Model Makers - The Professionals, Part 2: Mold Making - Hard and Soft Molds

In the newest excerpt, Susannah Shaw continues her look at mold making with hard and soft molds.

Stop Motion by Susannah Shaw. Reprinted with permission.

The Sculpt

If the puppet is to have a foam latex, or silicone covered body, you will need to first sculpt your model to make a mold to cast these materials in. This is called the sculpt or the maquette. You may also need to make some hard parts for your model: sometimes feet, or hair, or even faces may need to be hard. The general rule is: If you are casting a hard piece, you will need a soft mold (silicone) and if youre making a soft piece, youll need a hard mold (plaster, resin or fiberglass).

Make your armature. You may want to make separate parts of the body that can join together. In this case its useful on the armature to glue (epoxy) brass sleeve tubing at these points (arms to hands, neck to head). Its best to use a very firm clay for your maquette, as details and fine lines have to hold as it goes through the foaming or molding process. Blair Clark, visual effects supervisor at Tippett Studio prefers a Chavant clay. Mackinnon & Saunders in the U.K. use Harbutts, now made by Newclay Products. Others use Plastiline. Build up the clay and sculpt to the right shape.

Sculptor Stuart Sutcliffe, working at Mackinnon & Saunders, sets a mirror on the other side of the character he is sculpting, so that he can check the figure for symmetry. When you look at things, you tend not to see any discrepancies, your eye gets used to it. But with a mirror, the image is reversed, it confuses your brain and you can suddenly see all the discrepancies: theres a big lump on that side, or theres a sharper curve there than the other side.

Stuart Sutcliffe at work. Courtesy Mackinnon & Saunders. Photo © Susannah Shaw.

Textures

For textures such as wrinkly skin, dinosaur skin or fabric you can use ready-bought stamps from various sources or make your own taking latex casts off any surface: old leather, almond stones, bark, leaves, stone. To make good facial wrinkles on Plasticine, cover it first with cling wrap, then mark with a sharp edged tool, it just softens the sharp edges.

Undercuts

The presence of undercuts, i.e., a corner or curve that will be problematic when trying to release the mold, is probably one of the most important aspects of mold making. To assess how many pieces you will need for your mold you will need to look the model over very carefully to see if there is an undercut. Dont rush this stage. You will need to work out whether you will need more than two divisions for the mold, and where those divisions should come.

Seams

You will also need to think carefully about where the joins come on your model. This is because when you first take your cast out of the mold, you will inevitably have excess foam or silicone around the join (flashlines), which will need to be cut or sanded away. It would be unfortunate to design your mold so that the seam comes over the face, or some other exposed area. The sides of the body are generally easier to clean up.

The different elements need to be worked out the body might be cast in foam latex; the head and the hands might be cast in silicone, which means theyll need separate molds. For maintenance purposes, if its a series, hands need recasting on a regular basis. Because the wires in the fingers are heavily used, they should have a separate mold so you dont have to cast the whole body each time. The body should only need to be cast once it should last for a whole series, especially if its got a costume over it.

Texture stamps. Courtesy of John Parsons. Photo © Susannah Shaw.

Making a Hard Mold

Making a plaster or resin (hard) mold for foam latex or silicone:

Make a bed of potters clay or any clay of a different base to your sculpt potters clay is water-based and soft enough to bed an oil-based clay (Plastiline, Chavant) sculpt in without spoiling the detail.

Build up walls around the bed to the height you need, with card, foamcore or LEGO. LEGO is versatile, re-usable and can easily be found at yard sales. You can build up the walls to whatever height you need.

Bed the sculpt into the clay, making sure the clay comes up to your division marks. Ensure that the clay fits exactly around at your mark; it must seal all the way round the model. Cling wrap can be placed underneath the majority of the sculpt before it is embedded in clay. This is to make the clean-up process easier when preparing to make the other parts of the mold. Any creases can be easily touched up on the sculpt.

At this stage you also need to make key or location points that will ensure your mold halves fit exactly together. These can be made using small cones of clay, or make a dip with a marble at several points in the clay around your sculpt. (Make sure you dont sink the marble any further than the halfway mark, or youll have an undercut problem!)

You will also need to put in channels to allow the excess foam latex or silicone to escape when you press the mold together.

Plaster Molds

Plaster is cheap, non-toxic and quick. It can crumble if handled a lot. Cristacal or Ultracal is recommended. Health and safety warning: Ultracal has lime in it wear gloves when handling. Keep away from eyes.

To make a plaster mold, brush on your first layer of plaster, thisll ensure plaster has got into every corner. Coat the sculpt with layers of plaster, each coat being added when the previous has become warm. (Plaster warms up as part of the chemical process of hardening.) When cool again, it is set; turn over and take away the clay.

Coat the first half of your mold with a release agent a petroleum jelly like Vaseline is the cheapest and most effective. Then repeat the plastering process over the other half of your sculpt.

If there are more than two parts to the mold (this can depend on the shape of your model), you will need to repeat the process for each part.

The problems of undercut: (a) wrong, (b) correct. © Alec Tiranti Ltd.

Resin Molds

Fast cast resin more expensive, this is a polyurethane-based resin therefore quite toxic, but useful for series work and features as it is very strong.

Fiberglass resin (or GRP, glass reinforced plastic) uses a catalyst (and an accelerator if required) and is built up in layers with a fiberglass matting. Useful for series work, very durable, but toxic.

Epoxy resin mold brush on the first layer, and then pour on the remaining resin. Make sure your box is tight, as you wont want resin leaking over your furniture. The resin can be mixed with metal powders such as aluminum for strength, e.g., a two-part 50:50 mix sets in about five minutes (depending on type purchased). Consult with the manufacturer.With these polyurethane-based resins there are certain safety measures you should protect yourself with. Wear a mask to avoid inhaling fumes. Use a barrier cream on your hands as prolonged use can cause dermatitis. Wear goggles. If there is any chance of the resin making contact with your eyes. All the containers should have instructions on them.

(a) Body mold (left); (b) Glove mold. Courtesy of ScaryCat Studio.

Making a Soft Mold

Silicone Molds Silicone is very versatile. As it is so tear-resistant you dont need to worry about undercuts, you can pour the silicone into a containing box, and, once set, release the cast from the mold with one cut and maneuver your cast out. Silicone requires a catalyst. It can also be used for Plasticine as a press mold.

Plasticine Press Molds Making repeat models in Plasticine can be useful; the Plasticine is built up layer by layer in the mold. Rex the Runt was made with silicone molds. The silicone is tough and quick to release, making it useful for series work. Fastcast resin or plaster could also be used to make a press mold. For hard press molds you would need a reliable release agent. Soapy water, washing up liquid or petroleum gel can be painted into the empty mold as a release agent to help remove the Plasticine after it has been pressed into the mold.

There are seven basic rules for mold making:

For a hard cast, use a soft mold; for a soft cast, use a hard mold.

Plan your undercuts.

Think ahead with seams/flashlines.

Remember to add location/keys to your mold pieces.

Remember your release agent.

Remember to block vents after casting.

Dont rush!To read more about craft skills for model animation, check out Stop Motion by Susannah Shaw, Burlington, MA: Focal Press, 2004. 206 pages with illustrations. ISBN 0-240-51659-1 ($34.95).

Susannah Shaw is program development manager for the Animated Exeter Festival. She was head of Bristol Animation Course from 1996 to 200 at the University of the West of England and former camera assistant at Aardman (working on Close Shave among other films).

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