Patent ID: 7939009

Claim:
Method for making concave objects in polymeric material, comprising: compression molding of preforms comprising an upper neck which maintains the same form of the final concave object and a hollow body, joined to the neck, and the subsequent deformation treatment of the hollow body under hot temperature wherein the hollow body is dilated to assume a greater capacity and a correspondingly reduced thickness; wherein the compression molding of preforms comprises: providing a mould having a matrix comprising a first matrix part ( 20 ) to form the outer surface of the upper neck ( 91 ) and a second matrix part ( 30 ) to form the outer surface of the hollow body ( 92 ), and a punch to form the inner surface of the preform, the second matrix part ( 30 ) comprising a deformable wall ( 31 ); inserting a metered mass body of polymeric material within a matrix cavity, the reference mass value of the metered body being calculated such that an error results as an excess mass of polymeric material, and pressure inserting the punch into the matrix cavity with progressive penetration movement of the punch relative to the matrix until, at the end of the penetration movement, punch ( 11 ) and the matrix cavity give rise to a molding chamber ( 7 ) which confers the desired shape to the preform ( 9 ), wherein, in the penetration final step, the polymeric material completely fills the matrix cavity, and thereafter, further punch penetration movement relative to the matrix causes a pressure increase of the polymeric material, which pressure increase causes the elastic deformation of said deformable wall ( 31 ), thereby increasing the thickness of the entire perimeter of the hollow body transversal section, and absorbing the excess mass error of the metered mass body.