Multiple intersecting 'Shells' or 'Sub Objects' : printable?

Hi all,

I am very curious how the form1 printer will handle polygon objects made up of multiple intersecting pieces. The terminology can vary between software, but in the software I use, combining more than one polygon object creates a new single object, with multiple “shells”. This can be a really great technique to keep certain parts separate for editing.

I have had mixed results printing objects on Shapeways with multiple intersecting shells. After a lot of back and forth it seemed like the main problem was specifically when faces from the intersecting shells touched in way where the normals on each  shells touching faces were pointing in the opposite direction.

When I asked about printing polygon objects with multiple intersecting shells at the Inside 3d printing expo, I was told that the form1 will not support this.

However, reading on the FAQs here I found the following that is giving me hope that this is possible after all.

In the support section, it reads as if the support structures are combined as intersecting shells of the main object you are printing:

Distance from Model

This value defines the distance between the support structure “tip” and your model. Since support structures are actually attached to the model, this value is negative. You can think of negative values defining how deeply the support tip is “embedded” in the model. Changing this value will affect the touch point size.

Also, in the designing for printability section it reads as if to say mutliple intersecting shells are OK, so long as they are combined into one object:

Use Single Body (or Mesh) Settings

If your final model is the combination of multiple sub-models, make sure to save it as a single body (or mesh) in your CAD package. Otherwise there may be some irregularities in your print if the intersecting layers of the sub-models are not correctly recognized by PreForm Software.

Can the Form1 print objects with multiple *intersecting* shells, are these considered ‘problem objects’?



I generally find that the PreForm software does a good job warning you when you’re trying to print a broken file.  That’s important, because a bad STL can have a wide range of negative effects on how a model prints.  The sort of intersecting shells that you’re describing would have a problem known as self-intersection.  You can find a bit more information here on why self-intersecting models are not good geometry for 3D printing:

The documentation you’ve included above recommends that you use your CAD package to save your model as a single body / mesh.  That way, instead of intersecting shells of different parts, you’ll end up with one part with just one shell that encompasses the intersection of the parts.  (FYI, the “distance from model” is really a different feature that has to do with the support structures that the PreForm software makes, not the kind of intersecting shells you are concerned about.)

You should  also check out the support page here on how to go about repairing STL models: for more advice!

Thanks for the reply Martin! I’m definitely curious to see what errors will pop up in the software and what will result from trying to print a single object with multiple shells. I have printed these types of objects before, with success. If you were to save a model as a single body / mesh, would the software remove the geometry that intersects, the way a boolean operation would? I am trying to understand the effect you are describing. Thanks again!

This is an area that we are working on at the moment.  We will be improving our support of broken and multi shell STLs soon.  Stay tuned :slight_smile:

That’s great Maxim!

I wonder if you would regard a multi shell STL as a printable object (I hope!),  or as a problem to be fixed in some fashion?

I would love to see them supported “as is” with no need for editing (even automatically) of the STL.

At any rate, I look forward to software updates! :slight_smile:

Lets say you have two sphere’s that overlap. Would the area where they overlap be printed as a solid area or become negative space?