Have you encountered the often dilemma, “Should I do single-stage or two-stage mixing for my compound; which machinery to use?” – do not be surprised! You are not alone.

Single-stage mixing is considered for productivity reasons (and cost-effectiveness). While Two-stage mixing gives a better dispersion of the finer size blacks. And interestingly, for some compounds with high levels of blacks, even three or more mixing-passes may be necessary.

Rubber mixing as a subject would have been quite simpler, if we could answer this topic effortlessly. Unfortunately, it is not!

(Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

Single-stage mixing in an internal mixer is a cost-effective solution but difficult for all compounds. If the compounds have high filler loadings, you may be forced to mix in two-stages due to the high amount of shear and heat generated in the mixing cycle. If you use peroxide cures or are mixing expensive FKM, then you must be even more worried of the batch temperature.

Most experts feel two-stage mixing, with short time spans for each of the mixing stages, is helpful.

One school of thought advocate an open two-roll mill for second-stage mixing because the dispersion of the batch and the mastication is higher (than an internal mixer). Open mills, though slower, are safe for short scorch compounds.

A traditional mixing line comprises of an internal mixer above a dump mill then one (or two) mill before the batch off cooling line.

Internal mixers are high-capacity rubber compounding machinery. Hence they need to be supported by open mills with advanced features to keep pace with production. The rotors of these mixers operate at high-speed to maximise dispersion of the bulk ingredients and dump the batch at high temperatures. Curatives, blowing agents, etc are added on the open mill and final homogenization happens on the last mill before batch off. Also, adding the cure system on downstream mill eliminates the batch contamination problem from “leftover’s” trapped (between the rotor end plates and ends of the rotors) in the internal mixer. These open mills are recommended to have peripherally drilled rolls to take out heat of the compound before adding heat sensitive curatives.

Open mill mixing is operator dependent and hence quality of compound varies from beginning of shift to end of shift. (Read about Stock Blender). As compared to rubber mixing in a closed environment, the probability of “fly loss” is high in open mills. Hence, an alternate school of thought propagates second-stage mixing also performed in an internal mixer. This can be at a lower speed, energy and dump-temperature configuration setting on mixer.

Single-stage mixing in an internal mixer is possible, when you mix and drop the batch within 120⁰C. The present range of internal mixers have advanced designs to effectively control batch temperature. With many designs and rotor geometries for faster mixing, accompanied by quicker cooling features, mixers like tandem mixers allow traditional two pass to be reduced to single pass cycle. (I will cover newer mixing lines with Twin Screw Sheeter, Dump Extruder, etc in different posts). As a side note, if you opt for single-stage mixing with internal mixer; the Intermeshing Type Mixer has the best quality and efficiency.

Single-stage mixing is not always cheaper and two-stage mixing is not always better. The best way to decide is to make a cost-benefit analysis between the two processes for the different polymers that you work with. Quantify how much of your product defects are linked to poor dispersion. Analyzing them, you have your customized solution to mix effectively.

Summarizing, there is no one best way for all compounds. Your mixing process has to be designed to the polymer; depends on the viscosity of the elastomers used, the quantity of filler, mixing temperature, machinery employed, time at every stage of mixing and desired physical properties for the end use product. If you get your “desired” characteristics in a single-stage mixing, adopt it or wisely opt for two-stage mixing.

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