Solid Sheet Extrusion: Extruder Equipment
Bayer products can be extruded on all modern, conventional, single-screw extruders. A sheet extrusion line typically consists of at least one extruder and one sheet extrusion die. Following this is a roll stack, which is comprised of three rolls that calibrate and cool the sheet with their surfaces. Beyond the roll stack is a roller conveyor, draw-off rolls for air cooling, and cutting or sawing devices. Several other optional machine components that perform various operations can also be included in a sheet extrusion line.
Figure 1. Solid sheet extrusion line.
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 150 mm) are commonly used. The length-to-diameter ratio (L/D) of vented extruders should be 30:1 to 33:1, and in the case of double-vented extruders, up to 40:1. If vented extruders are not in operation, shorter screws (for example, 25:1) 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 in the following table have been achieved with different resins. Be sure not to exceed the recommended maximum melt temperatures.
| 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 Pump
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).
Sheet Extrusion Dies
Conventional dies are equipped with coathanger manifolds, restrictor bars, and if necessary, a flexible lip. Adjustments are usually performed manually,though dies are available with automatic adjustments.
Sheet extrusion dies with membranes, which are a relatively new technology, can be used in place of restrictor bars. Flow channel wall thickness can be reduced significantly with membranes, making it deformable. There are several other advantages to using membranes:- Influencing the melt stream is considerably easier with membranes.
- Intrusion between the die body and restrictor bars is not an issue with membranes, so there is no degradation of the melt.
- Formation of bands on the product.
- Complete cleaning of the die is easier.
Membranes are available for single-channel dies, multi-channel dies, and coextrusion adapters.
Coextrusion Adapters
When using a coextrusion die, you can use either fixed adapters, which have a set geometry, or adjustable adapters, which are called slide and blade adapters. The advantage of the latter is that the melt streams are separated from one another. This allows you to more easily process resins with different viscosities or processing requirements.
Figure 3. Coextrusion die (courtesy EDI Europe GmbH, Wiehl-Hübender, Germany).
Roll Stack
After the extruded melt leaves the die, it is picked up by the roll stack. The rolls cool the sheet, correct for minor gauge variations, and give the desired surface to the sheet. Most rolls stacks consist of three rolls arranged horizontally or at a 45° angle. Roll stacks are usually adjustable, giving you the ability to better match cooling conditions and sheet thicknesses. Rolls are typically heated with water or oil.
Downstream Devices
Additional machine components are located downstream from the roll stack and complete the extrusion line. The specific machines will depend on your application.
- Sheet conveyor.
- Laminating machine for double coating with protective film for sheets with optical qualities (used primarily for Makrolon® polycarbonates).
- Longitudinal and transverse separators.
- Cutting devices, which are typically operated with circular saw blades or shears. The number of saw teeth, the cutting speed, and other settings will vary depending on the design requirements of your material.
Storing Finished Sheets
Stack cut sheets manually or with a commercial stacking unit on pallets. To reduce moisture absorption and keep the sheets clean, wrap each sheet in polyethylene (PE) film. Use a single layer of 8 to 10mm PE film or, for better moisture protection, use two layers of 3 to 4mm PE film.
If you have any questions about extruder equipment, you can reach the technical team at BMS using the Contact form.
