X-ray vision makes masterbatch safe

Dr. C Frank, Sikora, discusses the use of an x-ray scanner for pellet, flakes and film inspection as well as analysis in the compound and masterbatch industry

Abstract

The quality standards in the plastic manufacturing industries are permanently increasing. Therefore, plastic material has to be inspected and analysed for different types of defects before it enters the final product. Contamination, defects or inhomogeneities down to a size of 50µm in the material have to be detected. This is achieved by a modular X-ray scanner and analysis equipment that can either be used continuously ‘online’ or for sample testing. In this way, transparent, opaque, coloured or black pellets and flakes as well as thick coloured films and tapes can reliably be inspected and analysed. Consequently, highest material quality and stable production processes can be assured.

Rising quality demands & challenges

For industrial production, highly pure and flawless plastic material is an important quality aspect. Due to the continuously increasing requirements, it is necessary to detect and monitor defects and impurities of decreasing size in plastic and intermediate products. Impurities of 50µm may already cause damage to production systems, such as to the crosshead of the extruder, or to the end products with high follow-up costs.

In the different processing steps for the production of plastic products, defects may repeatedly occur. This affects the processes of material producers as well as the compound and masterbatch producers, the processing industry, the recyclers and the whole supply chain. Accordingly, it is necessary to control and monitor the quality of the material used before it is further processed.

Today, there are various systems and technologies available in order to inspect materials in laboratories or during production. Most of the technologies are based on optical film inspection technologies, where a batch sample is extruded to a film. [1], [2], [3]. There are several limitations to these optical inspection systems:

Optical inspection systems such as pellet inspection systems only inspect the surface of pellets, not the inside the pellet. This applies especially for opaque and coloured pellets.

Optical film inspection is only applicable for transparent, thin films. Coloured pellets cannot be inspected without accepting major drawbacks. The only possibility to inspect coloured material through film inspection is to produce a very thin film. However, this means that only a very low quantity of pellets can be inspected. Therefore, the results of thin film inspections are not representative. 

The limitations of the today’s optical inspection systems require new ways and technologies to meet fully the requirements of the plastic industry. With new X-ray test technologies and methods production related parameters such as inhomogeneity, contamination and defects, material differences, cross contamination, etc can be analysed.

X-ray technology to inspect and analyse

By the use of X-ray technology, both transparent as well as non-transparent plastics are inspected for contamination. The basic principle is the different attenuation of raw material and impurities or defects respectively. The attenuation (µ) of the X-rays is mainly determined by the nuclear charge of elements as well as of the thickness of the material to be inspected [4]. It is proportional to the atomic number raised to the 3rd power (µ~Z3).

Plastics consist mainly of carbon (Z=6). Thus, they have only a very low attenuation. An iron contamination with comparatively strong attenuation (Z=26) can be detected clearly. An additive, for example titanium dioxide, also affects the attenuation. Titanium dioxide agglomerates significantly contrast in the dispersion with the surrounding material. This is possible because the titanium (Z=22) in the titanium dioxide strongly contrasts with the plastics.

With a specific developed X-ray camera system, it is possible to detect contamination or defects during continuous and discontinuous inspections, during film inspection or as an “online” continuous quality monitoring system. X-ray cameras are taking pictures of the plastic pellets or of the film, which are then processed by mathematical algorithm. The mathematical algorithm clearly identifies the difference in the attenuation, even for a contamination size down to 50µm.

X-ray to monitor and optimise quality

The production processes in the plastic industry are complex. Different materials are mixed together to receive different end products and there are many sources of contamination and other defects, which can enter the product. X-ray technology does not only detect metallic impurities in plastic pellets but gives also information on the condition of the overall production process. With X-ray, it is for example possible to identify cross-contamination of different plastic materials in one production process. This cross-contamination can be clearly visible, even if a plastic pellet has the same colour, but a completely different attenuation. A main advantage of X-ray technology is that it is colour independent.

Further tests show, that it is also possible to detect an organic contamination, for example a piece of tissue, as well as a small metallic contamination. It is also possible to see air holes inside, which could lead to the conclusion that some parameters in the production process need to be adapted.

X-ray can also be used to detect agglomerations of additives in polyethylene pellets. These agglomerations can lead to major problems when they are used in a further production process to manufacture final consumer products.

The determination of the correct degree of foaming or expansion is also a possible analysis, which can be realised by X-raying the respective pellets. In contrast, an optical inspection of the pellets would have given the information that all the pellets are flawless. However, an incorrect degree of foaming or expansion of the pellets causes major defects in the end product, which can be prevented by using X-ray.

In a further test, we used black pellets with different metallic contamination in order to prove the efficiency of X-ray technologies. We clearly identified contamination of the size of 50µm or 100µm in black pellets.

With the X-ray analysis it is also possible to detect contaminants in flakes. The following picture shows small and major metallic contaminants in flakes (image 1).

An optical analysis would not be able to provide these detailed results as the flakes are reflective and therefore would disturb the picture analysis of an optical camera. With the X-ray analysis system, it is possible to detect metallic contaminants and also classify their size. Due to this information it is possible to determine the quality degree of the flakes.

The same principle can be used for coloured thick film inspection and analysis. As already mentioned, optical film inspection needs transparent thin films and therefore cannot be used for an inspection and analysis such as shown in image 2.

Here the film has a dark color and is several millimeters thick. An X-ray analysis system is able to detect smallest contaminants in the film and therefore gives information on the contamination level of the overall material.

Conclusions

Quality standards in the plastic manufacturing industry are constantly increasing. Contamination, inhomogeneities or any other defects need to be detected and analyzed. Today, even the smallest contamination need to be detected during the production process. With the X-ray technology, we are able to detect contamination of 50µm.

Different inspection and analysis requirements need a modular concept approach for the equipment. Such equipment can be either used as a discontinuous lab scanner for pellets and flakes or as a continuous quality monitoring device at a production line. In this case the equipment is installed as an ‘online’ quality monitoring and analysis device and can be used for pellet and flake inspection as well. The images demonstrate the modular concept of an X-ray scanning and analysis device.

The concept is applicable for different material types as for instance TPE, TPU, etc. or any coloured masterbatch or recycling product. The detection is independent of the colour of the pellet or the colour and the thickness of the film or tape. Therefore, the material quality can be constantly monitored and, hence, significantly improved.

References

[1] Tomra demonstrates optical sorter, http://www.plasticsnews.com/article/20131030/NEWS/131039997/tomra-demonstrates-optical-sorter#, (10/2013)

[2] Laura Tarrach, OCS GmbH, Pellet Scanning in “Free Fall”, Kunststoffe international (12/2010)

[3] Satake, Pellet Sorter, http://www.satake-usa.com/documents/Satake-Pellet-Sorter-Brochure.pdf, (2014)

[4] Robert Fosbinder, Denise Orth, Lippincott Williams & Wilkins, Essentials of Radiologic Science (01.02.2011)