A study on surface finishing and weld line strength in micro injection moulding using a variotherm and visual mould

In temperature cycle injection moulding (TCIM) of thermoplastics, the mould cavity surface is heated to a temperature close to the glass transition point of the resin before melt injection, and then cooled after injection is complete. In order to study the weld line developing process and its influence on surface finish in micro injection moulding, a variotherm system mould was used. The effect of TCIM on mechanical properties in the region of the weld line is also the subject of the present work. In this mould, a visualization design was integrated. A tensile bar sample and a flat cavity with an obstacle were used for the study of two main types of weld line. The tensile specimens were produced applying different processing parameters. Weld line depth and surface roughness are determined using an atomic force microscope. Weld line strength of micro tensile samples were tested on the micro tensile test machine. Results showed that the shape of weld lines and roughness are mainly influenced by mould temperature and its interaction with the injection speed. It is necessary to increase the temperature to 10°C higher than Tg to obtain a glossy surface without weld lines. The cooling time of the injected part also affects the weld line depth. Elevated mould temperature and low injection speed were shown to improve weld line strength for each material in the experiment. Weld line strength was shown to be inversely proportional to the impingement velocity. Candidate reasons include air entrainment in the mating surface and superheating of the air compressed air in the region affecting the polymer behavior. The comparison between conventional injection moulding process and TCIM shows how the variotherm system is suitable to be used to improve the mechanical properties and to completely remove the weld lines of plastic parts.