The extruder filter is fixed to a porous or grooved protective plate, which creates a seal between the extruder and the mold. The pressure generated by a clean filter is small, maybe only 50 to 100lb/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 will affect the temperature of the molten material
When replacing a clogged 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 melt temperature may affect the overall dimensions of the product. For example, in a flat mold, cooler molten material may make the sheet thinner in the center and thicker around the edges. This situation can be corrected by automatic or manual adjustments to the mold.
Behind the filter converter, a gear pump is equipped to ensure that the molten material enters the mold stably, which 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, because 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 filters will increase the temperature of the PVC molten material and easily degrade. This requires materials with better thermal stability, thus increasing the cost of materials. If a PVC-specific filter converter is used, the cost will also increase. Therefore, most rigid PVC processors either avoid using filters or use coarse filtering devices without converters to only filter larger particles of impurities.
Steel wire is the most commonly used metal filter material for extruders.
Generally, the mesh size of the filter (or the number of wires per inch) ranges from 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 device type looks the same from both sides, a symmetrical arrangement (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh) is sometimes used to ensure that they cannot be turned upside down.