Preparing a sculpt for mouldmaking

[Peter_Rabbit]

I’m working on a project to use blender to produce moulds for plastic injection. I’ve encountered two issues I’m having trouble resolving:

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First issue: We’ll be using two part moulds, either made from aluminium or polyester (neither of which are particularly flexible). As a result, the model can’t have any faces which are concave to the mould line. So if the top half of the mould lifts away along the z axis, all concave details (like eyes and mouths) must face in that direction. If any face the Y or X axes they can snag on the mould and make it difficult to demould the part.

This wouldn’t be a problem with purely mechanical models, but some of them will be organic high-res meshes which I’ll be modelling with sculpting tools. As a result, hidden faces might be formed on the sides of the model without being immediately obvious.

Is there any way to retopologise a model in such a way that only detail visible to one axis is preserved? I’ve included an illustration to show what I mean.

This is no good because the angled recess will make it difficult to demould (assuming the mould lifts off towards Z)

This is what it should look like after being processed.

I had an idea about writing a script that casts parallel rays from a particular axis and creates vertices where they intersect with the model (a bit like a 3D scanner) but it’d be much easier if some tool for this already exists.

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Second issue: I need some way to select the mould line. This is the point on the model at which the two halves of the mould meet. Basicly the most extreme set of vertices running in a ring around the model. Again, not such an issue with mechanical models, but with high-res organic ones it’s a pain in the ass.

For example, on this model I need to select the blue line, as those are the most extreme vertices on the X,Y plane. Obviously on a model that simple I could just do it by hand, but the sculpts I’m making look more like this.

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Any help on either problem is greatly appreciated. Or if there’s some other way of doing this that’d be great too.

[JA12]

There’s a mesh analysis tool on 3d view properties in edit mode. Overhang is one of the display modes which can help to visualize those areas along an axis. Whichever way you prepare those areas, that could help to visualize the areas that need fixing, along the chosen orientation. The display tool doesn’t support n-gons afaik, so get rid of those if there are any.

[chrisd]

@Peter Rabbit"

Is there any way to retopologise a model in such a way that only detail visible to one axis is preserved? I’ve included an illustration to show what I mean.

I had an idea about writing a script that casts parallel rays from a particular axis and creates vertices where they intersect with the model (a bit like a 3D scanner) but it’d be much easier if some tool for this already exists.

If you render a depth map of your model, you could apply that rendering to a high resolution grid as a displacement map (heightmap). This will eliminate any undercuts, and the draft angle is the result of the transition from light to dark values. The camera axis essentially becomes the Z axis.

One word of caution about using a mesh model for cnc machining … some shops will insist on a nurbs model. It would be best to find out if the shop can use a mesh before doing a lot of work in Blender.

[Peter_Rabbit]

@JA12

There’s a mesh analysis tool on 3d view properties in edit mode. Overhang is one of the display modes which can help to visualize those areas along an axis. Whichever way you prepare those areas, that could help to visualize the areas that need fixing, along the chosen orientation. The display tool doesn’t support n-gons afaik, so get rid of those if there are any.

Thanks that’s really helpful, I’ll try it out.

Any suggestions for selecting the most extreme ring of vertices?

[Peter_Rabbit]

@chrisd

If you render a depth map of your model, you could apply that rendering to a high resolution grid as a displacement map (heightmap). This will eliminate any undercuts, and the draft angle is the result of the transition from light to dark values. The camera axis essentially becomes the Z axis.

That’s an interesting approach, I’d never have thought a displacement map could work like that, but if it works it works.

@chrisd

One word of caution about using a mesh model for cnc machining … some shops will insist on a nurbs model. It would be best to find out if the shop can use a mesh before doing a lot of work in Blender.

Thanks, good to know, I haven’t worked with nurbs since roughly 2004 in 3DS Max, lol. Right now we’re experimenting with several approches for making the moulds, only some of which require a CNC mill, so we might be able to dodge this.

[titipuchal]

Hi, i been making that kind of things often. I made this shader some time ago, is very basic, not will show you the transition zone in the parting line, neither will let you choose the demould vector but if is in x, y or z axis. If some body have any idea to solve this shortcomings, would be great.

Cheers

[chrisd]

@Peter Rabbit"

Second issue: I need some way to select the mould line. This is the point on the model at which the two halves of the mould meet. Basicly the most extreme set of vertices running in a ring around the model.

It seems like the Freestyle renderer does this to some extent (see countour/external contour) https://docs.blender.org/manual/en/dev/render/freestyle/parameter_editor/line_set.html?highlight=external%20contour

I don’t know if its possible to convert the selection made by the Freestyle renderer back to a an edge selection on the model?

[Peter_Rabbit]

@chrisd

It seems like the Freestyle renderer does this to some extent (see countour/external contour) https://docs.blender.org/manual/en/dev/render/freestyle/parameter_editor/line_set.html?highlight=external%20contour

I don’t know if its possible to convert the selection made by the Freestyle renderer back to a an edge selection on the model?

I see what you mean. I hadn’t considered that approach. Might be worth looking into alright, thanks!

[Peter_Rabbit]

@titipuchal

Hi, i been making that kind of things often. I made this shader some time ago, is very basic, not will show you the transition zone in the parting line, neither will let you choose the demould vector but if is in x, y or z axis. If some body have any idea to solve this shortcomings, would be great.
[ATTACH=CONFIG]477632[/ATTACH]
Cheers

Thanks, that might be handy alright. At this point though I’m thinking I’m just going to have to write a script to do it.