Abstract: Crystallizable mainly amorphous thermoplastic material is crystallized at a temperature exceeding the glass transition temperature of the material (13) by subjecting the material at the elevated temperature to compressive forces. The material thickness as a rule is reduced by at least 20%, preferably by at least 40%. To achieve this a plunger (20) and a mould (30) are moved to a minimum distance at which the thermoplastic material (13) located between the plunger and the mould part is subjected to compressive forces. The plunger and/or the mould part are provided with elements (29, 39) for the setting and maintaining of a temperature specified in advance of the surfaces (23, 33) by which the plunger and the mould part respectively abut against the thermoplastic material.

Abstract: A body (1) of oriented thermoplastic material is converted through heating in an area adjoining one boundary surface of the material portion to a layer (10) of substantially non-oriented material. A film (2) is fixed to the fixing layer (10) by pressing the film with a compression and heating element (50) and the necessary heat is supplied so that the film (2) will be fixed to the fixing layer (10). Since the film is bonded to substantially non-oriented plastic material, the desired detachability in the bond between the film and the material portion (1) is achieved.