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The Influence of the Filter Screen of the Extruder on the Product(Part 1)

Sep. 16, 2020

During the extrusion process of the extruder, the molten material is conveyed to the die through the filter. The filter screen allows the material to be filtered and can improve the mixing effect of the material. However, the filter screen can also cause fluctuations in the process, causing the back pressure and the temperature of the molten material to rise and sometimes decrease.

The custom Single Extruder Screen is fixed on a perforated or grooved protective plate, which can form a seal between the extruder and the die. The pressure generated by a clean filter screen is relatively small. As the pressure increases, the amount of impurities in the resin trapped on the filter screen increases, thereby blocking the filter screen.

1 The filter screen will affect the temperature of the molten material

When the blocked filter 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.

When extruding round products, these changes may not cause serious problems, but when extruding flat or irregular shaped products, changes in the temperature of the molten material may affect the overall dimensions of the product. For example, in a flat mold, the colder molten material may make the center of the sheet thinner and thicker on the periphery. This situation can be corrected by automatically or manually adjusting the mold.

Behind the filter screen converter, equipped with a gear pump that can 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 screen is replaced still needs to be resolved 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.

Custom Single Extruder Screen

Custom Single Extruder Screen

2How to choose extruder screens

Steel wire is the most commonly used metal filter material for extruders. Although stainless steel is more expensive, it can avoid rust. Nickel alloy filter screen is used to avoid corrosion by fluoropolymer or PVDC.

Under normal circumstances, the filter mesh (or the number of wires per inch) is 20 to 150 or more. The filter with 20 mesh is relatively coarse; the filter with 40-60 mesh is relatively fine; the filter with 80-150 mesh is very fine.

Most filter meshes are woven in squares, with the same number of wires in each direction. The Dutch braiding method uses thick metal wires in the horizontal direction and is specified as an even number. The filter screen made by the Dutch weaving method does not need to install parallel screens in the filter device to play a fine filtering effect.

The pore size of the filter with the same number of meshes is determined according to the diameter of the wire, and it is not necessary to be exactly the same. For example, a 20-mesh filter made of metal wire with a row spacing of 24in and a diameter of 0.02in has an opening of 0.01in on each side; and a metal wire with a row spacing of 30in and a diameter of 0.01in is made 20 mesh filter, the opening on each side is slightly larger, 0.04in. This is because the filtration of the fine wire is not fine enough, and it is more likely to block (1in=25.4mm).


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