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Jeffg1

Stubborn BB Core Not coming clean

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I had a BB core that was clogging repeatedly so I took it out of service and just today started taking a deeper look.  I have performed about 5 of the machine guided cleanings using PLA and cleaning filament, which produced gobs of yellow and black bits from both hot pulls and cold pulls.   But in the end, I am now getting "clean" cold pulls however you can see from the images that there is still evidence of some sort of material that is caked on the cylindrical portion of the print head, and some very tiny bits in the cone shaped portion. 

 

My concern is that if I put the nozzle back into service in this condition my problems will continue since we know that perturbations in the flow of a BB core will cause issues with PVA extrusion and subsequent clogging. 

 

Beyond the repeated hot and cold pulls, I have also tried heating the empty nozzle (currently at 300 degrees (previously spent 30 minutes at 275 degrees) in hopes that I could convert these bits to carbon and be able to pull them out.  So far after pushing about 1 meter of PLA and about 1 cleaning filament through repeated hot and cold pulls I have not been able to get any of these bits free, and each cold pull is similar to the shape shown in the attached images. 

 

Any advice would be greatly appreciated!

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Further bake-out for another 1.5 hours at 300c yielded more black carbonized bits, and finally a clean cold pull with the full shape of the nozzle. 

 

Planning to run a test print and see how it goes.  I think for now the moral is that if your cold pull is clean but rough and not the exact shape of the nozzle, you may have some stubborn bits that are stuck to the nozzle wall that need to be burned off.  Probably not a big deal on an AA nozzle but from my experience this has caused failed prints on PVA in a BB nozzle. 

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This is fascinating.  I'm amazed that I still learn basic stuff like this after years of printing.  Great post.

 

You might want to buy some nylon.  I hear it does a better job of cleaning.  I've used nylon for cold pulls occasionally.  It's great stuff.  I do it roughly at 120C (it has a higher softening temp).

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I am following this advise with one exception. I am kinda leery of going to 300°C on the core. But I am letting it cook a bit before I go in with my nylon materials. I can say that letting cook per your above steps has improved the cleaning function.

 

I am not a chemist, so, my original guess would have been that it would have hardened the material  to the metal at those temps. But it does seem to be working. I am also worried about damaging the plastics in the printhead at that temp.

 

I would love to have a materials specialist clarify this for us on what is actually happening and the potential damage to the core going to 300°C.

 

But, the letting it cook at the moment is making a difference.

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Just wanted to follow up and let the OP know that this has been very helpful. Even though the Polybox keeps the filament from crackling and sizzling, it still with carbonize in the nozzle.

 

The letting it sit at high temp for a while has been very, very helpful and wanted to thank you for sharing. My PVA is doing much better than before. :+1:

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I am not sure if the following is good advice for the UM3, so it would be best for an UM-engineer to comment.

 

For cleaning my UM2 nozzles, and for scraping off the ashes, I have rounded and sanded the tip of a long brass M3-threaded rod (=3mm outer diameter). So the tip is now round and smooth, and not so likely to damage the inner core. When inserting it from above into the nozzle, I can very gently (!!!) scrape the side walls of the core and of teflon coupler, to remove loose debris and ashes. I used a brass rod, no steel, because steel is too hard, and more likely to cause damage to the brass nozzle. And I never brutally bang the rod into the nozzle, always scrape very gently.

 

If the residu baked in the nozzle is shiny and hard like glass (like some PET materials do), then this is not going to work. But for rough, rather loose and carbonised material, it works well for me.

 

Also, for cleaning the nozzle opening, I have sanded down a needle to 0.39mm, cut off the sharp tip, and rounded it. So I can insert it from below into the opening in case there would be a partial clog, to gently push it out. However, if done brutally, this could very easily damage the brass nozzle opening (since the needle is steel, even though soft steel). So I normally don't do it, unless I suspect there are some particles partially blocking the nozzle opening. And you definitely have to cut off the sharp tip, and round it, otherwise this is a guarantee on nozzle damage.

 

Loose debris can then be removed with compressed air first, and atomic pulls.

 

But as said, this is not official advice, I don't know if it is applicable to UM3-printers too; and it might be a good idea for an UM-engineer to comment. It requires common sense and being very gentle. So, for example if I had to run school printers, I would not let every pupil do it...

 

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Even with Brass, I would be leery of putting it into the nozzle to try to break away debris.  From a clean cold pull you can see from the shape of the plastic coming out that the inside surface of the hotend and nozzle is very smooth and does not have much in terms of defects or machining artifacts. 

 

When you put metal in there, even brass, I would be concerned that over time you will start adding scratches and defects, which will accelerate the wear out of the hotend and the defects will create pockets for molten plastic to adhere and remain, where it will break down over time.  I also think the defects will create additional friction, which will make future cold pulls more difficult.  I think the safest bet is to only put plastic into the hotend. 

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