Tutorial: Mold Making I

At the moment I’m pouring molds of various sizes for several projects and thought it would be a good opportunity to put a tutorial together for reference. Particularly since several visitors have asked me about mold making in the past.

This tutorial focusses on ‘garage’ one sided mold making with off-the-shelf RTV and casting with resin/plaster products. Using one-sided molds you can create spectacular terrain and modelling details to improve your gaming table.

Two sided molding is possible with these techniques but I find the results are often disappointing. For high-detail two sided molding you really need professional and expensive pressure vulcanisers and spin casters imho.

I learned mold making simply through Googling the web and trial and error. Hopefully this tutorial will save you some expensive mistakes and be at least as useful as others already out there.

As I live in New Zealand the mold making products I use are fairly specific to the Australian market. My materials are all supplied by TopMark in Auckland who are excellent to deal with if you happen to be a NZ mold maker.

For our US visitors I’d recommend checking out the Hirst Arts site mold making page and/or related forums for US product suggestions.

Create Your Masters

The first step to creating a mold is having something you want to copy in plaster or resin. You should expend a considerable amount of time creating your masters and you should aim to make them as sturdy and as highly detailed as you can.

Make them sturdy because it’s always nice to keep your masters after molding them for future modification and possible remolding. Molds created with most RTV products do not have an infinite life so being able to create additional molds from your master can definitely be useful. Particularly if you’re a hobbyist seller like myself. In fact the pieces I’m molding here have already been remolded several times.

Make them highly detailed because once you have successfully created a mold you can easily reproduce them, producing tens or hundreds of casts with ease (and a little time). So it’s worth applying all your modelling skills and equipment in the creation of a master worthy of molding.

A final word on masters: Copyright applies to all original works. It’s illegal to copy and sell work derived from other individuals/companies without their explicit contractual permission. Don’t do it. Create your own masters. Take it from me it’s infinitely more satisfying molding your own work.

Making the Mold Pour Box

1. Assemble the materials for your mold pour box.

The pour box is a watertight container that holds the RTV around your masters while it sets. I build all my pour boxes from 3mm MDF, foam card offcuts, masking tape and Klean Klay modelling clay.

MDF (medium density fibreboard) is a pressed wood product with a fine grain. I use 3mm MDF for most of my terrain basing so have a lot of smaller offcuts left which I use for forming the base of the pour box.

MDF is good for the base because it’s stiff and flat. The base will form the open face of the final mold so I want it as flat as possible as I use a ‘smooth cover’ technique of casting. This is where a flat piece of plastic or glass is laid over a poured mold to ensure the cast pieces have flat bottoms.

2. Arrange your masters for molding.

RTV is expensive so you want to arrange your masters with minimal wasted space. However you also need to leave at least 4-5mm (or 1/6th of an inch) between each piece inside the mold and a little more for the external walls, say 5-6mm. Set RTV is quite flexible, so you need relatively thick internal and external walls to ensure your cast pieces aren’t deformed by the walls bowing when you pour resin or plaster into the mold.

Obviously the best thickness to use varies depending on the set strength of the RTV product you’re using and these estimates may be a little generous. I notice molds from professionals like Bruce Hirst have 2-4mm internal mold walls.

Once you’re happy with the arrangement glue each master down with a small dab of superglue. It’s important the masters are firmly fixed to the MDF base. RTV is a dense liquid, so light masters can float up before the mold sets which in the worst case can leave you with a solid block of RTV you’ll have to cut your masters out of!

If you ever want to remold or modify your masters chances are you’ll want to get them off the base. A small dab of superglue can be broken by twisting the master off the MDF. However if you’re confident you won’t rework the masters then definitely glue them down solidly because it won’t hurt.

3. Assemble the pour box.

Foam card is ideal for this because it’s breakable foam sandwiched between two layers of thin card. You can cut through one layer of card and some of the foam while leaving the other layer of card intact and then fold it to form watertight corners.

Take a continuous strip of foam card that’s long enough to entirely surround your masters. Stand it on the base and cut the outer side (from the centre) of the foam card and fold it in towards the masters, so the intact layer of card forms a watertight inner corner. Rinse and repeat for as many corners as you need.

Don’t feel obliged to make square or rectangular pour boxes. Many of my molds are irregular shapes simply to avoid wasting Ultrasil filling empty corners. The only important thing is there should be sufficient space between your masters and the foam card so the RTV will form a decent external mold wall.

Once you’ve cut the foam card into a shape you’re happy with, tape it down to the MDF using strips of masking tape around the outside of the box. I usually also tape the final edge of the foam wall shut too.

Enough masking tape would probably be sufficient to stop the RTV from escaping from the pour box but it’s pretty viscous stuff and can flow out or under the smallest gap. For this reason I also run a bead of Klean Klay around the entire pour box, making doubly sure the edge of the foam card wall is nicely sealed down to the MDF base and the final open edge is completely sealed.

Klean Klay is a reuseable modelling clay for use with RTV molding. Normal plasticene and some other modelling clays contains sulphur which will stop any RTV that contacts it from curing properly. This is a bad thing. You can also create temporary masters from Klean Klay, as well as use it for sealing off half of a master for two sided molding, so it’s handy stuff to have around. It’s pretty cheap too and you should be able to pick it up from any supplier that also stocks RTV molding kits.

Right! That’s your pour box made, next comes estimating the amount of RTV you need and pouring the mold. This is covered in the following post

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