Method for manufacturing synthetic resin containers

A method for manufacturing a bathtub, comprising the steps of:

• (a) pre-forming an inner surface layer component comprising a polymethyl methacrylate resin sheet by thermoforming onto a mold using a vacuum forming method;
• (b) fitting the inner surface layer component onto a male die;
• (c) forming a reinforcing layer by inserting a molten synthetic thermoplastic resin into the inside of a female die wherein the female die further comprises at least one independently movable portion;
• (d) fitting the male die onto the female die and bringing the male die and the female die close together to compress the reinforcing layer within a cavity defined between a back surface of the inner surface layer component fitted on the male die and the female die;
• (e) injecting compressed air into the reinforcing layer through at least one air nozzle formed in the female die while enlarging the cavity between the male and female dies so as to correspond to a predetermined thickness of the synthetic resin container thereby forming hollow portions within the reinforcing layer and after a lapse of a predetermined time period, moving the independently movable portion in a direction so as to enlarge said cavity between the male and female dies to form a reinforcing rib within said hollow portions;
• (f) causing the compressed air to press the reinforcing layer against the entire back surface of the inner surface layer component and the female die;
• and
• (g) cooling the synthetic resin of the reinforcing layer to solidify the same and releasing the bathtub from the dies.