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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Despite these advantages, the HFE’s are getting outdated in many applications.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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7 Quick Tips About Batch Weight Calculation For An Internal Mixer

Internal mixer is a standard rubber machinery for volume mixing in both tire industry and non-tire rubber industry.

When you use one, your most elementary requirement is to calculate the batch weight for your respective mixer model. Because when mixing rubber compounds, you should understand that different compounds based on the same polymer might require different batch weights. And different polymers will almost certainly require different batch weights.

Bainite Make Intermeshing Mixer

Image Courtesy: Bainite Machines Pvt Ltd

Here’s 7 quick tips for you to fix the batch weight for your rubber mixing. (Updated on 23rd Dec 2015: Flip through this post in our digital edition and download here)

1) Theoretical Equation

The thumb rule is the theoretical equation

W= NV x SG x FF

where W – Batch Wt [kg]; NV – Net Mixer Volume [dm³]; SG – Specific Gravity (density) of the mixed batch [kg/dm³]; and FF= Fill Factor.

Generally, most mixer manufacturers share this calculation with you. But remember, what they give you is only a theoretical number. This is only a starting or reference point and you need to arrive at your own mixing batch weight for your compound recipes, following some of the other tips stated below.

2) Net Mixer Volume (NV)

Since Internal mixer has a fixed volume mixing chamber, knowledge of the net volume (in liters) is required. This can be obtained from the manufacturer directly or in some cases from their literature for their various models.

When the mixer is used regularly (or if you have procured a used-mixer) the effective volume increases due to wear on the rotors and mixing chamber. If not compensated for this inside wear, your batch volume will be effectively too small leading to insufficient ram pressure on the compound, poor dispersion and longer mixing times. Annual measurements of chamber are recommended to update your batch weight correctly.

Excessively worn out mixers will have to be rebuilt or reconditioned (Read our posts on mixer rebuilding – Top 25 Things You Should Know to Discuss with Mixer Rebuilder and 17 Essential Questions to Select the Right Rebuilder for your Internal Mixer)

3) Guesstimate the Fill Factor (FF)

If you have a Tangential Mixer (aka Banbury) , then your FF can range between 0.70 and 0.85. And for a Intermeshing Mixer (aka Intermix), your FF can range between 0.62 and 0.70.

Knowledge of the fill factor is necessary because an under-filled mixing chamber results in the ram bottoming out too soon. This reduces the pressure on the rubber stock and increases the mixing time. An over-filled chamber leads to unmixed ingredients staying in the mixer throat. This creates a mess under the mixer when the batch is dumped.

For example, NR-rich compounds in an intermeshing mixer has a fill factor of around 0.65 while for the same compound in a two-wing tangential mixer, it is about 0.75. This compound will have an increased FF of about 0.78 for a tangential mixer with four-wing rotors. Each polymer also has its ideal fill factor and that varies again with Mooney viscosity and filler system.

Fill factor of a mixer depends on the age of the machine, wear and tear of the rotors and chamber, the rotor type, rotor speed, rotor friction ratio, nature of elastomer, ratio of elastomers/ fillers, mixing sequence, kind of polymers, fillers and individual SG of the ingredients in your recipe, viscosity of ingredients, etc. Generally, the lower the compound viscosity, the fill factor is higher.

Hence, we initially guesstimate the FF before stabilizing on the figure later on through actual trials.

4) Estimate the Specific Gravity (SG) of your Compound

You can estimate the density of your compound by multiplying the quantity of each ingredient with its individual density (you can get this figure in any compounding handbook or ingredient supplier literature). Sum up your individual results and then divide this number by the total sum (usually phr). The result will give you the estimated density of the compound. (Mathematically, this is the weighted average calculation).

For example, lets consider a sample recipe (I got this recipe from a web search) as below:

Recipe Ingredients  Volume Density Volume x Density
      (L) kg/L kg (or PHR)
SMR 10 106.4 0.94 100.0
Zinc Oxide 1.8 5.55 10.0
Stearic Acid 2.2 0.92 2.0
N550 Carbon Black 27.8 1.8 50.0
Oil 10.9 0.92 10.0
Antioxidant TMQ 1.9 1.08 2.1
Antiozonant DPPD 1.6 1.22 2.0
Sulphur 0.1 2.07 0.2
TBBS 1.6 1.29 2.1
TMTD 0.7 1.35 0.9
Total 155   179.3
Compound SG  (179.3/155) 1.16

Calculating, the SG of this Compound mix is arrived at 1.16 (=179.3/155).

5) Know your Internal Mixer

Knowing your internal mixer – its capabilities, design features like rotor (tangential or intermeshing), ram (pneumatic with dedicated air supply at the plant or hydraulic), variable speed capabilities of the motor, SCADA, PLC, automation and control features, etc.

Rotor speeds are critical because you can use higher speeds at the initial mix and then reduce the rotor speed to allow the batch to “knead” well.  This will allow you to get both your dispersion and distribution tasks of mixing right. Hence, when selecting a mixer explore variable speed drives since it give you advantage in your mixing process.

(If you are planning a new purchase, read and download our Questionnaire for Internal Mixer Selection)

Similarly, think of ram pressure.  If your ram pressure is too high you will cause excessive heat build up and poor flow of ingredients across the rotor tips. In intermeshing mixers, this will also cause internal pressure within the mixing chamber and might cause mixer failure. If ram pressure is too low, then you will not get the ingredients down into the rotors and this will result in poor mixing. (Read more about Hydraulic Ram here)

Banbury Mixer

Image of HF Mixer

6) Watch the Ram Action

After the above reference calculations are done and mixing initiated; watch the ram action during the mix. The ram should start high, move up and down about an inch or two and bottom out when mixing is complete. Good mixing practice dictates that when the ram bottoms out about 30 – 45 seconds before the batch is dumped, you can be assured that the chamber is properly filled and mixed compounds will be of high quality.

You need to observe the position of the ram by watching the tell-tail rod attached to the top of the ram. Hence, this requires more of practice and experience than theoretical knowledge.

If you have a good mixing system with controls and feedback features, you can correlate the position of the ram with the current and rise in temperature – these are important to get an optimized batch size and high quality of mix.

7) Optimize Your Mix Batch Size (…Do Not Maximize)

The key to successful mixing is optimizing your mix batch size, and not maximizing. And good mixing is a form of art.

Most mixer users want to get the most out of their internal mixer (quite natural!) and they test its capabilities to the full. Finally, when they get poor mixing, they wonder if they have done the right investment! 

If you try to take your batch size to the upper limits of the mixer’s “capacity” as specified in the manufacturer’s manual (that is usually a peak magical figure) and you have raw material variations such as particle size or bulk density changes in your fillers, this can lead to poor mixing (dispersion and distribution of ingredients).

The right batch size will be smaller, but your internal mixer throughput is increased by shorter mixing time and thus more batches in the same period. Thus, optimizing your batch weight will allow you to get consistent batch quality and repeatability that are of paramount importance to your (or your customers’) downstream processes.

The key factors that will influence your mixing optimization are compound formulation, ram pressure, mix procedure, mixing speed and rotor design.

Each mixer is different and it would be very difficult to determine the optimized fill factor without actually conducting several mixing trials. Experience is a key to good mixing.

Summarizing, when mixing rubber compounds, different compounds require different batch weights. These 7 tips will help you calculate the optimized batch weight for your compounding recipes on an internal mixer quicker.


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