Pre-heating of thermoplastic sandwich materials for rapid thermoforming

The thermoforming of thermoplastic sandwich structures based on the same ma trix polymer for the face sheets and the core requires precise temperature control both before and during forming. Two opposing requirements must be s atisfied: the face sheets must be heated to above the glass transition temp erature of the polymer, T-g, whereas the temperature of the core should not approach T-g in order to avoid the collapse of the foam cells. A numerical model was used to determine the optimal thermal parameters for thermoformi ng using an inverse method. The conductivities of the skins, the interface, and the foam were first calculated by the inverse method and then implemen ted in a thermal analysis. It was shown that the thermal requirements for t he skins and the foam cannot all be fulfilled using a classic one-step heat ing route; therefore, a two-step procedure was proposed. The numerical tool allowed the principal heating parameters, namely the temperature of the he ating plates and the holding and transfer times, to be determined.