Abstract
Abstract The uses of plasticity tests, especially rapid simple tests, are discussed, with particular reference to the limitations imposed by the complex plastic behavior of unvulcanized rubbers and stocks, leading to the conclusion that simple tests are likely to be of use only for routine control of repeat, nominally identical batches. A summary is given of information collected from the industry to show under what conditions a plasticity test is required to be used in factory control. The efficiency of a plasticity test is defined, and a discussion given on how the type of instrument (rotation, extrusion or compression) and the conditions of a compression test affect both efficiency and rapidity, thus showing what conditions furnish the best form of test. It is concluded that the parallel-plate compression test is the most suitable, provided the test-piece is precompressed to a small thickness so that it can be quickly heated to the test temperature before applying the load; other factors considered are the size of compression plates and of test-piece, use of release medium (e.g., thin paper), degree of precompression, preheat period, load, compression period, and temperature. Existing rapid plastometers are described, including full details of the new Wallace modification of the Hoekstra instrument, which embodies the recommended conditions of test.
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