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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Despite these advantages, the HFE’s are getting outdated in many applications.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.


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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Top 25 Things You Should Know to Discuss with Mixer Rebuilder

The first step to rebuilding your internal mixer is to select the right rebuilder. (Check out 17 essential questions to select the right rebuilder for your internal mixer). After, the mixer is disassembled and cleaned, a joint inspection (between the rebuilder and you/your company representative) should be conducted at work site.

During this joint inspection, you will need to take or approve decisions on different critical components of the mixer. Some of these could be rebuilt, few repaired, very few reused as it is and some will have to be replaced fully. If you choose your options wisely here, you avoid being shortchanged on the final scope of work and get the best Return on your Investment (ROI)….after all you would not want to pay for something that you can reuse!

Internal Mixer

Internal Mixer: An image from the web

So what are the 25 key things you should prepare for discussions on rebuilding your internal mixer? Lets list them.

  1. Chamber (Drilled) SidesReuse or Replace. If the bore is out of tolerance and/or surface worn out, this component cannot be reused.
  2. Rotors – Reuse or Replace. Some rotors can be reused by rebuilding the profile if they are not too old or not excessively worn out of shape.
  3. Rotor End (RE) PlatesReplace. Rarely can this component be rebuilt.
  4. End Frames – Reuse. In most of the mixers, this component can be reused unless the end frame casting has cracked.
  5. Bearings – Reuse or Replace. Rebuilders will recommend replacement on a cautionary note, however exceptions are possible if the bearings are new or not damaged during disassembly.
  6. Dust Stop Assemblies – Replace. Always
  7. Oil injector – Replace. Always
  8. Couplings – Reuse. Because these are long-life components.
  9. Door SupportReuse. In most of the mixers, this component can be reused unless there is a crack.
  10. Drop Door Shaft –  Reuse. In most of the mixers, this component can be reused unless excessively worn out.
  11. Drop Door (Door Top) – Replace. This component usually exhibits higher wear and tear. Hence, unless it is new, usually recommended for replacement.
  12. Latch Assembly – Reuse. In most of the mixers, this component can be reused as it is.
  13. Linear Actuators – Reuse. Rebuild and service them after dismantling.
  14. Throat Plates – Replace. This component wears out due to constant contact with floating weight.
  15. Floating Weight (Ram) – Reuse or Replace. Depending on the condition.
  16. Hopper – Reuse or Replace. Rebuilders will recommend replacement together with mixer body. Depending on the condition, this can be examined for reuse.
  17. Hopper Front, Rear & Side Plates – Reuse.
  18. Hopper DoorReplace. Always.
  19. Piston Rod, Plates & Cup Seals – Replace. Always.
  20. Grease & Oil Lube Assemblies – Replace. Always.
  21. Hydraulic Cylinders – Reuse. Change the seals.
  22. Hardeners & Fasteners – Replace. Always.
  23. Seals & Bushes – Replace. Always.
  24. Thermocouples – Replace. Always
  25. Rotary Joints – Replace. Always

Initial budget estimates notwithstanding, these decisions on “Reuse or Repair” that you take on the components (after disassembly and joint inspection) impact the final cost of mixer rebuilding. The cost could either increase or decrease depending on the trade-offs you are willing to take. If you have chosen a good rebuilder, he will guide you towards an informed decision – finely balancing the cost and the optimum restoration of your mixer.

Have you rebuilt your internal mixer? If so let us know your experience.

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