A method comprises the following steps:



providing a pre-processed element made of at least one polymeric material,
introducing the pre-processed element in a female mould part having a forming cavity,
shaping the pre-processed element between the female mould part and a male mould part, the female mould part and the male mould part being movable relative to one another along a moulding direction so as to form a concave object from the pre-processed element.




The forming cavity has a transversal dimension measured transversely relative to the moulding direction, the forming cavity further having a transition zone in which the transversal dimension passes from a larger value to a smaller value, the forming cavity further having a bottom.
During the introducing step, the pre-processed element is placed on the transition zone, so that the pre-processed element rests on the transition zone at a distance from the bottom of the forming cavity.