All casting involves pouring liquid metal into a cavity where it cools and solidifies. In sand casting that cavity is formed in green or resin sand. “Green” refers to the sand containing moisture from the clay binder, and not to its color. Resin sand is where the sand is coated with a resin binder.
The cavity is made in two halves. A pattern replicating half of the part being made is placed into a box and sand is packed around it. The box is turned over and the pattern removed, leaving a cavity in the sand. The process is repeated to form a cavity replicating the other half of the part in a second box.
The two boxes are stacked together and metal is poured in through a hole in the upper box. Once it solidifies, the boxes are opened and the sand broken away to reveal the cast part.
Internal cavities in the cast part are created by placing cores in the sand mold. These are removed after casting, so the patterns must be designed to make this possible.
Beneficial Characteristics of Sand Casting
The process can be automated for low-cost, high volume production
Suitable for a wide selection of materials
Green sand permeability helps reduce porosity and nonfills
Downsides to Sand Casting
Slow cooling and solidification (sand insulates,) results in a coarse microstructure
Minimum wall thickness is around 3/16”-¼”
Patterns need a draft angle of 1- 2° to release from the sand, so perfectly vertical walls aren’t practical
Parts may need considerable machining to produce tight-tolerance features like mounting holes and bores