Parallel plate compression plastimeters

There are basically two forms of parallel plate compression plastimeter (a) with both compression plates much larger than the test piece (Figure 6.2(a), plate test) and (b) with one or both plates of approximately the same diameter as the test piece (Figure 6.2(b), disc test). 

In the past, parallel plate compression plastimeters have been quite widely used for measuring rate of cure and methods have been standardised. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity or recovery or some combination of these, is then plotted against time of heating to give a scorch curve. An even more time consuming procedure was to measure tensile properties as a function of cure time. 

The principle of the compression plastimeter is very simple - the test piece is compressed between parallel plates under a constant force and the compressed thickness measured. This simplicity accounts for the early adoption of this type of instrument and its subsequent continued popularity. The work of Williams16 led to the first widely used parallel plate instrument and eventually to various modified forms all working on the same principle. Apart from simplicity, the compression principle has no real inherent advantages but a number of disadvantages ... 

ISO 732324 specifies a parallel plate test based on the Williams plastimeter with plates 4 cm in diameter. The test piece is 2.00 0.02 cm3 in volume and can conveniently be a cylinder 16 mm diameter and 10 mm thick. As discussed above, a close tolerance on volume is necessary for this type of plastimeter. The test piece is preheated for 15 min (the temperature of test is usually 70°C or 100°C) and compressed under a force of 49N. The thickness of the compressed test piece is measured in mm and this value multiplied by 100 quoted as the plasticity number. The preferred time of application of the force is 3 min. The correction to the standard in 2003 was to change the tolerance on the force from 0.05N to 0.5N. 

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