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Working with resin Resin cast parts and complete resin kits are an increasingly common feature in the modelling world. They have been around for a while and are only now begining to become more mainstream and widespread. They differ fundamentally from the traditional injection moulded plastic kits made by companies such as Airfix.To start with resin is much denser and consequently a heavier material. They are also often cast in relatively thick sections which often makes a resin model very heavy compared to the usual plastic one. The process of casting resin is not as sophisticated as injection moulding polysytrene and this results in the resin castings needing to be cleaned up and finished off before they can be assembled. There are two main areas that need to be looked at for the resin modeller. One is the condition of the material and the other is the inaccuracies caused by the moulding process, such as flashing. Firstly, the resin is cast normally by hand, and is introduced into the mould with no assitance and relies on gravity to fill all the corners of the mould. The common problem with resin casting is that many air bubbles get trapped in the material and the resin sets before they can be released. When the cast is released from the mould the air bubbles that were on the surface of the mould appear as small holes. These holes need filling to be hidden from view when the kit is painted. It is not uncommon to have quite a large number of these small holes. The moulds used in injection moulding are solid heavy metal affairs capable of withstanding high pressure injected plastic and have no flexiblity at all. A resin mould, on the other hand, is made of silicone, not dissimilar to that used around sinks and baths. It is dependent on being flexible as the mould is bent and twisted to release the casting before it springs back to its original shape. These moulds wear out pretty quickly and it is common to see signs of wear and tear, with the inevitable drop in quality of the casting. Because the moulds are so flexible the joining edges between two sections of a mould cannot be forced together without the mould being squashed out of shape. This results in resin entering the cavity of the mould but then seeping out between the layers of silicon. When the cast is released this resin appears as a very thin "extra" edge on some mouldings, this is known as "Flashing". These flashings need to be removed, (a sharp knife with careful small strokes) and the blanked out cavities should be cleared away. Below is a good example of a resin moulded lorry cab. The windows should be clear but are covered in a very thin layer of resin. To clear these start at the corner of one of the windows and carefully pull it down to the bottom edge. Do this on all four sides and the centre section will fall away. Any small little imperfections may then be cleaned up with a blade, needle file or wet and dry. Easy enough on a lorry window but when doing something like the steering wheel great care should be taken because resin is more brittle and generally less forgiving than plastic so take great care not to break off the smaller sections. Start in the middle of one of the sections and slowly break off little bits working outwards. When the bulk of the flashing has been removed, again finish off the remaining remnants with a fine blade or very fine needle file.

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Wednesday 20 August, 2008

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