Process for preparing a hollow, rib-reinforced laminated structure

A process for preparing a hollow, ribreinforced, laminated article consisting essentially of:

• a. placing two sheets between opposing mold platens, the sheets being aligned such that the sheet surfaces are in interfacial contact, at least one of the sheets being a thermoplastic material heated to its thermoforming temperature and at least one of the sheets being provided with grooves or integral projections, on the surface which contacts the other sheet, which form fluid passageways, and at least one of the mold platens being provided with a mold cavity to form a shaped article having ribs;
• b. closing the mold platens to contact the sheets and position the mold cavity contiguous to the thermoplastic sheet which is at its thermoforming temperature;
• c. introducing a fluid into the fluid passageways to distend the thermoplastic sheet into its mold cavity forming a shaped article having ribs, while the closed mold platens maintain contact between the sheets in the nondistended areas;
• and
• (d) adhering the sheets in the nondistended areas to form a laminate.