Measurement method of plastic shrinkage rate and analysis of its influencing factors
ZHOU Bi da, JIANG Shao fei, LI Ji quan*
(Key Laboratory of Special Purpose Equipment and Advanced Manufacturing Technology, Ministry of Education, 
Zhejiang University of Technology, Hangzhou 310014, China)


Abstract: Aiming at the problem that uncertainty of shrinkage rate value of material, the measurement method of shrinkage rate and its influence factors were studied, the calculation method of shrinkage rate was summarized. A method to calculate the shrinkage rate was proposed, which was based on shrinkage of the plastic parts measured by the thermal expansion instrument. The influence of the molding process and heat treatment on the shrinkage rate was discussed, and the relationship between the crystallinity and shrinkage was analyzed. The parts were molded by different molding process from isotactic polypropylene. The influences of molding process and heat treatment on shrinkage were discussed, the shrinkage was measured and calculated both before and after heat treatment. DSC was introduced to measure the crystallinity to analyze the effect of crystallinity on shrinkage rate. The results indicate that the measurement and calculation method of shrinkage rate is correct and validated by comparing with the shrinkage rate from molding experiment. The rapid heat cycle molding and heat treatment increases the shrinkage rate because of the increases crystallization, which should not be ignored in the design of injection mold.

Key words: shrinkage rate; molding experiment; crystallization; rapid heat cycle molding(RHCM); heat treatment