The filter is a very important part of the extruder. Only by filtering through the filter can qualified products be produced. The extruder filter is used for filtering and blending in the production of various viscous materials and products such as plastics, chemical fibers, rubber, hot melt adhesives, adhesives, coatings, and mixtures. The extruder filter has a mesh type and a mesh belt type. The extruder can replace the filter without interrupting production through an automatic screen changer, saving labor and time. The product performance is stable, and the full-automatic screen change is achieved without operation, increasing the effective filtering time and reducing production costs.

Overview of Extruder Filters

The largest production bases of extruder filters are mainly Anping, Hebei, Shanghai, Jiangsu and other places. Anping is a gathering place for filters, and the filters and mesh belts of Jiangdu in Jiangsu Province, as well as the filters of Shanghai Minzhe Company, are all good. Domestic extruder filter products have accurate filtration accuracy and stable and reliable quality. It has reached the level of similar products in Japan, Germany, France, the United States, etc., and can replace imports.

Select metal filter materials for extruder filters

. Although stainless steel is more expensive, it can be used in some PVC production lines or other occasions to avoid rust. Nickel alloy filters are used to avoid corrosion by fluoropolymers or PVDC.

Generally, the mesh size (or the number of wires per inch) of the filter is 20 to 150 or more. Filters with 20 meshes are coarse; 40 to 60 meshes are fine; 80 to 150 meshes are very fine.

Most filter meshes are woven in a square grid, with the same number of wires in each direction. The Dutch weave method uses coarse wires in the horizontal direction and specifies an even number, for example, 32 x120 wires/in (1in=25.4mm). Filters made with the Dutch weave method can achieve fine filtering without the need for parallel screens in the filter device.

The aperture of filters with the same number of meshes is determined based on the diameter of the wires, and it is not necessary to be exactly the same. For example, a 20-mesh filter made of wire with a row spacing of 24 inches and a diameter of 0.02 inches has a 0.01-inch opening on each side; while a 20-mesh filter made of wire with a row spacing of 30 inches and a diameter of 0.01 inches has a slightly larger opening on each side of 0.04 inches. This is because the fine wire is not fine enough and is more easily blocked (1in=25.4mm).

Generally, the filter is installed in such a way that the coarsest filter faces the protective plate and the finest filter faces the extruder. For example, the arrangement of the filter from the protective plate to the extruder may be 20 mesh/40 mesh/60 mesh, because this arrangement prevents the filter from being blocked and can "blow" impurities into the openings of the protective plate.

If the finest screen has 80 mesh or more, to prevent the screen from being blocked by the rotation of the molten material or large impurities, a coarse screen can be placed in front (such as a 20/100/60/20 mesh arrangement). Because this type of device looks the same from both sides, a symmetrical arrangement is sometimes used to ensure that they are not reversed (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh).

Some processors deliberately install the screens upside down so that the coarse screen filters larger particles of impurities on the upstream surface first. They believe that this method allows more molten material to pass through the side and causes less material to decompose on the upstream surface of the protective plate.

Impact

The filter screen of the extruder is fixed to a porous or grooved protective plate, which forms a seal between the extruder and the die. A clean filter produces a low pressure, perhaps only 50 to 100 lb/in2 (1lb=0.4536Kg, 1in=25.4mm). As the pressure increases, the amount of impurities in the resin trapped on the filter increases, thus blocking the filter.

The filter affects the temperature of the molten material

When replacing a blocked filter, the pressure will drop suddenly, and the temperature of the molten material may also drop, causing the size of the product to change. 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 changes may not cause serious problems when extruding round products, but when extruding flat or irregularly shaped products, changes in the temperature of the molten material may affect the shape of the product. For example, in a flat mold, the cooler molten material may make the center of the sheet thinner and the periphery thicker. This situation can be corrected by automatic or manual adjustment of the mold.

After the filter converter, a gear pump that can ensure that the molten material enters the mold steadily can prevent the above problems from occurring. However, the temperature change of the molten material after the filter is replaced still needs to be solved by adjusting the mold. At the same time, since the gear pump is easily damaged by hard impurities, the gear pump also needs to be protected by a fine filter.

The reason why some rigid PVC processors are reluctant to use filters is that the filter will increase the temperature of the PVC molten material and easily degrade, so a material with better thermal stability is required, which increases the cost of the material. If a filter converter dedicated to PVC is used, the cost will also increase. Therefore, most rigid PVC processors either avoid using filters or use coarse filters without converters to filter only larger particles of impurities.

Steel wire is the most commonly used metal filter material for extruders.

Generally, the filter mesh (or the number of metal wires per inch) is 20 to 150 or more.

You can put a coarse filter in the front (such as a 20/100/60/20 mesh arrangement). Because this type of device looks the same from both sides, it is sometimes arranged symmetrically (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh) to ensure that they are not reversed.

Filter precautions

Since wire filters are prone to rust, they should be stored away from moisture, otherwise rust will appear in the extrudate. More seriously, rusted filters can easily break and allow filtered impurities to leak through, so the filters should be stored in plastic bags or rust-proof paper.

When PVC material is being extruded, never place the exposed filter on the extruder head. This is because the hydrochloric acid in the air near the die head will corrode it. The best practice when extruding PVC is to install a fume hood above the die head and be sure to place the spare filter away from this place, or even bag it and place it in the freezer.

When purging and cleaning the protective plate, it is easy to cause the rewinding phenomenon, which damages the sealing surface and makes the sliding surface mismatched, resulting in leakage. This will not only make the cleaning work time-consuming, but the leaked molten material may affect the thermocouple value, damage the heater, and sometimes even cause fire.

For a single-exhaust, two-stage extruder, if the back pressure at the filter becomes larger, the output of the second stage will be reduced, but the output of the first stage will not be affected. When the back pressure increases to about 2500 lb/in2 (1lb = 0.4536Kg, 1in = 25.4mm), the molten material from the first stage begins to slowly leak into the exhaust hole. To avoid this phenomenon, be sure to replace the filter when the pressure is low. In addition, the gear pump behind the filter also has the function of inhibiting the leakage of molten material into the exhaust hole.

If the filter filters a large piece of debris (such as paper), it will suddenly fail completely, which will cause the pressure of the extruder to reach a peak quickly. Therefore, in order to prevent the above situation from happening, there must be a safety system such as a safety diaphragm, a safety pin or an alarm connected to a pressure gauge.