This will be a quick introduction on moulding and casting. There are plenty of books out there that will go into more detail. I don't have any experience with high temperature rubber moulds or white metal so I will not be discussing these. If you have more detailed question e-mail me and I will try and help
MOULD MAKING MATERIALS
There are times when you want to make a copy of an existing kit part. There are several ways of doing this. Here are some of the mould making materials
RTV - room temperature vulcanized rubber. Material is mixed with a catalyst. The time to set up ranges from a few hours to a few days. Holds detail and complex shapes very well but is expensive.
Vinamould - a rubber type moulding compound that you melt in an oven then pour onto your part. Due to the high temperatures of this product, plastic and styrene parts are out of the question. There are several different temperature grades which changes the flexibility. The good thing about this stuff is that it is re-usable. Make a mistake on a mould and you can melt the mistake to start over again
The distributors of this product in Canada is Model Builders Supply (MBS)
Moulder Builders Supply
40 Englehard Drive
Latex Rubber - This is an air dried rubber that has to be painted or brushed on in layers. Holds detail well but not as good as the RTV. Due to the time of the latex to cure this can be a long process to make a mould.
Silicone Sealant - Caulking compound found at the hardware store. Similar to the latex rubber method this has to be put on in layers. Stinks like hell but is relatively cheap. Very good if fine details are not needed. Good for the rough stages and if you are on a budget.
Plastercine - For a quick simple shape Plastercine can be used. Smash the part into the Plastercine to get the shape then pour the resin in.
Resin - I have used casting resin (Alumilite) as a mould making material once. Due to the inflexibility of the mould, getting the cast part out may be a problem.
A friend buys his supplied from a taxedermy store. Says the costs of the resins and mould materials are a lot cheaper.
Once we have chosen a moulding material we now need to make a box for the part to sit in. This box is needed to hold the mould material in. The most common material used is Plastic sheet but others have used Lego Blocks, cardboard, metal sheets etc. Depending on the part shape we will want to make a one part or two part mould.
Most of the moulds we make are for relatively small parts. When you get to larger parts the techniques get a bit trickier and reinforcements will be needed for the mould to keep it's shape
ONE PART MOULDS
This is used for simple part or parts that have one flat side. This flat side is mounted, taped etc on the bottom of the box we made earlier and then the mould making material is poured on top. Depending on the mould material you may need to put some sort of release agent on the master part. After curing we rip the box apart and then take the original part out. We can then start making replicas. Relatively simple.
TWO PART MOULDS
This is used for more complex parts such as figures. After we make our box we lay Plastercine on the bottom. The item is then forced slightly into the Plastercine. The Plastercine is then moulded around the figure until there are no gaps. I find dentist tools to be very helpful here. The reason we do this is that we don't want the mould material to get underneath. There are a lot of theories about where to make the mould lines. This is a whole other story that I won't go into detail here and is better learned from book or from others who can show you visual examples.
NOTE: Plastercine - some people have warned that this stuff will react with the mould material preventing it from curing. The chemicals in the Plastercine that keeps it flexible is supposed to be also working on the mould material keeping it from curing. I have had no problems so far. Haven't tried Silly Putty
When we are satisfied that there are no gaps we add keys to the mould. I generally use the end of a paintbrush to make indents. The more the better since this will keep your moulds aligned during casting. Also at the same time you should be thinking about runners. See the tips section for definitions of these terms.
Pour in the mould material and wait for the material to cure.. Once this has cured you will break off the bottom of your box and leave the sides intact. Take out all the Plastercine and clean this up as much as you can. When this is done you should be ready to pour your second part of your mould. Some people have suggested taking the master out of the mould and put some release agent all over it then put it back into the first mould. The theory is this will prevent the next pouring from getting in between the first mould and the master.
Pour the second half of the mould and let it cure. When ready you can soon make replicas.
There are a few casting materials on the market Alumilite, Castolite, Fiberglass resin etc. Alumilite is a very quick (few minutes) casting material but a bit pricey. Castolite is also quick and slightly less costly but I find it doesn't reproduce surface detail as well. This stuff has a different viscosity so it may be better for certain types of moulds. I have tried Fiberglas resin and have had some good results. The curing time is in the order of several hours so there is plenty of working time. I just hope you are not in a hurry. (also the stuff stinks) Some people do not sure it because they say the resin never really cures and that it stay sticky. I have done several casting with no problems. Once the casting has been primed you could not tell the difference from one cast in other materials.
Most of the items you can get from a hobby store but there are other sources that may be cheaper. Fiberglass or marine resin you can get at an automotive store or a store that deals with only resin.
All of the above casting materials are two part with a resin and a hardener. These are also exothermic (they give off heat when curing). Be warned that sometimes they can get very hot and burn you
Each has a slight shrinkage and there have been worries about long term stability but I have had no problems with cast parts so far.
These methods are messy so be sure to wear rubber gloves all the time. Some are very stink and you may need some sort of ventilation
Which one you use will depend on personal preferences
I did notice that some casting materials take several weeks to fully cure.
For the one part mould it is relatively simple to cast parts. Pour the resin into the hole. Squeeze or poke out as many air bubble as you can. One cured pop out the part. Depending on the mould and resin a release agent may be needed
For two part moulds casting is a lot more complex. The first item is to make sure your mould halves are securely tied together. The most common thing to use are rubber bands. The more the better as this keeps your mould from moving around. You have to get the resin in somehow so there have to be channels or runners into the mould. These are funnel shaped items cut into one half of the mould. These are made in areas that are not noticeable. For figures this would be the legs. Do not squeeze the mould too much or yo umay deform it and get deformed parts from the casting.
There should be at least two channels present. One to pour the resin in and a second one for the air to come out.
It may take several tries before a half decent copy is made
You may find the channels aren't big enough to let the resin in quick enough. You may also find a lot of air gets trapped in certain areas of the mould. It may be necessary to cut channels in places where air can be trapped.
To get bubble free casting the big guys do pressure casting and vacuum moulding. A vacuum chamber is used to cure the moulds. This gets rid of most of the bubbles and makes the mould stronger.
Pressure vessels are used during casting to squeee air bubbles into almost nothing. This can almost guarantee the cast item if usable.
the average modeller can not afford or justify a vacuum or pressure chamber. With techniques and tips a lot of problems can be eliminated to that your casting process can be very successful with out the pressure vessels.
Release agents - on master and moulds
As a general rule I use a release agent all the time. This will make the part come out of the mould easier and make the mould last longer. A lot of people swear by talcum powder. I have used petroleum jelly with no problem. Just make sure this is put on in a very light coat. Too much and some detail may be lost
To get an even thinner layer of petroleum jeyy a friend has suggested mixing this with paint thinner (Varsol). This is very thin and can be painted on. When the thinner evaporates all that will be left will be a very thin layer of petroleum jelly. I haven't tried this method yet but I will in the future.
Brushing on the mould material - Your castings are only as good as your mould. During the mould making process there can be air bubbles trapped. These can cause imperfections in your castings which have to be cleaned up. To prevent this I usually brush on the first coat of mould material. This way you can inspect for air bubbles before you pour the rest of the mould material on. Be careful when brushing on the mould material because you run the risk of taking off the release agent from the master
Learn where the air bubble will gather - After a few castings you will learn where air bubbles get trapped. Channels or your pouring process can eliminate some of them. Staking the mould also releases some trapped air bubbles. If you still have problems there are several things you can do. The major manufacturers place the moulds into a pressure chamber that shrinks any bubbles. Some place the items into a vacuum chamber that will cause the air bubbles to boil out. For the home caster these may not be feasible. What I do is I pour resin into the mould halves where the air bubbles may be trapped. I then close up the mould and pour the rest of the resin in. This is a bit tricky for the resins like Alumilite and Castolite due to the quick curing times. The Fiberglas resin has a longer curing time so you can work the mould longer. A messy process but your casting will be better
Openings - some parts have holes or openings that have to be cast on a two part mould. Determine before hand which side of the mould you want to have it on. You can use the plastercine to make the barrier but a cleaner method is to place a tape barrier (ie scotch) This will give you a smoother mould and may make the casting process easier
Learn from the masters. Look at other peoples casting and look at where their parting lines are. Try and figure out how they got their air bubbles out.
Runners - These are channels that are made into the mould for the resin to be poured in or for the air to escape. Try and visualise where the resin will be flowing. This will allow you to determine of there are possible locations of air being trapped. Don't cut a million runners into your mould or you may be spending all your time cleaning up your castings
Keys - These are indents made in one half during the mould making process. This will allow two part moulds to line up. They don't have to be very deep but the more the better
Master - The original part that you are trying to copy
Undercut - This is when a master part curves under itself. Most mould materials are flexible and you can stretch them slightly to release parts. Most moulds can handle slight undercuts with no problem but larger ones may rip or even trap the part in the mould.
Some people have problems with making copies of other peoples works. This is a big worry in the large scale figure world. I don't agree with taking someone else's hard work and recasting them to sell. People complain about the high costs of the originals or the rareness as justification for making copies. The worry here is that the recasts may be inferior to the original or that the person who made the original figure would be discouraged from making new figures.
There are those who even say if you want another part (ie wheel) you should buy the entire kit to get them. Not always possible if the kit is out of production
If making parts are for personal use I have no worries about casting away.
Not very easy or cheap. For small parts it may be cheaper to buy a new kit. For complex parts or scratch built items these methods can be very helpful. A usefull technique that may be beneficial sometime in your modelling future.