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.

Date of Award	1976
Original language	English
Awarding Institution	Aston University