 Switching over from the injection phase to the holding pressure phase is the method used most frequently. Most injection molding machines with cavity pressure measurement feature this type of switchover as an integrated function. The switchover is actuated not by the screw position or the injected volume but by a threshold value for cavity pressure. In this way, the effects of viscosity and metering fluctuations on the switchover point are minimized. In addition to switching over at a specific pressure, it is also possible to use an intelligent system for the purpose of the switchover. Such systems automatically detect volumetric cavity filling and send a signal instructing the injection machine to switch over from injection to holding pressure. During injection these systems analyze the cavity pressure curve and detect the characteristic rise in pressure that occurs on transition from the filling phase to the compression phase.
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Cavity pressure sensors are also used in the control of special processes in order to minimize process fluctuations. With fluid injection technology (GAIM and WIT), introduction of the fluid is started when the mold cavity pressure has reached the relevant level; a delay time is taken into account. The switchover from injection to compression in an injection compression molding process can be performed in various ways, using the screw position, the injection volume or also a force that is exerted by the melt on the embossing stamp, for instance. Especially in cases where the surfaces are more complex, the switchover point must be exactly right if undesirable marks are to be avoided. In this type of process, too, using mold cavity pressure to determine the switchover point leads to consistently high quality in the molded parts.
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