How best to deal with bridges when printing?

Hello All

I wonder if anyone could tell me how best to deal with bridges?

Do I want it to run faster over the bridge? slower?

Extrude more? less?

I have a bridge of about 21mm I wish to span. I am using PLA and Skeinforge.

Thanks

Advice:

With PLA, temperature is critical. You want to find out how low you can go without the extruder jamming and use a temp just slightly (a few degrees) above that for you bridges.

Speculation:

After that you need to find the sweetspot for speed/flow and this will likely depend on your nozzle.

experiment.

run with faster feed rate over the bridge. it is easier to modify feed rate rather than change flow rate just for the bridge. experiment a little. try 1.1-1.2 feed rate over bridges.

IMHO

PLA is much softer than ABS when hot, so won't span gaps straight but will bend down while printing.

You can test with support-structures or print a solid bow under the bridge with a small gap to the bridge and manually cover the top surface of the bow with grease or similar to prevent fusing ...

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My testing yesterday found when speeding up the feed rate on bridges did not really work, but when it was doing the infill (my test piece was a 2mm wide wall) the infill would span the gap!! - it would be strait which almost makes me think the feed rate should be reduced or the flow rate increased when bridging...

interesting. Anyway - I need more tests.

My settings (skeinforge 41) are "infill in direction of bridges" (Carve). And 0.8 ratio for bridge feed/flow rate (Speed). And it works out beautifully extruding black PLA from ultimachine at ~180deg C.

There is a 13mm bridge on this print (the bottom clamp), and it's pretty much flawless. [www.thingiverse.com]

Wade's extruder body also prints really nice with these settings, and that part has been a pain for me to print before.

Edited 2 time(s). Last edit at 05/16/2011 04:26AM by Nudel.

Hi Nudel

Sorry for being thick but could you clarify your ratio settings? Are you setting your Bridge feed rate multiplyer @ 0.8 leaving the bridge flow rate multiplier at 1 (giving a ratio of 0.8 as I understand it) or are you setting both variables to 0.8?

I would imagine setting the feed rate 0.8 and flow rate 1 would give roughly what I saw when the infill (very short zigzags) seemed to cross the gap when straight processes did not.

Both the bridge flow and bridge feed is set to 0.8. So it runs 80% of the regular flow and feed speed across bridges. I believe this was pre set in the profile I got from ultimachine (?) and it works really well for me.

I have tried to tweak the feed rate manually, but can't get good results with it, and it counters the idea behind volumetric extrusion.