Processability Testing

There should be no need for processability testing. If one controls adequately all the raw materials and all the processes prior to the tests, then the resultant materials will all have predictable processability. In the real world, one can try, but never achieve, this state of perfection, so that some processability testing may be essential for process control purposes. The progression from (a) dumping a mixed batch at constant cycle time to (b) dumping at a fixed temperature to (c) dumping at a fixed unit work shows a progressive attempt to produce a more consistent material in spite of the imperfections of the raw material and the process. No one processability tester is adequate for all purposes. The choice of processability testers for process control depends upon the polymer and the subsequent process to which the material or mix is to be subjected. Factors which can be significant are times, shear rates, temperatures, and whether the test modifies the relevant properties. Most of the published information relating to process control relates to controlled experiments rather than to processing under normal factory conditions. However, it is probable that much information on factory experience remains unpublished in order to avoid disclosure of commercially valuable information or, on the other hand, to avoid admission of mixing failures. The cost of purchasing and running the processability testing equipment has to be balanced against the costs of not recognizing a bad batch and the cost of delaying further processing while the tests are made. For these reasons no general recommendation can be made except that a single piece of data is insufficient to characterize either a polymer or a mix. Any factory will probably need several types of processability testing if it wants more than a very cursory view of the material being processed. The choice of a processability test for development of mix composition depends upon the degree of information which it is economical to acquire. If an order calls for an isolated 400 kg of material for a noncritical application, it may be worth doing no processability testing at all. On the other hand, if a factory is to produce hundreds of tonnes per week of a single type of mix, it is well worth while discovering considerable detail about the characteristics of that mix. In this case, equipment providing fundamental data over a wide range of shear rates and at small and large strains may avoid carrying processes so near to a critical condition that any slight change in material or process may cause a catastrophe. Many factory activities fall between these examples and thus require individual consideration.