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):
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.
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.
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:
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: