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Lüdenscheid Plastics Institute
“Oh no, the coffee machine is broken.” – This early morning realization is certainly not a good start to the day for many people. “How could this have happened?” is the question that arises for the person concerned. And they first try to remedy the situation in their own kitchen before going to a specialist dealer. Those with an affinity for technology may unscrew their coffee machine directly and discover in this case that a latching hook has broken off.
All cases of damage begin in one way or another and, depending on the application and severity, it is important to quickly identify the cause and define remedial measures. This is where the Materials and Damage Analysis department at the Kunststoff-Institut Lüdenscheid comes into play.
For a systematic approach to damage analysis, it is immensely important to first gather all relevant information. In the example of the defective coffee machine, for example, it is important to find out what material the defective component was made of, when it was manufactured, how high the failure rate is and under what circumstances the defect occurred. Only by knowing as many circumstances as possible can a case of damage be reconstructed and possible solutions defined. It is also important to be able to provide suitable reference samples, ideally from the same batch and with the same component history.
In order to rule out a material mix-up or contamination of the material during the production of the component, an IR spectroscopy was carried out in this case. It showed no significant impurities in the material and confirmed polyoxymethylene (POM) as the material. As the IR spectrum cannot distinguish whether it is a homo- or co-polymer, a DSC analysis was also carried out. Based on the melting point, a POM-C could be identified and here, too, there were no abnormalities such as foreign materials. Both analyses were carried out for the failed part as well as for the reference part. A comparison of the melting enthalpies of the samples determined from the DSC analysis showed slight differences between the samples.

Recording IR spectra in the analytics laboratory
Source: Kunststoff-Institut Lüdenscheid
Microtome sections were also carried out on the two samples to evaluate the material structure. This showed that the material structure of the drop-out sample had significantly coarser crystallite structures and a thicker amorphous surface layer than the good sample. This indicates a comparatively colder mold wall temperature and a higher melt temperature in the drop-out part, whereas the finely distributed, uniform microstructure in the good part indicates a more optimal temperature control of the injection molding process.

Microtome section of niO structure
Source: Kunststoff-Institut Lüdenscheid
Supplementary gel permeation chromatography (GPC) was then used to check whether the process conditions had led to material damage to the POM, which could have a detrimental effect on the mechanical properties of the ratchet hook in addition to the deviating microstructure. Ideally, samples are taken from different process stages for such an analysis. In this case, samples of the granulate before and after drying as well as a melt cake and a component from the current production and the two iO and niO samples already examined were available.
The GPC analysis determines the molecular weight distribution of the polymer, which can be used to estimate whether there is a shortening of the polymer chain length and thus material damage. In this case, a comparison of the tested samples showed that there were no significant differences in the polymer chain length distributions of the tested samples during current production, meaning that the temperature conditions proved to be gentle on the material during current production. The good part also showed a similar distribution, whereas the reject part showed a clear decrease in the average molecular weight. In this case, it was therefore possible to determine that an error must have occurred during process control, which led to the deviating microstructure and also resulted in material damage.
The Kunststoff-Institut Lüdenscheid is happy to assist companies with material and damage analysis. In addition to the DIN EN ISO 17025 accredited laboratory, the institute has access to an extensive network in order to be able to offer an analysis package tailored to the company. In addition, the material and failure analysis department works closely with the other departments of the institute, such as application technology, so that competent contacts are also available beyond the analysis.
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