TESTING 1-2-3
Predicting Casting Dimensions With Computer Process Modeling
Accurately measuring the high temperature dimensional changes in molding materials has shed new light on additional sources of dimensional variation in final casting dimensions. JERRY THIEL AND SAIRAM RAVI, UNIVERSITY OF NORTHERN IOWA, CEDAR FALLS, IOWA
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s liquid metal solidi- fies, it contracts first in volume and then physical size. Accu- rately predicting the final dimensions
has been accomplished with a set of simple rules along with significant tribal knowledge. Although the age of creating master patterns with wood are mostly behind us, the industry still uses trial and error when produc- ing patterns to meet specific casting dimensions.
Tis method was aided by the use
of “shrink rules,” and more recently, CAD geometry volume compensation,
ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.
“Predicting Casting Dimensions With Computer Process Modeling,” Jerry Tiel and Sairam Ravi, University of Northern Iowa, Cedar Falls, Iowa.
Background—As liquid metal solidifies, it contracts first in volume and then physical size. Accurately predicting the final dimensions has been accomplished with a set of simple rules along with significant tribal knowledge. Although the age of creating master patterns with wood are mostly behind us, the industry still uses trial and error when producing patterns to meet specific casting dimensions.
Procedure—Predicting casting dimensions requires accurate thermos-physical properties of molding materials. A high temperature aggregate dilatometer was used to measure the thermal expansion at bonded sand cores from room tem- perature to 1600 C (2912 F). The thermal expansion results were used in a process simulation software to predict dimen- sional changes in molding materials. Step-cone castings were poured and measured with a CMM to compare actual and predicted results.
Results and Conclusions— The difference between the actual casting dimensions and the theoretical casting dimensions from the shrink rule is apparent. The actual casting dimensions depends on the section thickness of the casting to a large extent. From this research, it was demonstrated that final casting dimensions can be estimated using accurate bonded sand thermo-physical properties data. Although the cases presented are restrictive, it provided solid insight into more
accurate predictions of final casting dimensions based on molding aggregates. 36 | MODERN CASTING July 2016
neither of which accurately predict final casting dimensions. Tis method of producing patterns is very costly, often requiring as much as 40% of the original pattern cost for dimensional adjustments. Previous researchers have studied the effects of various geometry and areas constrained by cores. Previ- ous research in sand expansion has identified several mechanisms whereby changes in the mold and core dimen- sions render current single volume percentage decreases inaccurate in determining final casting dimensions. By understanding the dimensional
changes sand undergoes during rapid heating and the rigidity of the mold-
ing media, accurate predictions can be made to the final casting dimensions given a specific pattern size.
Question Can computer process modeling be used to determine casting dimen- sions from room temperature pattern dimensions?
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Background As the ability to cre-
ate castings true to designs improves, reductions in machine stock can be made to
decrease cost, improve reliability and speed production.
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