Thursday, November 05, 2009


An end to ooze II

Ages ago, I put code to reverse the extruder at the end of plotting a sequence of line segments into the Java host software, thinking, "That might be useful someday." I set the default so that it reversed the motor for 0ms to turn the feature off, and forgot about it. Seeing Erik and Nop's post below, I thought I'd better finally try it out...

Above is the result - perfect! (Ignore the dark splodge in the middle of the square; that was crud on the extruder falling into the build; I should have cleaned it first.) Normally there'd be loads of string between the L-shaped nozzle-wipe bit and the block being made. Now there's none.

I'm running at an XY feedrate of 3000 mm/minute. At that rate setting ExtruderN_Reverse(ms) to 200 and ExtruderN_ExtrusionDelayForPolygon(ms) to 400 seems to work nicely.

The delay figure is how long the extruder runs before starting an XY move; this allows the polymer to move back down the barrel to take up the slack.

Lovely! :-D
It looks good but why are the two delays not equal to put the filament back where it was?
The start-up delay deposits a small splodge to stick things down and make sure that everything is running right before moving in XY. The code always places this in the middle of a solid region on a part of the hatch pattern, even when it's doing outlines (in that case it then plots a bit of the hatch up to the outline then plots that). That way it gets a good start, the outline is clean, and the splodges are buried in the finished solid out of the way.
What are the specs on your stepper, and the diameter on your pinch wheel? My first attempt at a stepper driven pinch wheel won't go much above 1000 mm/min, but I don't think my stepper was up to snuff.
Good code design then ;-) That's a fairly huge free-fix!

If ooze is totally eliminated do you think the wipe fence would be needed any more?
Wade - It's a NEMA 17 with a 5mm knurled shaft. The motor is this one:

which has a holding torque of just over 0.4 Nm. That seems to do 3,000 mm/min quite comfortably.

Ed - Great that you got the knives back! I think we'll keep the fence for a while at least: it adds only a trivial time to the build, and it clears the extruder's throat before each layer...
i was going to try adding the code to do that myself once i got my reprap up and running, but i am glad to see that some one beat me to the punch there, i think it makes a big difference in the quality of the output.

i just wish i could get the parts i need faster :(
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