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Note: The room-temperature vulcanizing silicone rubber used in moldmaking is expensive, and mistakes can be costly. In addition, many of the materials used in casting can be potentially hazardous. I take no responsibility for any losses you may incur by following these instructions, and please follow all safety precautions when molding and casting. When using casting resins, make sure to work in a properly ventilated area (or wear a respirator), and wear latex of vinyl gloves to avoid contact with the liquid components. Keep all molding and casting materials away from children.. About recasting: Obviously, the instructions below can be used to recast existing kits. I do not condone or advocate this activity. But the fact is, recasters will (and do) get the info somewhere. I offer this information to those who wish to make castings of their own prototype sculptures (or want to cast small parts to duplicate them for dioramas, etc). Thanks to: Tom Seiler and John Ulakovic, both of whom shared invaluable knowledge on the molding and casting process. Part I -- Moldmaking: Getting Started First you will need to gather your materials for moldmaking. Non-sulfured,
oil-based modeling clay (Klean Klay) The most important material you need at this stage is Klean Klay. This is a brand of non-sulfured modeling clay. Most modeling clay you find at the store has sulfur in it, and this will harm the cure of your silicone. Thus you must use Klean clay or make sure the clay you use is sulfur free. Klean Klay can be bought in 1 lb. packages from Micro-Mark (which carries tons of great modeling supplies).
Klean Klay can be purchased in 1 lb. packages from Micro-Mark (www.micromark.com) Foamcore makes a good mold box, but for smaller molds I use .040 styrene. Whatever you use should be essentially waterproof. The clear tape will help bind the sides and bottom of the mold box together and seal it. The sculpting tools you will need will help smooth the clay around the edges of the prototype and the mold box--I generally use a dentist's spatula--it's a great shape. You can also make your own spatula tools from copper wire or steel music wire. For this demonstration I will make a mold of my Slimer Mk II sculpture. The sculpture was sculpted in parts (body, 2 arms, upper and lower jaw) to make molding and casting easier. You may have to cut your prototype into parts to make molding it easier. I can't advise you on this, but use common sense. Do some simple two-part molds to get an idea for how it works, and then you will start ot figure out how you need to cut up your prototype, if necessary. It also helps to study other model kits to figure out how they were molded. First, I cut a piece of foam core for the base of the mold box. Next I will need to suspend the Slimer prototype in a clay bed.
Foam core base, prototype and blocks of Klean Klay You want to suspend your prototype about an inch off the surface of the mold. Why do this? The main reason is to that when you pour the silicone, air bubbles suspended in the rubber will start to rise.
Making a two-part mold in two pours If you start with a clay bed for the first pour, the bubbles will rise AWAY from your prototype, toward what will be the outside of your mold. Later, you will flip the whole thing and clear off the clay and do a second pour, and again, the bubbles will rise away from your prototype. If bubbles get lodged against your prototype they can create problems in the surface of your mold and affect all subsequent castings (believe me, I learned this the hard way). When embedding a prototype in clay, you must avoid undercuts that can "lock" your prototype in the silicone mold.
Undercuts and how to deal with them In the illustration above, note the undercut. When the silicone flowed into this gap it formed an obstactle to removing the prototype (and all subsequent castings). You must identify undercuts and deal with them. Shallow undercuts are usually not a problem as the silicone is flexible enough to pop out of them. But for deeper undercuts you will have to take action. Sometimes they can be filled in, but other times you may have to reposition the prototype or cut parts off so that undercuts can be avoided. I deliberately sculpted the Slimer Mk II in five parts to make it easier to avoid undercuts. Now that the Slimer is suspended, let's start building up the clay bed.
Yep, I got the base too small I realized the base was too small, and increased the size of the foam core. You need about an inch clearance on every side. Above, you can see that I am adding more clay supports. Important note: when building your clay bed, use chunks of clay and don't worry if there are small gaps between them. This actually makes the clay easier to remove later. Notice that I'm creating little "pillars" of clay to support the weight of both Slimer and the silicone which will be poured over him.
Beginning to form the surface of the clay bed Above, I;ve created a slab of clay that will serve as the top of the clay bed. This needs to be roughly at the halfway point around your model. Notice the support pillars under it.
Continuing both the supports and the bed surface
Working my way around
Working outwards now Now I'm getting close to the edges of the mold box. You want the supports to go all the way out to the edges. Notice how the Slimer is about halfway embedded in the clay.
Detail of clay bed meeting the prototype At this point I'm smoothing the surface of the clay bed with my fingers. There's still no airtight seal at the surface of the prototype but there will be soon.
Angle view of the developing clay bed
Creating airtight seal at the edges of the prototype At this point I switch to a spatula tool and start carefully smoothing the clay up against the prototype. This is essential so the liquid silicone doesn't creep down between the clay and the prototype.
Here is a homemade spatula tool that is useful, too I make a lot of my own tools. To make the spatula above, I heated steel music wire with a propane torch and then flattened it against a jeweler's anvil with a hammer. Then, using the flat of a Dremel cut-off wheel as a grinding surface, I shaped it into a flat, rounded spatula. Then I used sanding blocks to smooth the steel spatula. Then it was back to the propane torch. I heated the wire just above the spatula and added a "dog-leg" curve to make it easier to use. Then I added a handle and the tool is ready to go!
Detail of creating the airtight seal
Smoothing near the seal with my finger
Cleaning the prototype with 90% isopropyl alcohol and a brush Now that you've created the seal around the prototype, you can clean any bits of clay off the prototype by gently scraping the clay off with a tool and using a brush dipped in 90% isopropyl (rubbing) alcohol. The alcohol dissolves the clay quite well. You can also smooth the seal where the clay meets the prototype with alcohol and a brush.
Adding a mold box side with clear strapping tape Now it's time to build the rest of the mold box. Note that the sides need to be tall enough to allow about 3/4 of an inch to one inch of silicone above the highest point of the prototype. Use tape to secure the side to the bottom.
Adding a pour spout Here I am adding a clay form that will eventually become a pour spout so that I can pour resin into the mold. Every mold needs both pour spouts and air vents (sometimes called gates). The pour spout should be cone shaped to allow easy pouring of resin, but should taper as it meets the model.
Refining the pour spout I used a tool to improve the taper. When you begin casting, you will cut off the resin pour spout and this is easier when the taper is smaller. Obviously it still needs to be big enough to allow the easy flow of resin into the mold.
Continuing to refine the pour spout Here I have raised the wall and am making sure there is an airtight seal against the wall.
Adding an air vent Now it's time to add an air vent. This is to allow air to escape as you are pouring resin into the mold. Otherwise air will bubble out through your pour spout which is messy. I'm using brass rod, but you can also use flexible aluminum wire or other materials.
How the pour spout and air vent work Use more clay to seal the air vent to the wall and to the prototype.
Time to add another wall of the mold box
Adding clay to create a seal Add bits of clay to create an airtight seal against the walls of the mold box.
Smoothing the seal
Tools for creating keys In order for the mold halves to fit together tightly, you need to create keys. You can use any rounded object to create the keys. Once again, I created a tool from acrylic rod that works really well. Take a length of acrylic rod and, using the sanding drum of the Dremel tool sand it until it is rounded. Then use fine sandpaper to smooth the surface of the rod. You can also epoxy ball bearings to brass tubing to create an effective key tool.
Press the rounded point of the rod into the clay Twist the rod as you press it into the clay to make a smooth key dimple. You want the dimples to be about 1/2 to 1/4 inch deep.
A perfect key dimple When making key dimples, don't get too close to the prototype.
Using two different sizes of tools, key out the whole surface
Finished with the mold box This is the finished mold box for Slimer's main body. Before we proceed to the pouring of the silicone, let's take a look at some of the other clay bed setups I had to make for this model kit.
Building up the clay bed for Slimer's right arm This would prove to be the most complex mold I've had to make so far. It really takes some imagination to figure out how to mold complex parts like this. I will walk through the steps. Note above that I have detailed both the top or the mold box (arrow), the pour spout ("P.S.") and air vent ("A.V.").
Contouring the surface around the hand I had to create a contoured surface for this mold that would follow all the fingers of the hand. I viewed this as creating kind of a "catcher's mitt" web between the fingers.
Another view of the contours
Smoothing out the surface
Closeup of the hand When creating any kind of mold, you want to build up the clay bed to the halfway point of the prototype. Here I've carefully built up the clay bed from behind the hand, bringing up the clay bit by bit with spatula tools so that the fingers are half-embedded in the clay. Time-consuming but worth it.
Adding air vents with aluminum bonsai wire In my articles on sculpting I have sung the praises of bonsai wire, the aluminum wire used to train bonsai trees. Here it is again--the flexible wire is highly useful for making curved air vents for contoured molds. Above, I created a larger vent that follows the curve of the fingers, with smaller vents feeding from the tip of each finger into it.
Closeup of the hand air vents I use tweezers to press the wire halfway into the clay. Note that I've also used clay to blend the tips of the fingers into the wire, and to join the bits of wire to each other. The large wire vents to what will be the top of the mold.
Finishing out the mold box I've added the pour spout (base of the arm at left), key dimples and all but the last mold box side. As noted, this is a very complex mold, but it was the only way to get the job done. Note the cutout sections on the side and bottom. This will help create better mold halves.
Lower jaw mold box Here's the mold for the lower jaw in progress. As you go, you need to be aware of the top of your mold and where problem areas will occur. I realized after adding the two cone-shaped pour spouts (one is actually an air vent) that air bubbles would form in the tongue (rounded mound at center). So I added a wire vent there.
Upper jaw mold box This is Slimer's upper jaw section. Again, note how I vented it. It's helpful to imagine the section of prototype filling with liquid resin from bottom to top and where air pockets might collect (usually at high points that are not vented). Then add vents to these potentially problematic areas. I'll probably still get some air bubbles in that tooth at the very top, because I didn't place the vent at the very highest point on the tooth. Oops.
Animated resin flow diagram This animation shows how resin (yellow) and air bubbles (white) will flow into the mold and how most of the air will flow out through the vents. There are still a few potential problem areas, as shown by the white gaps on the top two teeth. You have to try and visualize where air will collect and then vent those areas.
Left arm mold box If you thought the other arm was complex, this one was WORSE, because the fingers on the hand curled over the palm. I decided it was best to cut off half the hand (with a jeweler's saw) and make a separate mold for it. This shows the arm with the two fingers removed, and the mold is a lot more simple as a result. Note the placement of the air vents.
Left hand mold box Here are the missing two fingers! They got their own mold. Note that I vented one finger into the other. That's it for this installment! In Part II, we will pour the silicone to make the molds. Go to Moldmaking & Casting Part II |
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of the questions I'm most often asked is "how can I mold and cast
my sculptures to make model kits?" I've created a step by step tutorial
to show you how. This isn't a complete guide, but it should be enough
to get you started.
