Plastic extruder also belongs to one of the types of plastic machinery. Extruder according to the direction of the head material flow and the Angle of the screw center line, the head can be divided into a right Angle head and an oblique head. In the extrusion process of the extruder, the molten material is transported to the mold through the filter screen. The filter allows the material to be filtered and can improve the mixing effect of the material. However, the filter can also cause fluctuations in the process, resulting in an increase in the back pressure and temperature of the molten material, and sometimes a decrease.

When replacing the blocked filter, 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 irregularly shaped products, changes in the temperature of the molten material may affect the overall size of the product. For example, in a flat mold, the cooler molten material may make the sheet thinner in the center and thicker around the edges. This can be corrected by making automatic or manual adjustments to the mold.

Behind the filter converter, equipped with a gear pump that can ensure that the molten material enters the mold stably, can prevent the occurrence of the above problems. 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, because the gear pump is easy to be damaged by hard impurities, the gear pump also needs to be protected by a fine filter.

Some hard pvc processors do not want to use the filter is that the filter will make PVC molten material temperature rise and easy to degrade, so the need for better thermal stability of the material, thereby increasing the cost of the material. If the use of PVC special filter converter, it will also increase the cost. So most hard PVC processors either avoid the use of filters, or use a coarse filter device without a converter, only filter larger particles of impurities.

How to choose the filter

Steel wire is the most commonly used metal filter material for extruders. Stainless steel, although more expensive, can be used in some PVC production lines or other occasions to avoid rust. Nickel alloy filters are used to prevent corrosion by fluoropolymers or PVDC.

In general, the mesh of the filter (or the number of wires per inch) is 20 to 150 or more. 20 screen filter is thicker; The filter of 40 ~ 60 sieve holes is relatively thin; The filter of 80 ~ 150 sieve is very fine.

Most filters have a checkered mesh weave with the same number of wires in each direction. The Dutch braid method uses thick wires in the horizontal direction and specifies even numbers, for example, 32x120 wires /in(1in=25.4mm). The filter obtained by the Dutch braiding method does not require a parallel screen to be set up in the filter device, which can play a fine filtering role.

The aperture of the filter with the same number of screens is determined according to the diameter of the wire, and it is not necessary to be completely consistent. For example, a 20-screen filter made of wire with a row spacing of 24in and a diameter of 0.02in has a hole of 0.01in on each side; And by the line spacing of 30in, diameter of 0.01in metal wire made of 20 screen filter, its each side of the opening is slightly larger, 0.04in. This is because the filter of the fine wire is not fine enough, and it is more likely to block (1in=25.4mm).

In general, the filter is installed in such a way that the coarsest filter is facing the protective plate, while the thinnest filter is facing the extruder. For example, the arrangement of the filter from the protective plate to the extruder may be 20 screen /40 screen /60 screen, because this arrangement prevents the filter from being blocked and can "blow" impurities into the opening of the protective plate.

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

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

Understanding and correct use of plastic machinery, can better improve the efficiency of plastic machinery.

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