Switchover to Holding Pressure

Depending on the cross section of the molded part and the corresponding flow resistance of the melt, a pressure increase is measured during mold filling. After switch over to holding pressure, the melt is compressed, and there is a rapid pressure increase. Because of the pressure transfer in the polymer melt from the flow path end up to the sensor position near the gate, it takes some time for the increase in pressure to be measured, and that is why a cavity pressure-dependent switchover to holding pressure tends to be too late. 

Differing theories discussed primarily in the U.S. and termed Decoupled Molding contradict the physical principles of process optimization since it always leads to a stagnation of the melt and thus to undesirable process conditions or to different boundary layer thicknesses (= different shrinkage conditions) due to conscious deceleration of the filling before the switchover to holding pressure. 

Quite similar is the switchover to holding pressure above a pressure threshold value with using a cavity pressure sensor. Once the optimal change is determined, it cannot react to viscosity variations. Both methods have the disadvantage in that the optimal switch over point is determined with difficulty, first using of a mold-filling study, and the switch over point then changes in the course of production. 

The proper method to control drum foamovers during a rapid switchover is to control coke drum pressure by pinching back on the outlet vapor valve to maintain the pressure in the full coke drum. While the operator at the switch valve is diverting the feed to the empty drum, he observes the full coke drum pressure. As this pressure falls during the switch, he closes off the vapor outlet valve from the full drum to hold the pressure within 5 psig of normal operating pressure. Using this technique, the switchover can be accomplished in about 20 minutes. 

Modern controis of an injection molding machine switch from a speed-controlled injection phase to a pressure-controlled holding pressure phase. For this purpose, various methods are available, which differ mainly in terms of their accuracy. For all methods, however, the goal is to switch over the target when the cavity is volu-metrically filled. If the holding pressure switchover is done too early, a momentary drop in pressure usually occurs, and the cavity is filled under holding pressure in an undefined way. If the holding pressure switchover is done too late, the melt is still compressed in the injection phase, which results in pressure peaks, overloads, or so-called residual pressure. Both phenomena usually prevent the production of an optimum molded part. 

Figure 11.6 Peak temperature profiles at gate and near end of fill for three sets of MuCell process conditions. The time to peak temperature changes with process conditions and should provide a reliable trigger for switchover from velocity control to pressure controlled hold time.

To make sure the stable state of resin in the process optimization test, the process monitoring system (PMS) was applied in a wide range in this study. For each critical step of the test, the way to utilize the PMS dq ended upon the phenomenon in the mold and the purpose of the test. The dependent variables such as switchover point and holding time were properly set with the aid of the PMS. Based on the relationship of the state of the resin with pressure and temperature, a new PMS index was proposed. The index showed the best correlation with the part weight among various indices through the correlation analysis. 

The process optimization includes the initial condition search. In order to optimize the process, the control variables should be selected properly. When the control variables are poorly selected, the process optimization may end up with unexplainable result. For the best result, the characteristics of the process variables should be carefully understood. The process variables can be classified as machine variables and material variables, as shown in Table 1. The state of the resin in the mold is closely related with material variables such as resin temperature, resin pressure and viscosity, etc. However, the material variables are hardly measured and controlled directly. The machine variables can be classified as independent, dependent and restrictive variables. The control variables for the process optimization should be chosen among the indepmdait variables. The depmdmt variables such as switchover point and holding time are to be properly found. These variables can be set in the initial condition searching stq>. For the purpose, the PMS provides the detailed information in the mold. 

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