Semi-permanent mold uses sand cores, or non-reusable inserts to create cavities, hollows, channels, passages.
Permanent mold casting is the low-turbulence, gravity-induced method of creating custom metal components with a reusable steel mold.
Semi-permanent mold casting is exactly the same process, but with the addition of a non-reusable component: A sand core. This slight difference essentially combines permanent mold casting with sand casting.
A sand core is a one-time-use insert to create cavities, hollows, channels, passages in a casting. While you only need 1 permanent mold to create 100 parts, you need 100 sand cores to create 100 parts!
Sand cores are custom structures made of sand and a binder. Core makers use different recipes and ratios of binders depending on the sand core’s application.
Creating a sand core is similar to creating a sand mold. Today, some sand cores are even 3D printed.
Before each pour, a sand core is placed in one half of the mold. The mold is closed, then molten metal is poured inside. The metal fills the empty space without penetrating the sand core.
When designing castings, it’s important to keep in mind the flexibility and opportunity that sand cores provide. However, keep in mind two things:
Therefore, we cannot cast a hollow ball.
Also, keep in mind that the internal sand surface finish of your casting will be lower quality than the steel-mold-given external surface.
Since you must make a disposable sand core for each casting you wish to produce, the process goes from permanent mold to semi-permanent mold.
When the casting cools, you are left with a solid aluminum part formed around a sand core. The sand must be removed by bake-out or shake-out.
The casting can be heated to a high temperature until the sand core “bakes out” of the casting. This can process can be paired with heat treat or degassing to save manufacturing time.
To remove the sand core, the parts are aggressively vibrated or shaken until all the sand separates from the metal.
Since 1946, Batesville Products has been manufacturing permanent mold (and semi-permanent mold) aluminum castings. Ask our experts anything!