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Packing model is to analyze the packing/holding process in the injection molding. During the packing process, the relation between pressure and time is illustrated as below.
The parking/holding pressure in the injection molding process
Basically, a packing pressure is applied while the molten polymers solidify inside the mold cavities. The density of polymer compensates for the volumetric contraction behavior of plastics from molten state to solid state. The packing pressure can be set as the original injection filling pressure (one-time injection pressure), or it can be larger, up to two-times its injection pressure. During the packing process, since the cavity is completely filled with plastics, it has a high back pressure. Moreover, the injector screw rod moves forward only a little bit during the packing stage, plastics is therefore slow flowing, which is referred as packing flow. The cavity reaches the highest pressure during the packing stage and solidification begins due to the low temperature of mold surfaces. The packing stage continues until the gate solidifies and gets frozen. The cavity has already been filled up with plastics during the packing process. Therefore, the pressure can be transmitted from the gate effectively. Additionally, along the thickness direction, the polymers close to the mold surfaces would first cool and solidify so that their volumetric shrinkage will occur first. However, before the gate solidifies and gets frozen, the polymers fill the mold in the thickness direction under the effect of packing pressure. Also, polymers near cooling channels have higher viscosity and higher flow resistance due to a lower temperature. Inrush of hot molten polymer from the gate cannot be easily penetrated; therefore the melt temperature at those areas would keep dropping and result in a cool material zone. Under this class of circumstances, transmitting packing pressure becomes more difficult. In high temperature areas, polymers have low viscosity and low flow resistance, which make molten polymer easy to replenish the volumetric change and to maintain those areas at a high temperature. The differential flow resistance generated by the temperature difference would force the packing flow to follow a specific low resistance path and proceed with the pressure transmission process. Packing pressure must be sufficient high enough to overcome the gate resistance so as to achieve shrinkage compensation. The viscosity around the gate area increases gradually during the packing process. The gate is also freezing to have a stronger resistance. Moreover, increasing the packing pressure and prolonging packing time would delay the solidification time for polymers. This will enhance pressure transmission and reduce the volumetric shrinkage rate of plastic parts. An excessive packing pressure is likely to be accompanied with mold-sticking problem. Furthermore, plastic parts would have an excessive residual stress, flash and bleeding problems. Insufficient packing pressure would leave plastic parts with larger shrinkage and empty holes problems. There are many factors that affect the development of parts in injection molding during packing process, such as the packing time, melt temperature, mold temperature, the types of plastics materials, the design of runner and gate. It is very difficult to catch them just by trial-and-error. Packing module can be a powerful tool to deal with this task.