Improving weldline strengths of injection moulded parts by ultrasonic oscillation

Injection moulding has been one of the most important methods for the manufacture of plastic products. However, there are several unsolved problems that confound the overall success of this technique. The forming of weldlines wherever polymer flow fronts meet is one of them. In this report, a method for ultrasonic oscillation assisted injection moulding was proposed to improve the weldline strength of moulded parts. A dumbbell cavity equipped with an ultrasonic oscillation unit was used to mould the parts. An amorphous polystyrene was used for all experiments. Experiments were carried out on an 80 ton reciprocating screw injection moulding machine. After moulding, the strengths of the weldline were measured. The effects of various processing parameters were investigated. The experimental results suggested that ultrasonic oscillation could improve the weldline strength of injection moulded parts for melt temperatures up to some optimum temperature; after that, the effect of assisting oscillation was found to relatively limited. Additionally, the morphological observation at the weldline interface was carried out under a scanning electronic microscope. The result suggested that the increase in the weldline strength was mainly due to the molecular entanglement across the weldline interface. With the ultrasonic oscillation, the optimisation of weldline strengths can be obtained by an appropriate combination of the melt temperature, ultrasonic oscillation delay time and oscillation time.