Evaluation of Weld Line Strength in Low Density Polyethylene Specimens by Optical Microscopy and Simulation

Investigation on the effect of different processing parameters onto injection moulded low density polyethylene (LDPE) had been done. The LDPE was moulded into dumbbell shape with weld line to better understand the flow front of the polymer melt and where two flow fronts meet. Three parameters were chosen to shear modify the LDPE, which were injection speed, injection pressure and mould temperature. Tensile testing was being done onto the dumbbell samples to evaluate the strength of the weld line produced. Differential Scanning Calorimetry was being done onto the weld line region to obtain the degree of crystallinity but was found to be not a good method for strength evaluation. Measurement of the thickness of the weld line under optical microscope showed that it was related to the tensile strength of the dumbbell samples with inverse relationship. Simulation of the polymer melt flow was also being done using CADMOULD to visualise the effect of the chosen processing parameters onto the polymer flow front and was found to be able to evaluate the weld line strength.