3D Printing Forum

Sometimes a model has a narrower section whose strength must be enough not to break in reaction to the sticking force at the build face. Is there a rule-of-thumb ratio? For example I am drafting a shape where the cross section area of the built resin will be 50% of the area "above" it, and don't want it to break with the unsticking force. Brace supports are not an option in this case. I use PFTE coating on the sheet which seems to help.

I don't think there is any ratio as such. I've always found that as long as you have a large area on the build plate and no large increased areas in printing it should be fine. I had to print a part 4 times, exact part, print number 3 failed, all I was left with was the supports, the rest of the print was on the FEP sheet. Print 4 was OK. If I'm in doubt I do a small test print so as not to waste time and resin.
Obviously the best idea is to only print a small area at a time by rotating the part to reduce this.
Do you have an image of the part?
Lez

The overhang on my part is difficult to visualise on the whole part but I've cut it in half so you can see the cross section with the little overhang.



Actually I will try the whole part, with the starting plane the flat bottom. But I have also cut it in two halves and the starting plane will be the rear face as in the image. I.e. the frame half will "hang" down as built and although long and narrow it never gets wider at any point.
Can you picture that?

Sorry I can't see the image.

Lez0 wrote: ↑May 25th, 2023, 11:24 am Sorry I can't see the image.

Hmmm. It shows for me. Here is a link https://1drv.ms/i/s!ArugkEfpRQyChN4SRj3 ... w?e=w38wnw

But, I think I see my original question was misguided. I was thinking of the relative sectional areas overall, but the important factor is the angle of any overhang. In my shape there is a right-angle flange: that means when the first slice layer of the flange is formed on the FEP sheet, there's no way it can be pulled off without having bracing buttresses or other supports. And I can't have those because I could not remove them later with enough precision.
Cheers
Nelson

EDIT: Grrr! Just realised that even attaching to the plate by the edge there is still a section with a right-angle flange that will tend be difficult to unstick from the FEP surface. Re-thinking.

Further to my above, here is the full shape and cross section.
The only way to avoid the fine flange from being broken seems to be to cut the model into 8 parts: diagonally and vertical and horizontal, and build so each piece hangs perpendicular, starting the build at the mid-frame cross section, and finishes at the mitred corner.
Any other suggestions?:

Another approach may be to form in two parts separated as shown below. I would delete the small radii. The narrow part would be fragile to remove from the plate, and the outer edge would be as rough as the build plate. But it could be manually smoothed and rounded. I think I'd fit the two parts together as soon as they came off the plate so the thick part would support the thin part - then they could be post-cured as a single unit.

OK, this is a tricky part to print and end up with what you need. The normal approach would be to print it with the large area on the build plate and add supports to the small overhang. The problem with this is that small area that overhangs would not have a good finish.
I don't know how big it is, so I'm assuming there is plenty of room to angle it on the printer.
I would look at turning it over, then tilting it at an angle of 30 - 45°. add supports to the faces that you can see in the plan view above.
You would be able to remove the supports easily enough and clean up the top face. The cross section of print would not be too large as it's at an angle.
One other thing, I would core it out as you have a large area, but then I don't know the size.
Lez

Thanks for the suggestion. It is about 45m wide and high.
I think if I can get away with not needing to clean up supports I would prefer it. It is still able to be modified in Freecad so re-doing it in two parts as shown either side of the red line in the previous post, looks like a viable option to me.
Nelson

Yes splitting it in 2 is an option, don't forget to leave a small gap between the parts otherwise they might not go together.