![]() ![]() ( I will cover newer mixing lines with Twin Screw Sheeter, Dump Extruder, etc in different posts). 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. ![]() The present range of internal mixers have advanced designs to effectively control batch temperature. Single-stage mixing in an internal mixer is possible, when you mix and drop the batch within 120⁰C. This can be at a lower speed, energy and dump-temperature configuration setting on mixer. Hence, an alternate school of thought propagates second-stage mixing also performed in an internal mixer. As compared to rubber mixing in a closed environment, the probability of “fly loss” is high in open mills. Open mill mixing is operator dependent and hence quality of compound varies from beginning of shift to end of shift. These open mills are recommended to have peripherally drilled rolls to take out heat of the compound before adding heat sensitive curatives. 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. Curatives, blowing agents, etc are added on the open mill and final homogenization happens on the last mill before batch off. The rotors of these mixers operate at high-speed to maximise dispersion of the bulk ingredients and dump the batch at high temperatures. ![]() Hence they need to be supported by open mills with advanced features to keep pace with production. Internal mixers are high-capacity rubber compounding machinery. Reference Image Courtesy: Bainite Machines Open mills, though slower, are safe for short scorch compounds.Ī traditional mixing line comprises of an internal mixer above a dump mill then one ( or two) mill before the batch off cooling line. 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). Most experts feel two-stage mixing, with short time spans for each of the mixing stages, is helpful. If you use peroxide cures or are mixing expensive FKM, then you must be even more worried of the batch temperature. 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. Single-stage mixing in an internal mixer is a cost-effective solution but difficult for all compounds. (Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here) Rubber mixing as a subject would have been quite simpler, if we could answer this topic effortlessly. And interestingly, for some compounds with high levels of blacks, even three or more mixing-passes may be necessary. ![]() While Two-stage mixing gives a better dispersion of the finer size blacks. Single-stage mixing is considered for productivity reasons ( and cost-effectiveness). 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. ![]()
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