The vast majority of scale model kits made today are made from plastic (polystyrene). Most modellers will, sooner or later, come acoss other materials and cast polyethylene resin is one of these. Working with polyethylene resin requires different methods and products. This tutorial is a guide to dealing with this material.
Background information about cast resin can be found in the article ‘Model Kit Materials’ and so we will not repeat it here.
Whether you are building a complete resin kit, or using an aftermarket kit to convert a standard plastic kit, you will find that the resin parts are likely to need more work on them than the more normal injection molded plastic parts that you may be used to.
The quality of plastic kits on the market is very good and most modellers have become used to snipping a plastic part from the sprue and, with little or no clean-up, putting it on the model. Unfortunately that will not be the case with resin.
Resin parts are cast from a liquid and may well come still attached to the casting block. If this is so, then they need to be separated. If the attachment point is thin then it might be separated with repeated passes from a sharp hobby knife. However, if the attachment point is thick it will need to be cut off with a fine saw, sometimes called a razor saw, which is designed for hobbyists. Normal saws available from hardware stores cannot be used as the teeth of the saw will be too large. The sawing process can be difficult and time-comsuming, especially if the link between the part and the casting block is large, but there is no way to avoid it. You may wish to try using a motor tool to speed up the process, but great care is needed when doing this. If too much friction is generated, the resin may melt.
The greatest difficulty can be cutting away the casting block without damaging the part. Sometimes it is better to cut away the bulk of the casting block, leaving a small amount behind that can be trimmed away with a modelling knife.
Note that whenever cutting resin like this, or sanding it, there will be a fine dust produced which is very bad for the lungs. Wear a filter mask and clean up your work area afterwards.
When the parts have been removed from the casting blocks, they need to be cleaned up. Any remaining lug where the part was attached to the casting block will need to be cut away with a knife or sanded/filed away. There is also likely to be a seam that will need to be removed with a sharp blade.
Examining All Parts
Once the parts have been removed from the casting blocks and cleaned up they need to be examined.
One potential fault is warping. Check whether the part has become distorted. For example, if it is the chasis of a vehicle place it on a flat surface and see if it sits right or whether it rocks back and forth. If you find a part has been warped then it is possible to sometimes undo the damage by applying gently heat, such as boiling water or even a hair dryer which softens the resin and makes it possible to reshape it. Clearly care needs to be used when applying heat in this way to avoid injury.
A second fault is air bubbles. Sometimes tiny air bubbles can be trapped in the mould whilst the resin is setting and this might mar the surface detail. These tiny holes are sometimes called ‘pin holes’. This fault can be rectified with filler and sanding. Remember that fillers designed for styrene plastic will not adhere to resin. However, if the pin holes are tiny, almost any type of filler will work well.
Another thing to check for is the need to drill any holes. Injection molded parts will probably have holes molded into them, but it is more difficult to create holes that go right through a part when it is cast. It may be that the resin part has an indentation to show where a hole should be, so that the modeller can drill it out completely.
Standard polystyrene cement which is perfect for conventional styrene models is absolutely useless for resin parts. Poly cement works by slightly dissoving the styrene plastic, but it wll not dissolve resin and so will not work at all. When gluing resin parts to each other, or to plastic, you will need to use either two-part epoxy glue or cyano (superglue) adhesive. Both of these work well, so it is down to individual preference.
Cyano is the most convenient because it does not have to be mixed and so is probably the first choice for many modellers. However, the epoxy cement will probably produce the strongest and most reliable bond.
Whichever glue you use, it will only work if the surface is prepared properly. Both types of glue need a dust-free and grease-free surface, so wash the parts in warm water with detergent and dry them thoroughly. The bond will probably be stronger if the surfaces to be joined are roughened slightly with sand paper.
Once you have glued the parts together, there may be a need for filler. The normal fillers intended for polystyrene such as Squadron ‘Green Stuff’ and ‘White Stuff’ will not adhere to resin because they are designed to ‘melt’ the surface of polystyrene. This does not mean that they cannot be used in certain situations, but you should be aware that they may flake away if spread thinly. Epoxy putties such as Milliput, or other fillers that have a natural tackiness, should normally used in preference when filling resin parts.
Using resin parts does provide the modeller with additional challenges, but there are also additional rewards. You have the opportunity to make an unusual or even unique model. Using resin also gives you the opportunity to hone and develop your modelling skills. Give it a go. Try starting with a simple conversion kit to enhance or modify an injection molded kit and build up to a full resin kit.