Plastics - 100 % Quality Injection Molding - KISTLER - #9

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www.kistler.com 9 The Pressure Profile and its Interpretation During the mold filling phase, the different characteristics of plasticized amorphous or semi-crystalline polymers are immaterial as long as their viscosity is identical. However, both materials display a different compression behavior. A higher amount of semi-crystalline material than amorphous material needs to be introduced into the cavity at the beginning of the holding pressure phase to achieve a pressure build-up. When the melt cools down during the holding pressure phase, more semi-crystalline material must be introduced into the cavity to compensate for the volume contraction and to prevent voids in the finished part. Specific characteristics of amorphous thermoplastics During the holding pressure phase of injection molding processes involving amorphous polymers such as polystyrene (PS), acrylonitrile-butadiene-styrene (ABS), styrene- acrylo-nitrile (SAN), polymethyl methacrylate (PMMA), polycarbonate (PC) and polyvinyl chloride (PVC), the mold cavity pressure drops to ambient pressure levels parallel to the declining part temperature in the wake of an increasing viscosity and the corresponding deterioration of the pressure transfer from the area in front of the screw. Specific characteristics of semi-crystalline thermoplastics Due to an initially sufficient pressure transfer during injection molding processes involving semi-crystalline materials such as polyethylene (PE), polypropylene (PP), polyamide (PA) and polyoxymethylene (POM) there is almost no change in the mold cavity pressure during the period after the compression phase and the onset of the crystalline melting point. After that, however, the significant volume contraction during crystallization brings about a sudden drop in the pressure. The duration of the holding pressure phase depends on factors such as the wall thickness of the molded part, the degree of crystallization and the processing parameters. The crystalline melting point of semicrystalline materials for example, is dependent on the prevailing cavity pressure. Cavity pressure Time Cavity pressure Time