As far as black specks are concerned, this type of material contamination is often caused by thermal overload, while metallic impurities are mostly due to damage and wear of screw, barrel and pelletizing unit during the compounding process.
In further processing, the cause is often sought in the machines and processes, while the raw material is rarely included in the analyses. Risks resulting from contamination are breakdowns in high voltage cables and damage and blockages in the hot runner system and the mold in injection molding systems. In this respect, a continuous quality control in the production process that fulfills a 100 % control of the amount of pellets to be processed is essential. SIKORA AG has developed a special online inspection and sorting system that combines X-ray with optical technologies and, therefore, automatically detects and separates color deviations and metal inclusions down to a size of 50 μm. By using the system, repairs and follow-up costs can be avoided and costs for machines and personnel as well as down times and scrap can be significantly reduced.
Quality assurance at plastic manufacturers, compounders and processors
Smallest process variations in compounding plants can already cause changes in the technical and optical characteristics of the pellets and in further processing to the future product. Thermal and mechanic overloads of the polymer, contamination caused by the production environment or a work overload of the staff can result unnoticed in contaminated pellets in the form of black specks, yellowing, burns, metal, dust etc.
During pellets sampling, which is a common method for the industry, contamination often remain undetected because the sample amount in relation to the delivered quantity and to the actual amount of contamination is too low. Furthermore, contamination can occur pooled in one or several bundles and also homogeneously spread over many bundles. Therefore, the taken samples are mostly used for simple offline incoming goods inspection.
In the past, plastics with optical defects or contamination were often sold with discounts to compounders or processors. The goods were modified or processed into other compounds with lower dosage. Alternatively, they were processed into products with non or only small optical demands. Charges with metallic contamination were demetallized by means of inductive metal separators and reapplied for a new purpose. While first optical sorting systems were primarily designed and used in Europe in case of an emergency scenario at the manufacturers, a service sector in the area sorting has been developed since the 2000s. The pellets to be sorted are transported to the service provider in their target packaging, where they are optically sorted, newly packed in original bundles and picked up again from the manufacturer a few days later.
This procedure is inconceivable for manufacturers of high quality and extremely pure raw material such as polyethylene for cable applications. The quality demands in the area of e+e (electric + electronic), in medical or aerospace are that high that any risk of foreign contamination must be eliminated in order to avoid image and consequential damages in the millions. In addition, for mass production of plastics with very high quality demands and for compounders of technical raw material, a 100 % online inspection with automatic sorting is required.
Online inspection and sorting of plastic pellets
The online inspection and sorting system developed by SIKORA AG does not only use optical technologies, but also combines those optionally with an X-ray camera. Therefore, even the smallest, in raw material included metal particles can be detected by the system. Furthermore, black specks and color deviations are detected by the optical cameras. Faulty pellets are separated immediately after detection via compressed air.
