RTV Moulding & Casting in Resin

This is a reprint of a 'How I do it' moulding walkthrough that Simon from BoltAction posted to Frothers- cleaned up and archived here for posterity. As he states in the course of the description- there are many ways to do this, this is just what works for him.

Making an RTV mould

Stuff you need to make an RTV mould: masters, a tile or bit of glass, double sided tape, scalpel, lego blocks and a pot.

The mould taking shape, build one corner first and sit the master on the tape, then build the walls around the master. Here you see 900 quids worth of King Tiger, a gate and a humber armoured car body.

Build the walls around the master keeping it tight to the model. You want enough silicone in the wall so it's rigid enough to stay upright, but not too much that it's hard to get the casting out. Experimentation is the key. build up so you go one brick level higher than the highest point. They're all in there nice and snug - the tape is NEC approved, meaning it comes up without leaving a residue. You can see on the right I am using a little card box as a mould box, for some turrets and little trucks I made. Now off out in to the potting shed for the dirty work.

Here is the mould tray lined up on the bench - you can see all sorts of shite here that is useful to the resin caster - stirring sticks, old pots, scales, mould release, dust, pokey things, MG distributor, coffee mug, old moulds, paint stripper, etc, etc - keep as much crap as you can, it helps. Rubber or vinyl gloves help too, especially for the resin.

Here's the mould release, spray this all over your masters and let it dry off. It helps to get it out, especially if you use pressure.

This is the resin and catalyst - mix it up in an old pot. Mix it well and use enough catalyst, I always buy two pots of catalyst and use waaay too much. There's nothing worse than finding a wet patch in the middle of a mould. Mix it well, scrape the sides and bottom, mix it well. Did I mention - mix it well!

Here I am pouring the RTV in. Start at the top end and prop up the tray so the RTV flows from one end to the other taking all the big fat bubbles with it. Pour in the longest thinnest stream you can to pop as many bubbles as you can.

Here's the whole thing filled up - see the bubbles on the surface. I run a proddy thing around the edges of the master a few times to push more bubbles around and hopefully up. Fill to about 4mm above the highest point of the master.

Slap it in the pressure pot - if you have no pot then you need to take a lot more care when filling the mould box up with RTV. You need to push RTV into all the crevises that will hold bubbles and get the bubbles out. Run it around, in and out, tipping the mould box around, fill slowly. It used to take me about an hour to get a single track mould for the T34/85, as I had to push RTV into every hole in every wheel - now I pop it in the pot and it's done in a few hours. The pressure pot does not make bubbles dissapear unless they have somewhere to go. If there's a bubble trapped under a concave area it will get a bit smaller, but it will stay there. Ideally using a vaccum is the best solution for making moulds - but you do run into problems with frothing (see what I did there?) and you need to make your mould box a lot bigger to accomodate the frothing RTV before it settles back down.

Here's the compressor pumping away - set to about 40 PSI and leave it connected to the pot but off overnight - try to get the pot done up as tight as you can so the pressure is there in the morning.

Here's the pressure gauge on the pot. In the morning it's all set and finished, then comes the decanting and moulding.... Until then it's time for me to grab an ouzo and paint some Poles!

The moulds are now set and turned out - here the card boxes are ripped to shreds and the contents disgorged.

Here I've started to dismantle the lego blocks and pulled the small masters out of the silicone.

Here's the humber armoured car and the five bar gate out of the lego tomb. You can see the base of the KT, notice the plasticard wall that creates the hole for pouring resin in and gives a nice sharp lower edge to the body - also notice the curved corners to stop the mould tearing. The gate will need to be cut out of the RTV with a scalpel because it has holes between the cross bars that will cross link.

The masters out of the mould - bit of a dull shot. You can see I snapped a mudguard off of the Humber. Tut tut.

Here's the moulds ready for casting - notice the stuff all to hand. Resin , scales, pot, 2.5mm allen keys ( a great prodder tool) and gloves. On the left is a selection of my gravity moulds for simple stuff like walls and fences. I always make sure I have a few empty simple moulds kicking around to dump the dregs of the resin in when you've popped the moulds in the pressure pot. I have a single tank trap mould to get those last little drips out - waste not, want not!

OK< here's the mould poured - tried to get a pic of the pouring but it's to fast to piss about with a camera. You need to know your moulds and how to pour into them to prevent bubbles. I tend to do the first cast by just pouring in resin and seeing what happens when it's cured. This tends to show where the bubbles will collect. In the case of the KT, there were only two tiny bubbles in the very front corners of the upper deck - now I know where to push my prodder! You fill up to just under the lip of the entry hole - the miniscus takes care of the rest and you end up with no waste and nothing to sand off. SOme of my moulds need to be stood on their ends when you pour resin in - the 251 is a prime example. You stand it on the nose, pour in to the nose and then lay it down as it fills up - means you get no trapped air in the front section. Vehicles with thin mudguards need a 2.5mm allen key run around them to remove bubbles. Finally I give the whole thing a couple of squeezes and twists before popping in the pot.

And there ya go - twenty minutes later (shorter in the summer) a king tiger pops out - perfect in every detail. resin goes through three different stages when it cures. Firstly it gets like toffee - bend it slow and it bends, push it quick and it snaps. Then it hits a stage where it is VERY brittle, it just crumbles when you stress it, finally it cures fully and is very durable. Don't try to de-mould it when it's brittle. I pop my dregs moulds first and if they crack I leave the good stuff for a while. You can get the moulds out of the pot after ten minutes, they're cured as much as they need in the pressure.

That's about it!

Some errata from the original thread regarding materials and design:

We tend to use plasticard or perspex or foamex for our mould bases and walls. Plasticard and perspex weld together without leaks if you use solvent adhesives like Polyweld or Dichlo. Foamex glues like shit to an army blanket with superglue. Then again we do have all these materials constantly laying about, so I can see how lego would work for those of you without a full scale workshop

As has already been said, painting silicone/RTV onto your masters is a must if you don't have pressure of vacc systems. Use a cheapish brush and clean it afterwards with cellulose thinners if you want to avoid slinging every brush you use.

Axson (as illustrated) produce a good quality range of RTVs and urethanes, as do Alchemie, Mouldlife and Tiranti. We've dealt with them all in the past and never had problems, so go with price and ease of contact and you shouldn't go far wrong.

If you're in the USA you can get a pretty good starter set from Alumilite - I used to use this RTV for scalextrics tyres - it has a low tear strength though.

I use Axson cos the rep called around and dropped me some samples off. I tend to go the way of as many free samples when I start anything.

With WWII vehicles the moulds don't last that long - they are not designed for casting in one piece - OK there are cast hulls and turrets about, but they weren't cast with all the extra crap in place, like we do. SO moulds tend to rip and tear. If you're doing sci-fi or fantasy stuff then you can design with DFM in mind. For some of the best vehicles designed with casting in mind take a look at Old Crow - superb looking vehicles and superbly designed for casting.

I have found after talking to a number of casters that the techniques used for buildings and large pieces tend to be quite different from those for small pieces.

What I'm showing here is not really that good for stuff that's much bigger than a big tank. You need to start part filling and adding adding voids if you're doing stuff like buildings - our two Soviet Izbas are made with a different mould technique, not that different, but different.

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