Abstract
The shrinkage of crystalline polymer products formed in injection molding is evaluated, on the assumption that the specific volume in polymers can be represented by Spencer-Gilmore's empirical equation, and compared with experiments. The values of the solid state parameters in Spencer-Gilmore's equation are estimated from thermal expansion and compressibility data, where the dependency of specific volume on die temperature is considered. The parameters for the melt state are determined using the results of specially designed injection moldings, which give the polymers very different temperature and pressure histories from each other. The mean specific volume can be represented with terms involving the integral of the specific volume along the thickness direction. If the time of gate seal and pressure change are measured, the mean specific volume can be calculated from this equation, where the temperature distribution of the polymer in die the cavity is simulated in the thickness direction. On the other hand the specific volume can also be obtained experimentally. Good agreement is obtained between the simulated and measured values for the mean specific volume and the shrinkage.
