Direct Pinch Print Quality Question

I have been tinkering with different extruder configurations, and decided to try a 1.75mm direct pinch set up with a UBIS hot end. unfortunately my spare UBIS is a 3mm (discovered this after already doing the conversion) so i was forced to use my direct pinch set up with 3mm instead of 1.75mm for now. after E calibration i started printing slowly around 30mm/sec and what i got was ok-ish, accurate, but what i noticed is that the Extruder motor is moving so slowly you can actually see the pulses in the extruded filament on the side walls of prints (see attached photo). (please speak up if i am reading that wrong)



I figured then if i speed things up, perhaps with a little luck E motor would be pulsing faster and thus might not show as much,. nope, its still there, and iv hit a speed wall of ~40mm/sec on the 3mm ceramic UBIS where if i push any faster E starts to skip beats randomly. I an not sure if its from e not being strong enough since I am using one of those 2/3 sized motors, or if the thicker than planed filament is hitting a point where its being pushed faster than its able to melt the thicker filament and at some point thicker filament reaches the nozzle for a second and causes a momentary jam which shows as random voids in the print. now i have upped the temp hopping it would remove the problem, and it does seem to help a little bit, but ultimately that only got me to ~45mm/sec before the voids start again, so I suspect its a little of both, either way for me 3mm direct pinch resulted in little more than a neat photo showing what not to do.

(corrected my fuzzy math) I am pondering ordering a 1.75mm UBIS or j-head mk-5 to try my original intent, but I am wondering, will changing the filament diameter really get rid of this pulsing i see on my side walls now ( I suspect it will ~1/2-1/3 the effect since it will take ~ 3x the number of steps to move the same volume of filament), or is this just a side effect of direct pinch not being geared down enough to smooth out the pulses? (because you can actually feel the pulses on the side walls, there little micro ridges that just futz with accuracy especially in the corners). or if i really want nice clean smooth even traces, should i just head back to a geared extruder set up and give up on a lighter direct pinch?

Or is there some other way i can tune that pulsing out of the equation? (micro stepping is maxed out)

Thanks - Alex
[www.thingiverse.com]

Edited 1 time(s). Last edit at 12/21/2013 11:35PM by AlexCphoto.

I don't know why it'd pulse like that. The only thing that I can think of is perhaps your gearing on your filament drive may be too low.

After a bit of googling i found this:

Quote
Nudel
Have you checked the tension on the extruder idler, and cleaned the teeth of the filament driver (hobbed bolt/gear, depending on your machine)?

If the teeth are partly clogged (due to too high or too low idler tension) it can slip at the same place, producing a pattern that repeats like that.

Try that.

Thanks for trying TTN, but nope, this one had nothing to do with the filament tension.

Apparently its really tricky (for multiple reasons) to try to direct pinch 3mm. One of the reasons (the one i hit) is specific to do with how you need to move 3mm about 1/3 as much as you have to move 1.75mm to get the same volume of plastic out of your nozzle. turns out with 3mm direct pinch on the standard/generic diameter hobbed stepper shaft hub the steps are so slow that you can in fact see the individual pulses of the stepper motor in the extruded filament. its very evident and you can actually feel the pulses ballooning the filament out the side of the print with each pulse if you run your finger over it, its practically like the ribs of a record.. All i replaced was the hot end (switched from a Ubis 3mm to a jHead 1.75mm) and the hot end mount plate, i changed nothing else on my set up, (didn't even adjust E steps on this first print just to see) and the difference is obvious. (Left 1.75mm / Right 3mm)



Now there is a smaller diameter hobbed Stepper HUB that claims ~30% more "strength" that would also increase the number of steps needed by ~30%. I have it here but due to its smaller diameter ill need to change the design of my direct pinch extruder to get things to line up properly (it also has a problem of not having a second set screw, and since its so small the one set screw it does have sticks out hitting the bearing (though i sappose that last one would not be a problem for 3mm)), so likely not something I am going to try any time soon, plus i never really intended to have this bot be 3mm so now that its going in the right direction, and printing much better than i have ever seen it print, I am happy even though if i look extremely close (with a magnifying glass) i can still just barely see the pulses, but there much tighter now and almost invisible to the naked eye.

* I suspect that the larger diameter filament causes a larger pressure wave through the melt zone, as each pulse on a 3mm is ~three times the pressure as a 1.75mm due to the "plunger" surface area being ~3 times as large and each pulse being the same distance regardless of filament diameter. While i will say that my brief encounter with 3mm (went through an full spool) did give me an appreciation for the accuracy curve of 3mm (if 3mm has x diameter variance off its allot better than 1.75mm being x diameter variance off) ultimately i still like 1.75mm better. the only other argument left for 3mm over 1.75mm is humidity absorption, in theory 1.75mm has more surface area and thus can absorb moisture quicker than 3mm,. but i use proper filament bags with de-humidifying so never have an issue with that anyway. YMMV

Happy i tried it though, always neat to see cause and effect first hand =)

Edited 1 time(s). Last edit at 01/04/2014 12:41AM by AlexCphoto.

Interesting read. Didn't cross my mind at all.