Mold filling cycle

From Equation 14.7, it is seen that for a constant flow rate, the applied pressure at the gate must increase with time to a certain power. In practice, the initial part of the mold-filling cycle is one of increasing applied pressures to maintain a constant flow rate. 

PET/elastomer blends have been used commercially either filled or reinforced. Owing to slow crystallization of PET in these blends, unfilled grades would be very difficult to mold. In warm molds, the cycle would be too long, the parts would tend to stick and distort when removed, making the process expensive. In cold molds, amorphous parts with low heat deflection temperatures, would be produced, reducing the usefulness of the blends. 

Fig. 24 Schematic diagram of EPP molding production cycle using the pressurized filling method...

The mold-filling process is simulated in order to determine whether they can be produced with the selected molding compound. The relevant parameters include the injection type and position, flow lines, the injection molding pressure and temperature, and the expected cycle times. 

Not lower than 85°C (185°F). Mold filling and article quality improve with increasing temperature. High mold temperature increases cycle time only marginally, as the glass transition temperature is at 145°C (293°F). Pre-drying... 

The mold must be vented to allow for gas escape such vents must be placed near weld lines and also near the last areas to be filled. Typical vents are slots 6 to 13mm (0.236 to 0.512in) wide and 0.01 to 0.03mm (0.0004 to O.OOlin) deep such slots are located on the mating surface of one of the mold halves. If a negative pressure device is available it may be possible to vent the mold into the water channels. This can speed up mold filling, reduce component burning and, reduce the cycle times. 

The modeling of resin curing is one of the most important issues in the analysis of compression molding because the cure cycle constitutes a major portion of the total process cycle time and an incompletely cured part should be discarded. Material flovy in mold filling is also an important topic because knit line formation can be avoided by optimizing the precharge location. Moreover, mold filling analysis is indispensable to predict the fiber... 

The cycle time is the total of the three stages mold preparation and preform preparation, mold filling and resin cure. Mold filling time is predicted by using the flow models. Cure model is used to determine the degree of resin cure and if a database is available which relates the degree of cure to the mechanical properties, the user can decide when to demold the part. 

Mold fill analysis programs affect both productivity and quality. The cycle time usually can be reduced and the part is of a higher quality due to minimum molded-in stresses. Lower molded-in stresses usually... 

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