HOW ARE RESIN KITS MADE?
Although recently superseded by 3D printing, cast resin components were a popular element of model kits for decades, particularly among smaller manufacturers. Cheap, lightweight and easy to work with, polyurethane-based resins can be cast into intricate shapes without the need for expensive machinery.
The quality of resin castings depends wholly on the quality of the original master and the moulds. Alas, a consistent quality across all manufacturers has been lacking and I’ve had the (dis)pleasure of wrestling with some truly awful resin kits over the years. Air bubbles trapped in the material are a common problem, while some castings may feature heavy deposits of ‘flash’ which must be removed carefully before assembly.
When they’re done well, however, resin kits can be a joy to build and most often come in the form of one-piece bodyshells, leaving modellers to expend most of their energy on building the underframe and adding smaller details. Indeed, resin has a number of limitations and the design of the kit should take these into account. A naturally soft material, it is liable to warp unless cast in fairly thick sections and it’s not the best material for rendering finer details such as buffers or brake pipes. This explains why most of the best kits include etched brass and whitemetal chassis, bogies and detail fittings.
Prior to recent developments in 3D printing, resin provided a convenient means of rendering kit components, especially one-piece bodyshells. The best kits offer brass, whitemetal and plastic underframe components, such as this package from DC Kits.
Resin bodyshells have allowed modellers to recreate unusual locomotives, often using RTR chassis as a basis. This Silver Fox Models gas turbine uses a modified Hornby Class 47 chassis.
To produce a resin-cast component, a ‘master’ must first be produced in plastic or metal. The master is then placed inside a liquid-tight container, into which is poured cold-cure silicone rubber. For hollow items such as bodyshells, an inner and outer mould is required. Silicone has the potential to form itself around even the most intricate details and shapes, allowing high-quality reproductions if the work is carried out properly. Naturally, the quality of the original master and the skill of the mould maker has a direct effect on the finished product.
The polyurethane resin itself is created by mixing a liquid resin with a hardening compound and this fluid is poured into the silicone moulds. A chemical reaction occurs within the liquid resin – generating heat – which helps the curing process, but it also degrades the moulds, thus explaining why resin kits are usually offered in short runs only.