Influence of blow molding machine filter on plastic extrusion molding

During the extrusion process of the blow molding machine, the molten material is conveyed to the die through the filter screen. The filter screen allows the material to be filtered and can improve the mixing effect of the material. However, filters can also cause fluctuations in the process, resulting in increased and sometimes decreased back pressure and molten material temperature.

The filter screen of the blow molding machine is fixed to a perforated or grooved protective plate, which allows a seal between the blow molding machine and the die. The pressure generated by a clean filter is less, maybe only 50-100lb/in2 (1lb=0.4536Kg, 1in=25.4mm). As the amount of impurities in the resin trapped on the screen increases, the pressure increases to clog the screen.

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

When replacing a clogged filter, the pressure can drop suddenly and the temperature of the molten material can 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 blow molding machine can be adjusted, and the linear speed of the blow molding machine 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 product form factor. For example, in a flat mold, the cooler molten material may make the material thinner at the center and thicker at the periphery. This situation can be corrected by automatic or manual adjustment of the mold.

After the filter screen changer, a gear pump is equipped to ensure that the molten material enters the mold stably, which can prevent the occurrence of the above problems. However, the temperature change of the molten material after the filter screen is replaced 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 screen.

The reason why some rigid PVC processors are reluctant to use filters is that the filters increase the temperature of the molten PVC material and are prone to degradation, which requires materials with better thermal stability, thereby increasing the cost of materials. The cost will also increase if the filter screen changer specially used for PVC is used. Therefore, most rigid PVC processors either shun the use of filters or use coarse filter units without converters that filter only larger particles of impurities.

2. How to choose a filter

Steel wire is the most commonly used metal filter material for blow molding machines. Stainless steel, although more expensive, can be used in some PVC production lines or other applications to avoid rusting. Nickel alloy filters are used to avoid corrosion by fluoropolymers or PVC.

Typically, filter meshes (or the number of wires per inch) are 20-150 or more. The 20 mesh filter is relatively coarse; the 40-60 mesh filter is relatively fine; the 80-150 mesh filter is very fine.

Most filter meshes have a square weave with the same number of wires in each direction. The Dutch weave method is to use thick metal wires in the horizontal direction and specify an even number, for example, 32x120 pieces/in (1in=25.4mm). The filter net made by the Dutch weaving method can play the role of fine filtration without setting parallel screen nets in the filter device.

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

In general, the filter device is installed in such a way that the coarsest filter screen faces the protective plate, and the thinnest filter screen faces the blow molding machine. For example, the screen arrangement from the protective plate to the blow molding machine may be 20 mesh / 40 mesh / 60 mesh, because this arrangement can prevent the filter screen from being blocked and can "blow" impurities into the protective plate. inside the opening.

If the thinnest screen is 80 or more mesh, in order to prevent the screen from being blocked by the rotational motion of the molten material or large impurities, a coarse screen can be placed in front (such as 20/100/60/ 20 mesh arrangement). Because this device type looks the same from both sides, a symmetrical arrangement (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh) is sometimes used to ensure that they cannot be reversed.

Some processors purposely put the filter upside down, allowing the coarse filter to filter the larger particles on the upstream surface first. They believe that this method allows more of the side molten material to pass through and less decomposition of the material on the upstream surface of the shield.

3. Matters needing attention on filters

Since the wire mesh is easy to rust, avoid contact with water during storage, otherwise rust will appear in the extrudate. What is more serious is that the rusted filter is easy to break and the filtered impurities will leak. Therefore, the filter should be stored in a plastic bag or anti-rust paper.

When extruding PVC material, never place a bare screen on the head of the blow molding machine. This is because the hydrochloric acid in the air near the handpiece will corrode it. The best practice when extruding PVC is to install a fume hood above the die and be sure to keep the spare screen away from here, or even bag it and place it in the freezer.

For the blow molding machine, if the back pressure at the filter becomes larger, the output of the second stage will be reduced, but the output of the first stage will not be affected. When the back pressure increased to about 2500lb/in2 (1lb=0.4536Kg, 1in=25.4mm), the molten material from the first stage began to leak into the vent hole. To avoid this phenomenon, be sure to replace the filter when the pressure is low. In addition, the gear pump behind the filter device also has the function of inhibiting the leakage of molten material into the exhaust hole.

If the filter catches a large piece of debris, it will suddenly fail completely, which will quickly peak the pressure on the blow molding machine. So, in order to prevent this from happening, it is necessary to have safety systems such as safety diaphragms, safety pins or alarms connected to pressure gauges.