Numerical investigation on the mechanism of polymer multilayer gas assisted co-extrusion precision molding process

Based on the t raditional co-extraction，an advanced multilayer gas assisted co-extraction precision molding technology (GACEPMT) was put forward. The research shows that GACEPMT can not only implement the automatic and accurate control of the product dimension，but also save energy. A comparative analysis on molding process and die swelling process of traditional co-extraction and gas assisted co-extrusion was implemented by the stable finite element numerical algorithm established. Based on the above analysis，the mechanism of eliminating the co-extrusion die swell was disclosed. The research results show that the multilayer co-extrusion die swell is caused by the viscoelastic melt secondary flow，and depends on the direction and strength of the melt secondary flow. The direction of the melt secondary flow depends on the sign of secondary normal stress difference，and the strength of the melt secondary flow is proportional to the secondary normal stress difference. The gas assisted segment of a gas assisted co-extrusion die can reduce and eliminate the melt first or secondary normal stress difference by wall slip of air cushion，which eliminates the melt secondary flow near the exit of die. The goal of eliminating the co-extrusion die swell was implemented by removing the melt secondary flow. So the GACEPMT can accurately control the shape and size of gas assisted co-extracting multilayer composite products and the shape and size of the gas assisted die are completely identical . Moreover，the results show that the extracting pressure can be reduced above 30% compared with traditional co-extraction.