in an on - line annealing process for rigid laminated sheets of reinforced thermoplastic material wherein an annealing chamber is maintained at a desired temperature by introducing heated air to impinge on a deflection plate within the annealing chamber , annealing of the thermoplastic sheets is improved by introducing heated air through a pair of elongated perforated tubes within the annealing chamber so that heated air is diffused so as to flow evenly across both sides of the rigid thermoplastic sheet as the sheet passes through the annealing chamber .