? over wire extrusion.. sparkout problem

[BSouders]

I work for a company that is producing circuit wire and building wire. With our smaller wire [(14-1, 12-1, 10-1 solid conductors) and (14-19, 12-19, and 10-19 stranded wire)] we are having a rise in sparkouts. We are extruding a PVC inner and a nylon jacket. We seem to keep getting pin holes in the insulation, which cause a sparkout and a sparkout causes scrap. Over the last year, the rate of sparkouts have risen significantly. We have been researching the problem and can not find any correlation between any changes in the process that we have made. We buy PVC resin from Georgia Gulf and color chips from Teknor color. We are trying to see if there has been some change in their products that can relate to our problem. IF ANYONE has ever seen this problem rise in wire and cable industry and found a way to correct it, please give me some advice. We are about to pull our hair out on things to try. Thanks

Brian

 

[patprimmer]

I can think of a number of possibilities, but I have no direct experience with cable coating.

The pin holes are probably caused by one of 2 actions:-

1) Jells or non meltable solids in the resin, causing very localised variations in melt flow, the solid or jell to hang up, and a disruption to melt flow.

2) Out gassing.
Out gassing can be from any number of sources, but mainly from coatings or contaminants on the wire, or moisture or or other volatiles in the resin, or in the additive packages such as the colour or stabilisers, or tie layers if used.

Something as simple as storing the wire in a cold area, then getting condensation on it prior to entering the cross head for instance.

Other possibilities are:-

A change in ambient conditions, like a door being left open that was always closed.

A worker handling the wire with damp hands whereas before the used gloves, or dry hands.

A worker has had a slight change of duty, and now handles something else as well as the wire or resin, and is causing some cross contamination.

The wall thickness has been reduced.

The clearances and taper angles within the cross head have been changed.

The effective pressure or dwell time or temperatures in the cross head have been changed.

The screw profile in the extruder has been changed. If to sever, it might increase production rates, but cause some degradation through shear, or if to gentle, it might not expel all air or volatiles from the melt.

If the extruder has a vented screw, a change of compression and decompression ratios, or a change in ambient conditions at the vent.

That is all I can think of off the top of my head right now.



Regards
pat pprimmer@acay.com.au
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[gambro]

Cleanliness is the top consideration in wire coating. The reason for your sparkouts is most likely due to formation of microvoids resulting from some sort of contamination.

One cause could be water. Is the wire completely dry? How high is the air humidity at the extrusion site? Another idea is to pre-dry the PVC to get rid of any surface moisture in the PVC granulate/powder.

PVC can release Hydrogen Chloride acid gas if it is not properly processed. The processing temperature should be high enough for good fusion but not so high as to release acid.

Impurities/dirt either on the wire or in the plastic material might also lead to such problems. Also, extruder wear or wear from the wire when it is run through the coating die can lead to metal particles being transferred to the plastic material.

Do you buy the PVC compound or do you compound it yourself? If yes, insufficient dispersion of additives (internal lubricants, plasticizer, stabilizers, fillers) could be the cause. Do you use carbon black? How high is the level of impurities in the carbon black?

Supply of clean resin is not enough to ensure good product. How is the plant environment at your facility? How do you guard against in introduction of contaminants?

 

[BSouders]

To answer a few questions: We do not buy our own resin. We buy our PVC compound through Georgia Gulf. They have been our supplier for the last 2 years. Prior to this APEX I believe was the primary supplier. The resin is not colored, it is a natural white color. We thought that maybe the color chips that we add (made by teknor) might be having some type of dispersion problem. That was blown out of the water though when looking at sparkout reports by color/sizes of wire. The natural white is just as high as say the black wire in some days in relation to number of sparkouts. Here in Kansas, the air is not very dry. Just yesterday morning when I walked out to leave for work, I didn't know whether to breathe the air or drink it. It is muggy, humid and sticky. Possibly there is some type of water forming on the copper wire in the staging area. The copper wire is drawn, bunched and stranded here at the same facility. After an order has been run in the wire mill, the wire is staged on teh cement floor in front of the extrusion area. To the post above, Wall thickness has been reduced over the last 2 years in cost saving means. We still pass all of UL's test and have set standards to meet or the material is scrapped and re-ran. Thanks for the replies, it is very helpful. I suggested these things to our senior engineer and a couple other people.

Brian

 

[BSouders]

We have a pre-heater on one of our lines that we run wire on. The pre-heater works by charging the wire with electricity which in turn heats the wire before the PVC and Nylon is extruded onto it. The temps that the wire is heated to, I'm not sure of. But it would be a start. Does anyone think that this would be an effective way to eliminate any moisture on the wire? Thanks

Brian

 

[patprimmer]

Yes, and it might help the bond, however if it is to hot, it might degrade the PVC and cause out-gassing.

Are you still using the same type, brand and grade of nylon, or did you change from say type 12 to type 6.

Are some wires coated in PVC and others are coated in nylon. or is the nylon on top of the PVC, say just to give termite resistance.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.