Who here thinks that there should be a scorch guard feature with a variable retract setting in the firmware? It would make more hotend designs possible and would probably help with idle nozzle ooze in multi-hotend 3d printers.
What exactly is “scorch guard”?
It would back up the filament if the nozzle is inactive for a certain timeout. It may determine the timeout based on filament specs or it might just say a specific amount of time.
I made up the term.
So you want to pull the filament out when not in use to prevent scorching?
I think just cooling the hotend whenever it isn’t in use would probably work out better. It would be a real trick to pull all the plastic out, and if you failed the little bits that were left would be alone with all the heat capacity of your hot end, and quickly char worse than they otherwise would have.
I do like the idea of keeping a hotend hotter than the temperature at which the plastic burns though. As long as you keep plastic flowing fast enough that it doesn’t get to that temp itself there’s no harm to it.
Perhaps we’ll see that a ways down the line if we ever get very low heat capacity, rapidly heat/coolable hotends.
Why not use a prime pillar or dump bucket as it pushes any over cooked filament out onto a sacrificial part or waste hopper. Pulling the filament back would mean you have to re-prime before using that nozzle again anyway. Most slicers have a standby temp for inactive extruders to reduce the ‘scortching’ issue.
This is an area of hot ends that I really hope people pursue soon. The ability to retract cleanly (as close to the hot area as possible) so that oozing and scorching doesn’t happen.
So, how do you do that? I don’t see how you can mediate a clean retract if the plastic is molten so you’d have to be able to quickly cool past glass transition and then reheat just as fast. Perhaps the polished interior of the Prusa nozzle would improve the likelyhood of a clean retract.
Related question - what problems do people have with scortching? I find an occasional purge with nylon clears any in-nozzle build up quite nicely.
@Tim_Rastall I suspect a much higher temp , yet more restricted temperature zone closest to the tip of the nozzle would do it. I just don’t know the best way of achieving that.
I expect that with a super-shot melt zone AND short thermal transition you could get close to the behaviour that @ThantiK is proposing.
This may well be useful in multi-nozzle printing, which interests me.
The issue as I see it though is that if we start at a state where the hot side is very hot, well above melting temp of the filament. Then plunge filament in quickly, you are going to get a high amount of initial “scorch” before your hotend reaches steady-state extrusion, where the extrudate is exiting the nozzle at the desired temperature.
Another thing to bear in mind is that you can’t mess with (shorten) the length of the final orifice (0.4mm diameter, 1mm length in the default E3Dv5) without impinging on laminar flow characteristics, so you are always going to have a certain amount of molten plastic scorching inside the small orifice hole.
@Sanjay_Mortimer That’s why I mentioned low heat capacity. If we had some nutty hotend in the future that could ramp temps up and down by 10-15 degrees during extrusion, we could ramp down every time we had to stop extruding and back up every time there was a reasonably long extrusion.
Now that I think about it we could probably do something similar with today’s hotends and gcode that intentionally maximizes continuous extrusions.
I’ve actually considered making the thermal break on Kraken (water cooled multi-nozzle hotend in development) have a higher thermal conductivity so that nozzles can drop in temperature more rapidly when not in use to stop oozing.
The water cooling system will have a huge excess of cooling capacity, so the extra heat transfer won’t be an issue, and the heater cartridges have plenty of oomph.
I’d be very interested in seeing that.
Also, I don’t have time before my trip, but when I get back Wednesday I think I’ll try printing a single layer across my whole bed and ramping speed/temp up. My current extruder is pretty crap speed wise, so it’ll be interesting to see if it can actually make a difference.
Temp makes a massive difference to top-speed.
I’ve printed PLA at 280C - through a large 0.6mm nozzle printing a big object. It was the only way to get the melt-rate up high enough to pump out enough volume for the 0.5mm high layers, with 1mm extrusion width at a high feedrate (100mm/s infill)
Then I’ll definitely try it. Particularly with my new .6mm* E3d nozzle coming in.
(It’s a 0.6mm nozzle, not 0.7mm coming with your E3D order)
Do give it a go, you’ll be surprised how much heat it takes to get enough melt rate to feed the 0.6mm nozzle.
I’ve been working on designs for a low thermal mass concept for a hot end to allow rapid heating/cooling for a while, but what I have in mind requires manufacturing techniques that have not been used for 3d printers before, and I’ve been having trouble finding a shop to do it.
Float the idea by the guys at http://thenewfleshworkshop.com They fabricate all kinds of crazy things. They tend to be problem-solvers and they work with many different materials.
Aside from the heat capacity and conductivity of your hot end, you need to make sure the pid in your firmware and your heater are up to the task as well. Your asking for short response time, no overshoot, and no steady state error. Tall order on a control system, bit if it’s tuned correctly, and the hardware’s up to it, this would be an awesome development