Many hobbyists delve into the world of metal casting to produce unique pieces. Some of these items are decorative while others serve a specific function. For large production runs of more than 1,000 pieces, consistency in each casting is important. So permanent moulds would be machined from a metal that can withstand molten brass or create wax patterns for an investment casting process. However, most hobbyists and short production runs can achieve acceptable results through the inexpensive, yet effective, sand casting process.
Create a pattern. A variety of materials are suitable for this purpose. Wood is the classic choice, but you could also fashion a pattern from polystyrene (styrofoam). Just be sure that whatever material you choose can withstand being pressed into a box of sand. Also remember to make the pattern larger than the finished casting since brass occupies more space in its molten state than as a solid. For brass, the linear expansion coefficient is 0.0000104 inches per degree Farenheit. For castings that require control over dimensions, be sure to take this expansion factor into account for creating the pattern accurately. Assuming a room temperature of 72°F and a melting point of 1,710°F, you will need to allow all dimensions of the pattern to be at least 0.017 inches larger than the solid casting.
Mix sand and clay, a process that is likely to require a few trial runs to achieve the perfect mixture. Sand usually requires the clay as a binding material to help it retain the mould impression, yet the sand acts as tiny vents to carry away any gases created by the casting process. So, some experimentation with the right ratio of clay to sand may be needed to obtain the desired casting quality. This is part of the learning process that makes this fun, despite the challenge.
Place sand and clay mixture in box. If you have a two-part mould, you will need to fill two boxes with this mixture. Be careful not to pack the sand and clay mixture in too tightly, or you may find it hard to press the pattern.
Place and press pattern. Take the pattern for the casting and place it in the sand mixture. An alternative is to partially fill the box with sand mixture, place the pattern in the box, and pack around the pattern. If you have a one-piece mould, remove the pattern and this process is complete. For a two-part mould, you will need to press patterns in both the cope and drag, the technical terms for the top and bottom parts, respectively. With a two-part mould, be sure to carve out runners and make provisions for the gate and vent. The latter provisions keep gasses from becoming trapped in the finished casting.
Assemble the mould pieces. If the mould consists of only one piece, then this process is complete. For more complex moulds, the cope and drag need to be assembled. Be sure to align the cope and drag to reduce the parting lines that will show from the casting process. If there are internal features for the casting, be sure to place the sand core in place, holding it steady with brass chaplets. These are pins made from the same material that will enter the mould in a molten state and become part of the final casting. Once the mould is assembled, it is now ready to receive molten brass.