Creating Internal Cavities in Sand Cast Parts
Cavities in parts made by sand casting, like valve bodies, pump housings, and engine blocks, are there mainly to channel, guide and hold fluids. They can also reduce finished part weight and save on material. In principle they could be machined out of the solid casting, but that would be slow and expensive. A better approach is to incorporate them into the part when it’s cast, so reducing, even eliminating, machining operations.
These cavities are created by cores placed in the sand mold. How these cores are made, held in place and removed is the subject of this blog.
An Overview of the Sand Casting Process
Sand casting is an inexpensive means of producing parts as small as a few inches in size and ounces in weight or weighing hundreds of pounds and measuring several feet. It can be used with both ferrous and nonferrous alloys although not with superalloys. (These generally need vacuum melting and pouring equipment.)
The cost advantage of sand casting results primarily from the simplicity of the mold-making process and the low cost of the mold material. “Green”, (so-called because of its moisture content,) or resin sand is packed around a pattern slightly larger than the part to be produced. The pattern is scaled-up to allow for shrinkage during solidification and to provide draft angles and machining allowances.
The mold is made in two halves so the pattern can be lifted away from the sand. If the part being sand cast needs internal cavities, these are created by placing cores in the space left in the sand when the patterns are removed.
The halves of the mold are then assembled and clamped together. This forms an internal cavity for the metal to fill.
Metal is poured in through channels in the sand, and allowed to cool. Once solidified, the mold is opened and the sand broken away. The cores are also broken up so they can be removed.
Cores are usually made from sand like that used to form the mold. This may be either a green sand or dry sand bound together by a resin. There are various types of resin binder, some of which are activated by heat and others chemically.
The core is produced in a core box. Similar to the casting process, this is a box containing a cavity the size and shape of the core needed. Sand is blown in at high velocity, which causes it to pack together at a consistent and controlled density. The resin is then activated and once the sand is bound together the box is opened to release the core.
Holding Cores in Position
Cores are held within the mold cavity in two ways:
- Adding features in the mold called core prints
- Using metal supports called chaplets.
Core prints are shelves or pockets at the outer extremities of the cavity that carry the core. Cores are lighter than the metal being poured and will float up if not held in place. Cores must be strong enough not break under the forces imposed by the liquid metal, yet sufficiently fragile that they can be broken out after metal solidification.
Chaplets are metal stands that hold the core. These become part of the cast part and where possible are made from the same metal. Chaplet placement must be considered carefully during part design to avoid creating areas that reduce the strength of the cast part.
Consult Experts at Impro When Your Parts Need Internal Features
Cores are some of the most difficult aspects of sand casting to get right. Their size, shape and strength must be appropriate for the sand mold and part design, and they must be located in a way that stops them moving as metal flows into the cavity.
Balancing these sometimes conflicting objectives requires a detailed understanding of the casting process and a great deal of experience. If you need sand cast parts with internal features, the specialists at Impro can help. Contact us to start that discussion.