INTRODUCTION
This will be a quick introduction on molding 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 molds 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
MOLD 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 mold 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
Vinamold - a rubber type molding compound that you melt in an oven then pour onto your part. Due to the high temperatures of this product, plastic an dstyrene parts are out of the question.
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 mold.
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
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 mold making material once. Due to the inflexibility of the mold, 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 mold materials are a lot cheaper.
Once we have chosen a molding material we now need to make a box for the part to sit in. This box is needed to hold the mold 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 mold
Most of the molds 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 mold to keep it's shape
ONE PART MOLDS
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 mold making material is poured on top. Depending on the mold 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 MOLDS
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 molded around the figure until there are no gaps. We don't want the mold material to get underneath. There are a lot of theories about where to make the mold lines. This is a whole other story that I won't go into detail here.
NOTE: Plastercine - some people have warned that this stuff will react with the mold material preventing it from curing. The chemicals in the Plastercine that keeps it flexible is supposed to be also working on the mold 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 mold. I generally use the end of a paintbrush to make indents. The more the better since this will keep your molds 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 mold 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 mold. Some people have suggested taking the master out of the mold and put some release agent all over it then put it back into the first mold. The theory is this will prevent the next pouring from getting in between the first mold and the master.
Pour the second half of the mold and let it cure. When ready you can soon make replicas
CASTING MATERIALS
There are a few casting materials on the market Alumilite, Castolite, Fiberglas 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 molds. 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)
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
CASTING
For the one part mold 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 mold and resin a release agent may be needed
For two part molds casting is a lot more complex. The first item is to make sure your mold halves are securely tied together. The most common thing to use are rubber bands. The more the better as this keeps your mold from moving around. You have to get the resin in somehow so there have to be channels or runners into the mold. These are funnel shaped items cut into one half of the mold. These are made in areas that are not noticeable. For figures this would be the legs.
There should be at least two channels present. One to pour the resin in and a second one for he 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 mold. It may be necessary to cut channels in places where air can be trapped.
TIPS
Release agents - on master and molds
As a general rule I use a release agent all the time. This will make the part come out of the mold easier and make the mold 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 mold material - Your castings are only as good as your mold. During the mold 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 mold material. This way you can inspect for air bubbles before you pour the rest of the mold material on. Be careful when brushing on the mold 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 mold also releases some trapped air bubbles. If you still have problems there are several things you can do. The major manufacturers place the molds 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 mold halves where the air bubbles may be trapped. I then close up the mold 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 mold longer. A messy process but your casting will be better
Openings - some parts have holes oropenings that have to be cast on a two part mold. Determine before hand which side of the mold 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 mold and may make the casting process easier
TERMS
Runners - These are channels that are made into the mold 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 mold or you may be spending all your time cleaning up your castings
Keys - These are indents made in one half during the mold making process. This will allow two part molds 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 mold materials are flexible and you can stretch them slightly to release parts. Most molds can handle slight undercuts with no problem but larger ones may rip or even trap the part in the mold.
ETHICS
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.
CONCLUSION
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.