EuroWire November 2020

Technical Article

Inspection and sorting of plastic pellets as a guarantee for visually and functionally perfect products By Hilger Groß, business development and sales manager, and Rebecca Zachau, corporate communications, Sikora AG, Germany

Introduction Black specks and colour deviations often lead to increased scrap rates in the production of plastic injection moulded parts, but also in the manufacture of profiles, cables and pipes. The same applies to metal inclusions in the pellets, which leave traces in the end product and the processing machines,

▲ ▲ Figure 1 : (from left) Plastic pellets with contamination; contamination detected by optical camera; contamination detected by X-ray camera

increasing costs. As far as black specks are concerned, this type of material contamination is often caused by thermal overload, whereas metallic contamination often originates in the cutting tools of the pelletising plants. 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 to the mould in injection moulding systems. In this respect, continuous quality control in the production process that fulfils 100 per cent 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 colour 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 downtime and scrap can be significantly reduced.

Figure 2 : Cross section of an extra-high voltage cable with discharge ▲

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 no (or only small) optical demands. Charges with metallic contamination were demetallised by means of inductive metal separators and reapplied for a new purpose. While the 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 of sorting has 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 and electronic), in medical or aerospace are so high that any risk of foreign contamination must be eliminated in order to avoid problems 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, 100 per cent online inspection with automatic sorting is required.

Quality assurance at plastic manufacturers, compounders and processors

The smallest process variations in compounding plants can cause changes in the technical and optical characteristics of the pellets and the future product. Thermal and mechanical overloads of the polymer, contamination caused by the production environment or a work overload of the staff can result in undetected contaminated pellets in the form of black specks, yellowing, burns, metal, dust, etc. During pellet sampling, which is a common method for the industry, contamination often remains 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.

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November 2020