Though a variety of material choices are available for making molds, the general method of construction more or less remains the same. The most simple and economical material is a superior grade of casting plaster, such as the one purchased from a art supply store. If you opt for this mode, make sure that the plaster is totally dry prior to pouring the molten lead in the mold or otherwise an explosion may occur.
For speed, simplicity and convenience, hobby metal professionals generally design their parts, so that a simple, single-piece and open faced mold will work. This rules out the problems of dealing with a sprue, cope, risers, and alignment. The side of the metal, which is exposed to the atmosphere will have a layer of crystallized metal on top, however when the castings are machined all across, it creates no difference.
Some of the important steps involved in forming a sand mold are -
- First, select a flask of adequate size to permit an inch or two of sand between the mold cavity and the wooden sides of the flask. For two part molds, large size is considered as better. More weight helps keep the metal in place and hold the cope from floating up and providing leakage between them.
- Position the upside down drag on a molding board, which is adequately big to contain it. Position the pattern in the drag upside down and sprinkle them with parting dust.
- Riddle sufficient sand into the flask in order to cover all parts of the pattern minimum ½ inch deep. After this press it into steady contact with the pattern using your hands. After this has been done, you can heap sufficient unriddled sand into the drag to cover the pattern by 2 or 3 inches.
- Use a small ram to pack the sand steadily around the drag and go around it twice. After this has been done, use a bigger ram to pack the sand over the remaining mold and add two or three inches more sand, and pack again. Repeat the process till the time you have packed sand to about 1/2 inch above the flask. You do not have to hammer the sand with the ram, simply thump it firmly. The objective here is to create an even density all across the mold.
- At this stage, the excess sand can be strike off by cutting down with the striker to the edges of the flask, and scraping it off to produce a smooth surface level with the flask. Take multiple cuts, taking out merely one or two inches at a time. If you like to fill in any uneven regions with more sand, pack it gently, and strike it off again. For bigger parts, it is a better to vent the sand under the pattern.
- Disperse some loose sand (1/4 inch deep or so) over the mold, position the bottom board on it and rub it in with a circular motion to produce a complete solid contact between the flask and sand. At this point, you can pick up the mold by holding the bottom board (and the molding board) in good contact with the flask. Turn the mold over and remove your molding board.
- Vent the mold by the vent wire straightaway down into the sand to the bottom board, once every square inch or so all across the surface of the mold, but no closer than ½ inch to the pattern. Gently brush or blow off the inevitable particles of sand, which will be broken up around the vent holes when finished.
- If this is a single-part mold, you can rap and draw the pattern now. Other than dry-out time, you are finished. In this case, after drawing the pattern, very gently, using the corner of a bent knuckle or the little corner where the thumb meets the palm, compress and round off the top edge of the mold cavity just a tad to reduce the risk of small pieces of sand breaking off and falling into the mold. Don't do this to two-part molds though.
For two-part molds, here are the remaining steps:
- Mark off the gating on the drag. Simply draw it into the sand - it will transfer to the cope sand. Apply the parting dust all across the drag and pattern in an even manner and then place the cope onto the drag, and repeat steps 3 and 4.
- Vent the cope sand using the vent wire and cut the sprue if you do not mold it in (recommended). Lift the cope and complete it by cutting any risers or gates and the funnel shape of the sprue's pouring basin.
- Metal casting professionals recommend to dampen the sand around the pattern with a small brush before rapping and drawing the pattern. After rapping and drawing the pattern, cut the drag gates and remove any loose particles from the drag.
- In the final stage, mildly place the cope back on the drag, and cover the sprue and risers to stop any foreign material from falling into the mold.
- Left the mold to dry out for a week or so.
When readying to pour, allow the melt get hotter than just liquid. The more hotter it is, the more fluid it will be while pouring that will help to keep away cold shuts and improper filling of the mold. Allow the aluminum to get beyond just shiny liquid.
While designing the pattern, don't forget that smaller and thinner parts of the metal will solidify first, and the thicker parts of the casting will render liquid to the smaller ones as they solidify. Hence the thicker parts will tend to contract towards the edges, producing a shrink, or void, in the center. To ensure homogeneity, make sure to place a bigger riser, or reservoir near the larger parts of the cavity to facilitate the supply of liquid to the casting as the metal solidifies.