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.
Modeling of Pressure-Induced Melt Removal Melting in Vane Extruder for Polymer Processing
The assembled vane extruder with seven groups of vane plasticizing and conveying units (VPCUs) was designed to study the melting phenomenon and mechanisms in vane extruder. There were two melting stages during the melting process, including the solid-rich melting stage and the melt-rich melting stage, and there were no obvious boundaries between the solids and melts. The pressure-induced melt removal melting was the dominant melting mechanism in the solid-rich stage. The theoretical model of the pressure-induced melt removal mechanism was established. The melting percentage was defined to evaluate the boundaries between the solid-rich melting stage and melt-rich melting stage. The melting percentage is proportional to a three-quarters power of the temperature at stator's inner surface, a quarter power of solids’ pressure, as well as inversely proportional to the rotating speed of rotor. By comparing theoretical calculations with experimental measurements of the melting percentage of material low-density polyethylene extruded by the first VPCU, it was indicated that the theoretical model and experimental measurements were consistent with the same tendency.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths