Testing for uniformity of compound

Compound testing has two aspects, one is for quality and the other is for reproducibility of the mixing process. Even though both aspects are intimately related, they will be discussed separately. 

The uniformity in terms of reproducibility involves batch-to-batch as well as in-batch variation. The required size level of the uniformity depends upon the downstream method of processing such as extrusion and injection moulding as already discussed. If the size is in the order of mm, the compound may be extruded from a capillary rheometer at a constant speed and the pressure oscillation may be taken as a measure of uniformity. 

For a larger size non-uniformity, for example, variation within the batch of internal mixers may appear as a pressure oscillation observed by a pressure-tap at the chamber wall of a mixer [13]. It may also appear as an oscillation of the mixing torque (see Table 3.1 [24]). However, before using these oscillations as a measure of degree of nonuniformity, an independent check by actual sampling of the contents must be done, because the oscillation is also related to the way in which the compound is moved around the mixer. For the batch-to-batch variation, previously mentioned two-point measurements of moduli may be used. If the requirement is not very critical, even a one-point measurement may be adequate. 

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