Lightweight resin molded product and production method thereof

A method for producing a lightweight, hollow resin molding, comprising:

• injecting a melt of a fiber-containing molding material which comprises a fiber-containing thermoplastic resin having a fiber content of from 10 to 70% by weight and in which the fibers have a mean fiber length of from 2 to 50 mm, into a mold cavity of a mold equipped with a rib-forming member;
• or
• injecting and compressing the melt into a mold cavity of a mold equipped with a rib-forming member;
• then expanding a volume of the mold cavity;
• and
• introducing a gas into the melt of the fiber-containing molding material after the start of the cavity expansion;
• thereby obtaining said resin molding which comprises at least one reinforcing rib in a hollow area.

A lightweight, hollow resin molding, obtained by a method, comprising:

• injecting a melt of a fiber-containing molding material which comprises a fiber-containing thermoplastic resin having a fiber content of from 10 to 70% by weight and in which the fibers have a mean fiber length of from 2 to 50 mm, into a mold cavity of a mold equipped with a rib-forming member;
• or
• injecting and compressing the melt into a mold cavity of a mold equipped with a rib-forming member;
• then expanding a volume of the mold cavity;
• and
• introducing a gas into the melt of the of a fiber-containing molding material after the start of the cavity expansion;
• thereby obtaining said resin molding which comprises at least one reinforcing rib in a hollow area;
• and
• wherein said resin molding has an expanded structure formed by utilizing a spring-back function of said fibers which are entangled in the resin molding;
• wherein said reinforcing rib has a greater thickness than an outer periphery of said resin molding.