Thermoforming process

A method of thermoforming in which a thermoplastic resin sheet heated to be softened is brought into contact with an inner circumferential surface of thermoforming female die formed generally into a bowl-shaped recess, the method comprising the steps of:

• communicating a rim forming recess formed generally into a shape of a bowl-shaped recess with a bottom opening and forming a bottom wall of the bowl, an outer wall, a bottom wall and an inner wall of a rim of the bottom, wherein the rim forming recess includes a first portion spread outward so as to be continuous from the bowl bottom and a second portion continuous from the first portion and having a reduced inner diameter, the first and second portions forming the outer wall and the bottom wall;
• supporting a bottom plate inside the rim forming recess so that the bottom plate is moved forward and rearward relative to the rim forming recess;
• bringing the thermoplastic resin sheet heated to be softened into contact with an inner circumferential surface of the bowl-shaped recess and immediately thereafter, moving the bottom plate to the second stop position so that a rim is formed;
• spreading the rim out from the bowl bottom opening and reducing its inner diameter to form a spread rim outer side wall surface and at the same time forming a bottom wall of the rim formed by a flat portion where the inner diameter is reduced;
• welding the rim inner wall surface turned up in the bowl bottom opening such that the rim forming recess has a generally triangular section and a cavity is formed inside the rim;
• and
• retreating the bottom plate to the first stop position.