Saturday, January 12, 2008


Bed Corner Printout Success

It took me 3 goes. One cable pulled, and a stuck filament roll; but I got it printed in the end. Thanks to Steve for helping with the STL file. The object took 5.8 hours to print in PLA using Darwin hardware and Adrian's latest fixes. It has really picked up steam now and the quality has improved as the nozzle moves faster - as predicted.

No hint of curling on this part either.

If you look inside the holes on the left, you can see some stringing, but it is now more like annoying cobwebs than a barrier that needs drilling out with a cordless drill. Just as well - I did stick an 8mm drill bit in my hand last time.

Here's the view from above, with a clearer view of the stringing. The thick stuff is caused by a plotting error we're looking into, and the fine, spider-web stuff is what the more persistent stringing issue now looks like.

I've tried to bend the part, and it doesn't break despite the plastic flow being a little low, so we should be good to give it a go in a real Darwin.

Vik :v)

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as far as the fine stringing, how hard would it be to program the head to avoid holes when moving to its next destination? would stringing still occur if the head did not move straight to the next place?
This will minimise the problem, but if you're printing a vertical 'U' then there's only one way to go between the two arms!

How's your programming? :)

Vik :v)
How long does the stringing last once you have turned off the flow and how wide is it in comparison to the normal deposition width?

Could you, when you know you have to move to a new spot, try to walk back across 'old' ground. If the new target is another island, then continue round and round in your current island until the stringing should have stopped; then move and start again on the new island?
Nice. The PLA gives it a Fortress of Solitude look. It'll be interesting to see a complete Darwin with PLA parts.

Do you thing the internal taper will cause problems with the anti-backlash spring?
hugh: In the shot you can see a huge string connecting the part to the printhead. That's how much it'll string for. You don't want that deposited randomly, and you don't want to hang around until it is finished. Moving fast solves the problem nicely.

Steve: The internal taper is fine with the spring. I've fitted it on a piece of studding and played with it to check.

Vik :v)
Perhaps the only way to contend with the stringing problem will be a modification of the extruder head. How air tight is it? Would a tiny vacuum tube running to the head be too much hindrance? Or a small cylinder with an additional piston? One of these could be used to create a momentary back-draw on the liquid polymer in the extruder tip just prior to crossing a hole. Then reversed to push the liquid back into place to start drawing again. Or if the impeller thread is on a stepper motor one could briefly run it backwards a couple of steps, that might work to withdraw the goo.
It's a stepper motor, and yes we have a version of the hardware that will reverse it. Not tested yet though. Just had a power cut 6 hrs through an 8 hour print job. Arrrgh! Well, I had to go out to attend a truck vs. power pole call anyway...

Nobody hurt, just truck & power pole. Power restored 5 hours later.

Vik :v)
Um scratch, stepper motor and add reversible motor. Obviously standing in sun all day guarding power cables has weakeneded der brane.

Vik :v)
What's the current power drain for a Reprap? If it's not too high, perhaps a PC UPS might help with power cuts.
About 6A. I actually run off a 5A-rated supply but it power-limits when the Z-axis cuts in.

Vik :v)
The only other option I can think of is particularly ugly, have a solenoid driven pin or flap that slices the string off as it closes the tip of the extruder head just prior to crossing the hole.
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