Understanding the Tooling You Buy
With every quote you receive from a foundry, there will most likely be a line item specifying the tooling required to produce your casting along with its estimated cost. Tooling types (commonly referred to as pattern equipment) and costs can vary greatly between foundries, so it is important that you understand exactly what you are buying. The following tools are common in the sand casting process.
To produce the overall shape of your casting, a pattern will be required. The pattern itself is a positive shaped, full-sized model of your casting, along with any core prints (more on cores in a bit) and the required gating and riser system. Depending on the size and weight of your casting and the flask size of the mold, the pattern may have more than one impression of the casting. When molding sand is compacted around the pattern, it creates a negative shape in the mold, which when assembled and poured, produces your casting.
The patterns can be made from wood, plastic, or metal depending on the annual volume of pieces you intend to procure. The patterns are mounted onto an aluminum or wood board that creates the parting plane and separates the cope and drag mold halves. With higher production volumes, the pattern is often cast integrally into an aluminum matchplate, which offers increased durability along with the ability to have contoured or offset parting lines.
Cope and drag patterns and mold boxes for the air-set molding process are similar types of foundry patterns. Where a matchplate has both the cope and drag pattern mounted to the same pattern plate, cope and drag patterns are mounted to separate pattern plates, since the production process requires the mold halves to be made separately.
If your casting design contains internal passages, you will need to procure a core box, or perhaps multiple core boxes depending on part complexity. A core box contains the negative shape of the internal casting passage, and when filled with sand, creates a core that is placed into the mold. The core is supported in place by the core prints that are formed in the pattern.
Core box types vary depending on the shape or complexity of the core along with the process used to produce the core. The core box can have one or more cavities depending on the physical size and shape of the core. Materials used for core boxes vary from wood and plastic to metal for higher production parts. Iron core boxes are often constructed when the core shape dictates the use of a heat curing core process, such as the shell process. Shell cores often also require that ejector systems be added to the core box to facilitate the removal of the core from the box. When ejector systems are required, be aware that they can add significantly to the total cost of the core box.
The simplest type of core box is called a dump box. Envision a child making sandcastles at the beach, filling a bucket with sand, turning it over and pulling the bucket off. In this analogy, the bucket is basically how a dump core box works. The core box is filled with prepared core sand, compacted, and then simply turned over and the box is pulled off, creating the shape of the core. In the production of large castings, two dump cores are often pasted together to create one large core. This is done to make it much easier to handle the core box due to its size.
The most common type of core box is a two-piece box, usually referred to as a split core box. The simplest shape it will produce is a core that is often called a pin core; one that is cylindrical in shape. Split boxes have two or more alignment pins in them to correctly locate the halves when they are together.
Some casting designs require the use of loose pieces in the core box or pattern. Loose pieces are removable parts that are manually withdrawn from the core after the core is removed from the box. The loose piece creates a shape in the body of the core (such as an undercut) that could not be produced by any other method.
With the advent of 3D sand printing, highly complex molds and cores can be produced without the need to build complex tooling. You should consult with your foundry’s engineering team as to what method will be most economical for your particular casting design. CS