Lost foam casting using dimensionally self-stabilized pattern

Details Lost foam casting using dimensionally self-stabilized pattern Lost foam casting using dimensionally self-stabilized pattern

1999-03-26

2001-07-24

Dunn, Tom (Department: 1722)

Metal founding

Process

Shaping a forming surface

C164S045000, C164S246000, C264SDIG009

Reexamination Certificate

active

06263950

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to lost foam casting and, more particularly, to lost foam casting using a dimensionally self-stabilized polystirene foam pattern.
BACKGROUND OF THE INVENTION
The lost foam casting process is a well known method of producing metal castings of complex shape wherein an expanded polymeric foam pattern is embedded in a mold typically comprising unbonded foundry sand, and molten metal is poured into the mold to evaporate and displace the pattern in the mold. The dimensions of the casting closely reflect the original dimensions of the foam pattern that the metal replaces. Thus, it is important to use patterns as dimensionally accurate and stable as possible.
The polymeric foam pattern is obtained by molding pre-expanded polystirene or other polymer beads in a pattern mold to impart the desired configuration to the pattern. For example, a commonly used material for making polymeric foam patterns comprises expandable polystirene (EPS) raw beads that contain a blowing agent that typically includes mostly normal pentane with other alkanes also present (e.g. some raw beads are supplied with up to about 30% by weight alkanes other than normal pentane) and that have a raw bead size distribution with over 90% of the beads having a bead diameter in the range of about 0.2 to 0.5 millimeters. These are referred to as T-beads and are needed to provide a satisfactory pattern surface and to allow formation of thin-walled patterns, such as for example only, patterns with wall thicknesses of approximately 3 to 5 millimeters for casting vehicle engine blocks. The EPS raw beads are pre-expanded at a temperature above the softening temperature of polystirene and above the boiling point of the blowing agent. The pre-expanded EPS beads then are molded into the desired configuration in a pattern mold that is steam heated to further expand the beads and then water cooled to stop the expansion process after the pattern is formed to shape. The pattern then is removed from the mold.
Upon removal from the mold into the ambient atmosphere, such polystirene foam patterns are known to initially grow in size as air diffuses into the pattern and then to shrink in size. In the past, conventional lost foam casting practice involved storing the molded polystirene foam patterns for an extended amount of time (e.g. 30 days) at room temperature until dimensional equilibrium was approached, and then to proceed with use of the patterns in casting molten metal. Another practice involves preexpanding EPS beads, molding the beads to form a desired pattern, and then subjecting the pattern to oven aging to rapidly bring the pattern to stable dimensions.
A method to more rapidly stabilize pattern dimensions is described in U.S. Pat. No. 5,385,698 where pre-expanded EPS beads are expanded from dense raw beads and heated for a time prior to molding to form a desired pattern.
Another attempt to rapidly dimensionally stabilize the patterns described in U.S. Pat. No. 4,816,199 involves pre-expanding EPS beads, molding the expanded beads to form a desired pattern, and then subjecting the molded pattern to subatmospheric pressure in the range of 2-20 inches Hg for at least 5 hours to rapidly bring the pattern to stable dimensions.
An object of the present invention is to provide a method of lost foam casting using an improved dimensionally self-stabilized polystirene foam pattern that permits direct use of the pattern in the lost foam casting of molten metal without the need for the post-molding, dimension-stabilizing pattern treatments described above.
SUMMARY OF THE INVENTION
The present invention provides a method of lost foam casting that uses a foam pattern molded from polystirene beads expanded from raw polystirene beads that have a raw bead diameter selected to provide a satisfactory pattern surface and to allow formation of thin-walled patterns, and that include a relatively slow-diffusing blowing agent in an amount of at least about 40% by weight of the blowing agent present in the raw beads to render the pattern inherently more dimensionally stable over time after pattern molding. The weight percentage of the blowing agent is with reference to the dense raw polystirene beads prior to preexpansion to produce the expanded beads. The dimensionally self-stabilized pattern can be used directly in the lost foam casting of molten metal without an intermediate post-molding dimension-stabilizing pattern treatment. For example, the pattern can be embedded in a mold comprising refractory particulates, such as foundry sand, and molten metal can be gravity or countergravity cast in a manner to replace the pattern in the mold.
An illustrative embodiment of the present invention involves molding of the pattern from polystirene beads preexpanded from raw polystirene beads that have a raw bead size distribution with bead diameters in the range of about 0.1 to about 0.6 millimeters to produce patterns with thin walls and satisfactory pattern surface finish and that include isopentane as a relatively slow diffusing blowing agent alone, or together with normal pentane (n-pentane) as a relatively fast-diffusing blowing agent (diffusivity properties being with respect to the polystirene matrix of the molded pattern) to render the pattern inherently more dimensionally stable over time. When isopentane and normal pentane are used together, the isopentane blowing agent is present in an amount of about 40% by weight or more of the total of the blowing agent, preferably about 50% to 70% by weight of total blowing agent, present in the raw beads. The dimensionally self-stabilized patterns exhibit reduced shrinkage over time after molding and an extended shelf life before pattern shrinkage occurs beyond a preselected pattern shrinkage tolerance. The patterns can be used in the lost foam casting process during this extended shelf life without the need for any intermediate stabilizing treatment, such as long term ambient aging, accelerated oven aging, or accelerated vacuum treatment, used heretofore to rapidly stabilize post-molding pattern dimensions.
The above and other objects and advantages of the present invention will become more readily apparent from the following detailed description taken in conjunction with the following drawings.


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