​The tandem extruder at PPI consists of a 34 mm co-rotating intermeshing twin screw extruder (mixing extruder) feeding a 40mm single screw extruder (cooling extruder). Polymer pellets or powders are meter fed into the hopper of the twin screw extruder. The blowing agent is injected in the twin-screw extruder through either a syringe pump or dual piston HPLC pump at 20 L/D of the first extruder, after a complete melting of the material is achieved.  The screws of the twin-screw extruder are specially configured to ensure good mixing of polymer and blowing agent.  The barrel and the die temperatures of the single screw (cooling) extruder are controlled using four separate oil heaters to achieve proper cooling and temperature control.  The die is mounted on the barrel of this cooling extruder.