Abstract: A process for producing reinforced laminate plastic panel structures by thermoplastic compression employing internal mold mandrels which are significantly embrittled during or subsequent to the compression step. Hat stiffened panels formed from two thermoplastic resin sheets may be produced by using elongated mandrels formed of ceramic particulate material adhered together by a binder such as an alkali metal silicate. The mandrels are arranged between a skin sheet and hat stiffener sheets with the respective sheets disposed between the platens of a hydraulic press or other suitable compression molding mechanism. Sandwich laminate structures can be formed employing free thermoplastic resin sheets with a first set of ceramic particulate mandrels interposed between one adjacent pair of sheets and a second set of such mandrels interposed between the other adjacent pair of sheets.