XLPE curring fail reasons

hi
i have problem in the xlpe curring that when i use single master batch color the curring pass the test when i use double master batches like Green/yellow the test is fail can you help me to find the reasons

thank you

magdy mansour

magdy

i there,

You really should be giving us the details of how you are achieving the cross-linking reaction such as by the use of pre-compounded insulation containing organic peroxide, organic peroxide injection or cross-linking by using vinyl silane as a grafting agent (Sioplas or Monosil processes). We cannot see what you see and we cannot understand what you think is obvious and do not tell us.

Assumptions
(1) Our response will assume that you are using a pre-compounded insulation containing organic peroxide and that you are using two extruders (one master, one slave) with the smaller slave extruder introducing a green ground stripe (or something similar) using a green color masterbatch with an appropriate amount of clear organic peroxide bearing insulation compound (Or perhaps you just use regular polyethylene and just rely on organic peroxide migration from the adjacent insulation after the extrusion process.) when you fail the cross-linking process.

(2) We also assume that you watch the shelf life of XLPE insulation compound so that there is no reason to suspect the compound itself. Likewise, we assume that your XPLE compound supplier is competent and reliable so that there will be no significant variances in each compound run.

(3) Finally, we assume that this is a horizontal or slope CV line using saturated steam for the heat and pressure needed to achieve the cross-linking process. Please confirm this along with the gage size or conductor cross-section in mm^2. Thank you.

Response

Our first reaction is therefore that you should be focusing on the differences between the two processes and that means:

(1) The occasionally used second striping extruder (slave).
(2) The interconnection pressure tube.
(3) The special striping crosshead.
(4) What happens when you set up the slave extruder and the special striping crosshead.
(5) What happens when the system is running.

The questions are as follows:

(1) Is the equipment that adds the color masterbatch at the slave extruder as least as accurate as that used on your main extruder? (Thereby giving both the proper amount of masterbatch and organic peroxide bearing base compound?) (Gravimetrics)

(2) Is the L/D ratio the same on each extruder? Is the temperature control on each extruder the same, accurate and set to the same temperature profile? Are the controllers checked for accuracy on a regular basis? Are they self-tuning PID? See Note 2. Are the compound melt temperatures the same?

(3) Is the striping crosshead in good shape and does it really have good temperature control (Self-tuning PID)? See Note 2. Does it have hot spots because it is not properly insulated and is the neck in good shape? See Note 1

(4) Is it possible that the striping crosshead has a large metal mass, inadequate heating and no insulation so that the plastic temperature is significantly reduced through the crosshead. Could the conductor be pulling too much heat out of the crosshead? Is the conductor preheated and the conductor temperature well controlled throughout the whole run?

(5) Is the slave extruder including the screw and barrel, in really good shape with no pre-cure found in the breaker plate, screens or on the screw when the extruder is cleaned. See Note 1

(6) Is the connection tube between the slave extruder and the striping crosshead properly sized (diameter) for it's length so as to prevent pre-curing by excessive shear in the tube itself? You need at least a 20:1, L:D ratio. See Note 1

(7) Is there accurate temperature control (Self-Tuning PID) See Note 2. and insulation on the connection tube to prevent hot spots. See Note 1


Resolving the above questions will not necessarily allow the cross-linking process to occur in a steam-cure line but they are indeed the proper questions to ask of this line to make sure you have the extrusion processes well under control. With the exception of the pre-cure issues, they are appropriate to a moisture cure line as well. A dry cure line can have additional issues so you really need to tell us the type of line you have.

Share the process analysis with others in your company so that your people really learn something from this experience.

Note 1
Operators sometimes turn down temperatures to stop pre-curing from occurring and this can confuse things a bit.

Note 2
PID is shorthand for proportional, integral and differential control. It is also sometimes referred to as proportional, rate and reset control. For further information check your automatic controls textbook from your engineering university days. The recommended controllers are self-tuning.

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

How did you test the curing? hotset or gel content?
What percentage of color you added inside?

Normally, a few percentage of color concentration will not produce curing failure whether in water cure or peroxide cure, you should look back at your test method and dumbbellsample.

Grillo