Option to inset / chamfer the extra thickness created by the "no raft" support

As probably most of us know, when you add a support to a part oriented directly on the build platform in “no raft” mode, it looks like it simply adds a extra thickness equal to the “Z-compression adjustment” parameter to the bottom of the part, which is great

After a few calibration cubes, it’s quite easy to figure out the amount needed to get the Z-accuracy you want, however, that leaves us with the dreaded elephant’s foot effect (I think i’s due to stray laser light curing too much resin in the early layers).

I’m about to write up some scripts to pre-process my STL or openscad files to manually add the extra bottom thickness but with an inset/chamfer of up to 1-2mm to avoid the elephant’s foot or to aid in removal,

It would be fantastic if we cold control the inset/chamfer of this generated support in Preform so I don’t have to regenerate my STL file every time I need to change orientation (for example).

(Yes I’m aware I can just rotate the part and add supports, but there are other problems with that, except for the elephant’s foot, the part comes out a lot better on all of its faces printed directly on the build platform.)

PS. I measured the z-compression factor to be about 0.2mm on my Form 3 with black v4 resin.

Let us know how this goes in practice.

“Elephant’s foot” is a delightful name for the effect you’re describing. I call it “compression” and “bleed”, and documented it with a rough diagram here.

There are a few challenges to reducing the amount of bleed (or elephant’s foot) that “oozes” out onto the build platform around the footprint of your part. First off, keep in mind the first few layers are overcured with multiple laser passes in order to gain sufficient adhesion to the build platform. You’re correct this (among other things) multiplies the amount of “stray laser light” that accumulates around the edges.

The build platform is also “pushed” harder against the PDMS for the first few layers, at least for the Form 2, for the same reason. I’m not sure if the Form 3 has a similar mechanism (e.g. I hypothesize it might overtension the flexible tank membrane at the start of the print or cause the tank to be pushed down harder against the rollers of the LPU, but haven’t seen any official remarks about this from Formlabs).

On the Form 2, I’ve found the best way to reduce bleed is to adjust the Z Fine Tuning of the printer until you get a) results that are the correct dimensions in the Z axis, and b) prints that adhere “just enough” to the build platform (strong enough to stay attached, but loose enough that you can pop them off without much resistance). I’ve found (a) and (b) tend to intersect nicely, and I’ve managed to all but eliminate bleed for many kinds of direct-on-base prints.

Unfortunately the Form 3 has no such adjustment yet. :frowning:

I gather you want to counteract the effect through proactive design tweaks by chamfering the edges of your model where it contacts the build platform. E.g:

      |         |
      |  model  |
       \       /
        \     /
|       build         |
|      platform       |

The problem with a model like one above, is that the first few layers do not share the same footprint. The area under the point of the model will be affixed solidly to the build platform. But as you move up through the next few layers and the footprint expands, the edges won’t have a good base below them to “grab” onto. When I’ve tried this, I get crumbling around the edges and a worse result than a bit of bleed.

That’s why Preform has an “Early Layer Merge” setting under Advanced Settings when you generate supports (or in your case, when you generate the extra base thickness). It merges the first few layers of your model to create an aggregate silhouette, then extrudes that silhouette toward the base. You’ll need to play with that parameter (along with Z-Compression Correction) to keep Preform from undoing what you’re trying to accomplish.

The result might look something like this:

      |         |
      |  model  |
       \       /
        |     |
|       build         |
|      platform       |

That gives cleaner edges, and cooperates with the printer’s behavior of overcuring the first few layers.

It’ll take some experimentation to determine the ideal Z-Compression Correction and Early Layer Merge for your part. When doing these sort of experiments, I often model it all outside Preform for fine-grained control, then plop my STL onto the build platform and skip supports generation altogether. But that might just be out of old habit (since I began doing this before Preform exposed those options).

Hope this info helps!

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Hey @rkagerer!

Yeah, I’ve seen that post you’ve written in the past, it was really informative and actually drove ths feature request.

I’ve actually printed several tiny (2x2x3mm) blocks directly on the build platform to test the effect of cutting an inset on the bottom, and as far as I can tell I had to inset the bottom 0.2mm (on my Form 3 the z-height is always compressed by about 0.2mm, so I try to always include a 0.2mm platform the design when I want to print directly on the platform (see example here: https://www.thingiverse.com/thing:4547424)

It would just be nice to be ale to set this inset value on pre-form for things I don’t have the CAD files for (or again, use a mesh editor or write a program to process the STL… but I feel this should be an option in the slicer.

Yep there’s certainly room for Preform to help here. Nice lens cap, btw!

As I understood (correctly?) from this thread: Z-Compression Correction question
There are several factors influencing how much the Z-Compression Correction needs to be:

  1. Type of printer (I guess values are different for Form 2, 3, 3L…)
  2. The tolerance on the hardware parts and assembly of the printer / vat
  3. The composition of the resin: (I guess some are more compressible than others)
  4. The total surface area of the first layers: (The bigger the area, the less compression can happen given the same spring force)
  5. Layer height: (may be curing parameters alter the amount of compression of the resin)
  6. (what else?)

hat else?)

Except for 2, I guess this value could be calculated by Preform at some extent.
Now it just a field left for the user, but the amount of experimentation needed is not only a lot and expensive, but also needlessly duplicated by every user.