Paper to the Polymer Processing Society European Meeting, Strasbourg, 29th-31st August, paper 5-3.

S F Bush

Introduction

To view the introduction, please click on the link: ModellingLace

Paper at the Polymer Processing Society European Meeting in Prague, 21st-24th September 1992, Paper 6-06.

S F Bush with O K Ademosu, D R Blackburn and F Yilmaz

Introduction

To view the introduction, please click on the link: LFRmoldings

Paper to the 3rd International Conference of the Polymer Processing Society, Stuttgart, paper 05-7, 11th-15th April 1987.

S F Bush

Summary

Arguably the main problems in injection moulding reside in post-moulding distortion, manifested as shrinkage, warping and sinking. A major contribution to these problems is the existence of non-equilibrium conformations or shapes of the polymer chains in the post-mould state. As is well-known, the degree of this non-equilibrium condition varies throughout the moulding, usually being greatest in the wall regions closest to the gate. This variation arises essentially from the many large differences in strain rate experienced by a chain flowing into and within a mould, coupled with the fact that production rates of cooling do not, in general, allow chains time to fully relax while still in the fluid state.

During the injection process, the conformation of the chains, particularly their degree of entanglement, determines, with the temperature, the local viscosity. Both viscosity and conformation change with time and from point to point. In fact chain shape (and therefore viscosity) is the product of local conditions inherited from upstream. Mathematical models of injection moulding, however, customarily treat viscosity, where it is not taken as constant, as an empirical function of the local variables usually a principal strain rate and temperature.

By contrast the approach adopted by this paper is to employ a formulation which treats chain conformation and viscosity as dependent variables to be calculated within the computation along with the usual microscopic variables, namely bulk velocities and temperature. The chain shape is characterised by variables whose local rates of change are functions of the shape variables themselves, the velocity gradients and temperature. The chain shape in a finite region of the mould at any instant is then obtained as the resultant of the local rate of change and values of the chain shape convected into the region from upstream. The local viscosity and elasticity are then obtained. In this way we both obtain a prediction of molecular orientation and account directly for the viscosity and elasticity memory.

The results of this formulation are presented as simulations of three-dimensional injection mouldings of a variety of basic shapes. The simulation model is part of a design system which includes a graphical interpreter of mould cavity detail. Velocities, temperatures, pressure and molecular orientation are displayed by the post-processor as colour-graphic contours at various stages of mould-filling.

See also the section on Polymer Morphology & Fibre Reinforcement Mechanisms.

Report to T & N Materials Research Ltd.

S F Bush

Summary

A large number of 3 mm plaques have been made and tested using commercial resin formulations and a variety of potential reinforcements.

The resin formulations were modified by the addition of a diol, ethylene glycol, or triol, glycerol, and initially different catalyst concentrations were explored. These modifications were designed to alter the cure rate and flexural modulus with unpredicted effects on the strength.

The reinforcements used were available commercial types: 1.5 mm fibres sized for urethane applications; 3 and 6 mm fibres sized for olefine applications; hammer milled glass of approximmately 150 μ in equivalent diameter; wood flour of maybe 20 to 50 μ equivalent diameter.

A number of post-curing regimes have also been explored. These involve some hours of heating at 100 and 150 ^oC broadly corresponding to temperatues likely to be attained near an engine.

Flexural strengths and flexural moduli have been determined by 3 point bending of 60 x 10 x 3 mm strips. Reasonable consistency has been obtained particularly when allowance is made for the occasional cavity present on the breaking section. For this reason it is thought that the strength results are conservative, tending if anything to underestimate the potential strength.

See also the section on Polymer Morphology & Fibre Reinforcement Mechanisms.

Report to Lucas Electrical Systems Ltd.

S F Bush

Summary and Conclusions

1. As commissioned, the report covers the following points: the experimental observation with strain-guages of the strains developed in a typical knob assembly; an estimate of the strains likely to be developed in the assembly by reference to the dimensions of the cap and case as found from moulded specimens; an estimate of the likely stresses resulting; and finally the implications for crack development.
2. The strain-guages fitted performed well and reproducibly on the same specimen over a period of months. The estimates of strain are based on the considerable deviations from design recorded on six specimens and compare fabourably with the observed values. The application of the lubrication oil supplied did not make any apparent difference over a period of days and weeks on the specimen supplied. The experimental approach adopted appears satisfactory for the class of moulding supplied.
3. The estimate of stress developed is very tentative, based as it is on point property data supplied by the manufacturer for the grade of ABS used. The tensile stresses in the worst fit case correspond to about 30% of quoted yield strength.
4. This stress would probably be sufficient to propagate a microcrack of about 0.5 μ, which could correspond to a distinct, but invisible-to-the-naked-eye weld line feature. A macromolecular defect of say 0.06 μ corresponding to a good weld might require about 100-120% of quoted yield strength to propagate.
5. A crack could thus result from a combination of circumstances: a more distinct weld line than usual, probably arising from a small unnoticed change of moulding conditions, together with a larger-than-average strain in the blister region of the case, of the magnitude estimated. The design should thus avoid weld lines in tension-stressed parts of the assembly, especially if as is sometimes the case, they correspond to the coolest part of a mould.

 
See also the section on Polymer Morphology & Fibre Reinforcement Mechanisms.