Uniform wall thickness This solves the problem of uneven mold shrinkage stemming from difference in wall thickness. The thick portion is designed uniform by reducing wall thickness, but the core is made as large as possible. A small core may be heated so that the molding cycle becomes longer. Thin wall thickness Products should be designed to have thin wall thickness in the range that causes little problem in product functions, as thin wall thickness is advantageous for the molding cycle. Symmetrical shape If the product has a shape such that a circumference rib and core shape may cause a temperature differential between the stationary platen and the movable platen, warpage may occur by unbalanced mold shrinkage stemming from a rib tip or a temperature difference. In this case balance the mold shrinkage by employing symmetrical shape. Reinforcement by ribs Warpage cannot be avoided in some product configurations. In the case, reinforcement ribs are made in order to prevent inclination and warpage. Rib thickness is set 1/3-1/2 product base.