Start/Stop extrusion delays - advice?

I built an SAE linear bearing Prusa with Wade's geared extruder for 3mm filament and Mk IV .35mm hot end, driven by a RAMPS board.

Surprisingly, I didn't really have any hardware issues, it's all come down to getting things dialed in to make a good print. I'm using Sprinter firmware, Pronterface to feed G-Code to the printer and manually control it, and slic3r to generate G-code.

The issue I'm running into is getting my extrusion to start and stop cleanly. I've found that a temp setting of 210-220 is giving me the most consistent extrusion, but there is still a lag. If I manually extrude a few mm, the extruder drives, and a fraction of a second later, a string of plastic feeds out of the nozzle, and keeps flowing out until a half second or more after the extruder has stopped driving.



Here's a hollow pyramid I printed at 220C - judging by the top, that temperature was too high. As you can see there are gaps in the edges of the puramid, where the flow through the nozzle started too late (it's a bit more apparent on the back legs) and there's still plastic flowing as the head moves over to the next leg, getting stretched out into strings between the points. The base, looks good in comparison.



Here's a hollow pyramid I ran at 210C. It had the same stringing problems and gaps in the edges, but I cut them away to get a view of the bottom - to see that the infill looked good - which it did because the extruder was continuously running, rather than starting and stopping which is where the lag causes problems.



Also, the temp seems a bit high, for the PLA's specs but looking at the hot end with a thermal camera, I can see the nozzle face looks a bit cooler than the header block area, despite the fact that they're milled out of a single piece of brass. That makes me wonder if I've got more gooey plastic in the middle of the nozzle/head and it may be a bit cooler at the tip of the nozzle, bottlenecking things.

It seems like the near-liquid plastic is being springy enough that it's building pressure at the start delay, then releasing it at the end.

I've considered the .35mm nozzle diameter, rather than .5mm could contribute here - the PLA needing to be warmer/more flexible to squeeze down through it from 3mm, contributing to needing a higher temp, contributing to the springiness problem. I chose .35mm hoping to work more detail, after having read of folks using that size successfully, and most of the people recommending against it sounding more along the lines of "I did .5mm and it worked great" than "I did .35mm and it didn't work well, so I switched to .5mm and it's great." I also figured it would be easy enough to drill the nozzle out to .5mm, as opposed to buying another nozzle to try going down to .35mm later.

Any other advice on things to tweak to improve print quality based on these images would be greatly appreciated as well.

Thanks!

Increasing travel speed might help.
What about retraction ?

You don't say what speed you are printing

It's quite hard for Slic3r to do that pyramid as it will be too fast at the top.

But to help the stringing issue-

For a print speed of about 50mm/sec
Have reverse speed set to 30mm/sec and 1.2mm filament reverse
Travel speed of 150+

Let us know how that prints

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[richrap.blogspot.com]

220º is pretty hot for PLA. Unless you have a fan pointed at the print, the layers aren't going to cool properly. It also looks like your retraction settings aren't aggressive enough, which is contributing to the stringing. RichRap's advice is good. I'd start again with a slow speed (30 mm/sec), a normal temperature (185º), and RichRap's retraction settings and try again. Then increase the speed slowly until you've got it going at a good clip.

I also have 0.35mm nozzle on my extruder (MakerGear hotend). I have similar issue but not exactly the same. If there is pause in printing it starts to extrude plastic with delay. I solved that issue on beginning on print enabling skirt, but id does not solve delay in print. For example I tried to print 2 pulleys for reprap prusa metric and almost fully miss a tooth at the place where extruder starts after it is moving between parts.
So I ordered 0.5mm nozzle already (even 0.75 to try) and I hope that will solve the problem.

PS Actually I would take it back. After enabling extruder retraction and tuning it I got very good results.

Edited 1 time(s). Last edit at 02/12/2012 08:15PM by rGlory.

Wow, thanks for the excellent replies.

All my attention got whisked away from printing to help launch a new company. I'm trying to get some time back on to tweaking on the printing.

I increased retraction, with no great improvement, but will play some more with that.

I figured with .35mm if I end up not being happy, It'll be easy to dill it out to .5mm, or pick up another J-Head.

Thanks again!

Following up from the "you're printing too hot" comments. I had a similar issue; I had turned the temperature right up because otherwise I didn't get consistent flow.

I found that the better way of solving this was to get the bed *really* level and to get it to just the right height where the plastic touches the bed as soon as it comes out but doesn't get smudged. In this way, the adhesion of the plastic to the bed means that the plastic is getting pulled out, and you get a more consistent line at lower temperatures.