METHODS FOR PREPARING A FORMED CELLULAR PLASTIC MATERIAL PATTERN EMPLOYED IN METAL CASTING

Abstract: Specific types of formed patterns and core assemblies, wholly or partially formed from certain destructible cellular plastic materials have a decreased tendency to form nonvolatile residue during the casting of metals such as stainless steel. Superior castings are thereby obtained without resort to uneconomic casting methods.

Description: METHODS FOR PREPARING A FORMED CELLULAR PLASTIC MATERIAL PATTERN EMPLOYED IN METAL CASTING This invention relates generally to so-called "lost foam" methods for casting metals. More specifically, it concerns methods for preparing various novel specifically defined heat-destructible shaped- foam patterns for use in replica-casting of metals (particularly low carbon steels) by the lost- foam technique (particularly techniques involving "top gating") . Lost foam casting essentially involves pouring molten metal into a heat-destructible pattern of cellular plastic material (or foam), while the pattern and its entry port(s), or "gate(s)", are essentially surrounded and supported by highly compacted refractory material such as sand. In the past, commercial processes have mainly involved the use of foam patterns in which the plastic material was polystyrene. However, there are problems with use of expandable polystyrene (EPS) in lost foam casting, also called evaporative pattern casting, where the pattern or core assembly is partially or wholly EPS. One problem is that carbonaceous nonvolatile EPS residue floats on molten iron and becomes trapped inside the cavity formed by the decomposing polymeric foam. The large amount of residue results in carbon- containing voids, called carbon defects, weak points and leaks through the casting. This leads to inefficient manufacturing and component failures. A second problem with EPS molded patterns or core assemblies is that of shrinkage. An EPS molded part with a hydrocarbon blowing agent, such as pentane,. loses most of the blowing agent in a period of one month or less at room temperature. Simultaneous with the loss of blowing agent, shrinkage of the molded parts occurs. This dimensional change is undesirable, especially if molded parts are to be stored for an extended period or if the tolerance of the cast part is critical. Recent-ly published Japanese Patent Disclosure Kokai No. 60-18,447 has working examples concerning the use of foam patterns prepared from polystyrene or several copoly ers derived from raw materials including methyl methacrylate and alpha-methyl styrene, in casting iron and aluminum by the "bottom gate" casting technique. It also has broader general allegations. For example, it proposes that the lost mold substrate can be a homopolymer of methyl ethacrylate, and that the molten metal may also be zinc, brass, or steel. Prior art methods of lost foam casting have now m been found to be inadequate and unable to prepare superior metal castings for many types of metal (such as steels having a very low carbon content) and/or many types of casting technique (such as "top gate" techniques involving the use of downwards flow of the 10 molten metal into the heat destructible pattern, rather than merely "bottom gate" techniques involving upwards movement of the molten metal). This invention overcomes many of the 15 deficiencies of the prior art. In its broadest aspects, this invention relates to the use of one or more processing conditions or limitations which have been found to be critical. These conditions (none of which are expressly or inherently disclosed by 20 aforementioned Japanese Kokai) include, but are not limited to the following: (1) the casting of steel having very low carbon content; (2) the use of a "top gate"; (3) the use of a plastic material containing an mm average total aromatic component within the plastic's molecules of less than 3 weight percent based on the total weight of . plastic material; and (4) the use of pre-foamed particles (immediately prior to being molded) which particles have a broad "molding window 0 time range" (as defined hereinaf er). A first broad aspect of the invention is a method for preparing a heat-destructible shaped-foam pattern intended for use in replica-casting of a metal 5 casting by the lost foam technique, by steps including ( 1 ) preparing foamable beads from a mixture of a plastic material and a blowing agent; (2) heating the foa able beads to form pre-foa ed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the product from step (1) includes plastic material containing a majority of repeat units of the formula: fCH 2 -CR*(C00R) wherein R is selected from the group consisting of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalkanes having 3-6 C, and R' is selected from the group consisting of CHo and C2H5 and wherein the pre-foamed beads used in step (4) have a ' molding window time range of at least 5 seconds as determined by a test wherein said beads are expansion- molded in steam at a temperature that is 21°C above the glass transition temperature of the plastic material, and wherein molding window time range is defined as the difference in time between the maximum period under which good molding occurs and the minimum time under which good molding occurs for a molded foam having a density within the range of from 1.35 to 1.6 pounds per cubic foot. A second broad aspect of the invention is a method for preparing a heat-destructible shaped-foam pattern intended for use in replica-casting of a metal casting by the lost foam technique, wherein the metal to be cast is an iron base alloy, a steel, a stainless steel or a stainless steel alloy having a carbon percentage, after casting- of up to 1.8 weight percent; by steps including (1) preparing foamable beads from a mixture of a plastic material and a blowing agent; (2) heating the foamable beads to form pre-foamed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the plastic material has a majority of repeat units of the formula: j . CH 2 -CR'(C00R)i wherein R is selected from the group consisting of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalkanes having 3-6 C, and R' is selected from the groups consisting of CHg and C2H5. A third br ad aspect of the invention is a method for preparing a heat-destructible shaped-foam pattern intended for use in replica-casting of a metal casting by the lost foam technique, by steps including (1) preparing foamable beads from a mixture of a plastic material and a blowing agent; (2) heating the foamable beads to form pre-foamed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the product from step (1) includes plastic material containing an average total aromatic component within the plastic's molecules of less than 3 weight percent based on the total weight of plastic material, and the plastic material has a majority of repeat units of the formula: fCH 2 -CR'(C00R)f wherein R is selected from the group consisting of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalk ' anes having 3-6 C, and R' is selected from the groups consisting of CH-^ and C^Hg. The technical advantages of this invention are illustrated by the discussion below and a comparison of the Examples and Comparative Examples hereinafter. The ability to produce defect-free castings using a top gated pattern in a multi-pattern cluster is 0 a major advantage of this invention. While bottom gating, side gating, and combinations of top, bottom and side gating may also be useful in certain circumstances, the use of top gating has the following four major advantages. 5 1. Better handling of clusters in the dipping, drying and flask loading steps. 2. Less breakage' during sand compaction as a result of sand pressure of the gate area 0 where the foam cross section is typically small. (During compaction sand flow is frequently down the flask walls, across the bottom and up the center. Bottom gated patterns situated near the bottom of the 5 flask are thus subject to considerable pressure during this step which, if too severe, may break the pattern connection to the cluster at the gate. With top gating 0 the cluster may move at the bottom slightly without concern for breakage.) 3. Since the sprue is shorter the metal yield (of useful cast metal from molten metal) is c correspondingly higher. 4. Risers, if needed, are filled with hotter metal and thus can be designed smaller, again resulting in a higher metal yield. m It should be noted that, firstly, with pattern materials prone to generating carbon residues, bottom gating results in the defects occurring on the upper surfaces of the casting. Top gating on the other hand has been found to create a tendency to cause carbon 10 defects to occur "within" the casting as opposed to on its upper surface. This poses a serious problem for parts used under stress where internal carbon defects may function as 3tress raisers in the final part leading to mechanical failure. Elimination of internal 15 carbon defects is thus an essential key to being able to cast parts with top gating^ and an unexpected advantage of this invention. Secondly, casting trials have generally shown 20 that top gating places "more severe demands" on the foam pattern than bottom gating. This is because in the final phases of metal filling the foam adjacent to the gate (which is the last to be displaced by molten mm metal) that portion of the foam has a tendency to collapse before filling with the metal is complete. This type of failure is clearly serious because the resulting castings fail to completely replicate the- pattern. 0 We have now found, very surprisingly, that the tendency for foam collapse to occur during metal casting of top gated patterns is strongly correlated with the moldability of the pre-foamed resin as 5 determined by the size of the "molding window" obtained in standard test procedures described hereinafter. Even with the benefit of hindsight it is still m not clear as to why the molding window time range of the pre-foamed beads is critically important (over and above the requirement that the shape of the molded pattern conform to the shape of the metal item that is to be cast). However, the discussion below is now 10 given as a partial and hindsight explanation of our surprising finding. Firstly, for a resin to be successfully molded it must expand rapidly when heated to a temperature 15 above the glass transition temperature. Since diffusion of blowing agent is β accelerated during heating, the retention of blowing agent during pre- expansion and molding " is a critical factor in determining the minimum density at which the resin can 20 be molded. The measurement of blowing agent retention following heating to a temperature typical of that used in pre-expansion is thus a useful index of the resins expected performance in molding. 25 Two major factors control the rate of blowing agent loss from the poly(methylmethacrylate) (PMMA) resins used in our invention at temperatures above the glass transition temperature. 30 1. The barrier properties of the polymer, and 2. The uniformity of the nucleation of the resin. "Barrier properties" of the resin during mm expansion are highly dependent on the molecular weight distribution of the polymer. According to the present invention the optimum molecular weight distribution appears to be obtained in the polymer when a level of crosslinking corresponding to one crosslink per weight average molecular chain is incorporated. The resulting molecular weight distribution is then very broad, including some network polymer which is insoluble in solvents which will dissolve the uncrosslinked polymer. -Ideally the soluble portion of the αrosslinked resin will have an apparent weight average molecular weight of about 250,000 ± 30,000. Poly-dispersity of the material should be 2,7 or greater. "Uniformity of nucleation" is also important. If the pre-expanded bead has a uniformly fine cell structure consisting of cells with diameters from 30 to 180 microns when the absolut-9- density (as opposed to bulk density) of the beads is about 1.5 pounds per cubic foot, optimum retention of blowing agent will be achieved provided the polymer in the foam has acceptable barrier properties. In some circumstances, if for example the amount of blowing agent added to the monomer mixture is excessive, phase separation of the blowing agent from the polymer may occur in the late stages of polymerization rather than during quenching at the end of the reaction. Since the polymer is still soft at the former stage the blowing agent which phase separates can diffuse readily and collect in .pools much larger than the microscopic nucleation 3ites which are formed during normal quenching. During expansion, each of these large pools of blowing agent becomes a discrete cell. In the "prefoamed" state these large cells make the foam particles vulnerable to damage and resultant loss of blowing agent. In the process of molding, as described elsewhere, pre-expanded beads are placed in the mold cavity of a steam jacketed, vented mold tool. During steaming the beads expand a second time, collapsing the voids between the originally spherical foam beads. The pressure exerted by the foam is contained by the pressure on the tool and leads to inter-particle -fusion. If the steaming time of the mold cycle is too short, fusion is incomplete, the part is heavy from water remaining in the voids, and mechanical properties of the foam will be poor. If the steaming time is excessive the foam pattern will loose some of its blowing agent and the pattern will shrink back from the walls of the mold cavity. If the density is not too low, between these two times there will be a time range sufficient to provide acceptable quality, well-fused, full-size patterns. If one attempts to mold a resin at too low a density, shrink-back will occur before fusion -has been completed. In -this case there will be no combination of time and temperature (steam pressure) which will yield an acceptable pattern, that is, a molding window does not exist. The molding window for a given density for a given pattern represents the combination of times and temperatures (steam pressures) which yield acceptable molded parts. Since the size of the molding window is a function of the barrier properties of the polymer as well as the character of the nucleation, the size of the molding window provides an index to the moldability of the resin. In general an excellent correlation may be obtained between the size of the molding window and the bead expansion-blowing agent retention vs time at 130 degrees C. plot. Resins which (1) expand slowly, (2) fail to reach a high volume ratio, (3) expand rapidly and then suddenly collapse, or (4) exhibit rapid loss of blowing agent also tend to have a small molding window at useful densities. Molding window plots for many resin formulations were determined. Many of these resin formulations .were further evaluated in casting trials. From the molding windows trials and corresponding casting trials, it was concluded that the foamable beads used in step (2) of the invention preferably have (i) a volume increase by a factor of at least 20 after a period of 5 minutes, and more preferably by at least 30; (ii) a maximum volume expansion of at least 60; and (iii) a collapse occurrence no sooner than within 30 minutes, more preferably no sooner than within 60 minutes; all wherein the foamable beads are subjected to hot air in an oven at a temperature of 25°C above the glass transition temperature of the plastic material. The following test method was used to determine the "volume increase after 5 minutes"; "maximum volume expansion"; and "time of collapse occurrence." A sample of beads having a weight of 0.5 gram is placed in a 1 gram aluminum weighing dish. The dish containing the sample is then placed in the preheated oven at the predetermined temperature for the predetermined time. The hot air is mildy-circulated through the oven at a rate well below that at which fluidization of the particles would occur. It should be noted that a separate sample is required for each individual test. Volume expansion of the beads is determined by conventional liquid displacement tests, after the beads have.been cooled back to room temperature. Air temperature is 130°C for typical PMMA resins having a glass transition temperature of about 105°C. Examples Concerning the Effect of Molding Window Time Range Tables 1A and 1B taken together provide one example of the correlation between molding window time range (Table 1A) and the casting performance (Table 1B) of top gated patterns having graduated "ease of casting." The molding window time range was determined for six different PPMA resins using a vented, block mold with part dimensions of 2" deep x 8" high x 8" wide. The mold was mounted on a mold press with a vertical parting line. The tool Cmold) was vented on the two 8" x 8" faces-with a square array of vents on 1 3/16" centers, 49 vents per side. With the exception of Resin # 2 all of these materials have, in other tests, shown acceptable ' performance" in bottom gated casting configurations. The metal poured was ductile • iron. Shape A (in Table 1B) was the least difficult shape to cast, and Shape D was the most difficult. Table 1A Prefoamed Beads Used to Prepare Pattern Resin ft 1 2 3 4 5 S Molding 11 2 15 18 • 18 14 Window* Density, 1=5 1.5 1.38 lo57 1.48 1. pcf** Cell Medium Large & Fine Large & Fine Medium Size Fine Small Small Fine Blowing*** 113/114 113/114 113 113/114 113 113/114 Agent * Molding window time range determined at 20 psig steam, time in seconds. ** Density of pre-expanded" resin used in molding window determination. * ■ *-*"U3 •• denotes the DuPont Freon ® 113® or 1,1,2-trichloro-l, 2.2- trifluoroethane; and "114" denotes the DuPont Freon ® 114 or 1,2- dichloro-1,1,2,2-tetrafluoroethane. Table IB Casting Results**** Shape A Fair Poor Good V V GGooood V Good V Good Shape B Poor Poor Fair Good V Good Shape C - - Good Good Good V Good Shape D Poor Poor Poor Poor V Good **** Casting results: In all cases the ductile iron castings showed no surface defects due to lustrous carbon. The gradation of performance of the resin indicated relates to the tendency for the foam to collapse during the pouring of the patterns in a top gated configuration. Casting Shapes A to D had the following configurations.. A. 11.5" dia. flange with open cylinders 7.5" and 3.5" O.D. attached to opposite sides. 3. Same as A but all diameters increased about 30%. C. 18" dia. flange with hemisphe ical, cap on one face and support posts on the other. D. 8.5" OD x 6.12" ID open cylinder attached to a 14" x 1.44" flange. 10 Surprisingly, a cellular plastic material having a majority of repeat units of the formula shown in all broad aspects of the invention yields less nonvolatile carbonaceous residue than expected. Even 15 more surprisingly, the use of a cellular plastic material of poly(methyl methacrylate) , one embodiment of this formula, in lost foam " casting, results in the nearly total absence of the defect-causing nonvolatile carbonaceous residue. 20 This absence or near absence of carbonaceous residue and the resulting casting defects allows the use of cellular plastic material patterns with higher densities. Increased density affects the patterns' 25 compressive strength, surface hardness, and stiffness. This increased density translates directly into improved casting tolerances and less stringent handling requirements especially in the sand filling and 30 compaction steps. This absence or near absence of residue also allows the casting of low carbon steel, stainless steel and alloys of these steels due to a decrease in carbon -,m pickup from the molded cellular plastic material patterns into a molten metal. An excessive carbon pickup will result in a loss of corrosion resistance in stainless steel .and a loss of physical strength in low carbon high alloy steels. ,- When casting aluminum, defects due to polymeric residues, while not visually observable, are detectable at folds and fronts where molten aluminum coming from different directions meet. The defect, in this case, is a thin layer of polymeric residue which reduces the 10 cast part's integrity by causing weak points and leaks at the folds and fronts. Thus, due to the nearly total absence of non¬ volatile carbonaceous residue, the cellular plastic 15 materials of the -present invention are useful in the preparation of patterns, wholly or partially composed of a destructible portion. These cellular plastic materials may be polymers, copolymers or interpolymers having repeat units, of the aforementioned formula and a 20 formed pattern density of 0.7 to 5.0 pounds per cubic foot. Pyrolysis Screening Trials 25 Various preliminary screening trials were performed. In particular, certain plastic materials, based on pyrolysis temperatures which approximates actual casting conditions, but absence the presence of a blowing agent, have now been tested and shown to have reduced amounts of carbonaceous nonvolatile residue. These plastic materials include styrene/acrylonitrile copolymers, poly(alpha-methylstyrene) , poly(methylmethacrylate) , poly( 1-butene/S0 2 ) and -,,- poly(acetal) , as discussion below. To obtain an indication of the amount of carbonaceous nonvolatile residue present for a given material, a technique was adapted from rapid pyrolysis analysis methodology used to study the decomposition of polymeric materials. The method uses a weighed sample of about 1 milligram of the polymer to be tested. The sample is placed in a quartz capillary. The capillary is installed in a platinum coil contained in a sample chamber. The sample is pyrolyzed by passing a current through the platinum coil. Pyrolysis gases are trapped in a gas chromatograph column for later separation and identification by rapid scan mass spectrometry. Following pyrolysis, the residue remaining in the quartz capillary is Weighed to determine the weight percent residue yields Table 2A indicates pyrolysis residue yields at ' two different pyrolysis conditions as shown in Table " 2B. The second column of pyrolysis conditions with an approximately 700°C temperature rise per second is believed to more closely approximate metal casting conditions. TABLE 2 PYROLYSIS RESIDUE YIELDS % Residue Polymer Poly(Acetal) 0.5 • Poly(methyl methacrylate) 0.8 3.2 Poly(l-butene/S0 2 ) 3.8 Poly(alpha-methylstyrene) 2.2 Lightly crosslin ed expandable polystyrene 6.2 15.1 Ξthylene/acrylic acid copoly er 8.6 Styrene/acrylonitrile copolymer with 1,1,2-trichloro-l,2,2-tri£luoroethane 9.3 11.55 Poly(ethylene terphthalate) 11.0 Polycarbonate -* 26.4 52.8 TABLE 2B PYROLYSIS CONDITIONS Heating Rate l β C/sec 700 β C/sec Maximum Temperature 1400°C 1400°C Hold at Maximum Temperature 6.7 min 18 sec Atmosphere Air Nitrogen Flow During Pyrolysis None None Pretreat ent Temperature 50 β C 50°C Capillary Tube Configuration Open tube Inlet .end closed Decreased amounts of residue are necessary for those cast metals having a low carbon specification. This speci ication is found for some grades of stainless steel. Those polymers having low residue are useful in the casting of such grades- of stainless steel. It is believed that the type of monomer(s) and desired polymer(s) have an affect on the tendency for carbon formation to occur during the pouring of ferrous castings. The formation of carbon during the pyrolysis of polymers is largely a kineticly controlled phenomena. Polymer decomposition via unzipping, as is believed to occur in methyl- and ethyl-methacrylate as well as in alpha-methyl styrene, results in a very rapid lowering of the average molecular weight of the polymer. The low molecular weight fragments which are formed are highly volatile, and if a liquid, have a very low viscosity. Their escape from the pattern region is thus rapid compared.to the rate of escape of the much larger polymer fragments formed by the random cleavage mechanism. Thus PMMA and PMMA/alpha- methylstyrene (AMS) copolymers are expected to exhibit lower carbon formations than polystyrene on pyrolysis at 1400 degrees C. Another factor that enters into consideration is the propensity of the monomer molecules to form carbon. In this regard, molecules containing an aromatic group are generally more prone to carbon formation than those without. Oxygen in the molecule also serves to reduce to carbon yield by tying up carbon in the decomposition products as CO or C0 2 . These trends are seen clearly in the residue yields reported in Table 2A. These considerations lead us to conclude that PMMA containing less than 3% of aromatic-group- containing monomer units will yield a lower amount of carbon residue than the PMMA/AMS copolymers prepared in the working Examples of the aforementioned Japanese Kokai. Preferably, the cellular plastic materials have a majority of repeat units of methyl methacrylate: H f f J ■ c- -C-7- 1 1 H c=o f 0 a ! Most preferably, the cellular plastic material is composed of at least 70 percent by weight of methyl methacrylate repeat units, excluding any blowing agent. Cellular plastic materials to be used for lost foam casting suitably have a glass-transition temperature within the range of 6θ°C to 140°C. Preferably, the glass-transition temperature is about 100°C. The R group must not include aromatic nuclei, such as, for example, phenyl, naphthyl, or toluoyl, because these typically yield carbonaceous residue. The R group also must not include groups prone to ring closure during heating, such as, for example, -Cs≡N and -N=C=0 which also yield carbonaceous material. Further, in one aspect of the invention the plastic material contains an average total aromatic content within the plastic's molecules of less than 3 weight percent based on the total weight of plastic material. Examples Concerning Aromatic Content of Foam A casting similar to that designated as "Shape A" in Table 1B above was poured with ductile iron using a top gated sprue system. The pattern was prepared using a 50:50 mixture of expanded polystyrene and PMMA pre-expanded beads. Compared to a PMMA pattern of similar density, the polystyrene-containing pattern when poured produced a casting with an unacceptably high level of carbon defects. In a comparative experiment a 2" x 8" x 8" b.lock of foam with a density of about 1.5 pcf consisting of a copolymer prepared from a monomer mixture containing 30 parts of styrene and 70 parts of methylmethacrylate was poured with ductile iron. The block was oriented horizontally and gated along the bottom edge. The resulting casting showed a moderate level of carbon defects on the upper horizontal surface compared to virtually no carbδn defects on a PMMA block gated and cast in the same manner. From discussions- with foundrymen and literature references it is known that expandable polystyrene (EPS) when used as a pattern material in steel castings, results in carbon pickup of from 0.15% to greater than 0.5 . With EPS patterns the carbon frequently occurs in segregated locations causing a localized failure to meet composition and performance specifications. In addition to carbon pickup, lustrous carbon defects and carbon occlusions are sometimes • observed in steel castings made with EPS patterns. By analogy with the ductile iron results described for 50:50 and 30:70 polystyrene/PMMA systems, lower aromatic contents are expected to-reduce but not eliminate the problem of carbon pickup in low carbon steel alloys. The examples below relating to the pouring of PMMA patterns with steel confirm that carbon pickup can reach an acceptably low level when the aromatic content of the monomer is essentially zero. EXAMPLES OF STEEL CASTINGS MADE WITH PMMA FOAM PATTERNS Steel is commonly defined as an iron base alloy, malleable under proper conditions, containing up to 2 percent by weight of carbon (see McGraw Hill"s "Dictionary of Scientific Terms," Third Edition, 1984). There are two main types of steel — "carbon steels" and "alloy steels." According to a British Alloy Steels Research Committee definition "Carbon steels are regarded as steel containing not more than 1.5 weight percent manganese and 0.5 weight percent silicon, all other steels being regarded as alloy steels." Alloy steels may be divided into four enduse classes: (1) stainless and heat resisting steels; (2) structural steels (which are subjected to stresses in machine parts); (3) tool and die. steels; and, (4) magnetic alloys.• Step casting patterns were assembled from pieces cut from 2" x 8" x 8" PMMA foam blocks. Densities of the foam patterns were 1.1, 1.5, and 1.9 pcf. A martensitic stainless steel with a base carbon content of 0.05% was poured at a temperature of about 290Q degrees F. (1580 degrees C). Hot melt glue was used top assemble the foam step-blocks. The blocks were packed in a bonded sodium silicate sand. Carbon pickup at 0.01" and 0.02" depths into the upper surfaces of the first and second steps of the casting amounted to 0.01 to 0.06% net at all three densities. At the third step (top of the 6" thick section) carbon levels ranged from 0.12 to 0.19% representing a carbon pickup of from 0.07 to 0.14%. The sectioned castings after etchin .showed no signs of carbon segregation. Another step block was poured with a high strength, low alloy steel, (nominally 1% Ni, 0.75% Cr, and 0.5% Mo) with a base carbon content of 0.16%. A rubber cement was used to bond the foam pieces into the step block configuration. Foam density was 1.5 pcf. Carbon levels in samples milled from "cope" surfaces ranged from 0.01 to 0.22%. On the first and second steps carbon levels were 0.08 to 0.14%. Based on these results it was concluded πafc PMMA can be used as pattern material with low allay steel without detrimental carbon pickup. Top gating of patterns to be poured with, atea - is expected to require highly collapse resistant foam: as in the case of ductile iron poured with top gating:. Acceptable blowing agents must have a suffi¬ cient molecular size to be retained in the unexpaπded- bead as well as adequate volatility to cause the beadsi to expand at a temperature in the range of 75°C to 150°C, preferably between 100°C and 125°C. The solubility parameter of the blowing agent should preferably be about two units less than the solubility parameter of the polymer to assure nucleation of a fine-cell cellular plastic material. A wide variety of volatile fluid blowing agents may be employed to form the cellular plastic material. These include chlorofluorocarbons and volatile aliphatic hydrocarbons, such as, for example a mixture of iso- and normal-pentane. Some considerations exist though and include the potential of fire hazard, and the loss of blowing agent over time, which may cause dimensional stability problems. For these reasons, chlorofluorocarbons are preferred. Some .of these chlorofluorocarbons include, by way of example and not limitation, trichlorofluoromethane, dichlorodifluoro- methane, 1 , 1 ,2-trichloro-1 ,2,2-trifluoroethane and 1,2- dichloro-1 , 1 ,2,2-tetrafluoroethane and mixtures of these f uorochlorocarbons. The preferred blowing agent is a mixture of 1 , 1 ,2-trichloro-1 ,2,2-trifluoroethane and 1 ,2-dichloro- 1 , 1 ,2,2-tetrafluoroethane. This mixture is preferably present in an amount of 40 to 50 weight percent 1,1,2- trichloro- 1 ,2,2-trifluoroethane and 50 to 60 weight percent 1 ,2-dichloro-1 , 1 ,2, -tetra luoroethane by mixture weight. .- Preferably, chlorofluorocarbons or chloro- fluorocarbon mixtures are present in the cellular plastic material in an amount of from 14- to 28 weight percent by total combined weight of the cellular plastic material and chlorofluorocarbon and most preferably 20 to 24 weight percent. The density of the formed destructible portion of the pattern after forming is generally in the range of 0.7 to 5.0 pounds per cubic foot. Preferably, the density is in the range of 1.0 to 2.2 pound3 per cubic foot. The use of a crosslinking agent in the prepara¬ tion of the plastic material is preferable, but not required. These crosslinking agents may include, by way of example and not limitation, divinylbenzene, ethylene glycol dimethacrylate and diethylene glycol dimethacrylate. The crosslinking agent is present in the plastic material from 0.00 to about 0.08 weight percent by total weight. Preferably, when the ero3a iπk.i∑ιg agent is divinylbenzene, the crosslinking regen : • .__-__. resent in the plastic-material at about 0.04 * " by total weight. ' " Er?e£era-_.ly there -are about 0.5 difunctional 0 crosslinking " agent- molec.ules--..per weight average polymer chain. The-, use of a crosslinking- agent improves the molding: characteristics of the- cellular plastic 5 mat rial... y reducing blowing agent diffusion and loss at mσ dirrg temperatures-, thus_ rendering the cellular pi sre, material less-susceptible to premature, collapse.. 0 While the use of a crosslinking agent may reduce cellular plastic material expansion rate, this decrease in expansion rate may be partially or wholly offset by decreasing the base molecular weight of the plastic material. This base molecular weight is the 5 molecular weight which would be normally obtained in the absence of a crosslinking agent. The use of a suspending agent and one or more initiators may also be required in the preparation of 0 the plastic material. The suspending agents may include, by way of example and not limitation, methyl cellulose, polyvinyl alcohol, carboxymethyl methyl cellulose and gelatin. 5 The initiator may be one or more peroxides which are known to act as free radical initiators. The initiators may include, by way of example and not limitation, ammonium, sodium and potassium persulfates, hydrogen peroxide, perborates or percarbonates " of sodium or potassium, benzoyl peroxide, tert-butyl hydroperoxide, tert-butyl peroctoate, cumene peroxide, tetralin peroxide, acetyl peroxide, caproyl peroxide, tert-butyl perbenzoate, tert-butyl diperphthalate and methyl ethyl ketone peroxide. The use of a chain transfer agent in the preparation of the plastic material is also preferable, but not required. These chain transfer "" agents may include, by way of example and not limitation, iso-octyl thiαglycoater and carbon tetrabromide. Preferably the chain transfer agent is carbon tetrabromide. The use of a chain transfer agent in the preparation of the plastic material in combination with the initiator allows the polymer molecular weight to be controlled independently of the rate of heat generation in the polymerization. The chain transfer agent reacts with the growing polymer chain end, terminating the chain growth but also initiating the growth of a new chain. A chain transfer agent is thus valuable in highly exothermic polymerizations, since it allows initiator levels to be changed while still obtaining the desired molecular weight through an opposite change in the amount of chain transfer agent used. For example, in a system with CBrj j as a chain transfer agent and tert-butyl peroctoate (t-BPO) as an initiator, a two-fold decrease in t-BPO requires an approximately 20 percent increase in the CBr_ j chain transfer agent level to maintain about the same molecular weight. On scaling a reaction from a smaller to larger reactor, it has been found that initiator levels may need to be lowered to avoid an excessive temperature differential between the reaction mixture and the vessel cooling system. The following weight percents of materials yield resins with molecular weights in the range where expansion rate, time to- foam ^ collapse, and ultimate expansion are all excellent. Weight Percent Based on MMA Monomer Number of Experiment CBr, t-BPO 1 . 41 .70 2 .47 .23 3 .50 .11 In addition to the benefits described above, 15 resins made with a CBrj j chain transfer agent have a lower temperature at which thermal degradation begins than resins made with lOTG chain transfer agent or chain transfer agents of lesser activity. 20 The general process steps for obtaining a cast metal part utilizing a pattern with a molded destructible portion are the following: (A) Prepare the Pla3tic Material: The mm formulations are prepared in a one gallon reactor having agitation. Aqueous and organic phase mixtures are prepared. The aqueous phase having water, carboxymethyl methyl cellulose (CMMC), and potassium dichromate (K 2 Cr 2 0γ) is prepared in a one gallon wide 0 mouth bottle and is transferred to the reactor by vacuum. The organic phase mixture, having monomer, initiator, chain transfer agent and blowing agent is prepared in a shot-add tank. The shot-add tank is pressurized to about 80 psig (pounds per square inch 5 gauge) with nitrogen and the organic phase is pressure transferred to the reactor. Following the completed loading of the organic and aqueous phases into the reactor, the organic phase is dispersed and sized by agitation for about 30 minutes at about ambient temperature and at s pressure that is slightly above atmospheric. 10 The reactor is heated to 80°C (Centigrade) and is held for about 6 hours. The temperature is then increased to about 95°C for about 1.5 hours. The temperature is then increased again to about 110°C for about 4 hours and is followed by cooling to ambient 15 temperature. Heating, and cooling rates are about 0.5°C/minute. After cooling the plastic " material, now in the form of . beads, the reactor is emptied and the beads are 20 washed with water. The .beads are then vacuum filtered and dried at ambient conditions. Table 3 contains formulation and process information for several runs. • 25 30 35 TABLE 3 Run 1_ 2 3 4_ Water , g ( grams ) 1246 1246 1246 1246 Methyl Methacrylate, g 976 976 976 974 l,l,2-trichloro-l,2,2- -trifluoroethane, .g (F-113) 176 174 183 176 l,2-dichloro-l,l,2,2-tetra- fluoroethane, g (F-114) 217 203 207 209 Carboxy ethyl methylcellu- lose, g 3.3 3.3 3.3 6.6 κ 2 Cr 2°7' ~ 1.5 1.5 1.5 1.5 t-Butyl-Peroctoate, g 4.56 4.56 4.56 4.56 t-Butyl-Perbenzoate, g 1.70 17.1 17.1 1.9 Name of chain transfer agent IOTCS* 1 * I0TG (1) CBr. (2) CBr. {2) Weight of chain transfer agent, g 3.0 5.06 3.1 4.0 Divinylbenzene, g 0.0 0.0 0.0 .419 Revolutions per Minute for agitator 180 220 220 220 -Mw x 1Q~ 3(3) 371 301 199 264.8 -Mn/-Mw ( ) 2.5 2.1 2.4 3.6 Volatiles, percent 23.7 22.85 23.9 22.85 (1) Iso-octyl thioglycoate (2) Carbon tetrabromide (3) Weight - average molecular weight (4) Number average molecular weight/weight-average molecular weight (B) Pre-expand the Beads: Use steam or dry air to pre-expand the beads to "pre-foamed" beads having a loose-packed bulk density about equal to 10 percent greater than the planned density of the parts to be molded. Zinc stearate in an amount of about 0.04 to about 0.40 weight percent by total weight may be added as an antistatic and antifusion aid. Preferably, the amount is about 0.10 weight percent zinc stearate. One example of a typical ύnexpanded bead resin and its properties are as follows: Resin Poly(methyl methacrylate) Volatiles (as 1,1,2- 22.8 weight percent trichloro-1 ,2,2-tri- fluoroethane (F-113) and 1 ,2-dichloro- .1 ,1 ,2,2-tetra- fluoroethane (F-114)) Divinylbenzene 0.043 weight percent Molecular weight about 265,000 (weight average) Expansion volume, ratio 24 " .6 of unexpanded beads to expanded beads after 5 minutes at 130°C Expanded density after 1.5 pounds per cubic foot 5 minutes at 130°C Unexpanded bead size -30 + 60 mesh range (250 to 590 microns) A typical operating cycle for pre-expansion based on the use of a horizontally adjusted drum expander with a steam jacket heating system is as follows: STEP FUNCTION TIME 1 Inject beads into preheated 18 0.1 minute gallon expander. A typical charge size is 0.5 pounds. 2 Preheat beads 1.4 minutes 3 Inject 75 cubic centimeters water 0.1 minute while pulling a vacuum of 10-12 pounds per square inch absolute (psia) . 4 Release to atmospheric pressure 0.5 minute and hold. 5 Return to vacuum at about 7 psia 0.3 minute and hold. 6 Discharge pre-expanded beads. 0.75 minute By varying the time for " expansion or the steam pressure, the density of the expanded beads can be modified. With the operating conditions indicated, the following densities are obtained: PREHEAT STEAM PRESSURE BEAD DENSITY 3 minutes 24 pounds per square 1.3 pounds per cubic inch gauge (psig) foot (pcf) 1.4 minutes 24 psig liδ pcf (C) Age the Pre-foamed Beads: If direct contact steam heat is used during the prefoaming or pre-expansion step (B), the beads should be allowed tb dry thoroughly before molding. Drying usually is complete within 24 hours when beads are stored in a netting storage hopper. (D) Mold the Pre-foamed Beads: Molding is generally done on an automatic machine with each step precisely timed. Steps include, but are not limited to: pneumatically filling the mold with beads, passing steam through the mold to heat the beads, cooling the mold with water, and demolding the part. " A typical molding cycle is as follows: STEP FUNCTION TIME Fill mold with beads 5 seconds pneumaticall . Steam both sides with 12 to 24 seconds 13 psi steam. Steam moving -side wi£h 12 3 seconds psi steam. Steam stationary side with 3 seconds 13 psi steam. Water cool to -about 120 6 seconds degrees Fahrenheit (°F) Vacuum dwell to remove 4 seconds water. 7 Cool dwell. 90 seconds 8 Water cool to about 90°F. 6 seconds 9 Vacuum dwell. 6 seconds 10 Cool dwell. 90 seconds 11 Eject part. The above cycle produces acceptable , smooth- finished , distortion-free parts with a molded density of 1 .35 to 1 .4 pcf after drying when using pre-expanded beads having a bulk density of 1 .5 pcf . (E) Age the Molded " Part: Even with the optimum molding conditions, some moisture is retained in the part. Aging 24-72 hours at ambient conditions removes this water. Alternatively nearly all of the water may be removed in 4-10 hours by drying the molded parts in a circulating air. oven heated to 50- 60°C. During the aging step the molded part will achieve final dimensions which will vary only slightly over an extended period of time. (F) Assemble Pattern Parts: Many complex parts such as manifolds and cylinder blocks are molded in several sections to accommodate constraints on the foam mold design. These are now assembled typically by conventionally gluing with hot melt glue. ' Due to the fact that the molded part of.-cellular plastic material employed in the present invention stabilizes at final dimensions quickly and varies in its final dimensions only slightly over an extended period of time, no special precautions are required to assure that all molded parts are at the same stage of aging as long as they are completely dry, as may be required with molded parts of a cellular plastic material not employed 1 in the present invention. (G) Refractory Coat The Pattern(s): The purposes of the refractory coating are: (1) to provide a finer_ grained surface than would generally be obtained if the coarser sand directly contacted, the foam; (2) to prevent molten metal from flowing out • into the sand; and (3) to allow molten polymer, monomer and pyrolysis gases and liquids to escape rapidly during casting. The refractory coating is similar to core washes used widely in the foundry business. Typically the refractory coating consists of fine mesh refractory particles suspended in a water or alcohol slurry with suitable surfactants to control viscosity and assure good wetting. Core washes may be applied by dipping, spraying or brushing on the slurry. Following application the refractory coating is cured by air drying at -ambient temperatures or elevated temperatures up to about 60°C. The porosity and surface properties of the refractory in the coating are very important parameters since they affect the pressure in the mold during pouring and the retention of metal inside the mold. Both factors directly influence the final quality of the molded part * (H) Attach Molded Parts to Gates, Runners, and Sprues: Hot melt glue may be used. Since gates, runners, and sprues must also have a refractory coating, it may be desirable to make the complete assembly before applying the refractory coating as described in step F. (I) Pack Foam Pattern(s) Attached to the Needed Sprue(s) Assembly(s) in Sand in a Flask for Pouring: In this step, the refractory coated parts and sprue assembly having a deep pour cup with about 8 to 12 inches free board above the sprue is supported while dry, loose foundry sand containing no binders is poured into the flask. Optionally, the flask can be vibrated on a 1 to 3 axis vibration platform during filling and for a period after filling is complete to tightly pack the sand around the pattern. (J) Pour the Casting: Pouring is done with standard procedures used for other casting methods, such as the "green sand" method. The rate of pouring must be rapid enough to keep the sprue filled to the surface of the sand. The sizes of the gates and runners are optimized to give the best fill rate at the static head obtained with a full sprue. ( ) Allow the Casting to Solidify and Cool: Care should be taken not to jar the flask before solidification is completed. (L) Shake Out the Flask: In this step the casting and sprue system is removed from the flask either by pulling out the casting or by dumping out the sand and removing the casting. (M) Cleanup of the Cast Parts: This may include air or water jet cleaning, shot blasting and machining of flange faces. A preliminary inspection to reject off-spec parts should be done. (N) Complete Machining: Drill and tap .holes, cut 0-ring grooves, etc. (0) Quality Check: Test parts for leaks, defects, dimensional specs, etc., prior to assembly and use. Additional Examples Additional Examples of the invention concerning factors such as type of chain transfer agent, and the ability to cast articles having a very low and uniform carbon content throughout the casting are given below. Example 1 Four formulations of a poly(methyl methacrylate) cellular plastic material are prepared having the following properties: Number 1 2 3 4 Molded 1.43 1.35 1.35 1.40 density pcf Molecular 371,000 265,000 301,000 199 000 Divinyl Benzene 0.0 0.043 0.0 0.0 ' Agent Volatiles 23.7 22.85 22.85 23.9 (as F-113 plus F-114, weight percent) Chain IOTG CBr 4 I0TG CBru transfer agent Molded cellular plastic material blocks 8 inches (in.) by 8 in. by 2 in. of the above formulations are used to make the desired patterns, sprues and r .nners. The parts are assembled into a complete casting pattern system and refractory coated. The patterns are then packed in a flask with sand. The patterns are packed, for this example, with their thickness in a vertical direction. The patterns are: Thickness Length Width 2 in . 8 in . 8 in . 1 in . 8 in . 8 in . 1 /2 in . 8 in . 8 in. 1 /4 in . 8 in . 8 in . 8 in. 4 in . 2 in . All formulations are-τ_sast in each thickness, with the exception of formulation number 1 which is not cast in the 2 in. and 8 in., thickness. The 8 in. thickness pattern is gated at the bottom of the pattern and at approximately half the thickness of the pattern. Ductile iron, having about 3/5 percent carbon, at approximately 2650°F is used for all patterns. The reduction in carbon defect is readily apparent in all the castings, which have no visual surface carbon defects. The lack of carbon defect in the 2 in. thick and 8 in. thick patterns, in particular, indicates an important advantage in using the method of the present invention. This advantage is the capability of providing carbon defect-free castings with a wide variety of gating systems. Due to the lack of carbon defects and residue, there is no need to optimize the gating system to avoid carbon defects, thus saving time and money. Example 2 Three formulations of a poly(methyl methacrylate) cellular plastic material are prepared having the following properties: Block Number Molded density pcf 1.33 1.36 1.66 Chain transfer agent CB ^ CBr^ I0TG Molded cellular blocks of the above formulations are used to make the desired patterns, sprues and runners. The parts are assembled into a complete casting pattern system and refractory coated. The patterns are then packed in a flask with sand. Stainless steel, having about 0.035 percent carbon is used for all patterns. The final carbon percentage at each of five points in each of the cast patterns is then determined in duplicate. The results are presented in Table 4. TABLE 4 Block Number 1 2 3 Final Per<cent Carbon Aft ' er Casting Determination First Second First Second First Second Points 1 0.048 0.053 0.082 0.067 0.105 0.056 2 0.040 0.049 0.043 0.049 0.083 0.052 3 0.042 0.039 0.041 0.039 0.085 0.064 4 0.056 0.045 0.050 0.047 0.055 0.052 5 0.048 0.051 0.062 0.057 0.075 0.085 The final carbon percentages are within the specification percentage of carbon for many stainless steels and stainless steel alloys, although for the specific stainless steel of this example, the carbon percentages exceeded the specification carbon percentage of 0.040, due at least in part to the fact that this particular stainless steel had about 0.035 percent carbon prior to casting. Although only a few embodiments of the present invention have been shown and described, it should be apparent that various changes and modifications can be made without departing from the scope of the present invention as claimed.

Claims: CLAIMS: 1. A method for preparing a heat-destructible shaped-foam pattern intended for use in replica-casting of a metal casting by the lost foam technique, by steps including ( 1 ) preparing foamable beads from a mixture of a plastic material and a blowing agent; (2) heating the foamable beads to form pre-foamed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the product from step (1) includes plastic material containing a majority of repeat units of the formula: £CH 2 -CR'(C00R)} wherein R is selected from the group consisting of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalkanes having 3-6 C, and R' is selected from the group consisting of CH3 and C2H5; and wherein the pre-foamed beads used in step (4) have a maiding window time range of at least 5 seconds as determined by a test wherein said beads are expansion- molded in steam at a temperature that is 21°C above the glass transition temperature of the plastic material, and wherein molding window time range is defined as the difference in time between the maximum period under which good molding occurs and the minimum time under which good molding occurs for a molded foam having a density within the range of from 1.35 to 1.6 pounds per cubic foot. 2. A method for preparing a pattern, as recited in Claim 1, wherein the cellular plastic material has a majority of repeat units of the formula: H CH H C=0 0 and wherein the plastic . material has an apparent weight average moleular weight in the range of 220,000 to 280,000 and a poly-dispersity of at least 2.7. 3. A method for preparing a pattern, as recited in Claim 1, wherein the destructible portion is a cellular plastic material of poly(methyl meth¬ acrylate). 4. A method for preparing a pattern, as recited in Claim 3» wherein/ the cellular plastic material of poly(methyl methacrylate) is prepared with a chain transfer agent of carbon tetrabromide. 5. A method for preparing a pattern, as recited in Claim 4, wherein the cellular p.lastic material of poly(methyl methacrylate) has at least one entrapped blowing agent. 6. A method for preparing a pattern, as recited in Claim 4, wherein the cellular plastic material of poly(methyl methacrylate) has at least one entrapped chlorofluorocarbon. 7. A method, as recited in Claim 6, wherein the chlorofluorocarbon entrapped in the cellular plastic material is present in an amount of from 14 percent to 28 percent by total combined weight of the cellular plastic material and the chlorofluorocarbon. 8. A method, as recited in Claim 7, wherein the chlorofluorocarbon entrapped in the cellular plastic material is present ill an amount of from 20 weight percent to 24 weight percent by total combined weight of the cellular plastic material and the chlorofluorocarbon. 9. A method, as recited in Claim 8, wherein the chlorofluorocarbon is 1 , 1 ,2-trichloro-1 ,2,2- trifluoroethane. 10. A method, as recited in Claim 8, wherein the chlorofluorocarbon is a mixture of 1 , 1 ,2-trichloro- 1 ,2,2-trifluoroethane and 1 ,2-dichloro-1,1,2,2-tetra- fluoroethane. 11. A method, as recited in Claim 10, wherein the destructible portion of the pattern has a density of 1.0 to 2.2 pounds per cubic foot. 12. - A method, as recited in Claim 11, wherein the metal to be cast is a steel alloy, a stainless steel or a stainless steel alloy having a carbon percentage, after casting of 0.1 weight percent to 0.5 weight percent. 13- A method, as recited in Claim 11, wherein the carbon specification, of the metal as cast, is less than 0.1 weight percent. 14. A method, as recited in Claim 1, wherein the metal to be cast is aluminum. 15. The method of Claim 1 wherein the replica- casting uses at least one top gate for feeding molten metal towards the foam pattern and wherein the molding window time range is at least 12 seconds. 16. . A method, for preparing a heat- destructible shaped-foam pattern intended for use in replica-casting of a metal casting by the lost foam technique, wherein the metal to be cast is an iron base alloy, a steel, a stainless steel or a stainless steel alloy having a carbon percentage, after casting of up to 1.8 weight percent; by steps including (1) preparing foamable beads from a mixture of a plastic material_ and a ' blowing agent; (2) heating.the foamable beads to form pre-foamed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the product from step (1) includes plastic material- containing a majority of repeat units of the formula: fCH 2 -CR » (C00R)} wherein R is selected from the group consisting, of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalkanes having 3-6 C, and R' is a. selected from the group consisting of CH3 and C2H5. 17. The method of Claim 16 wherein the steel casting has a carbon content of from 0.05 to 0.5 weight percent. 18, A method for preparing a heat-destructible sfiaped-foam pattern intended for use in replica-casting of a metal casting by the lost foam technique, by steps including (1) preparing foamable beads from a mixture of a plastic material and a blowing agent; (2) heating the foamable beads to form pre-foamed beads; (3) optionally cooling and aging the pre-foamed beads; and (4) heating the pre-foamed beads in a mold under conditions sufficient to form a molded shaped article having a closed cell structure; wherein the product from step (1) includes plastic material containing an average total aromatic component within the plastic's molecules of less than 3 weight percent based on the total weight of plastic material, and the plastic material has a majority of repeat units of the formula: £CH 2 -CR » (C00R)i wherein R is selected from the group consisting of alkanes having 1-4 C carbon atoms (C), hydroxy alkanes having 1-4 C and cycloalkanes having 3-6 C, and R' is selected from ;-the groups consisting of CH and C2H5. 19. A method according to Claims 1, 16 or 18 wherein the foamable beads used in Step (2) have (i) a volume increase by a factor of at least 20 after a period of 5 minutes; ii) a maximum volume expansion of at least 60; and a collapse occurrence no sooner than within 30 minutes; all wherein the foamable beads are subjected to hot air in an oven at a temperature of 25°C above the glass transition temperature of the plastic material.

Inventors: JOHNSON DAVID RICHARD, MOLL NORMAN GLENN, JOHNSON, DAVID, RICHARD, MOLL, NORMAN, GLENN
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: B22C9
IPC codes: B22C7, B22C9