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Packing and discharge in injection molding
Author(s) -
Koita Y. T.
Publication year - 1974
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760141206
Subject(s) - plunger , materials science , mold , composite material , reciprocating motion , molding (decorative) , flow (mathematics) , orientation (vector space) , mechanical engineering , mechanics , geometry , mathematics , physics , engineering , gas compressor
In a reciprocating‐screw injection‐molding machine, the screw or plunger forces the plastic into the mold. It remains in the forward position under pressure for a fixed time during which plastic flow into the mold or ‘packing’ takes place. After the timer runs out, the screw moves back while rotating thus releasing the pressure on the plastic in the mold. At this time, if the gate has not frozen, flow of plastic out of the mold or ‘discharge’ takes place. In this study, clear rigid PVC, poly (vinyl chloride), was molded with varying amounts of packing and discharge. Photoelastic stress patterns for parts observed between cross‐polaroids suggested that packing and discharge give rise to high frozen stresses due to molecular orientation in the gate area. Mechanical strength tests on the molded parts show that these high stresses are a source of weakness. Short forward times for the plunger significantly improve the mechanical properties of the parts in the gate area and at the same time have a significant influence on part dimensions, especially in the gate vicinity. Laminations observed at the gate interface can also be reduced by controlling the extent of molecular orientation introduced by packing and discharge.