Surface Defects
Defect:
- Cold flow: molten metal flows too slowly and cools before filling the mold
- Cold lap: two streams of molten metal meet and do not fuse together, creating a seam on the surface of the casting
- Chill: area of the casting that solidifies too quickly, resulting in a hard, brittle surface
- Non-fill: mold cavity is not completely filled with molten metal
- Swirls: marks on the surface of the casting caused by turbulent flow of molten metal
Cause:
The leading edge of the metal flow is too cold and laps together.
Laminations Defect
Defect:
Cause:
Mostly caused by poor metal flow control, but other causes are possible.
Gas Porosity Defect
Defect:
Cause:
Gas trapped in the metal flow during die fill.
Blisters Defect
Defect:
Cause:
Gas trapped just under the surface of the metal during die fill.
Flow Porosity Defect
Defect:
Cause:
Metal flow is too slow, too cold, or has a poor flow pattern, leaving spaces (porosity) between solidified metal flows.
Shrinkage Porosity Defect
Defect:
Cause:
Cast metal takes up less space when solid than when liquid. This creates spaces (porosity) in the casting, especially in hot spots.
Heat Sinks Defect
Defect:
Cause:
Shrinkage cracks just under the surface.
Leakers Defect
Defect:
Cause:
A loose dendritic structure inside the casting is exposed by an opening in the casting skin. This opening provides a leak path. Leakers are another version of shrinkage porosity.
Cracks and Tears Defect
Defect:
Cause:
Many causes, including shrinkage cracks on the surface, casting being stretched in the die, mechanical stress at die opening, ejection, or from trimming.
Inclusions Defect
Defect:
Cause:
In aluminum, inclusions are often oxides caused by poor melt-cleaning and furnace cleaning practices. They can also be caused by furnace refractory. Iron aluminum inter-metallics in zinc alloys can lead to polishing and machining problems.
Carbon Buildup Defect
Defect:
Cause:
Carbon buildup is a defect that can occur in castings when deposits from lubricant, or water mixed with lubricant, accumulate on the die surfaces.
Outgassing Defect
Defect:
Cause:
Bubbles appear on the casting surface during a painting or finishing operation. This is caused by a leak path that develops through the casting skin when the casting is heated during finishing, allowing the heated and expanding trapped gas to escape.
Edge Porosity Defect
Defect:
Cause:
Either shrinkage of the molten metal as it cools and solidifies, or the presence of trapped gases.
Bending and Warping Defects
Defect:
Cause:
A combination of design flaws and operational inconsistencies.
Flash Defect
Defect:
Cause:
A combination of high metal temperature, poor die fit, and inadequate machine locking during high-pressure casting.
Stained Castings
Defect:
Cause:
Contamination of the molten metal with foreign substances, most commonly die lubricant but potentially other materials as well.
Waves and Lakes Defects
Defect:
Cause:
Early solidification of metal flow during casting, typically observed in decorative zinc castings. This occurs when the initial metal flow cools rapidly, forming a separate skin that does not re-melt during subsequent filling. The surface of this area exhibits a finer grain structure and a slightly different appearance compared to the rest of the casting.
Drags Defects
Defect:
Cause:
Deformation of the casting during ejection due to undercuts in the mold. Undercuts can result from buildup on the die surface or die erosion caused by solder.
Die Erosion
Defect:
Cause:
The cause of die erosion, cavitation, and burn out can be high metal velocity, bubbles in incoming metal, or high oxide or silicon content in metal.
Solder Defect
Defect:
Cause:
Aluminum or magnesium can combine with die steel, causing cast metal to stick to the die surface. In zinc, the zinc forms a layer on top of the steel.
Deformation from Ejector Pins Defect
Defect:
Cause:
The casting is still soft and adheres to the die during ejection, causing it to bend or deform as the ejector pins attempt to push it out.
Excessive Flux Defect
Defect:
Cause:
Excessively applied flux can lead to increased porosity and surface corrosion. This can be identified by submerging the casting in clean water overnight and observing for white spots in areas with porosity, or by examining a fracture through the porous area.