A rubber part to be over-molded was cracking and failing when the second component was injected (Figure 1). The molder reached out to SIGMA to analyze the possible reasons for this failure. In order to consider all possible factors producing the problem, the part was simulated including all its components: metallic insert, elastomeric part and over-molded plastic, reproducing over several molding cycles exactly the times and process parameters actually used in the production floor (Figure 2).
The initial filling analysis showed a large temperature gradient in the part, where the outer regions had a higher temperature than the inner section. In the thermal analysis it was noticed that the cold plastic insert used was acting as a barrier for heat transfer, and therefore impairing the appropriate curing of the failure region (circled region in Figure 3). The curing analysis revealed that after a molding cycle of 383 s, the curing gradient in the part was severe and the average curing gradient in the part was 75%.
It was proposed to pre-heat the insert to 100°C. The same thermal, filling and curing analysis was conducted. In the new scenario, the temperature in the part at the end of filling was more uniform, and the critical region had a temperature 15°C higher. The more homogeneous temperature and the overall larger amount of energy available in the system increased the average curing degree in the part from 75% to 82%, therefore reducing also the curing-induced warpage. The combined effect of a higher curing degree achieved in the affected region (which increases the local mechanical resistance) as well as a lower warpage-induced stress eliminated the cracking.
The warm inserts solved the problem. Additionally, the molding time was reduced down to 370s, 13% compared to the first configuration.