A method for making products made of plastics comprises the steps of moulding a piece made of polyethylene PE, or made of polypropylene PP, and mechanically folding in a guided manner a flap portion of the aforesaid piece at a folding temperature that is greater than ambient temperature and less than the alpha-type intermediate physical transition temperature of the material. The method reduces the spring-back of the folded portion and the yield in the folding zone.