A Simplified Model for the Melting of Polymer Pellets under Compression in a Twin-Screw Extruder
Plastic energy dissipation (PED) is an important mechanism involving the melting of polymer in a twin-screw extruder. In the present study, the melting behavior of polymer pellets under compression is studied by a simplified model with the finite element method, based on the fact that mechanical energy from the deformation of a single pellet is converted into heat energy. The model shows that the heating of the pellet is not uniform. Under compression, the contact area of the pellet yields first, and the deformation in the pellet depends on the compression ratio and properties of the pellet. The simulated results show that the temperature in the volume around the line passing through the contacting point and the center of the pellet is higher than the temperature further from the contact area.
Plastic energy dissipation during compressive deformation of individual polymer pellets and polymer particulate assemblies
Plastic energy dissipation (PED) of individual polymer pellets as well as of densified polymer particulate assemblies may be the dominant heating/melting mechanisms in intermeshing corotating twin-screw extruders (Co-TSEs). In the present paper, the amount of PED is evaluated by conducting the uniaxial unconfined compression experiments on both molded cylindrical polymer samples and polymer particulate assemblies. The compression experiments were extended to a much higher deformation (strain) range. The results of this work clearly demonstrate the significant contribution to melting from even a single deformation of individual pellets. One severe deformation of PP pellets can provide almost one sixth of the energy required for their melting. For PS pellets, one severe deformation alone can provide sufficient energy to heat up the pellets to above their glass-transition temperature. On the other hand, the mechanical energy input during the compressive deformation of particulate assemblies is much lower than that of solid polymer samples, depending, to a great extent, on the degree of the particulate densification/void ratio of the particulate system. There are two types of PED in the melting section of Co-TSEs, localized PED of individual pellets in the partially filled kneading section and global PED of consolidated particulate assemblies in the fully filled section. The mandatory deformation of solid particulates, caused by the interaction of kneading paddle pairs, and the associated PED for both individual pellets and consolidated particulate assemblies, provides very efficient heating/melting of polymer feedstock in Co-TSEs. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 287–298, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10033
Modeling of the plasticating process in a single‐screw extruder: A fast‐track approach
A new mathematical model has been developed to analyze the entire flow field of a single screw extruder under steady-state conditions. Intended as a rational design tool for practising engineers in the polymer processing industry, the model contains no partial differential equations and hence does not require the use of numerical solution techniques. To achieve generality, a generic approach is proposed and has been adopted in the derivation of governing algebraic equations from general conservation laws covering channel geometry, polymer flow speed, equivalent radius in a pipe, material properties, power consumption and heat transfer. The model makes no use of empirical factors or correlation. The validity of the new model has been assessed by comparison with published experimental results. Good agreement was achieved with respect to its ability to predict (a) the solid-bed width profile; (b) the axial pressure profile and (c) the temperature and pressure of the melt pool at the extruder exit. Furthermore, the model can predict other information including the solid-bed velocity in the axial direction and the power consumption. The work has demonstrated the potential of a fast track approach to designing helical extruder screws, while maintaining a level of accuracy comparable with more complex 3D models but without the penalty of computational efforts.