The basic concept of the sand casting process is to use a mold to create the casting. The mold contains a negative impression of the casting as the molten metal will be poured into the mold. To create the mold, a pattern is used. The pattern helps design the outer walls of the mold. However, many parts are designed to create complex internal structures where there may be spaces that the molten metal should not enter.

For example, there may be a part with an internal hole that is used to slide the part into a complex assembly system. A rod will pass through this hole to hold the part in place. To create this area, a core will be used during the casting process.

What is a core?

A core is an insert that is placed into the mold. The sand is compacted and pressed, then poured over the pattern. The role of the core is to direct where the molten metal will flow to fill the cavity in the mold. Some cores can be very complex. They may have internal angles and geometric shapes to create complex holes and channels for the molten metal. They may also be solid to create internal cavities, or hollow so that the core allows the metal to fill other parts.

What are cores made of?

Cores will be made using the same resin sand used in the casting process. Sand is placed into a core box with the desired internal pattern of the part. Depending on the manufacturer's capabilities and equipment, the sand is hammered, heated, or blown into the core box. The core is then removed from the core box and placed into the mold.

Cores can be made using both hot box and cold box methods. The hot box method is where resin is added to the sand and a catalyst is used. Heat is used to activate the catalyst as the core hardens. The cold box method involves using a catalyst that does not require heat activation as the resin and sand harden to create the core.

The core and mold are made using the same resin sand because the resin sand can withstand the temperature of the molten metal without damage. Once the metal cools, the mold is broken. To remove the core inside the solid metal, the machine will perform a vibration process that shakes the part and simultaneously breaks up the sand. There is usually a hole in the mold so that the sand can be shaken out and poured out. If this hole is not needed for the finished part, this hole can be sealed during the finishing process.

How is the core held in place?

Since the core will be inside the mold as the molten metal is poured into the mold, it will move or rise from its designated position. The core holder is made of the same metal as the cast part. This metal will hold the core in place until the metal is poured into the mold. The core seat that touches the molten metal will become part of the actual part once the metal cools. The core is then shaken apart and removed.

Another way to hold the core in place is to make the core longer than the mold itself. This method is used when the space that cannot be filled with metal will extend beyond the outer walls of the mold. A groove placed in the sand mold will hold the core in place.