Hi,
I am a very beginner in 3D printing.
I’ve created box and lid design in fusion. Coils inside the lid (1) hanged down, but the coils on the box seem OK.
What’s the problem? How can I fix the problem?
Btw lid works.
Setup:
A1 Mini, Creality Hyper Petg Gray, at 245C, 70C, profile: Generated from Generic PETG
It is very hard to print steep concave overhangs. The coils on the box were n the other hand are supported by the convex curvature. Think of the plastic coming out of the nozzle as rubber band. When you print an overhang on the inside it naturally tries to pull inwards. If it hasn’t much contact area to the layer below because of the steep overhang, it will just not attach. The next layer then has even less or no surface at all to stick to.
What you can do:
In addition to what has been said, please remember to dry the PETG properly. This isn’t optional - it’s a necessity. This applies even if the PETG is new and you’ve just unpacked it. While this isn’t always necessary for PLA - it depends on the situation - it’s safe to say that PETG almost always needs to be dried.
I just looked up your filament on the Creality website - taken from the product page; link at the end
Please also check to make sure your filament profile values are correct:
In your comment, you mention 245°, but you must specify the minimum and maximum temperatures. I don’t understand how you came up with a value in between? You must enter a minimum and a maximum. 
for printed threads you can use more starts which gives a steeper angle, which makes overhangs easier
(4 starts means there are 4 independent threads each offset 90 degrees to each other, instead of a single thread)
If your design allows it, orient the lid so the threads are facing upward instead of downward. This gives every thread layer support. Hotter PETG droops more on overhangs. Lower the printing temperature. Try 235–240 degree C.
I think your filament and slicer settings are OK, but I would design the part differently, following @Alex_vG’s suggestions.
Instead of shaping the thread profile as an arc, try a truncated triangle. The lower layers will provide better support than a semi-circle, just as a chamfer overhang prints better than a convex fillet.
When I design threads, I usually only design the male half. Then I use it as the “tool” to Cut a perfectly matching female thread using Fusion’s Solid→Modify→Combine function. Adjust clearance with Modify→Offset faces.
Thank you all!
I made a few tweaks. I changed the circular coil to a triangular one (which I also chamfered). I shortened the coil stand’s inner lid a little. It worked.
I couldn’t eliminate the drying option. I don’t have a tool to dry the filament right now. I added holding grips at #3 and #4, which don’t make any change to the coil printing performance.
As a result, changing and chamfering the coil significantly affects the printing result. It is enough for me now.
