Metal founding – Process – Shaping a forming surface
Reexamination Certificate
1998-10-05
2001-02-20
Pyon, Harold (Department: 1722)
Metal founding
Process
Shaping a forming surface
C164S045000, C164S235000, C164S516000
Reexamination Certificate
active
06189598
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lost foam casting and, more particularly, to method and apparatus for lost foam casting of metals and alloys, especially aluminum and its alloys, with substantially reduced fold defects.
BACKGROUND OF THE INVENTION
Lost foam casting of metals and alloys involves pouring molten metal or alloy onto a vaporizable polystyrene pattern residing in a bed of foundry sand so that the molten metal vaporizes the pattern and displaces it in the foundry sand to form a casting. Lost foam casting is described in such patents as U.S. Pat. Nos. 4,085,790; 4,616,689; and 4,874,029. In the past, the polystyrene pattern has been coated with an thermally insulative, gas permeable refractory layer or coating prior to being embedded in the foundry sand. The gas permeable refractory coating allows pattern decomposition vapors to vent into the foundry sand and also helps prevent collapse of the sand as the pattern is vaporized. U.S. Pat. Nos. 4,482,000 and 4,448,235 describe polymer-modified refractory coatings for use on polystyrene lost foam patterns to reduce entrapment of pattern decomposition vapors in the molten metal as it replaces the pattern.
In the lost foam casting of aluminum alloys using polystyrene patterns having core-forming passages that are filled with foundry sand when the pattern is embedded in the foundry sand bed, the gaseous decomposition products of the foam pattern have been found to cause gas holes and so-called fold defects in the cast component proximate the core-forming passages, especially those passages located remote from the initial melt/pattern contact region. The gas holes are usually rounded and detectable upon x-ray examination of the cast component and affect casting quality in dependence on their size and quantity. Fold defects generally are not detectable on x-ray examination but are found when castings are test fractured. Fold defects generally are considered nearly two dimensional, constituting very thin void regions or seams of unbonded metal forming a plane of weakness that results in low strength and low ductility of the casting. In the past, attempts to reduce fold defects in lost foam aluminum alloy castings have involved casting the molten aluminum alloy at very high temperatures of up to 1450 degrees F, for example. However, although fold defects may be reduced by use of high melt casting temperatures, they nevertheless still have been found to be present in a problematic percentage (e.g. 10-50%) of lost foam castings that are produced in a high volume lost foam production environment such as, for example, cast automotive cylinder blocks and heads. Moreover, use of such high melt casting temperatures produces coarse grained lost foam castings that reduces their mechanical properties.
Production engineers have been constantly attempting to eliminate or substantially reduce fold defects in lost foam castings but, to-date, there has been no generally reliable solution to the problem of fold defects. As a result, lost foam cast components of less than optimum quality and higher than desired lost foam production costs still are encountered.
An object of the present invention is to provide method and apparatus for lost foam casting of aluminum and its alloys as well as other metals and alloys using a vaporizable pattern in a manner to substantially reduce or eliminate fold defects in the cast component.
SUMMARY OF THE INVENTION
The present invention provides a method of lost foam casting wherein thin, elongated void regions or seams of unbonded metal (i.e. fold defects) in the casting are reduced or eliminated. The present invention involves selectively contacting one or more surfaces of a destructible pattern proximate to which one or more fold defects are prone to form in the casting with a material that is effective to reduce or eliminate fold defects in the casting proximate the surface(s).
One illustrative embodiment of the present invention involves selectively contacting the pattern surface with a material, such as for example only, silicone, that is anti-sticking relative to the usual gas permeable refractory layer that is subsequently applied on the pattern so that the refractory layer does not tightly adhere or stick to the pattern surface as the pattern is destroyed during casting. The material allows the pattern to shrink away from the refractory layer at the selectively coated area and decompose without accumulation of pattern decomposition material (e.g. coalesced polymer decomposition liquids) at the refractory layer proximate the pattern surface that can generate fold defects in the casting. In a particular embodiment of the present invention, the material can comprise a layer of silicone or other material that reduces adherence or sticking of the refractory layer to the pattern surface proximate to which fold defects form in the casting.
In another illustrative embodiment of the present invention, the pattern can be coated with the usual gas permeable refractory layer with, however, the pattern surface proximate to which fold defects are prone to be formed in the casting left uncoated for direct contact with the refractory mass (e.g. foundry sand) when the pattern is positioned therein. The refractory mass directly contacting the uncoated pattern surface avoids harmful accumulation of pattern decomposition material proximate the pattern surface that can generate fold defects in the casting.
In practicing embodiments of the present invention, the entire destructible pattern preferably is cleaned with a cleaning agent prior to being contacted with the anti-sticking material or the refractory mass in the manner described above.
The present invention substantially reduces or eliminates fold defects in lost foam aluminum alloy and other metal/alloy castings and, if desired, permits lower melt casting temperatures to be used with concomitant improvements in casting microstructure (e.g. finer dendritic arm spacing) and mechanical properties.
Advantages and objects of the present invention will be better understood from the following detailed description of the invention taken with the following drawings.
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Chandley George D.
Zhao Qi
General Motors Corporation
Lin I.-H.
Pyon Harold
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