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

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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