During the extrusion process of the extruder, the molten material is fed to the die through the filter screen. The filter makes the material filter and can improve the mixing effect of the material. However, the filter can also cause fluctuations in the process, causing back pressure and molten material temperature to rise, and sometimes decrease. The filter screen of the extruder is fixed on a porous or grooved protective plate, which makes it possible to form a seal between the extruder and the die.

A clean filter produces less pressure, and as the pressure increases, the number of impurities in the resin trapped on the filter becomes greater, thereby blocking the filter. The filter screen will affect the temperature of the molten material. When the blocked filter screen is replaced, the pressure will suddenly drop, and the temperature of the molten material may also drop, resulting in a change in the size of the product. In order to maintain the same size of the product, the screw speed of the extruder can be adjusted, and the linear speed of the extruder can also be adjusted. These variations may not cause serious problems when extruding a round product, but when extruding a flat or irregularly shaped product, variations in the temperature of the molten material may affect the dimensions of the product. For example, in a flat mold, the cooler molten material may make the sheet thinner and the periphery thicker. This situation can be corrected by automatic or manual adjustment of the mold. Behind the filter screen changer, a gear pump capable of stably entering the mold of the molten material can prevent the occurrence of the above-mentioned problems. However, the temperature change of the molten material after the filter screen is changed still needs to be solved by adjusting the mold. At the same time, because the gear pump is easily damaged by hard impurities, the gear pump also needs the protection of a fine filter. Some rigid PVC processors are reluctant to use the filter because the filter will make the PVC molten material temperature rise and easy to degrade, so that the need for good thermal stability of the material, thereby increasing the cost of the material. If the use of PVC filter screen converter, will also increase the cost. Therefore, most rigid PVC processors either avoid the use of filters or use coarse filtration devices without converters to filter only larger particles of impurities. How to choose the filter wire is the metal filter material commonly used in the extruder. Although stainless steel is more expensive, it can be used in some PVC production lines or other occasions to avoid rust. Nickel alloy screens are used to avoid corrosion by fluoropolymers or PVDC. In general, the screen mesh (or the number of wires per inch) is 20 to 150 or more. The filter screen of 20 mesh is relatively coarse; the filter screen of 40~60 mesh is relatively fine; the filter screen of 80~150 mesh is very fine.

Most filter screens have a checkered mesh, with the same number of wires in each direction. The Dutch weaving method uses thick metal wires in the horizontal direction and is specified as an even number, for example, 32x 120 pieces/in(1in = 25.4mm). The filter made by the Dutch weaving method can play a fine filtering effect without setting a parallel screen in the filtering device. The aperture of the filter screen with the same number of mesh is determined according to the diameter of the metal wire, not consistent. For example, a 20-mesh filter screen made of wire with a row spacing of 24in and a diameter of 0.02in has an opening of 0.01in on each side. The 20-mesh filter screen made of wire with a row spacing of 30in and a diameter of 0.01in has a slightly larger opening of 0.04in on each side. This is because the filtration of the fine wire is not fine enough and it is easy to block (1in = 25.4mm). In general, the installation method of the filter device is: the coarse filter screen faces the protective plate, and the fine filter screen faces the extruder. For example, the screen arrangement from the protective plate to the extruder may be 20 mesh/40 mesh/60 mesh, because this arrangement can prevent the screen from being blocked and can "blow" impurities into the openings of the protective plate. If the fine screen is 80 or more mesh, in order to prevent the screen from being blocked by spinning of the molten material or large impurities, a coarse screen may be placed in front (such as a 20/100/60/20 mesh arrangement). Since this device type is the same from both sides, a symmetrical arrangement (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh) is also sometimes used in order that they will not be reversed. Some processors deliberately turn the filter upside down so that the coarse filter filters the larger particles of impurities on the upstream surface first. They believe that this method allows a lot of side molten material to pass through, and the material on the upstream surface of the protection plate is less decomposed. Since wire strainers are prone to rust, moisture should be avoided during storage, otherwise rust will appear in the extrudate. Seriously, the rusty filter is easy to break and the impurities filtered out are leaked. Therefore, the filter should be stored in a plastic bag or anti-rust paper. When the PVC material is being extruded, the exposed screen must not be placed in the extruder head. This is because the hydrochloric acid in the air near the die will corrode it. The best way to extrude PVC is to install a smoke exhaust cabinet above the die head, and to keep the spare filter screen away from here, and even place it in the freezer after being bagged.

When purging and cleaning the protection plate, it is easy to reverse the winding phenomenon, so that the sealing surface is damaged, and the sliding surface does not match, resulting in leakage. In this way, not only will the cleaning work be time-consuming, but the leaking molten material may affect the thermocouple value, damage the heater, and sometimes even cause a fire. For a single-vent, two-stage extruder, if the back pressure at the filter becomes larger, the output of its section will be reduced, but the output of its section will not be affected. When the back pressure increases to about 2500lb/in2(1lb = 0.4536Kg,1in = 25.4mm), the molten material from the section begins to slowly leak into the vent hole. In order to avoid this kind of phenomenon, the filter screen should be replaced when the pressure is low. In addition, the gear pump behind the filtering device also has the effect of molten material leaking into the exhaust hole. If the filter filters a large piece of debris (such as paper), it will suddenly fail, which will cause the pressure of the extruder to peak. Therefore, in order to prevent the above situation, there should be a diaphragm, a pin or an alarm connected to the pressure gauge.