Ikosaederstern d199|18
model can be downloaded:
Love it !
Kind of like a raspberry pi shadow logo .
That’s a lot of stringing. What’s the material?
PET-G … and i am using a 0.6 nozzle which, as i suspect, causes more stringing.
@Ulrich_Baer yeah but not THAT much stringing… Looks like you need to increase your retraction distance and the amount of material to re-prime after a retraction. However, this looks pretty small and could just be TOO small for a .6mm nozzel. If you REALLY wanted it to look nice, a .2mm nozzel and .1mm layer height with the right retraction settings would look nice.
@Kevin_Danger_Powers you can belive me that i already have testet all possible settings and you can not get much less. A smaller nozzle (and a new PTFE inliner to make the resorvoir smaller) would be a solution. Retraction is just for reducing the pressure but strings are pulled out of the molten. Even if you would be able to suck in it wouldn’t help. I am not sure if a needle valve (within the nozzle) could work, probably not. You can do something with the temprature and how fast the travel move is so the string rips but the gaps are around 8mm. But if you managed to print with PET-G without strings i am interessted in your setup.
I’m pretty sure retraction DOES suck in… hard to prove conclusively, but if you let the hot end cool fast after retraction and then cut it in half with a mill (which I have done), there’s a big air pocket inside the nozzle. Molten polymers are viscoelastic and there’s every reason to think the melt pool has some tensile strength at high speeds. (Would be a similar effect as die swell.) The retraction air bubble doesn’t clean the nozzle walls though, so there’s still some residual molten material to ooze down under gravity.
Another factor I’ve found to majorly impact stringing is airflow pointed at the nozzle. You can draw out a string that cools and pulls plastic out of the nozzle tip. Same retraction settings can work perfectly with no airflow but string with high airflow.
@Ryan_Carlyle https://www.matterhackers.com/articles/retraction-just-say-no-to-oozing - alslong there is no gasket between filament and Hotend there will be no negative pressure.
@Ryan_Carlyle regarding the Airflow - you can have stringing due to Airflow - you also can have less by cooling the string fast enough so it rips. Afaik strings happen if plastic is hotter - but you are right that if i direct heat onto that the string it will also contract. But this cause the whole object to soften (thin structures can only be printed in PET-G with lot of cooling)
@Ulrich_Baer Matterhackers is wrong. Polymers and wax are fundamentally different materials with different liquid phase behavior. By the “no gasket” logic, there’s no way to generate positive pressure either. Then how does extrusion work? If you do the math on how hard extruders push at stall, there’s thousands of psi / hundreds of bar of pressure in the hot end. How does that seal in and not just blow liquid filament out the top of the heat break?
The answer is that the solid incoming filament shears high-viscosity semi-molten plastic against the heat break walls in the transition zone, and that shear zone creates a pressure seal. Same thing for screw extruders. Shearing high viscosity fluid generates pressure.
@Ryan_Carlyle if you compress a liquid (molten plastic) it will seal itself to the filament (die swell as you said and as it is pressed upwards into cooler area so it acts as a seal for molten plastic) - But not for Air - and there are not hundred of bars pressure, maybe 3 or 5. The Nema17 has 0.025Nm detent torque onto a 1.75mm filament - lets say 1N on 0.5cm radius onto an Area of 2.4mm²~ 0.4N/mm² = 4bar.
But you are right about the seal but when retracting this seal breaks as now not hot plastic try to get out but air is trying to get in. But you could just add a gasket to prevent this. However you already said (just as i) that this wouldn’t help reducing stringing.
@Ulrich_Baer your torque and force math are way off. And die swell has nothing to do with the sealing mechanism.
A good extruder like a bondtech can push 100N. So, 400 bar. Normal printing forces are lower but it’s still a LOT of pressure trying to blow back up the heat break. Most hot end function explanations don’t have a good way to contain this much pressure without backflow. If you push filament in and then pull it right back out, you can see the semi-molten filament DOES try to flow back out, but the incoming solid filament viscous-shears it back down at steady state. Similar mechanism to how screw extruders generate pressure, but linear shear and small clearances instead of helical shear over a long screw.
@Ryan_Carlyle right i used detent torque and got the translation wrong… my bad! However every capillar has thousand bar inside - as this is needed if diameter gets small. But if you are right you could put the nozzle into molten plastic - retract and end up with plastic sucked in - i tested this it is not working on my extruder. Or just extrude 2mm in mid air and retract or pull the filament - nothing went back - please try this yourself. Maybe you get 0.1mm back if you are lucky. Maybe your filament is sealed better if the diameter of your PTFE inliner is tight. but that would cause jamming as filament diameter changes about 50µm .
@Ryan_Carlyle just got another point … while you can extrude with a lot of pressure (and this is needed). Sucktion will stop at 1bar so maybe this is just not enough negative pressure to get plastic back in as you will only create a vacuum inside the heated chamber.
@shauki you are welcome, I sent a lowpoly version to your mail (the org has over 50mb and there is no use for it in a small print)
@Ulrich_Baer you ever play with silly putty? And pull hard and fast? It doesn’t flow at high speeds, it holds tension until it reaches brittle failure. Viscoelastic polymers have significant tensile strength at high speeds. You ever pull out a long section of PLA when unloading the hot end? You’re drawing out long polymer molecules in tension. So you’re not necessarily limited to 1 bar of suction when you retract fast. How much suction do you really get…? No idea, and probably no way to measure it. Might be minimal or zero. I just know there’s an air pocket in the nozzle after retracting and that’s physical evidence of something suction-like or tensile happening.
If the nozzle tip does see suction, air will channel around extruded plastic to get into the hole. There’s like a factor of a billion viscosity difference between air and molten plastic, so air is definitely going to want to flow up the nozzle more than extruded plastic does.
Honestly not sure how you could prove or disprove any of this without a see-through nozzle and high-speed video.
@Ryan_Carlyle the “air” pocket was connected to the nozzle opening or? Maybe thermal shrinkage played a role too. From my experiments i can only say that retraction take the prestress (the polymer acts like a spring) and not suck something in - the retraction also happens when the noozle is on the surface (closed) . Also you are refering to Non-Newton fluids but polymere are the oposit they chemical binding loosens when sheared https://en.wikipedia.org/wiki/Shear_thinning in PET you have this (when cold) as BOPET (Mylar) it gets softer if bend (irreversible).
Here’s an air pocket shot. You can see the air channels up the center of the melt pool and a lot of plastic sticks to the sides (where it continues to gravity-ooze)
I did this a long time ago, but I’m pretty sure for this test I just pushed filament in by hand and yanked it out at the same time I killed the heat, so it’s not a super representative air bubble. Real retraction of 2mm or whatever would not be so much air.
missing/deleted image from Google+
Actually went back and checked some notes, and that hot end was from a mid-print heat creep jam after a retraction. I killed the print as soon as the extruder started clicking.