concretesubmarine.com/ FORUM

Metal casting in a foundry is a complex process. Test methods are in place to help check casting quality assurance. These inspection methods help identify potential casting defects. Casting, like other manufacturing processes, is susceptible to a variety of defects, whether sand or lost-wax casting. Casting defects represent undesired anomalies in the manufacture of metal castings. Different types of casting defects include surface defects, inclusion defects, forming and casting defects, and cooling defects.

very rough metal

Some casting defects, such as very rough surfaces, are visible to the naked eye.

Surface defects

Surface casting defects are visible to the inspector. These include very rough or uneven surfaces; "veins" or "rat tails" caused by mold cracking at high temperatures; "elephant hides," which wrinkle from rapid cooling; burnt sand; and peeling defects. Mold release defects are metal flakes on the surface due to gas trapped in the mold.

Inclusion Defects

Slag

Slag inclusion is a defect in which a non-metallic material creates pockets or band-like inclusions in a casting. Slag is an important part of the melting process in furnaces and is required for quality castings because chemical processes in silica and calcium-based slags change the viscosity of the metal. During the pouring process, however, foundry workers must keep the slag away from the mold itself. Just before pouring, the slag is usually skimmed off, but not all of it is captured at this stage. The gate or gate of the mold also needs to be formed to capture it. Failure at any step in the process could allow the slag to contaminate the metal. Additionally, metal oxidation can be a problem for metals exposed to elevated temperatures for extended periods of time. Slag will form inside the mold. Oxides are not the only slag: carbides, sulfides or nitrides can also be defective.

Sand inclusion

Sand inclusions are common. As with all sand castings, expect a small amount at surface depth. However, excessive sand inclusions can damage the casting. In this defect, the sand in the mold is trapped in the metal.

If the surface has a lot of sand inclusions, it may indicate the need for mold cleaning or a different styling system. In foundry sands that have been baked for stability, molds can become brittle in the oven for too long. Perhaps a better option is no-bake molding or investment casting. This defect may also indicate that there is not enough adhesive, or the wrong kind of adhesive, to form a solid shell. Another possibility is that the sand is not filling enough.

Forming and pouring defects

A piece of metal on a blue background has a small miss-run turf on the exterior wall

Misalignments may be small and look like mechanical damage, but rounded edges will make them disappear.

Defects that leak or run incorrectly

These casting defects occur when not enough molten metal is introduced to fill the mold. Open risers are sometimes used to tell when the mold is full: if they are misplaced, the casting may be under poured. Occasionally, early icing during the pouring process can lead to misplacement. In a poor run, parts of the casting are incomplete, often with rounded edges, and the metal freezes before it reaches the mold walls.

Punch or floating core defects

Defects of this type can cause castings to be dimensionally wrong. Molten metal applies pressure, and if the cores are not stabilized or the mold is not clamped properly, they will lift up, deforming the casting.

flicker defect

Flash at joints is common in castings but must be removed. Flash is the thin skin that forms when liquid metal seeps between the closed joints of the mold. If the mold is not stable, flash can be severe enough to deform the casting.

Die mismatch or die shift defect

This can happen if the upper and lower dies or the top and bottom of the dies are not aligned when closed. The resulting castings are often comical because the top and bottom of the mold are askew.

The word DIGITAL is cast metal. The first three letters are ambiguous due to casting issues.

Die instability can cause detail problems, but it can also cause fluctuations in larger dimensions.

Sand instability defect

Sand instability, insufficient filling, or problems with the foundry sand mix can result in a loss of casting detail. This problem can affect fine details, such as lettering or decoration, or it can cause the edges of the entire cast to wobble.

Cold lap or cold close

These occur when the temperature of the mold or molten material is too low. Instead of fast flow throughout the mold, part of the metal flow slows down and begins to harden. This icy bulge becomes an obstacle surrounded by other metal swirls: like a rock in a river, there are often disturbances on the downstream side of the obstacle. The visible cold lap round lip creates a permanent discontinuity in the casting surface. Such defects are visible on the surface and sometimes small enough to be ground or filled, but cold cuts or lap joints can penetrate deep enough to threaten the structural integrity of the casting.

cold shot

Cold bullets are also caused by premature freezing of some metals. They look like small iron balls or teardrops, fixed and suspended in the material around them.

Large grey metal casting with holes scattered across the raw surface

Porosity or pitting has small round holes near the top of the mold caused by trapped gas.

Pinholes and pores or stomata

This can be caused by the gas in the mold pushing the molten metal to leave voids or bubbles as it cools. If the mold is not porous enough to allow gas to escape, trapped gas may be caused by the conditions of the mold. Rusty, hydrogen-rich scrap metal charges are more prone to pinhole defects because they bring in more hydrogen. Porosity is not the same as shrinkage, although both leave small holes in castings and can be found in the same piece of metal. Air holes tend to leave regular, bubble-like pinholes near the top of the mold.

cooling defect

Shrinkage rate

Shrinkage cavities are caused by insufficient metal volume in the mold. As the metal cools and shrinks, small holes remain throughout the casting. These holes are usually jagged compared to the smooth floating pinholes caused by the gas.

An industrial wheel is cut in half on the x-axis and shown with jagged cavities through the metal

Shrinkage cavities reduce the strength of the casting.

Shrinkage

These are like shrinkage cavities because there is not enough metal to fill the space as the casting cools and shrinks. However, this defect is often a more serious structural problem. Often, these shrinkage cavities look like large, jagged cracks in the center of the metal volume. The casting is cooled from the outside in, so the cavity can be formed where it cooled last. This is not always the case: Shrinkage depressions visible on the outside of the casting may appear, depending on the infrastructure of the gates, runners and risers that support the casting. These depressions are usually not jagged or open, but where the metal appears to be dented compared to the intended shape. By changing the casting design, with tapered walls and smooth corners, it is possible to repair objects that suffer from a lot of shrinkage,

cooling defect

Cold casting defects can occur if the mold is too cold, or if the casting is removed from the sand too quickly. Rapid freezing can cause casting surfaces to become very brittle as if they had been air quenched. When the defect is mechanical rather than structural, brittleness is eliminated by heat treatment, and annealing the casting may be able to rescue it.

Cross section of a metal casting showing cracks through the edges

Hot tearing indicates the fragility of the cooled metal to crack under shrinkage stress.

hot tears or cracks

Cooling too quickly can also produce hot cracks or cracks. These are similar to shrinkage cavities, but in metals are more band-like voids caused by rapid shrinkage. Castings with hot tears or cracks have poor mechanical properties.

Cooling deformation

This is a common defect in very long, thin castings, but can also occur in other shapes. In this defect, the metal casting deforms during cooling and deviates from its intended shape, causing it to exceed specified tolerances.

an examination

Dimensional tolerance or surface finish issues can often be detected by visual inspection and measurement. Mechanical properties testing can also be performed simply. Other issues, such as porosity and shrinkage cavities, are internal to the casting. Test methods can be used to catch internal problems so that castings do not fail unexpectedly under load.