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Studies 'clear air' on EPC.

The large audiences attending to the various presentations of research into Expendable Pattern Casting (EPC) attest to the continuing interest in this relatively new casting process.

In an AFS project, researchers at the Southern Research Institute, Birmingham, Alabama, compared the casting characteristics and accuracy of EPC to aluminum and iron castings made in nobake and green sand molds.

The Institute's project compares casting dimensional stability of the three processes by precision measuring 300 iron and 160 aluminum test castings made by commercial foundries. It also detailed the importance of pattern design to finished casting success.

Foam patterns allow design and process flexibility not possible with conventional casting methods. Eliminating the need for bonded sand molds, bonded cores and parting lines contribute to improved casting accuracy while reducing cleaning room labor and energy costs.

Dimensional results varied widely between EPC iron castings and sand cast test parts. Nobake castings were shown to be more accurate and green sand less accurate than EPC castings, but EPC offers preparatory savings over the other mold systems tested. Results showed EPC accuracy can be improved by better control of foam shrinkage and more precise tooling adjustments.

Comparing EPC to conventional molding processes, P. Guichelaar, University of Missouri-Rolla, explained what occurs during EPC mold filling. Discussing a recent study on the effects of metal velocity, mass and energy balances on the decomposition of foam and their effects on casting quality, he concluded that during aluminum mold filling, foam patterns "melt" but don't immediately decompose into gaseous products. Instead, the liquid polystyrene is pushed to the interface between the liquid metal and the refractory coating, infiltrating the coating and eventually decomposing at the coating/sand interface.

Guichelaar also addressed concerns with pyrolysis-related defects in ductile iron castings made in EPC molds. The so-called artifacts that resemble a convoluted sheet are generally composed of a carbonaceous material intermingled with complex magnesium silicates. The artifacts were found to derive from the chemical action precipitated by heat from molten metal and the reaction of magnesium with silicate phases in the melt.

In aluminum casting, most of the decomposition product is initially liquid. Therefore, Guichelaar said, an understanding of the effects that coatings have on how the liquid polystyrene will wet and infiltrate the coating material is essential.
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Title Annotation:96th AFS Casting Congress Milwaukee; expendable pattern casting
Publication:Modern Casting
Date:Jun 1, 1992
Words:377
Previous Article:Porosity control is key to quality.
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