TPT November 2015

Article

Sikora AG

Today, there are various systems and technologies available in order to inspect materials in laboratories or during produc- tion. Most of the technologies are based on optical film inspection technologies, where a batch sample is extruded to a film [3] [4] [5] . There are several limitations to these optical inspection systems: • Optical inspection systems

Picture 3: Vacuoles, metallic and organic contamination

Picture 4: Inhomogeneities/agglomerations inside the pellets

X-ray cameras take pictures of the plastic pellets or of the film, which are then processed by mathemati- cal algorithm. The mathematical algorithm clearly identifies the dif- ference in the attenuation, even for a contamination size down to 50µm. By combining the analytical results of the mathematical algorithm, the system clearly identifies, for example, contaminants in plastic pellets (Picture 1).

such as pellet inspection systems only inspect the surface and are not able to look 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 through a film inspection and analysis system 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 today’s optical inspection systems require new ways and technologies to fully meet 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 the inside of a plastic pellet or coloured thick films By the use of X-ray technology, both transparent and 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 upon the nuclear charge of elements as well as of the thickness of the material to be inspected [6] . It is proportional to the atomic number raised to the 3 rd power (µ~Z 3 ). 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 “at line” continuous quality monitoring system.

Picture 5: Incorrect degree of foaming or expansion inside the pellet

Further possibilities with X-ray to monitor and optimise material 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 also gives 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 (Picture 2). 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, which in this example was probably a piece of tissue, as well as a small metallic contamination. It is also possible to clearly see that one pellet has an air hole inside, which could lead to the conclusion that some parameters in the production process need to be adapted (Picture 3).

Picture 6: Metallic contamination in black pellets

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N ovember 2015