Dielectric Analysis: Online Monitoring of the Degree of Crosslinking
Dielectric analysis (DEA) could revolutionise quality control in elastomer injection moulding. Initial tests show that the cross-linking reaction is detectable, which enables inline monitoring and precise demolding with a defined degree of cross-linking.
Initial situation / problem:
The mechanical component properties of injection-moulded elastomer components are largely determined by the degree of cross-linking. A qualitative or quantitative determination of the component properties has so far only been carried out after production with a large time delay, during which rejects may be produced. Determining the cross-linking status of elastomer components as a new quality parameter integrated into the mould can significantly minimize this risk. Dielectric analysis (DEA) is an established method for determining the degree of cure in epoxy resin processing, but has not yet been verified for elastomer applications.
Objective:
The aim of the project is therefore to investigate the DEA method with regard to its suitability for recording the crosslinking process during elastomer injection molding. If the DEA proves to be suitable, components can be demolded on the basis of the DEA measurement signal with a defined degree of cross-linking to ensure consistent component quality.
Solution and results:
To this end, a temperature-controlled offline test bench with integrated DEA is being developed and manufactured. This test rig can be used to simulate variable process conditions in order to evaluate the penetration depth as well as the temperature and pressure dependency of the measurement signals. Using natural rubber (NR) and styrene-butadiene (NBR) mixtures, the cross-linking reaction is investigated at different temperatures. In general, the test setup can be used for measurements of any compounds on an organic and inorganic basis. Tests with NR mixtures have shown that the incipient reaction can be detected by dielectric analysis. Compared to rheometric measurements, there is usually a time offset, which is due, among other things, to the different sample geometry and the difference between the chemical and mechanical influence of the cross-linking reaction. Finally, the results are transferred to a sensor concept in the mold. Process engineering investigations are required to clarify which positioning in the cavity appears sensible, how high the load capacity of the sensor is with regard to the shear and pressure loads that occur and which general design guidelines can be derived.
Project data and thanks
The IGF project 22803N of the Research Association for Plastics Processing was funded by the Federal Ministry of Economics and Climate Protection (BMWK) via the DLR as part of the program for the promotion of joint industrial research and development (IGF) on the basis of a resolution of the German Bundestag. Our thanks go to all institutions.
We would also like to thank NETZSCH Process Intelligence GmbH for providing DEA analytics and Momentive Performance Materials GmbH, Vorwerk Autotec GmbH & Co. KG, W. Köpp GmbH & Co. KG and Freudenberg FST GmbH for providing material, carrying out tests or scientific support.
Project duration: 01.07.2023 – 30.06.2025
Project partner / funding:
