PE Extrusion

[samenriq]

Good Day!
Hope for assistance if anyone can help identify what can cause this kind of screw wear for extruding HDPE pipe.
We use HDPE pipe grade PE 100 grade resins and add black or blue masterbatch.
Can mixing color masterbatch result to this kind of wear?
Or is that a screw metallurgy concern?
Please assist to help us understand what we need to correct.
Best regards, Sam

PE 45mm screw.jpg

DSC00653.jpg

DSC00661.jpg

[Len]

First some questions:
1. How old is this screw? Is/was it designed for PE processing?
2. Was it used before? And if so, what resins were previously processed on it?
3. Is your color M/B only color or are your also running FR M/B on this screw?

Observations:
1. The screw looks like a single stage screw; feed, transition, meter sections w/out a vent or a second transition section. Is this true?
2. Most PE screws are 3:1 compression ratio and at least 20/1, L/D.
a. What are the dimensions; compression ratio and L/D of this screw?
3. The screw looks like it could use a re-plating. It looks very rough and pitted to me.
4. You are getting some form of plate out (yellow residue) just behind the feed flight. Have you identified this residue material?

I can't formulate a reply w/out some basic information.

Len

[samenriq]

Dear Len,
This single screw is less than 2 years old. Was brand new when acquired. We have been buying from the same supplier but the last two purchase was problematic, the other one broke and the latest one, this was how it looks like. We have screw from other older machine from the same supplier but did not have this manifestation of wear. We suspected that the supplied screw was not made to German standard as we used to know. But when we made the complaint to the supplier since this was the second screw already, We were told that :
" the majority reason of wear on KME45-36 was abrasion due to different melting points of base material and colour master batch. Therefore, wear was concentrated in area “end of grooved bush/beginning of barrel”.
Was told because the masterbatch base material was LLDPE. That's why we can't understand how can it gives this wear when the same extruder was suppose to be able to produce pipes from HDPE, LDPE, PPR materials.
We believe that the metallurgy of the screw was defective to start with. That's why I'm hoping maybe someone from the community maybe have some experience on this behavior to validate our thinking.
Best regards, Sam

[Len]

Sam,

The screw manufacturer's excuse for erosion and wear is unfounded. I've never seen a screw in such a bad condition after 24 months of running PE resin. The screw looks like it was corroded, eaten away by acid or caustic material, certainly not normally present in PE pipe compounds. If you were running PVC or Delrin I could understand such an appearance, especially if these materials were degrading under your process com\nditions. But, not PE.

Secondly, it is not unusual to use a LDPE concentrate or M/B in a HDPE resin. In fact, it wise to select a lower, easier melting LDPE as a carrier for the color, and run it in a higher melting (lower MF) HDPE for good dispersion.

This screw should process LDPE, HDPE resins and concentrates w/out such pitting and roughness! I agree, the plating was not done properly from the beginning. Also, I suspect the base metal used to make the screw is inferior.

I take it this was his second chance to supply a screw that would not break or in this case corrode due to poor metallurgy of the screw and plating. I'd start looking elsewhere for screw replacement.

Len

[samenriq]

Dear Len,
Thanks a lot for the reply
We are really hoping to get replies from the community that are not related to us so we can get an unbiased reply.
But to reply based on the pictures shown and compared to their excuse that "abrasion was due to different melting points of base material and colour mater batch".
I also don't think that there are any components inside the PE master batch that can result to this kind of damage? Is my assumption correct? because otherwise, the wear should look like something sand the surface off unlike in the pictures showing pitting. Isn't it?

Best regards, Sam

[Louis]

The wear appears to be either a chemical reaction to the screw surface, or severe abrasion.

The "yellow" material I assume is one of the additives in your master batch. That said, it appears that it is a powder of some sort and not part of a color concentrate or master batch in the typical sense of the word. The first photo shows a close up of the second turn of the screw and it too exhibits the same pitting as that seen in what appears to be the transition section of the screw, although not nearly as bad as evidenced by the second photo, which shows the transition and first few turn in the same view. This leads me to lean towards abrasion as the material is more tightly packed in this area than the first few turns at the feed.

I am betting that the metering section shows little or no signs of wear as the ingredients are well mixed and lubricated by the time it reaches that section.

To help define the problem, can you tell us if the same material has ever run on this machine with a different screw or on another machine without experiencing a similar issue.

As to the different melt temperatures of the various ingredients, I doubt that has nothing to do with the wear. It looks more to be a mechanical issue as the leading or pushing edge of the flight appears to be worn down thinner in the one photo indicating abrasion, whereas a chemical reaction might affect the complete screw in a uniform pattern

As to the screw “breaking”, that is another story and would require more details such as screw speed at break, motor amps, and barrel temperatures at a minimum. Not knowing those details makes it difficult to put the blame on the manufacturer.


Hope that helps

[samenriq]

Dear Louis,
The reason why we're confused because we have used the same material to an older line and it does not manifest the same wear. That's why we're thinking how can the screw behave differently when we're using the same extruder from the same supplier who also supplies the screw. And we are thinking maybe the older screw came from original German origin while the newer ones came from their China source.
And the reason for the wear they're telling us was that the difference in the melting points of base material and masterbatch as quoted in their email reply.
We also produce PVC pipes with TiO2 and CaCO3 and colorant and the wear we are accustomed to look like the surface was sanded off and the screw diameter is reduced.
And because of those previous experiences we find it hard to accept that from the screw pictures that is was result of wear due to production and we are more inclined to suspect that something may have probably went wrong with their nitriding or surface treatment of the screw during their production.
Thanks and regards.

[Louis]

Based on your feedback and analysis of the probelm it does seem to indicate a defective feedscrew. I suppose an anaysis of the metal might prove or disprove that, but at what cost?

One last question samenriq, are the screw designs from the older line the same as this one? Compression ratio, L/D, flight depths and helix angle?

[samenriq]

Dear Louis,
Yes they are, 45mm screw diameter and L/D 36.

[Len]

Dear Louis,
Yes they are, 45mm screw diameter and L/D 36.

Thank you.

I asked for that information in my first post.

Len

[zaheera zaleq]

Gentlemen,

The pictures indicate the same thing happened on my lab 40mm screw extruder 5 years ago (if my memory still in a good shape . And the medium is PE too. So, my supervisor sent the screw to our engineering team and what they discovered was the screw actually a new coated-secondhand-screw. which mean if you can bleach this screw inside some handsome acids, u will find the real form of it (and probably will make you heartbroken). but first you must find the outer material of the screw so it will help you to choose the best solution for the bleaching process. you can ask your chemist if you have one.

I hope my sharing will help. thank you

Zaheera

[samenriq]

Thanks Zaheera for sharing your experience

[zaheera zaleq]

Most welcome sam