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I have an STL file of a bone, and I want to create a mold to fill in with bone cement for a drilling exercise.

However, a midline cut through the mold doesn't work as the bone is atypically shaped. I would like to create a plane that follows either one of the bone's surfaces to create a mold that separates well.

Shape of the bone

Shape of the bone

This doesn't work, as you might expect:

This doesn't work, as you might expect

It should look like this:

It should look like this

Image source: The third photo from Cranioplasty Welcomes 3D Printed Molds for Reconstructive Surgery in Russia

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I would like to thank @hfcandrew and @MagWeb for their in-depth explanations:


Quoting from the original forum post:

Here's how I do moods from shapes like this needing curved parting lines:

Ohne TitelX.png

Step: Finding the parting line of the shape: MM's Overhangs shader (drag the sphere with the red bottom onto your object) is pretty useful if it is set to paint regions at its max angle. You can modify this angle in Analysis > Overhangs. Set AngleTresh to max (89°) and cancel the tool. MM remembers this setting to use it for the overhang shader:

Ohne TitelXX.png

Now drag this shader from Shaders onto your object.

Ohne Titel X2.png

Use Edit > Transform to rotate the object to an orientation where you get a meaningful border of red/white. You want to find a position where there are no white islands within a red area or red islands within white.

Ohne Titel X3.png

Now using the ViewCube switch to Bottom view. Make sure that you do not navigate for the following: Go to Select, select something (which region doesn't matter at all) and do Modify > SelectVisible:

Ohne Titel 2.png

Run Modify > SmoothBoundary. At its default settings, it will create a face group too:

Ohne Titel 3.png

Now you've found the parting line.

  1. Design a parting surface:

From above you've got a face group. We want to use its boundary to deform a plane primitive:

For easier view drop the first shader from Shaders onto your object.

Go to Edit > CreatePivot. Set PlacementMode to SnapToGroupBorders. Double-click at the group boundary several times to get pivots at that boundary like this:

Ohne Titel 4.png

When done go to Select and double-click the face group to select it. Run Edit > FitPrimitive. By default, it will insert a square object. Set a high MeshDensity and make sure to check CreateNewObjects. Scale up the object using the transform widget. It needs to be about two times its fitting size:

Ohne Titel 5.png

Accept. Now there should be a FitPrimitive1 object in your scene. Leave Select and hide the shape object.

Activate the plane and select all of it (Ctrl/kbd>+A). In Select go to Deform/Wrap.

In Wrap, double-click next to a pivot to create a red sphere. Drag this handle to the pivot to make it snap to it. Do the for all pivots to deform the plane:

Ohne Titel 6.png

After accepting Wrap, you should discard all pivots. You may make them visible in the ObjectBrowser by clicking the pivot icon in the bottom-left of the browser. You may now show the source shape to check the parting surface:

Ohne Titel 7.png

Next is to construct the molds from that.

  1. Construct solid molds:

At first, we need to create an offset shell from the source shape. Its OffsetDistance will define the width of the flanges later on. I use Edit > MakeSolid in Accurate mode which allows me to add an offset:

Ohne Titel 8.png

Drag the transparency shader onto this result (top-right in Shaders) and make a copy of the parting surface:

Ohne Titel 9.png

SelectAll of one FitPrimitve an use Select > Edit > Extrude to give it a thickness including the ghost shell to one side.

Ohne Titel 10.png

Do the same on the other FitPrimitive with a negative offset

Ohne Titel 11.png

Make a duplicate of the source shape. Hide all objects but one pair of source shapes and FitPrimitive.

Activate the extruded FitPrimitives as the first and hold down Shift one source shape as the second object. Run BooleanDifference:

Ohne Titel 12.png

Do the same with the second pair.

Ohne Titel 13.png

If the molds' sizes are bigger than your printer's volume it's to cut the halves. To do this Edit > Combine both halves to one object for now and run Edit > PlaneCut at Slice(KeepBoth):

Ohne Titel 14.png

Run Edit > SeparateShells to split all of the mold's parts again.


From this point, I modified the technique described for my case.

I had to re-mesh and smooth edges quite a few times in Meshmixer to obtain a completely solid object without any holes, which enabled me to make a boolean difference.

Here is my re-meshed bone and the plane cutting through it: Bone with plane cutting through

I made two copies of the fit primitive, and extruded them in separate directions: Raw extruded mold

While this is not an elegant solution, I sculpted each mold piece to have a thicker mold that I can cut into orderly pieces, so I can print and compress the bone cement in between.

Green monster mold of nightmares

I cut a few planes through the mold, obtaining this orderly shape. Note the bone within the mold

Now to print it:

Separate pieces

How they fit

Obviously, it would be better to create some mechanism to improve alignment(Ball/hole or some pins?).

But here is the end product, not dried yet:

End product 1

End product 2

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    $\begingroup$ Please edit and include the relevant part of the link, if possible. However, in Magweb's answers alone, there are 28 images spread over 5 posts/steps - so probably not feasible. $\endgroup$
    – Greenonline
    Apr 9 at 7:04
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    $\begingroup$ Nice of you to come back and post an answer, however, at SE we expect answers to be self containing, not link only. As links on the net tend to rot, we need a summary of the answer in the link which will be retained on our own servers. Please edit your answer with a summary from the link, pictures are welcomed and can be inseterd to be hosted from our own image servers (also retained with the SE site). $\endgroup$
    – 0scar
    Apr 9 at 7:07
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    $\begingroup$ I assume that the hfcandrew post is this one? Is that the correct post? That is just one animated gif, so it would be possible to post that - however, the image is too large to post (exceeds 2MB), I have just tried it. I have also tried to resize the image, but not had much luck. I guess this will have to be a link only answer. +1 anyway, for coming back with the answer :-) $\endgroup$
    – Greenonline
    Apr 9 at 9:02
  • $\begingroup$ I modified my answer using a combination of the original post and my own production process. Hopefully, this will help anyone who is trying to print a bone/organic material two-piece mold. Thanks for the pointers. $\endgroup$ Apr 9 at 11:59
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    $\begingroup$ Excellent, thanks for updating the question! Hint, you may want to loo up non-planar printing, this may help create better molds if they have a shallow indentation! $\endgroup$
    – 0scar
    Apr 9 at 12:15

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