Repair, maintenance and use specifications of large blow molding machines

During the extrusion process in a blow molding machine, the molten material is conveyed to the die through a filter. The filter filters the material and improves mixing. However, it can also cause process fluctuations, leading to increased backpressure and melt temperature, or sometimes even decreased backpressure.

The blow molding machine's filter is fixed to a porous or slotted protective plate, creating a seal between the blow molding machine and the die. A clean filter generates low pressure, perhaps only 50-100 lb/in² (1 lb = 0.4536 kg, 1 in = 25.4 mm). As the amount of impurities trapped in the resin increases, the pressure increases, leading to filter blockage.

1. Filters Affect Molten Material Temperature

When replacing a clogged filter, the pressure can drop suddenly, and the melt temperature may also drop, resulting in changes in part dimensions. To maintain consistent part dimensions, the blow molding machine's screw speed or linear speed can be adjusted.

When extruding round products, these variations may not cause significant problems. However, when extruding flat or irregularly shaped products, changes in melt temperature can affect the product's overall dimensions. For example, in a flat die, cooler melt may result in a thinner center and thicker edges. This can be corrected through automatic or manual mold adjustments.

A gear pump after the filter changer can help ensure a stable flow of melt into the die and prevent these problems. However, mold adjustments are still necessary to address temperature fluctuations after filter changes. Furthermore, because gear pumps are easily damaged by hard impurities, they also require protection from fine filters.

Some rigid PVC processors are reluctant to use filters because they increase the temperature of the PVC melt, making it more susceptible to degradation. This necessitates the use of more thermally stable materials, which increases material costs. Using a filter changer specifically for PVC also increases costs. Consequently, most rigid PVC processors either avoid using filters or use coarse filtration without a changer, filtering only larger impurities.

2. How to Choose a Filter

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

Typically, filter mesh sizes (or the number of wires per inch) range from 20 to 150 or more. Filters with 20 mesh are coarse; those with 40-60 mesh are fine; and those with 80-150 mesh are very fine.

Most filter meshes have a square weave, with an equal number of wires in each direction. The Dutch weave uses coarse wires in the horizontal direction, with an even number of wires, for example, 32 x 120 wires per inch (1 inch = 25.4 mm). Filters made with the Dutch weave provide fine filtration without the need for parallel screens within the filter unit.

The aperture size of filters with the same mesh size is determined by the wire diameter and does not necessarily need to be identical. For example, a 20-mesh filter made from wire with a 24-inch spacing and a 0.02-inch diameter would have openings of 0.01 inch on each side. A 20-mesh filter made from wire with a 30-inch spacing and a 0.01-inch diameter would have slightly larger openings of 0.04 inch on each side. This is because fine wire does not filter as finely and is more prone to clogging (1 inch = 25.4 mm).

Typically, filters are installed with the coarsest mesh facing the protective plate and the finest mesh facing the blow molding machine. For example, the mesh arrangement from the protective plate to the blow molding machine might be 20 mesh/40 mesh/60 mesh. This arrangement prevents clogging and allows impurities to be "blown" into the openings in the protective plate.

If the finest filter has 80 or more meshes, a coarser filter can be placed in front to prevent it from being clogged by the swirling motion of the molten material or large impurities (for example, a 20/100/60/20 mesh arrangement). Because this type of setup looks identical from both sides, a symmetrical arrangement (20 mesh/60 mesh/100 mesh/60 mesh/20 mesh) is sometimes used to ensure they are not reversed.

Some processors intentionally install the filters upside down, allowing the coarser filter to filter larger impurities on the upstream surface first. They believe this method allows more molten material to pass through from the sides and reduces the decomposition of material on the upstream surface of the protective plate.

III. Precautions Regarding Filters

Since wire filters are prone to rust, they should be stored away from water, as this will cause rust to appear in the extrudate. Worse still, rusted filters can easily break, allowing filtered impurities to leak through. Therefore, the filters should be stored in plastic bags or rust-proof paper.

When extruding PVC, never place an exposed filter near the die head. This is because hydrochloric acid in the air near the die head can corrode it. When extruding PVC, it's best to install a fume hood above the die head and keep spare filters away from this area. They can even be bagged and stored in a refrigerator.

For a blow molding machine, if back pressure increases at the filter, the second-stage output will decrease, but the first-stage output will not be affected. When back pressure reaches approximately 2500 lb/in² (1 lb = 0.4536 kg, 1 in = 25.4 mm), molten material from the first stage will begin to leak into the vent. To prevent this, always replace the filter when the pressure is low. Furthermore, the gear pump behind the filter also helps prevent molten material from leaking into the vent.

If the filter filters large pieces of debris, it will suddenly fail completely, causing the blow molding machine's pressure to spike rapidly. Therefore, in order to prevent the above situation from happening, there must be a safety system such as a safety diaphragm, a safety pin or an alarm connected to the pressure gauge.