Trobuleshooting: two part mould with plaster shell

[follox]

Hello! Sorry to introduce myself with a question - I swear if I ever read anything I'm actually qualified to answer I'll give it a shot! I'm quite good at failing at casting, but I'm a bit in love with it, which makes it a bit expensive and frustrating... SO I wanted to ask for pointers before I go too far with this one!

This is the latest problem I set myself - it's basically a u-shaped snake (the side you don't see is similar to the one you do). A poured box mould would use more latex than I can really justify in that negative space so having stared at some tutorials I think I should use a split mould with a plaster shell to support it. It is made of cured super sculpey and I've built up potters clay to what I think is the mid line all the way round.

This is what I plan to do - can somebody with some experience please critique it and tell me what I'm certainly doing wrong and how to avoid it?!

1) paint on a thin layer of latex (currently have Tiranti T20)
2) when cured, slap on some thicker layers until it's bulked up
3) build up a wider rim of clay and then layer up some plaster on top of the latex
4) flip the lot, dig out the clay and clean it off as best I can
5) blast the lot with mould release or vaseline
6) repeat painting then layering silicone
7) repeat plaster case
8) remove the original
9) ...profit

As I was typing that I realise I've not made any pouring channels or air vents in the clay at this first stage - see what I mean?! I'd really appreciate any advice along those lines that might save the day! Enormous gratitude if you can help in any way

[follox]

Added 'holes', if it'll help...

[mangozac]

Hi Follox, welcome to RA!

It sounds like you've got a pretty good handle on the situation! I can't really suggest anything different - even your gating and venting look good. I'd pour into the head gate and let the other one be a vent.

Have you used latex before? I've got no experience or advice about it. Make sure you've got some kind of keys for the plaster to grab the latex with. Also keys in the plaster do the two halves align properly.

Please keep us posted!

[RageofAchilles]

Hi, I take it that you mean T20 silicone, not latex?
I have bought many products from tiranti, I started with them back in 1990. If I were you I would also pick up some Thixo if you don't already have some. When doing a mould like this thixo will allow you to add a very thick layer in one go, as it turns the silicone into a spreadable paste.

http://www.tiranti.co.uk/EdgeImpactShop ... +Additive+

a side note for anyone thinking of using latex and Urethane resin- They stick to each other without tonnes of mould release and the exothermic reaction of Polyurethane curing seriously damages latex

you'll find another recent post on this forum about making a similar mould for a helmet visor, you might want to check it out

[follox]

[mangozac]

Hang on, you're not using oil-based clay? The clay will dry out and go hard? That is not ideal and can potentially cause issue with silicones (is T20 addition/platinum or condensation/tin cure?). Sulphur-free oil-based clay is the ideal medium of choice.

[follox]

Just a potters clay, as recommended by someone whose brain I picked when I threw a hissy fit at the sticky horrible plasticine I was using which I couldn't clean off the models. No good?

(it didn't cause any issues when I used it to make walls for my last (rubbish) split moulds - but it would be nice to use something that doesn't dry out I guess, if you have any suggestions?)

[follox]

[RageofAchilles]

if you don't use Thixo most of the silicone will run down the sides and pool around your sculpt, even multiple layers will leave the top of the sculpt as the weakest part of the mould. Thixo lets you slap great blobs of silicone on an inch thick at a time.

by all means go ahead and paint on your first layer, but it really would be better on dry clay with a wax release agent, wet or damp clay will muck about with the chemical reaction curing the silicone and you may end up with an expensive gungy mess.

I used to have trouble with Tiranti's RTV101 silicone when I used it with a sulphur based plasticene. The silicone in contact with the plasticene wouldn't cure. So to be safe mix a little T20 and apply it to a piece of clay, hopefully you wont have the same problem.

As for your gate issue, so long as the gate and the vent are at the same height on the finished mould when your pouring , the resin will naturally level out

[mangozac]

Potters' clay is a terrible idea. Just get some quality, sulphur-free plastecine. It must have been total garbage plastecine you've used in the past - good stuff is easy to work with!

As RageOfAchilles said, because the top of the gate and vent are at the same level the resin will level out and everything will be fine.

[follox]

Ok see this is where I was confused - because my gate and my vents in that picture aren't anywhere near the same height. The two at the sides touch the floor and the one on the nose is just waving around. When I remake it I need the nose tube to reach to the floor I guess - which means the final mould will look something like an inverted 'W' (but with each tube the same length)?

One of you is telling me dry clay, the other plasticine - help! Is it a terminology mix-up? I can't see how my clay surround drying will help, as I'll have to crack it off when I come to do the other side.

I guess the potter's clay is out, then - I'll see if I can dismantle this without damaging the snake. Would something like this be a right to use?

Thanks for your help. This is exactly why I wanted to troubleshoot before I did anything stupid I couldn't undo

[mangozac]

OK yeah once your tubes are the same length it will be fine.

RageOfAchilles isn't actually telling you to use potter's clay - he's just saying that if you do you really wont it to be dry so that it doesn't cause any issues with the silicone chemical reaction. The plastecine clay you linked to will be perfect.

[RageofAchilles]

[follox]

Gotcha, thanks so much for clarifying I hate feeling so dim - one day I want to nail this though, because I love the process (well, the bits that work, when they work. If they work).

I'll check that vid when I'm not (ahem) AT work.

This is the other plasticine I was considering, but there's no weight mentioned - besides, if you reckon the Tiranti stuff is the business I may as well get that.

[mangozac]