When I was first approached to write this article, I was very hesistant. While molding and casting have many benefits in the customizing world, it also has a darkside. I'm not going to lecture you on the ethics of Copyright laws, but please try to remember that someone put a lot of sweat and tear into their property. I've seen too many examples of people casting the works of someone else and selling it for a profit. You don't know how it feels until you've had done to you. Unfortunately, I've been a victim of this. I traded a custom piece to a supposed friend only to find out that he made molds of it and resold the casts. Not only was he making money off of my hard work, he was also taking credit for work that didn't belong to him. People, there are better ways to make money.
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The Mold Box: the first thing you need to do is make your mold box. There are no set guidelines but a few simple rules might help you out.
a)The box should be large enough to hold your figure as well as leaving 1- 1.5" on each side of the figures. This is done give stability and support to your mold. If there's not enough mold material, there's a chance that features/parts will be distorted in the casting process or even worst, the mold could tear or collapse.
b) The box should also be 2.5 - 3 times thicker than the figure or parts that's being molded. You'll see why this important later on.
The box itself can be made from numerous different materials. Two of my favorite are Legos and carboard. Legos are great in that you can constantly change the size and shape of your box without much effort. This will help to save mold material ... which isn't very cheap. In addition, the Lego plastic doesn't stick to most mold material which make it easy to demold and clean up. My only advice here is to use crosslinking patterns with the bricks to increase its stability as well as reduce the chances of leaking.
Mold boxes can also be made from simple cardboard. I use those free USPS priority boxes for this. Cut out the appropriate size for the box and use either a glue gun or packing tape to seal the box. Again the object here is to minimize the area of teh mold box without compromising the stability of the mold itself.
If you're just doing a one-piece mold of a head, plastic or paper cups works very well.
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Fillers: when making a 2-piece mold, you only want the initial mold to cover only half of the figure. As such you need to add something else to fill in that other half. Any type of clay will work for this as long as the clay doesn't permanantly stick to the mold or figure. A lot of modelers use sculpting clay, but I prefer Play Doh. Play Doh is cheap, but more important, it's easy to clean up afterward. Add your filler to the bottom of the mold and press the figure into the filler until half of your figure is sumerged in the Play Doh. Check to make sure the Play Doh seals completely around the figure so your mold material won't leak through to the lower half of the figure Using pen or pencil, punch a few holes around the periphery of the Play Doh. This is done to create alignment slots to connect the two halves of the mold. At this point you can spray figure, filler and mold box with a mold release agent (note this step is not needed if you're using silicon mold material). Wait 30 minutes for the spray to dry and then you're ready to pour the mold material.
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Mold Material: there are numerous mold materials out there with different viscosity, shore hardness, stability and thermo properties. What you decide upon will depend what you're trying to achieve (as well as your budget). You can find a review for the different types Smooth-On mold material at:
http://www.smooth-on.com
For basic run of the mill resin casting, I use either Smooth-On PMC 121/30 Dry. This is the same stuff that McFarlane Toys use for their prototype works. These RTVs are simple to work with (1:1 mixing ratio), captures intricate details very well and fairly durable. The mold itself is usually good for 10-15 castings before damage occurs (of course this really depends on the complexity of the mold as well as the fine details). If you want to do mass production casting, I recommend either Dow Corning silicon, Smooth-on Smooth Sil 910 or porcelain molds, but those will be covered here.
Estimate how much mold material you'll need. Volumes can be calculated using simple geometric equations such like V= height x width x length for rectangular shapes or Pi x Radius square x height for cylindrical shapes.
With Smooth-On 121, you just mix equal volumes of part A (clear/ yellowish liquid) and part B (brownish liquid). Make sure to mix them throroughly but avoid generating airbubbles. It's not necessary to degas the RTV, but if you have access to a vacuum chamber, degas for 2-3 minutes at 29cm Mercury. Pour the mold material into the box. To avoid air bubbles in your mold, pour slowly and 12 - 24" above the mold box. This is done to create a thin stream that should burst any air bubbles as it travels downward to the box. Try to pour into the lowest point in your mold. Doing this will reduce the chances of trapping air bubbles. The mold material should cover approximately 0.5 - 1" above the figure. If the mold is not thick enough, it might distort or even collapse after extended use. If you have a pressurized or vaccum oven, use it. If not let your mold material cure at room temperature for 12-16hrs. Please note that some RTV (especially silicon ) doesn't cure very well below 65F. After the mold has cured (let's call this half mold #1), remove it from the box and clean out the Play Doh/ filler. Once mold #1 is clean and dry, put it back into the bottom of the mold box. It'll act as filler this time. Put your figure into mold #1 and make sure it fits snuggly. Spray the figure and mold #1 with a mold release agent .... you don't want the second half of the mold to stick to the top half right??? I usually add an additional thin layer of vaseline to mold #1 to ensure separation. Pour the mold material as before and wait until it cures.
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Casting Resin: like mold materials, there are numerous types of casting each with their own unique properties. The most common resin used is polyurethane. This is similar to the plastic used by toy manufacturers. There are different polymer resins each with different properties such as strength, flexibility, weight, etc, etc. Smooth-on has a good review of their products:
http://www.smooth-on.com
The two resins I use most are Smooth Cast 300 and Clear Cast 200 (transparent resin)
Casting: before you can actually start cranking out casts, there are a few preps steps you must do first. If you didn't create a pour hole during the molding stage, you need to use a dremel and drill it in. I normally pick an inconspicous spot on the figure to put in the pour hole. For more complex figures with lots of undercuts, you might consider multiple pour holes.
You should also add in air vents at this point if needed. Air vents are basically thin holes that are put/ drilled into your mold to help trapped air escape. The most common area that air gets trapped is the hands and fingers. As your resin flows down the arm past the wrist, it goes into a narrow passage. If that passage is too narrow, the resin blocks the escape of air. This will result in incomplete castings with big holes or missing fingers. If air doesn't get trapped, the resin will fill in completely. Howevers, it's a lot easier to break or sand off these pieces (spruce as they are commonly called) than it is to fill in missing parts after the cast cures.
One of the biggest problems encountered in casting is the formation of bubbles on the surface of your casts. Unless you have access to injection molding, it's a fairly common problem. Here's some on things you can do to reduce and eliminate this:
a) spray or brush on a thin coat of mold release agent to the surfaces of both molds. This is an important because the spray reduces the surface tension on the mold material and allows smooth coverage by the resin (a liquid). The analogy I normally use is that some RTV acts very similiar to a newly waxed car. If you spray that car with water, you'll notice that the wax created a high surface tension and causes the water to form tiny dropletes rather than completely covering the surface. Your resin is initially a liquid and will act in the same manner. The mold release also facilitates the demolding process. This is crucial if you're not using a silicon mold since polyurethane resin and and will stick to polyurethane RTV. Nothing really sticks to silicon ... well except silicon itself.
b) heat your mold prior to casting. Heated air or gas tend to rise and by preheating your mold you can eliminate some of the air bubles that might normally be trapped at the bottom. WARNING: some mold material are more thermostable than others so read the material sheet before you heat your mold .... you definitely don't want to melt your hard work.
c) dust you molds with a thin layer of baby powder. Again, this is the same principle as as the mold release spray .... reduction of surface tension. I haven't use it since I normally use a spray release, but some of my friends will swear by it.
d) tap or vortex your mold after you've poured the resin in. Tapping the side and bottom of the mold might dislodge some of those trapped air bubble from the surface of your mold.
e) pour your resin at a steady and even pace. Pour the liquid resin into the deepest part of the mold. This will reduce the chances of trapping air pockets. Rotate your mold to help the resin fill into those difficult undercut/ negative zones. If you pour it too quickly, you have a higher chance of trapping air bubble. If you pour it in too slowly, the resin might start to cure before you're done.
Most polyurethanes have a pot life bewtween 4-7 minutes (the time you can work with it until it starts to cure). Completely curing takes anywhere from 15-30 minutes depending on the resins. You can also buy accelerators to speed up cure time.
f) monitor your resin. Most resins are moisture sensitive and only last about 1 year or less after it's been opened. Using expire resin can often result in "sweated" cast. I don't know the exact the exact chemical process but the cast itself often secretes a oil liquid that never fully dries and very hard to paint over.
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Advance Casting
Once you get the basics down, you can explore new tricks. Ever wanted to cast wood-like product? With walnut shavings and a Filler, you can recreate wood grain patterns. How about cold casting? There are filler that have aluminum or iron shaving that adds a metal sheen as well as weight to your cast. You can also do numerous other trick such as styrofoam casting by diluting SC300 with water. There are also rubber glass that cure clear and can easily be crumbled to look like ice or glass for your dioramas. You can also cast latex and different types of rubbers. There are many avenues out there and it's up to you to explore them. Remember, part of the fun of customizing is discovering new techniques and tricks. Best of luck.
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Tung