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Submitted by AndrewM on April 27, 2008 - 12:22am.
The simplest and, generally, least expensive method for creating a mold is through layering the mold compound on your blank. This method is good for simple objects that are flat on the largest side, without any major indentations or thru-holes parallel to the flat surface. Thru-holes are not an immediate disqualifier, however, if you are willing and able to machine the hole in your finished object. Just plug the hole with a little wax or clay prior to creating your mold, and drill it out afterwards.
The easiest and best application of layered molds is for small objects with an outside surface area less than one cubic inch. Layered molds are also useful for when you are creating a mold from only a portion of an object, such as when you only want to use the face from a gargoyle statue, as opposed to the entire head or statue. Larger molds can also be created using the layering technique, but under most circumstances you will want to use a one piece poured mold.
The main choice for hobby level mold making is to use liquid latex as the mold compound. Liquid latex is available through most craft and hobby stores, and is relatively inexpensive given the amount of latex that comes standard (usually enough for several dozen layered molds). Application of liquid latex is as simple as using a small paintbrush and "painting" it onto the your blank. A word of warning, liquid latex has the tendency to crack and bubble while drying when applied in too thick of an application; as such it makes a very poor choice for poured molds.
Thin layers of latex will generally dry at room temperature in about 45 minutes. You may use a hairdryer to speed the process up if you are in a hurry, although you should always allow the first two coats of latex to dry naturally. After each layer dries, you will want to apply a new coating of latex to the mold. For objects less than one cubic inch in surface area, 12 to 15 coats will usually suffice. For larger objects more will be needed. Make certain you continue to apply each coat to the flange area as well.
If you find that there are some "soft spots" in your completed mold, usually seen as nearly transparent areas when the blank has been removed, you can apply additional latex to the outside of the mold. Just be very careful not to warp the mold when doing so or, better yet, place your blank back into the mold prior to applying additional latex.
A liquid latex layered mold, depending on the thickness of the walls, is usually good for a dozen castings or so. After each casting the latex seems to break down a bit, eventually leading to a warped, pitted, or otherwise unusable mold. The more casting resin the mold holds (i.e. the bigger the object you are making), the quicker the break down, mostly due to the heat generated from the casting resins.
There are other choices for materials you can use when making a layered mold. In a pinch I have used rubber cement for very small molds, as well as aquarium sealant, which is one part silicone cement, for midsized molds. For very large castings lacking fine detail, fiberglass can make an excellent, long lasting mold.