Extrusion

Introduction to Extrusion

Extrusion plays a prominent part on the plastics industry. Extrusion is a continuous process, and can be adapted to produce a wide variety of finished or semi-finished products, including pipe, profile, sheet, film and covered wire.

Extruder

Extruder is the most important part of Extrusion line. In order to produce satisfactory extrudate it is necessary to apply heat to the granules in order to soften them and make the resulting melt capable of flow under some pressure. This is carried out rotated in the barrel by means of gear box and variable screw drive or Eddy current motor.

 Therefore the screw barrel has following functions :

• pumping
• heating
• mixing
• pressurizing

In order to make each function as effective as possible it is normal practice to divide the screw into 3 zones:

• feed zone at hopper end
• compression zone (transition) at the middle
• melt zone (melting zone) at the die end

 The function of the feed zone is to collect granules from the feed hopper and transport (pump) them up the screw channel. At the same time the granules should begin to heat-up and compact and build up pressure as they advance towards screw tip (die end). For efficient pumping the granules must not be allowed to lie in the screw channel. They must therefore show high degree of slippage on the screw channel surface and a low degree of slippage on the barrel.

The maximum delivery of granules by the feed section may be achieved by :

• A relatively deep channel
• a low degree of friction between the granules and screw
• a high degree of friction between granules and barrel wall
• an optimum helix angle (20 degree for LDPE)

Parts of the Extrusion Line

You can get small idea about Extrusion line from following image.

 Parts

1. Screw Motor System
2. Hopper
3. Screw Barrel
4. Screw
5. Heaters & Thermocouple
6. Land & Core
7. Die
8. Vacuum Calibration System
9. Calibration & Cooling Bath
10. Bath Doors
11. Haul-Off System
12. Cutting System

Control Dimensions & Defects

At Pipe Extrusion,

Dimensions

• Increase or Decrease Mean Outer Diameter  :  Controlling Vacuum Pressure 
• Increase or Decrease Wall Thickness  :  Controlling Haul-Off (Puller) Speed
• Increase or Decrease Length  :  Controlling Cutting System Limiter

Defects

Ballooning  :
If there is a Haul-off speed variation, some parts of pipe will increase Outer Diameter. This is the Ballooning. Then you have to adjust Haul-off .

Ripples  :
 If you facing this defect, you have to control Calibration & Vacuum.

Grooved Lines  :
If you facing this defect, first you can use Purge Compound. But if defect continued, you have to control Die system.

Holes  :
If you facing this defect, first you can speed-down the Haul-off. But if defect continued, you have to control PVC Compound by controlling Filler content.

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