Hard particles in Sioplas

Dear all,

I am process guy presently facing problem in triple extrusion sioplas line for hard particles.What are the root causes and what should be temperature profiles for correct extrusion? Presently we are facing problem when line is stopped and then restarted.
Any suggestion is welcomed

Regards
Parag Kulkarni

Hello Mr. Kulkami,

Could you please give us some more information.

1) Where are you finding these "hard particles" In the finished product? In the screen packs? On the screw?
2) Which layers have these "hard" Particles? The inner shield? The outer shield? The insulation?
3) In Asia, I have seen the term "hard particles" applied to precured or pre-crosslinked polyethylene. Is this what you are describing?
4) I presume you have a dry sioplas system. This means that you are using properly maintained and calibrated gravimetric mixers together at each of the extruder hoppers. Please advise.
5) When you mix the carbon color concentrate with the base polyethylene and the catalyst masterbatch for the semi-conducting layers, are you properly drying that carbon color concentrate first to a -40C dew point? Conversely, if you are buying premixed semi-conducting thermoplastic polyethylenes, are you drying those compounds to -40C dew points?
6) Are all the catalyst masterbatches well within their shelf life and once mixed together at the hopper, are the components used quickly and well within the storage recommended time period. At the end of any run, are the mixtures still remaining in the hoppers thrown away?
7) Finally, is your plant situated in a tropical, high humidity location?

Thank you.

Dear Mr Stewart-Hay,
Thanks for reply.
Please find my answers :

1) Where are you finding these "hard particles" In the finished product? In the screen packs? On the screw?
Ans : Hard particles are found in the finished product of the sioplas line.

2) Which layers have these "hard" Particles? The inner shield? The outer shield? The insulation?
Ans: Outer shield.

3) In Asia, I have seen the term "hard particles" applied to precured or pre-crosslinked polyethylene. Is this what you are describing? Ans :No these are not in the precured or pre-crosslinked polyethylene .

4) I presume you have a dry sioplas system. This means that you are using properly maintained and calibrated gravimetric mixers together at each of the extruder hoppers. Please advise. Ans : Yes, You are right.

5) When you mix the carbon color concentrate with the base polyethylene and the catalyst masterbatch for the semi-conducting layers, are you properly drying that carbon color concentrate first to a -40C dew point? Conversely, if you are buying premixed semi-conducting thermoplastic polyethylenes, are you drying those compounds to -40C dew points? Ans : We are not actually measuring dew point of the dryer but we are drying the material at 75 Degrees C.

6) Are all the catalyst masterbatches well within their shelf life and once mixed together at the hopper, are the components used quickly and well within the storage recommended time period. At the end of any run, are the mixtures in the hoppers thrown away? Ans : Master batches are within shelf life but components may or may not be used quickly as this line is not frequently run and it is not feasible to throw away mixtures of the hopper at the end of the run.

I hope this will give fare idea of the problem.

Hello again,

The first thing I must mention is the dryers. This is especially true for the black semi-conducting color masterbatches, or conversely for the premixed but thermoplastic semi-conducting polyethylene.

All polymer dryers must be of recirculating dehumidifying design. The extruder hoppers must be closed so that the required dew point is achieved. Recirculating hot air without a dehumidifying section is not the answer as the moisture is always there. Moreover, the measurement of the dew point is the standard, not the hot air input or output temperature.. It is very good practice to lower the dew point to -40C for all polymers just before extrusion.

The second thing deals with the left over mixed material in the extruder hoppers.

1) You must have enough storage in the hopper to achieve the proper drying time.
2) You should take the left over, dried and mixed components from the hopper and store them in a sealed (air tight) metal drum so that no moisture from the air can enter.
3) Then you can store the mixed material for a very limited period of time as defined by your catalyst masterbatch manufacturer. Proper management of this mixed material is extremely important.
4) Use up this premixed compound first on the next run but again dry it to a -40C. dew point.

Now you have stated that these "hard particles" are not precured XLPE and that you have no trace of them in the outer shield extruder screen pack.

a) Are these "hard particles" so small and already in one of the constituents that they just pass through the screens?
b) Is there no trace at all of the hard particles at all in the constituents?
c) If these hard particles are forming somewhere in the extruder and/or crosshead then;

i) Review the thermal profile of the extruder and crosshead and make sure that there are no hot spots especially at the transfer tubes from the extruder to the triple crosshead.
ii) Immediately contact the masterbatch catalyst. manufacturer and send them samples of the "Hard particles" embedded in the outer shield samples . Perhaps they will be able to identify these "hard particles" for you.
iii) Immediately repeat this process ii) above with the color masterbatch manufacturer.

I have actually witnessed very fine sand from sandblasting at the polymerization reactor/ mixing plant embedded in the supplied polymer but in this instance, it choked the extruder by filling the screen pack.

This is just about all we can do from afar.

Mr. Parag
May I know the maximum RPM of outer semicon extruder and what was the RPM at the time of process. How the extruders are cooled? liquid or air. Are you using the similar materials for both the semicon layers? Is it happening all the time or with specific voltage grade.
As Mr.Peter J Stewart-Hay, suggested you should have check the temperature profile and hot points along the connections. Please check for wire mesh size also. If you give me the Temperature profile and extruder profile i may help you in establishing the process.
some time it is not the material, it may be the extruder screw and profile, the cross head geometry. Are all three extruders and cross head from same supplier and how the material flow in third layer?
Just find answers of these questions, i hope you will reach on some conclusion soon.
regards,