Long gone are the days when making a scale model involved just plastic. Modern kits are often multimedia with resin, metal, wood and other materials in addition to traditional styrene plastic.
This means that a modeller needs to know how to fix together different materials and perhaps one of the most difficult combinations is gluing metal to metal. Cyano (superglue) or epoxy resin adhesives are most commonly used, but these are far from ideal in some situations.
An alternative method is to solder metal parts together. Although this is not easy it will provide a very strong bond and in a few cases is the only practical solution. This tutorial will guide you through the process.
|CAUTION: Soldering requires the use of an electric or gas powered soldering iron which will be extemely hot. In addition to the risk of personal injury there is a danger of causing damage to property, or starting a fire, so extreme care should be exercised and it should only be attempted by adults.|
The most common glue used in scale modelling is polystyrene cement which comes in a variety of consistencies from thick tube glue to ultra-thin liquid cement. For gluing plastic (styrene) it is ideal. For glueing metal to itself, or other materials, poly cement is utterly useless.
The adhesives most often used for metal are two-part epoxy cements, or cyano adhesives, neither of which are ideal for the following reasons:
Problems with epoxy resin adhesives:
Epoxy glues have to be mixed and usually it is necessary to mix far more than you need, so it is wasteful. When mixed they have to be used within a specific time before it cures. This time can be anything between 30 seconds and 24 hours depending on the make of epoxy, so you need to choose the curing time suitable for each task. Until the epoxy cement has started to harden, it has very little holding power, so any joint has to be held firm with clamps or similar.
Another problem with epoxy is that it is very messy and stringy – a bit like gluing with treacle. It is difficult to get a professional looking finish and often more glue ends up on fingers and the wrong parts of the model than on the joint. Finally, when it is dry, any excess is very difficult to remove (it is best carved away before it fully hardens).
Problems with cyano adhesives:
The disadvantages of epoxy mean that cyano (superglue) adhesives are more often used where one of the materials to be glued is metal. This can work well where there is a good surface area and a rigid joint. For example, when fixing a metal plate on to the side of a vehicle, a good bond can be achieved because there are two large flat surfaces that will not move. Even so, it is critical to make sure that there is no oil or grease on the metal, since any greasy surface will repel the cyano and prevent a good bond being acheived. In fact, to be on the safe side it is best to use fine wet-and-dry paper on the metal to be bonded to ensure there is a good key.
Unfortunately, cyano is unlikely to work very well when there is a small surface area, or when there might be movement in the joint. One example of this might be where a flat photo-etched part is being bent to form a box. Where the edges meet there is hardly any surface area, so very little for the adhesive to grip on to. In these cases, it is necessary to use thick gap-filling superlue and to apply it where it is unlikely to be seen. When the joint might move such as when forming a metal framework, the metal parts can bend and flex as is in the nature of metal to do so. However, the gap-filling superglue will be rigid and is likely to crack and split so the joint fails.
Great care is needed when using cyano as it’s ability to glue human skin to itself is legendary.
The alternative is to use a method that provides a very strong, almost invisible joint that will not crack if the joint moves and that method is soldering. Soldering is particularly useful when making three dimensional objects out of metal such as boxes, or metal railings such as the stowage bins often seen on AFVs.
Soldering Tools and Materials
The whole process uses exactly the same tools and materials as soldering electrical and electronic parts, so if you have done that – or know somebody who does electronics as a hobby then you will have a head start. The items needed are:
This is used to melt the solder and is simply a way of applying heat to a small area. Most soldering irons use mains electric for power, but there are some that use a small butane gas canister. The tip of the iron needs to be small because you will be applying heat to small areas.
An alternative to a traditional soldering iron is to use a tiny flame blowtorch sometimes sold in cooking shops for creating the crispy sugar coating on creme brulees. However, I would caution against using a naked flame when trying to apply heat to tiny modelling parts – the risk of something going horribly wrong is too great.
This is a soft metal that is normally sold as rolls of wire. It will be widely available from hardware and electrical stores. The wire comes in different thicknesses and you will probably need fairly thin wire.
Solder wire normally contains a proportion of lead so should be handled carefully and hands should be washed afterwards.
This is a gel or paste that will normally come in a jar or tube. The flux makes the solder run along the metal join and adhere to the metal. Without flux the solder will sometimes form into little balls and simply roll off the metal, or it might attach itself to the soldering iron.
Clamps and holders:
When soldering, you will find that you need at least four arms and 30 fingers all of which are heat resistant. Seriously – you will need some way to hold the items to be soldered steady while at the same time holding the soldering iron and the solder. Since the items being soldered will get very hot they need to be held in a vice, or clamps, or similar. Masking tape may also help to hold the parts together until the solder bond is set.
Step By Step Guide
Although no two soldering operations are quite the same, there are some general principles to follow which are demonstrated here as a brass photo-etched mudguard is constructed. The mudguard is made from several piecies, some of which have to be bent to give them shape, but eventually there is no getting round the fact that some of the tiny parts have to be joined together.
1. Assemble tools, parts and prepare work area
Before starting out make sure you have all the tools and materials you will need. Get all the parts you are going to solder together. It is possible that you may put the hot soldering iron down on the work surface or that melted solder will drip, so make sure that your work surface and indeed the floor is well protected.
The tip of soldering irons can become soiled with residue from previous soldering so occassionally will need clearing of debris with a file and/or ‘wet and dry’ paper.
2. Prepare the parts
Soldering will only work on clean grease free metal. Wiping all the parts with a paper towel dampened with white spirits will get rid of any oil or grease and rubbing it with fine ‘wet and dry’ paper or steel wool will give a good key for the solder to bind to. At this point it is a good idea to check that the metal parts have been cleaned up properly i.e. any stubs from the fret have been filed away.
Electric soldering irons can take a long time to heat up properly so switch on the iron in plenty of time.
3. Clamp the parts together
The parts to be soldered need to be held together until the solder hardens. Also the entire assembly needs to be held steady and you will not be able to do this yourself because you will need both hands for soldering and the parts will become too hot to hold. Therefore, it is necessary to arrange some system of clamps and clips to hold the assembly firm.
4. Dry run
Do a complete rehersal of how you are going to solder the parts together. Work out how you will hold the soldering iron and the solder. Which direction will you move them both? Where will any excess solder drip off? Can you move around safely without knocking anything over and are you able to return the iron to it’s stand when you have finished?
These are all questions best answered before you start.
5. Do the soldering
Start by painting a thin layer of flux over the joint that you want to be soldered using an old paint brush. Then hold the iron against the back of the joint trying to get as much of the iron in contact with the parts as possible. After a few seconds, push a the end of the solder wire on to the front of the joint. If the joint is not yet hot enough nothing will happen, but when the joint does reach the magic temperature the solder will suddenly turn to liquid and run along the joint.
The temptation will be to keep pushing the solder wire into the joint and it will keep melting. You will actually need very little solder and if too much is used it will either run off or will build up excessively. This does not matter if the joint will be invisible, but should be avoided if the joint will be visible. It is almost impossible to remove excess solder without remelting it and starting all over again.
If you are holding the parts together with masking tape, then it is likely that the glue on the tape will melt as it gets hot and the tape will come off. Try to arrange the tape so that there is always some tape away from the point of contact of the soldering iron.
The solder will melt and then resolidify in a couple of seconds. If you are not happy with the joint you can remelt the solder and move it around with the tip of the soldering iron, or you can add extra solder. When you are happy with the result leave it for a few minutes to cool down and fully harden.
If you are attaching two flat pieces together, then a thin layer of flux should be painted on the faces to be joined. The solder will be drawn into the join by capillary action when it melts.
REMEMBER TO SWITCH OFF THE SOLDERING IRON!
6. Test and tidy up
Test the joint by giving it a little wiggle. It should be strong but there is no way to know without trying it and it is better to find out before you attach it to your model. The visible part of the joint may need to be cleaned up with a file or ‘wet and dry’ paper.
If the join is not perfect on the visible side then there is no reason why you cannot use a model filler or putty to improve the look of the joint. However, the same rules about using glue on metal apply to putty – you will need to use a filler that does not rely on a reaction to the styrene plastic. Appropriate fillers for metal are gap-filling supperglue, or epoxy putty such as Milliput and these will both make the joint even stronger.
Soldering is not easy and does require practice. However, it does provide a very strong bond in situations where conventional glue will either not work at all, or will produce a weak bond. When creating a metal stowage basket from scratch using metal wire, the last thing you want is for joints to start springing apart when it is being attached to the model. A soldered joint will give some security that it will hold in place even with relatively rough handling.
However, do take care because it is so easy to burn yourself or furniture with a hot soldering iron.