Extrusion: Screens & Screening

By Allan Griff

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Q: A quick question regarding staining of carbon black yarns. Can you think of CB building up on the screen and when the pressure reaches a certain threshold pushing the CB and staining the yarn black or do you think CB is unlikely to get stuck there? Our screens are european "2/14-88" but not sure how to translate this into your numbered mesh. Can see you an influence of the type of CB and the particle size in it getting stained?  
A: I'm not sure what is meant by "staining." If it is a black product, how can it show a stain?
In any case, carbon black is sold in varying particle sizes, and I suppose some might be big enough to get caught on a fine screen. The supplier of the colorant (or colored compound) can be asked about the particle size. If the problem is indeed traceable to agglomerates (clumps of smaller particles) any improvement in dispersive mixing will help. You should also know for sure the hole size. It sounds like you have 2 14-mesh and an 88-mesh, and it it's European it's holes per cm, so that corresponds ot 2 35-mesh and one 220-mesh. That's a big difference, and I'd expect problems with the 220/88 blowing into/through the 14/35. If you send me a picture of the two clean screens with a ruler in the picture, I'd be glad to count wires and judge the gauge for you. The likely explanation of black particles caught on the screen is degradation behind the screen, perhaps on the screw tip or root, or even in the feed. You can tell the difference, as if it is the feed it will start right away and the pressure increase will be steady. If it being made in the system, perhaps because the screw is running slowly, it won't show at first, but after a while pressure will build up at an accelerating rate.

Q: What kind of screen packs do we need for FEP, PE and PVC compounds?
A: FEP probably needs a non-ferrous material, usually a nickel alloy. The material supplier should advise here, but since FEP is fully fluorinated, I suspect it will need the special metals, not only for screens but for all surfaces in contact with the melt. PE is easy: regular steel is OK, but protect from rusting. If you worry about that and don't use them up too fast, go to stainless steel. PVC is a little more controversial: steel is OK if the system isn't run too hot, and not left with the head full and the screw stopped for more than a few minutes. Stainless steel is better. Many PVC extruders do without screens (and their benefits) in order to keep melt temperature lower and thus allow a cheaper (less stabilizer) material.

Q: A lot of companies that buy our screens ask what mesh configuration they should use for the certain material that they run. We've found it is pretty much trial and error. Is there any mesh combo's your recomend for certain materials? 
A: There is no relation of mesh to material, except perhaps for PVC as explained above, where a lighter pack means lower melt temperature. The meshes depend on the degree of filtration desired, which in turn depends on the needs of the application. Also, as a tighter pack means faster clogging (more frequent changes) and better mixing (more pressure/temperature), these factors will enter into the decision. You could say that no-one ever got fired for starting out with a 20-40-80 pack, but that should be questioned and modified if needed, once the process is established.

July 27, 2007