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EPC research reinforces benefits.

The American Foundrymen's Society research project on expendable pattern casting (EPC) that began in 1989 is a good example of the benefits of long-term R&D involving: financing, technology development and technology application and transfer.

The EPC process uses a foam pattern produced by expanding gas-filled plastic beads in a precisely machined, heated aluminum tool that can be complex and contain internal passages. It eliminates conventional green sand tool wear and cope/drag mismatching.

A 100-lb iron casting made in a nobake mold typically requires 200-500 lb of sand and 2-5 lb of organic binder. Organic binders generate toxic emissions as molten metal pyrolyzes binder compounds. in contrast, the same 100 lb EPC casting requires 0.3 lb of foam for the pattern and far less organic materials.

EPC foam patterns are coated with a refractory slurry which is dried, then backed by dry unbonded sand in a steel flask. The sand supports the foam and fills internal pattern passages that in conventional castings have to be formed by separately molded and bonded cores. The EPC mold is poured the same other foundly processes. The molten metal displaces the foam to produce a metal duplicate of the pattern.

The Society's EPC R&D program began in 1989 at the Southern Research Institute, with task participation at the Universities.of Wisconsin, Alabama, Missouri-Rolla and Purdue. The magnitude of the project required a consortium of researchers and sponsors, each of whom brought skills from many disciplines.

Studies of patterns, coatings and sand technologies were paramount. Mathematical and physical modeling were managed by the University of Missouri-Rolla, while foam pyrolysis research and studies of the mechanism of degradation were conducted principally at the University of Alabama. In plant studies of casting precision, sand casting properties and the preparation of manuals on the process and defects were conducted in 12 participating foundries.

An air gauging system was developed to minimize pattern distortion caused by the weight of the stylus in conventional coordinate measurement machines (CMMs). The air gauge was moved by stepping motors to make dimensional measurements on parts. Accurate to 0.0005 in., the device determined the effects of bead density and bead type on pattern precision. It also measured the dimensional accuracy of castings made from various foams.

A second air dimensioning system capable of making 30 simultaneous measurements was devised to determine the effects of bead type and processing on foam mold accuracy.

Many EPC defects were examined to isolate the causes of coating failures leading to penetration. Methods to minimize penetration are being evaluated. A coating wickability test was devised to correlate wick pyrolysis products with gas and lap defects.

Mold distortion is a major cause of rejected EPC castings. A distortion strip was developed to measure distortion when it occurs during compaction. Otherinstrumentationmeasuresthefullcycle acceleration intensity and amplitude during compaction.

A significant accomplishment was the development of a compaction gauge to measure the rate of fill and densification of sand in a flask. Used with a distortion strip, it determines compaction variables favoring sand fill and densification with minimum pattern distortion. These instruments are used to map flasks for regions of poor sand flow and high distortion. Mathematical modeling of aluminum and iron solidification is still in development.

Future of EPC

The EPC process is an applicable technology for producing many types of castings. Component production is often simplified by eliminating cores and controlling casting wall thicknesses. It offers opportunities to consolidate parts, reduce machining, minimize assembly and reduce waste and disposal costs. Without proper controls, however, EPC can produce defective castings. A pilot line at Southern Research Institute conducts systematic process variations and develops prototype castings

It has been proposed that AFS' efforts in EPC technology continue to:

* increase understanding of the foam

pattern making process; study the effects

of tool temperature, steam pressure

and time, and how bead variables

affect fusion and part precision;

* understand coating formulations,

batch-to-batch consistency to control

weight, minimize veining and penetration

defects;

* standardize gating to minimize fill

defects;

* develop instrumentation and test

methods to monitor and improve

compaction;

* transfer project information to program

participants through in-plant

evaluations and demonstrations of

new technologies.

The EPC project is designed for participation by casting users, tool/foam producers, foundries and suppliers to provide a tangible understanding of the benefits of EPC. Sponsorship provides access to the best available technology and the opportunity to discuss technical needs with other program participants.
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Title Annotation:expendable pattern casting
Author:Twarog, Daniel L.
Publication:Modern Casting
Date:Feb 1, 1993
Words:734
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