Expanded or expansible plastic material for the fabrication of s

Details Expanded or expansible plastic material for the fabrication of s Expanded or expansible plastic material for the fabrication of s

1997-12-05

1999-12-28

Batten, Jr., J. Reed

Metal founding

Process

Shaping a forming surface

164 45, 164235, 521 79, 521 94, B22C 702, C08J 900

Patent

active

060068183

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the field of manufacturing casting pieces by lost pattern molding.
The first process involved in this technique is manufacture of a tray made of an expanded plastic, which will be shaped to yield a pattern that will be shaped like the metallic piece. After preparing the molds and the pouring basin, also made of expanded plastics and joined to the pattern with glue, the caster applies a refractory coat with uniform thickness on the pattern, and after the coat has dried, the pattern is placed in a casting box. The actual casting is done by filling the box with sand, connected to a system of self-hardening bonding agent commonly used in foundries, until the pouring basin touches the upper part of the mold. After the sand has settled, the caster can proceed to casting the metal directly upon the expanded plastic.
This process is generically known as "lost casting"; it is more specifically intended for manufacturing prototypes, for single pieces or pieces in small series, but of large size.
The second process, also involved in this technique, is fabricating a series of patterns by expanding an expansible plastic. A cluster of patterns is obtained by gluing patterns onto the molds and the pouring basin. The cluster is then submerged in a refractory coat, or such a coat is sprinkled on it, and it is dried at low temperature; it is then placed in metallic containers, and sand, without bonding agent is incorporated up to the upper level of the pouring basin by vibration. The caster then directly proceeds to casting the metal upon the expanded plastic.
In English, this process is known as "lost foam"; it is more specifically meant for manufacturing repetitive pieces of small or medium size.
In the following, therefore, the term "lost pattern" will refer to patterns made of plastic and used in these two processes for manufacturing casting pieces by molding.
These processes are well known to casters and are thus based on elimination by gasification of a pattern, comprised of expanded plastic, during casting the metal, the latter taking on the shape of the mold while the pattern is gasified and disappears. It is often found that the pieces manufactured according to these processes have defects such as lack of metal adjacent to areas of carbon accumulation, in addition to changes in the chemical composition of metals in ferrous alloys.
The expanded or expansible plastics used for manufacturing lost patterns are copolymers or polymers such as: economical, but has the highest available carbon content; other polystyrenes with a more or less high density can also be used. g/liter, where the price is much higher than the price of polystyrene.
Although the previously described processes are used very widely by casters, there has as yet been no explanation for the difficulties encountered here. It is probable that these difficulties are due to gasification of the plastic, which is essentially made of carbon and hydrogen constituting the lost pattern [lost foam]; it is decomposed due to the high temperature of the molten metal. This decomposition is preceded by a liquid front of plastic that is propelled by metallostatic pressure; in a reducing atmosphere, cracking causes the formation of carbon deposits known as "glossy carbon" to casters.
For ferrous alloys with a low carbon content, such as ordinary steel, carbon steel or manganese steel, the unoxidizable, special or alloyed steels have high potential solubility in terms of carbon; the defect resulting from the aforementioned processes is represented by a recovery of carbon that modifies the alloy.
For ferrous alloys with a high carbon content, e.g., such as gray iron, white iron with spheroidal graphite and alloyed, carbon solubility is inadequate, resulting in carbon structures that form metal marks on the sides and ceiling of the mold; these changes change the intended shape of the piece and render it useless. Moreover, the phenomena the may modify the metal's structure can result from carbon that is solubilized in the liquid metal, in particular

REFERENCES:
patent: 3884708 (1975-05-01), Burkert
patent: 4633929 (1987-01-01), Santangelo et al.
patent: 5041465 (1991-08-01), Sonnenerg
patent: 5403866 (1995-04-01), Kato et al.
patent: 5787958 (1998-08-01), Shivkumar et al.
Abstracts of Japanese Patent Publication 59-218239 Published Dec. 8, 1984.

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