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Generally, mixing machines can be considered as rather perverse objects in that repeatability is not a fundamental characteristic. In the case of plastics or inverted plastics, where the viscosity of the material may depend upon its position in the mixer, or in cases where the viscosity varies over wide limits during the course of the mix, stagnation points may develop, so the time necessary to produce a desired degree of homogeneity can fluctuate widely from batch to batch. Where rapid tests of the material are not available and where strict limits on homogeneity are demanded, these problems are generally resolved by extending the mixing period, sometimes a factor of four or five times that which may be actually necessary. This solution represents inefficient use of expensive capital equipment and may result in a product that is not completely mixed and therefore must be rejected.
Manager, Reactor-Monitoring Center, Tracerlab, Inc., Richmond, Calif.