Making Moulds- Introduction
by cardyfreak
Making moulds of objects and casting them might seem like a complicated process but it is a lot easier than you think. Sure, you need to take a bit of care to get the best results but that is true of most things.
Having the correct tools to hand is probably the most important piece of advice I can give. Particularly when casting, time is of the essence. You don't want to be rummaging around for things at the critical moment when pouring silicone or resin so always make sure you have everything you need laid out in front of you neatly. When your mould tips over and resin starts pouring out all over your table, you'll be glad you can lay your hands on some kitchen towel without having to go rummaging through the kitchen!
The basics
Here is a list of some common terms and their definitions as I understand them. They might be incorrect, but I'm pretty sure they're close to what's right.
RTV silicone- Room Temperature Vulcanising silicone. This is a type of rubber. Most master and production moulds are made out of organic rubber. These are very hard wearing and are used to make many casts of a shape using low melting temperature alloys, traditionally pewter or other lead alloys. This is a bit excessive for what we need, so The Lord giveth RTV.
Also know as 'condensate cure silicone', RTV uses the moisture in the air and/or an additive catalyst to cure (vulcanise) the silicone rubber into a solid shape. Places with high humidity cure faster than ones with low humidity, and the exposed surfaces will set first. However, some silicones with a catalyst will set at a uniform rate. If we pour silicone over an object we can make a mould.
Depending on how much catalyst we add we can vary the stiffness of the finished mould. Your RTV will generally come with mixing instructions for varying ratios of silicone to catalyst.
For reproducing figures and fine detail we should use the minimum amount of catalyst advised as this will make a softer mould that is better suited for capturing fine detail and easy release of the cast item. For larger moulds we should use more additive to make the silicone stiffer as it will maintain a uniform shape and reduce the chances of warpage across the mould. So for minis use a low mix ratio, for terrain pieces like bulkheads and larger use a higher ratio. If the ratios are incorrect the mould will fail. Too little catalyst and the mould won't take shape properly. To much and it will be brittle and difficult to de-mould without tearing.
Mould- This is what we use to capture a shape and subsequently reproduce.
Cast- The process of reproducing a shape using a mould and casting material, and the resulting product.
Casting Chamber- A box or other shape that is used to contain the silicone whilst it sets. Can be made from a variety of materials, including Lego, foamcore, plastic and cardboard.
Foamcore and card are fine for open faced moulds but I find them a little bit of a pain to use as two parter casting chambers as they can break apart when releasing the first stage of the mould, and recreating the exact shape of the mould can be difficult. Lego gives you a constant size and shape to work with, making your work neater and giving you more confidence with the materials at your disposal. Head to the local Lego shop and buy some bits. Get a base piece and a bunch of 4, 3 and 2 stud pieces, with a couple of ones for versatility. A small tub and base costs about £11. That's what I've got in the example below. I can make about four casting chambers from a small tub, to fit one mini each.
Resin- Polyurethane resin is a common casting material. It's more expensive than plaster but sets a lot quicker and harder. It's ideal for casting figures and capturing fine detail. Use gloves when handling this stuff cos it's a swine to get off your hands!
Resin generally comes in two parts, the resin and the hardener. These are combined (usually a 50:50 ratio) and a chemical reaction takes place causing the clear resin mixture to go hard and opaque. This is why the thicker parts harden first; the chemical reaction is stronger in these deep sections. You'll notice if you make bases that the centre will cloud over before the edges. This is the reaction at work. It is an exothermic reaction, meaning it gives off heat. You'll feel this through the mould when you cast.
Also, resin leeches the moisture from the mould and over time will perish it. We can counter this to some degree by using release agent and mould nourisher to protect the mould but eventually it will become unusable. You should generally leave a mould for about a week to finish setting up if you want to cast using resin.
Other resins include epoxy and acrylic.
Pot Life- This is the amount of time you have to mix and pour the silicon or resin after it has been combined with the catalyst/hardening agent. The more catalyst, the shorter the pot life.
De-mould Time- This is the amount of time you have to leave the casting material to set before taking the cast item out of the mould.
Release Agent- A silicone or wax based product we coat the mould in to protect it and ease the release of a cast. Also vital for making two-part moulds, as the only thing that will stick to silicone is silicone, so the release agent acts as a barrier between the two faces of the mould. This will be discussed later.
Plaster- Everyone knows plaster. Plaster of Paris will work perfectly well in moulds, but is very brittle and takes a while to set. Generally I would advise not to use Plaster of Paris as we need something more robust. Better still is Dental plaster. This is the stuff dentures are made out of. It sets rock hard and is really cheap- less than £20 for 25kg. A general rule is if has the word 'Stone' or 'Die' in it, it'll be rock solid when it sets. Notable British plasters are Diestone, Herculite and Crystacal.
This suits terrain casts more than miniatures, but some plasters can capture fine detail. They can be quite heavy compared to resin. Plasters will not damage the mould in any way, and can be used in a mould immediately as compared to resin, where the mould should be left for about a week.
Open Faced Mould- This is a one piece mould where the casting agent is poured into the top of the mould. Best suited for simple shapes like bases, barrels, barriers or single sided bulkheads. Care should be taken when selecting an object to open face mould to ensure the object doesn't have any significant overhangs. This would prevent the original object from being removed from the mould so you'd know if this happened.
Two Part Mould- This is a mould that has two pieces, or 'faces', typically capturing the details of the front and back of an object. This is better for moulding complex shapes. Most of your miniatures will have been cast like this. The telltale sign is the mould line running around the central axis of the mini. This is where the two faces of the mould meet. Care must still be taken when selecting candidates for this method of moulding. One piece mini's with no gaps through them are ideal; for the sake of example, Cawdor or Van Saar. But the newer range of necro minis with the optional weapons wouldn't be any use if the weapons were attached. The silicone would get between the mini and the weapon and would stop the mini from being released from the mould. The solution to that would be to not attach the weapon before making the mould. Gaps between the legs are fine, it is only gaps between parts of the mini which would make de-moulding impossible that we have to look out for.
