How to stop Prusa Slicer from extending supports into parts of the model where I won't be able to remove them?
The issue is easier describes with illustrations than explained (see attached figures).
Ihave an enclosure with a slot for a mounting bracket. Perpendicular to that slot there is a hole for a bolt which will hold the enclosure in place and prevent the box from jumping off the bracket (see fig. 1). The slot has a "gabled roof" by design to avoid the need for supports within the slot (see fig. 2). The slot is only 1.7mm wide so won't be able to clean it out and remove and supports.
Figure 1: The entire enclosure in my CAD program
Figure 2: Cross section of the enclosure box with the "gable roof" at the top of the slot for the mounting bracket
However, I want to have supports for the holes for the bolt and the washer. It's easy to remove the supports with a drilling bit after the print. However the slicer creates supports for the washer pocket (fig. 3) and the bolt hole (fig. 4), which is fine, but also "extends" the support through the slot (fig. 5+6). How can I avoid that?
Figure 3: Washer pocket with supports after slicing
Figure 4: Bolt hole with supports after slicing
Figure 5: Additional undesired extra support within the slot seen from below
Figure 6: Additional undesired extra support within the slot between both holes
As you can see I have already marked the entire inside of the slot, incl. the "roof" to block supports (fig. 7) and only the upper part of both holes is marked for supports (fig. 8). Still, Prusa slicer creates a "connecting" support between both holes.
Figure 7: Slicer settings to block supports from the entire slot
Figure 8: Slicer settings to create supports for the upper surface of the hole
Right click on the part > add modifier Select and position a suitable shape then right click on the modifier > change type - two of the options are support blocker and enforcer, choose and slice.
Sometimes it is easier to set where you want support and choose support for enforcers only, sometimes the other way around.
Cheerio,
RE:
Have you tried to add a support blocker?
Right-click your model and select "Add support blocker" -> "Box"
And then edit size and position of this block to fill out the slot in your model.
It could also work to just have it at the bottom of the slot which will block supports being generated on the buildplate.
Edit: Late to the party, it seems...
RE: How to stop Prusa Slicer from extending supports into parts of the model where I won't be able to remove them?
The following is all general principles. To be more specific to your model we would need a copy of your 3mf project file saved from Prusa Slicer. With that said...
It is going to be difficult and you will need to be careful with how/where you paint your support ENFORCERS. I almost never use painted blockers anymore. I do occasionally use a mesh designated as a blocker to cover large areas, especially if the geometry is bas and has intersecting faces which often stop any kind of painting working properly.
First you have to understand that a Blocker is not a keep out area. All you are doing with both Enforcers and Blockers is defining surface faces that you do/don't want supported. That is all. After that the support generation algorithm takes over and using all the other support settings you have it then generates the support.
This means if you have an area that IS designated as needing support then it will build down from there to any surface below that to achieve that support, even if its covered in a blocker.
Next point you have to remember is that even with Snug type supports, the support will expand slightly out from the surfaces it is supporting, same with the base of the support. So make sure you don't have the support First Layer Expansion setting anything other than zero (even then there still a little bit). Don't use Grid supports except where you want masses of overkill supports generated.
It will also tend to connect nearby islands of support, with snug support you can somewhat control this by changing the Closing Radius setting, setting it very low can dampen the effect. Also adjusting the Pattern radius to give denser support can help if you are supporting smaller areas. By carefully making the painted Enforced area smaller/larger and slicing you can get much more control over where exactly supports are used.
It is a learned skill, judging how the painted enforced area/the algorithm and the geometry all interact. At the moment its very iterative. Which is why when support control is critical its sometimes easier to model your own into the model.
Personally for holes like that I wouldn't even bother with support. Its not needed and you can ream the hole out with a drill bit after printing. Those supports are not going to do a thing to help that geometry unless you have the 'Top contact Z distance' AND the 'XY Separation between object and support' set so low that its actually touching. And if that's the case you won't be able to remove them unless you have a multi extruder setup and use materials that don't stick like pla/petg or soluble supports.
If the slight bit of droop at the top of a hole or your washer pocket is a problem for fit then model in a small sloped roof like area at the top. That way it will slice nicely and the washer will fit properly. Same can be done at the bottom spot too to compensate for the way slicing works.