Rubber & Tyre Machinery World

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Why We Love Twin Screw Sheeter (And You Should, Too!)

Twin Screw Sheeter replaces the dump-mill and sheeting mill combination in a traditional rubber mixing line (an image you had seen in my earlier post – Single-Stage or Two-Stage Mixing?). This means you could visualize Twin Screw (Extruder) Sheeter, as a rubber machinery that accepts mix compound directly discharged from an internal mixer into its hopper chute and converts it into a continuous, seamless rubber sheet that is then fed into a Batch-Off Cooling Line.

For those who have been following my blog, you have already viewed a video of this equipment in action in my earlier post Rubber Mixing Room.

When you explore this equipment for purchase, you should not be surprised with different OEM’s calling it in similar sounding names. For example, you will get a Conical Twin Extruder (CTE) with Roller Head from Colmec SpATwin Screw Roller Head Extruder (TSR) from KobelcoTwin Screw Discharge Extruder (Convex™) from HF Mixing Group or simply Twin Screw Sheeter (TSS) from other rubber machinery manufacturers like Bainite Machines.

In construction, they all appear similar as shown below.

Kobelco Make TSR

Kobelco Make TSR with description

For reading simplicity, let me address this machinery simply as “TSS” for the rest of this article.

You will find the TSS to be ideal for conventional and diverse applications including tire manufacturing, custom compounding, hose & belt manufacturing and technical rubber goods production.

So, here’s why we love Twin Screw Extruder Sheeter (And, I feel, You Should, Too!).

  • Energy Saving: Rubber compounding is a energy-intensive process. So, any technological advancement that has the potential to reduce energy consumption receives my first preference (and I hope you will agree with me here). Let me help you with a quick back of the envelope calculation. If you are using a 270 Liter Tangential Internal Mixer, you are engaging at least two units of 26″x84″ two-roll mills in the downstream section. Each 26″x84″ two-roll mill, requires around 180 kW (minimum) motor power – totaling to 360 kW (=180 x 2) only for the mills. For a similar capacity mixer, a TSS downstream will not seek more than 300 kW power (again, there is energy-efficient models available here). So, this rough calculation, when a TSS replaces the traditional dump-mill with sheeting mill set up, straightaway gives you 16.7% savings in energy (60 kW less).
  • Labor: The second aspect is the reduction is labor cost. Unlike two-roll open mills (with or without ), where you will need two separate operators, a TSS can be set up to perform sheeting function of rubber sheet without an hands-on operator at its vicinity. Even if not fully automated, you do not need an operator once the discharge of rubber sheet from TSS is fed into a Batch-off.
  • Reduced Contamination: In open two-roll mixing mills, your rubber mixing is exposed to the environment and it is difficult to control any dirt or moisture absorption by the compound during milling process. In a TSS, this is eliminated. Your rubber and its recipe constituents are mixed and sheeted-out in a closed environment under temperature controlled conditions right from the time you feed it into your internal mixer. Hence, with reduced contamination, you get a guaranteed higher quality of your mix compound.
  • Self-Cleaning Feature: The Screw and Barrel of the TSS is at a downward inclination (15º) angle from the feed chute section to exit of the roller die head. This incline ensures that compound flow to the exit of the barrel is reinforced and no material remains inside the TSS – hence, the self-cleaning feature.
  • High Mixing Line Efficiency and Productivity: When you install a TSS , your compound batch from the internal mixer is converted into a continuous sheet and the working of TSS can be automatically synchronized with rubber mixing line speed. This in turn, improves the mixing line performance making it more efficient. The continuous sheeting without operator involvement increases your mixing line throughput and overall productivity. Original Equipment Manufacturers (OEM) can offer you customized TSS models beneath internal mixers with throughput capabilities from 500 Kg per hour to 21000 Kg per hour (….and that’s a vast range by all means).
  • Effective Temperature Control: Your rubber compound discharge temperatures from TSS is reduced while sheeting out the material because no additional work (hence no additional heat) is introduced into your compound. Additionally, there is circulation of tempered (or chilled) water inside the conical twin screws, barrel and the peripherally drilled rolls of the roller die. This flowing water facilitates an effective heat exchange to take away the heat from the rubber mix and reduces the compound temperature at the discharge sheeting section.
HF Twin Screw Extruder

HF Make Twin Screw Extruder

  • Compact Layout: Most manufacturers offer various drive options, making the design of the TSS very compact yet sturdy. This means that a TSS can be accommodated under most internal mixers starting from the lowest production range of 16-25 Liter capacities based on the OEM standards.
  • Easy Maintenance: Further, the screw tips of the energy-efficient conical twin-screws do not touch each other and hence there is minimized wear of the screws. A rapid action hydraulic cylinder arrangement for clamping and moving the roller-die calender on rails facilitates the cleaning of the screw tips and insides of the barrel tip during your scheduled maintenance. Also, the TSS does not require external pushers, as in case of single-screw dump extruders. These features make a TSS maintenance easy for you.
  • Additional Features: With increasing trend of Silica usage in rubber compounding, you need to be cautious of the metallurgy and surface treatment characteristics of any rubber compounding machinery you buy. Hence, explaining the major ingredients of your recipe to your OEM is of paramount importance. For example, in TSS you can seek rollers that has hard-surfaced rolls if you are processing silica compounds. This will minimize the compound sticking to the TSS roll and increase its life.

Lastly, this physically very sturdy and robust, rubber machinery is designed for intrinsically safe mixing line operation.

Summarizing, with its capabilities for conventional and diverse applications, a TSS is emerging as the standard downstream equipment in the rubber compounding process for masterbatch and final mixing lines. And that is why we love Twin Screw Sheeter.

How about you?

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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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Peripherally Drilled Rolls or Centrally Cored Rolls?

Do you use centrally cored rolls or peripherally drilled rolls in your two-roll mixing mills? Or a combination of both?

Roll selection for a mixing mill is of decisive importance for the quality of many high-tech products manufactured by the rubber industry. Open two-roll mills in rubber processing are recommended when quick cooling for the batch being mixed is sought, say for example in final mix compounds.

Generally, these rolls are made of Chilled Cast Iron (CI) through a process of vertical casting. Chilled CI has greater resistance to deflection and uniform heat transfer characteristics. Depending on the presence or absence of alloy, the hardness of the outer working (chill) zone could be in the range of 460-650 HV with a thickness 12-20 mm.

Basis the application, manufacturers take extreme care on the properties of the rolls that include breaking strength from journals and core material, thermal conductivity, surface quality and wear resistance of the roll, overall machining and surface quality.

As these rolls operate at high speeds, precise concentricity with proper balancing of rolls is a prerequisite for efficient utilization of material and energy. The surface quality of the rolls is crucial for the products to be produced. The smoother and more precise the rolls, better the product.

Viscous deformation of the rubber compounds occurs between the rolls of mills during mixing and mastication. This generates heat that needs to be removed through effective cooling. Hence, water circulation passages for cooling are an essential feature of the roll design in rubber mills. These passages allow a pre-defined circulation of the cooling agent (mainly water) and ensure that the temperature can be kept within a prescribed tolerance over the entire face length of the rolls.

Two designs are normally available – centrally cored rolls and peripherally drilled rolls. Peripherally drilled rolls are possible for diameter greater than 150mm (or 6 inches). The cross-section of a centrally cored roll is easy to visualize. But, ever wondered how the insides of a peripherally drilled roll looks like?

Well here is with a sectional view with water flow.

Peripherally Drilled Roll

The water entering into the roll is cooler (blue colour) and as the heat transfer occurs, the water temperature rises gradually (red colour at exit).

The efficiency of heat transfer is relatively higher in the case of peripherally drilled rolls than in centrally cored rolls due to close proximity of the water channels to the roll surface. In peripherally drilled rolls, the passages for heat exchange are provided approximately 25mm under the roll surface and can vary nominally between manufacturers. Reputed roll manufacturers like Walzen Irle, Leonhard Breitenbach and Karl Buch, in their decades of existence, have built their own standards. Roll manufacturers are also available in China, Taiwan and India for various sizes.

The manufacturing processes and costs involved in producing a peripherally drilled rolls is relatively high, hence they are priced higher than cored rolls. Your choice of peripherally drilled or cored rolls depends on the quality of rubber processing required in mixing mills and the marketability of your rubber products for a price that profitably covers your investment.

Any state-of-the-art Calender in rubber processing also use peripherally drilled rolls for its stated advantages.

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