Twin Screw extruders comprise a pair of parallel screws, rotating inside a barrel with an 8-shaped cross-section. At the exit end, each half of the barrel converges into a short conical section, each with a die at the apex.
Co-rotating twin screws and/or extruders are used in many branches of industry for producing, preparing, and/or processing highly viscous materials. They find a wide variety of applications, especially in the plastics, rubber, and food industries. Co-rotating twin-screw machines usually have modular configurations and are thus quite flexible for adapting to changing tasks and material properties. Well-founded knowledge of machines, processes, and material behavior is required to design a twin-screw extruder for economically successful operations.
Counter-rotating twin-screw extruders are used for a variety of plastic products and processes. It is interesting to note that closely intermeshing, counter-rotating twin-screw extruders can be designed to pump materials in a non-drug flow manner in locked C-shaped chambers. Only this device (and ram extruders) can convey via positive displacement, as compared to the drag flow single-screw extruder and semi-drag flow co-rotating twin-screw extruder. There are two distinct and separate families of counter-rotating twin-screw extruders: • High-speed and energy input, with both intermeshing and non-intermeshing designs, melt the polymer early and are designed as mass-transfer devices, with the primary applications being mixing, devolatilization, and reactive extrusion (HSEI). • Low-speed, late fusion, intermeshing with either parallel or conical screws are designed to avoid energy input and do not melt the materials (typically PVC) until the middle or latter part of the process section (LSLF). High-speed, energy input (HSEI) counterrotating twin-screw extruders can be intermeshing or non-intermeshing. The co-rotating intermeshing mode, as previously discussed, dominates the compounding market, having captured over 90% of current installations. Counter-rotating designs are primarily used for specialty applications—such as high-level devolatilization and reactive extrusion. By contrast, low-speed, late fusion (LSLF) counter-rotating twin-screw extruders are primarily used for PVC and other shear-sensitive formulations that benefit from a design that minimizes energy input combined with pumping uniformity. These devices are often inadequate to perform energy-intensive processing. As implied by the category, the LSLF counter-rotating twinscrew extruder operates with lower rpm than its high-speed cousin. Just like any extruder, control parameters for the counter-rotating twin-screw extruder include screw speed, feed rate, temperatures along the process section, and vacuum level. Monitor-only parameters include melt pressure, melt temperature, and motor amperage. The motor (AC or DC) inputs energy into the process via interacting twin screws imparting both shear and energy. Higher screw speeds result in more shear for a given screw design. Barrel sections are electrically heated and cooled by liquid or air, depending upon the machine configuration and heat-transfer requirements of the process
