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Kathryn

Possible to check for clogs before print job?

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I have an Ultimaker 3 printer that for the most part performs extremely well with little need for hovering/constant supervision. However, there are times where a print will be started, the prime blob and first layer will print successfully, and then I'll come back 20 minutes later to find that the filament has stopped extruding. This most commonly happens with PVA, and from what I can tell from my subsequent hot/cold pulls it is because the nozzle did have a blockage in it, but for some reason the print initially started just fine.

 

That said, is there any way to see if a nozzle is clear or blocked before starting a print job? Or are there any good tips on avoiding blockages entirely?

 

One strategy I've been using recently is backing the filament out of the nozzle when it is not in use, which so far has helped somewhat. That said I also run longer print jobs with PVA support where the nozzle may be sitting idle for a period of time once the primary material no longer needs support to finish the print, which means there's a higher risk of blockages for the next print job.

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The only way to do that, as far as I know, is to perform hot and cold pulls on a regular basis to remove all the gunk and burnt particles inside the printcore that could be responsible for clogs and such. And, of course, when not printing, do not leave the filament inside.

 

I believe that the printer does some retraction when a nozzle is not used for printing, like, when nozzle 1 is printing and nozzle 2 sits idle, but I may be wrong.

 

Basically, regular maintenance, check-up and cleaning of the printer.

 

Also, proper storage of your PVA to avoid it going bad. This thing is a absorb ambient humidity like a sponge, and can turn bad very quickly. So, it must be kept in a closed box with some sort of dehumidifier that keeps humidity level below 55%. Else the filament will absorb ambient moisture and that can lead to bad prints and clogging, as it becomes mushy then all the humidity it absorbed is release when it is heated up for print, creating bubble and such. If you hear some sort of sizzling and crackling sound when PVA is extruded, it means it has gotten too much humidity in it.

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Yeah that does seem to be the most effective solution- I was hoping to avoid having to do a hot/cold pull every time I start using the PVA again, but it's such a finicky material that may just be what it takes. 

 

Good news is we do already have a small dry box that the filament sits in during printing, and it is true that that has helped substantially with avoiding clogs in the middle of a print.

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You shouldn't have to do a hot/cold pull every time you print with PVA. I print with PVA quite often, but sometimes I won't use it for a week or two and replace the BB core with an AA for whatever reason, and when I put the BB core back in, it usually prints PVA without troubles. I don't think I've done a hot/cold pull on the BB core for a month or two now, and the last print I did in PVA this week came out perfectly, and I hadn't used PVA or the BB core for a couple weeks now.

 

When I put it in and put the PVA, I let the printer do the purge when the PVA is loaded, the first few centimeters are leftovers from before that come out all bad and cracked and sizzling and slightly burnt, then the 'new' PVA from the spool comes out, all clear and nice, and that's it.

 

Do you have a drybox for storing the PVA when you're not printing? It's very important that PVA is stored in a low-humidity environment when not printing, because that's when it can absorb a lot of moisture. And the tiny packet of dessicant isn't enough once the PVA is out of the sealed plastic packaging.

 

Also, I'd try and check the BB core as well, if you have clogs that often. Maybe try with another BB core if you have one handy, just to see if you have the same problems. It's possible the Bb core is faulty or damaged.

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Yes, the filament is always sitting in a drybox even when it is feeding into the printer.

 

I've also observed that crackling of bad filament during the initial purge, and it is true that that usually clears most of the crud out of the way before the print starts. It's interesting to hear that you haven't had to do a hot/cold pull in a month or two- it may be that in that case scheduled maintenance of the print cores may take care of the problem effectively and that the earlier problems we experienced were entirely due to saturated filament. 

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That's very possible. I had many problems with PVA before finding out how quickly it would absorb moisture. Putting my spools of PVA in dryboxes for storage and printing solved them.

 

If I'm not mistaken: PVA, PLA and Nylon are the filaments most sensitive to humidity that should be kept in dryboxes all the time. The rest should be ok, though, personally, put everything in drybox just to be sure. I lost enough work hours before.

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One thing I will do once in a while is to do a 'move' on the filament so that I can see how well and thick the material is, see if there is any caramel colouring (Burnt PVA) or major curls. I also listen to how the material will print and look at the previous print to see if the material is not printing well on hygroscpoic moateroals. PVA specifically should print fairly clean and not with wispy, lace patterns in the walls. Nylons should not be cloudy or popping sounds.....etc. Materials that are starting to clog will start to not give a clean noodle of filament, maybe thinner, curling a bit etc. Bits stuck near the nozzle exit will cause the material to actually curl back up onto the nozzle. This can be cleaned many times with something to pull it off the tip. But if left there, it will create a point that material can grab and stick and cook.

 

But on PVA, it is a good idea to do at least a few hot pulls every now and then just to be sure.

 

It can also become a major issue if you leave the PVA in the nozzle when doing prints that do not require the PVA. The heat exchange between the nozzles can heat up the core just enough to slowly bake it real solid and that is a pain to clean. If I am going to be doing a print with no supports, I will back the material out the nozzle a bit to try and minimize that.

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That sound like good advice as well. I have noticed that it's fairly common for the filament to start curling up when it first comes out of the nozzle so I've been doing my best to keep the nozzle itself clean using tweezers, which tends to help a bit. 

 

I'll look into doing more hot pulls though- those are typically pretty fast so it'd be less of an undertaking to have to do that before every print.

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