Polcarbonate "burnt knots"
I'm printing with rigid.ink 1.75 mm Polycarbonate using the advice here: basically a 0.93 extrusion multiplier, 0.9 extrusion width, 275 C nozzle, 120 C bed, first layer Z as low as I dare, glue stick applied to the heat bed and a brim on the print. It all works fine except...
...every so often a knot of burned filament develops and deposits itself on the printed surface (see picture attached). The nozzle hits this and, given the hardness of polycarbonate, the print is loosened from the bed and either warps or comes away entirely.
Any suggestions on what I might change to stop this happening? The nozzle has been cleaned with floss and on the outside before starting the print, this knot develops during printing.
[Nozzle is P3D Apollo 0.4 mm (60% conductivity improvement over standard brass).]
RE: Polcarbonate "burnt knots"
Update: one thing I hadn't done was reduce all speeds to 30 mm/s or less and I'm cautiously optimistic that that has fixed the problem; at least, it hasn't happened in the last two prints.
RE: Polcarbonate "burnt knots"
I tried the same Thick layers printing with the same Rigid.Ink PC and could not get reliable results and got burnt bits as you describe.
Ended up printing normal PC settings, but at a temperature of 295C, no cooling fan and slowed right down on the speed.
I'm at work at the moment, but will try and post my settings when I get home later today.
I still can't get reliable bridging though with this PC.
Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet.
RE: Polcarbonate "burnt knots"
Thanks for that, glad to see it's not just me. Slowing down to 30 mm/s may have helped but the problem did still occur and I think what has helped me more in the end is to increase the amount of glue stick applied and to even not clean if from the bed between re-prints. I needed to complete 5 prints of an object about 10 cm high and, with this additional, adhesion the bed attachment was enough for the nozzle to kick the burnt-bit obstructions out of the way.
Since what I'm printing is only a mould for something else I don't care about the brown marks in the print. Would be very interested to see how your settings vary from the standard Prusa PC ones.
RE: Polcarbonate "burnt knots"
Still at work...
I printed onto Black Buildtak which was a big mistake, it practically fuses to the surface and is nigh on impossible to remove.
Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet.
RE: Polcarbonate "burnt knots"
As promised.
Print Settings all as per 0.2mm Quality Mk3 except speed. All set to 20mm/s and non print moves 100mm/s
Filament Settings Nozzle temps 295 all layers and bed temp 110 all layers. Cooling OFF all layers.
No change to Printer setting.
These settings work for me with no enclosure except for bridging which I have not been able to get right yet.
Also just got another Buildtak sheet, but going to try something else first (Blue Masking Tape) since PC adheres too well to Buildtak.
I have been printing certain parts for Skelestruder using 100% infill except the cooling nozzle, and all have come out perfect, but these have been for gearing which I did using 0.25mm nozzle, parts where temperature was an issue before (PETG melted!) and where the extra rigidity of 100% infill I deemed necessary. I printed parts for home, but only without overhangs due to the drooping issues which I have not resolved yet.
Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet.
RE: Polcarbonate "burnt knots"
I had a look at that P3d Apollo nozzle, looks interesting BUT...
There,s always a but. It's aluminium which is why the better thermal conductivity, this particular nozzle also has a coating on it which should make it last 6x as long as brass, but I'm not sure about how well it will hold up to temperatures of 295 Deg C like I have been using.
It is made from a 7075 Aluminium alloy, and here is my first concern. Anyone whom is familiar with metals will tell you all about screwing aluminium into aluminium. It will, but you try getting it apart again after some thermal cycling. Maybe the coating helps, but I still have alarm bells ringing in my head over this.
Then there is softening temperature and this report on the material from NASA
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720022809.pdf
especially this bit
These are the temperatures and times to hold the alloy for it to soften before forming. Now the recommended temperature to remove a nozzle and or tighten it is 280 Deg. From the chart above it looks like it will soften from 218 deg instantly enough to be formable (Extruded / drawn), so I would worry that at 280 or more it may be too soft to maintain dimensional strength. Definite danger of threads picking up whilst trying to screw / unscrew the nozzle in my mind.
Can you post a little detail on your experience with this nozzle so far to maybe put my fears to rest as it looks interesting apart from my worries.
KR
Andy
Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet.
RE: Polcarbonate "burnt knots"
Thanks for the detail: I have a vague idea that I'd tried increasing the nozzle temperature to 295 C before but that the printer (MK3) had failed to reach that temperature when asked. If that works and doesn't leave burned knots then it would be much easier; slow is not a problem for me provided the darned thing doesn't fail.
On the nozzle front, I was recommended it by @bobstro after I needed to change my original Prusa brass nozzle due to a persistent blockage. I've not had any problems that I would blame on it yet, at least not when printing ASA, but I've not attempted to remove it, which might be problematic given what you say above. Maybe @bobstro can comment?
RE: Polcarbonate "burnt knots"
I was kinda hoping that @'ing bobstro would make this forum notify him but I don't think it has and I can't find a "send private message" option either.
RE: Polcarbonate "burnt knots"
Hi Rob. 
click on Bobstro's name, to the left of a post see red oval, above! and you should be offered a PM window!
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Polcarbonate "burnt knots"
Ah, that's how you do it, silly me, message now sent, thanks.
RE: Polcarbonate "burnt knots"
I don't print PC so I can't really offer up any first-hand insights. My hottest prints are with 3DXTech PETG at 270C. I have no issues swapping out the P3-D Apollo nozzles at 285C and have many hours of prints with them. I do use some Loctite silver anti-seize paste on the threads when mounting a new nozzle per a suggestion by Joan. I'd reach out to P3-D via the contact form on their website and ask. They do have some testimonials on their website... somewhere, but I can't find them now.
This may be a good use for E3D's Nozzle-X. It's still hardened steel, but coated and rated for high temps. It won't have the good thermal characteristics though.
RE: Polcarbonate "burnt knots"
Update: I thought this would be a good time to try my new P3-D 0.40mm Apollo nozzle. The sizing seems a bit off on this one. It fits very tightly in my 7mm socket, unlike the previous 0.60 and 0.80mm versions I purchased, and it was uncomfortably tight fitting into the heater block. I'm going to contact P3-D. I've been very happy with my P3-D nozzles to date, but this is a concern. Hopefully it's a one-off problem.