apparatus for making a foamed plastics product . a foamed plastics product is formed by introducing into an at least substantially fully plasticated material a series of discontinuous gaseous bubbles while said plasticating material is under a substantial and often variable pressure , accumulating said material and intermixed bubbles in an injection zone while maintaining said pressure thereon and thereafter injecting said material into a mold whereupon said pressure is sufficiently released to permit expansion of said gas bubbles and the formation of a foam thereby . said gas is introduced , as aforesaid , by a series of separate injections wherein the pressure on the gas being injected rises from substantially zero to a level potentially in excess of the highest pressure expected to be developed within the plasticating means . thus , regardless of the pressure existing in the plasticated material at any given moment , gas will be injected into said plastic material when the pressure on the gas attains a suitable differential over the pressure in the plasticated material at any given moment and such injecting will be in substantially uniform quantities regardless of the pressure existing within said plastics material at the time of any given injection thereinto . the apparatus aspects of the invention contemplate injecting the gaseous material into the plasticated plastics material by a high - speed positive displacement pump , as a piston pump or a diaphragm pump , wherein each stroke of the pump injects one bubble of gas into the plastics material .