Resin model kit casting

What it is

Once you can make a mould, you stop being limited to the parts in a box and start producing your own, which is why casting your own resin parts is the point where model building shades into small-scale manufacturing. Resin model kit casting is the practice of making silicone moulds of master parts and casting copies in polyurethane resin, used to duplicate components, produce conversion parts, or even run small batches of original kits at home.

The reason people take it up is independence. Wargamers cast extra terrain pieces and bases, modellers reproduce a part they only have one of, conversion builders make multiples of a custom piece they have sculpted, and small creators run short editions of their own designs. Instead of buying every part, you become able to make them, which opens up scratch-building, kit-bashing, and original design in a way nothing else does.

The process is a two-material craft. You build or sculpt a master, encase it in liquid silicone rubber that cures into a flexible mould capturing every detail, then mix and pour fast-setting polyurethane resin into that mould to produce hard, detailed copies. Managing air bubbles, mould design, and the brief working time of the resin are the core skills, and a pressure pot or vacuum chamber is often used to banish bubbles for clean casts.

It rewards the technically minded. There is real chemistry and craft in getting flawless, bubble-free castings, and the ability to reproduce parts at will is genuinely empowering for a serious modeller.

Fun facts

Professional and serious home casters cure their resin under pressure in a pressure pot, because squeezing the curing resin shrinks any trapped air bubbles to invisibility, which is how commercial resin kits achieve their flawless, bubble-free parts.
The silicone used for moulds and the polyurethane resin used for casts both cure through chemical reactions with very short working times, sometimes only a couple of minutes for fast resin, so casters must mix and pour with practised speed before the material sets in the cup.
Because a good silicone mould can produce dozens of copies before wearing out, a single sculpted master part can become an entire squad of identical wargaming figures or a run of conversion parts, effectively letting a maker manufacture their own components at home.

How it works

Start by preparing a clean master and planning the mould, because the cast can only ever be as good as the mould, and the mould only as good as the master. Use a part with no undercuts that would trap it, seal any porous master, and decide on a simple one-part or a two-part block mould depending on the shape. Think about where the resin pours in and where air escapes, since poor mould design is the root of most casting failures.

Make the silicone mould carefully to capture every detail. Mix the two-part silicone precisely by weight, stir thoroughly but without whipping in air, and pour it slowly in a thin stream from one corner so it flows over the master and pushes air out ahead of it. Let it cure fully before demoulding. A well-mixed, slowly-poured silicone mould reproduces astonishing detail and lasts for many casts.

Then cast resin quickly and manage the bubbles, because polyurethane resin sets fast. Mix the two parts in the right ratio, stir fast but briefly, and pour immediately into the mould before it begins to thicken, since you often have only a minute or two of working time. Air bubbles are the enemy, so pour in a thin stream, and if you have a pressure pot, cure the cast under pressure to crush any trapped air to invisibility.

Demould, trim, and repeat. Once cured, flex the silicone to release the hard resin cast, trim any flash and pour stubs, and the mould is ready for the next copy.

Benefits

Make Your Own Parts at Will Engaging Material and Process Craft Reproduce Rare or Custom Pieces Run Small Batches of Original Designs Teaches Mould Design and Chemistry Reduces Need to Buy Every Part Deeply Satisfying When Mastered

What you need

Here's what to gather before you start. The essentials are marked.

Two-part silicone rubber: for making the moulds

Polyurethane casting resin: for the cast parts

An accurate scale: for mixing by weight

Mixing cups and stirrers: for precise mixing

Master parts: to mould, sculpted or existing

Mould-release agent: to protect moulds and aid demoulding

A pressure pot (optional but ideal): for bubble-free casts

FAQs

What do I need to start casting resin?

Two-part silicone, casting resin, and accurate mixing tools. You make a flexible silicone mould of a master part, then pour polyurethane resin into it to make hard copies, so you need both materials, an accurate scale or measures for mixing, cups, and stirrers. A master to copy and a mould-release agent complete the basics. A pressure pot is optional at first but the biggest upgrade for flawless, bubble-free results.

Why do my casts have air bubbles?

Air gets trapped during mixing and pouring. Stirring whips in air, and pouring too fast traps it in the mould, leaving bubbles and pinholes in the cast. Mixing gently, pouring in a thin stream from one corner, and designing the mould so air can escape all help. The definitive solution is curing the resin in a pressure pot, which shrinks any trapped bubbles to invisibility, as commercial casters do.

How many copies can one mould make?

Often dozens before it wears out. A well-made silicone mould is durable and can produce many casts, which is what makes casting so useful, a single sculpted master can become a whole squad of figures or a batch of parts. Eventually the fine detail and surface degrade with repeated use and resin exposure, but a good mould gives plenty of copies before it needs remaking.

Is resin casting safe to do at home?

Yes, with proper precautions, but it demands care. The silicone and especially the polyurethane resin give off fumes and the uncured resin is an irritant, so you must work in a well-ventilated space, wear gloves and a respirator suited to the materials, and avoid skin contact. Cure and dispose of waste responsibly. With the right protective equipment and ventilation it is a safe home craft, but it is not one to do casually without them.
⚠️ Polyurethane resin and silicone release harmful fumes and can sensitise skin, so always use a suitable respirator and gloves, work with strong ventilation, and keep all materials away from children and food areas.