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Help! Prusa i3 Hotend Snagging on Prints

Posted by Thinkyhead 
Help! Prusa i3 Hotend Snagging on Prints
December 28, 2013 11:05AM
I've been having an issue with the tip of the nozzle getting snagged on overhangs when printing some objects, which throws off the y axis. For example, when printing i3 y corners, the through-holes tend to produce overhangs that rise a bit above the current layer, and they almost always snag.

So first, this is a pretty long nozzle with a cylindrical protrusion at its end which is prone to catching. Second, since this is an i3 with vertical rods and the nozzle sits forward of the rods, it forms a lever such that if it gets snagged while the y bed is moving towards the rear the nozzle is drawn downward, exacerbating the friction between the nozzle and the object. And in fact, every time this happens it is when the bed is moving in that direction, towards the rear. The nozzle catches on a hole, is drawn downward into the hole by leverage, and grabs the part till it pops out with a loud thunk, often stopping the y bed for a good 10mm or more.

I tried to address the levering issue in my extruder design by adding a piece to hold the bottom of the nozzle forward at the level of the bottom rod, and it seemed to help, but it didn't eliminate the problem.

So, I'm sure I could fix this completely just by replacing the nozzle with one that has a rounded end instead of one with a nipple, but I'd like to see if there's something I can do otherwise, since this was a pretty pricey nozzle and I don't look forward to eBaying it as a used item. I have some leeway to increase the voltage on the y motor, so I will surely do that much. But what else might I be able to do? Has anyone else had this problem? Any advice would be greatly appreciated!

| Lead Developer of Marlin Firmware
| Help support my work at Patreon and Elsewhere.
Re: Help! Prusa i3 Hotend Snagging on Prints
December 28, 2013 12:07PM
It's not real clear what's happening here, but I'll make a few guesses (in no particular order):

1) You are extruding to much plastic and it's building up on the nozzle. When you turn a corner, the buildup gets left behind. - quick fix calibrate the extrusion rate.
2) You have something loose and your frame is moving. Simple fix - frame stabilizer thread rods.
3) You have a lot of ooze and as you change layers you get a blob. Simple fix - retraction settings.
4) Your bed is loose / binding and it tips. Simple fix - tighten things up / clean / lubricate
5) Your Z axis is slipping. Simple check - run a Z axis cal.
6) Your frame and rods are loose and moving as you print. Simple fix - tighten everything up.
7) You have wet filament. Simple check - look for bubbles and listen for pops as you extrude.
Re: Help! Prusa i3 Hotend Snagging on Prints
December 28, 2013 07:55PM
Thanks for the suggestions! Actually, mechanically everything is tight, the machine has stabilizing fins, and the extrusion and retraction are spot-on, producing smooth vertical surfaces in PLA. Nor is the bed expanding from heat, since I'm printing on cold blue tape. The filament is really dry, too. This is definitely not an issue of blobs, but a side-effect of some kinds of overhangs. Despite the extrusion being perfect, the overhangs on the upper third of these 8mm sideways holes lift up slightly due to the tension of the cooling plastic. Really, this would not be an issue at all if not for this nozzle's design, which is why I'm leaning towards just replacing it. Meanwhile I will try printing these objects with other orientations to put the onus on the X axis instead.

A typical trouble spot...

Edited 1 time(s). Last edit at 12/28/2013 08:22PM by Thinkyhead.
Re: Help! Prusa i3 Hotend Snagging on Prints
December 29, 2013 09:57PM
I have exactly the same construction on one of my hot ends. It does not snag on anything ....
Re: Help! Prusa i3 Hotend Snagging on Prints
December 30, 2013 03:41PM
I have exactly the same construction on one of my hot ends. It does not snag on anything ....

And you are also using a Prusa i3? Here in this linked video you can see the construction of my printer. Perhaps you can comment on any significant differences.

Edited 1 time(s). Last edit at 12/30/2013 03:41PM by Thinkyhead.
Retraction and Lift
December 31, 2013 08:54PM
One thing I haven't used in my slicing settings for a while is Lift on Retraction which raises the Z axis whenever a retraction occurs, or with some slicers, only when crossing perimeters. I recall from way back in the early days when I was first learning to do 3D printing with PLA —oh about 8 months ago— that my prints didn't collide as much with Lift enabled, for obvious reasons. Even when the print was blobby (which my current prints certainly aren't) a lift of 200µm was enough to prevent parts getting knocked around, especially important if you're printing on bare glass. I also notice that this snagging issue occurs less often with layer heights of 200µm or smaller, I presume since any flaws are smaller. I'll bet that if I just enable Lift and re-slice the parts for which I've had problems, this should stop happening. I stopped using Z lift this summer in part because my direct-drive extruder and new (non-leaking!) nozzle eliminated ooze, but also because Lift makes the Z motors work a lot, and I had worried about losing Z steps.

Another thing that can cause collisions is allowing the Z motors to turn off during the print, because they tend to fall into the nearest whole step (or so I've been told). With "Lift on Retraction" enabled, the motors are unlikely to ever sleep, so they can't fall out of step. But I'm pretty sure that even with "Lift on Retraction" disabled, I have the firmware set up not to let the Z motors sleep. In any case, I can check the final heights of some of my prints and see if they are a millimeter or more too short, which would implicate lost Z steps. My sense is that this isn't involved in my particular issue, but it's something the casual reader might want to check if they experience this kind of situation and run across this thread.

Anyway, I'll mess around with Lift and see whether it helps!

Edited 1 time(s). Last edit at 12/31/2013 08:55PM by Thinkyhead.

| Lead Developer of Marlin Firmware
| Help support my work at Patreon and Elsewhere.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 05, 2014 08:18AM
Do keep us posted. This is the only problem I have with my MakerFarm i3. It works great printing along at 70mm/s but every so often I have a print where the Y axis digs into the print. It always happens on a travel move where the carriage is moving toward the back, just as you describe. My prints are generally quite good and everything is tight, it's just that the nozzle sliding over the surface of the print will sometimes catch on a piece of plastic curling up on a thin tower, or get forced into a crevice created by the edge of some strangely-sliced infill. It only happens once in every several hours of printing so it's hard to reproduce it.

I have one of the J-Head IV-B hotends that has the longer tip on it. Apparently at some point they went from a 1.25mm nozzle orifice length to 0.5mm. Looking at the two side by side, the newer smaller nozzle looks like it would be less prone to getting stuck in things due to it having a smoother profile with the rest of the hotend.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 06, 2014 12:44PM
I'm running a MakerFarm i3 with their J-Head part of the time and their Magma the rest of the time.....

The only time I see anythig strange, it's because I'm extruding to much filament. As long as the extrusion is set up right, nothing bumps or bashes.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 06, 2014 01:35PM
My extrusion should be dialed in pretty well. I do have ooze issues for sure though. What are your retraction settings?

I run 1mm @ 13mm/s. If I try to go above 15mm/s, the stepper stalls out (tried swapping drivers, stepper, changing current). I've also tried up to 2mm retraction but couldn't tell much difference other than I started to get restart problems.

It doesn't help that my filament is always somewhat moist from Florida humidity too.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 06, 2014 10:12PM
Depending on the filament vendor and the type of filament I run anywhere from no retraction to a 1 mm. In some cases I do a 1/4 to 1/2 layer raise on the Z. I have never seen an extruder stepper stall out. That's with the standard MakerFarm motors and drivers set to the standard Vref's or to 1/2 the standard Vref's.

If you have wet filament, you need to dry it out .... there's no way to get consistent extrusion with bubbles in the plastic.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 08, 2014 09:20AM
For keeping the filament dry, save those heavy plastic bags and silica dessicant packs, and close them up tightly. You might want to double-bag them in humid florida.

I haven't been printing much this past week, but I should be getting back into it over the next couple of days as I'm helping out with a delta build that will require a few new parts. I've enabled Z lift and updated my saved config files for both the latest Slic3r and the latest Cura. Fortunately I put together an AppleScript droplet to batch-slice STL files using a selected .ini file in Slic3r or Cura, and this has saved me many hours in my tuning process.

I will let you know how it goes, as I try to slice a group of the worst-offending parts.
Re: Help! Prusa i3 Hotend Snagging on Prints
January 08, 2014 06:20PM
Once the filament gets moist, a dry out in a warm oven (or something similar) is about the only way to dry it out. Something in the 5 to 10 hour range at a temperature as high as the filament will tolerate should do the trick.
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