PET preform molding technology
In the preform molding process, the best conditions is the possible low temperature, the possible short cycle time, evenly and completely melted, the possible less IV and AA , transparent possibly. Process condition related is:
It refers to the barrel temperature in the injection machine and the hot runner temperature of PET preform mold . On Molding process, only 30% of the heat come from the external heating, 70% is from the internal shearing heat, so in addition to the appropriate barrel coil heating, the molding also need much shear heating.
Injection and packing
Injection is to overcome the resistance of the flow , the melt is filled into a mold. On the preform molding, the best process is three stages speed and pressure setting, with descending order.
If injection speed is too slow, shearing isn’t enough, the melt would cool down before the full filling, which result in short shots; Fast filling and poor venting on the cavity would result in insufficient filling and severe shrinkage.
Packing has two important roles: to prevent backflow of melt and to ensure proper cooling under pressure (enhance the cooling effect). Too high pressure will cause excessive filling and make molds open, high internal stress and crystallizing. While too low pressure will cause shrinkage, preform deformation(poor cooling).Gate problems, such as holes, voids, and etc., because the cooling rate of decline at the gate.packing time should be appropriate, for too short packing will cause holes.
Pressure releasing is to reduce the pressure within the hot runner, to prevent the gate stuck and poor needle valve operation . But too much will cause shrinkage, drawing and hole.
Back pressure is a force applied to the melt plastic,driven by the rotation of the screw activated by hydraulic cylinder system. its function: to strengthen PET plasticizing , eliminate air bubbles. Start from 0,after preform ready then increase gradually until there is no bubble inside,at that point,it is a proper stage.Too high back pressure would lead to high shear, bad shape, gate blockage and etc.
Buffering material is the unfilled material in front of screw after each injection , too little will cause poor shape, while too much will cause PET to decompose. The amount is adjusted gradually from small to large, until no whiting or crystallization .
PET raw material is opaque, while the preform is transparent , the reason is because of cooling. Poor cooling will reduce the cooling rate , lead to shrink ,preform deformation and longer cycle time.In order to avoid those situation, the following should be noted: proper water treatment, regular waterways cleaning, the water flow rate and pressure check ,core and cavity cleaning and etc.