Feedscrew

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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.