Visualization analysis of pellet-conveying behavior around feed throat in injection molding machine I: analysis of unstable pellet-conveying phenomenon

摘要

Stable pellet-conveyance around the feed throat of the heating cylinder of an injection molding machine is an especially important element in precision molding. In this study, we observed the pellet-conveying behavior of several resins using simultaneous visualization equipment for both the feed throat area and the heating cylinder in order to clarify the mechanism for the occurrence of unstable pellet-conveyance in PBT. Moreover, we studied the correlation between the friction coefficient and temperature of the pellets using a friction measuring machine, and obtained the following results: 1) The visualization experiment confirmed that PBT pellets have a small axial velocity component and a large rotational velocity component compared with other resins, and showed an unstable velocity distribution with changes in the screw revolution rate. When the screw stagnated during the charging process, an airspace occurred temporarily in the solid-conveying area of the heating cylinder. At the same time axial velocity component of the pellet decreased while the rotational velocity component increased. (2) The friction measurements confirmed that the friction coefficient of the resin pellet depends mainly on the temperature. Moreover it was shown that the reverse motion of the screw during the charging process forms an inverse temperature area where the temperature of the screw surface becomes higher than that of the inside wall surface of the cylinder. From these results, it was suggested that unstable pellet-conveyance in PBT results from the changes in friction force balance in the solid-conveying region due to the both of the temperature reverse and the singularity of friction coefficient of PBT.