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Rubber Extruders And Extrusion – Special Supplement

Rubber industry only had Hot Feed Extruders until 1950’s. Then came the Cold Feed Extruders and other variations as we see them today.

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.

Life today is mostly spent trying to catch up with latest developments. However, in the process the basics and fundamentals are sometimes overlooked. So, in this Knowledge On-The-Go Special Supplement, we bring to you a ‘collector’s edition’ on Extruders and Extrusion incorporating the fundamentals.

(Our digital editions are available on two platforms – Youblisher or Yumpu)

Extruders_Special Supplement

Click on the Image to Flip Through The Digital Book

Download PDF Here

I hope you find this special supplement informative.

(Disclaimer: All the pictures and statements in this special supplement are sourced from web or shared by respective companies. All copyrights belong to actual owner. Rubber Machinery World does not independently verify them nor will vouch for their genuineness, hence will not be liable for any misrepresented data. The images are used here for representation purpose only.)


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Editor’s Pick: Extruder And Extrusion Process (Part 2)

This is Part 2 of Extruder And Extrusion Process by Dr.S.N.Chakravarty.

Comparison between Hot Feed & Cold Feed Extruder

Hot Feed Extruder Cold Feed Extruder
L/D ratio varies from 4: 1 to 5: 1

In many cases the screw has increasing pitch.

L/D ratio varies from 15: 1 to 20:1.

The screw has decreasing pitch. The screw has increasing root dia with more flight depth at feed end which increase from feed to discharge.

Skill of warming mill operator and extruder operator is required to control uniform temperature and viscosity of stock. Warming mill not needed. But for preparation of strip mixing mill is needed.
Compound change requires cleaning/ changing of warming  mill and quick breakdown of new compound. No such requirement
Amount of scrap till correct extrudate comes out is dependent on operator’s skill. First few meters are scrap rough and nervy; there after uniformity attains and is less dependent  on operator’s skill.
Lower HP in extruder, but requires warming mills. Higher HP in extruder is required; but overall HP is less and no requires no warming mills.
Wearing of scroll is less. Wearing of scroll is faster.
More floor space is required. Less floor space is required.

Extruder Output Comparison

Maximum output lb./hr at equal rpm

Extruder inch

Screw Diameter mm rpm Hot feed Cold feed

3.5

90

70

900

1100

6.0

150

51

2990

3080

8.0 200 46 4620

5060

One of the basic characteristics of all the extrusion process is the clearance between Barrel and screw. This is generally permitted to a limit in order to :

  • control porosity
  • control extrusion temperature

Each manufacture has their own design of screw. However the basic function remain same. An adjustment can be made for volume occupied by the flight lands. Screw design has a direct bearing on out put and quality of Extrudate . Output rate depends  upon the screw design and geometry of extrudate .

Output of extruder is dependent on size of extruder i.e. the ratio of L:D. First estimate of out put is expressed as :

Output = A.(L/2).R.d

A         =          Cross sectional area of extruder

L          =          Lead Length

R         =          Revolution per minute

d          =          density of rubber

Generally these exists a liner relationship between RPM & output. Other factors also have bearing on out e.g. Pitch height , base depth, width and flight. This is expressed by following

Formula 1

Each of those parameters has an effect on extrusion characteristics and output.

However net out pat will be governed by the extension characteristics  desired by user for type of compounds. For example if temperature is a limit, this is to be optimized for each compound and shall be a crucial factor for out put.

Die Design

The die is the piece of metal which controls the final shape of extrudates. Generally die making and die design depends on experience, however a good control on compound properties gives better extrusion .

It is necessary to get a swelling factor at each point of profile for a particular compound. The same varies from centre of die to extreme ends. Die swell is defined as ratio of extrudate section & cross-section of die. Depends on Polymer, carbon – type & phr, viscosity,  Lead angle (Bevel ) of die etc.

Cold Feed Extruders

In recent year there has been an upward trend in production of cold feed extruder. In general sense an extruder which is fed with the compound at a temperature  above ambient may be said to be a hot feed extruder. On the other hand any extruder where compound is fed at ambient temperature or below may be called a cold feed extruder.  Some of the differences may be classified as under :-

FEED HOT FEED COLD FEED
  Prewarmed hot feed above room temperature No need to feed hot compound. It is fed at room temperature.
L : D Ratio Generally hot feed extruders are having 5: 1 or 4: 1 Generally vary between 12: 1 to 16 : 1
Mills A set of mill is required No mills are required
Floor space requirement More Less – Because of no mills.
Power & water consumption More Less
Temperature control Compound is homogenised on mills & is a manual job also. Compound is homogenised with better control .
Feed system Through a feed mill Must-as metal pieces may harm the pins.

Pin Type Cold Feed  Extruder

Most of the Cold Feed Extruders are now equipped with metallic pins fixed along the circumference in full length of screw. These pins help in homogenizing the compound. These number may be optimized by process & if required pins from last zone i.e. towards head may be blocked. These pins are generally 8 to 10 in circumference  in 10 rows will have 8 X 10 =  80  pins.

Dr. Chakravarty can be reached on kpspltd@gmail.com


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Editor’s Pick: Extruder And Extrusion Process (Part 1)

Dr.S.N.Chakravarty sent me this informative piece on Extruder And Extrusion Process.

Here is Part 1 of this two-part series.

Introduction

Extruders are machines, which shape rubber to a profiled strip by forcing it through a die. In the simplest form an extruder consists of four basic components viz.

  1. a device system
  2. a barrel
  3. a ram or screw for forcing the rubber through the barrel and
  4. a head holding the die which ultimately gives the desired shape.

Extruder drive system comprises of an AC or DC motor along with a reduction gear unit using V-Belt drive for power transmission or connected directly using suitable couplings. The extruder drive has to turn the extruder screw at the desired speed. It should be able to maintain a constant screw speed because fluctuation in screw speed will result in throughput fluctuation, which in turn will cause fluctuations in the dimensions of the extrudate.

The fundamental  distinction between two types of extruders is in mode of operation:

  • Continuous: Delivers rubber in a continuous manner and has a rotating member or screw; here the pressure is produced by a screw.
  • Discontinuous: Delivers rubber in an intermittent fashion and has a reciprocating  ram or screw, these type of extruders are ideally suited for batch type processes such as injection moulding.

Ram Type Extruders

Barwell Ram Type Extruder

Barwell Ram Type Extruder

In ram extruders, a quantity of warm compound is placed into the cylinder, the die is attached to cylinder and ram pushes the compound through the die to form a profiled section.

Advantages:

  • Extrusion can be carried out at lower temperatures.
  • Difficult compounds can be extruded.
  • Easy to clean.
  • Useful to short runs.
  • Useful for compounds which need to be strained through gauge for quality products requiring completely contamination free material .

Screw Type Extruders

A screw extruder consists of five components (a) drive system (b) a feed hopper ( c ) a screw rotating within (d) a barrel (e) a head and die. The basic principle is that the screw carries material from the feed hopper by acting as a conveyor or a hump providing pressure to extrude or force the material along the machine barrel through the head and the die.

Screw Type Extruder

Screw Type Extruder With Description

  • Feed hopper: the purpose of which is to receive material and pass it down to the flights of the screw. It is mostly supplied in the form of strip.
  • Barrel: within which the screw rotates, the usual clearance is approximately 0.40 mm. Normally, the barrel is fitted with a detachable liner in the form of a sleeve which is highly wear and corrosion resistant made of hardened steel . The barrel is made double walled for steam or water circulation so that a constant temperature is maintained in the extruder head.
  • Screw: A conventional extruder screw has three geometrically different sections.
    • Feed section (closest to the feed opening) generally has deep flights and consists of approximately 1/5th of the length of the screw. The material in this section will mostly be in solid state.
    • Metering section (closest to the die ) usually has shallow flights and consists of approximately 2/5th of the length of the screw . The material in this section will mostly be in molten state.
    • The third section, which connects the feed section and the metering section, is called as compression section. In going from feed section towards the metering section, a compression of the material in the screw channel takes place, which is essential for the proper functioning of extruders. The compression favors streamline flow and helps to eliminate air and also ensures a constant pressure in the head.

Extruders are usually designated by the diameter of the extruder barrel. The ratio of relative output of extruders varies as the square of the screw diameter . Thus the output of a 60 mm extruder will be 2.2 times that of a 40 mm machine.

An additional designation often used is the length to diameter (L/D) ratio. This is an important factor in the selection of extruders to match process requirements. The length of a rubber extruder depends on whether it is a hot feed or cold feed extruder.

Hot feed extruders are usually very short about 3D to 5D while cold feed ones range  from 12D to 20 D .

Screws are made of steel alloy forging, heat-treated to a machinable  hardness and hard-chrome plated. The flight lands are surface hardened to approximately 600 Brinnel. The screw is internally bored to facilitate water-cooling.

  • Head: the purpose of which is to equalise the pressure from the screw and barrel and to transport the compound smoothly at equal pressures and speed to the die.
  • Die: The purpose of which is to give the compound the desired shape. The extrudates shrink along their length and increase in thickness and width, the behaviour being termed “ die swell “ which depends upon (i) rheological characteristics compound  (ii) shape of head and extrudate (iii) pressure in the head (iv) the head and compound temperatures .

The  extruder is operated in such  a manner so that temperature is gradually more from feed to discharge, the die being the hottest part.

Cross Head Extruder

This is used to cover hose, wires, tyre bead wire and the extruder function here remains same as for regular extruder and cold feed type is preferred. The extruder barrel, screw and head can be heated / cooled to desired temperature. The material to be coated comes at 90° angle to the extruder screw system, and rubber thus makes a 90° turn.

In the conventional screw extruder, the rubber compound is conveyed in three states, as solid, then as a mixture of solid and melt and finally, as a melt. The movement of the material in the barrel is the resultant of four flow mechanisms. The two of these are the drag flow and transverse flow.

The drag flow results due to the forward conveying action of the material towards the die, produced by the relative motion between the screw and the barrel while the transverse flow results in a circulatory flow which is important for heat transfer and mixing but is not contributing directly to the  extruder output.

In Part 2 of this article, you will read more of COMPARISON OF EXTRUDERS.

Dr. Chakravarty can be reached on kpspltd@gmail.com


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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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