PlusTio

Die Design - 1. The role of a die is to from the melt into a desired linear product : fibres, films, sheets, profiles, rods , etc . 2. The die is a channel, whose profile changes from that of the extruder bore to an orifice, which produces the required form. 3. The dies can be classified using different criteria.     Example - Considering cross section of the extrudate one may recognize dies to produce -     • Solid Cross - Sections (Click Here)     • Hollow Cross - Sections 4. Another classification scheme is based on the die attachment to the  extruder barrel -     • Straight - throught Dies - (Click Here)      • Cross - heat dies-  (Click Here)     • Offset dies - (Click Here)

Breaker Plate - 1. This is performed metal disk,  with a multitude of small diameter holes. 2. The breaker plate is placed in between the screw and the die assembly. 3. It's principal function is to provide support for the screen pack . 4. The breaker plate helps also to convert the spiral flow of the polymer melts leaving the screw into a linear, axial one. Screen Pack - 1. Screen Pack is usually a multilayered,  nearly symmetrical sandwich of metallic wire screens.  2. It is placed between the screw and the breaker plate,  using the latter for physical support. It's  role is to eliminate the particulate contaminants.  3. The screen pack composition is always based on trade-offs, there is need to eliminate particulates using screens with high mesh,  but at the same time this create flow restriction,  it reduced the throughput, thus low mesh would be preferred.  4. Decision should be made as for as the maximum tolerable size of the particulate contaminan

Screw Cooling - 1. Larger screws are bored though for passage of heat transfer medium. However, care must be used not to over-cool the screw. 2. The cooling may be freeze a layer of plastic on the screw root, reducing the channel depth thus producing more shear at a cost throughput. 3. This may  also reverse the required relationships between the friction coefficient (low friction coefficient on the screw, high on the barrel),  further reducing the drag flow. 4. Furthermore,  there is a danger that the material staying a long time near the screw root will degrade,  contaminating the product. 5. It is important to remember that the conveying ability of the screw is controlled by the friction coefficient ratio : f(barrel)/ f(screw) it is important to maximize this ratio. 6. Under normal circumstances the minimum value of the ratio that guarantees conveying is 1.4 7. Screw cooling may be recommended to prevent decomposition of heat sensitive materials. 8. However, it should b

                    : Barrel Cooling : 1. Barrel cooling is needed to prevent overheating that may cause degradation. 2. For small extruders fans that blow air over or around the barrel are used. 3. Other cooling system used include . 4. Cooling channel inside the barrel wall. 5. Find on the barrel or on the heaters to speedup that transfer. 6. A watet-fog spray over barrel. 7. Continuous, controlled vaporization of liquid (water) 8. Copper tubing carrying cold water is sometimes used.               *Hopper Cooling Systems* • Water-cooling is used to cool the hopper throat to prevent bridging and to protect the rubber parts present in the screw support  assembly.

Steam heating : 1 . The high specific heat and latent heat of vaporisation of water makes steam an excellent heat transfer medium. 2 . Howevery, 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 effect.

Fluid Heating System : 1. The Heating fluid, that is most commonly used for extruders is oil. It may be heated by any suitable means (mainly electrical). 2. The heating system consists of a heater a circulating pump,  a surge tank, and a heat transfer channel in the extruder barrel.

Band Heaters : • They consist of Ni-chrome or other resistance wires mica or ceramic insulated,  then encased in steel cover.             *Mica insulated * 1. Flexible , supplied as a single piece . 2. Can withstand a load of 23-31 KW/m2 3. Shorter service life 4. Less expensive            *Ceramic insulated * 1. Rigid,  supplied in 2 halves. 2. Can withstand higher heating load 3. Better services life 4. More costly

√ The insulated heating elements are cast into semi-circular or flat aluminum blocks,  which are machined to match the surface to be heated.

Heating Systems ( Element ) - There are three methods of heating extruders : 1. Electric. ( Three Type heating ) 2. Fluid . (click here) 3. Steam Heating. (click here) Electric Heating : (a) . Induction Heaters. (click here) (b) . Cast-in Heaters. (click here) (c) . Band Heaters (Two Type Heaters). (click here)          (i).  Mica Insulated          (ii). Ceramic Insulated √ The electric heating is most commonly used due to : • Accuracy • Reliability • Easy to hook up.                  *Cooling Systems*     1 . Barrel Cooling. (click here)   2. Hopper Cooling. (click here)   3. Screw Cooling. (click here)

1 . AC Current passed through coil thus setting up a magnetic flex. Heat is generated from the resistance offered to the eddy current set up by the flux. 2 . The barrel is heated directly by its resistance to the induced current.                   * Advantages *    √ Accurate  Control of Temperature.    √  Good provision for cooling the barrel.     √   No possibility for hot or cool spots.                      * Disadvantages *     √ Relatively  high cost.        

Thrust Bearing - The screw fits into a thrust bearing located behind the feed hopper. The function of the thrust bearing is to absorb the thrust force acting on the screw inside the extruder barrel. 1. The bearing prevents  the screw from moving  backward . 2. Bearing life-time depends on the pressure and screw speed. For high speeds, oversized being is needed. 3. For twin screw extruders several smaller bearings joined in one shaft is used. (Thrust Force = Extrusion Pressure X Cross Sectional Area of the screw )

Drive Moter : 1. Drive moter provides the torque that turns the screw, and thus provides power to push out the material. 2. DC-Motor are widely used. 3. AC-Motor is used fluctuations in output (surging)  may occur.   <Gear Reduction And Torque Transmission> 1. The role of gear reduction and transmission is to reduce the motor speed (1750-2000 rpm) to workable screw speed (15-200 rpm) and transfer the motor torque to the screw. 2. For the variation of screw speed over a wide range without reducing it's  torque,  strapless speed changers are used. 3. In the older and smaller extruders,  V belts offered direct coupling,  to transmit the motor torque to the reduction gearbox. In modern machine the transmission is achieved by direct coupling.

Ex truder Screw : 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 Single Screw Extruders (SSE) * 1. Plasticating the material (softening,  melting and mixing the melt). 2. Conveying 3. Pressurizing and pumping .            * Advantages of Single Screw Extruders (SSE)* 1. Uniform heating 2. Continuous process 3. Better mixing than in ram extruder 4. All types of resins can be processed. Extruder Barrel : 1. Barrel is the cylindrical chamber in which the screw rotates. 2. Hopper is fixed to the top of the barrel. 3. In addition,  barrel may have one or several vents and feed ports. 4. Barrel is provided w

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. Flow in the 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 the chamfer on the down going side), as well on the type of screw and the barrel. 3. The solid conveying in the feed hopper usually depends on the gravity flow of particulate polymeric material moves down the hopper by its own weight. 4. Some materials have poor flow characteristics and may block the flow (know as arching or bridging). 5. This is more likely to occur when the particle shape is irregylar, the particles are small, and the internal friction on the bulk material is high. 6. Highly compressible bulk material are also susceptible to solids conv

√ The Single Screw Extruders is a relatively inexpensive machine for small or medium size production lines. √ For line with the throughput exceeding 10 tons per hour (10/h), the capital costs of SSE and TSE are comparable. SSE that can produce 40 Tons/hour can be custom-built only few manufacturers. √ The Single-Screw Extruders consist of screw rotating in heated barrel or cylinder to which the material is fed.   1 . Feed hopper.  (click here)   2. Extruder Screw and Barrel . (click here)   3 . Drive system (motor,  gear box,  transmission). (click here)   4. Thrust Bearing. (click here)   5. Heating and Cooling Elements. (click here)   6. Screen pack and Breaker plate. (click here)   7. Die . (click here)   8. Temperature and pressure  controls. •Single Screw Extruders parts and function-(in short notes) 1. Screw Pump :    Combination of screw &Barrel 2. Hopper :   Funnel like device,  mounted on hopper throat. Holds a constant reserve of material

√ Ram Extruder is discontinuous Process √ It uses a ram or piston operating in a cylinder √ The material is a softened by thermal energy from the outside heaters √ After softening the material is forced out by the ram movement.  After that ram retracted,  the cycle is repeated.               < ADVANTAGES > √ There are no violent agitation of the material. √ Simple process control. √ Material that can not under go shearing can be processed. √ Material having high viscosity ( PTFE, UHMHDPE ) can be processed. √ Power consumption is low.               < DISADVANTAGES   > √ Large temperature gradient between the ram cylinder wall and the interior of the plastic bed. The gradient may lead to degradation of the surface layer to the resin. √ Low throughput √ Discontinuous process.

√ Orientation of the barrel (vertical & horizontal) . Mechanism of the flow generatio n ( screw, disk, drum, barrel, two-stage disk Extruders, etc . ) √ The other type of extruders are being rarely used, mostly for specific reasons ( the multi-screw machines have been popular for the large volume compounding  of PVC . ) Two Type Extruders : 1. Batch Type : (a).Ram Extruders. (click here) (b).Reciprocating Screw Extruders. 2. Continuous Type: (a).Screws less extruders =              √ Disk   Extruders             √ Drum Extruders             √ Other Extruders (b). Screw Extruders =         √  Single screw Extruders (SSE)          √  Twin  screw  Extruders  (TSE)          √  Multi Screw  Extruders       

1 . Continuous process 2 . In principle, the plastic raw material is plasticated by means of a screw plastications unit and the molten material is continuously pumped out of through a standard orifice (die) in order to take the shape and then the shape is set by cooling/sizing system. 3 .The required materials (Polymers,  Additives, Re-inforcements, etc.) are fed through a feed hopper into a barrel.  4. The polymeric material soften by the heat from the external heaters and generated by frictional  force.  5 .The energy needed to convert the cold particle into a hot viscous melt depends on the specific heat, heat of melting (crystalline polymers), the maximum temperature of the extrusion.  6 .The transfer of heat into individual pellets by thermal conductivity.  Thermal conductions is important in removing the heat from the melt in order to solidify it.  7 .The pressure required for the continuous extrusion can be generated by the extruder alone,  or by attached