Due to the chemical activity of rubber compounds, storage temperature and -time of the material prior to processing do have a significant influence on the processability of rubber compounds in injection molding.
The aim of this work is to quantify storage-induced material changes in the laboratory as well as to evaluate how these changes affect the injection molding process. Furthermore, two key process indicators (KPIs) are presented which allow the evaluation of the material condition inline.
Therefore, a batch of a NBR model compound filled with carbon black was produced to be used for injection molding experiments in its fresh state as well as for repetitive experiments after a storage period of three months at cooled ambience. This is generally the maximum storage period for commercially processed rubbers.
Several process settings were varied systematically to identify the most important parameters regarding the processing stability of the compound. Simultaneously to the injection molding experiments, the material was characterized in the laboratory for its changes in rheological, thermodynamic and curing behavior in dependence on the storage time.
While a significant viscosity increase with storage time was observed, the incubation time of the rubber decreased which diminishes the possible processing window. The results of the injection molding experiments clearly show that the material changes observed in the laboratory can also be monitored in the injection molding process. With two KPIs based on machine signals, the inline evaluation of the material storage condition is possible and allows timely triggering of counteractions to ensure process stability as well as the estimation of expectable part quality.