PVC Cable Cracks

Hello everybody,

I already read a past topic ( 2008 called cable cracks) could be the same situation but we're experimented cable cracks on PVC, the situation is as follows:

We ran this material early April this year and didn't show any problem related to cracks.

We ran two weeks ago same material with same compound lot and shows cable cracks at the moment that we performs the cold impact test ( the crack on small gauges is radial but on .75mm2 and above is longitudinal).

The process parameters on both situations are exactly the same ( same temperature profile 190 C , same line speed 800 meters/min , same master batch percentage, same cable cooling temperature 29 C ) the only two variables that I can see is:

April plant weather conditions was around 36 C and no Humidity on environment....November around 15 C and 30 % humidity.

The second variable is the material age and storage conditions ( is stored just in a plastic bag inside a carton box ).

Could somebody tell me about if core preheater could help with this condition??

Extrusion line is a 60 mm SAMP 24:1 with a barrier screw.

Thanks a lot and regards to all forum.

Angel

Hello Angel,

Two points:

#1 - Since you have not told us that the stored PVC was heat sealed in the polyethylene bags we are going to assume that it wasn't and that it was just stored and continued to absorb moisture from the atmosphere. We always recommend that at extrusion, plastics be dried to a -40 dew point with a proper dessicant type dehumidifying hopper dryer. (Recirculating hot air driers without moisture removal are of no value at all. All they do is heat the plastic while recirculating the moisture.) Was there any indication of moisture on the surface of the wire? (Roughness)

#2 - We have no idea as to the shelf life of your PVC compound. If you purchased this compound, you should get that information from the manufacturer. If you manufactured that PVC compound yourself, your chemical laboratory should be able to tell you that. Generalized opinions however don't count unless they are backed up with facts or measurements.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Hello Peter,

First at all thank you for your very quick reply, the details for the two points described below:

1.-Actually in our process we uses 3 different compounds, PPE is dried at 180 F for 3 hrs before put into a extruder. PVC ISO is dried at 100 F /3 hrs. and PVC is dried also at 100 F / 3 hrs....We did this because we experimented in the past big amount of leak sparks and lumps and it fixed the problem for this new PVC compound. Because it was a trial, we didn't dry because was just a small amount.. At first look ( surface, tensile, elongation ) seemed pretty good.

2.-Based on Supplier information this PVC shouldn't have degradation but as you said this information must be backed with Lab information.

Thanks again and regards to all Forum.

Angel