Injection mold flashing of liquid crystalline polymers

✍ Scribed by Kurt F. Wissbrun

• Publisher: Society for Plastic Engineers
• Year: 1991
• Tongue: English
• Weight: 709 KB
• Volume: 31
• Category: Article
• ISSN: 0032-3888
• DOI: 10.1002/pen.760311510

✦ Synopsis

Abstract

Thermotropic polyesters, such as Vectra (Hoechst Celanese), have excellent moldability for intricate parts that require high precision of form, such as electronic connectors. Two apparently contradictory aspects of molding behavior contribute to the moldability. On the one hand, the low viscosity of the liquid crystalline polymer (LCP) at high shear rates favors ease of filling molds that contain long, thin paths. On the other, parts molded from LCP have little or no flash to interfere with the functioning of the parts.

There has apparently been little work on the rheological aspects of flash formation. An approximate analysis is made by considering that the flash is the result of melt being extruded from the mold cavity into a slit at the mold parting line. The driving force for the extrusion is the injection pressure. The flow is assumed to be isothermal until solidification occurs, at a time that depends on the thickness of the slit, on the thermal diffusivity of the melt, the melt and mold temperatures, and on the solidification temperature of the material. The viscosity is assumed to have power‐law dependence on shear rate. It is found that when the aspect ratio (length to thickness) of the flash is small, its length is strongly dependent on the magnitude of the pressure drop at the contraction from the cavity to the slit.

At the minimum pressure required to fill a mold, the flash length is predicted to be independent of the rheological and thermal properties of the melt, except for the power‐law exponent. Differences in end correction can, however, account for different tendencies to flash at equal moldability.

Comparison of the model with Richardson's analysis of freezing in a cavity suggests a correlation of the thermal properties of the melt with his parameter c, which is related to mold filling ability. Tests of the model and possible refinements are suggested.