Plastification & Melting in Twin Screw Extruders | Technovel
The Importance of Plastication and Melting in Twin Screw Extruders
In a twin screw extruder, the two screws apply intense shear stress to the material in a repeated manner. This produces several heat generation phenomena that interact in complex ways, driving the plastication and melting of the resin. The plastication and melting stage accounts for a large share of the extruder’s total energy consumption. It also has a strong influence on the final dispersion state and on product properties, which makes it a critical process step. Optimization of the plastication and melting section, together with operating conditions, is therefore a central theme that determines the performance of the entire twin screw extrusion process.
Plastication and melting inside a twin screw extruder does not rely on a single heat source. Several heat generation mechanisms act at the same time within the extruder, and the resin melts as these mechanisms overlap in complex ways. In general, the heat that contributes to plastication and melting can be organized into four categories: (1) heat from plastic deformation, (2) heat from viscous dissipation, (3) heat from friction, and (4) heat conducted from the barrel heaters.
Heat from Plastic Deformation
When plastic pellets are compressed and deformed by the screws, the crystalline and molecular structures inside the material undergo deformation. Part of the external energy applied during this plastic deformation converts into heat and raises the material temperature. In the solids conveying zone in particular, pellets that have not yet fully melted are deformed under strong compressive stress against one another, so heat from plastic deformation tends to become pronounced.
Heat from Viscous Dissipation
Once the resin begins to melt, the material behaves as a viscous fluid. When this fluid is subjected to strong shear, internal friction converts mechanical energy into thermal energy. This phenomenon is called viscous dissipation. In a twin screw extruder, the kneading and mixing zones generate very high shear rates, so heat from viscous dissipation becomes very large. With high viscosity polymers in particular, viscous heating often emerges as the dominant cause of resin temperature rise. Viscous dissipation raises resin temperature efficiently, but under excessive shear it can also trigger thermal degradation and a drop in molecular weight, so careful optimization of operating conditions is essential.
Heat from Friction
Inside a twin screw extruder, the molten resin, the screws, and the barrel are in continuous contact and relative motion. The friction produced by this contact is another important source of heat. In the solids conveying region, unmelted pellets repeatedly contact and slide against the barrel and screw surfaces, and the resulting frictional heat contributes to the rise in material temperature. This frictional heat assists material heating in the early stage of melting and accelerates the transition from pellet to melt.
Melting by Heat from the Barrel Heaters
External heaters are installed on the extruder barrel, and heat is supplied from the barrel wall to the material based on set point temperatures. Conduction from these heaters plays a key role at startup and under low rotation conditions. In a twin screw extruder, the internal heat from mechanical shear is large, so once operation stabilizes the heaters often shift to a role of temperature maintenance and fine adjustment. In the early stage of melting, however, plastication and melting cannot proceed in a stable manner without sufficient external heating.
Visualizing Plastication and Melting Inside a Twin Screw Extruder
Technovel’s specially configured ULTnano15 uses a vertically split barrel option, which allows direct observation of plastication and melting behavior inside a twin screw extruder. In the plastication and melting zone, solid pellets are conveyed by the screws and gradually compressed. Plastic deformation, friction, viscous dissipation, and conduction from the heaters overlap in complex ways, and the pellets can be seen turning into a continuous melt.
This kind of visualization is highly useful for understanding what actually happens inside a twin screw extruder, and it offers valuable insight for screw configuration design and for the optimization of operating conditions.
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ABOUT THE PUBLISHER
Technovel Corporation — Extrusion Machinery Specialists
Osaka based Technovel specializes in extrusion machinery. We built the world’s first horizontally multi screw extruder, and our Quad and Octa screw extruders now serve diverse industries. Our twin screw range runs from the world’s smallest 6 mm lab unit, through our best-selling 15 mm model, to large production machines. This column shares the know how behind them.