Process for producing hollow bodies of thermoplastic material

A process for producing a hollow body having a wall which partially comprises foamed thermoplastic material, by an extrusion blow molding procedure, including:

• co-extruding a preform having at least first and second layers of which at least one layer comprises foamable plastic material;
• foaming said at least one layer of foamable material;
• disposing said preform in a blow molding mold, the mold comprising a plurality of mold parts and at least a portion of the extruded preform material being squeezed off the preform by squeeze edges on respective ones of said mold parts when the mold is closed on the preform;
• and expanding the preform in the mold until it bears against inside surfaces of the mold parts, using an internal increased pressure which is such that the at least one foamed layer is compressed only to an immaterial degree.

A process for producing a hollow body formed by a wall means comprising a plurality of layers of which at least one said layer is of foamed thermoplastic material, comprising:

• providing a generally tubular preform comprising a plurality of layers of which at least one is of foamable thermoplastic material;
• foaming the foamable material of said at least one layer;
• disposing the preform in a cavity of a blow molding mold enclosing the preform, the mold comprising a plurality of mold parts defining a mold cavity and at least a portion of the extruded preform material being squeezed off the preform by squeeze edges on respective ones of said mold parts when the mold is closed on the preform;
• and
• expanding the preform in the cavity in the mold enclosing the preform by introducing a fluid under pressure into the preform in the mold, thereby to cause at least substantially the whole of the outside surface of the preform to come to bear against the inside surface of said mold cavity, to mold the preform to the desired shape, the pressure produced within the preform by the fluid being controlled such that said at least one layer of foamed material remains at least substantially unchanged in thickness by the compression effect of the fluid pressure in the preform.