Comparative study on internal and external release agents - Evaluation of process parameter variation on demolding stresses

The use of Internal Release Agents is one of the most promising strategies to reduce cycle time and therefore production costs in Resin Transfer Molding processes. Hereby, the Internal Release agent is mixed into the resin system. During the injection and the cure-cycle, the Internal Release Agent drifts to the mold surface and builds a passivating layer. This strategy supersedes the need for release coatings and thereby discards the coating of the mold surface by an external release agent before injection. The occurring effects are dependent on the process conditions during the injection and the cure-cycle. However, the knowledge of influencing factors and occurring mechanisms is restricted. Today the process parameters for internal release agents are mostly set by trial and error. In this investigation the demolding stress is examined with a tensile test. Therefore, a pick-up plate is designed to manufacture tensile-test specimens and the dependency of the demolding stress on four process parameters is analyzed: amount of release agent, temperature of mold-surface, number of demoldings, and the kind of a reinforcement structure. Accordingly, the trials show four results. First, a higher amount of internal release agent lessens the demolding forces, whereas a repeated coating with external release agents does not improve the release behavior. Second, a higher temperature leads to lower demolding forces. Thirdly, the number of demoldings do not show an effect on the demolding stress. Last, differences in the demolding stresses are detected by a comparison of three glass fiber architectures even though the reinforcement structures are not in contact with the surface of the tooling.