Computer Tomography of Plastic Parts
The technique of computer tomography that has become commonplace in the field of medicine can also be employed for analysis of plastic parts. In this way, the first parts out of a mould can be examined in a nondestructive manner.
Wall thickness variations and accumulations of material at the ends of flow paths can result in defects, density fluctuations and voids in plastic parts. Since these inhomogeneities can often be the starting points for microcracks, they represent potential weak spots in the component.
To date, the presence of a void was often noticed only in the course of a failure analysis after problems appeared during life expectancy tests or when tests that accompanied production were not passed. Then, thermal test procedures were generally employed to clarify the material’s identity and exclude serious processing errors. If these tests did not provide clear results, destructive tests were conducted in order to determine the cause of the problem. Both the effort required for such destructive tests and the results of these tests depend to a great extent on the experience of the individual performing them and a reliable analysis of possible void-prone regions in engineered plastic parts.
In the current issue of the magazine Kunststoffe, Dr.-Ing. Steffi Illner, an employee of Stiebel Eltron GmbH & Co. KG, Eschwege, reports on the use of computer tomography to examine plastic components. Since the beginning of 2003, Stiebel Eltron has used high-resolution quantitative computer tomography (pQCT) from Stratec GmbH, Pforzheim. The procedure detects density changes, inhomogeneities and voids in the part in a non-contact and nondestructive manner. To achieve this, a cross section through the object being examined is generated with the aid of x-rays. These are produced in a rotating x-ray tube, focussed and then directed through the test specimen. A detector ring located opposite the x-ray tube records the absorption profile emitted by the test specimen. By mathematically folding a multitude of absorption profiles from different angles, cross-sectional images are calculated. The result is a two-dimensional cross section through the specimen in which surfaces with different densities are represented by different colours. With the aid of commercially available 3-D software, a three-dimensional representation can then be generated from a large number of cross sections. The resultant 3-D model can be handled like a conventionally designed model and examined virtually with regard to material inhomogeneities.
The advantage of the procedure is that the first parts out of the mould can already be tested nondestructively. The processing window for manufacturing can be established on this basis and subsequently validated at any future date. Comparison of an FEM load analysis with inhomogeneous regions in the component during the preproduction phase can then provide a reliable assessment of the risks.
Since the original moulded part remains intact, it is available for additional testing. After suitable life expectancy tests have been completed, further examination can provide information as to how and to what extent voids will affect the properties of the component under actual use conditions.
All thermoplastics can be examined with pQCT. Testing of two-component parts and plastic assemblies is also possible in an uncomplicated manner, if the materials employed exhibit sufficient density differences. Overall, computer tomography represents a reliable instrument that has proven its usefulness in series production for all-around quality control of plastic parts subject to high loads.
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