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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 SpA, Twin Screw Roller Head Extruder (TSR) from Kobelco, Twin 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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A Dummies Guide to Rubber Extruders

Rubber extruders have a varied field of application. So, when you come across a rubber profile, strip, hose, cable, wire, cord coating, tire tread, v-belt, tube, or blank remember that they are only a few handful of products manufactured using extrusion process. 

And your main goal of extrusion is to get the highest output at good quality of product.

Based on the convenience and usage history, you need to know of the two types of rubber extruders viz. Hot Feed Extruders (HFE) and Cold Feed Extruders (CFE).

And further, you have variants within Cold Feed Extruders viz. Plain Type, Pin Barrel Type and Vent Type Extruders. The recent most advanced adoption that you will come across is that of co-extrusion lines.

When you choose a rubber extruder, you should deliberate and discuss extensively upon few key things i.e the design, material technology and manufacturing accuracy of screw, barrel and die-head. While, I would cover more on them in a later post, I would like you to remember that very few manufacturers world-wide can guarantee you a well-designed and precisely manufactured rubber extruder.

Hot Feed Extruder (HFE)

Rubber industry only had Hot Feed Extruders until 1950’s. HFE’s extrude your rubber compounds at reduced temperatures.

Hot Feed Extruder with Dual Head

Bainite Machines Make Hot Feed Extruder

The screw depth of a HFE is relatively larger and you get a consistent output due to its short screw design. L/D ratio is mostly in the range of 4:1 to 6:1 which keeps your rubber compound dwell time and its temperature increase to a minimum.

Each HFE has an hopper and feed roller section with spiral undercut liner that allows your compound to enter the extruder easily. The feed roller on a hot feed extruder allows your compound to pass the scraper knife, directed around the roll and then fed back into the hopper. The feed roll bearings are placed in positions to prevent contamination. You can vary the output by changing the screw speed using variable speed drives.

Despite these advantages, the HFE’s are getting outdated in many applications.  Because the rubber that is fed into a Hot Feed Extruder needs to be pre-heated or warmed using two-roll mills to achieve the required degree of viscosity and temperature that facilitates smooth flow of rubber, its compaction and extrusion through the die.

And that made experts introduce Cold Feed Extruders.

Cold Feed Extruder (CFE)

Cold Feed Extruders are designed and manufactured with specially designed screws best suited for cold feeding of rubber. You can discuss with your manufacturer and avail various options of screws for a wide range of compound and extrusion applications.

While manufacturers offer L/D ratio up to 24:1, the most preferred by end-users is generally in the range 12:1 to 18:1.

Pin Type CFE

Pin Type CFE Image from Web

For feeding the cold rubber, it is recommended that you use a feeding conveyor with metal detector to remove metal particles. This avoids damage to the screw or barrel. In some plants, I find the sensitivity of the metal detector calibrated to a low value that it virtually renders the detector useless.

Every CFE comes with a Temperature Controller Unit (TCU) that controls the barrel temperature so that the shape and size of the extruded products are uniform.

Variants in cold feed extruders along with their uses are

  1. Plain Barrel Type Cold Feed Extruder – These CFE’s as the name suggests have a plan barrel and used in manufacturing of hoses, blanks, fluorocarbon rubber, butyl rubber, etc.
  2. Pin Barrel Type Cold Feed Extruder – These CFE’s have around 80-100 pins protruding out of the barrel towards the screw center. These pins enhance the mixing and dispersion of your rubber as it is kneaded between the barrel and screw. And the result is processed rubber with outstanding homogeneity and extrudate quality. This flexibility in Pin Type Cold Feed Extruders endears to all making it a universal extruder for many rubber compound formulations involving varied applications. Hard rubber compounds also can be processed because of high extruder torque.
  3. Vent Type Cold Feed Extruder – Vent type or vacuum type extruders were developed for production of non-porous profiles and hoses. These CFE’s have a custom-built screw, and a degassing barrel with a vacuum pump attached to vent bubbles out of extruded compounds.
Triplex Extruder

Nakata Make Triplex Extruder

Co-extrusion: Customer-specific customization and usage complexity demands led to the introduction of co-extrusion for manufacturing of various profiles. And so you today have Simplex, Multiplex (Duplex, Triplex, Quadruplex, and Quintuplex) and Roller Head technology. Multiplex lines of piggy-back type of 2,3,4, and 5 layers have a compact construction.

Roller-Head-Extruder

Berstoff Make Roller Head Extruder

Roller Head Technology involves a combination of extruder with preform head and two-roll calender. They offer twin advantages of – high uniformity of the material thickness over the entire sheet width with absence of air traps even at higher thicknesses (~ 20mm thick as against conventional calender lines that give max 3mm thick sheets with or without air traps) and excellent homogeneity of the material produced.

Both these characteristics are important for high-quality rubber products such as tire components, V-belts, conveyor belts, tank linings, cover sheets, blank sheets and roofing sheets, etc. For even thickness across the entire sheet width of the roller head, there are three options that can be used alone or in combination with one another – roll crowning, roll crossing or roll bending, that will compensate for the elastic deflection of rolls. (I will cover more on these in an another post)

In today’s world you will see that usages of these technologies are overlapped. For example, in tire industry, you can notice that
Tread & Sidewall are extruded using Simplex, Duplex, Triplex, Quadruplex Lines
Apex are extruded using through Simple & Duplex
Inner Liner are extruded using Simple, Duplex & Roller Head Technology

Rubber extrusion is in itself a vast subject, however if you know the above terms and types of rubber extruders, you have made a good beginning.

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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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