Dr.S.N.Chakravarty sent me this informative piece on Extruder And Extrusion Process.
Here is Part 1 of this two-part series.
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.
- a device system
- a barrel
- a ram or screw for forcing the rubber through the barrel and
- 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
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.
- 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.
- 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 email@example.com
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