Mould Holding pressure
The holding pressure represents the pressure exerted on a molded part during a secondary pressure stage. As already demonstrated, there is rarely any correspondence between the set hydraulic pressure and the pressure in cavity (Fig.3.8). The level and time of the holding pressure are of major importance to the dimensional accuracy and quality of a molded part. They determine how exact the cavity surface is duplicated. While the optimum pressure level can easily be established by controlling dimensions, the molding pressure time is frequently guesswork. However, the cavity pressure will provide reliable information.
As long as the sprue, the gate, or any other narrow passage of the melt flow has not yet frozen, changes in the level and time of the holding pressure will have an effect on cavity pressure. After the gate is sealed (frozen off), no further influence can be exerted. Figure 3.15 shows profiles of hydraulic pressure and cavity pressure foe different holding-pressure times. It can be seen in this example that a holding-pressure time of more than 8 seconds no longer causes a cavity pressure change near the gate because the gate was sealed within this time. Extending the holding-pressure time beyond 8 seconds does not affect the quality of the molding and would only waste energy.
In case the cavity pressure cannot be measured, the maximum effective holding pressure time may be determined by systematically monitoring part weight and holding-pressure time. With increasing holding-pressure time, the weight of the molding reaches a maximum value (Fig.3.16); the weight no longer increases after a maximum effective time. A longer holding time has only an infinitely small impact.
Post by SINO Marry