when injection - molding a molten thermoplastic resin into a molding having different thicknesses , a void inducing member is set in a cavity corresponding to heavy sections . then gas pressure is applied passing through a gas passageway to a tip end of the void inducing member , thus forming a void nucleus in the resin adjacent to the tip end of the void inducing member . the void nucleus develops to a void by shrinking force of the resin as the molten resin cools .