Shrinkage rate
Shrinkage holes are different from pores with round, smooth surfaces; they appear as jagged, angular edges. It is normal for metal to shrink as it cools and solidifies. However, if shrinkage is uneven, distorts the shape of the part, or creates internal voids, it becomes a defect. Common shrinkage defects are:
• Open shrinkage defects. They appear as depressions (depressed surfaces) or holes (tubes) on the surface of the cast product. Open shrinkage occurs when the metal alloy shrinks unevenly and air is drawn into the mold.
• Closed shrinkage defects. These appear as holes inside the casting where the molten metal is heated unevenly. They can appear in microscopic or macroscopic form. Macroscopic shrinkage looks like jagged lines or marks and is visible to the naked eye. Micropores, on the other hand, look angular and can be seen with a microscope.
• Warpage. This shrinkage occurs during or after the metal solidifies and changes the shape and size of the part. They cause the metal to bend into flat or larger parts.
Causes
• High metal concentration in a specific area of the mold.
• Injection pressure is too low.
• Poor runner and gate design.
• Uneven pattern of metal solidification.
• Temperature differences at different locations of the molten metal.
• Extremely high pouring temperatures.
Remedial Actions
• Use simpler casting geometries with improved runner and gate design.
• Use simulation software to improve runners to ensure optimal filling of the cavity.
• Increase metal injection pressure.
• Insert cooling coils, fins, or internal cooling to ensure proper heat dissipation.
• Clean metal surfaces to remove contaminants.
2. sink
Sinking is a downward pressure impact on the casting surface that does not accurately replicate the mold design. When there are subsurface cavities, depressions appear on the casting surface. These depressions appear along the thick wall areas of the metal surface. You can easily identify these casting surface defects by visual inspection because the water groove reflects light.
Causes
• Uneven wall thickness of the casting often leads to uneven solidification shrinkage.
• Partially overheated molds cause the casting to solidify slowly in these overheated areas.
• Insufficient cavity venting, which causes gas to compress between the liquid metal surface and the cavity surface.
• Low injection pressure.
• Short holding time and poor charging effect.
Remedies
• Optimize the casting structure design to ensure uniform wall thickness.
• Improve the venting performance of the mold cavity.
• Avoid overheating the mold cavity and give the metal a longer holding time.
• Increase the metal injection pressure.
