Undercuts in (Silicone) Mould for creature bait

Hi everyone,
it is the first time that I want to make a bait and the second time I was using SubD.
The result is a model of a anomalocaris I want to pour (still WIP):

Does anyone have experiences or could give me some advices on the mould, the minimal thickness, etc?
Is that amount of undercuts possible with a silicone mold?
I found some references which seem to have undercuts in the mold(the middle one):

Draftangle of mine:

And of course the file:
20220220_Anomalacris.3dm (18.7 MB)

Thanks everyone for every comment and have a nice sunday!

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It will be very challenging to mold that, especially the (thin) overlapping spaces between the fins/wings and at the tail. I think you can make it a lot easier on yourself by simplifying that a bit and making sure it is largely an two piece up and down mold, like the examples you show (draft direction in Z)
So if possible straighten the two ‘antennae’ and insert some wire in the mold to bend it in shape after molding.

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Thanks for the reply @Gijs;
Sounds like the most practicable solution. Probably the antennae will hang down anyway.

Just out of curiosity, do you think the crab in the middle is also just cut by one horizontal plane passing the legs? Or maybe it is not a plane? Looks like the whole body was just in on of the halfs.

Hi Baris,

Back when I was a teenager I used to cast lead soldiers in silicone molds. The cut line was never planar: it followed the silhouette line. Don’t forget to include registration pegs in your silicone to make sure the mold parts align correctly.

Lead, being heavy, was good at filling the mold. I presume that you won’t be using lead in this health and safety era, so what are you casting in? You’ll need a more substantial sprue for resin and the like. The biggest problem with your model will be avoiding air pockets in all those protrusions: you’ll need a vent hole in the mold at the high point of each one.

Silicone comes in different grades of hardness: softer allows larger undercuts but won’t last for as many casts and can deform, so ensuring it is well supported is important.


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Hi @jeremy5 ,
thanks for the various inputs!

My thought are now: if I don’t have undercuts, I maybe could just print the mold in an SLA printer(?).
And try the undercut version with silicone:

I am not really the person that concerns about this, just for laziness I wouldn’t do it, I guess.

that’s a really good point, I wasn’t aware of!

Thanks again Jeremy!

I tried several things:

  • First try with tin-cure (condensation cure) silicone:

    The SLA printed positive form suffered from inhibition caused by the sulfur in the resin, so the silicon never cured completely.
    I made a UV-Chamber for the SLA to cure completely, since in Germany in February there is hardly any sun, and I am kind of impatient:

    I read in 3d print forums, that it should work very well. I would not confirm this, but it helps.
    -Second try, now with platinum-cure (addition cure) silicone:

    Due to missing vents, the wings did not get filled with material, when pouring the plastisol(just as @jeremy5 predicted):

    -Third try:
    I did not make a picture of the mould, this is how the positive looks like:

    it came out better, but still problems due to trapped air:

Then I decided to try printing directly the (negative) mold. Therefore, I laid the Anomalocaris nearly completely flat (as @Gijs proposed).

They came out completely, altought the vents did not print out, due to a modelling error, so I gave it a next try. I got confident with this technique, so I tried it with the full undercuts of the original model:

There is still a little air trapped in the tail part. Probably some more trying on material temperature or using a syringe could get rid of this.

I hope the fishes going to like it as I do!
Have a nice weekend everybody!