Profile Extrusion: Extruder Equipment
Bayer products can be extruded on all modern, conventional, single-screw extruders. For profile extrusion, the thermoplastic resin is melted in an extruder and pre-shaped in a die. The profile is given its final contours in the downstream sizing section, where it is cooled at the same time. An extrusion line consists of an extruder, sizing table, take-off unit, and automatic saw. Other stations can be added, such as a delivery table, a punching or printing facility, and other special processing units.
Figure 1. Extrusion line (Extruder: Kuhne, Sankt Augustin. Downstream equipment: Ide, Ostfildern).
The production of high-quality profiles requires a correctly designed profile die in combination with an appropriate sizing process. The choice of resin is equally important—high-viscosity grades are best for profile extrusion.
Extruder Screws
Extruder screws receive pellets from the feed hopper and move them forward in the extruder barrel toward the die. The screw generates frictional heating that helps to melt and mix the resin. Screw diameters and geometries affect the melt temperature, viscosity, and throughput rates of the resin.
The choice of extruder size, meaning the screw or barrel diameter, is primarily governed by the required throughput. Most of the extruders used in profile extrusion have a screw diameter in the range of 1.77 to 2.36 inches (45 to 60mm).
Three-section screws (for feed, metering, and compression sections) are typically used for profile extrusion. The screw length-to-diameter ratios (L/D) range between 1/25 to 1/33 with a screw pitch of 1 D. Longer screws are usually equipped with a vent zone and/or shear and mixing elements.
The screw geometry, specifically the flight depths in the feed and metering sections and the length in the compression section, should be selected to best suit the material being processed. Flight depth ratios of 2.5:1 to 3:1 (feed section/metering section) have been used successfully in profile extrusion.
Use the following values as a guide for screw flight depths in the metering section of the extruder:- 45mm screw diameter: 2.2 to 3.2mm flight depth
- 60mm screw diameter: 3.0 to 4.0mm flight depth
The drive power of the extruder for processing engineering thermoplastics should be adequately dimensioned.
Coat the plasticizing unit with universal corrosion and abrasion protection to reduce equipment wear. This can be achieved by using bimetal cylinders coated centrifugally with a lining on an iron-chromium-nickel-boron basis and screws that are armored with stellite ribs.
Profile Die
The melt is initially shaped in the profile die. The dies must be optimized to avoid dead spots and to obtain a uniform flow rate through the entire die orifice. Coextrusion dies are used for extruding combinations of unplasticized and plasticized components or reinforced and non-reinforced materials.
Sizing Unit
Since the extruded profile is cooled and given its final contours in the downstream sizing unit, the sizing process is key to producing high-quality profiles.
First, match the die and the sizing unit to achieve a stable melt-shaping process and dimensional stability of the profile.
Second, consider the following factors when designing the length, geometry, surface, and cooling method of the sizing unit:- Line speed
- Dimensional tolerances
- Material or melt properties
Third, use dry sizing sections when processing engineering thermoplastics.
And finally, consider cooling the profile in water after sizing. If you use water cooling, separate the water bath from the sizing unit so no water can be drawn into the sizing unit.
If you have any questions about extruder equipment, you can reach the technical team at BMS using the Contact form.
