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  1. #1
    Join Date
    Apr 2020

    Is my model unprintable?

    Can I share my experience so far?

    I have created 2 separate models, using Progecad (similar to Autocad) both based on a similar idea.

    The first is a townscape with each building represented as a block with a flat or pitched roof. Some of the buildings, e.g. churches, have more complex forms. The whole is set on a base, which is modelled as a 3D solid, 3mm thick.

    When I create a .STL file and upload it to 3D Hubs, I get a printability report of:
    1656 non-manifold edges
    604 boundary edges
    8563 intersecting faces

    Nevertheless, despite these “errors” I have been able to get an acceptable 3d print, in which just a few of the pitched roofs do not achieve the expected form.

    I think I understand the concept of non-manifold edges. Each of my building blocks is a 3d solid with a minimum of 6 faces: bottom, top and 4 sides. If the roof is pitched, I've constructed it from 3d faces on top of the walls. Therefore each roof/wall junction has 3 elements: the wall surface, the flat top surface and the roof 3-d surface. I think these are the non-manifold edges.

    I could explode each of the blocks into 3dfaces and delete the flat top surface below the pitched roof. But I think there would still be non-manifold edges where the walls meet the base, unless I could somehow subtract the floor area of each block from the base. However, both of these operations would be very laborious, particularly the second one. As I say, without the “corrections” the model appears to be to some extent printable.

    The second model is also a townscape, but this time it is on a hillside so the base is not flat. I have created a stepped base where each step overlaps the one below by a small amount. I suspect that each junction between the steps may be an “intersecting face”. Additionally, the landscape, rising from one step to the next, is constructed in a series of 3d faces. Again there may be circumstances where the junctions between the stepped base and the 3d faces of the landscape are non-manifold edges. The printability report this time identifies:

    3342 non-manifold edges
    984 boundary edges
    18340 intersecting faces

    Now the report tells me that there are too many errors for the model to be printed.

    When I run the STL file through Meshlab, I can easily delete duplicated faces and non-manifold faces and vertices. But this leaves my model full of holes. Some of the roofs have lost their gable ends, so the roof surfaces are now too thin to print – like a ridge tent with open ends.

    Surely it's possible to make a model with a solid base, and then place forms on that solid base, albeit that this would make non-manifold junctions? And if this were possible, would it not also be possible to place roofs on 3dsolid blocks, each one comparable to placing a form on a base?

    How does the .stl file "know" whether a form is solid or hollow?

    Can anyone give me any guidance?
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  2. #2
    Join Date
    Apr 2020
    Is my question too complex? Does no-one know anything about this?

  3. #3
    Join Date
    Jul 2020
    Solids in an STL file are defined as a set of triangular facets. A facet is defined by its three corners, and a normal vector that tells the software which direction the facet is facing. Per Wikipedia, "the facet normal should be a unit vector pointing outwards from the solid object."

    If you have a solid defined where some of the normals are pointing inwards, it's hollow. If all the facets are pointing outwards, it's solid.

    A somewhat common problem I've had will some CAD software is that they can get confused (or make it easy for me to get confused) about which side the facets are facing, and generate inconsistent or nonsensical sets of facets that don't reflect what I think I'm seeing on the screen.

    Hope that helps a bit.


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