Rheological and Curing Characteristics of Thermosetting Plastics as Related to Injection Moulding

Abstract

This thesis discusses the measurement of the flow characteristics of thermosetting plastics relevant to injection moulding. Three basic techniques have been used, a capillary viscometer, a variable torque rheometer and an oscillating disc rheometer. The capillary viscometer measured’ the shear stress/shear rate characteristics of a compounded melt prepared in the injection barrel and the results correlated well with those of a capillary viscometer fitted in place of the - nozzle of the injection machine. Phenolic, urea and melamine formaldehyde materials show a power law relationship between shear stress and apparant shear rate up to shear rates of 1O3 sec-1. At very low shear rates, phenolic and melamine materials show Bingham flow behaviour merging into laminar flow. At high shear rates, the phenolic shows plug flow as the melt emerges from the injection nozzle and mould gate. The capillary viscometer gave different results when used with granular (i.e. non-worked) materials and only gave a general indication of the material viscosity. The variable torque rheometer gave an empirical measure of the duration of fluidity of a thermosetting material which correlated well with results obtained during injection moulding trials. Only the oscillating disc rheometer viscosity recorded changes throughout the complete crosslinking reaction. The dynamic torque measurement is different to the continuous shear viscosity measurements made with the capillary and torque rheometers. _ There is a general correlation between their measurements but more important is the possibility of correlating the oscillating torque value (after separation into elastic and viscous components) with the crosslinking process and the chemical reactions involved during the material formation and moulding processes. The oscillating disc rheometer enables the cure rate of a material to be determined as well as the chemical kinetic reaction constants. An Arrhenius type relationship exists between the temperature, and gelation and cure time. The thesis shows how the results of each of the three techniques correlate well with the injection moulding process that the three techniques form a necessary, complementary combination of data and which are not mutually replaceable. These techniques enable the injection moulding behaviour of materials _ to be predicted, although injection moulding results show that the process itself involves a complex interaction between machine, mould and material and that none of the proposed or current techniques will provide a precise measure of ‘injection mouldability'. The proposed technique does however, provide a better Foundation on which to base material control and evaluation studies.