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Continuous mixing of rubber compounds

Continuous mixing processes are in the field of plastics processing and for the manufacture of thermoplastic elastomers state of the art.

The production of rubber compounds however is traditionally carried out batchwise in an internal mixer. For this purpose based on defined compound recipes and depending on the selected fill factor, raw materials such as polymers, fillers, additives and curing chemicals were charged taking into account the order of mixing and the process parameters of the mixing cycle. This discontinuous mixing procedure however influences the process and results in batch to batch variations in mixing quality affecting the properties of the final product.

At the DIK we are able to use a co-rotating twin-screw extruder for continuous mixing of rubber compounds. The continuous feeding results in some advantages compared with the internal mixer mixing process such as consistently high mixing quality, fewer steps in the process, a marked reduction in energy consumption and a reduction of dust creation.

To optimize the mixing quality, it makes sense to adapt the following actuating parameters to the respective recipe:

• Dosage and form of feeding of the rubber
• Dosage of other mixing components
• Design of the screw geometry
• Adjustment of process parameters (e. g. revolutions per minute, cylinder and screw temperature)

The quality control is carried out on one hand online by analysing process parameters such as torque and melt temperature and offline by the evaluation of compound properties through sampling at specific time intervals. Typical parameters to investigate are e. g. the dispersion of fillers, the efficiency of distributive mixing of mixing components using Laser Induced Breakdown Spectroscopy - LIBS, Mooney viscosity or indicated values due to dynamic mechanical stress.