Luzbot 6 Printing Error - Noobie Help Please!

Hello-

I’m using a Lulzbot6 that our wonderful library makes available, but provides no support. The software it uses is Cura for PC.

I’ve made 4 prints so far, only 2 of which worked out. The other two prints encountered the following problem. If you can offer advice, I will really appreciate it and I will share it with the library and other users here.

Problem: All is going fine, and then (sometimes after more than an hour of all goodness), I’ll notice that the print head is just hovering above the printed object, but not extruding filament. In one such case, I caught it within a 30 seconds of the error and hit pause, but I have been unable to figure out how to back up the print head to the last correct print spot and then resume printing from that point.

So, the whole thing is a fail at that point, despite all being fine until the error.

I have opened and looked at the console debug and read the docs, but I can’t find any info on this (which would seem like a relatively common issue).

Questions:

  1. What’s the most common reason for the filament to suddenly stop extruding mid-print?
    more importantly,
  2. When I notice that something went wrong like above, what is the correct process for basically ‘resuming’ printing from the last good spot?

THANK YOU for anyone who can help! I have several things that will take 5+ hours to print (all the while sitting there in the library as required) and, I’m very afraid that I could have this error in hour 4 and no way to pickup from the last good print spot!

~ Your Friend Scantron

Probably the main cause is retractions where the hobbed bolt in the extruder chews enough of the filament that it can no longer grip it to force it into the extruder.
You can check that by losening the filament clasps (while the toolhead is hot) and removing the filament and inspecting it. Is there a big “bite” out of it? If so, clip off the end of the filament above where there are any chew marks and reinsert.

If that’s what happened, there’s several things that can cause it, including too many retractions in a short period of time or having the tension too tight (the 2 spring loaded screws pressing the filament into the hobbed bolt in the extruder). Another thing to check is the actual hobbed bolt. If it was chewing plastic, it’s probably full of it and needs to be cleaned or it won’t be able to grip anything very well. There is a little wire brush tool that came with the Taz 6 to do that. Be sure to advance the extruder so the bolt rotates a bit so you get all 360 degrees of it clean.

As for pausing, I generally avoid it and use the “filament change” feature from the main menu on the Taz LCD. That will pause it, raise the extruder up and back a fair bit, reverse the filament out a bit and prompt you to change it. That’s a good time to inspect things. After you put them back together, hit the knob on taz to resume printing (or whatever the prompt on the screen says). It will purge a bunch of filament in place and then it will move back and resume printing from where it left off.

What type of filament?

Thank you! So, the machine will know how far to go back to restart?

If I notice that the print head is hovering above the piece and not extruding, it’s usually been doing so for at least a few minutes (before I notice). So, if I hit “filament change” at that point and the resume, won’t the machine just return to the point where I hit “filament change,” rather than go back to the previous place where it actually stopped extruding?

polylite PLA 2.85mm

Thanks for any help you can offer. (See my response above also, re: returning to actual spot of error).

Thanks!

Alas, there is no practical way to “back up” the printer to recover from the scenario you describe. It’s pretty near impossible to accurately determine which layer went wrong, even with the use of a micrometer, or counting layer lines in the print – if you’re off by even one you’ll have a damaged part. And even if you got the correct layer, determining where in that layer things went bad is likewise impractical to impossible – and then, you’d have to find that spot in the layer in the gcode!

By the time you accurately determined the above information, you might as well have spent the time to figure out why the printer does this, and prevent the problem in the first place (and that’s what the previous posters are, IMO, trying to do – prevent the re-occurrence since for all practical purposes you cannot recover from this). Sorry…

Thanks! That really helps as I was indeed, overly focused on recovery rather than preventing the error.

Yes, unfortunately as mwester said, there’s no easy way to go back to where the printer stopped extruding.

Without upgrades/modifications to the heatsink fan, PLA will always be affected by heat creep with these printers. Try lowering the extrusion temp to reduce the affect of heat creep… the recommended print range should be on the filament spool or the manufacturers website.

Try a different type of filament, PETG is probably the next easiest filament type to print. ABS can be finicky about the initial nozzle height, but once its dialed in there are few issues.

Thank you! [added characters…]

Your filament jammed. There are several reasons why this might happen but it’s hard to know the actual cause without a lot more information.

Do you know what sort of material was being used to print?

Cura LulzBot Edition has a set of recommended profile settings for materials that they have profiled on their printers. This is usually a good “starting point” … but I usually find even these benefit with a bit of tweaking.

There is a “hobb” gear on the print-head … this is the thing that feeds the filament into the extruder. There is an adjustable (sprint-tensioned) “idler arm” that applies pressure on the filament to keep it snug against that hobb gear to help it feed.

If that is too tight… it can chew up filament. If it’s too loose… the filament will slip. I typically set mine to roughly 1/3rd of the tension possible (anything close to around 1/2 tension is probably ok).

Next up is temperature. When you use the Cura software it needs to know what type of filament is being used. This matches the filament to a printing profile meant for that filament type. It is how it knows what temperatures to use for the print nozzle (as well as several other things). If the filament wasn’t warm enough, it wont want to extrude easily and can jam.

Next up are “retractions” and this one is often to blame. I’ll try to explain what this means…

The filament feeding into the print-head is cold and solid. It is pushed into the “hot end” where it heats to printing temperate and takes on the gooey liquid state. As the machine is printing (laying down filament) is continues to push filament into the hot end and extrudes it. But what happens when it gets to the end of that printed segment and needs to move to some other area of the part to continue printing? If it were to just “move” without doing anything… that soft gooey-liquid filament would continue to ooze out of the nozzle during the non-printing move. This results in “stringing” (very fine filaments – almost like spider-web – are left behind). To reduce this stringing & oozing, the printer “retracts” by running the hobb gear backward a short distance. The purpose of this is to relieve pressure so that the filament wont continue to ooze.

When I print PLA (probably the easiest type of material to use), I find the retractions should be just 1mm … or maybe 1.5mm.

If you set retraction values in Cura to be too high, two things happen…

  1. You can end up pulling air into the extruder (which wont damage anything) but that air heats up. As it heats up it expands. So when you start to print again, it can create a bubble which “pops” and it ends up spitting out a bit of filament. Again… no harm is done to the printer, but it doesn’t leave a very nice finish on your part.
  2. If your part has lots of short segments that results in frequent retractions, the hobb gear ends up moving back and forth over the same short bit of cold filament so many times that the gear starts tearing up the filament. It gouges a wear-mark in the side of the filament and gets stuck… and now you have a filament jam.

Check the teeth in the hobb gear and make sure they are clean. The cold filament wont stick to the teeth… so a few puffs from a can of compressed air will easily clean them. But if those teeth are jammed with filament then the hobb gear wont grip the filament and it will start slipping.

How to check Retraction settings:

When you are on the “Prepare” panel of Cura LulzBot Edition (along the top you’ll see a the words “Prepare” and “Monitor”) the right side of the prepare window has the filament and print settings. Make sure “Print Setup” is using “Custom” (not “Recommended”) because if you use Recommended then all the settings are hidden from view.

In the “Custom” view, scroll down to the section that says “Material” and this is where you can see the retraction setting. The setting you care about is “Retraction Distance” (for PLA that should be less than 2mm … and I usually find that 1mm works best on my printers).

The value below that is “Maximum Retraction Count”. For some strange reason most of my profiles have this default to “99” (which is a terrible idea). I set it to 10 (and I reduce it even more if I have problems). There is one more value called the “Minimum Extrusion Distance Window” and for me that default to 2mm.

What it means:

If my “minimum extrusion distance window” is set to 2 and my “maximum retraction count” is set to 10, then it means the printer is counting how many times the print job orders a retraction over any given “2mm” long length of cold filament (this is before it goes into the hot-end). If the hobb gear has to run over that same bit of filament MORE than 10 times… then any additional retractions will simply be ignored. (Because excessive retractions will chew up the side of the filament and result in a jam). Setting this value to the default of 99 is … lunacy (if your hobb gear grinds over the same short length of filament 99 times… you will surely have a jam). Just occasionally I’ll print a problematic material that is particular soft (PVA is like this) and even 10 is too much.

Thanks for the fantastic help! Really appreciate you and all those who took time to help me out. Humans are cool!