Plastic Extruder and Extrusion Equipment

A machine for producing more or less continuous lengths of plastic sections.

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.

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.

There are at least three defined sections in a basic feedscrew, and if specifically engineered to accomplish a definite purpose, they can have additional sections.

1. The feed zone takes resin from the hopper and conveys it along. During the journey, resin pellets encounter friction from feedscrew surfaces, barrel surfaces, and each other. This mechanical friction is about 85% of the required heat, so it is critical that the drive equipment to turn the screw have the HP capabilities to overcome friction AND turn the feedscrew at a steady and controlled rate. Some extruders can continue to plasticate materials long after their external heat sources are shut down.

2. The compression zone is next. Here, the channel depth between screw flights diminishes and the result is to pressurizethe now melting resin. Friction, barrel heating, and compressionin this stage should complete the melting process. Two important design parameters are associated with this zone.

a. The compression ratio is measured as the channel depth at the end of this zone divided by the channel depth in the feed zone. Different compounds or operating pressures require different compression ratios.

b. The length of the compression zone affects the rate of compression. These two parameters will be different for different compounds.

3. The metering zone has a constant channel depth and primarily exists to further mix molten resin. The end result is a smooth consistent melt with uniform temperature.

4. In some processes, a de-gassing or devolatizing section is required. This is a shorter zone that immediately follows the compression zone. Channel depth is suddenly increased, and the resulting pressure drop causes a release of any gas, which can be vented or drawn off via vacuum pump. The remaining melt is re-compressed and metered.

Mechanical screw design also requires the selection of high-grade materials and precision machining. The screw must fit tightly in the barrel to prevent excessive back-flow or drag flow ofresin due to excessive gaps between the screw flights and the barrel surface. It must not be so tight that it contacts the barrelsurface itself, causing grooves and other damaging effects.

As if the tight tolerances were not enough of a challenge, some materials require extra processing and are best handled in twin-screw extruder. Here, two screws are tightly mounted in a “figure 8 ” type barrel, and the screw flights are designed such that they avoid grinding each other during rotation. The screwscan be designed to operate co- or counter-currently.

Co-current operation adds a degree of mixing to the process and would be advantageous where, for example, green and blue pellets need to be mixed as extrusion occurs to get a melt that has an aqua hue. The resin is carried from the first screw to the second between each flight.

Counter-current operation serves to convey the melt in a smooth predictable manner and helps eliminate pressure pulsing. Due to machining and operation demands, this equipment is more expensive to build and maintain than single screw extruders, so itis reserved for special extruding needs.

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.

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.

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

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.

Historically, DC drives and motors have been the ideal drives forextrusion.

Their relative advantages are listed here.

• DC drives and motors offer wide constant torque speed ranges (20:1).
• DC has been the simplest choice of design when considering choices between AC, DC or servos.
• They offer smaller sizes at larger horsepower ratings (>60HP).
• DC drives are easily retrofitted to existing DC motors.

On the technology front, AC drives/motors are coming into theirown as good extruder candidates. With the continual develop-ment of PWM technology and more rugged AC motor designs,more and more extruder manufacturers are looking for AC solutions. AC drives/motors offer the following advantages.

• Dynamic response with vector operation. Recent designs employ sensorless vector operation and give high speed response yet require no feedback.
• AC motors require minimal maintenance (no brushes or commutators) and are suitable to harsh environments. (Elevated temperatures, dust, volatiles, etc.)
• Motor designs for extruder duty units feature high overload capabilities and very wide constant torque speed ranges.

Regardless of the choice between AC or DC for an extruder,Reliance Electric has the right products and technologies to pro-vide good solutions.

• Microprocessor-based regulators;
• Easy-to-configure drives with quick-start capabilities;
• Control from any number of sources: local, remote, network, serially to a PC;
• AC and DC motors that are specifically designed as extruder duty; and
• Easily modified with a wide variety of optional kits available for those extra special applications.