Your first casting just came out of the mold covered in rough, sandy patches. The surface looks terrible—nothing like the smooth finish you expected. You check the sand moisture. It’s at 3.2 percent, right in the normal range. So what went wrong?
Green sand casting defects damage your castings in nine distinct ways, each caused by specific problems with sand composition, mold handling, or metal pouring practices.
You can identify most defects through visual inspection before any machining starts. Surface defects show up immediately—rough texture, holes, cracks, or excess metal. Internal defects require X-ray or ultrasonic testing to detect cavities and porosity hidden inside the casting.
The nine main defect types are:
Gas defects happen when trapped air, steam, or combustion gases can’t escape through the sand during the pour, creating holes and cavities in your casting.
Blowholes and pinholes are the most common defects in green sand casting. You’ll find them as round or oval cavities—blowholes measure 3mm or larger, while pinholes stay under 3mm. Both appear on the casting surface or just beneath it.
Metal penetration happens when molten metal pushes between sand grains or chemically fuses with the sand surface, creating a rough, sandy texture that’s extremely difficult to clean.
This defect shows up as a sandpaper-like surface finish with sand grains embedded in the metal. In severe cases, you’ll see a thick layer of fused sand and metal that requires grinding or blasting to remove. The roughness extends beyond normal casting texture—you can feel individual sand grains locked into the surface.
Shrinkage defects form because metal contracts as it cools from liquid to solid, and if you don’t feed that volume loss with additional molten metal, you get cavities and porosity.
Most metals are less dense as a liquid than as a solid. Iron shrinks about 3-4 percent by volume during solidification. Steel shrinks 4-5 percent. Aluminum shrinks 6-7 percent. That volume has to come from somewhere.
Shrinkage manifests in two forms: concentrated cavities (large voids) or distributed porosity (sponge-like networks of small holes).
Shrinkage cavities appear as large, irregular voids typically located at the last point to solidify in the casting. They often have a characteristic tree-like or V-shaped appearance with dendrites (crystal growths) visible inside. The cavity surface looks rough and oxidized because it formed as the metal was partially solidified.
You’ll find shrinkage cavities in hot spots—thick sections, junctions between thick and thin walls, and areas farthest from the gates where metal enters. These locations stay liquid longest while the surrounding metal solidifies.
Shrinkage porosity shows up as scattered small holes distributed through a region rather than one large cavity. Under magnification, porosity looks like a sponge with interconnected voids between the metal grains. This happens when the metal doesn’t solidify uniformly—different areas freeze at different times, trapping small pockets of shrinkage between them.
Sand expansion defects occur when the sand in your mold heats up during the pour and expands, creating compressive stresses that buckle or crack the mold surface.
Silica sand expands when heated. The expansion is small at first, but at 573°C, quartz sand undergoes a crystalline transformation that causes sudden expansion of 1.2 percent by volume. Since the sand right against the metal cavity gets much hotter than the sand further out, you get differential expansion—the hot inner layer tries to grow while the cool outer layer restrains it.
That compressive stress has to go somewhere. The sand surface buckles, cracks, or lifts up, and molten metal can flow into those cracks and defects.
These three defects are all caused by sand expansion, but they look different and occur in different situations.
Rattails appear as long, shallow, irregular grooves on flat or slightly curved surfaces. They look like someone dragged a stick across wet sand—a V-shaped or U-shaped depression that might be 2-5mm deep and 5-10mm wide. The surface inside the groove is usually rougher than the surrounding casting.
Rattails form when compressive stress in the sand creates a small crack parallel to the surface. The crack opens up slightly, metal flows in, and you get this characteristic groove. They occur most often on flat plates, large flat surfaces on castings, and horizontal surfaces in the drag (bottom mold half).
Buckles look similar to rattails but are broader, shallower, and straighter. Think of a wide, gentle depression 10-20mm wide but only 1-3mm deep, running in a fairly straight line across the casting surface. Buckles appear on large, flat horizontal surfaces where the sand has room to move.
The difference between a rattail and a buckle is mostly geometric. Both come from the same mechanism—surface expansion creating compressive stress—but buckles form when the sand lifts in a broader wave pattern rather than cracking.
Scabs are the most serious expansion defect. They appear as rough, raised areas on the casting surface with a thin layer of metal covering a cavity filled with sand. When you grind off the surface layer, you find a honeycomb structure underneath—porous metal mixed with sand particles.
Scabs form when the expansion stress actually lifts a section of the sand mold surface away from the base sand, creating a gap. Metal flows into this gap. The sand that lifted up sits on top of the metal, gets partially fused into it, and creates the characteristic raised, crusted appearance.
You’ll find scabs on cope (top mold) surfaces more than drag surfaces because gravity helps pull the sand layer down in the cope. Large, flat casting surfaces are most vulnerable.
Misruns and cold shuts are flow-related defects where the metal didn’t properly fill or fuse in the mold cavity.
Both defects waste castings completely—there’s no fixing an incomplete casting or one with a weak cold shut line running through it. The metal already solidified wrong, and you can’t melt it back together.
Misruns occur when the molten metal stops flowing before completely filling the mold cavity, leaving an incomplete casting. You’ll see unfilled sections, typically at the extremities of the mold (the areas farthest from where metal enters) or in thin sections that freeze quickly.
The casting just ends—a normal surface that suddenly stops, often with a rounded or bulbous edge where the metal froze as it was flowing. Thin sections like ribs, fins, and flanges are most prone to misruns because they have high surface area relative to volume and freeze fast.
Cold shuts happen when two streams of metal meet in the mold but don’t properly fuse together because they’ve cooled too much by the time they meet. You’ll see a visible line or seam where the streams met, often with a slight gap or crack along the junction.
Cold shuts create weak spots in the casting. The metal on either side of the cold shut line didn’t fully bond at the molecular level. Under stress, the casting will likely crack along that line. Even if it doesn’t crack, the cold shut shows up as a cosmetic defect and a stress concentrator that weakens the part.
Both defects trace back to insufficient fluidity—the metal’s ability to flow easily through the mold before freezing.
Sand inclusion and erosion defects occur when pieces of the mold break off or wash away and become embedded in the casting surface.
You’ll find these defects as rough spots, areas of excess metal, or actual sand particles fused into the casting surface. The sand can appear on any surface but is most common on the cope (top) surface where gravity pulls sand downward into the molten metal.
Cuts and washes are erosion defects where the flowing metal literally carves a channel through the sand mold surface. You’ll see excess metal in a groove or channel pattern, often near gates where metal velocity is highest. The eroded sand washes downstream and gets trapped in the metal elsewhere.
The casting has extra metal in the eroded area that you have to grind off, plus sand inclusions wherever the eroded sand particles traveled before getting trapped.
Sand holes appear when loose sand particles fall or wash into the mold and fuse into the casting interior. You find them when you machine the casting—the cutting tool suddenly hits a void containing sand. The void might be 3-10mm in size, filled with compacted sand grains.
Drops and tears happen when sections of the mold break off and fall into the metal. This typically occurs on the cope surface or on vertical mold walls that don’t have enough strength to support themselves. The casting has an area of extra metal with sand embedded in it, corresponding to where the piece of mold broke away.
Root causes: insufficient mold strength and excessive metal velocity.
Mold handling defects happen when you damage the sand mold during pattern removal, mold assembly, or transportation to the pouring station.
These are the most frustrating defects because they have nothing to do with your sand properties or metal quality—you’re breaking perfectly good molds through mechanical damage.
Drops and tears show up as irregular chunks or pieces missing from the mold cavity surface, typically on vertical walls or undercut features. When the broken sand piece falls into the mold cavity, you get extra metal in that area with sand embedded in it. If the broken piece falls out of the mold entirely, you get an oversized, rough section on your casting.
Pattern sticking causes surface tears when the pattern pulls sand out of the mold as you try to remove it. You’ll see rough patches, torn areas, or entire features ripped away. Sticking happens when green strength is too low, when you didn’t use enough parting compound, or when the pattern doesn’t have adequate draft.
Spring-back is a less obvious defect that occurs with automated molding machines using high squeeze pressures. When you compress sand to very high density (hardness above 95), the sand actually springs back slightly after the pressure releases. This changes mold dimensions and creates internal stresses in the sand that lead to cracking and expansion defects.
