Due to the complexity of the casting process, there are many opportunities for things to go wrong, resulting in casting defects or unwanted irregularities in the metal casting process. Some defects can be tolerated, some can be fixed, but some must be eliminated. In order to determine the nature of the defect; examine the probable cause and take appropriate corrective action and its location; appearance; shape and size must be determined.
All defects will fall into one or more of the following categories.
Inclusions
Inclusions classified as negative defects may be the result of foreign particles (usually non-metals such as slag) contained in the casting. Inclusions can also form from missing areas of metal in the casting, resulting in small, round, irregular or angular craters. In addition, inclusions may be depressions or voids of various sizes; usually there are traces of ceramic and refractory on the surface.
There are many possible causes of inclusions; some may occur early in the casting process, such as cracks in the wax mold allowing ceramic fragments to enter the mold cavity; in some cases, ceramic material suspended from the shell may fall off during the casting process .
tears
Also known as shrinkage cracks, hot tear defects appear as jagged cracks with an irregular path, and can occur when molten metal is constrained from shrinking by a ceramic shell mold during cooling or solidification. Casting designs may sometimes need to be modified to reduce major section variation and prevent thermal cracking.
Machine error
Classified as a negative defect type, misalignment can occur when the metal cannot completely fill the mold cavity, leaving a very smooth, rounded edge. Possible reasons are that the metal is too cold; the shell is too cold or the filling is too slow.
cold close
Cold shut occurs when the two metal streams do not fuse together properly in the mold cavity, resulting in discontinuities or weak points within the casting and cracks. Potential causes are low pouring temperature, slow pouring speed or too cold shell.
Gas/porosity
The gas or porosity appears as a round, smooth-walled cavity; it may exhibit slightly oxidized surfaces with different diameters, and it is during cooling that bubbles form within the casting. This is because most liquid materials can hold large amounts of dissolved gas, which is expelled as the metal solidifies. Blowholes may appear on the casting surface in the form of blowholes, or blowholes may be trapped inside the metal.
run out
Spill occurs when molten metal leaks out of the mold during the pouring process, causing the part to not form properly leaving voids, which in most cases are smooth in appearance, but can also be rough and irregular of. Potential causes are thin ceramic shells, shell dewaxing cracks, or handling in foundries.
shrink
Has a jagged or linear appearance, shrinkage defects when the casting section changes (such as thick or too thin); at the feed gate of the casting; or when the thick metal solidifies during cooling, making it impossible to use standard feed metal to compensate for shrinkage.
Positive Metal/Splash
Spatter, classified as a frontal defect, may be caused by molten metal "splashing" onto the casting. Fortunately, this type of casting can often be removed without damaging the part. In some cases, this defect may be due to a failure of a weak area of the casing during the dewaxing or casting process, causing metal to leak into the voids of the casing.
wax damage
Wax damage is known as negative and positive defect types and is often mistaken for a running error. However, wax damaged parts will develop sharp edges instead of the more rounded edges seen in misalignment. As the name suggests, one possible cause is improper handling of the wax model (negative), but there can also be positive defects due to dripping wax from the wax component.
convex
Not always detectable by the naked eye, but can be detected by measurement, and the bulge can be identified by the gradual thickening of the casting wall. Potential reasons are that the wax pattern is too close to cause bridging of the ceramic shell or high metal static pressure.
Understanding casting defects and their causes is critical to managing casting quality. For more information on investment casting defects, click here.