a device for producing moldings of plastic material having a cavity therein filled with a fluid comprises a mold cavity , an injector for injecting flowable plastic into the mold cavity , an injector for injecting a pressurized fluid into an interior portion of the plastic , and a side cavity connected to the mold to receive flowable plastic melt expelled by the injected pressurized fluid . the side cavity may be randomly lockable and may comprise a closing stuffer which rests flush with respect to the mold cavity interior walls .