In the extrusion process of the extruder, the molten material is transported to the mold through the filter net. The filter net makes the material filter, and can improve the mixing effect of the material. However, the filter can also make the process fluctuate, resulting in the rise of the back pressure and the temperature of the molten material, and sometimes it can be reduced.
The filter extruder is fixed on a porous protective plate or groove, so that a seal is formed between the extruder and the die. A clean filter produces less pressure, probably only 50 to 100lb/in2 (1lb=0.4536Kg, 1in=25.4mm). As the pressure increases, the amount of impurities in the resin intercepted on the filter is increased, thus blocking the filter network.
The filter network affects the temperature of the molten material
When the blocked filter is replaced, the pressure drops suddenly and the temperature of the molten material may decrease, 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 be adjusted. These changes may not cause serious problems when extruding round products, but when extruding flat or irregular products, the change of melt material temperature may affect the size of products. For example, in a flat mold, the cold molten material may make the center of the sheet thinner and make the periphery thicker. This can be corrected by automatic or manual adjustment of the mold.
In the rear of the filter, a gear pump is equipped to ensure that the molten material enters the mold steadily, which prevents the above problems from happening. However, the temperature change of the molten material after the replacement of the filter still needs to be solved through the adjustment of 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 fine filter network.
PVC processors reluctant to use filter some hard, the filter will make PVC melt temperature easy to degrade, so you need a better thermal stability of materials, thus increasing the cost of material. If you use a PVC - specific filter network converter, it will also increase the cost. So the most hard PVC processors or avoid the use of filters, or use a coarse filtering device without converter, only filter larger particles of impurities.
How to select the filter net
Steel wire is the most commonly used metal filter material for extruder. Although stainless steel is more expensive, it can be used in some PVC production lines or other occasions to avoid rust. Nickel alloy filter network is applied to avoid corrosion by fluoropolymer or PVDC.
General, filter mesh (or the number of wires per inch) ranged from 20 to 150 or more. The 20 filter mesh coarse; 40 ~ 60 mesh filter is fine; 80 ~ 150 mesh filter is fine.
Most filters are woven mesh grid, the same number of wires in each direction. Holland knitting is a method of adopting wires in the horizontal direction, and the provisions for the even number, for example, 32x120 /in (1in=25.4mm). The filter net obtained by the Holland weave method does not need a parallel screen in the filter device to play a fine filtering function.
If the same number of filter aperture is according to the diameter of the metal wire to determine, not necessarily exactly the same. For example, the row spacing of 24In, filter wire of 0.02in diameter was made of 20 mesh, the each side of the hole is 0.01in; and the spacing is 30in, the wire of 0.01in diameter was made of 20 mesh filter, the each side of the hole slightly larger, 0.04in. This is because the filaments of the fine wire are not thin enough and are more likely to block (1in=25.4mm).
In general, the installation of the filter is: the coarser filter screen facing the protective plate, and the finest filter in the face of the extruder.
For example, from the protection board to filter the extruder arrangement may be 20 mesh /40 mesh /60 mesh, because this arrangement structure can prevent the filter blocked, and will be opening "into the" impurity protection board in.
If screen fine for 80 or more of the mesh, in order to prevent the filter blocked rotation of molten material or large impurities, it can be a rough filter in front of (such as 20/100/60/20 mesh arrangement). Because this type of device from both sides to see the same, so in order to ensure that they will not be reversed, sometimes using symmetrical arrangement (20 mesh /60 mesh /100 mesh /60 mesh /20 mesh).
Some processors will deliberately filter installed upside down, let the rough filter to filter the upper surface of large particle impurities. They believe that this method allows more side melt materials to pass through and make less materials on the upper surface of the protective plate decompose.
Attention to filter network
Because the steel wire filter is easy to rust, so in storage to avoid moisture, otherwise the rust will appear in the extrusion. What is more serious is that rust filters are easy to break and filter out impurities. Therefore, we should store filters in plastic bags or rust proof paper.
When is extrusion of PVC material, must not be exposed to the filter screen is arranged in the extruder head. This is due to the corrosion of hydrochloric acid in the air near the die of the head. The best way to squeeze out PVC is to install a smoke hood over the die head, and make sure that the spare filter net is far away from here, and even placed in bags.
When blowing and cleaning the protective plate, the reverse roll phenomenon is easy to occur, which causes the cover to be damaged, and the slip surface is not matched, and the leakage is produced. In this way, not only will the cleaning work take time, but also the leaked molten material may affect the numerical value of the thermocouple, and it will also damage the heater, and sometimes even cause the fire.
For single exhaust, two extruder, if the filter net pressure becomes larger, so the second output will be reduced, but its output is not affected by the first paragraph. When the back pressure increases to about 2500lb/in2 (1lb=0.4536Kg, 1in=25.4mm), the molten material from the first section begins to leak into the exhaust hole. In order to avoid this phenomenon, it is necessary to replace the filter when the pressure is low. In addition, the gear pump behind the filter also has the effect of inhibiting the leakage of molten material into the exhaust gas hole.
If filter into a large debris (such as paper), it will suddenly completely failure, this will make the extruder pressure rapidly reached the peak. Therefore, in order to prevent the above situation, there must be a security system such as a safety diaphragm, a safety pin, or an alarm connected to the pressure gauge.