A plastics injection molding machine includes an injection mold having a cavity for molding a part with channels of relatively larger and smaller volumes. Plastics material and pressurized gas are injected into the cavity and are forced to flow into the channels. A flow restricting element is selectively extendable into the smaller volume channel to decrease the rate at which the plastics material flows into the smaller volume channel in order to balance the distribution of plastics material and gas among the channels, and develop a part with substantially uniform cross-sectional material wall thickness desired gas penetration length in hollow channels.