15 years ago there were investigations about measurements in the melt. A high dependence on pressure and temperature was demonstrated and final the measurement for wall thickness was not recommended.
Directly behind the die, the pressure is constant. With PVC Extrusion a measurement is common, but still this position is not practiced. By Polyolefine (PE) it is in common to pull down the pipe diameter between the die and the cooling and therefore measurement is not recommended. With PVC it should be possible, with vacuum assistance, to achieve a good transducer signal coupling. The precondition is high temperature proven transducers. Measurement using fixed transducers is inexpensive, while reversible scanners need more effort.
This position is already successful applied in production. As is further described in the chapter mechanics, the transducers are mounted into the sizing sleeve.
This position is commonly preferred. The ring within the fixed transducers is mounted directly behind the sizing sleeve (gif 12k). This position gained an early centering information, especially for large dimensions.
Fixed transducers can be mounted almost, without problems in every spray vacuum or full water bath. Thus a reversible scanner is very problematic. For scanners problems could occurr because of a small installation space and vacuum proofed devices. Since a failure occurs during production, it is not possible to maintain this without a stop to production.
This is the most highly recommended position for a scanner. Here so-called Immersion (flanged) scanner (gif 12k) are common. These are small compact units and are flanged at the end on the bath. Also sviwel arm scanner (gif 12k) are applied here.
If a very early measurement is demanded, for instant in connection to a thermal die centering device, special short (2 meter) vacuum baths are designed. Here, the second bath is installed as a vacuum bath too.
Also measurement chambers (gif 15k), especially for small dimensions, are here applied here.
This position, in front of the bath, is ideal in preventing the occational problems resulting from position 6, regarding vacuum sealing and handling within the Extrusion start up.
With the above positions, the pipe is still very warm and a further contraction causes a thickness and diameter change. As the pipe is thicker the cooling needs more time. In this case a manual calibration can only be done very late, thus it is a disadvantage regarding optimum material savings.
For large dimensions it is recommended, to measure as close as possible to the die, for instant with fixed transducers at position 4. , to gain an early centering. A new idea is to support the fixed measurement, by the use of an additional second measurement, with a reversible scanner at position 8. This support can make the relevant quality record.
Behind the Position 8 there still appears a shrinking of the pipe, but this is small and can be adjusted due to the system's parameter. Because of sagging, caused by large dimensions, the measurement within the vacuum is not recommended as a quality recording.
The measurements above were discussed regarding vacuum Extrusion lines. In pressure calibration lines the sizing sleeve is directly connected onto the die, thus Pos. 2 is not relevant any more. For the other measurement positions the same general arguments are relevant.
Different aspects are present only in pos. 4 , because there is no vacuum applied and so a reversible scanner can be recommended. This scanner has a similar design to the immersion-flange-scanner. Experience has shown the key benefits to be that it is, user friendly, accurate in measurement, simplified in its mechanics and reliable in operation.
Rolf Diederichs 06. March 1996, email@example.com