Strategies For Improving Surface Quality And Cellular Structure Of Microcellular Injection Molded Parts And Mechanisim Analyses

The microcellular injection molding(MIM)using supercritical fluid as blowing agent can dramatically reduce product weight.It can also shorten molding cycle time and reduce injection pressure.The products molded by MIM have several advantages,such as high strength/weight ratio,high accuracy of shape and size,high dimensional stability,low shrinkage and deformation.However,microcellular injection molded parts usually have obvious surface defects,such as swirl marks and silver streaks.So they can not be directly used in applications where a better surface quality is required.Three methods were used to improve cellular structure and surface quality of microcellular injection molded parts in this dissertation,including rapid thermal cycling molding(RTCM),processing parameters adjusting and injection compression molding(ICM)with mold temperature control.Mold cavity surface temperature was controlled by RTCM to improve surface quality of microcellular injection molded polyformaldehyde(POM)cover plates.The effect of mold cavity surface temperature on cellular structure and surface quality of microcellular injection molded POM cover plates was studied.The formation mechanism of rough surface on microcellular injection molded parts and the mechanism for improving surface quality by controlling mold cavity surface temperature were preliminary analyzed.The results showed that raising mold cavity surface temperature could improve surface quality of microcellular injection molded parts.Compared to 40 °C,raising mold cavity surface temperature to 150 °C could reduce surface roughness of cover plates by about 85%.Microcellular injection molded acrylonitrile-butadiene-styrene copolymer(ABS)parts were molded at different gas dosages.By controlling the cell nucleation rate of polymer/gas solution during the filling stage through changing gas dosage,the cell nucleation in advancing melt front was decreased to improve the surface quality of parts.The results demonstrated that the surface quality of parts was greatly improved and was similar to that of solid parts when gas dosage was 0.13%.Microcellular injection molded ABS parts were molded at different mold temperatures.The results showed that compared to 40 °C,raising mold temperature to 85 °C could reduce the surface roughness by about 70% at area far away from gate on the part.But there were some blisters on surface of part molded at a mold temperature of 100 °C.Microcellular injection molded ABS parts were molded by using ICM technology to study the effect of injection compression parameters including shot size,compression distance and compression force on cellular structure of parts.The results demonstrated that unfoamed solid area in core layer of parts was broadened by raising shot size and compression distance.More uniform cellular structure appeared in core layer of parts by increasing compression force.Microcellular injection molded ABS parts were molded by using ICM technology at different mold temperatures.It was found that part molded at 80 °C had better surface quality and cellular structure.A method combining mold temperature control with ICM technology to improve surface quality and cellular structure of microcellular injection molded parts was proposed for the first time.The research on improving surface quality and cellular structure of microcellular injection molded parts in this dissertation is of great significance and can widen the application fields of microcellular injection molded parts.