Sadly, for critical prints (ones I need only this ONE of) the technique I've been using is sitting in front of the printer watching for the offending pieces and snipping them off while the print head is otherwise occupied. Takes timing, to be sure. And it's boring as all heck. But, it's helped make those one-off prints successful enough to be used in their utilitarian roles.
The types of situations I've dealt with that way include:
- blob on the side nozzle that hangs down and sticks to and existing built-up surface: I use a pair of bent-tip tweezers. I wait patiently with the tips near the nozzle. As the nozzle gets right to the edge of the infill area, I squeeze the tweezers down over the surface of the nozzle, trapping the glob between them. This lifts the head up for a moment, which is why I only do it during infill. The other crazy thing I do to help prevent stuff sticking to the print head is to use a paper towel folded numerous times (so you don't burn your fingers). I put 3-4 drops of TriFlow teflon-infused light oil onto it, and I wipe clean the hot print head. It will "steam" a bit as the lube boils off, but it seems to leave a bit of the PTFE on the head, and stuff doesn't stick as much.
- blob sticking up from decreasing-radius problems. In this case, the extruding happens at a constant rate, and as the circular structure (a small hole, a boss, etc.) gets smaller as the head gets to the center, more and more extra material accumulates. These will **sometimes** solve themselves over time, but the hard lump makes the head lift, and that essentially cause the lump to travel along the wall length. For those, I use a thin pastry spatula heated to just under the plastic melting temp, and use that to gently flatten the hump...while the print head is elsewhere.
- Ooze...hateful ooze. When doing multiple parts in a run, or other places where the head has to make an inbound perimeter crossing, if the (entry) move point is always the same, and there's any ooze during that move, life is going to be a mess. I've seen it where the ooze blob deposits itself onto the last-layer in that area. The machine then draws the perimeter, gaining height and a bump where the oozy move came across that perimeter. It's painful to watch, really. The next layer's transition jump will hit that bump, and sometimes deposit ANOTHER ooze blob, making the problem worse and worse. I've had it knock a print free from the PEI super-grip bed. Needless to say, that killed the entire print the first time. Now I retract better, and keep that hot spatula happy if I see the head working to get over the accumulated bump.
- strings of filament that didn't adhere properly to the previous layer. Those, I use a pair of small semi-flush wire cutters (e.g., Lindstrom model 8156). I cut the threads...but have to be careful and not pull on it, especially when printing the rubbers (Ninjaflex, TPE, etc.). The cutters need to be pretty well-kept, since the extruded thread can fit inside a pretty small nick in the blade and not be cut...which can mean the print head is now heading straight for that point again as it forms a perimeter.
- blobs of material extruded during moves due to slightly higher-than correct residual pressure in the head, and/or plastic that just ozzes (again, like TPE will drain right out of the nozzle if I wait too long).
- The whole "start of print" time. When the head initially tries to home, it's sometimes oozing. I use the tweezers-on-the nozzle method as it heads z-negative toward home, so that as it reaches the bed, there is not really any space for significant ooze. Prior to starting the print, I also extrude 3-5mm onto a piece of paper or something (not on the clean bed is the point), and then retract the amount set in the slicer. This helps with pre-print ozze. And, to make sure the juice is flowing, I add a 1-layer, 3-thread skirt. That way, even if there was ooze that emptied the nozzle a little, that void will be long gone by the time that first layer skirt is done.
- Wipe. In some materials, in some designs, I've found that if I let it wipe the nozzle (on existing infill) as it starts a long rapid move, the amount of ooze occuring during the move will not be enough to futz the print.
I do try to avoid stopping the print for any reason. The only way I've ever gotten anything useful after a failed print is to cleave off the broken layers, and then start an entirely new print at or near that level. I've even switched materials using that process, simply gluing the top and bottom together
.
Regards,
Thom