Melt Temperatures Inside the Extruder

Melt temperatures in the extruder tend to be highly nonuniform because of the low thermal conductivity of polymers. Therefore, it is more efficient to heat the polymer by viscous heat generation than by heat from the barrel heaters. The actual melt temperatures inside the machine can be quite different from the barrel temperature. Also, temperature peaks within the machine are often much higher than the bulk average melt temperature. 

Melt temperatures inside the extruder are difficult to measure because one cannot use an immersion melt temperature sensor along the barrel because the probe will be sheared off by the screw flight. Downstream of the screw, the melt temperature is normally measured with an immersion probe these can be made with adjustable depth of the probe see Fig. 8.106. 

In reality, the temperature distribution is dynamic in other words, melt temperature changes with time. These changes can be significant, but short-term (0-10 seconds) temperature changes cannot be measured with a conventional melt temperature sensor because the thermal mass of the probe is too large. Infrared melt temperature measurement allows detection of rapid (millisecond range) melt temperature fluctuation [114-118]. 

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For temperature measurements on the emerging extrudate, contacting-t)rpe measurements are not suitable because of damage to the extrudate surface. For non-contacting temperature measurements, infrared (IR) detectors can be used. The intensity of the radiation depends on the wavelength and the temperature of a body. Non-contact IR thermometers can be used to determine the temperature of the plastic after it leaves the die. IR sensors can also be used to measure the melt temperature inside the extruder or die see Section 4.3.3.2. 

These are normal temperatures for HDPE and should not cause degradation under normal circumstances. However, it was suspected that the melt temperatures inside the extruder were much higher than the barrel temperature. Therefore, a thermochromic powder with a color transition temperature of 250°C was added to the feed. With this material, a very clear discoloration was noticed in the material exiting the die. Next, a thermochromic powder was added with a color transition temperature of 300°C. This material unexpectedly resulted in very clear discoloration just as in the first material, indicating that stock temperatures in the extruder exceeded 300°C. 

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