This is a really good tutorial, nice and clear. Can you estimate how much this costs roughly? And how many casts you will get with that bag of dental plaster? It looks like fun to do and I am sure it is cost effective.
You can get different volumes of the silicone and resin but if you are just wanting to try it out I'd get a kilo of both silicone and resin. This'll set you back about £30. The release agent is a fiver I think, but if you've got silicone release spray or wd-40, that'll do. Even cooking oil would do but that's messy and I wouldn't advise it.
How many moulds from a kilo of silicone? You'd get eight of those moulds I've made there, two faced moulds for single mini's are about 50ml per side so you'd get five full moulds. A kilo of resin will give you quite a few of those barriers, I reckon you'd get twenty plus. It obviously depends on the size of the things you're casting.
The dental plaster would last an age. You get 25kg for £17 so you could get an absolute ton of things out of that. You can get smaller bags, so id try a five kg bag of plaster to make sure its suitable for the objects you're casting. The draw back is the time to set. It's about an hour for de-mould time I think, but resin is only 10 minutes. I'd keep the resin for mini's and use plaster for terrain.
You could mould some barrels, say six, then cast them in resin, then make another mould with the six originals and the resin ones to make a twelve barrel mould, then you would be able to make 18 barrels in one sitting using plaster. Since it takes so long to set up you might as well do them in bulk lol!
But this would use up your silicone quite quickly. But once you have a working mould it'll last for ages. If you only use plaster you'd get infinite casts from it with proper care of the mould. I'm not sure how many resin casts you'd get before you notice major mould degradation. My most used one is my barrel mould that I've only used resin in and I've gotten about ten casts so far with no signs of damage. Proper care will always extend the life of the mould so it's hard to quantify.
To make the mould last as long as possible you should always use release agent between casts, and when finished give the mould a scrub with warm soapy water to remove any residue of silicone or the chemicals produced by the reaction. These things will attack the mould and shorten its life.
That's an excellent tutorial. I particularly like the idea of using Lego for the casting chamber. Wish I'd thought of that before casting my ice walls. They ended up bulging and looking like large butt-plugs because I didn't reinforce the walls, causing the weight of the liquid resin to push out the the sides of the mould. We learn from our mistakes!
Looking forward to seeing how you do the two-part moulds!
Mmmm butt plugs. Tutorial please!
I'm working on the two parter at the minute, just waiting on things setting. I've written it up, just need to format it. Hopefully get it finished by the weekend, then ill do the casting tutorials. Joy-gasm!
Two part moulds are a little more labour intensive than open faced moulds. We have to consider a few more mechanics of the casting process to make a successful mould.
The first, and most critical, provision we need to provide is a way to line the two parts of the mould up consistently. If the mould is off centre by even 1mm, the cast will look terrible. The way we ensure the correct alignment of the faces is by using 'keys'. These are simply a device that will only allow the two faces to engage in the correct way, keeping the mould lined up perfectly. They are absolutely essential to the success of your mould, but are very easy to provide. I will show you how to do this later in the tutorial.
Because the shape of the item we are moulding is usually more complex than in open faced casting, we may need to provide vents for air to escape from the extremities of the object. If we don't provide adequate provision for air to escape, the resulting air bubbles will prevent the casting material from filling the mould completely, resulting in a write off cast. Think of the way the casting material will be filling up from the bottom of the mould; it will be displacing the air within, so think of anywhere the air could get trapped. This is where we will need to provide a means of allowing the air to escape.
And don't be tempted to try to stick to many things into the mould. Keep it simple. You multiply the chances of things going wrong if you start to crowd the mould. I'd recommend doing one item per mould. If its weapons you're doing, you can add more in obviously, but for figures I'd do them individually.
One more thing about weapons; remember the shape the old sprues used to come in? A central trunk of white metal with the weapons branching off like a tree? That's the set up I'd go for if I was doing weapons.
These are the main considerations for two part moulds over open faced moulds. Particularly with 'keying' the moulds, the success of the mould is entirely dependant on it. Venting is a bit more forgiving, as if the venting we initially provided when making the mould turns out to be inadequate after the first cast, it's easy to widen the vent channels with a hobby knife. Even if we forget to put them in whilst moulding we can easily supply them after the fact.
I like to mould my minis with the base on them. It gives a bit of an indication that the mould is filled adequately when the resin starts to fill the space where the base would be without it pouring all over the place. Obviously if you use scenic bases you can attach the mini to this and when you cast it, it will come out with the base already attached. Handy!
Obviously, I didn't actually take a mould of this mini- that's copyright infringement. The photos are purely instructional to show how to bed something into the casting chamber and provide the required mechanisms for keying and venting.
So without further ado, let's get stuck into it.
What you need-
Lego for the casting chamber
A base for the Lego
Some plasticine or modelling clay
Cups and containers for measuring and pouring
Silicone tipped spatulas for stirring
First of all, build the first two rows of your casting chamber. Make it big enough to have a gap of about half an inch around the mini. Then push in your plasticine/ clay. You can level it by pushing down on the plasticine with a piece of card or something, but it's not essential. As long as its roughly smooth and level we will be okay.
Now sink the mini halfway into the clay. Try to line it up so every piece of the reverse of the mini is in contact with the plasticine, but every part of the front of the mini is sitting proud of the plasticine. This will prevent silicone from seeping around the back of the model, and mean that both faces will have an imprint of a full side of the mini. If any part of the mini gets completely encased in one side of the mould, you won't be able to free the original. You can take some plasticine out if you need to. In this example, the leading leg is too deep in the plasticine so i will remove some. You can vary the minis depth and angle any way you want, just make sure you keep the above guidance in mind. And be aware of the feet, ankles in particular. They are easy to get sunk into the plasticine and trap the mini. Remember though that the mould will be quite flexible so small overhangs aren't a massive problem.
Now figure out if and where you need to supply vents. The ends of weapons that are sticking out tend to be the most likely place you'll need to provide vents. Sink a toothpick into the clay so it creates a channel from the tip of the mini to the edge of the mould.
Now we provide the 'key' for the faces to line up. Using the end of a pencil, thick brush, or any other shape, press into the plasticine in various places around the mini. Put plenty in, you can't have too many, but you can have too few. We're not pressing all the way down to the bottom of the chamber, were just creating littles pits that will help the two sides locate consistently.
Next, use your clay shaper, sculpter or toothpick to press the plasticine flush against the outline of the mini. This will help to eliminate unsightly mould lines. If we don't do this you'll end up with a MASSIVE line around your mini, ruining some of the detail, particularly the face. Just press the plasticine gently so you can't see a gap around the mini. You don't need to smooth the plasticine into the mini, just close the gap.
Build up the rest of the casting chamber. Another two rows should suffice.
Mix up your silicone. 50mls will be enough. Ill be using a mix ratio of 100:4 to make a relatively stiff mould. I had done a 100:2 ratio but it took about three days to set fully. If you live somewhere warm and humid it'll set quicker, but the guidelines for setting times are calculated at a constant temperature of about 25•, and its never been that warm in the north east of England since the late Cretaceous period. Remember to always pour silicone in a long, thin stream to eliminate as much aeration as possible.
Give the mini a good coating of silicone. This will ensure all of the detail is captured. The bristles of the brush get into the recesses of the detail, knocking the air out of it and coating it with silicone. Don't be shy, slap it on.
With that done, we can pour the silicone into the mould. Use a long, thin stream, starting in a corner of the casting chamber. Don't pour directly over the mini, let the silicone flow over it naturally. This will displace the air leaving no air pockets.
Now that that's done, we wait. Let the silicone set-up over night. DON'T TOUCH. You'll distort the mould.
12 hours has passed. Stick the mould in the fridge for fifteen minutes. This will chill the plasticine making it stick to itself better, allowing you to remove it without it leaving a coating all over the mould. If you are using modelling clay you can ignore this step (I think).
Disassemble the chamber and peel the plasticine off the mould. Don't worry if the plasticine pulls the mini or vent sticks out with it, we can easily re-locate them by flexing the mould a little as we insert it back into its place. If there are any signs of the silicone seeping past the mini, cut them off. The face should have no little raggy ends sticking out. But don't cut off the keying!
If you have any plasticine stuck into the details of the model you can remove this with some terps/alcohol. My release agent is basically paraffin so brushing this over takes the plasticine out. Bear in mind though that release agent will leave a layer of wax on the mini- to much can affect the capture of detail.
Build up the chamber again and insert the mould with the minis and vents facing up, ensuring the mini is fully engaged with its mould.
Now give the whole of this side a liberal coating of release agent, avoiding the mini as much as possible. When that dries give it another. Then when that dries, give it one more. It's important that the silicone is well coated, as the wet silicone of the second face will bind with the first if there is no barrier between the two. And remember to coat the key pieces.
Mix up another batch of silicone to the same ratio as before, coat the mini with a brush as before and pour the silicone in using the same technique as before.
Leave it another 12 hours.
Now lets have a look and see what we've got. Disassemble the chamber and remove the block of silicone. Peel the two sides away. You shouldn't use too much force to do this. The silicone may still setting up so it is possible to deform the mould if you are too heavy handed. You can use a knife to cut along the mould line if you need to, but if you haven't used enough release agent the mould will be unusable. Cut it up, retrieve the mini, and start again. If it does split, congratulations! You've got a two part mould! You little fucking beauty, you!
Don't know if the information helps but fibreglass resin doesnt stick to most types of plastic ice-cream containers. This might be true for silicone/resin mixes too. Glad wrap also makes a great 'releasing agent' if detail is not much of a concern.
Do you fancy having a crack like? It's a laugh. I've got all sorts of crap moulded, I've got some pine detailing strip from homebase that's going to be getting done soon, vents, air con units, a cool little bone shrine thing I liberated from the top of a missionaries staff, rakes of shit. I'm like a silicone-ejaculating magpie. I blast that shit over everything!