Weak link

[jemc750]

Love this model because kids have so much fun playing with them. I have printed this in various sizes and mostly had zero issues. I tried it at 250% and had a weak link between the leg section and the next one up, which caused the model to break in half. All other joints were strong and smooth. This happened on 4 different attempts. At this scale the slicer did add small supports in the joint, I tried a 5th one without support across the joints and none of the joints held up. Thank you

[DrLex0]

This must be a problem with the print settings you're using at the 250% scale, because:

• You have no problems with the smaller prints, even though the smaller this model is printed, the less material is in the links and the weaker they are (of course assuming that when scaling up the print, the number of perimeters is scaled up as well, see below).
• All the links in the model have an identical design, so if only a particular link consistently fails, it's because it is not being printed like the others. This is not the fault of the model. I'm not going to tweak the model to cater for one particular quirk of one particular printer with specific print settings.
• You are the first to report this kind of failure in the 5.5 years that this model is published…

The failure looks as if layer adhesion in your print is very weak.
Things you can try:

• Increase print temperature. From the photos, this looks like a ‘silky’ PLA filament, which usually requires higher temperatures than regular PLA to obtain good layer adhesion. Also, because the print is larger, everything has more time to cool down before the extruder passes over it the next time, and more heat is needed to compensate for this.
• Reduce cooling fan speed if possible. Too much cooling will cause poor layer adhesion. Too little will cause curling of overhangs, so use the lowest speed that avoids this.
• Use more perimeters (contours). You need thicker walls at 250% scale than at 100% to get the same relative strength. It probably doesn't scale up linearly, so if you've printed 100% scale with 3 perimeters, then 7 or 8 perimeters (3×2.5 = 7.5) is probably a bit excessive, but I would use at least 5 perimeters.
• Disable supports, or at least prevent your slicer from adding supports in the regions of the links.

[jemc750]

Thank you for the input, I will try your suggestions.

[grawlinson]

I just printed this with PETG at 270% and the parameters suggested in this issue (also did gyroid infill instead of grid). Took a little over 10 hours on a X1C, and I'm fairly happy with the result.