Under Pressure

The initial die build and balancing are vital for positive long term effects. The re-strike station, particularly on thicker or formed materials, often exerts a concentrated force near the exit side of the die. Unbalanced pressures can cause the die to “kick,” resulting in premature fatigue, excess wear and subsequent cracking or breakage of die components. Optimal balance throughout the die is achieved by paying close attention to pressure readings from the film and making adjustments in the gutting station area. This is usually accomplished by varying the stripper pressures when balancing infeed to output die ends.

In their testing, Graenert and Eeles used Pressurex film to analyze the pressure on and around the cutting edges of a progressive die assembly (shown in Figure A). Compressive forces act on the microcapsules of the sensor film just before cutting through the strip and film. This pressure causes the sensor film’s microcapsules to burst, producing a high resolution “topographical” image of pressure variation across the contact area.

Fig. B: Cut through of Pressurex sensor film reveals stripping and gutting punch forces. The line scan represents cross-sectional forces. Click the image to enlarge.

Similar in thickness to ordinary paper, Pressurex changes color directly proportional to the amount of pressure applied and reveals whether pressures are evenly distributed (shown in Figure B). Precise pressure distribution is determined by comparing the exposed sensor film to a color calibration reference chart.

In Graenert and Eeles’ test, the film showed the areas which were doing most of the work in this die station. As shown in Figure C, the film allowed them to observe the precise pressure distribution and actual pressure values in the die and take action to correct imbalances or excessive pressures before die-build completion. The cutting edges in the gutting operation compared to the restrike forms showed similar pressures and were adjusted.