A method for the injection molding of internally hollow parts includes the steps of: injecting melt of thermoplastic material in the inner space between a punch and a matrix; opening a valve in order to place the inner space in communication with a secondary compartment; stopping the injection of the melt and opening a valve for supplying nitrogen in the inner space in order to create a cavity within the part being molded; stopping the supply of nitrogen and simultaneous perforation of the part being molded in the section of the connecting duct for the outflow of nitrogen from the mold and the subsequent possible collection thereof; and supplying pressurized air for washing towards the atmosphere after stopping the supply of nitrogen and the simultaneous perforation of the part being molded in the section of the connecting duct.

Methods of making interior panels having integrated airbag deployment doors for motor vehicles are provided. In one example, a method of making an interior panel comprises contacting a molten resin in a molding tool cavity with a plurality of juxtaposed pressurized gas streams to form a series of holes, bubbles, knit lines, or combinations thereof in the molten resin. The molten resin is solidified to form a substrate. The series of holes, bubbles, knit lines, or combinations thereof define a frangible tear seam in the substrate.