PROCEDIMENTO DI INIETTOFUSIONE DI MATERIALE TERMOPLASTICO

Foaming in part or all of the molded product is controled by enlargement of the tool cavity depending on tool cavity parameters such as pressure and/or temperature and/or material flow. Cavity volume change occurs either during the injection phase, during the foaming phase or during both phases. Cavity enlargement extends over the whole duration of one or both phases or over part of one or both phases. Foaming during the injection phase may be prevented by cooling the injected polymer in the tool cavity. In the same tool(4,4a) localized areas of the semi-finished molding(8) are then heated(4e) to reactivate the blowing agent and induce foaming(8b) while simultaneously an enlargement of the cavity is achieved through retraction(4f) of moving slides(4d). Slides are shaped so that in their final retracted position they define the final outer shape of the foamed area(8b) of the finished item. A semi-finished product may also be molded in one tool while preventing blowing agent expansion and then moved to a second tool where local areas are heated to reactivate the blowing agent. Simultaneous slide retraction in the tool allows foaming of heated areas as before. In the latter process the tool may have expansion chambers with heating elements which allow uncontroled expansion of the reheated semi-finished product, thus filling the chambers. For production of objects with constant or almost constant cross-section over their entire length a tool is used which comprises a shell-like part(1) with an injection channel which move together with the injection unit and a piston which remains in a fixed position during cavity enlargement. Volume change may be effected by pistons which project into a tool cavity. Either the pistons can be retracted while the tool remains fixed or vice versa.