This is a brief summary of the different materials commonly used in scale model kits, describing their characteristics and differences.
Model kits are probably older than many modellers realise and were first made from wood. However, the hobby did not take off until the injection molded process became widely used and it was possible to mass produce kits in tens of thousands. This made them widely available and relatively cheap.
For most of the general public, the mention of a model kit will bring thoughts of plastic parts on sprues and this is still the most common material. However, scale modelling has come of age and there is much more to the latest kits than a few sprues.
The most common materials to be found in model kits these days are:
- Injection molded polystyrene plastic
- Cast polyurethane resin
- Metal photo-etched frets
- Metal gun barrels and ammunition – turned brass or aluminium
- Rubber, resin, styrene and metal tank tracks
- Metal cables
Materials In Detail
Injection Molded Polystyrene Plastic
This is by far the most common material and many kits consist of this and nothing else. Model kits based on the injection molding process have been produced since the 1950s and there are now literally thousands of kits available in all scales.
Beads of polystyrene are forced under considerable heat and pressure into large metal molds. The beads become liquid and flow along channels (that become the sprues) into the parts themselves. When the mold halves are released ejector pins push the parts, held together by the sprues, out of the mold. At this point the plastic is still soft and needs to be handled carefully, but after a few minutes the plastic will become quite rigid.
The plastic can be produced in any colour including clear parts for windows and cockpit canopies. Manufacturers used to produce models in the colour of plastic that they felt the model was most likely to be finished in so that it did not need painting. These days almost all models are primed and painted so the colour of plastic is largely irrelevant but different manufacturers still have their favorites.
One of the features of this process is that the metal molds are very expensive to produce, although when the process is started it is possible to quickly produce many thousands of models from each mold. This means that injection molded kits are normally only made by large companies and the tendency is to stick to subjects that have a wide appeal and where the manufacturer can be certain of good sales.
Until recently, one disadvantage of the injection molding process was that using a rigid mold in two halves meant that complelx pieces could not be produced. Because the two halves of the mold had to be pulled apart the pieces themselves could not have any undercuts. The result of this was that the complex parts had to be produced in several piecies, or they had to be simplified.
Dragon have managed to get around this problem to a large extent with the slide mold process that uses molds with more than two halves. This has allowed them to produce extraordinarilly complex parts such as complete tank turrets with details stamped on both the inner and outer sides. However, this process is complex and kits that use this technology are appropriately expensive.
Nevertheless, even using conventional molding technology standards have increased over the years and it is surprising the level of finesse that can now be found in some injection molded plastic kits.
Resin is the little brother of injection molded plastic. Resin kits initially were produced as aftermarket accessories to injection molded kits.
Due to the considerable expense of creating injection molds, only well known subjects were initially produced by this method. Small enterprising companies saw that there was a market for helping modellers convert injection molded kits to less popular subjects and so began to produce conversion kits. Later as resin became accepted as a viable modelling material entire kits were produced in this medium.
Two liquids are mixed and poured into semi-rigid rubber-like molds. When the mixture has hardened, the molds are separated to reveal the parts.
The molds are made by casting around an original and they can be produced much cheaper and easier than the large complex injection molds. This allows them to be produced by small companies, indeed it is possible to buy all the substances required to cast resin in the home and this is regulaly done by advanced modellers.
Unfortunately, the molds do not last long and the process is very labour intensive. This makes resin kits far more expensive than equivalent injection molded kits. However, there are many unusual subjects where resin is the only option available and turning to resin allows modellers to produce a truly unique model.
The quality of resin kits can vary enormously. The resin shrinks a little as it sets and the manufacturer needs to take account of this. Resin parts can also be subject to warping and excess flash. On the other hand, when done well, resin parts can show a level of detail that beats the best injection molds. Resin is particularly favoured for producing figures because it is ideal for reproducing the folds in clothing and the undercuts present behind ears. Resin is also very popular with aircraft modellers to improve the details of cockpit interiors.
Resin parts often require more work than injection molded kits. The parts are often attached to a casting stub where the resin liquid was poured into the mold and this has to be removed. The dust produced when cutting and sanding resin can irritate the lungs, so either wet sanding has to be done or a face mask should be worn. Furthermore, polystyrene cement will not work on resin parts, so either epoxy glue, or superglue (cyano) has to be used.
Photo-etching is a process used to make very thin or tiny metal parts. It is used to make parts that are either too small or too delicate to be made with either the injection molded or cast resin processes. Examples of this include buckles on seat belts, scale tool clasps and hinges that actually work, ventilation grills and thin armour plate.
PE parts were originally produced as after-market accessories to allow modellers to super detail their kits. PE kits have become more and more extensive and some replace many of the kit parts.
Recently, the larger companies producing injection molded kits such as Tamiya and Dragon seem to have taken the view ‘if you can’t beat them, join them’ and have started to produce kits that contain PE frets for those parts that cannot be produced by the injection molded process.
The parts to be produced are drawn on a computer with a CAD drawing program. This is printed on clear film. The film is placed over a photographic plate and exposed to light so that an image is left on the photographic plate. The photographic plate is then soaked in chemicals to develop it. It is then soaked in strong acid which etches the metal on those areas. This is then soaked in other chemicals that dissolve the metal leaving behind a thin fret of tiny parts.
The process lends itself to small scale production and like resin parts is often carried out by small companies and can even be done at home. A video showing the photo-etch process is available on YouTube http://www.youtube.com/watch?v=QjgH250bUrA
PE parts are not easy to use and some modeller’s hate them. The parts are very tiny so are a challenge to handle and glue. Like resin they do not respond to polystyrene cement so epoxy or cyano adhesives are normally used (although for tiny parts gloss varnish works well).
Although PE parts are by nature flat, they can be rolled and folded to make boxes, fenders etc. To do this you will almost certainly need to invest in folding tool such as Etch-Mate or Hold And Fold. Regrettably these are not cheap.
As well as allowing modellers to use very tiny and very thin parts, PE is useful for producing damaged parts that can add extra realism. PE metal fenders and ammo boxes can be bent and punctured in a realistic way that is difficult to reproduce in plastic.
It is worth noting that sometimes an aftermarket PE set will be made for a particular model kit in which case it is easy to get the parts needed for your kit. The PE parts set may even quote the serial number of the kit it is intended to supplement. However, some aftermarket PE kits are more generic and might just state that they are applicable to a particular aircraft or vehicle. Sometimes, a PE parts kit may not refer to a particular vehicle at all but may be a collection of ‘tool clamps’, ‘seat belt buckles’ or similar that are appropriate for many models. The more vague the description is, the more careful you will have to be about using the parts on your model. There is little point going to all the trouble to super detail a model tank if you end up putting German tool clasps on an American Sherman tank.
It is also worth noting that just because a part is made from photo etched metal, it does not automatically make it better than the kit part. Advances in the injection molded process mean that some plastic parts are now every bit as good as PE replacements. It has become quite common to use only part of an after-market PE set. Some kit manufacturers will provide some parts in both injection molded and PE forms so the modeller can choose which is the best rendition of the original.
Further information about using photo-etched parts will be covered in a separate article to be published later.
Metal Gun Barrels and Ammunition
Small gun barrels in plastic are normally produced as a single piece, but they will have a mold line to remove which can be difficult when the barrel may be no thicker than a human hair. They will also be extremely fragile and prone to snap off, furthermore the muzzle will probably be solid.
It is therefore no surprise that when modellers were offered the opportunity to buy beautifully lathe turned metal barrels with hollow ends and sometimes even rifling, they jumped at the chance and these have become very popular.
These metal barrels are difficult to produce and a tank main gun may cost half as much as the model, yet there are armour modellers who consider them essential. Whether this is the case is debatable. A plastic barrel may take more work to glue and sand, but when it is painted it often looks exactly the same as the metal replacement. This is not the case with smaller barrels such as those used for machine guns. These are so tiny that it is difficult to produce them in plastic and the metal replacements are often noticably better quality and have more finesse.
Initially, metal barrels were sold as separate aftermarket accessories. However, the leading model kit manufacturers have now started to include them in their kits which makes them much more affordable.
With regard to ammunition, complete shells or cartridge cases can add to the realism of a model or dioarama. Clearing the mold lines and painting dozens of small shell cases is no fun. Fortunately, most common types of ammunition can now be obtained in metal turned on a lathe. These are extremely realistic and if made of brass, they do not even need to be painted.
Many military vehicles carry winch or tow cables and the twisted metal strands are difficult to replicate in plastic. Some of the leading manufacturers include metal cable with their kits and it can be purchased separately as an aftermarket accessory.
Tank tracks can be made out of a flexible rubber-like material, resin, injection molded, or metal. They will be the subject of a separate article, so are not covered here.