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Influence of filter net of Blow moulding machine on plastic extrusion molding
- Sep 25, 2018 -

Influence of filter net of Blow moulding machine on plastic extrusion molding

During the extrusion process of the blow molding machine, the molten material is conveyed to the die through the filter mesh. The filter screen allows the material to be filtered and the mixing effect of the material can be improved.
However, the filter screen can also cause fluctuations in the process, resulting in back pressure and molten material temperature rise, and sometimes reduce. The filter screen of the blow moulding machine is fixed on a porous or grooved protective plate, so that a seal can be formed between the blow moulding machine and the Die head. Clean filters produce less pressure and may only be 50-100lb/in2 (1lb=0.4536kg,1in=25.4mm).

As the amount of impurities in the resin trapped on the filter is increased, the pressure increases to block the filter.

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

When the blocked filter is replaced, the pressure drops abruptly, and the temperature of the molten material may decrease, resulting in a change in the size of the product.

To maintain the same size of the product, the screw speed of the blow moulding machine can be adjusted, and the linear speed of the blow moulding machine can be adjusted. These changes may not cause serious problems when extruding round products, but when extruding flat or irregularly shaped products, changes in the temperature of the molten material may affect the product's dimensions. For example, in a flat mold, the cooler molten material may make the material center thin, and make the periphery thicker.

This condition can be corrected by automatic or manual adjustment of the mold. After the filter converter, equipped with a can ensure that the molten material stably into the mold gear pump, can prevent the occurrence of the above problems. However, the changes in the temperature of the molten material after the replacement of the filter will still need to be solved 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. Some hard PVC processors do not want to use the filter because the filter will make the PVC molten material temperature rise and prone to degradation, which requires better thermal stability of materials, thereby increasing the cost of materials. The use of PVC-specific filter converters will also increase costs.

Therefore, most hard PVC processors, either avoid the use of filters, or use a coarse filter without a transducer, only to filter the larger particles of impurities.

2. How to select the filter screen 

Steel wire is the most commonly used metal filter material for blow moulding machines. Although stainless steel is more expensive, it can be used in certain PVC production lines or other occasions to avoid rust.
Nickel alloy filters are used to avoid corrosion by fluoropolymer or PVC. In general, filter screens (or the number of wires per inch) are 20-150 or more.
20 Screen Eye filter mesh is coarse, 40-60 screen eye filter screen is relatively thin, 80-150 screen eye filter is very fine. Most screens are grid-woven, with the same number of wires in each direction. Dutch weaving method is the horizontal direction of the use of coarse wire, and defined as an even, for example, 32x120 root/in (1in=25.4mm).
With the Dutch-style weaving system of the filter, do not need to set up in the filter device parallel screen can play a fine filter effect. The aperture of the filter mesh with the same number of screens is determined by the diameter of the wire, and there is no need to be exactly the same. For example, a filter of 20 sieve eyes made of wire spacing 24in, 0.02in diameter, the opening of each side is 0.01in, and the line spacing is 30in,
A 20-mesh screen with a diameter of 0.01in of wire, with a slightly larger opening on each side, is 0.04in, because fine wire filters are not thin enough and are more prone to clogging (1in=25.4mm). In general, the installation of the filter device is: the most coarse screen to the protection plate, and the most fine screen is facing the blow molding machine.
For example, the screen arrangement from the protection plate to the blow molding machine may be 20 sieve eye/40 screen eye/60 screen eye, because this arrangement structure can prevent the filter to be blocked, and can "blow" the impurities into the opening of the protection plate. If the most fine screen is 80 or more, in order to prevent the filter from being blocked by rotational movement or large impurities in the molten material, a coarse filter can be placed on the front (e.g. 20/100/60/20 sieve eye arrangement).
Because this type of device looks the same from both sides, in order to ensure that they are not reversed, it is sometimes used in symmetrical arrangement (20-screen/60-eye/100-eye/60-eye/20-eye). Some processors deliberately reverse the filter, allowing coarse filter to filter the upstream surface of large particulate impurities.
They believe that this method can allow more side melt material through, and the protection plate upstream surface of the material less decomposition.

3. Considerations for filter screen

Since the wire mesh is prone to rust, avoid contact with water in storage, otherwise rust will appear in the extrusion.
What is more serious is that the rust filter is prone to breakage and leaks the filtered impurities, so the filter is stored in plastic bags or rust-proof paper. When extruding PVC material, the bare screen must not be placed on the head of the Blow moulding machine. This is because hydrochloric acid in the air near the nose will corrode it.
The best way to extrude PVC is to install a fume hood above the die, and make sure to keep the backup strainer away from here, or even put it in a bag and place it in the freezer. For blow moulding machines, if the back pressure on the filter mesh becomes larger, the second segment of the output will be reduced, but the output of the first segment will not be affected. When the back pressure is approximately increased to 2500lb/in2 (1lb=0.4536kg,1in=25.4mm), the molten material from the first segment begins to leak into the vent hole. In order to avoid such phenomena, it is important to replace the filter when the pressure is low.
In addition, the gear pump behind the filter device also suppresses the leakage of molten material into the vent hole. If the filter is filtered to a large piece of debris, it will suddenly fail completely, which will cause the pressure of the blow moulding machine to reach its peak quickly. Therefore, in order to prevent the above situation, must have such as safety diaphragm, safety pin or the pressure gauge connected to the alarm and other safety systems.