As the name suggests, a mold made of sand is used to cast metal. The sand is cheap and reusable. Mixed with clay, it is adaptable and its grains are small enough to obtain an even surface on all of your casted parts.
This post will be illustrated with the frequency convertor seen below, a project that SurfaceID worked on a couple of years ago. The frequency convertor is an electronic outdoor unit which converts and relays data input into radio waves. It is protected against weather and interferences by two heavy clippable shells. Those shells are what called for sand casting.
The sand casting process is simple. You build a sand mold from the negative space created by two patterns and fill it with molten metal. Here’s an infographic showing the upper shell of the convertor:
The image below shows our four patterns—CNC produced tops and their respective bottoms—before sand is poured into the flasks. Notice how a gating system has been incorporated as part of the patterns. This will form a natural reservoir and tunnel into the sand which will help to pour our molten metal into the mold.
The X-ray of the frequency convertor is interesting because each one of those patterns is in view:
Now, we fill the flasks with sand. At this stage of the process, the mix is soft and easily covers our pattern. We ram the sand a bit so it’s not too loose and because the mix is wet, we let it dry.
We then assemble our two-parts casting flask, and pour metal through the gating system.
Pouring molten metal into a box of malleable sand isn’t immune to failure, and some of the casts had to be put into the foundry cemetary. Rough spots usually appear because of mold erosion; this is when worn out sand drop into the metal and decrease its strength. Metal penetration into the sand also happens when it is too coarse.
Here’s what the good ones look like once we break them out of their mold. The sand casting is done, but the shells will require a bit more work.
CNC machining is once again necessary for threading and other features that require utmost precision, like creating a gap for the waterproof seal. You can see the contrast of textures between the sand casted metal and the one that has been milled:
The shell is now usable. We finally put a coat of paint on it and the assembly of frequency convertors can start!
In conclusion, sand casting is both a versatile and cheap process. Pretty much everything involved in its operations can be reused. The surface finish won’t be as perfectly smooth as when using complex and expensive tooling like CNC or die casting, but if you need heavy-duty parts coming from a low initial investment, sand casting might be your best candidate.