New techniques for making accurate foam patterns.

New Techniques for Making Accurate Foam Patterns

The quality of evaporative pattern castings can be improved by producing more dimensionally accurate evaporative foam patterns.

As the practicability of manufacturing castings with the Evaporative Pattern Casting (EPC) process continues to gain acceptance, attention is now turning to improving the quality of EPC castings. The emphasis given to differing aspects of quality varies with the product application and the alloy cast.

With the automobile industry's increasing involvement with EPC though, accuracy of the castings has become the major measure of quality. Parts designers, striving for improved product performance, constantly seek to reduce casting tolerances.

Pattern Accuracy

Dimensional accuracy has many aspects: normal vector variation, warpage, concentricity, etc. The relative significance of these is part-dependent. Some parts require special solutions to their particular problems. However, a general approach to machine design and expanded polystyrene (EPS) processing can prevent some problems and ameliorate others.

A range of molding and bead expansion machines and auxiliary equipment, some of which are shown in Fig. 1, have been designed to produce accurate patterns. The following are the most important aspects considered in their design.

Shrinkage--This is not a problem in itself, but the variability of shrinkage is a major source of pattern inaccuracy.

Molding--In terms of both cycle-to-cycle variability and process methodology, molding can be a source of dimensional instability.

Tooling--Tool alignment, tooling flex, construction, material and design all play a role in pattern accuracy.

Demolding--At this stage, the pattern is easily deformed if it is not handled correctly.

These aspects can be minimized with an understanding of EPS and sound engineering principles. Accordingly, the factors influencing each of these aspects can be analyzed and controlled.

For example, in the molding process, which has a major influence on pattern accuracy, one of the factors is heat exchange. These are the controls affecting heat exchange.

Steam Quality--Dryers and insulation are used to maintain steam quality in molding.

Instrumentation--Sufficient accuracy is necessary in all sensing and monitoring devices.

Reactive Process Control--The process must be made robust to uncontrollable externals.

Tooling--Design, material selection and machining of tooling affect proper heat exchange.

Vacuum Cooling--The use of vacuum to quickly cool and dry the tooling provides better heat exchange.

Vacuum cooling and drying of tooling, however, offers other process advantages. For example, its effect on pattern dimensional stability is only now receiving attention. A study found a 67% reduction in linear variability over the traditional "wet" process, which uses water cooling alone. Figure 2 shows typical variabilities obtained by each method.

Flexibility

With consideration of the factors for the various process steps, it becomes apparent that easily exploitable machine flexibility is the key. This enables the machine to be "tuned" to suit any tool. In this way the maximum potential of a given tool can be extracted. This flexibility must be built into the machine from the point of initial design. Aspects of flexible design are:

Intelligent PLC/Reactive Software--The controller and software should be capable of controlling the machine functions as required at initial set-up. Both process step timings and the operation of functions should also be easily modified to suit new tooling.

Regulators/Flow Controllers--These should be employed on all fluid streams that may need to be pressure or flow controlled.

Filling Options--The machine should permit pressure, vacuum or venturi filling, or any combination of these.

Adaptable Handling--The role of the handling unit for precise pickup of delicate patterns is important in preventing distortion of the pattern. Figure 3 illustrates the possible effect of failing to support the pattern sufficiently.

Vacuum Pump & Tank--Fine control of machine vacuum service is necessary.

These design features were developed in consultation with foundrymen. And, it should be noted that the machines will often be used in foundries, rather than in molding shops.

While good, informative customer service can overcome the difficulties of start-up and new tool commissioning, it is important that operators be sufficiently trained to gain experience in the principles and functioning of these machines.

The great potential of EPC should not be held back by out-dated technology and inadequate machinery, or by a lack of information on the new. [Figure 2 & 3 Omitted]

PHOTO : Fig. 1. A bead preexpansion machine (top) and molding machine (bottom) are shown.