Line opening mold

Hi everybody!
I’ve a question on how I can difine the profile (silhouette) of a solid object in order to open a mold.
In rhino using “draft angle” command I can only visualise the part that are undercut but not extrapolate any line…

Someone has a solution? Best of all should be a script…
thks a lot,

Can you show us the shape of the mold?

Does Silhouette help?

asyou see in the attached images (It’s an example) the command don’t let extrapolate any line… and the silhouette command it’s not always fine…

Did you hide the model to see if the curve from Silhouette is complete around the perimeter of the model? some manual editing may be needed.

Yes, the line is not completed (often happned) and in any case is not what I’m looking for…

Hi @illo76,

for the missing curves after _Silhouette, you might check your model for naked edges using _ShowEdges, bad surfaces using _ExtractBadSrf and in areas where you get multiple curves, extract the surface and check if they have multiple knots using _RemoveMultiknot.

You may check your document unit and angle tolerances too. If they are set too coarse, _Silhouette sometimes makes problems. To go deeper you might provide the model for closer inspection.

Appart from all this, there is only the _DraftAnglePoint command which might help to get the points which form the curve you search for. To find more about it, run the command and press F1


thks Clement for your answer, better than nothing :smile:

I hope someone in future can resolve the question with a script or samething similar

Hi @illo76,

i´ve thought about scripting it too. But this (the creation of parting lines and curves) is probably something Rhino needs as a built in command.

Meanwhile i´ve been doing such splits by hand by drawing the points using _DraftAnglePoint with even distance, then create degree 3 curves through the points using _CurveThroughPt. Finally i pull the curves to the part, connect them using blends and then split the surfaces. It`s intensive but offers full control over the parting lines.


yes Clement,
maybe Rhino 6 will improve this command
your method is actualy the only one even if is very long and do not preserv from error…

Hi Marco,

I use the following method to split Orthopaedic shoelasts:

Create a mesh outline on the plane where you more ore less want to split is.
Rebuild this curve to degree 2 or 3 and offset the curve a a little to the inside.
Project this curve on the object, and find the mean curve of the 2 projectioncurves.
Pull this mean curve to your object and use it to split your object, or as a rail-curve to create a splitting surface.


All these mold making issues are handled by Solidworks, TopSolid or Cimatron which are much more expensive, require dongles or even monthly rental payments. I got very condescending remarks from a Cimatron agent concerning Rhino. But all Rhino needs to do to handle this heavyweight competition would be to add Pull direction choice, Parting line split, Parting surface, Add drafting and Create shut-off surfaces and Bob (McNeel) would be your proverbial uncle.

Paul - can you provide simple examples of the “all we need” items? If we have concrete examples, there’s always hope that we can cook something up.


Pascal - These three videos are as simple as it gets

As a pensioner I can’t afford Rhino updates, so this is for the greater good:)