SINGLE-SCREW EXTRUDERS (SSE)
The Single-Screw Extruder consists of a screw rotating in heated barrel or cylinder to which the material is fed. As shown in the figure, the following parts can be identified.
1. Feed hopper
2. Extruder Screw and Barrel
3. Drive system (motor, gear box, transmission)
4. Thrust Bearing
5. Heating and Cooling Elements
6. Screen Pack and Breaker plate
7. Die
8. Temperature and pressure controls.
Feed Hopper
1. The hopper purpose is to transfer a polymer into the extruder barrel, where it is compressed. This compressed forces the air out from between interstices of resin pellets or rubber chunks (air is expelled back through the hopper).
2. It also breaks up lumps and polymer agglomerates, creating a more homogeneous feedstock that can be readily melted.
3. Flow in hoppers depends on their type (gravitational or forced), bulk density of the material, the shape and location of the feed inlet (direct or tangential feed, straight or with a chamfer on the down-going side), as well as on the type of screw and the barrel.
Extruder Screw and Barrel:
1. Due to screw rotation the frictional heat is produced.
2. The material flow in the screw channel in a spiral pattern, with the mixing.
3. However, by contrast with the ram extrusion, here the thermal and frictional heat affects continuously renew the material surface, thus the thermal degradation is not as severe.
4. The softened material is forced out by the screw.
Functions of SSE:
1. Plasticating the Material (i.e., Softening, Melting, and Mixing the melt).
2. Conveying
3. Pressurizing and Pumping
Advantages of SSE:
1. Uniform heating
2. Continuous process.
3. Better mixing than in ram extruder
4. All types of resins can be processed
Functions of the Three SSE zones:
There are three zones in a SSE: Feed, Compression and Metering. In addition, there are the up-stream (feeding), and down-stream (cooling) zones.
Feed Zone
For good feeding the flight volume in the feed zone must be as large as possible, since the solids conveying ability of the extruder depends on the ratio of the friction coefficients on the barrel and on the screw root, frequently grooved barrel (cooled by circulating cold water and thermally insulated from rest of the barrel) is used in the feed zone. Its advantages are: increased throughput, greater extrusion stability and smaller effects of pressure variation at the die on the flow.
Feed zone comprises first 3-10 turns of the screw flight, located directly under the feed throat. The resin enters the extruder in this zone. Its functions are:
1. Collects the granules from the feed hopper
2. Compacts the granules and heats up them building up pressure as they advance down the screw.
3. Convey the material to the next zone.
Within the feed zone the extruder screw has a small, constant root diameter, i.e. larger channel depth than in other zones. This accommodates larger volume of the porous solid body, enhancing extruder throughput.
To prevent premature melting of the granules in the feeder throat (the effect known as bridging), the temperature in this zone is lower than in the others. Usually, hopper cooling is to be used. Furthermore, the grooved barrel section is usually provided with intensive cooling capabilities.
Compression or Transition Zone
The Transition zone follows the solids conveying zone. The purpose of this zone is to compress the solid bed and to provide intense friction between it and a barrel. Within this zone the channel depth progressively decreases, thus compressing the resin and forcing gases and volatiles from the melt to flow back to feed hopper.
Due to the progressive compression of the polymer bed in this zone, the material undergoes consolidation, then starts to soften and melt under the concerted action of the heater bands and the shear (mechanical) energy. Thus, melting is caused by the two principal forms of energy: the thermal, applied to the polymer by conduction from the external heaters through the barrel surface, and the mechanical converted to heat through friction and viscous dissipation. The friction coefficient of polymer on the metal surface (f) depends of pressure (increasing P by 10 increases f by a factor of 3 to 4), temperature (e.g., for PE: f = 0.6 – 0.002T 8C), and the sliding velocity. The conveying is controlled by the difference in the friction coefficient between the solid bed and either the barrel or the screw. It is important to maximize this difference.
The length of this zone very much depends on the type of polymer it is supposed to process. Screws for amorphous resins usually have longer compression zone than the semi-crystalline ones. In a SSE, this zone extends over a major part of the extruder, e.g., L(melt) = 10 –14D.
Metering Zone
This sector follows the compression zone and is characterized by a constant root diameter and shallow channel depth. The shallow channel depths ensure that high shear is added to the resin to melt the material homogeneously. In principle, at the entrance to this zone all solid particles should be melted. This Zone extends to the outlet of the extruder, thus it encompasses the screw pack, breaker plate and the die.
Once the resin is in the molten state, the extruder acts on it as a pump, transferring, homogenizing the molten polymers, and building up the pressure to the level required to force the material through the discharge nozzle or die. SSE is capable of delivering up to 70 Mpa of pressure.
Due to the high rate of shearing, pressure built-up so the resin can be pushed out of the barrel.
Metering zone gives the final mixing and ensures thermal uniformity.
In metering zone melt is brought to the correct consistency.
To obtain smooth extrudates, the melt should be well mixed originates from the resin back-flow. Since the back-flow is related to the melt pressure, there is a need to increase it by restricting the flow. The restriction can be increased by:
1. Decreasing the channel depth
2. Decreasing the channel width
3. Replacing the metering zone with a smear head.
4. Screw cooling.
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