Screw Nomenclature:
P - is the screw pitch, distance between the centers of a two adjacent flights.
C - is the channel depth o radial distance form the core of the barrel to the root.
© is the helix angle, defined as an angle between the flight to the transverse plane of the screw axis.
D - is the screw diameter, developed by rotating the flight about the screw axis.
Flight - is the helical metal thread of the screw.
R.D - is the root diameter
W - is the channel width
L - is the land width.
L / D Ratio = Length of Flighted Portions
Nominal Diameter
Nominal L/D ratios 22-24: 1. There are several advantages of long L/D:
1. Longer residence time
2. Greater output
3. Uniform output and greater mixing
4. Pumping at higher pressures
5. Greater melting with less shear and more conduction from barrel.
However, Short L / D also offers some advantages :
1. Short residence time that facilitates the processing of heat sensitive materials
2. Less space
3. Lower torque (hence high strength to screw and less horse power)
4. Lower cost.
Compression ratio is the ratio between the channel depth in the first flight of the feed zone to the depth in the last flight of the metering zone.
Compression Ratio ==>
Channel Depth in the First Flight of the Feed Zone
Channel Depth in the Last Flight of the Metering Zone
Depending on the extruded polymer, screw with the compression ratio varying from 1.5 to 4 should be selected.
Mixing elements
1. Several designs.
2. Incorporated in the metering zone of screw
3. Mainly to improve mixing, homogeneity
Extruder Screws:
1. General purpose screw,
2. PVC screw,
3. Nylon screw,
4. Two stage screw/vented screw,
5. Segmented screws is also available for special purpose.
Thrust bearing: When the screw is rotated the material is inside the barrel starts to move forward and the material give a back force to the screw opposing the material powers desertion. This frce is called thrust and bearing used to adsorb. The force is called thrust bearing.
Three type of thrust bearing:
1. Flat roller bearing,
2. Topered roller bearing
3. Spherical roller bearing.
- The screw fits into a thrust bearing located behind the feed hopper.
- The function of the trust bearing is to absorb the thrust force acting on the screw inside the extruder barrel.
- The bearing prevents the screw from moving backward.
- Bearing life-time depends on the pressure and screw speed. For high speeds, oversized being is needed.
- For twin screw extruders several smaller bearings joined in one shaft is used.
Heating and cooling elements:
Induction heaters: AC Current passes through coil thus setting up a magnetic flux. Heat is generated from the resistance offered to the eddy current set up by the flux. The barrel is heated directly by its resistance to the induced current.
Advantages:
1. Accurate Control of Temperature.
2. Good provision for cooling the barrel
3. No possibility for hot or cool spots.
Disadvantages:
1. Relatively high cost.
Cast in-heaters: The insulated heating elements are cast into semi-circular or flat aluminum blocks, which are machined to match the surface to be heated.
Band heaters: They consist of Ni-chrome or other resistance wires mica or ceramic insulated, then encased in steel cover.
Fluid heating system: The heating fluid, which is most commonly used for extruders, is oil. It may be heated by any suitable means (mainly electrical). The heating system consists of a heater a circulating pump, a surge tank, and a heat transfer channel in the extruder barrel.
Steam heating: The high specific heat and latent heat of vaporization of water makes steam an excellent heat transfer medium. However, this system is not frequently used because of low maximum temperature that can be achieved, a need of working with high pressure piping, frequent leaks of steam that require shutting down of heating for repairs, and corrosion effects.
Cooling: For cooling the mould is transferred to the cooling station while still rotating. The cooling should be made as quickly as possible to avoid the plastic part to shrink away from the mould. Otherwise the part will get distorted. Cooling can be done by air or water. To provide faster cooling cold water is sprayed over the mould.
Mould temperature control: Polyolefin’s are processed in chilled moulds. Engineering thermoplastics are moulded in heated moulds. Mould temperature of 70-80º C is common when good mould surface reproduction is required.
Barrel cooling: Barrel Cooling is needed to prevent overheating that may cause degradation. For small extruders fans that blow air over or around the barrel are used. Other cooling systems used include:
1. Cooling channels inside the barrel wall
2. Fins on the barrel or on the heaters to speedup heat transfer
3. A water-fog spray over barrel.
4. Continuous, controlled vaporization of liquid (Water)
5. Copper tubing carrying cold water is sometimes used.
Hopper cooling: Water-cooling is used to cool the hopper throat to prevent bridging and to protect the rubber parts present in the screw support assembly. The barrel temperature may be carried out upto upper mouth along with partial melting of feed causing a problem called lump function for which feeling is blocked and processing will also stop. The hopper cooling also prevents lump formation and thus gives the ease of processing.
Screw cooling: The cooling may freeze a layer of plastic on the screw root, reducing the channel depth thus producing more shears at a cost of throughput.
1. This may also reverse the required relationship between the friction coefficient (low friction coefficient on the screw, high on the barrel), further reducing the drag flow.
2. Furthermore, there is a danger that the material staying a long time near the screw root will degrade, contaminating the product.
3. It is important to remember that the conveying ability of the screw is controlled by the friction coefficient ratio:
F (barrel) / f(screw) ó it is important to maximize this ratio.
4. Under normal circumstances the minimum value of the ratio that guarantees conveying is 1.4.
5. Screw cooling may be recommended to prevent decomposition of heat sensitive materials
6. However, it should be carried out using the cooling fluid at the temperature above the softening point of the principal polymeric component.
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