Contributed by: Beth Foederer, PE Optex Process Solutions, LLC
Typically the OEM supplies a thermocouple for melt temperature readout at or after the discharge end of the extruder. The most common type is fixed depth, flush mount. This type is designed for the thermocouple to be in contact with the polymer melt flow to measure and feedback the melt temperature PV (present value). Additional melt probes less commonly used include adjustable depth thermocouple probes and infrared probes. These also are designed to contact the melt to measure and feedback the PV of the melt.
All of the melt temperature probes must be mounted in a port in either the extruder barrel or the polymer flow piping. This port can be located at any position in the system that provides a relatively smooth and straight internal flow surface. The port must be drilled through the steel component to the polymer flow channel so the probe can be mounted to be in contact with the molten polymer.
A flush mount probe is positioned such that the tip of the thermocouple is at the edge of the melt stream. While this is an ideal position to keep the melt flow undisturbed, it may not be a representative measurement of the average melt temperature. Figure 1 shows a typical flush mounted melt probe. Since polymers are poor conductors of heat, if the steel wall temperature is different than the bulk polymer temperature, a flush mount thermocouple will always feedback a PV close to the actual steel temperature (the control thermocouple setpoint). This type of thermocouple will be limited in how helpful it will be to troubleshoot or improve the process since it seldom gives feedback that is representative of the average bulk melt temperature.
FIGURE 1
A fixed depth thermocouple (Figure 1) is positioned with the tip into the melt stream. It does create some melt disturbance but provides polymer temperature feedback more representative of the average melt temperature. The depth into the melt stream is commonly from 3mm to 13mm (0.12-0.5”). If the probe depth is longer it is more prone to being damaged or bent due to the high polymer viscosity during lower temperature shut down, start up or purging. While this type of probe is more representative than the flush mount, it is still just a one spot measurement of temperature.
The location of these melt temperature readouts may be in a poor position, such as an elbow or turn. The velocity profile through the elbow as well as heating and control issues may significantly influence the measured temperature. The best location for the melt thermocouple is after all interacting devices such as the screw, valve, screen changer, static mixer and elbows and before final forming. It is best in a straight feed pipe section just prior to entering a feed block or die. Due to the polymer viscosity dependence on temperature; understanding the temperature variation before forming will allow control adjustments that may significantly improve the product quality and consistency.
The adjustable depth thermocouple significantly improves the range and accuracy of the data that can be gathered from the melt stream. The adjustable depth thermocouple is a much better tool for troubleshooting melt temperature problems as the tip of the thermocouple can traverse from the edge of the melt stream to the center and in most cases, beyond center. During normal operation, the tip can be left slightly in the melt stream or flush with the melt stream. For process analysis, troubleshooting or optimization; the thermocouple can be put into the melt stream by turning the thumbwheel (see photo, below). The temperature can be continuously recorded as the tip is moved to different positions across the melt stream. This data can now be analyzed to understand the position dependent temperature variation across the melt stream. The distribution of this variation can provide us insight into the system performance to guide us in adjusting the control set points to improve melt temperature uniformity. For example; a peak temperature at the wall (above the zone set point) may indicate too high a temperature setting at the discharge end of the barrel.
By using an exposed junction (fast response) thermocouple and the appropriate high speed data acquisition device, this thermocouple can also be used to evaluate time dependent temperature variation as well. The thermocouple can be adjusted to a specific depth and left over a longer period of time. The variation recorded over time by the data acquisition system will indicate time dependent temperature variation. This is often indicative of poor mixing, cycling heaters, or feeding instabilities. Again, the insight gained by the data collected can guide our adjustment of the operating conditions to improve uniformity.
One concern when using the adjustable thermocouple is that the user needs to remember to back the tip near to the wall when finished. If the tip is too far into the melt stream, the probe can bend when the machine is shut down or purged. It is difficult to remove this tool after it is bent.
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