Casting answers & advice.

Q Investment casting maintains the tightest tolerances among all casting processes. What is the most important step in controlling final dimensions?

Background: Perhaps more than any other casting process, investment casting has factors that contribute to the dimensional accuracy of its metal components. The tooling allowances for investment castings must compensate for thermal expansion, shrinkage, hot deformation and creep in each step of the process--the wax pattern, the shell mold and solidifying alloy. Due to restraint by geometrical features in the die, wax shrinkage and deformation make the largest contributions to the overall dimensional change between the pattern tooling and its corresponding cast part. Control of the wax pattern dimension is therefore a critical step in the investment casting process.

All the factors that determine dimensional changes associated with wax processing must be evaluated. The shrinkage of the wax after injection into the die can greatly affect the final wax pattern dimension. However, wax that is fed into the die during the dwell time compensates for some of the wax pattern shrinkage. Computer models of wax deformation could be useful tools for predicting wax pattern dimensions. However, the lack of proper material models and thermo-physical data for waxes has limited the use of computer models to accurately predict wax dimensions.

Recommendation: Using instrumented wax injection trials with a step block mold, recent research evaluated the restraint of geometrical features by the metal die and process parameters such as dwell time, platen temperature, injection pressure and injection temperature. Comparison of the experimental and numerical simulation results from this research provides several insights into the wax injection process that can help improve dimensional accuracy:

* wax feeding during the dwell time has a considerable effect on the total shrinkage of the wax pattern;

* the wax continues to flow into the die cavity to feed the shrinkage as long its the wax in the injection port is in a paste state;

* the injection port should be designed to avoid premature freezing during the dwell time since pressure is not transmitted after it freezes;

* either heat transfer models or wax pressure measurements may be used to determine the optimum dwell time (or injection port dimensions) to prevent premature injection port freezing;

* viscoelastic models accurately capture the details of wax pattern deformation;

* pure elastic models are unsuitable for predicting wax deformation because any distortion is completely relaxed once the pattern is removed from the die.

The information submitted in this column was supplied by the AFS Investment Casting Committee (4-1).

Q We produce a number of castings for a manufacturer of food processing equipment. We are requested by the customer to certify that the alloy has been approved by the Food and Drug Administration (FDA) for contact with food products. Where does one find the list of FDA approved materials for this purpose?

Answer: As storage as it may seem, the section of the FDA that has jurisdiction over these types of materials is the Office of Food Additive Safety because these materials are considered a contact "food additive." However, they do not have any listing of materials that have been approved for food contact.

If you must certify to the customer that the alloy is FDA approved, then it is necessary to request a ruling from the Office of Food Additive Safety by providing it with a complete chemical analysis of the alloy and a description of how and where the material will be in contact with food. For example, will it be working in an acid or alkaline atmosphere, at what temperatures will it be used? The request should be sent to the following address: Office of Food Additive Safety, U.S. Food and Drug Administration, 5100 Paint Branch Parkway College Park, MD 20740-3835.

You should expect a response back from the FDA in six months.

If you would like more information, read "Prediction of Wax Patterns Dimensions in Investment Casting," A.S. Sabau and S. Viswanathan, AFS Transactions, 02-103.