Cable crack

Hi,

I plan to run a black MDPE jacketing compound. I have produced the compound with 70% HDPE + 24% LLDPE + 6% Carbon Black masterbatch. My potential customer has manufactured a low voltage cable using the compound and found the cable to crack.

Please suggest any reason why this occur? Anything to do with the formulation? The Shore "d" hardness is found to be in HDPE category (62) instead of MDPE category. Does this have to do with the extra content of HDPE or non- uniform mixture occurring?

Any suggestion or help would be great.

There are many reasons why a cable jacket can crack including poor processing at your customer's plant. If indeed you are sure that this is your problem as the plastics compounder (In a tropical country.) then I would first suspect poor mixing. If that doesn't resolve the problem then abandon the linear low density polyethylene in favor of low density polyethylene and see if that resolves the issue. Hopefully you have a laboratory where various mixtures can be tried and assessed.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

(Question 1) When mention about poor processing at customers’ plant, is it got to do with the processing temp./ pressure? Or others? At this stage, I agree it should be poor mixing as we are still in the R&D phase for the compound.

(Question 2) Would you suggest any equipment that can perform for this mixing? Thank you for the suggestion.

(Question 3) Are there any reason LDPE could be a better resin than LLDPE for this case?

(Question 4) Why is it that the problem is more significant in a tropical country? Weather?

Thanks

Response to Question 1
Poor processing at your customer's plant.
First of all, you have told us zero about the cable other than it is low voltage and that the sheath is cracking. We don't even know how the cable cracked (Radially or with longitudinal splits.) Perhaps there is a metallic shield under the sheath and that was incorrectly formed and thus protruding almost completely through the sheath. Moreover we have no idea if your customer used the correct tooling and cooling procedures. Similarly we have no clue as to when the cable jacket failed. Was it during dereeling and was the full reel sitting out in the sun for a long period of time first? For all we know, this cable has much air space inside it and perhaps your customer applied the jacket eccentrically so that there is a extremely thin wall on one side of the cable.
We cannot see what you see or what is happening on the other side of the planet. Moreover, we do not read minds.

Response to Question 2
Improving your mixing process.
How can we recommend improving your mixing process when we have no idea what you are doing now? You must really do your own development at your plant. There are of course a number of very good machines available for mixing plastics.

Response to Question 3
Medium density PE from LDPE and HDPE mixed together.
I have quite a lot of experience processing compounded mixtures of high density polyethylene, low density polyethylene and a foaming agent properly mixed and pelletized to produce a high quality, medium density, cellular polyethylene compound. This compound worked well for us for many years. You might be having a bit of a problem properly mixing linear low polyethylene and high density polyethylene. I expect it will be easier for you to stop using LLDPE in favor of LDPE in this instance. Prove me wrong!

Response to Question 4
Tropical Country.
In general I relate polyethylene sheath cracking with cold climates and thus I was trying to make the point that sheath cracking in a tropical country is very unusual, at least to me.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com