Multi-Wall Sheet Extrusion: Extruder Equipment
Bayer products can be extruded on all modern, conventional, single-screw extruders. A multi-wall extrusion line consists of at least one extruder, a multi-wall sheet extrusion die with nozzels, a vacuum calibrator, and a take-off unit.
Figure 1. Multi-wall sheet extrusion line (courtesy Omipa, Morazzone, VA, Italy).
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.
Screw diameters can vary between 3.5 and 8 inches (90 and 200mm), although screws with diameters of 5 to 6 inches (120 to 150mm) are commonly used. The length-to-diameter ratio (L/D) of vented extruders should be 1/30 to 1/33, and in the case of double-vented extruders, up to 1/40. If vented extruders are not in operation, shorter screws (for example, 1/25) can be used.
In addition to conventional two- or three-stage vented screws, modern screw technology offers more options. Barrier screws or screws with special mixing and shearing devices are an example (see Figure 2). These types of screws are often combined with a grooved feed zone that improves the transport of the pellets through the barrel.
Figure 2. Examples of screw geometries of extruder screws.
Increased throughput and a good melt quality are the essential criteria for screw optimization. The temperature of the melt should be kept as low as possible. Under real-life conditions, the specific throughputs shown on the following table have been achieved with different resins.
| Table 1. Throughput relationship between screw diameter and die width | ||
| Screw diameter in inches (mm) | Die width in inches (mm) | Throughput in lbs/h (kg/h) |
| 8 (200) | 138 (3500) | 1500 to 2600 (750 to 1200) |
| 6 (150) | 98 (2500) | 1000 to 2000 (500 to 900) |
| 5 (120) | 83 (2100) | 800 to 1500 (350 to 700) |
| 3.5 (90) | 59 (1500) | 400 to 800 (200 to 350) |
Coat the plasticizing unit with universal corrosion and abrasion protection to reduce equipment wear. Use bimetal cylinders coated centrifugally with a lining on an iron-chromium-nickel-boron basis and screws armored with stellite ribs.
Melt Pumps
Modern installations for the production of polycarbonate sheets are generally equipped with a melt pump for continuous and pulsation-free supply.
Melt Filter
Bayer recommends using a hydraulically operated melt filter. Multiple screen combinations can be used, depending on the materials to be extruded and quality requirements. For example, a typical screen combination for Makrolon® polycarbonates when extruding solid sheets is screens with 30, 50, 140, and 30 openings per inch, respectively (0.6, 0.3, 0.1, and 0.6mm).
Multi-Wall Sheet Extrusion Die
Dies with specialized nozzles are used for multi-wall sheet extrusion. The flow channel geometry of the nozzle makes it possible to form multi-wall sheets with two or more layers. Multi-wall sheet extrusion dies may or may not have restrictor bars. In general, the dies have two variable restrictor bars as well as nozzle lips for both the upper and lower chord.
Figure 3. A Technikums multi-wall sheet extrusion die with nozzles.
Calibrator
The calibrator is positioned after the die and consists of up to six separately controlled cooling zones as well as several separate vacuum zones. The function of the calibrator is to set the final form of the sheet through cooling and vacuum-controlled shaping. The calibrator can be adjusted to adapt to varying conditions, such as different sheet thicknesses, melt temperatures, and production speeds.
Take-off Units
Take-off units pull the cooled and shaped sheet from the calibrator. They typically consist of two to six rubber-covered pairs of rolls.
Heating Device
A heating device, positioned after the take-off section, is equipped with infrared heating elements. If necessary, the multi-wall sheet can be reheated rapidly to eliminate internal stress, which could cause distortion.
Cutting Blades
Instead of the saws used for cutting extruded solid sheets, knife-like blades cut multi-wall sheets. Cold blades are used for edge trimming or separation in the longitudinal direction. Heated blades cut in the transverse direction. During transverse cutting, the cavities should not be closed completely in order to avoid a pressure rise in the support air.
If you have any questions about extruder equipment, you can reach the technical team at BMS using the Contact form.
