Plastic extruder

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Extruder A machine for producing more or less continuous lengths of plastics sections such as rods, sheets, tubes, and profiles. Its essential elements are a tubular barrel, usually electrically heated; a revolving screw, ram or plunger within the barrel; a hopper at one end from which the material to be extruded is fed to the screw, ram or plunger; and a die at the opposite end for shaping the extruded mass. Extruders may be divided into three general types—single screw, twin-or multiple screw, and ram—each type has several variations.

The major components in an extruder are discussed here.

1 Feedscrew
2 Barrel Chamber
3 Dies
4 Other Equipment
5 Power Transmission Equipment
6 Types of extruders
7 See also

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Feedscrew

As the only moving part in many extruders, feed-screws must do the job of moving the resins through the barrel chamber in asteady and predictable manner. As a result, and the feed-screw is critical to the design.

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Barrel Chamber

This thick-walled steel chamber that is expected to withstand high pressures ( 20,000 psig), is precisely machined for a tight fitwith the feedscrew, and has a hardened steel alloy on its inside wall to prevent wear and corrosion. Some barrels will also have agrooved feed zone to increase the frictional forces on the resin.

The barrel also is heated to facilitate melting of the resin. Although the major contributor to melting is friction, the heat asconducted through the barrel can serve as a "fine adjust" or vernier in temperature control and energy input. Electrical resistance heating is a common method employed. Advantages are that several temperature zones can be set up with multiple elements, and temperature profiles can be created as material requirements vary. When thermal needs are not so complex, steam heating via a jacketed barrel chamber. A jacketed chamber uses cooling water to prevent overheating of the melt in thevicinity of the die as well.

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Dies

The opening that allows plasticated material to form particular shapes is also a highly engineered part. Dies are designed to compensate for effects of shrinkage when a melt re-solidifies, two dimensioned size adjustments, and varying rates of solidifica-tion. Dies must be free from defects and scratches, otherwise themelt could show the defect's pattern. The flow of melt to the die typically follows a tapered path, with the die having a thickness associated with it. (See figure 3) This results in the melt undergoing a pressure drop as it exits the die, and this prevents unwanted build-up at irregular places along the die, which would spoil the product.

Dies can take on a variety of shapes and have adjustable openings. In the case of filament extrusion and others, multiple duplicate die patterns to extrude many strands in parallel can be found on a single die.

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Other Equipment

There are other parts of the extruder that deserve a brief mention.

Different hoppers are used for different purposes. Feed hoppers hold and supply resins to the feedscrews. Motor driven helical screws or vibrators help eliminate any bridging or arching of theresins that prevent the smooth flow from the hopper to the feedzone.

Mixing hoppers upstream of the feed hoppers compound any needed plasticisers and fillers to the required specifications.

Melt pumps can smooth the effects of pressure fluctuations that otherwise would result in uneven extrusions and resulting off-spec products. These help out in cases where multiple dies are on a machine, and can be individually closed off on the fly. The downside of melt pumps is their expense, plus they are extramoving parts that must be maintained in good condition.

As an alternative to a melt pump, there is a feedscrew design variation that adds an additional zone with screw flights with areverse pitch from the other sections. This serves to act as a surge suppressor

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Power Transmission Equipment

As mentioned before, the feedscrew is the moving part and it must be driven. Operation in a steady and predictable manner is vital to making quality extrusions. As friction represents about85% of the energy used in heating resins, this also means that the power transmission equipment must be capable of supplying the energy to overcome this friction, particularly if starting fromrest or recovery from a maintenance outage.

Good speed control is extremely important to assure that ade-quate resin is being fed to the process. However the ability tomaintain even pressures to get consistent flow is equally impor-tant. Good response to torque changes as well as steady speedcontrol of high friction loads is the challenge.

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