Advantages and disadvantages[edit]

Cores are useful for features that cannot tolerate draft or to provide detail that cannot otherwise be integrated into a core-less casting or mold.^[1]

The main disadvantage is the additional cost to incorporate cores.^[1]

Requirements[edit]

There are seven requirements for cores:^[2]

1. Green strength: In the green condition, there must be adequate strength for handling
2. In the hardened state, it must be strong enough to handle the forces of casting; therefore, the compression strength should be 100 to 300 psi (0.69 to 2.07 MPa).
3. Permeability must be very high to allow for the escape of gases.
4. Friability: As the casting or molding cools, the core must be weak enough to break down as the material shrinks. Moreover, they must be easy to remove during shakeout.
5. Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.
6. A smooth surface finish.
7. Minimum generation of gases during metal pouring.

Types[edit]

Many types of cores are available. The selection of the correct type of core depends on production quantity, production rate, required precision, required surface finish, and the type of metal being used. For example, certain metals are sensitive to gases that are given off by certain types of core sands; other metals have too low of a melting point to properly break down the binder for removal during the shakeout.^[2]

Green-sand cores[edit]

 

Green-sand cores

Green-sand cores makes casting long narrow features difficult or impossible. Even for long features that can be cast it still leave much material to be machined. A typical application is a through hole in a casting.^[2]

Dry-sand cores[edit]

The most simple way to make a dry-sand cores is in a dump core box, in which sand is packed into the box and scraped level with the top. A wood or metal plate is then placed over the box, and then the two are flipped over and the core segment falls out of the core box. The core segment is then baked or hardened. Multiple core segments are then hot glued together or attached by some other means. Any rough spots are filed or sanded down. Finally, the core is lightly coated with graphite, silica, or mica to give a smoother surface finish and greater resistance to heat.^[2] Single-piece cores do not need to be assembled because they are made in a split core box. A split core box, like it sounds, is made of two halves and has at least one hole for sand to be introduced. For simple cores that have constant cross-sections they can be created on special core-producing extruders. The extrusions are then cut to the proper length and hardened. More complex single-piece cores can be made in a manner similar to injection moldings and die castings.^[2]

Types of core:

• Cold Box
• half core box
• dump core box
• split core box
• left and right core box
• gang core box
• strickle core box
• loose piece core box