How to measure the internal temperature in extruded tubes and hollow rods, even during the production

In extrusion, the temperature distribution inside the product is a very relevant value whose knowledge can contribute to understanding a variety of effects or problems. However, measuring the internal temperature of an extruded product is difficult in most cases, because in a continuous process such as extrusion (with sometimes very long line lengths) it is often very difficult or impossible to introduce sensors into the inside of the product without damaging to extrudate (which also changes the thermodynamic situation).

Within the scope of various troubleshooting projects, the temperature distribution at the product inner wall was the parameter assumed to be essential for different production problems (in particular deformations or roughness). The sensor system (INVENT 360), shown above, was developed to prove the connection between the problems and the cause.

The sensor is equipped with a 360° temperature measuring system to measure the inside wall temperature and a LIDAR system (Light Detection and Ranging) to determine the exact position. The sensor also has a thermal insulation jacket and internal temperature monitoring. This ensures that the complex electronics of the system are not exposed to excess temperatures. In the event of too high temperature (e.g. due to too long residence times inside the product), the sensor automatically moves out of the product so that damage can be avoided. The sensor values recorded by the system are continuously communicated to the corresponding app via an integrated WLAN interface, so that all information are available at any time. In addition, the data is written to an internal memory card. In difficult communication conditions, communication and power supply can also be implemented with the aid of a supply line.

A caterpilar drive concept with integrated self-centring was developed as the travel drive, since the first prototypes of the system (cambered wheels) has sunk into the still soft molding compound due to the high surface and prevents the system from tilting inside the product due to the self-centring mechanism.


Internal temperature measurement during production

For measurement, the sensor is activated and inserted into the tube (or hollow rod) to be measured at the end of the cooling section. The LIDAR measures the distance of the sensor to the extrusion head and outputs this as a scalar value. Now the sensor can be moved to a set position inside the product via a position specification. An internal control loop then ensures that the sensor automatically maintains this target position by the travel drive operating at the same speed as the pull-off speed of the product.

Once the sensor has reached the set position, the temperature measurement can be activated. An integrated radiation pyrometer, deflected by a deflectiving, mirror, measures the temperature on the inside of the product. The temperature values are written to a database with the corresponding angle position and stored.

The data is then evaluated either live or subsequently on the PC.

Sensor as “window into the process”

Of course, a sensor itself or a measurement is never a solution to a problem, but sensors help to understand the causes of problems and to recognise connections that are not obvious. In the case of internal temperature measurement, this is often influenced by many different factors, the effect of which are often not assumed to be so immense. By using the system, various extrusion defects could already be precisely detected by measurements. At the same time, however, it was also possible in other cases to prove by such measurements that the presumed cause of the problem was not a correct assumption.

Typical extrusion problems due to temperature differences

  • Deformation
  • Sagging
  • Internal stresses
  • Roughness
  • Degree of crystallisation
  • Density distribution
  • Surfaces

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