Sand casting is a robust and cost-effective process for making parts in a wide range of ferrous and non-ferrous alloys. It’s also a complex process to optimize, because many variables are at work. Simulation helps, but issues can still arise. This blog looks at how sand casting experts go about diagnosing and resolving the most common issues.

The Casting Development Process

When presented with designs for a finished part, the foundry has to work out how best to cast it. That includes designing the pattern and cores that will form the mold cavity, plus the system of sprue, runners, gates and risers that will let molten metal flow in and fill it.

Simulating mold filling and metal solidification identifies potential problems before any tooling is made. After testing improvements, tools are made and the first parts cast. These are then analyzed to identify deviations in geometry, and to find any defects.

Inspecting the First Samples

Inspection of the first parts begins with a thorough visual examination. This looks for areas where metal hasn’t filled the cavity completely, (nonfills), surface finish problems, and other issues like cold shuts (lines where two flow fronts have failed to merge) and open blow holes.

Next, a dimensional inspection checks whether a machined part can be obtained from the casting. It covers key features and dimensions, and looks for pattern shift and deformation.

Casting any complex part includes forming internal features, and checking these dimensions requires sectioning. This also provides an opportunity to look for defects inside the casting.

Commonly Identified Issues

Almost all casting problems can be grouped under four headings:

• Gas porosity: Looks like bubbles in the metal, because that’s what they were.
• Shrinkage: Can appear as voids within the body of the part, as hot tears around regions where the section changes, or as geometric faults.
• Problems with mold sand and cores: Poor surface finish and geometry problems often stem from these aspects of the process.
• Oxides and inclusions: Cold shuts are one form of oxide problem, but oxides can also get in as metal is poured. Inclusions are non-metallic particles in the cast part.

Root Cause Identification

Making changes to the tooling and/or casting process requires an understanding of what caused the problems. The following list is not exhaustive, but covers the most common causes.

• Gas porosity: Either metal flowed into the cavity in a turbulent manner, capturing air bubbles, or there was moisture in the mold. In the case of turbulence, the solution is to slow down the flow by reducing the gate cross-section. However, there may also be a problem with the metal solidifying too quickly and not letting the bubbles rise to the surface. Moisture is a particular problem with green sand, as it’s needed in the clay binder that holds the mold together.
• Shrinkage: Metal contracts as it cools and solidifies. To allow for this, the metal feeding system includes a “riser”. This acts as a reservoir of metal that cools slower than that in the mold cavity and feeds back to make up for shrinkage. Shrinkage defects occur if this process doesn’t work correctly.
• Problems with mold sand and cores: Sand, especially green sand, needs some permeability to allow gases resulting from moisture to leave. If the sand is compacted too tightly, this won’t happen, resulting in poor surface finish and open blowholes. Movement of sand and cores can happen when metal flows into the cavity too quickly. Misalignment of top and bottom mold halves indicates worn or poor quality tooling.
• Oxides and inclusions: Oxides are a particular problem with aluminum and result when the cavity fills from multiple directions. They may also be brought in if metal is poured too quickly. Inclusions in sand casting are most often sand particles washed off the walls of the mold by fast-moving metal. They can also be particles of refractory material that came away in the furnace, ladle, or pouring channels.

Impro: Sand Casting Professionals

Sand casting is a complex process, so it shouldn’t be surprising that the first sample parts cast often have some problems. Careful analysis will determine the root causes, letting the foundry team refine the process and deliver high quality parts.

As a leader in metal casting, Impro combines the latest tools and technologies with the experience of foundry professionals to identify root causes and make improvements. Contact us if you need a source for high quality metal parts.