Processing Twin Screw
Processing Twin Screw Extrusion Systems for Biobased Resins Article contributed by Charlie Martin, American Leistritz Extruder Corporation, Somerville, NJ, USA Leistritz Bio-Screws H i g h speed, energy input (HSEI) twin screw extruders are emerging as the manufacturing methodology of choice to process bio-based renewable materials. Applications include compounding, reactive extrusion, devolatilization and foaming. The final product can be a pellet to facilitate accurate and consistent feeding of secondary processes. There is also a trend to bypass the pelletization step and to produce a film, fiber, sheet or profile from the twin screw extruder. This is referred to as direct extrusion. Although similar to traditional plastic processes, there are subtle differences that take into account the heat- and shear sensitivity of bio-based polymer compounds. HSEI twin screw extruders are starve fed with the output rate determined by the feeder(s). Feeders meter solids (pellets/ fillers, etc.) or liquids (including supercritical CO 2 ) into the HSEI twin screw extruder. The extruder screw RPM is independent and is used to optimize compounding efficiencies. Because the pressure gradient is controlled and can be designed to be zero at specific locations in the process section, materials are easily introduced into downstream barrel sections by a side stuffer that “pushes” fillers (calcium, talc, etc.) into the extruder. Downstream side stuffing is especially beneficial for shear sensitive fillers, such as woodfiber or starch. The controlled pressure profile also facilitates venting to remove moisture and other volatiles that are inherent with many biobased formulations. By utilizing multiple vents, it is possible to remove 20% (or more) volatiles. HSEI twin screw extruders process materials bounded by screw flights and barrel walls. Screws are segmented and assembled on splined shafts. Barrels are also modular, and uti- 14 bioplastics [06/02] Vol. 1
Processing lize liquid cooling. A typical process length to diameter ratio (L/D) is 32 to 48, with up to 60 L/D (or more) being possible to facilitate reactive processing and/or multi-stage devolatilization. The motor inputs energy into the process via rotating screws that impart shear into the materials. Segmented screws/barrels, in combination with the controlled pumping and wiping characteristics of the HSEI twin screw extruder, allows screw/barrel geometries to be matched to the process tasks. Solids conveying and plastication occurs in the first 1/3 of the process section. Screw elements for mixing and devolatilization are then integrated into the design. Discharge elements finally build and stabilize pressure to a die. Screw designs can be shear intensive or passive based upon the materials being processed. For instance, the early portion of the screw design can be shear intensive, if required for alloying, and the latter part less aggressive to accommodate the shear sensitive nature of many bio-based fillers or additives. Strand or die face cut pelletizing systems can be mated to the HSEI twin screw extruder to make pellets. Cooling may be in air or water, depending upon the composition of the formulation. Pelletization facilitates the consistent feeding, transport and packaging of the pellets to a single screw extruder or injection molding machine. As previously stated, HSEI twin screw extruders are also used to combine compounding/ devolatilizing with direct extrusion of sheet, film, fiber, laminates, profile and other extruded products. Typically, a gear pump or screw pump is attached to build and stabilize pressure to the die. The system utilizes a PLC to implement a pressure control algorithm program to maintain a constant die inlet pressure. Close to 100% of bio-based polymer compounds have been processed, at some stage, on a HSEI twin screw extruder. It is important to also recognize that the HSEI twin screw extruder is just another tool, albeit a powerful and versatile one, in the process. Integration of the upstream and downstream systems, in combination with the formulation, are all equally important to optimize the process. www.leistritz-extrusion.com bioplastics [06/02] Vol. 1 15
