LSR Molding: Identifying quality problems before they appear

 (c) SIGMA Engineering GmbH

Figure 1 - SIGMASOFT® Virtual Molding analysis considers the interaction between all mold components over several cycles.

Figure 1 - SIGMASOFT® Virtual Molding analysis considers the interaction between all mold components over several cycles.  (c) SIGMA Engineering GmbH
Figure 2 - The thermal analysis in the movable mold half revealed a large temperature gradient between the base and the tip of the silicon-nipple.  (c) SIGMA Engineering GmbH
Figure 3 - Curing degree after 30 s. The assumption of a constant 160°C mold temperature would have mistakenly lead to shorter cycle times than the ones actually required under the real mold temperature conditions.  (c) SIGMA Engineering GmbH

The demand for liquid silicon rubber (LSR) products is growing. Particularly in the medical and baby care markets, its high thermal stability and very good physiological properties make LSR the material of choice for an ever increasing number of applications.

However, molding LSR can be a challenge. In order to maximize profit and reduce scrap, it is important to get a clear understanding of the complete process and to anticipate possible problems, including the flow and curing behavior, as well as the tempering conditions through the complete molding process.

CVA Silicones, in France, reached out to SIGMA in order to get a better insight on one of its product applications. In this case, they wanted to build a four-cavity mold for a silicon-nipple application. The SIGMA engineer on charge, Denis Mercier, was confronted with the task of analyzing the mold behavior and the resulting part quality. The challenge was to completely analyze the mold performance, and to foresee possible quality issues that could arise during production.

In the simulation, the mold starts from room temperature and is “heated” by the heating cartridges until it reaches the production conditions. Once it is there, several molding cycles are virtually “run”, one after each other. In this way, a steady-state is achieved, just as in production, and under this profile the molding analysis of filling, post-pressure and curing stages is completed (Figure 1).

The analysis revealed that with the current tempering layout, the temperature distribution in the movable mold half presented large gradients, as seen in Figure 2: while on the bottom the temperature was 170°C, on the nipple tip it was almost 20°C lower. “This large temperature gradient induced variations in the curing behavior, compromising the cycle time”, explained Mercier.  

This application demonstrates how important it is to consider the “big picture” about the mold thermal performance. For comparison purposes, a “conventional” injection molding simulation was completed, assuming homogenous mold temperature. This simplified approach, assuming a homogeneous mold temperature of 160°C, predicted a curing time of 30 s (Figure 3). However, the analysis of SIGMASOFT® Virtual Molding demonstrated that after 30 s the part’s tip has regions where only 43% curing has been achieved. “Not considering the real thermal behavior of the mold would have led to costly decisions, where the part performance would have been compromised. Under this scenario, most likely a lot of iteration would have been required to find the reason for part failure”, described Mercier.      

Applying the SIGMASOFT® Virtual Molding to LSR molding allows identifying the reasons for possible quality issues early, saving effort and productivity loss when the production of the part starts. Identifying quality and processing issues upfront, even in the design stage, reduces the necessary iterations in part and mold development; as the mold development time is reduced, capacities are released for new projects.

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