An Investigation of Weldline Strength in Injection Molded Rubber Parts

Weldline is one of the typical molding defects and appears at the meeting points of multiple melt flows. The presence of weldlines in rubber products is regarded as a one of the most undesirable phenomena, since it may result in poor mechanical properties as well as unsightly appearances. Injection molding of large/complicated products are usually prepared by multiple compounds, which produces weldlines once the melt fronts are joined by impingement flow. In the present study, the rubber injection mold was designed and constructed in order to investigate the effect of processing parameters on weldline strength of natural rubber. The results revealed that an increased amount of CaCO3 did not affect the weldline strength. The difference in maximum tensile strength between weldline and non-weldline specimens becomes more pronounced with the high loading of SiO2 and CB. This was not solely associated with the changes in cure characteristic of rubber compounds but also the reduction of tack property and elasticity of the vulcanizates. In the case of hot weldline containing specimen, the results suggested that the longer the distance from obstacle the higher the weld line strength.