Casting defects are unwanted irregularities that appear during the metal casting process. Defects increase manufacturing costs and waste and can lead to products and parts that do not perform as designed. Knowing the most common defects that appear during the metal casting process lets you know what to watch out for.
The 6 Categories of Casting Defects
Most casting defects that occur can be categorized as one of five types: gas porosity, shrinkage defects, metallurgical defects, pouring metal defects, mold material defects, and casting shape defects.
When gases are trapped inside the molten metal during casting, porous areas are formed inside the part as the casting solidifies. These gases often show up in poorly vented areas of the casting and can occur when too much dampness is present.
Found below the part’s surface, blowholes are large cavities that often are not found until the part is machined or analyzed with x-rays. When covered by thin layers of metal, blowholes are called blisters. The presence of these holes in the casting is caused by gas entrapment.
2. Open Holes
When blowholes appear at the casting surface, they are called open holes, caused by gas trapped in the casting during pouring.
Smaller than blowholes, these pockets of porosities are typically less than 2mm in diameter and are often found in groups with many other pinholes. These groupings tend to occur in the cope (upper) part of the mold in poorly vented pockets.
As the metal used in casting cools, the size of the part shrinks. While this shrinkage is normal, careful consideration is required to prevent shrinkage in the cavity from occurring and ruining the part. Uneven shrinking can also leave residual stresses in the part, altering the part’s performance. Keeping the liquid metal in the mold at an even temperature is key.
4. Open Shrinkage Defects
Open to the atmosphere, air compensates for these defects as the part shrinks leaving surface deformities on the casting. Pipes and caved surfaces are two types of open shrinkage defects that occur on the surface of the casting.
5. Closed Shrinkage Defects
Similar to porosity, closed shrinkage defects occur below the surface, inside the casting. Hot spots and isolated pools of hot liquid are common causes of closed shrinkage defects.
During or after solidification, a casting can take on an unwanted change in dimension. This type of deformity can render the casting ineffective and is often more pronounced in large, flat sections of the casting.
Improper cooling can change the microstructure of the finished part in different sections.
7. Hot or Hard Spots
If sections of the casting are able to cool more quickly, hard spots can occur due to the change in microstructure. These hard spots can add difficulty to machining processes.
8. Hot Tears or Cracks
As the casting cools, cracks or tears can form typically in the form of irregular crevices in a branched pattern.
Pouring Metal Defects
Cold shuts, misruns, and slag inclusion can all occur if the molten metal used in casting is not prepared and handled properly. Too low of temperatures limits the amount of time the liquid metal has before solidifying.
If the molten metal is not properly heated it may not flow properly to all of the extremities, resulting in a misrun. Castings with parts missing are a clear sign of a misrun.
10. Cold Shut or Lap
A line or crack with a round edge on the casting surface is a good indication of a cold shut defect. This surface defect creates a stress concentrator that makes the overall casting weaker.
11. Slag Inclusion
Often called scabs, these irregular metallic crusts appear on the casting surface. Only a few millimeters thick, slag inclusion or scabs are similar to rat tails.
12. Cold Shots
Globules formed from splattering during pouring can become entrapped in the molten liquid.
Mold Material Defects
If the mold being used is not in good condition or is not prepared properly, the mold itself can lead to defects in the casting. Soft molds from a lack of ramming can lead to many of these defects,
13. Cuts and Washes
These areas of excess metal appear when the molten metal erodes the molding sand. Often the defect is tilted in one direction, showing the direction the metal flowed as it entered the mold.
When the grains of sand in the mold fuse with the molten metal, fusion defects occur. The casting will have a thin crust with a brittle, glassy appearance to it. This appearance will firmly adhere to the casting.
As the name suggests, swells appear as an enlargement of the casting. Swells typically have the shape of a light, smooth bulge on vertical casting faces.
16. Run Out
When liquid metal leaks out of the mold, you have a ‘run out’ defect. Because there is not enough molten metal left, the part typically appears incomplete or missing.
Irregularly shaped projections on the cope surface of casting are called drops. These defects occur when sand falls or drops into the casting while the metal is still in the liquid phase.
18. Metal Penetration
If the molding sand has gaps, liquid metal will often penetrate the mold. Look for a rough, uneven surface finish on the casting.
19. Rat Tails, Buckles, and Veins
Irregular lines or cracks on the casting are called rat tails or reins. When rat tails are really bad, they are called buckles. Typically these defects occur on the bottom surface of the mold.
Casting Shape Defects
Even when the mold is prepared properly, defects can occur during the casting process.
20. Shift or Mismatch
For casting to set properly, the upper (cope) and lower (drag) parts of the mold must line up correctly at the parting line. This type of defect is easy to detect as the casting will look as though the mold shifted at the parting line.
21. Flash, Fin, and Burrs
Any unwanted and excess material attached to a cast is considered a flash, fin, or burr. Typically a thin sheet of metal, a flash often occurs at the parting faces.
Many different aspects of the metal casting process can lead to a defect in the finished casting. Knowing the basics of metal casting defects gives you an idea of what to look for when inspecting metal castings.
Properly identifying the defect is the first step in determining how to prevent the defect from occurring again. Contact Transvalor USA to learn how casting simulation software can take casting defect mitigation to the next level.