Melting conduction with forced melt removal

6 CONDUCTION MELTING WITH FORCED MELT REMOVAL 

Removal of the melt, also discussed in Section 5.1, is made possible, in principle, by two mechanisms drag-induced flow and pressure-induced flow (Fig. 5.4). In both cases, the molten layer must be sheared, leading to viscous dissipation. The latter provides an additional, important source of thermal energy for melting, the rate of which can be controlled externally either by the velocity of the moving boundary in drag-induced melt removal or the external force applied to squeeze the solid onto the hot surface, in pressure-induced melt removal. 

This term is used for an extrusion operation where the barrel is heated for the start-up, but then heating is discontinued and the only source of heat is viscous dissipation. 

Finally, the continuous removal of melt has the added benefit of not exposing polymer melts to high temperature surfaces or regions for long residence times. 

From a mathematical point of view, problems of conduction melting with forced melt removal are far more complex than ordinary conduction melting, because they involve the simultaneous solution of the momentum and energy equations. Moreover, boundary conditions are often ill defined. 

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Conduction Melting with Forced Melt Removal, 201... 

When the melt emerges from the extruder die, its shape must be fixed within a short distance. The haul-off mechanism pulls the solid extrudate forwards, and either air pressure or vacuum forces the outer melt surface into contact with a cooled metal calibrating section. Once the outer skin of the extrudate has solidified, cold-water baths or sprays complete the process. The cooling section is relatively long because conduction is the only mechanism of removing heat from the extrudate calculation of its length is an important part of the process design. 

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