Rubber & Tyre Machinery World

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Rubber Dispersion Kneader – The Other “Internal Mixer”

In the world of rubber mixing, when “purists” refer to “internal mixer”, they either mean a Tangential Rotor Type (aka Banbury Mixer) or a Intermeshing  Rotor Type (aka Intermix).

While “practitioners” have a third category viz. Rubber Dispersion Kneader.

When you discuss with the practitioners (mostly in Asia) it is very common to hear the terms “Kneader Banbury”, “Kneader Mixer”, “Kneader Intermix”, “Intensive Mixer Kneader”, “Internal Mixer Kneader”, “Dispersion Mixer”, “Tilt Mixer” etc being used in the same context as an “internal mixer”.

So, what is this dispersion kneader and how is this different from an internal mixer?

Dispersion Kneader

Moriyama Make Dispersion Kneader

To start with, a kneader means a machine that specializes in kneading substances, something to a dough form. I suspect it originates from Germany when Heinz List, a pioneer of modern industrial processing technology, first developed a kneader reactor to process high viscosity materials.

The respective rotor, throat, chamber and floating weight designs are different in a dispersion kneader and batch mixer (or internal mixer) . While a mixer discharges the batch through a bottom drop-door, the kneader tilts 125-140 degrees to discharge the batch. Available in more customized capacities than batch mixer, this machine can be positioned on the ground level. While a mezzanine floor is required for mixers with drop door. For similar capacities, dispersion kneaders use lower power than mixer. The mixing time is higher than an internal mixer and hence production volumes are lower in kneader.

Though developed initially for mixing thermoplastics, dispersion kneaders have a unique place in the elastomer mixing industry. Users love the ease of cleanliness on this machine especially when they have to change the colour of their compounds frequently. These machines are also easy to operate and their varied applications include

  • Oil seal, Body seal, Gasket, Belt, Hose, Tube for Automobiles
  • Rubber for Electric wires
  • Conveyor Belts & Power transmission belts
  • Rubberized Rolls
  • Rubber based products like Plug, Cap, Glove, Dental for Medical line
  • Sporting goods like rubber ball, etc

Moriyama Japan (now merged with Nihon Spindle Manufacturing Co Ltd in 2014) enjoys a leadership status in Dispersion Kneader and continues to innovate regularly. Many regional players co-exist in India, China, and Taiwan catering to different categories of customers in rubber mixing industry.

Over a period of time, these kneader manufacturers have developed designs, features and automation for high quality and optimum mixing performance to position themselves closer to the internal mixer. Hence, the confusing terminologies (that I spoke of in the beginning) needs to be viewed in this context. The only caution being when trying to articulate, discuss and solve rubber compounding issues, it is very important to know exactly which rubber machinery is being used – is it dispersion kneader or internal mixer?

Happy Mixing!


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Single-Stage or Two-Stage Mixing?

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.

Traditional Mixing Line with Two-Roll Mill Set-up

Reference Image Courtesy: Bainite Machines

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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Should you buy a Stock Blender for your Open Mill?

The Stock Blender is an add-on accessory mounted on top of an open two-roll mill. It aids in rubber blending by pulling the mix compound from the front roll of the mill and guiding it to the nip gap between the two mill rolls.

A Mixing Mill With Stock Blender and Hydraulically Operated Mill Guides

Structurally, this machine assembly consists of a blender roll, a carriage unit with guide rolls, pressure roll and rear roll along with required automation.

The Blender Roll is a hollow roll with water passage for internal cooling. The surface of this roll is hard chrome plated, mounted on antifriction bearings at both ends and operates on a variable speed motor. The VFD allows you to tune for every compound or recipe, thus enabling the speed of the stock blender to follow mill speed. This feature is crucial to ensure that compound does not sack-in or tear-off when taken up and through the stock blender.

On the assembly frame is also a hollow pressure-roll mounted on antifriction bearings and has a swivel operating arrangement. Pneumatic cylinders actuate this movement. The Carriage with screw and guide rollers is equipped with two limit switches at either end of travel. These limit switches signal the carriage traverse motor to reverse at extreme ends during to & fro travel, while guiding flow of rubber to mill nip via blender roll.

A stock blender helps in homogeneous mixing and repeatability of the mixing operations. Many companies use them on the open mill (also known as dump mill) below the internal mixer. After masterbatch mixed in an internal mixer, curatives are added and final homogenization done on these mills. Because the mill helps incorporate undispersed curative or fillers that might have fallen off the ram or drop-door when the batch was dropped. In a different scenario, if you need to drop the mix batch at a higher dump temperature, you could use the stock blender to take some heat out of the batch and then add the curatives on the mill. Reduction in temperature is obtained by allowing the batch from the mill roll airborne to pass over the water-cooled blender roll before it returns back to the nip gap. When these mill rolls (centrally cored or peripherally drilled design) are properly cooled, the stock-blender allows you to reduce the batch’s temperature continuously for lower viscosity compounds.

Stock Blenders are also used on pre-warming mills before feeding the rubber sheet to a Calender.

A Stock blender enhances efficiency and productivity through reduced operator fatigue. Cutting down hot rubber many times, rolling the sheets and feeding them back into the nip gap of the mill rolls is a physically demanding task for the operator. Stock blenders eliminate this completely and improve your batch to batch consistency. The operator also benefits from a safe work-environment.

The operators, though need to be trained well. For an open mixing mill the stock blender can be used only when the batch has achieved a certain stage of homogeneity. When a batch from the internal mixer is dropped on an open mill with stock blender, the operator must pass the complete batch through the nip before diverting the batch to the stock blender. Else, there is a risk that only a part of the batch turns round the stock blender. A big lump,which is neither cooled nor mixed, would continue to ride and turn on top of the nip. This lump does not pass the nip until the operator evacuates the compound to second mill or batch off.

The stock blender designs vary among manufacturers. Hence, not paying attention to the design features can result in owner’s or operator’s nightmare. For example designs with ledges behind the rolls can cause compound to accumulate and these must be cleared out regularly to prevent contamination. Similarly, a ball screw with tube casing for carriage movement is superior to a conventional screw design and eliminates contamination from lubrication of the screw. Compact designs and increased level of automation with programmable carriage traverse movements make the stock blenders a joy to work with a high level of safety.

Summarizing, buying a good robust stock blender increases the mixing capacity of an open mill, improves the batch cooling, reduce mixing time and improves dispersion. With a trained operator, you can extract the best returns out of your investment in this rubber mixing machinery accessory.

 


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Questionnaire for Internal Mixer Selection

HF MixerLast week, the two-roll mixing mill criterion checklist was amongst the most visited and downloaded posts. It has generated an equally curious interest from our members, as to what similar details should they prepare for discussions with internal mixer supplier.

Hence, here is a quick checklist Internal Mixer Selection Questionnaire for download.

Internal mixers could be either Tangential or Intermeshing Type. Both have their respective advantages and disadvantages when it comes to rubber mixing and the mixer selection will depend on your final product and optimum quality-price trade-off.

Please remember, this criterion identification list is not exhaustive however it is definitely a good start if you are considering an internal mixer purchase. Subsequently, you may evaluate further on the features that the manufacturer/supplier is offering.

Let us know if you found the questionnaire beneficial to initiate a focused discussion with your internal mixer supplier.


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Two-Roll Mixing Mill – Criterion Check List for Buyers

Ever been in a situation, where you wanted to buy a two-roll rubber mixing mill but did not know the crucial questions to ask the supplier? Or terrified with the different formats of techno-commercial offer from various suppliers? Or flummoxed by what basic features really are required to meet your mixing operation? Or even more confused to decide on the different features offered on the mil?

Yes!!

Well, then you need a ready-reference-sheet of selection criteria to consult with the mixing mill supplier and have an intelligent discussion with your machinery salesman. (A questionnaire list available for download at the end of this post will help you freeze your criterion).

Some criterion on this check list are optional while most are crucial to aid you decide well. After the baseline requirements of your two-roll mixing mill are frozen, insist your potential suppliers to give you a quote focused on these criterion.

Of course, there are developments in two-roll mill. If your supplier offers additional features, treat them as options or extras to your core requirement. Subsequently, you can compare on various brand factors – including workmanship quality, supplier reliability, product aesthetics & durability, after-sales service and spares support to your market, price and discounts, etc to decide the most suitable supplier.

Buying a lowest priced mill without focusing on the features you need for your rubber mixing operation will lead you to post-purchase discontent.

Download your Two-Roll Mill Selection Questionnaire today and avoid buyer’s remorse.


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Internal Mixers Maintenance Chart

Internal Mixer is a favourite equipment for volume and quality rubber mixing world-wide. These could be Tangential Type Mixer (referred as Banbury) or Intermeshing Type Mixer (referred as Intermix).

HF Mixer

Image from HF Mixing Group

Users buy brand new internal mixers from the manufacturers. Or they get used-machinery from the secondary market. The used-machinery could be either from a trader or from another user. Reputed internal mixer manufacturers include HF Mixing Group, Kobelco, Bainite Machines, Carter, Yiyang etc.

Brand new mixers (and some times old ones too!) come with an Operation and Maintenance (O&M) manual. In such cases, the owner gets comprehensive information on the recommended maintenance practices of the mixer from this manual. Well maintained mixers have longer life, shorter break-down times and gives highest ROI.

In case you do not have the O&M Manual of your internal mixer, here is a template chart of recommended maintenance schedule. You may want to adopt this chart with some customization. Please download the pdf here – Recommended Maintenance Schedule For Internal Mixer

Let us know if you found this chart useful for your internal mixer maintenance.


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