Method for making a package in sheet material

A method for making a package in sheet material, the method comprising the steps of:

• providing a sheet (100) of packaging material, comprising at least one layer of paper;
• providing a mould (20) comprising a mould cavity (22) delimited by a forming surface (22A) and having a supporting surface (24) that surrounds the mould cavity (22),
• wherein the forming surface (22A) has a top edge (22B) extending along a pre-set closed path (K);
• positioning the sheet (100) over the mould cavity (22) and resting on the supporting surface (24);
• driving a withholding member (26) to press against the supporting surface (24) a portion of the sheet (100) resting on the supporting surface (24) itself;
• while the withholding member (26) presses the sheet (100) against the supporting surface (24), driving a forming member (28) provided with an operating surface (28A) having a shape complementary to that of the forming surface (22A) of the mould cavity (22) to push the sheet (100) into the mould cavity (22) and bring the sheet (100) into contact with the forming surface (22A) of the mould cavity (20), at the end of the action of the forming member on the sheet there being formed a hollow portion (102) surrounded by a perimetral portion (104) that extends along a predefined profile (K1) corresponding to the pre-set path (K),
• wherein the forming surface (22A) of the mould cavity (22) and the operating surface (28A) of the forming member (28) are configured for providing a region of the hollow portion (102), extending along the predefined profile (K1), that defines, with the perimetral portion (104), an angle (q) equal to or greater than 90°;
• and
• operating a pressure member (32) to press the perimetral portion (104) of the first sheet (100) against a contrast surface (34) so as to subject the perimetral portion (104) of the first sheet (100) to an action of stretching along a path parallel to the predefined profile (K1), wherein provided in the pressure member (32) or in the contrast surface (34) is an annular groove (34B) that extends in a direction parallel to the predefined profile (K1) of the perimetral portion (104) pressed between the pressure member (32) and the contrast surface (34), and provided on the contrast surface (34) or on the pressure member (32) is an annular relief (32B) having a complementary shape that is to penetrate into the annular groove (34B), the method comprising, via said annular relief (32B), pushing the perimetral portion (104) of the sheet (100) into said annular groove (34B) so as to exert an action of stretching of the perimetral portion (104) in a direction transverse to the predefined profile (K1).