During the extrusion of the Extrusion Machine, the molten material is conveyed to the mold through the filter. The filter filters the material and improves the mixing of the material. However, the filter can also cause fluctuations in the process, resulting in back pressure and elevated temperature of the molten material, sometimes reducing production.
The filter of the extruder is fixed to a porous or grooved protective plate to form a seal between the extruder and the mold. A clean filter produces less pressure and may be only 50 to 100 lb/in2 (1 lb = 0.4536 Kg, 1 in = 25.4 mm). As the pressure increases, the amount of impurities in the resin trapped on the filter web increases, thereby clogging the filter.
The filter will affect the temperature of the molten material
When replacing the blocked filter, the pressure will Sudden drops, the temperature of the molten material may also drop, causing 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. These changes may not cause serious problems when the round product is extruded, but changes in the temperature of the molten material may affect the overall dimensions of the product when a flat or irregularly shaped product is extruded. For example, in a flat mold, a colder molten material may cause the center of the sheet to be thinner and the periphery to be thicker. This can be corrected by automatic or manual adjustment of the mold.
After the filter inverter, a gear pump capable of ensuring stable entry of molten material into the mold can prevent the above problems. However, the temperature change that occurs after the molten material is replaced by 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 a fine filter.
The reason some hard PVC processors are reluctant to use filters is that the filter will increase the temperature of the PVC molten material and cause degradation, which requires heat. Better material stability, which increases the cost of the material. If you use a PVC-specific filter converter, it will also increase the cost. Therefore, most rigid PVC processors either shun the filter or use a coarse filter without a converter to filter only the larger particles.
How to choose a filter
A wire is a commonly used metal filter material for extruders. Stainless steel, although expensive, can be used on certain PVC lines or other occasions to avoid rust. Nickel alloy filters are used to avoid corrosion by fluoropolymers or PVDC.
In general, the filter mesh (or the number of wires per inch) is 20-150 or more. The filter screen of 20 mesh is relatively thick; the filter of 40-60 mesh is relatively thin; the filter of 80-150 mesh is very fine.
The mesh of most filters is woven in a square, with the same number of wires in each direction. The Dutch weave method uses thick wire in the horizontal direction and is specified as a double number, for example, 32 x 120 roots/in (1 in = 25.4 mm). Filters made by the Dutch weaving method do not require a parallel screen in the filter unit to provide fine filtration.
The pore size of the filter mesh with the same number of mesh eyes is determined according to the diameter of the wire, and it is not necessary to be completely identical. For example, a 20-mesh filter made of wire with a line spacing of 24 inches and a diameter of 0.02 inches has an opening of 0.01 in each side; and is made of a wire having a line spacing of 30 in and a diameter of 0.01 in. 20 mesh filter screen, the opening on each side is slightly larger, 0.04 in. This is because the fine wire is not sufficiently fine to filter and is more likely to clog (1 in = 25.4 mm).
In general, the filter unit is installed in such a way that the coarse filter screen faces the protective plate and the thin filter mesh faces the extruder. For example, the filter arrangement 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 “blown” the impurities into the protective plate. Inside the opening.
If the fine mesh is 80 or more mesh, in order to prevent the filter from being rotated by the molten material or blocked by large impurities, Place a rough filter on the front (such as the 20/100/60/20 mesh arrangement). Since this type of device is identical from both sides, in order to ensure that they are not reversed, a symmetrical arrangement (20 mesh / 60 mesh / 100 mesh / 60 mesh / 20 mesh) is sometimes used.
Some processors deliberately reverse the filter to allow the coarse filter to filter larger particles on the upstream surface. They believe that this method allows more of the side molten material to pass through and allows less material to decompose on the upstream surface of the protective sheet.
Notes on filters
Because the wire mesh is prone to rust, it should be stored Avoid moisture, otherwise rust will appear in the sever. More seriously, the rust filter is prone to breakage and the filtered impurities are leaked. Therefore, the filter should be stored in a plastic bag or rustproof paper.
When squeezing PVC material, do not place the bare filter on the extruder head. This is because hydrochloric acid in the air near the die of the head will corrode it. A good practice when ejecting PVC is to install a fume hood above the die and be sure to keep the spare filter away from it, even after it has been bagged and placed in the freezer.
It is easy to reverse the phenomenon when purging and cleaning the protective plate, causing the sealing surface to be damaged and the sliding surfaces to be mismatched, resulting in leakage. This will not only cause cleaning time, but also 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 second stage yield will decrease, but The production of a section will not be affected. When the back pressure is increased to approximately 2500 lb/in2 (1 lb = 0.4536 Kg, 1 in = 25.4 mm), the molten material from one section begins to leak into the vent. To avoid this, be sure to replace the filter when the pressure is low. In addition, the gear pump behind the filter device also prevents the molten material from leaking into the vent hole.
If the filter is filtered into large pieces of debris (such as paper), it will suddenly fail completely, which will quickly bring the pressure of the extruder to a peak. Therefore, in order to prevent this from happening, there must be a safety system such as a safety diaphragm, a safety pin or an alarm connected to the pressure gauge.
