The use of strain gauges for radial stress measurement during tableting

The influence of size, configuration and positioning of die wall strain gauges on the measurement of radially transmitted stress developed during tableting was investigated. Calibration of strain gauges attached to a cutaway die wall was achieved by compression of rubber-like materials in the die, Breon Polyblend 504 being more effective than red rubber for this purpose. Hysteresis of response observed when calibrating a thin-walled die was possible due to excessive distortion of the weakened die wall although the extent of this hysteresis varied with calibration material. The use of full bridge arrangements produced an increase in response when the compression site was moved away from the gauges. The opposite effect was seen when half bridge arrangements, using two active gauges, were used. The paradoxical effect observed when full bridge arrangements were used was shown to be due to straining of the compensating gauges. The dependence of die wall response on compact position was substantially reduced by the use of multiple gauges mounted along the die length and connected together to form a single gauge on each side of the die.