Melt temperature override

Another observation the learner will make about extruder operating features is that the melt temperature overrides the controller set point (of Barrel Zone 3 in this simulator) at very high screw speeds. This maybe caused when the cooling capability is inadequate (Fig. A.5) this example may help the learner understand the concept of shear heating through screw-generated friction. 

Figure A.5 The melt temperature (376 °F) is shown here overriding the barrel zone temperature...

Finally, it should be noted that duplex formation is driven by an enormous and favorable enthalpy change that overrides the entropic penalty associated with organizing the single strands. Thus, duplex stability is very temperature dependent, meaning that even duplexes that are very strong at room temperature (A eq > 10 M ) melt at easily accessible temperatures (70 to 80 C at fiM concentrations). Repetitive annealing therefore provides a facile and rapid route to self-assembled structures that are kinetically inert under ambient conditions. 

Heat transfer, in particular, can be the overriding transport process in many polymer processing applications, and the strong temperature dependence of the viscosity of polymer melts needs to be kept in mind. This, rather than the specific applications considered here, is the major lesson to be learned from this chapter. 

Before much engineering work was needed, it was required that a product be defined. There were several candidates. Two leading candidates were calcium polyphosphate and sodium calcium polyphosphates. Calcium polyphosphate was more difficult to make. There was probably no way that a product of this type could be made without heating raw materials to a clear melt at temperatures near 900 °C. Yields were not as high as with sodium calcium polyphosphate. An overriding property of calcium polyphosphate, making it a poor second choice, was its irritating prickly nature. 

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