Prediction of strain recovery during solid-phase forming of thermoplastics

An approach to predict the strain recovery behavior of polycarbonate (PC) and high impact polystyrene (HIPS) under isothermal and non-isothermal conditions in a solid-phase forming environment is presented in this paper. The constants A and n of a power law relationship of the form ?(t) or δ(t) = Atⁿ, fitted to isothermal creep and stress relaxation data, were determined over a wide range of temperatures for both materials. An expression for isothermal recovery was derived and compared to experimental data. Master curves and the resultant shift factors, obtained by superposing the stress relaxation and creep data (both in tension and compression) were used with the time-temperature super position principle to numerically evaluate uniaxial strain recovery under a specific temperature history. The average temperature history obtained by numerically solving for the temperature distribution in a disc, at an initially high temperature and in contact with a cold metal surface, was used for the non-isothermal case. The theoretical results were compared with recovery data obtained from non-isothermal backward extrusion tests with a temperature history similar to the one described above. Reasonably good agreement was obtained.