Wire mesh screens, used in most extruders, are usually square mesh - that is, same number of wires per inch or cm in both directions. The purpose of wire mesh screens is to filter out contaminants and also to build up pressure in the extruder.
Pressure in extruders can have dual effect- it can be both good and bad: it helps mixing and is particularly useful if screw speed is high and the residence time of polymer melt in the extruder is not very long. On the negative side, there are certain aspects of pressure that could hamper production rate - More pressure means more resistance to flow through the head and die; thus causing the motor to work harder and leading to an increase in melt temperature coming out of the extruder.
Screens are almost never reused. Though the screens can be cleaned and put back, they may get distorted or broken during removal and can lead to an additional danger of being put in backward (compared to the first use). In such a case, contaminant particles lodged in between the wires get driven forward out into the product when the line starts up.
Usually extruders of rigid PVC pipe and profiles do not use screens at all. These are the operations in which an increase in melt temperature may require a more expensive formulation containing more stabilizer.
Typically, the screens are supported on a breaker plate, a perforated steel disk that prevents the screens from being pushed into the head and die and serving as a seal between the head section (die and adapter) and the extruder barrel.
The exception to the square mesh screens is Dutch weave, where one direction uses fewer but thicker wires than the other. This arrangement results in a stronger screen with rectangular openings, thus bringing down the usage of wire mesh screens as only one of these is usually enough. In most extrusions, though, a screen pack is used, which is made up of several screens: a 20 mesh (20 wires per inch) is put up against the breaker plate, then perhaps a 40 mesh. In case of extreme levels of filtration required, an 80 mesh is used after the 40 mesh. Some very critical applications such as fibers, filaments and thin film use even finer screens, such as 200-350 wires per inch. Most screen packs are assembled with the heaviest screen next to the breaker plate, as heavier screens support the finer ones and prevent their blowing through from the differential pressure. In the reverse arrangement, the coarser mesh will catch most of the contamination and the pressure drop across the finest one will stay low, preventing a blow-through.
The wire gauge also plays a very vital part as the same mesh size can have bigger holes if thinner wire is used. Sometimes screens are doubled up with two 40s or two 80s - to get a tighter pack. If that is done, it is essential to set the screens at 45 degrees to each other, so that maximum and consistent filtration capacity is achieved.
Some packs are actually sandwiches, with a coarse screen on both sides, partly to protect the finest screen that otherwise might be exposed to the rotating flow of the melt coming off the screw, and partly to make a symmetrical pack which can be put in either way with the same result.
With small equipment (2.5" diameter or less), screens are changed manually. The line is stopped and the head opened, the old screens are taken out and the new ones put into the end of the barrel. Care has to be taken so as to not to block the seal line; otherwise leakage will occur during operation.
On many larger lines, or wherever opening the head or starting/stopping production is not so easy, an automated screen changer is preferred, basically reducing the downtime.
On machines where recycled material is being used and contamination levels are high, the changes are much more frequent and the value of a changer is higher. Even with these, the changing should be manually initiated, as timing may need to be adjusted to match ends of runs, shutdowns or color changes.
Some changers require the line to be stopped during the change, but many can be operated "on the fly," although some production time is still lost to account for the lower back pressure (either slowing down the screw or speeding up the takeoff). For these systems, it is good to have a pre-fill option, where the new plate and screens are pre-filled with melt just before pushing them in place. This avoids pushing air into the system, which might cause trouble or even breakage of the continuity of the product.
Self-cleaning changers have a device that passes over the upstream surface of the screens with a passage leading to the outside, so that the internal pressure pushes out a small amount of melt together with the contamination. They may operate continuously or intermittently as needed.
True continuous changers are also made, either the rotating or linear type. The rotators have a wheel with a number of plate positions, say 8 or 12, and the wheel is indexed into position when a new plate-screen combination is needed. This works with round screens but must account for the pressure change when the new screens are in. To avoid this, some devices use screens in D or pie-slice shape, which can be placed so close together that the wheel can turn slowly but continuously, always bringing clean screen into the flow path to maintain near-constant back pressure.
The linear changer works differently. A strip of screen comes off a coil and passes across the flow path, and as it leaves the other side the melt around it is frozen by the temperature-controlled chamber it must pass through. The screen is thus embedded in the "plug" and goes out with it, forced out by internal pressure. Nothing moves except the screen itself.
Most screens are conventional steel, requiring some attention to keep them from rusting in storage, which would mean more chance of blow-through when used. For relatively clean materials requiring fewer changes, stainless steel screens are used and their added cost is justified by the reduced worry about rusting.
For extrusion of PVDC and some fluoroplastics, special metals (usually nickel alloys) must be used not only for screens but for the entire interior of the system, as these polymers are too corrosive for regular steel or even stainless steel under extrusion conditions.