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.
Experimental and simulation studies of heat transfer in polymer melts
The heat transfer of a single solid cylindrical poly(ether imide) (PEI) pellet in polyethylene (PE) melt under simple shear flow was studied using experiments and numerical simulations. This included two approximate 2-D representations of the geometry, and a rigorous 3-D geometry matching the experimental conditions. A generalized power-law viscosity model for PE melt, and temperature-dependent conductivity and specific heat were used for PEI in the models. Viscous dissipation was included in the simulations. From the experimental temperature history, the mean heat-transfer coefficient h was determined to be 370 W/m 2 .K, with the assumption of a uniform convective environment. From the simulation, h was determined to be 1160 W/m 2 .K and 420 W/m 2 .K in the 2-D simulations with circular and rectangular pellet, respectively, and 250 W/m 2 .K for the 3-D simulation. Results from the 3-D simulation proved that the uniform convection assumption used to calculate h for the experiment was not acceptable.
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.