Making Sand Molds and Patterns

You played with sand when you were a kid right? The idea is simplicity itself. Make a void in sand matching the shape of the object you want to create. Pour in the casting material to fill the void and... voila! - we have cast the object. There are a few details to manage. On the left we can see a a freshly rammed sand mold that I made to cast the bases for the Gingery Mill. The drag side (the bottom half) is entirely sand with the cores sticking up from the part line. The cope side (the top half) forms the top of the bases and also contains the sprue. The cohesion of the molding sand holds everything together, so long as the mold is not jolted. This mold will soon be assembled and the metal will be poured in. At that point it is in the hands of the casting Gods...

Making Molds

This picture shows the three essential parts of the mold:

The drag and cope are made from butt joined 3/4 inch pine. The size is determined by the size of the pattern you wish to cast. The idea is to have 1-2 inches of molding sand surrounding the casting. Since ramming sand is not much fun (at least for me) you are best off with a mold just large enough for the casting. On the inner wall of both the cope and drag, a strip of wood has been nailed to hold the rammed sand firmly in place.

The alignment pins are designed to hold the mold halves in good alignment while being very easy to take apart. Having a relatively shallow taper on them is a good way of achieving this. There are typically two alignment pins on either side of the mold. They should be attached in an asymmetrical way. By doing this we won't make the mistake of installing the cope on the drag the wrong way around.

Alignment pins made from unseasoned woods have a tendency to shrink as they dry- causing alignment to become sloppy. It's not mandatory- but if you have well seasoned hardwoods available they would be the first choice for alignment pins.

The whole thing is painted to protect the wood- but DON'T paint the alignment pin bearing surfaces. If you do they won't fit together properly and they will be hard to take apart. Rubbing some candle wax on them is enough to both protect and lubricate them.

The drag and cope have some angle iron tabs screwed to the side that can be bolted together. This prevents the drag and cope from coming apart. For smaller castings these are not necessary, but the hydrostatic pressure of molten metal in a larger casting can float the cope off the drag and this is not a good thing. Bolting the mold halves together is cheap and easy piece of mind.

The molding board is just a piece of plywood the size of the mold, with some feet screwed to it. The feet allow you to get your fingers beneath it. This makes it easier to move and manipulate the mold.

Making Patterns

By time spent, making metal castings is mostly about woodwork, because the patterns used to create the sand void are made of wood. The wood working does not have to be structurally strong, but it does need to be dimensionally accurate. It should also have a smooth surface finish to make it easy to remove the pattern from the mold after the sand has been rammed. On the left we see three patterns that were used to cast the bases and bed of the Gingery Mill.

Soft woods such as pine and poplar are good woods to use. Manufactured woods (plywood and MDF) should generallly be avoided because they are harder to shape cleanly and the end grain will be harder to finish. Joints can typically be glued. This will be strong enough and you will avoid splitting the wood with a nail or screw.

When a pattern is made you should think about how it will release from the sand, and what surfaces need some draft (slight taper) either built or sanded into them. One of the trickiest parts of sand casting is getting the pattern out of the sand cleanly. Adequate draft and a smooth surface finish for the pattern will make this alot easier.

Auto body filler is used to create fillets for internal corners. This does two things- it prevents a sharp edge in the sand (which will tend to crumble) and it gives extra strength to the cast object. When a pattern is finished outside corners should be rounded over, inside corners should be filleted, and the whole pattern should be smooth. The side of the pattern that will end up on the mold parting plane should be completely flat.

The pattern should be painted to protect the wood and add to the general smoothness of the pattern. I use a couple of coats of spray primer followed by a couple of coats of spray varnish. E.g. Varathane. I think the solvent based paints give a better finish, but the water based paints will also get the job done.

Molding Sand

One of the first things you learn about sand casting is that you don't use regular sand- in fact the quality of the sand is a critical factor for the quality of the castings. It has to be wet enough to hold its shape, dry enough to prevent excessive steam, fine enough to cast detail, and coarse enough to allow steam to escape. It's a tricky balancing act. The Dave Gingery book talks exclusively about a "greensand" made from a sand, clay and water mix. The sand mold must be well vented to allow the steam to escape rather than creating cavities or blowing the molten metal back out the sprue.

I confess- I took the easier road by purchasing a 100lb box of pre-mulled "Jupiter Blend" molding sand. This is fine sand that has been mulled with an oil based binding agent called Petrobond. Because it has no water, there is no steam- and while some gases are liberated from the oil, I haven't found venting to be necessary. The sand has a somewhat oily texture and smells bad after it has been burnt by the molten metal- but it works well and eliminates some trickiness from the casting process. The sand closest to the molten metal will be burnt to black dry sand after casting. This used sand can be blended back into the mix or can be scraped away (as best as you are able) and discarded. Unburnt sand can be used again.

Ramming the Mold

Here we see two loose patterns that have been positioned in the cope half of the mold to be rammed with sand. For each pattern a strategy for ramming needs to be developed. Should you ram the cope or the drag first? Should you use a split pattern (one that comes apart at the cope/drag parting line)? In this picture the two bases for the Gingery Mill are being rammed into a single mold. The cope will be molded first, with the sprue at the top of the pattern. Then the cope will be turned over and the drag will be rammed on top of it. When the mold is completely rammed the mold will be turned right way up, the patterns will be removed, and the core of the bases will stick up from the part line of the drag.

To be successful, the patterns must separate cleanly from the sand. This is a delicate and tricky operation. The extra time spent on the patterns to ensure they were well drafted and smooth will pay off here. "Parting Dust" is applied to the patterns to make them easier to remove. There is a professional grade parting dust you can use, but talcum powder also works well and smells nice.

Here the bed pattern for the Gingery Mill has been rammed into the drag half of the mold. A piece of wood has been placed at the side of the pattern to form the gate upon which the sprue will be placed. The part line of the mold has been dusted to allow the cope and drag to separate cleanly when it comes time to remove the pattern.

Here's a closeup of the cope half of the Gingery Mill base mold. The sprue was created by placing a dowell at the top of the patterns during the cope ramming. This mold has had good separation from the patterns. There is some flaking around the pattern edges on the parting plane, and this will lead to some casting "flash". This is of little concern. A few minutes of post processing with a file will take these off the casting. In general you don't need to be too obsessive with bits of sand that are missing, but you should care about sand present in a place which should be void. Of course, when a core breaks off in the pattern that's a real drag, but somehow we cope.