Sand casting is a versatile and cost-effective way of making metal parts, but several aspects need close attention if defects are to be avoided and efficiency maximized. Here’s a look at the top four.

Patterns and Cores

The pattern produces the hole in the sand that forms the mold or cavity, so it’s shaped in the form of the part to be cast. Cores are shapes that create hollow regions within the cast part. They are placed into the cavity after the pattern is removed and stay there until the metal has solidified.

There are two challenges to address with the pattern. First, making a mold involves packing sand around the pattern, which must then be lifted out. That requires the mold be made in two halves, and that the pattern lifts out easily without disturbing the sand.

Making the mold in two halves means there will be a parting line around the edge of the casting where upper and lower molds meet. The part designer needs to be aware of this and decide where they want it positioned. Bear in mind while doing this, that undercuts aren’t allowed because they would stop the pattern lifting free from the sand.

In addition, vertical sides of the pattern need a slight angle outwards from the center of the part. This “draft” is to ensure that pattern lifts clear without disturbing the sand. Draft angles in sand casting are typically 1° to 2°, and must be considered during part design.

For cores, the considerations are similar. These have to come free of the core boxes that form the shapes needed, so again, they may have draft angles and parting lines.

Feeding System

The mold cavity formed by the pattern needs channels for metal to flow in through. These are:

• The sprue: A vertical channel that metal is poured into.
• Runners: Horizontal passages feeding metal into the cavity.
• Gates: Restrictions on the runner cross-section that regulate how quickly the cavity fills.

Filling rate is a key process variable to optimize. If metal flows in too quickly, turbulence incorporates air that becomes small holes or “porosity” in the metal part. However, if it fills too slowly, the metal will start to solidify before the cavity is full, resulting in “nonfill” defects.

Leading foundries use sophisticated software tools to model mold filling and so optimize the dimensions and placement of gates, runners and sprue.

Riser Design

The riser is another vertical opening in the sand, similar to the sprue. In this case though, the riser fills after the mold cavity is full.

The purpose of the riser is to form a reservoir of metal that remains molten as the metal in the mold cools. This means optimizing the surface area-to-volume ratio to reduce the cooling rate below that of the part being cast.

As the metal in the cavity cools, it also contracts. The riser supplies additional liquid metal to make up for this shrinkage. If the riser is too small or incorrectly located, nonfills and hot tear defects, (a form of crack), are likely.

As with the feeding system, advanced modeling software is used to optimize riser design and placement.

Sand

Traditionally, sand casting was done using “green” sand. Today resin sand predominates, although green sand is preferred for some parts.

“Green” means it contains some moisture. This is in the clay binder that holds the sand together around the pattern. The modern alternative is to coat the sand with a resin. Various types are used, most of which have a heat-activated binding mechanism.

Resin sand forms stronger molds, which can create thinner walls and finer detail in the cast part. In addition, there are fewer defects like inclusions, caused when particles are washed out by the flowing metal. It can also save on secondary machining, which for many parts offsets the additional cost. For parts that don’t need fine detail though, green sand may be satisfactory.

Experience Yields Results

While sand casting is a proven process, consistent success requires a high level of finesse. As a leader in foundry operations, Impro has the expertise and technical resources needed to maximize quality and efficiency. If you’re exploring sand casting for your next project, or you’d just like to learn more about our capabilities, contact us.