Already 2 years ago, the company MAILLEFER raised urgent questions about the possibilities for the development of a system for the detection and sorting of contaminations in plastic pellets.
This technology was originally planned for the inspection of XLPE pellets used for the production of medium, high and extra-high voltage cables as well as for underground and submarine cables. But also in other markets, such as the plastic processing industries or the hose and tube industry, the purity of raw material is a crucial quality aspect.
Also in the field of medical engineering, increasing quality demands concerning the reliability of medical products are reported, for example regarding hoses for endoscopy or ureteral catheters. If contaminations enter the production process, the risk of quality losses of the end product may be the result. Consequently, this leads not only to image damages of the company, but also to health hazards of the client. Therefore, the production of highly pure material, but also the continuous inspection of the granule used for the production process before it enters the end product, is of crucial importance.
In order to catch possible impurities in the pellets before they get into the product, manufacturers use screens which are positioned directly in the melt flow after the extruder, before the crosshead and, thus, prevent the contaminations from entering the product. But these screens can get clogged by contaminants after a certain run time and then the melt pressure in the extruder may increase significantly. Finally, the production has to be stopped in order to change the screens. An approach to increasing productivity is to omit the screens. This requires, however, appropriate methods to detect and sort out contaminated pellets at an early stage.
A solution to this problem is a system which inspects the pellets for purity before they get into the extrusion process. Currently installed devices use optical measuring principles that randomly inspect pellets for contaminations. Since this is a sample check only, not 100 % of the material is controlled. With optics, however, only the outside of the pellet can be inspected, while impurities inside the pellets remain undetected. Moreover, the inspection of the raw material with existing devices is not done under clean room conditions, so that new impurities can emerge from the ambient air or as a result of the conveyor belt used.
“We have a close cooperation with SIKORA for more than 20 years now, which brought us technologies such as the CSS 2 for the monitoring of the purity of the PE melt, or the ULTRATEMP 6000, a system for the continuous measurement of the melt temperature. It was a matter of course that we addressed SIKORA concerning the development of an inspection and sorting system”, explains Timo Mäkelä, Process Specialist for MV and HV products at MAILLEFER. “For MAILLEFER, it was important to develop a system, which inspects 100 % of the pellets and which sorts out reject material reliably”, Mäkelä goes on.
The list of requirements mainly included 8 points:
1. Material throughput of 500 kg/h
2. Detection of metallic and organic contaminations
3. 100 % detection of small contaminations ≥ 50 µm
4. Detection of contaminations on and inside the pellet
5. Transport of the pellets without conveyor belt, as a conveyor belt may cause additional contaminations
6. Flow of material inside the devices under clean room conditions
7. Reliable sorting of contaminated pellets
8. Assuring purity of raw material of all potential suppliers
In initial experiments, various sensors were tested in order to ensure a reliable detection of contaminations with a size of 50 µm. Very soon it became clear that SIKORA had the know-how to develop the appropriate technology to meet MAILLEFER’s requirements. With the newly developed PURITY SCANNER, metallic and organic contaminations not only on the surface, but also inside the pellet can be detected. The system applies for pellets as they are used at the production of hoses and tubes in the medical, automotive, aircraft or cable industry.
With the intelligent combination of X-ray and optical technologies, a system was developed which inspects 100 % of the pellets for purity before they enter the production process. Rejected pellets are sorted out reliably.
The transport of the pellets is carried out via a vibrating ramp. This transport system is hermetically sealed with the result that no external impurities get into the material flow.
“The PURITY SCANNER is the result of hard work and a trustful cooperation between MAILLEFER and SIKORA. With this system, the quality and consequently the reliability of the end products in the various industries can significantly be increased. This also improves the safety and usability for the end user”, Mäkelä sums up.