The temperature of the plastics melt is often measured with an immersion TC. The probe protrudes into the melt and reads the temperature at the point of the TC junction. To avoid conduction errors the junction should be thermally insulated from the base of the probe. The immersion TC is inexpensive, good for point measurement, and available with adjustable depth. One drawback is that the immersion probe changes the velocities in the channel and, thus, the melt temperatures. Also, dead spots may occur behind the immersion probe; this can lead to degradation in plastics with limited thermal stability.
Advantages of immersion melt temperature probe:
• Inexpensive
• Point measurement
• Available with adjustable depth
Disadvantages of immersion melt temperature probe:
• Changes the velocities in the channel
• Changes the melt temperatures in the channel
• Dead spots can occur behind the probe
A flush mounted probe can be used. This measures the melt temperature at the wall, which is usually the same as the metal wall temperature. As a result, this melt temperature measurement is not the most useful. Another probe has a straight protruding design; these probes are also available with adjustable depth so that temperatures at different positions in the channel can be measured. Yet another design has a tip on the probe that points upstream. The TC junction is located in the tip. The benefit of this design is that there is minimal disturbance of flow where the temperature is measured. This probe is also available with adjustable depth.
It is also possible to run a bridge across the channel with several probes attached to it. This allows simultaneous melt temperature measurement at several locations.
Barrel Temperature Measurement
The temperature of the barrel is usually measured with TC or RTD sensors pressed into the barrel; the sensors are generally spring loaded. Many temperature sensors are constructed with a metal sheath to obtain sufficient mechanical strength. As a result, significant conduction errors can occur in the measurement. This can be minimised by using a sensor whose temperature sensing element is thermally insulated from the rest of the probe. The accuracy of the measurement is strongly dependent on the depth of the well, the type of sensor, and the air velocity. For accurate temperature measurement the well depth should be at least 25 mm (1 inch) and the extruder barrel and die should be shielded from air currents.
Measurement error can be reduced by thermally insulated sensing tip
Measurement accuracy is dependent on:
• Depth of the well
• Type of temperature sensor
• Air velocity
