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Posted by psneddon 
May 28, 2016 08:35AM
Hello All,

I'm trying to print the arms of the T-Rex skeleton, but not having much luck. Could it be a retraction issue? The bottom part looks pretty good until it gets to the branching of the print.

I'm using Cura

Retraction speed is 30 mm/s Distance is 4.5mm

Trying to print this at:

Print speed 50mm/sec
Density 10%
Bottom/Top thickness .8mm
Shell thickness .8mm
Layer height .2mm

printed on I3 rework with .4mm nozzle

Any help would be greatly appreciated. Thanks to all.
open | download - image2.jpeg (480.9 KB)
Re: T-Rex FAIL
May 28, 2016 11:40AM
Do you have a part cooling fan? It looks like the usual pla overhang issues with not enough cooling.
Re: T-Rex FAIL
May 28, 2016 12:48PM
Yes, I have a cooling fan with a duct I designed to cool the PLA as it comes out of the nozzle.

It really helped with this bridging test:

Before duct:

After duct:

Not perfect, but a heck of a lot better than the original I3 rework printed fan duct. I was searching online for some answers and figured it may need a more powerful fan than the one that's on it now which is a 40mm 12V 0.08A fan.

I'm getting ready to try a bigger fan and modify the duct for it. The fan is about 70mm square rated at 12V 0.15A

Edited 1 time(s). Last edit at 05/28/2016 12:49PM by psneddon.
Re: T-Rex FAIL
May 28, 2016 01:59PM
It will be interesting to see if the bigger fan helps. I suspect that the biggest difference is going from none to some, but maybe some more will help smiling smiley

It does look like a tricky part - small layers so you can't slow down too much without it getting too hot, and very tall compared to the diameter. Not sure I could print that very well at all.
Re: T-Rex FAIL
May 28, 2016 02:03PM
50mm/sec is extremely fast for such a detailed piece. Use like 20 instead and lower temperature to 190.

Edited 1 time(s). Last edit at 05/28/2016 02:05PM by ggherbaz.
Re: T-Rex FAIL
May 28, 2016 11:07PM
50mm/sec is extremely fast for such a detailed piece. Use like 20 instead and lower temperature to 190.

I lowered the print speed to 20mm/sec and printed the upper part of the feet starting at 185 and lowering it about half way through to 182, but still no luck

I am currently printing the upper part of the feet again. This time at 10mm/sec and started at 182 half way through and then 180 for the last half. I also enabled cool head lift and increased the minimal layer cool time from 10 seconds to 15 seconds. I don't see any improvement though.

I don't think it's the stl file. It's Thingiverse #275091 and other people have printed this without any apparent problems.
Re: T-Rex FAIL
May 28, 2016 11:57PM
Next thing to try is to use a sacrificial part so you can give it more time to cool or try to use support material even if isn't needed.

One more thing to try is to play with the extrusion multiplier.

Edited 1 time(s). Last edit at 05/28/2016 11:59PM by ggherbaz.
Re: T-Rex FAIL
May 29, 2016 08:43AM
I thought the cool head lift was the same as adding a sacrificial part - the head stops printing and moves away from the piece for at least 5 or 6 seconds when it's printing the branched parts of the print?

I've never messed with the extrusion multiplier. I guess I could give it a try. Should I increase or decrease the multiplier? I guess I could try both ways.

I was also wondering about the bed level. I leveled the bed prior to printing using the single sheet of paper method and I adjusted the bed accordingly. I did notice however that during my last attempt (using the cool head lift method) when the head returned to the first leg and starting printing again, the leg was being pushed around a bit when it was printing and the second leg was more of a mess as well. This was the result:

I'm thinking maybe the cool head lift option is more prone to backlash (?) resulting in the head not returning to the proper height? Just guessing, but I don't think I'll be using that option again.

Maybe there's some slop in the belt tension somewhere. I think it's probably time to take it apart for cleaning anyway just in case.
Re: T-Rex FAIL
May 31, 2016 08:33PM
You are doing the calibrations with both hotend and bed at operating temperature? If not that might be part of the reason, but yes a backslash may cause the nozzle to dig into the printed part or extrusion is still too hot and swollen (lack of cooling). For the extrusion multiplier you would reduce it since it's a small piece. The cool head lift isn't giving you enough time to cool the part it needs to stay away little bit longer.
Re: T-Rex FAIL
May 31, 2016 09:53PM
I downloaded the arms to have a look at - it looks outrageously hard to print. It would be interesting to see some of the prints close up to see how well anyone has done. I also wondered if it might print better flipped over 90 degrees with lots of support.

Re: T-Rex FAIL
May 31, 2016 10:38PM
I'd replace your axial filament fan with a radial (blower) fan... axial fans are better at sucking than blowing. Trying to make them blow through a narrow duct opening makes matters worse.

I'd print the arms standing on end, with a good brim, pretty much as the OP did. But print both at once, spaced a long way apart on the bed, so that they have plenty of cooling time. Or maybe even print 2 or 3 copies at once, for increased cooling time and to double/treble the probability of getting one good outcome.
Re: T-Rex FAIL
May 31, 2016 11:40PM
Try the one with support and see what comes out, it might be difficult to remove the support from the thin arms, but printing in different angles has helped me print stuff that otherwise would be a nightmare.
Re: T-Rex FAIL
June 08, 2016 09:56PM
Thank you all for your welcome suggestions. I got around to trying it again with a few changes.

1. Changed the orientation of the print
2. Changed the flow rate from 100% to 95%

I forgot to change the layer height from my last print - the object was printed at .24mm layer height. The bottom layers are a bit sloppy (where the main supports were), but the overall result wasn't too bad considering.

Edited 2 time(s). Last edit at 06/08/2016 10:09PM by psneddon.
Re: T-Rex FAIL
June 08, 2016 10:08PM
That looks pretty good!
Re: T-Rex FAIL
June 08, 2016 10:11PM
That looks pretty good!

Thanks for your suggestion to try a different orientation. smiling smiley
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