Are there any issues with overlapping solid objects?

The question is this: how does the slicing software handle two solid objects that interest - does the software actually care?

Lets say I have a cube 5x5x5mm and a sphere dit a diameter of 5mm. I place the sphere on top of the cube and then I lower the sphere into the cube by 3mm

When you look at the model from the side you have a 5mm cube with a 2mm dome. Does the software simple slice both objects and print the resulting set?

The question that I am really trying to ask is whether I am forced to combine those two objects into a single watertight object or can I get away with simply overlapping them.

Some of my models have lots of little parts that are sometimes a real bugger to get to combine properly into a single solid model. I currently combine all the parts because I have always had to do so for my CNC but it would be great if this step could be eliminated because it is such a cumbersome operation.

I suspect that if parts had perfectly coplaner interface surfaces and they were stacked directly on top of one another that it would work, the issue is whether the software can resolve solid overlaps.

Thanks in advance.

BTW: I received the Form 2 two weeks ago and it it the most amazing tool in my armory. My CNC cutters and laser cutters have been relegated to the dinosaur era corner of the workshop :grin:

Preform uses nettfab to repair bad meshes, but for the best results in preform, use a hollow mesh that is already watertight.

PreForm’s slicing software is usually not too sensitive to this case. But you can check what it did using the slice slider on the right side.

Where you’re more likely to encounter problems is in support generation. The support generation algorithms use more information from the mesh than the slicing algorithms do. So keep an eye on what’s going on when you generate supports.

I would probably recommend continuing to merge your shells, just so you don’t have to always double check what PreForm is doing.

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Yes, it is fine to print a single .stl file that contains separate but overlapping objects.

They print as if they had a unifying boolean operation performed on them, but as [JasonSpiller] ( mentioned, you should only do this for small objects, as larger objects really need to be hollowed out.

Thanks. You know, I never noticed the slider on the right before. That is exactly what I need.

I generally create my own supports when I print my own models because I have absolute control over support placement. My models are designed for resin casting and then kit assembly so adding supports to the models is just another small step in the overall design process. Doing so allows me to position the supports appropriately in relation to the “face” side of the part.

Merging shells can be terribly frustrating at times though. I spent a couple of hours last night fiddling with a Rhino model that would not close. I understand what the problem is with the drawing but it is sometimes difficult to repair when the error occurs at a single point where 5 surfaces meet.

How sensitive is your software to shells with minor imperfections? I see that the software sometimes “repairs” shells. Does the software clearly indicate an error in a shell if it is unable to repair the fault?

I design and manufacture scale model trains. I design them in kit form so the parts are generally very small and the overlaps are tiny. I take a lot of care to ensure that that wall thicknesses are within limits. The idea is really just to save time by not going through the trauma of having to combine a vast number of little objects into a single watertight object if I don’t have to do so.

In that case, yes, it will work well for you.

It would be great if you could upload some photos. :slight_smile:

Yes, we love photos!

I have a vast catalog of HO scale (1:87) South African models that I designed over the years using CNC, laser, brass etching and lots of other technologies. I am in the process of developing the entire range in N scale (1:144) - For those of you who know model railways the scale of 1:144 was chosen for very specific reasons. Anyway, I am busy printing the first part sets for the fist test model. I created a fairly generic set of basic parts for a specific range of models. The generic parts will be printed first so as to get a baseline to work from. I am very big into modular and plug n play design / development / CAD etc so all my work is done in a way that is conducive to this. I will post pics of my N scale models here as they progress. You are welcome to look at my HO scale stuff on Facebook: Scalecraft - South African Model Railways (SAR / Spoornet)


Almost all of my prints are made from intersecting parts, just make sure the parts intersect a little bit rather than having the surfaces in the exact same positions. I don’t think the software does a Unify boolean operation, otherwise I would think it would take a while for it to import and process.

The only thing that can be an issue is that if you use automatic support generation it can end up putting supports inside the print for parts that are intersecting because it doesn’t necessarily detect they are supported already, this is rare though most of the time it’s fine.
Also, check to see if the support tools will work for you, I don’t recommend using it automatic, but it works very well to place the supports manually, the only issue I have with the supports system is that sometimes I’d like the support post to be further away from the print which isn’t something you can control.

I believe PreForm treats each object separately. So if they overlap, the laser beam will traverse the union of the overlapped geometry twice on each layer. From a printing perspective, this is basically equivalent to CSG during object creation. But the resin itself may be “overcured” in these overlapped regions. Whether or not that causes a problem with the final printed object, I couldn’t say…

This type of overcure makes the bases REALLY hard to remove. I had a lot of small parts and mashed the bases over each other and it was like crazy glue.

Hi hillcraft,

Randy_Cohen and SGuber make good points. I haven’t had any problems with my overlapping small prints on my Form 2, but if you do find that an odd print or two of yours is having any issues with over-curing, it would be good if you can share the details.

You probably won’t have any over-cured base problems as I believe that you would have aligned all your components externally and importing them as one .stl file.

I have created a lot of over lapping and connected piping. I usually forget that when two hollow pipes intersect a perfect hole is not created because the interior of a hollow pipe is open.
I then add a elliptical soap inside and at the intersection to cut out the opening!

I need better 3D modeling software.

I’ve never had an issue with overcuring. Or base problems, the small items are firmly stuck to the platform and easy to remove.

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I’ve been printing pretty much exclusively with Tough. 2 liters of Tough 01 and now I’m on to Tough 02. Maybe it’s just my printer, but I go through fits trying to get some prints off the platform (a cruel irony, since I could never get Tough to stick to the build plate on my Form1+). It’s unusual I don’t break the base trying to pry the print off, and I often leave little shards that I have to chisel off the plate after getting the print off (I actually bought a 2" chisel for this specific purpose). I don’t even use the FL removal tool, I bought a 10" “Insulation knife” (a long thin blade) now since I can get it started under an edge and it’s thin/tapered enough I can drive it between the print and the plate all the way from one side to the other of the print.

Tough 02 doesn’t appear to be any different than Tough 01 in this (or any other) regard.

Gotta be from over-curing, probably because my build plate Z reference is too low, the first few layers are compressed to the point the laser is basically just re-curing what’s already been printed. Nothing else makes sense.

I use the standard resins–I set the base to 1.5mm and it’s usually a bit flexible enough to where the larger bases won’t break and the small ones pop off easily enough. I just use a chisel exacto knife

Make sure your model sufaces do not touch as to create co-planer faces.
I have had trouble if the sufaces overlap too much. I try to keep the intersection less than one layer in height. For me this has been the sweet spot.

Co-planet faces will cause grooves on the lower surface extending all the way until they hit another surface.

I’m not sure if Preform actually sees then model as separate entities as stl format. I have my doubts it is that advanced. Usually I’d I have errors I will check them model in an outside program. What I have found is separate shapes that intersect severely cause self intersections that are hollow.

I haven’t found a mesh editor yet under $1000 that can automatically fix self intersections. However you can in many poly editors that have the option to grow a selection like Hexagon or Silo3d and cut paste those connected groups into actual shapes then Boolean them back together.

I did try meshmixer from Autodesk but haven’t run any of the models through yet. It does have some repair and analytics in it that is nice such as wall thickness. It’s free and probably at least worth looking at.

If you look at the model in Preform and go through the slices it will show what might be the laser path, which seems to show it treating each layer as a complete pieces even if the 3D objects are not welded together.

If you’re designing parts for 3D printing, it behooves you to learn how to use your CAD program correctly, meaning the use of Constructive Solid Geometry boolean functions to join components of the model together without leaving overlapping/intersecting/internal faces and/or shells.

If all you care about is how the outside looks, you can probably get away with being lazy. But the moment you want to do something like hollow-out an object you’re going to have problems with unjoined/overlapping geometry.

Also, PreForm appears to have difficulty with internal vertices and faces (which it has every right to have difficulty with). You may find that some supports go too deep in to a face of the object or even all the way through an object.

Taking the time to build your object properly up front often pays dividends down the road when you print it.

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