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Basic Extruder Machine
Every extrusion company has its own variation of plastic extruder, but the basic operation of all the machines remains the same. Firstly, there is the feeder zone, which transfers the pellets or powder resin from the hopper to the barrel. The plasticizing unit  consist of a feed screw, barrel and temperature controller where the resin convert into a molten form.
Other parts of the machine that requires special mention are:
Hoppers: Holds the resin till the supply is fed to the feed screw.
Motor driven helical screws: helps in smooth flow of the resin from the hopper to the feed zone.
Melt pumps prevent the uneven flow of resin especially when multiple dies are used. In addition, on a multiple dies, the pump can be individually closed off on the fly. Melt pump systems is expensive to install and maintain.
Alternative feed screw: These are feed screw used instead of melt pumps in order to act as a surge suppressor.
Consideration of designs: The design of the extruder machine depends on the types of material that is used and design of the product to be manufactured. A fragment of difference in the design will gravely effect the final quality of the product. Therefore, special attention has to be paid to the following areas:
Mechanical screw design: Requires the selection of high grade material and precision machining. The screw must be fit tightly into the barrel to prevent excessive back-flow or drag flow of resin due to the large gaps that are formed between the screw fight and the barrel surface. At the same time the screw should not be so tight that the machine parts like the groves and threading are damaged.
The design of an extrusion profile has a large impact on how readily it can be extruded. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section, this is called the circumscribing circle.
Dies can be designed to take a variety of shapes and should have adjustable opening. The dies are machined part which have a patterned opening. The dies must be clean and without defect, so as to prevent the formation unwanted product designs. For filament extrusion and other purposes, multiple patterns can be formed in a single die.
Dies of different types produce different dies :
| Circular cross-section |
weed trimmer line, fishing line and other circular line products |
| Donut-shape cross section |
a tubing die for pipes and tubes |
| Cloth hanger die |
wide and thin sheets |
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Temperature control
As extrusion technology advances day by day, various techniques are being used to control the temperature and energy input. In the plastic extrusion process temperature control is important because overheating will deform the product. Mechanical heating, electrical resistance heating and steam heating are few methods used to control temperature.
Deviation from the operation parameters can be compensated by adopting mechanical adjustments in the extrusion machine. For eg. a flexible lip can be fixed just before the die. |
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