Having trouble making a mould


Hello all. At the moment im trying to make a negative mould for my surf fin. I’m having some trouble making a solid box from where I would like to use boolean difference. I have outlined how my box should look like but i’m clueless as how to make it a solid. I hope one of you can help me. Hydrofoil 5 mallen.3dm (1.5 MB)


Your box isn’t quite as square and true as it first appears, which may be your intention. First thing I’d do is explode your curves so you can build surfaces from them, at the moment some are joined. Then I’d use the planar surface command to build your planar surfaces, loft for the ones that aren’t planar (the top surface). Then join the resultant surfaces into a solid.

Good luck.


Looking at your geometry closer, loft isn’t going to give you a split surface on top of the box. I ran the silhouette command and see that the split isn’t flat, so you’ll have to build that surface a piece at a time.


Also note that the wing object gets flagged as bad object…


Phoeww allright. This is quite difficult for me. I’m trying to find the thickest part of the wing to cut it in half. So to speak to get 2 perfect halfes. I’m really sorry if my questions are dumb, but how do I find this? Also Wim, you mentioned the object as flagged as a bad object. Im not sure if I can do something about that. All i need is this slice the wings in half. Please help

(Pascal Golay) #6

Hi Rik - if you know the mold pull direction, set a CPLane so that the pull is in CPlane Z, then use the Silhouette command to find the curve I think you are looking for. If the pull direction is World Z then Silhouette from the Top view should do it.



Don’t use boolean difference that is guaranteed to be the path to failure. Even thinking about boolean difference will lead you astray.

The first thing is to find the silhouette curve from top view.
Luckily the silhouette falls close enough to an isocurve that you can use splitsrf (split by isocurve) to split. You don’t want to split with the silhouette curve if you can avoid it as that will just result in messy parting surface.

After splitting the next step is making a good parting surface.
I used sweep1 to sweep horizontal lines along the edge. The
part of the parting surface that is away from the edge can be smoothed out to make a smoother parting. After the parting surface is made It can be joined to the mold cavity surface, A copy of the parting can be used for the other side of the mold.

The last step is make a frame by extruding and trimming and joining to each side of the mold. You need to make sure the parting extends past the frame.

hydrofoil_mold.3dm (381.1 KB)

The square part in the middle will probably need draft added.
The draft will need to match the parting you make for the wings. Its not centered on the parting of the wings so draft will need to be constructed from where it intersects the parting. One side will have more than the other.


Hello Jim,

I’ve tried to trace your steps but this is really next level. Just brilliant the way you made that mould.
Could you maybe make a video of how you made this exactly? Because I need a 2 part mould of the whole thing. I can pay you if you want for your time. I hope to hear from you soon.


I don’t know how to make a video. Here is a file showing the steps.
hydrofoil_mold2.3dm (565.2 KB)

I didn’t have all the original curves so this is not exactly the same as before.

The commands I used:

silhouette (from top view)
split by isocurve
Sweep1 (roadlike top style)
extrude planar curve

The Help file has good information if you are not familiar with any commands.


Hello Jim. I’m sorry to bother you again but it is still abit unclear for me

The redline indicates how I would like to cut the whole part.

In the beginning you said that the first thing I need is the silhouette from top view (the redlines). Then I have to use splitsrf to split. What what exactly am I supposed to split? Is the object I have to split the whole thing itself? And then the command splitsrf asks me what the cutting objects are… You mentioned that I shouldn’t split with the silhouette ( the red lines). What are my cutting objects? Hydrofoil 5 moulds.3dm (1.8 MB)


Yes the red curves look good for the parting.
Keep in mind your part is symetrical so you only need to build half the mold. The other half you can create as a last step using Mirror command. The surface in the middle can be extracted and after mirroring the two halves should join into a closed polysurface.

SplitSrf only works correctly if you select a surface. This is a bug that they stubbornly refuse to fix. You won’t be given the option to split by isocurve unless you have a surface selected and it won’t let you select the surface that you want if it is joined to another surface. It is a Catch 22 but we have to live with it. Rhino is a very capable program, but you have to learn to navigate around the booby traps.

The surface that you want to split is the wing on one side. You need to explode or use ExtractSrf to separate the wing so that you can select it for split by isocurves. When you split by isocurve you don;t use any cutting object. The split follows the parameterization of the surface. The silhouette curve lands close enough to a knot line so you click on that using knot or int
Osnap. You only have to split the leading edge of the foil,that’s enough to split it into two pieces.

The red curve you have drawn for the square part in the center looks about right for constructing the square part with draft. You will want to use planar curves and ExtrudeCrvTapered to build the center part with draft… You don’t want to use ExtrudeCrvTapered with nonplanar curves if you can avoid it.

The best way to make the square part with draft is to make i all drafted surfaces without fillets and them add fillets after.


I assume you want to maintain the same relative dimensions all the way to the tip.and eventually make an RTV silicone mold. The first problem is that none of your curves meet at their ends. The leading edge does not meet with the split line (red) for the foil. Build the foil on the XY plane and then add the anhedrel, mounts, etc. I cleaned up the intersections and moved the original foil to several locations along the outline, scaled them to length, and used the patch command to create the top and bottom surfaces and trimmed them to the leading and trailing edges… Took about 5 minutes. Here’s a couple of screen shots to give you an idea of how I did it.

Unless you have access to a 5 axis CNC mill I suggest you produce a master with 3D printing. I use Shapeways for all my masters.


Hello Hacketet. I do indeed have a 5 axis cnc machine available but i want to cnc the mould itself.
This was the thing i was trying to achieve

Hydrofoil 4 (1) (1).3dm (2.2 MB)


Looks good – hope my comment helped.


I have another problem. My last mould isnt as clean as I would like.

As seen on the picture, I’d like to cut a hole in my object. The red line indicates the outline. The gray arrow, indicates what i would like to delete. I tried project the lines on the object but that didnt work. I hope you know how to do this.
Hydrofoil 4 (1) (1).3dm (2.6 MB)


Hi Rik,

Move the red fin by 0.1mm towards the grey shape.

Explode the bottom grey shape and then select all the grey bits and trim out the hole.

Join the grey bits again.

Explode the red fin and then use the grey shape to trim the edges of the red part.

In the beginning this way of working will often lead you to the point where the original error/problem occurs and will aid you in building cleaner surfaces in future iterations.



I’m not clear on what you want to accomplish here. One guess is that you have two components here and you want to create a void in the gray one into which the foil will be inserted. I do things like this by placing a copy of the foil where I want it and then use the gray object to trim away the exterior portion of the foil. Then use the remaining foil surfaces to trim a hole in the gray object. You should be able to do this with the Boolean functions, but I’ve found them to be unpredictable.

If you are trying to join the red and gray objects and the area inside the red lines won’t trim, you need to look very closely at the edges of the red surfaces. If they were created with the patch command, it is not perfect and there are probably small areas where the top and bottom surfaces do not match Join the red surfaces and then use the command ShowEdges. Any naked edges will be highlighted. If you zoom WAY in you’ll see that the edges do not match – computers don’t understand “close enough” To fix these edges go to menu Surface/Edge Tools/ and use Join 2 Naked Edges. This will override the tolerances set in the options dialog – I usually use .0001” (or equivalent when drawing in mm) in the options dialog, but will usually be happy with .001”. Once you have plugged all the holes it should trim nicely.

You should be aware that Rhino doesn’t always make good trims because of tolerance problems. If you see a naked edge show up as a dot, use ZoomNaked and you may see a little triangular thing that shouldn’t be there. The best way I’ve found to handle this is to figure out what surface has the problem and then redraw it using another technique.

Finally, if this direct approach doesn’t work, use Curve/Curve from Object/Duplicate Edge, and use the resulting curves to make the trim. If the curves are not on the surface they will trim from your point of view, so it’s important that you are view is perpendicular to the surface or you might get a very strange looking hole.


The best way to analyze the problem is to use the Intersect command to find the intersection between objects you are trying to trim. Its best to work in parallel view mode so that you can zoom in to see what is going on.

There are 2 problems that are preventing you from doing what you are trying to do.

  1. After you remodeled your wing is no longer closed. On the trailing edge the top and bottom halves do not match. I used ShrinkTrimmedSrf on the top and bottom of the wing and then MatchSrf (set to position) to fix that.

  2. The parting split on the leading edge of the wing doesn’t match the parting split of the center object. That the main reason trim isn’t working. I modified the nose of the center and
    split it do that the parting matches the wing on both ends. (see file.)
    hydrofoilx.3dm (393.4 KB)

The magenta curve is the interestection of the center piece and the wing.
The green surfaces are the flat parting surfaces at the leading and trailing edge of the wing at the z level where the intersection curve meets the parting. The green curve is the parting split for the center piece.

With every thing lined up properly you can trim and join and mirror the parts to get what you want.

Note: you still don’t have draft on the center piece and that may be a problem for drawing the part from the mould. Also, you might consider modeling something this big with a looser file tolerance. A .001mm tolerance for something this big is unnecessary. It would be a lot easier if you work with a tolerance of .01mm.


I’m not entirely sure how you did this but it’s brilliant. You also mentioned I should make a draft from the center piece. What is a draft exactly? At the moment im incorperating the wing and centerpiece as part of the mould. The center part is good as is it I think?


When you make parts to be moulded ensure there are no undercuts, unless it’s planned and you’re using some sort of flexible mould, like silicone.

All vertical faces should have a slight angle (draft angle) so that the part will release from the mould easily.

Take a look at a plastic ice cube tray, the cubes aren’t perfectly square, they’re truncated to allow the ice to eject from the tray.

I’m guessing this will be a two part mould so that you can lay up the fin and main body so you want to ensure the split line is made in such away that the part will eject correctly: