PROBLEM TO BE SOLVED: To prevent void-based irregularities in plastic moldings with a thin wall from being formed.;SOLUTION: The void-based irregularities in the plastic moldings (40) with the thin wall like a drinking cup during injection molding, are prevented from being formed in the thin wall part (42) of the moldings. The thin wall part (42) is formed in the zones (25, 26 and 27) of a thin wall forming cavity part (20) located between adjacent flow channel chambers (21), and in these zones (25, 26 and 27), the thickness of the thin wall forming cavity part (20) increases in the internal main flow direction of the adjacent flow channel chambers (21) to the zones (25, 26 and 27). The formation of such irregularities in the air gap can be prevented from occurring by setting the dimensions of a mold cavity (48) so that the thickness of the cavity part (20) increases in the flow direction within the range of less than a threshold rate. The flow channel chamber (21) does not extend as far as the edge part (28) of the thin wall forming cavity part (20). In addition, the dimensions of each of the cut regions of the adjacent flow channel chambers (21) in the final zone (27) of the adjacent thin wall forming cavity (20) to the edge part of the mold cavity (48), are smaller than those of the upstream parts of the adjacent flow channel chambers (21).;COPYRIGHT: (C)2002,JPO
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