New screen technology for extrusion processes

During extrusion, the materials pass through the wire mesh screens on their way to die to offer filtering and better blending. However these screens also create process variables, increase back pressure, melt temperatures and in few cases affect the efficiency.

The mesh screens are supported by the breaker plate that has holes to produce the bond among die and extrusion. The clean screens hardly increase pressure by 50 – 100 psi. If the pressure level is high, it refers to the contaminant accumulation.

Check temperature

After the clogged screens are replaced, it results into significant pressure drop. It may also happen to the melt temperature. When extruding a material, these changes may not be troublesome however it may create problem in few cases such as during extrusion of flat or uneven shaped products, the change in melt temperature may deform the material. With specific die adjustments, this problem can be handled but, do not overlook it. You can also place a gear pump subsequent to the screen changer to prevent this issue and can maintain a regular output through the die. However the change in melt temperature subsequent the screen change may still need die adjustment. The gear pumps have small clearances that can be spoiled by hard materials, therefore the fine screens are required to secure the pump.

Instead PVC screens, the use of nickel mesh is suggested to save the cost.

Common materials for screens

Steel is a common material that is used in the production of extruder screens. It is costlier but utilized in few PVC lines to prevent the corrosion problems. Nickel alloys are utilized to prevent corrosion due to PVDC and fluoropolymers.

The screen mesh, or the count of wires per inch, ranges between 20 to 150. Many screens consist of simple square mesh structure with the same count of wires in every direction. The dutch weave has thicker wires in horizontal direction. The Dutch weave mesh follows fine filtration technique without the need of multiple thin screens as single dutch screen is sufficient.

Inconel Mesh, Inconel 690 Mesh

The screens made from same type of mesh are not usually same. The filters made from thinner wires are less fine and are susceptible to breakage. Conventionally the screen packs are configured with the coarsest screen opposite to breaker plate and the finest screen towards the screw. This structure secures the screens from damage and contamination.

For a finest screen of 80 mesh, the coarse screen may be positioned over it to avoid it to be twisted from the rotation of the melt or worn by big contaminants. As this kind of pack seems uniform from the both sides, it is produced from the symmetrical mesh to ensure that it cannot be installed backside.

Although some processors purposely configure screen packs in the back side. Keeping the coarser screen initially grabs the large size particles on the upstream surface. Observing this method, it permits more lateral melt flow and low deformation on the upstream layer of the breaker plate.

The steel screens corrode in storage and they should be protected from the moisture or the corrosion. The corroded screen can easily break and allow the passage of contaminants. In case of using PVC screens, they should not left open on the extruder because it can be attacked by the hydrochloric acid that is present in air. The best option is to use PVC with a fume hood on the die head and keep extra screens away.

The breaker plates can deform due to severe torch cleaning, if this occurs, sealing surfaces are deformed and sliding surfaces don’t interact adequately that results into secretion.

In addition of time taking cleanup, the secreted melt affects the thermocouple readings, damages heaters and in distinct conditions, it can catch fire and burn off.

The screens remain no- longer useful if a large amount of contaminants accumulate such as paper that results into rapid pressure change in the extruder. So you should be ready with a security system for example rupture disk, shear pins etc.

Which kind of contaminants you should be aware of?

Dirty screens describe what is going through the extruder. As soon as the contaminants are hold or gels, it is essential to find out from where they are coming. Gels that are elastic in nature, can travel through everything however the finest screens can be made in a resin unit or through the extrusion process. If you have large amount of gels in the beginning which quickly reduce, they are definitely home and not caused by the resin maker. It can be tough to find the source of contaminants.

The versatile wire meshes have become popular for example conveyor and process belts in the different applications. The root of this creative technique was decades old as wire weavers that successfully fulfilled the challenge to weaving technique.

Following the latest techniques and material combination, Heanjia Super-Metals develops the wire meshes for high corrosion applications. In the elevated temperature ranges the heat resistant material shows the maximum potential.

The reliable expansion of the material, new shapes and the stead adaption of the weaving techniques, Heanjia offers the characterized solutions.

The manufacturer of high precision meshes and, the world’s premier leader of wire mesh techniques, Heanjia serves the different sectors and fields. We offer the tailor- made high tech solutions that are far superior to classic weaves.

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