Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

Super easy question for those experienced I presume.

I've seen many tutorials where they utilise clay to embed a model at first, cast the other half, the flip, remove the clay and then cast the other half over the top of the silicone (with appropriate release/separating agent sprayed first).

The specific question I have is what is the use of the clay? Could I not just attach a small vent to the model at the bottom for pouring resin later, ensure the model is level and cast the first half of the silicone (even though some will be filling up from the bottom and thus underneath the model used for casting?) wait for it to fully cure and casting the other layer directly over the top?

My first assumption is that if there is an issue it can be because the model floats on the silicone however that doesn't seem to happen in the video's I've watched. To avoid this, would weighing the piece being casted down with a small weight on the top alleviate this problem?

Anyone with a super quick answer to this question and explanation why, it would be appreciated.

Cheers in advance

EDIT: FYI - ideally looking to AVOID using clay to be honest, I will do if I have to but I'd rather not, so if I can cast without then great, alternatively, are their any other options other than clay?

So the question is: why can't we cast the bottom mold first, thereby removing the need to use clay to set up a scene.

Not all 2 part mold yields perfectly planar arrangements because not all shapes are made the same. If you were to rely on gravity alone to get the perfect mold, then you would need to ensure all of the pieces are perfectly aligned along the z-axis, down the to last small bits. You will always yield a mold whose internal face is planar, and not necessarily a mold that will, say behave like a mold (have many different shapes & sizes).

Using the clay allows you to build "hills" and "mounds" that wrap around your pieces better, which allows you to design where the mold lines will exist.

You can try and setting up a scene without clay, and you will instantly understand, hands on, on exactly why you use clay for making 2 piece molds.

FWIW, I don't like working with plastic clays either, but it works.

If you can share with us exactly why you don't feel like using clay, maybe we can help you find an alternative.

 skchsan wrote:
So the question is: why can't we cast the bottom mold first, thereby removing the need to use clay to set up a scene.

Not all 2 part mold yields perfectly planar arrangements because not all shapes are made the same. If you were to rely on gravity alone to get the perfect mold, then you would need to ensure all of the pieces are perfectly aligned along the z-axis, down the to last small bits. You will always yield a mold whose internal face is planar, and not necessarily a mold that will, say behave like a mold (have many different shapes & sizes).

Using the clay allows you to build "hills" and "mounds" that wrap around your pieces better, which allows you to design where the mold lines will exist.

You can try and setting up a scene without clay, and you will instantly understand, hands on, on exactly why you use clay for making 2 piece molds.

FWIW, I don't like working with plastic clays either, but it works.

If you can share with us exactly why you don't feel like using clay, maybe we can help you find an alternative.

Clay is messy and I have a small working space that I require to be multifunctional (It's also my office desk) I'd 100% prefer not to throw clay into the mix with that tbh

Fine, I take your advice, I'll use clay, need to figure out a place to do it though... Or are there alternatives? Playdough for example?

Playdough works. What kind of clay are you using? The recommended plastic clay (the ones you bake cure) is a little bit messy (like get film-y thing all over your fingers messy) but not like "I dropped a full can of paint" messy.

Alternatively, you can use bluestuff/equivalent with a pot of hot water to the side. If you don't mind 1-time use, I've used model magic to form a few molds, but it cures pretty solid for what it is - I broke a few fine detail pieces (like swords) while pulling it out of the negative, but depending on your shapes it might work for you.

I have cast quite a number of items and figures with silicone to be used for making resin (polyurethane) casts. I have always found making a mold of the entire mini and then slicing along outer edges to split the mold in half and staggering the cut along the edges makes matching the two halves together easy enough. In the best case scenario I would leave the mold not cut all the way in two pieces, so it goes right together easy.

In looking over the process you describe, I think this is a much better way to get a cleaner mold, so I think i will definitely try that upon the next project!

Keep in mind, as long as you place pour spouts (or top venting holes) for any areas where air cannot escape as you pour the resin and give it a good vibration treatment for the resin to fill in all the holes, your castings come out all pretty well.

However, how many casts you will get really depends on the detail and how careful you are with removing the cast from the mold. I have done some small detailed minis and only got 3 or 4, but other fairly large molds where I have done dozens and the mold is still in good shape. in any case, this process is not really for mass production.

endlesswaltz123 wrote:

Clay is messy and I have a small working space that I require to be multifunctional (It's also my office desk) I'd 100% prefer not to throw clay into the mix with that tbh

Fine, I take your advice, I'll use clay, need to figure out a place to do it though... Or are there alternatives? Playdough for example?

Play dough shrinks when it dries out, and the silicon curing can pull moisture out of anything it touches, so this might be problematic.

Plasticene is better, as it's oil-based and doesn't dry out. If you don't want to mess about with the sometimes rather messy stuff sold for kids, you can buy packs of plasticene specifically formulated for modelers. It's (from the stuff I've tried) firmer and less oily.

It's been a while since I worked for a company that specialized in resin casting but I do remember we didn't use the likes of plasticine or clay in our molds. There is a product called modelling wax which we used for a multitude of things, mostly master model making. Unlike those mentioned it didn't get sticky like plasticine or shrink, it becomes workable with heat from your hands and when it cools it holds its shape. There are a few types; the tan is the softest and easiest to work with, also an oxide version which is very solid and closer to plasticine, which you can make more solid(it doesn't really need it to be honest)if you put it in the freezer.

We bought it from a company in England, Alex Tiranti, who specialize in working with resin. There is another company, Tomps, who do a lot of work for the film industry and the likes of museums(props and reproduction of fragile pieces).

Not entirely on topic but I hope it helps.

From what I remember, you have to be careful which clays you use for two part silicone moulds. Ones with sulfur in them can prevent the silicone from setting.

I usually use Oyumaru or silicone putty, which is thick enough that you don't pour it at all, thus it doesn't need any clay. Just press the part in half way, make your key indents, let cure, dust with starch and apply more putty for the second half.

^^ this.

Some "clay" (plasticene) types have sulphur compounds, and some others have various hydrocarbon chains that prevent some silicone rubbers from fully curing.

That's why specialist casting and moulding suppliers have their preferred "brands" of the stuff for the job. They have been shown to have few if any issues. Products like "Klean Klay" and the like.

Play dough can work. Yes, it shrinks if it dries out, but if you have nothing else, it will work for a one off. The bubblegum smell also partly counters the smells of curing silicone rubber, too.


As for mould release agents. There are a couple of aerosol spray types that are good. You can also use talc. Common unscented talc is available from mineral suppliers (it's used as an ingredient in the clay "slip" for ceramic casting) - but the regular 'baby powder' stuff (asbestos free**, if you can get it) also works. A light dusting. Shake some in, shake it around the mould, dump the excess out.

Water is the enemy of polyurethane resins. It causes the resin to foam up. the talc absorbs the water and reduces the incidence of surface bubbling.

** J&J in the USA got into a spot of bother after it was discovered that their flagship "baby powder" was contaminated by asbestos particulates. There's no real "safe" amount of asbestos, especially for small lungs.

https://www.smooth-on.com/
Has all your needs covered along w videos showing you how....


I'm not affiliated with them just have used their stuff before and was happy w the results.

I've been using plain old corn starch for my release agent and so far, can't complain. Big upside is that it's readily available and you'd be hard pressed to find a safer material.