On the Pipe Flow and the Ejection of Molten Plastics

Abstract

Many experiments on the flow of molten polymers have hitherto been performed by means of relatively short capillary tubes. The static pressure of the flow is measured indirectly by the load of plunger. In this paper the results of experiments made with the use of comparatively long tubes-not capillary-are described. As for the pressure, the elastic deformation of the tube according to the static pressure of the flow was measured directly by the wire strain gage. Polyethylen and polypropyrene were used as testing materials.The shearing stress vs. shear rate curve can be approximated by two straight lines as shown in Fig. 3. The shear rate at this crooked point of the flow curve corresponds to that of the maximum swelling ratio, and over the region of this shear rate appears the vibration of pressure.The swelling mechanism by Barus effect was investigated by introducing the Voigt model after McIntosh, on the other hand, and it was assured that the experimental tendency had good agreement with the theoretical ones as shown in Fig. 5. So it seems that Barus effect can be explained by a modified Voigt model of Fig. 6. The flow rate at which the smooth figure of ejection is obtained is calculated from Eq. (10). The facts thus obtained by this study will serve as the design for dies of injection machine.