Tips: Casting

Casting

EASY MOLDS

Make your own molds to reproduce small parts. Mix a small amount of epoxy putty, flatten it to about 25 mm thick pancake with a rolling pin or small round bottle, then coat the top surface with a non-stick cooking spray such as Chef Mate. Before the putty cures, press the original part into the putty and carefully lift it back out; make sure that the depression has a sharp outline. When the putty is fully cured, respray with Chef Mate and fill the mold depressions with either a 50/50 mixture of superglue and baking soda (for small parts) or 5-minute epoxy (for larger parts). When set you have the duplicate part you want.

WORKING WITH RESIN

Resin is one of several polyurethanes that are impervious to solvent cements. It is necessary to use either super glue or epoxy to join resin parts. Super glue is better because it is easier to sand. Epoxy can be rubbery and hard to sand. It is also best to lightly sand the parts to be joined before gluing. As a mold-release agent is used in resin casting, it is necessary to wash parts well in soapy water before gluing or painting. Where there are pinholes or other imperfections in the resin casting, fill these with gap-filling superglue and when hard, sand before priming with lacquer, enamel or water-based acrylics. If an area of the resin repels the primer, there may still be mold-release agent on it so wash or sand again and try spraying in light, dry coats.

AIR BUBBLES IN RESIN

Tiny air bubbles tend to spoil many resin castings and when painted these are very visible. One way to deal with this problem is to fill them with a thin mix of Liquid Poly and model filler such as Humbrol Filler or Revells Plasto. You can apply the mixture with a flat brush, removing any excess with a cotton bud damped with a little Liquid Poly. Another method is to use a thick acrylic paint applied in the same way, this time, any excess being removed from the surface using either Isopropanol or Tamiyas thinners. If you use gap-filling super glue, the trick is to use an accelerator on the glue, then sand straight away while it is still soft. Gap filling super glue sets immediately, but does not cure for an hour or so. That is when it gets really too hard to sand.

Softening plastic in hot water to bend or straighten a part also works with resin. But do not let the water boil. After a few minutes you should be able to unwrap the resin part, then hold it in the straightened position until it cools.

WORKING WITH RESIN

Polyurethane resin is a two-part plastic material which is pourable and cures quickly at room temperature, setting hard within minutes. It is a useful material for modellers and cottage industry manufacturers because it can be used to mold thin detailed pieces. However, while it is not a particularly potent toxic material, it can cause respiratory irritation if the fumes are inhaled. When working with polyurethane resin, always wear a mask, safety glasses and rubber gloves. Resin parts often feel oily and it is caused by a precipitate from the curing process. This film of chemicals is difficult to remove. Before painting, the model should be washed down with lacquer thinner or alcohol and then lightly sanded to provide a key for the paint to adhere to.

WORKING WITH RESIN

Resin is one of several polyurethanes that are impervious to solvent cements. It is necessary to use either super glue or epoxy to join resin parts. Super glue is better because it is easier to sand. Epoxy can be rubbery and hard to sand. It is also best to lightly sand the parts to be joined before gluing. As a mold-release agent is used in resin casting, it is necessary to wash parts well in soapy water before gluing or painting. Where there are pinholes or other imperfections in the resin casting, fill these with gap-filling superglue and when hard, sand before priming with lacquer, enamel or water-based acrylics. If an area of the resin repels the primer, there may still be mold-release agent on it so wash or sand again and try spraying in light, dry coats.