Advice on mould making with RTV silicone


New Member
Hi all,

I'm in the process of making another couple of Portal guns, following pretty much the same process as I did for this one.

This time, however, my intention is to make moulds of the front grip and rear shell using RTV silicone then cast the two components for each gun using polyurethane resin. This is my first experience of silicone mould making. Once the silicone mould is complete, I will create a fibreglass mother mould.

I've started slathering the silicone onto the front grip 'master' (thin coat first to make sure all features were covered properly, then one coat of thickened silicone) but I've realised I might be going about it slightly wrong, and I would like an opinion or two on the matter!

Because the entire outer surface of the front grip needs to be replicated in PU resin, it all needs covering with silicone. Based on the idea I need a two part mould, I made a 'wall' out of oil-based clay to allow me to separate the two silicone parts. Once the outer silicone section is build up enough, I will remove the clay wall, apply wax/petroleum jelly to the newly-exposed silicone and then start building up the layers of the second silicone section.

I think I've put the clay wall in the wrong place, though.

Although everything is nice and sturdy and easy to get to while there is a solid master underneath it, when I try and accurately re-form the two halves of the silicone mould to make a hollow shell, I don't think it will be easy due to where I've made the joint.


  1. Do I even need to make a two-part mold for this component? What if I just coat the whole front grip master in silicone, make a multi-part fibreglass mother mould, partially cut the silicone with a craft knife to extract the master, then cast with PU resin and expect to deal with some 'flashing' where the PU seeped into the silicone mould?
  2. Should I move the clay wall to the outer edge of the piece (red dotted line in last image)? As it stands, the outer silicone section (the one I've started on) stretches over the whole of the outside, over the lip and back up inside the part until it meets the clay wall.

Sorry for the long-winded post, but I really don't want to waste any silicone if I can help it!

Thanks for your time,


Don Z

New Member
Hi there!

You do not need to cut the silicone/ or make a two piece silicone at all.The silicone will be flexible enough to remove from the pattern and the castings no problem. I would recommend thickening up the silicone somewhat. I normally go for about 8mm thickness all over. On the last layer you should use washing up liquid on your hand to make a clean smooth finish on the silicone before it sets. You will need a multi-piece fibreglass mother mould. Probably in two pieces right down the middle. This also means you will need registration keys on either side of your silicone so that they lock back into the fiberglass when putting back together.


Sr Member
If you want to avoid the spikes on the silicone, dip your gloved hand into some rubbing alcohol, and smooth the silicone out.

Edit- just saw the previous comment. So its a "what he said"


Master Member
I agree with don, my own experience says put more silicone on to build up the wall thickness. you can just trim off the spikes with scissors before you put the next coat on.

when i mold smooth surfaced pieces like you have there i have a minimum of 10mm wall thickness
otherwise even if you think the your thin walled silicone is seated in the mother mold correctly you always get undulations which ruin the final product. if you want good molds you have to spend the pennies.
talking from my own agony!

as far as applying the thickened silicone, i would say apply it sooner than you did, you should be able to almost pour it onto your cast/mold, and while you move it around it will set up, with no spikes. if you apply it really thickend you always trap air bubbles which you don't want.

Last edited:


Sr Member
Congrats on your portal gun. It looks beautiful.

That said, i want to recommend going to YouTube and watching a few videos on mold making. You can find lots of information there.

I am not expert on portal guns, so I am a little unsure what this is. The first thing that comes to mind is that this is such a smooth surface, with no locking detail, that silicone is unnecessary, and a hard shell fiberglass tooling would be fine.

Being that you have already started the silicone, here are my only points: in order to get a good casting from this, registration will be key! By "registration" I mean fitting the two halves of the mold back together in the SAME orientation. (sorry if the is overstating the obvious). If you can cast several (silicone) registration keys prior to the final coat of silicone, when you brush on the final layer, place the keys around the surface, as silicone will adhere to silicone very well. These keys will register the silicone back into the jacket just fine.

Advice for the futute: selecting the parting line is the first step in any good mold. It will determine how effective the mold produces a part, and how much clean-up there will be to the part.
if I were to mold this model, i would start by mounting the model onto a sturdy board, and made the parting line where you have your red line drawn. It would divide the top and bottom right at the outer most edge. If the parting line were right at the corner, that would mean that cleaning up the flash would be as simple as taking a file to the two surfaces, where they meet at the corner, and done.
secondly, the parting surface should be no less than 1" (2 cm) in width. That would be 2cm for the silicone, and 2cm for the jacket, total of 4 cm. The parting line can be made of clay, as you have done. Clay is best smoothed witha tool, and not with your fingers. So getting (or making) a good rake is important. One can be made from a hacksaw blade in a pinch. It will give you a good straight edge to cut a good clean surface.

Sorry for more typing than you wanted, but those are my suggestions. Will you be making additional molds for this gun? If you would like to post pics and consult prior to starting, perhaps I can offer better advice for the future.

Good work on the gun. I look forward to seeing the finished effort.