Hot set/ permanent set

Sirs/ Madame,

I have been manufacturing XLPE compounds for quite some time.

However, recently, I encounter a problem whereby the finished product cable) of my customer failed to pass Hot Set and Permanent Set (curing 8 hours). I have looked at the formulation but find no clue to that. Nothing has changed in terms of production for my customers as well. Any suggestions? Also, are there any methods to improve Hot Set and Permanent Set results?

NB: At what circumstances would a wire/ cable vary in terms of their size (thickness/ diameter) under different production speeds?

Appreciate your help.

Do I understand that this is a moisture cross-linkable, Silane grafted XLPE compound? If so, I imagine that the problem is with the dry Silane masterbatch.

At your customer's plant:
- Increases in screw RPM decrease the degree of cross-linking.
- Increases in extrusion temperature increase the degree of cross-linking
- Increases in dry silane masterbatch content increase the degree of cross-linking.

With regard to the dry Silane masterbatch at your customers plant:
- The shelf life time of dry Silane masterbatch is approximately 6 months
- Once the packaging bag is opened, it should be used immediately.
- The bags have to be kept in cool and dry condition. This could be a problem in a tropical country.
- Transport and storage above 50 degrees C should be avoided. A distinct possibility in a tropical country!
- To control the feed rate exactly, a gravimetric dosing system should be used.

Insulation tensile strength and elongation at break can also be improved at your customer's plant by:
- Conductor preheating
- Gradient cooling.
- Tubing at the extruder if possible.
- Very high relative humidity in the sauna.
- Increasing the temperature in the sauna.
- Decreasing the amount of wire/cable on the reel that enters the sauna.

Diameter (thickness) variances of wire and cable are a result of:
- Slugging inside the extruder leading to variable output. (Melt position in the extruder moving around and out of control.)
- Worn screw and barrel.
- Extruder and capstan drives not tuned and or maintained and thus hunting around the set points.
- Extruder and capstan drives overheating and hunting. A distinct possibility in a tropical country!
- Screw cooling water control malfunctioning.

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Hot Set Test

- First the cable is cured using steam.
- Then a standard length of sample is selected, and a gauge length is marked.
- The thickness of the sample is measured and corresponding weight is calculated with the values given in the specification (Stress = 0.2N/mm²).
- The weight is attached to one end of the sample and kept in an oven maintained at 200°C for 15mins.
- The elongation is then measured. If the sample breaks, the sample is a failure.

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Please note that we have been provided with information regarding the acquisition of ovens for the hot set test (Measurement of OF Hot Creep) Please contact INHOL B.V./ PTL at www.inhol.com The contact there is Jacob Steendam. This email address is being protected from spambots. You need JavaScript enabled to view it.


In future please try to give precise information and please post electrical issues in the Electrical Section. Thank you.

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