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Effect Of Extruder Filter On Plastic Extrusion

  Effect of Extruder Filter on Plastic Extrusion

  During the extrusion of the extruder, the molten material is conveyed through the filter to the mold. The filter allows the material to be filtered and improves the mixing effect of the material. However, the filter can also cause fluctuations in the process, resulting in back pressure and molten material temperature rise, and sometimes will be reduced.

  The filter of the extruder is fixed to a protective plate of a porous or trough so that a seal is formed between the extruder and the mold. Clean the pressure generated by the filter is small, may only 50 ~ 100lb / in2 (1lb = 0.4536Kg, 1in = 25.4mm). As the pressure increases, the amount of impurities in the resin trapped on the filter becomes larger, thereby blocking the filter.

  The filter will affect the temperature of the molten material

  When replacing the blocked filter, the pressure will suddenly drop, the temperature of the molten material may also be reduced, resulting in product size changes. In order to maintain the same size of the product, you can adjust the screw speed of the extruder, you can also adjust the linear speed of the extruder. These changes may not cause serious problems when extruding round products, but changes in the temperature of the molten material may affect the dimensions of the product when extruding flat or irregularly shaped products. For example, in a flat mold, the cold melt material may make the center of the sheet thinner, leaving the periphery thicker. This can be corrected by automatic or manual adjustment of the mold.

  Behind the filter converter, equipped with a gear pump that ensures that the molten material stably enters the mold, the above problems can be prevented. However, the temperature change of the molten material after the replacement of the filter 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 fine filters.

  Some hard PVC processors do not want to use the filter because the filter will make the PVC melt material temperature and easy to degrade, which requires better thermal stability of the material, thereby increasing the cost of materials. If you use PVC-specific filter converter, will increase the cost. So most hard PVC processors either avoid using the filter or use a coarse filter without a converter to filter only larger particles of impurities.

  How to choose a filter

  Steel wire is the most commonly used metal filter material for extruder. Although the stainless steel is relatively expensive, but can be used for some PVC production line or other occasions to avoid rust. Nickel alloy filters are used to avoid being corroded by fluoropolymers or PVDCs.

  In general, the filter mesh (or the number of wires per inch) is 20 to 150 or more. 20 mesh mesh filter is relatively thick; 40 ~ 60 mesh mesh filter is relatively small; 80 ~ 150 mesh filter is very fine.

  Most of the screen mesh are woven in square, the number of wires in each direction the same. Dutch knitting method is used in the horizontal direction of the thick wire, and is defined as a double number, for example, 32x120 root / in (1in = 25.4mm). Filtration made by the Dutch knitting method does not require a fine filter function in the parallel arrangement of the filter in the filter unit.

  The same number of mesh filters are based on the diameter of the wire, and it is not necessary to be exactly the same. For example, a 20 mesh mesh filter made of a wire having a pitch of 24 in and a diameter of 0.02 in. Was 0.01 in each opening and made of a wire having a pitch of 30 in. And a diameter of 0.01 in. 20 mesh filter, the opening on each side slightly larger, 0.04in. This is because the fine wire is not fine enough to filter, and more easily blocked (1in = 25.4mm).

  Under normal circumstances, the filter device is installed: the most rough filter against the protection board, and the smallest filter is the face of the extruder.

  For example, the screen arrangement from the protective plate to the extruder may be in the form of a 20 mesh / 40 mesh / 60 mesh, since this arrangement prevents the filter from being blocked and can be "blown" into the protective plate Of the openings.

  If the finest screen is 80 or more, in order to prevent the screen from being blocked by the rotational movement of the molten material or large impurities, a rough screen may be placed in front (e.g., 20/100/60 / 20 mesh arrangement). Because this type of device is the same from both sides, it is sometimes possible to use a symmetrical arrangement (20 mesh / 60 mesh / 100 mesh / 60 mesh / 20 mesh) in order to ensure that they are not reversed.

  Some processors deliberately put the filter installed upside down, so that the coarse filter first filter the upper surface of the larger particles of impurities. They believe that this method allows more side of the molten material through, and the protective plate on the surface of the material less decomposition.

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