The present invention teaches an assembly and corresponding method for producing a plastic part having a sandwich mold including an upper half and a lower half. Each of the halves exhibit an opposing inner face which, upon assembling, collectively define an interior cavity corresponding to a configuration of the part to be produced. An insert sheet of a polymer material (typically a thermoplastic at room temperature) is placed upon a lower of the mold halves and prior to assembling the upper mold half. A heat source is communicated to the closed mold to cause the insert sheet to melt into the cavity. A cool source is subsequently communicated to the closed mold to cause the thermoplastic to harden within the cavity and prior to opening of the mold and removal of the finished part.