________________________
The following is a partial list of crystalline type materials which are completely compatible with Coumarone-Indene Resin and can be used therewith. Fatty acids, fatty esters, carnauba wax, candelilla wax, chlorinated polyphenyls, castor waxes, terpene resins, styrene resins, alkyd resins, phenolic resins, hydrocarbon resins, and olefin resins. Amorphous materials that are completely miscible therewith include polyisobutylenes, rosins, and liquids. The liquids will include the common type plasticizers such as dioctyl phthalate.
The following materials are not directly compatible with Coumarone-Indene resins, but can be mixed with one or more of the above materials in which they are compatible, and the mixture then added to the Coumarone-Indene resins: paraffin waxes, microcrystalline waxes, oxidized microcrystallines, polyethylenes (amorphous), Fisher-Tropsch waxes, oxidized Fisher-Tropsch waxes, ethylene vinyl acetates (amorphous), polypropylenes (amorphous).
The following example gives both a broad range and a preferred percentage by weight of a filled pattern wax using a petroleum resin.
EXAMPLE 9
Paraffin wax 125 - 180°F 5-25 15 Microcrystalline wax 140 5-25 12 to 190°F Thermoplastic cycloalkene resin 15-50 (Mixed pet. olefine resin) 33 Filler 5-45 40
The following example gives both a broad range and a preferred percentage by weight of a filled pattern wax using a rosin derivative.
EXAMPLE 10
Paraffin wax 120-180°F 5-50 26 Microcrystalline wax 1-50 2 140-190°F Castor wax 2-40 7 Polymerized rosin 15-40 35 Filler 5-45 (Pentaerythritol) 30
The materials given above and that are indicated as not being directly compatible with Coumarone-Indene resins are directly compatible with both the petroleum resins and the rosin resins. Compatibility is determined by heating the materials to a temperature of 30°F above their melting points, mixing the materials thoroughly and allowing them to cool to room temperature. If no precipitation nor layers are formed, the materials are considered compatible or mutually soluble.
As indicated above, the fillers of the invention are highly polar polyhydric alcohols of tetra substituted methane that is devoid of more than one nonpolar substituted alkyl group or groups totaling more than one carbon atom, and the alcohol radicals of which have an OH group for at least half of their carbon atoms. Such alcohols will include the hexahydric alcohols such as mannitol and sorbitol, the tetra hydric alcohols such as aerythritol, and lower alcohols such as methanol, glycol, glycerine, etc. Such compounds will include those having the following formula: ##SPC1##
wherein: R 1 is a lower alcohol having a carbon chain of up to six carbon atoms with an OH group on at least half of the carbon atoms; and R 2 is an R 1 group, a methyl group, a nitrate group, an amine group, or a halogen group. All of these substituted groups except a methyl group are polar groups which preserve crystallinity and provide high melting temperatures, whereas nonpolar hydrocarbon groups of more than 1 methyl group, as for example, an ethyl group, higher alkyl group or the substitution of 2 methyl groups drastically lowers the melting point and water solubility.
While the invention has been described in considerable detail, I do not wish to be limited to the particular embodiments shown and described, and it is my intention to cover hereby all novel adaptations, modifications, and arrangements thereof which come within the practice of those skilled in the art and are covered by the following claims.