Shorter Cycles for RTM Process
Series production of fiber-reinforced composite parts with a thermosetting matrix employs, among others, the so-called RTM process (Resin Transfer Molding). During process, the RTM molds have a relatively low, constant temperature in order to prevent overly fast curing during the injection phase. In turn, this results in relatively long cycle times.
A research project at the University of Heilbronn investigated whether the cycle time could be reduced by means of dynamic mold temperature control. This involved thermally decoupling the curing phase and the injection phase. During the filling phase, a low cavity wall temperature is maintained so that complete filling of the mold cavity is assured. Then, the mold temperature is increased to accelerate the cross-linking reaction.
For such variotherm processing of reaction resins, the mold temperature control must satisfy two important requirements. On the one hand, the temperature distribution across the surface of the mold cavity must be as uniform as possible to ensure uniform cross-linking of the material system throughout the cavity. In addition the active change of the mold temperature during production should occur as quickly as possible. Accordingly, the heating system and the entire arrangement should have correspondingly high heating dynamics.
Currently, variotherm temperature control systems using water as the heat transfer fluid are quite common. Hot and cold water is directly cyclically through temperature control channels in a mold insert to establish appropriate mold temperatures. Alternatively, electric resistance heating elements can be incorporated into the mold. For good thermal homogeneity, a conformal layout of the electrically conducting layer must be provided.
With electrical heating systems directly beneath the mold's surface, it is possible to achieve faster, higher and more uniform temperature changes than with water-based temperature control. Moreover, controlling to a certain temperature level is relatively easy to implement through regulation of the operating voltage. As a rule, however, fluid systems are still generally employed to cool the mold inserts.