What causes these artifacts?

jens55

I am printing little parts bins. I am printing two parts bin in one go. There is an outside wall and an inside wall (two total) and I am getting strange artifacts mostly on the inside wall.I have tried increasing pressure advance up to 0.1 and down to 0.04 (direct extruder) but there is no obvious change.
I am printing at around 65 to 70 mm/sec with PETG at 240 degrees.
The printer does the inside wall first followed by the outside. There are a few artifacts on the outside but I suspect that is the inside wall artifacts showing through. The vast majority of problems are on the inside wall. I suspect the problem is due to the printer switching from parts bin one to parts bin two.
When I print a single bin (instead of two), everything gets slowed down to make layer time long enough. Speed is basically halved. There are no artifacts - perfect print.
When printing two bins, there is basically no stringing between the two bins so the filament is not bleeding out of the nozzle during the switchover (if you look very close you can see just a whisp of material between the two parts but there is no stringing).
Looking at the picture, you can see that most of the time the print is done properly but in about 5 or 10% of the layers, when the filament is supposed to flow it doesn't and there is always a tiny burnt blob at the end of the missing section of the print layer.
I have no idea what I should try next to eliminate the problem. Maybe somebody here has seen this kind of a thing?
weird artifacts.jpg

o_lampe

@jens55 These are blobs that are collected outside the nozzle and overheat after a while. From time to time they find their way into the print.
Possible causes:
The nozzle runs through strings from earlier layers
The hotend leaks

You could find and eliminate the cause or wipe the nozzle every n-th layer. Silicone brushes are good wipers.

jens55

@o_lampe, while I agree that the blobs are accumulated filament on the outside (it is PETG after all), I question three things:

why is the happening at the same spot. I get these accumulations regularly but they happen at random locations and the blobs are MUCH bigger. They are big enough to fall off from gravity and rarely get incorporated into the print. The blobs here are tiny, always the same size and they are always incorporated in the print.
The line of missing filament layer always starts at the corner. Regular PETG blobs occur at random and are generally not preceded by a missing layer of filament. Normal blobs just happen, they might stick loosely to the layer that is just printing but generally they come off easy and are not incorporated into the print like they are here .
In this case the blobs only happen if I print multiple objects. If I print a single bin there are no such artifacts.
I am trying to understand what is happening here because it definitively isn't a normal PETG bugger.
If I had to take a wild guess I would think that the partial layer that is missing has accumulated at the tip and has burned and is then incorporated into the layer. Normal PETG blobs tend to accumulate on the outside of the tip but these things here look like they are accumulating at the very point where the filament exits the nozzle.
I will do another print at a lower temperature and see if that affects anything.

Thanks for your input!

Edit: In this case there is no leak at the hot end but if there was a leak, it would not pick the same area in the print to show up and would also show for single object prints.

oliof

could it be the blobs are at an unretract or end-of-wipe error, i.e. at restart of extrusion? if yes, you may be pyrolyzing the filament that's not extruded but lingering in the hot zone, and it gets pushed out when extrusion starts again...

jens55

@oliof, that seems quite possible and would likely result in exactly what I am seeing. The burnt filament could be sort of blocking the nozzle and preventing extrusion which gives the short section of missing filament. Eventually the pressure in the melt chamber gets high enough to push out the burnt plug and the filament starts to get laid down again after the plug comes out.
As mentioned earlier, I will try the print at a lower temperature and see if that helps. I can also try to increase retraction. I will report back .....
Thanks!

jens55

A follow-up:
It would appear that bringing printing temperature down from 240 degrees to 230 degrees solved the issue.
While I can not say with 100% certainty that this was my problem, the artifacts have disappeared for the time being.

dc42

@jens55 I'm glad you found a solution. 240C does sound rather high for PETG. I use 225C.