Abstract: A process and associated apparatus used to prepare a thermoplastic composite from a plurality of plies of thermoplastic resin prepregs. The prepregs are formed into a composite structure under two chambers; a rigid outer chamber (of any convenient size or shape) and a second flexible inner chamber containing a prepreg lay-up. The absolute pressures are reduced concurrently in a stepwise method in both chambers. This concurrent, stepped pressure reduction is accomplished at a rate which prevents the vacuum bag from moving far from the prepregs, and prevents wrinkles from forming in the prepregs. Pinching off the diffusion paths required for the removal of unwanted gases is eliminated. The lay-up is then heated and the absolute pressure in the outer rigid chamber is increased. This pressure constrains the flexible inner chamber during out-gassing of the thermoplastic resin, preventing wrinkles from forming in the prepregs.