Cap for containers, plant, method and forming device for making said cap

A method for making a cap, comprising, in sequence, the steps of:

• providing a cap for a container, the cap having a press-twist configuration and comprising:
• a metal capsule having a transversal wall of longest extension which extends transversely with respect to an axis of the container, and a lateral wall which extends up to a first height H1 with respect to the transversal wall about said axis of the container,
• a curling in a distal part of said lateral wall relative to the transversal wall,
• a sealing gasket with an annular shape for engaging with an opening of the container, the sealing gasket comprising a transversal portion, positioned in contact with the transversal wall of the capsule, and a lateral portion, positioned in contact with the lateral wall of the capsule and ending at a height H2 with respect to the transversal wall of the capsule;
• wherein the sealing gasket is made of a material which is free of PVC and wherein a ratio H2/H1 between the second height H2 and first height H1 is between 0.55 and 0.90;
• wherein the lateral portion comprises a first distal portion relative to the transversal wall of the capsule and the first distal portion relative to the transversal wall has a plurality of peaks configured to engage around a projection present close to the opening of the container to improve a closing seal with respect to the container;
• wherein the plurality of peaks are arch-shaped toward an interior of the cap;
• forming a disk by punching a metal sheet and deforming said disk to make the capsule;
• curling the capsule, in which the curling is made in the distal part of the lateral wall relative to the transversal wall of the capsule;
• forming the sealing gasket, comprising the steps of depositing a predetermined quantity of material free of PVC inside the capsule and subsequently deforming the material free of PVC inside the capsule to create the sealing gasket.