Adding break-off parts (for custom supports)
I am working on a tall model which needs supports for an overhang which occurs pretty high up in the print. Automatically generated supports would not only be very tall, wasting a lot of material and print time, but would also have to stand on other parts of the model which will be very visible in use. (I cannot use organic supports which could come in from the sides, since I need to slice with variable layer height, and PrusaSlicer can't combine that with organic supports.)
So I plan to add a custom "shelf" to my model on which the supports could sit. It would be connected to the model at an inconspicuous place, and tilt forward (at a manageable overhang angle) to reach the area below the structure which needs support. Auto-generated supports can then sit on top of that shelf. After printing I want to break off the supports (of course) and the shelf.
Is there a proven way to model and print sub-structures which stay connected during printing (carrying their own weight and then some), but can be broken off relatively easily afterwards? Can I just leave a tiny gap between two surfaces, to obtain two shells which are printed with a time separation, but are close enough to each other for one structure to stick to the other? How big should the distance be? Or is there some other proven design pattern?
The material will be PLA, in case that matters, and the contact area between the "shelf" and the main model can be made flat and approx. 10 cm² in size. -- Many thanks for sharing your experience!
RE: Adding break-off parts (for custom supports)
I've printed a lot of things that sound similar and never found a suitable solution, so spent time adding in thin 1 layer sheets at certain print levels to connect weak adjacent structures. Supports that build off the part never seemed to work well. They either fell off during printing or were so well attached they may as well have been a solid part of the part. So I did make single walled vanes that attached every few cm to the full height of a part to avoid any supports being built. Painful to remove - requiring careful Exacto knife skills to cut them off. Unfortunately, those were all one off projects I didn't keep around. Basically, add a triangular piece orthagonal to the part, use brims if starting from the sheet, or just attach them jutting out from the part for higher supports. The trick is to find the origin of the unsupported area, or let autosupports build from the vane (an inverted cone on the vane will build a base for supports).
Here's a quick example of a vane used as a support base:
But since organic supports were included in Slicer I now use them for many otherwise troubling support problems. In this example I had to play around with the final tree diameter 2mm was too small to survive to height, but otherwise worked fairly well. And I admit I went overboard on painting in supports where they probably weren't needed; but I wanted the print to work first time.
RE: Adding break-off parts (for custom supports)
Many thanks for the detailed reply, Tim! Looks like I have some experimenting to do. Multiple point(-like) contacts to the main object are probably the way to go. I have the advantage that I can attach the "shelf" in an area which will be hidden once the model is fully assembled, hence don't have to do a perfect job with the Xacto knife.
Oh, and sorry about posting this in the wrong forum section. It was meant to go into the "how do I print this" section of course.
RE: Adding break-off parts (for custom supports)
You're welcome. Pretty sure you know the drill when adding parts; stability is needed in more than one axis, a vane might become an L shape, or two vanes are better than one, lol, etc. My example is an example, I'd do things differently to minimize print materials, but it shows the concept.
One of my favorite slicer fails was a 4x4mm support that was 180mm tall, on my Mk3 bed slinger. lol. I was young in print years and trusted the slicer knew what it was doing... it was my first spaghetti monster. I tried tape and superglue to hold it together, but eventually, while I wasn't near the printer...