US 3498365 A
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United States Patent O Int. or. size 7/00 US. Cl. 164-349 6 'Claims This invention relates to cavityless casting processes and more particularly to improvements in the gasifiable patterns used in these processes.
In the cavityless casting process as disclosed in the Shroyer Patent No. 2,830,343, patterns of expanded plastic material such as polystyrene or a similar material are embedded in a forming material and are gasified without any appreciable residue by the molten casting charge. In practice, it has been found that the quality of the castings produced according to this practice has not come up to expectations, particularly the upper surface of such castings. Cope defects have occurred which are frequently caused by the reaction of the molten casting charge with the pattern material.
One cause of this cope defect has been attributed to the closeness of the decomposition temperature of the pattern material to the temperature of the molten casting charge. Efforts to overcome this have been directed toward the incorporation of substances into the foam material which act as depolymerization catalysts to increase the rate of pyrolysis as the molten casting charge is poured into the mold. Organic films have also been applied to the outside surface of the pattern to increase the burning rate of the pattern as the molten casting charge is poured into the mold. It has been found that to introduce sub stances into the foam material to decompose the material at a faster rate also produces adverse effects if a flame results from the faster decomposition rate. The flame produces an effect on the surface of the mold cavity which adversely affects the finish of the casting.
One of the principal objects of the present invention is to provide an improved pattern for the cavityless casting process that overcomes the above disadvantages.
Another object of the present invention is to provide a pattern for the cavityless casting process which improves the surface finish on the final casting.
Still another object of the present invention is to provide a gasifiable pattern which has improved decomposi tion or combustion properties with no flame characteristics.
' These objects are accomplished in a cavityless casting process wherein a cellular plastic pattern is to be embedded in a body of molding material. These patterns are formed to the shape of the article to be cast and have the characteristic of being gasifiable upon subjection to the temperature of the molten casting charge. In order to prevent the production of any flame during the combustion or decomposition of the pattern, it is treated by either adding flame inhibitors to the material or by coating the pattern with a flame inhibitor. When inhibitors are used, materials may also be added to increase the decomposition rate of the pattern material.
Materials-which can be added to the polystyrene during foaming which act to prevent the production of any flame are generally halogen containing agents like chloroparaflin, tri(2,3-dibrompropyl) phosphate, pentabromdiphenylether as well as antimony oxide. Since the cavityless casting process depends on the combustion or decomposition of the pattern to produce the casting, the production of castings having a good surface quality using ice a pattern containing a flame inhibitor was contrary to the accepted practice but proved successful. This result cannot be positively explained but must be attributed to the influence the added substance has on the gasifying process and the formation of residue.
In the first tests using a flame-preventative material, it was found that the foam material gasifies at a slower rate and it was necessary to lower the pouring speed. Materials which decompose into radicals at the casting temperature have also been added to the cellular plastic pattern material, producing an improved result. Examples of these materials are di-benzoyl, di-tert.butyl, di-cumyl-peroxide, dilauroyl-peroxide and tertbutylhydroperoxide or azo-diisobutyrontril. These radical forming substances accelerate the gasifying of the foam material and if the foam material also contains flame preventatives the surface qualities of the casting are further improved. As is well known, flame preventatives that contain halogen and organic peroxide cooperate synergistically.
In the Shroyer process, the production of a flame during the rapid decomposition of the pattern was not feasible because of the lack of oxygen in the casting pattern. Instead, the foam material gasified mostly into a form of monomer.
In order to accelerate or improve the rate of combustion or decomposition of the cellular plastic pattern, it is possible according to the present invention to add substances to the plastic material of the pattern which absorb the heat radiation of the molten casting charge. Substances suitable for this purpose are soot, graphite and the like if they have no unfavorable influence on the pouring melt. Otherwise it would be preferable to use organic dyeing materials which absorb the longer wave length of the spectral range or infra-red range; for instance, methyl violet or deep blue commercial plastisol dyeing material. These absorbing substances change the long wave radiation which goes out from the molten casting charge into heat in the plastic material of the pattern. This, together with the flame-repellent material and in a given case radical-forming additions, leads to a quick and complete gasifying of the model.
Through the appropriate selection of the heat absorption materials, it is possible to have the plastic pattern material immediately change from a cellular plastic material that is heated to its melting point relatively quickly into a gas form of sublimate. The thickness of the layer of cellular plastic material which is undergoing the transition will depend on the depth of penetration of the radiation. At the present time the difficulties experienced at the gasifying temperature where residue accumulates on the surface of the casting is attributed to the accumulation of plastic material because it must initially melt before transformation to a gas. Therefore, it changes first into a solid coking productinstead of into the gas, although in very small amounts, and takes a considerably longer time to change to the gaseous state. The molten metal will set before the complete gasification of the plastic material, leaving a void on the surface. This is only a suggestion as to the decomposing of the cellular plastic material but seems to explain the presence of residue on the surface of the casting charge.
As an example of the above, patterns made from a cellular plastic material were cast which consisted of foamed polystyrene with a specific weight of about 18 kilograms per cubic meter. The casting charge was cast iron having a casting temperature of 1350 C. The wall thickness was 50 mm. on the average and the weight of the casting pieces about 650 kilograms. The mold forming material was normal casting sand at the usual dampness. The casting was poured in as usual from the bottom up.
In the first test, castings cast with patterns of foamed polystyrene material without any additional materials resulted in castings with poor surface quality. There were pores, holes and burn places, especially in the upper part of the rough casting. Patterns of foamed polystyrene material with about 5% by weight tri-(2,3-dibrompropyl) phosphate evenly distributed throughout the pattern resulted in castings having an acceptable surface; that is, having only a few pores.
In the second test, patterns were made exactly as above except that all patterns were coated with a facing of 95% by weight of silica or silicon dioxide and 5% by weight of graphite powder. The patterns without the triphosphate material produced castings with poor surface finishes but did not have the burn places as indicated above. Patterns with the material had perfect surface finishes.
Further tests showed that up to 30% by weight of flame-preventative material can be added to the pattern material. The quota can be lessened according to the halogen content of the additive to about without changing the effect essentially. By further adding a radical forming substance from 2%5% by weight, the quota of the flame preventative can be reduced to about 6% by weight without having holes in the casting pieces. The holes appear very possibly through insuificient gasifying of the model. w
From the above it should be apparent that various changes and modifications can be made in the above disclosure without departing from the scope of the appended claims.
What is claimed is:
1. In a casting arrangement, in combination,
a molding material having embedded therein a body consisting essentially of a cellular plastic pattern having substantially the configuration of an article to be cast and adapted to be gasified at the elevated temperature of the molten casting charge which is to be cast to form said article, said cellular plastic pattern having distributed therethrough from 5% to 30% by weight of a flame-preventative material.
2. In a casting arrangement, in combination, a molding material having embedded therein a body consisting of a cellular plastic pattern having substantially the configuration of an article to be cast and adapted to be gasified at the elevated temperature of the molten casting charge which is to be cast to form said article, said cellular plastic pattern having distributed therethrough from 5% to 30% by weight of a flame-preventative material, and said pattern having a coating comprising about 5% by weight of a heat-absorbing material.
3. In a casting arrangement, in combination, a molding material having embedded therein a body consisting essentially of a cellular plastic pattern having substantially the configuration of an article to be cast and adapted to be gasified at the elevated temperature of the molten casting charge which is to be cast from said article, said cellular plastic pattern including a flame-preventative halogen containing material distributed throughout the pattern in an amount equal to 6% to 10% by weight, and a radical I forming substance in an amount equal to 2% to 5% by weight. Y
4. The combination according to claim 1 wherein said pattern includes .a heat-absorbing material.
5. The combination according to claim 4 wherein said heat-absorbing material comprises a material to absorb the infra-red range of the spectrum selected from the group consisting of soot, graphite and organic dye material.
6. The combination according to claim 5 wherein said pattern includes a radical forming substance.
References Cited UNITED STATES PATENTS I. SPENCER OVERHOLSER, Primary Examiner R. D. BALDWIN, Assistant Examiner US. Cl. X.R.