Melt Temperatures and Screw Design

Short-term melt temperature changes can be measured with fast-response thermocouples. A number of studies using a fast-response thermocouple mesh were conducted at Polymer IRC, School of Engineering, Design, and Technology at the University of Bradford, England [121-125]. In these studies, dynamic melt temperatures 

It is interesting to note that the melt temperature variation for the simple conveying screws is quite large. The melt temperature variation at a screw speed of 90 rpm is greater than at 50 rpm-this is true for all three screws. The melt temperature variation at 90 rpm for the simple conveying screws is about an order of magnitude higher than for the barrier screw. 

In simple conveying screws, the helical screw channel extends over the entire length of the screw without interruptions, slots, or barriers. As a result, these screws have poor melting and mixing characteristics. These screws are susceptible to unmelt this is a condition where unmelted particles reach the discharge end of the extruder. 

High melt temperature variations do not necessarily result in large pressure fluctuations. This is evident in Fig. 8.110 where the melt pressure variation is shown for the same three screws at the same screw speeds and barrel temperature profiles. 

The 220 barrel temperature refers to the temperature of the last barrel temperature zone the actual profile is 150-185-205-220. For the 200 barrel temperature the actual profile is 140-170-185-200 and for the 180 barrel temperature the actual profile is 130-155-165-180. For the simple conveying screws, the barrel temperature profile (BTP) has little efi ect on the pressure variation, and the pressure variation is quite small. For the barrier screw, the BTP has a significant effect on the pressure variation and the pressure variation is much higher-as much as an order of magnitude. 

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