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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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