Sinks are exterior die-casting defects resulting from shrinkage porosity. Surface depression is another common expression for this defect. As the name suggests, this casting oversight ends up causing outside dents, ranging from random to uniform shapes.
The development of shrinkage porosity close to the outer surfaces initiates the downward depression. Porosity due to shrinkage develops on exterior sides due to its retention of extreme temperatures. The die of molten metal alloy solidifies last on those spots.
Determining defects like sinks can help avoid or minimize the prospective damage. This article briefly described the mechanism, causes, effects, and countermeasures for surface depressions or sinks.
Causes of Surface Depressions
Shrink porosity begins forming at specific locations with the die starting up, maintaining almost the same die temperature. Distant heavier sections from the gate or a featured thin wall between heavier sections and the gate are mainly prone to shrinkage.
A thin layer forms during the castings’ filling as the porosity lies beneath the surface. The surface becomes extremely heated by that time with no cooling efforts. And those scattered shrinks at the bottom flat surface start combining to create a large void.
Looking into the mechanism brings up the following factors that may trigger sink defects in die casting –
- Considerably large differences in wall thickness.
- Improper structural design of the moldused for casting.
- Poor positioning of the gate to feed metal flows.
- Small/confined cross-sectional area of the gate.
- Low injection ratio while casting molten metals.
- Extremely high initial/local temps of the mold.
- Existing mold damages.
- Failure to completely release the entrapped gas/air.
Effects of Sinks in Die Cast Parts
The most concerning effects of having surface depressions on die casts include –
- Compromised/reduced aesthetics.
- Formation of metal beads on the surface.
- Further expansion of voids or dents.
- Additional costs in recovery and fix.
- Damaged integrity in critical sections.
- Eventual formation of visible cracks.
The solidified depressions may squeeze the very last segments of molten alloys to cause metal beads. Those liquids, being eutectic, mostly remain stuck between sinks and surfaces.
Prevention of Die-Casting Sinks
Sinks are sub-surface cavity that can be typically detected by visual inspection. It causes the affected surface to reflect light differently indicating the presence of sinks. The operator must take preventive measures to stop the shrinkage upon detecting the surface defects.
The following actions can eliminate or significantly lower the sink formation process:
⦁ Localized Colling Spray
Application of suitable cooling spray directly on the detected frost or roughness initiates an immediate decrease in temps. The spray must be applied to the compromised spot, leaving the surrounding regions untouched.
⦁ Heating the Alloys
If half of the die cast remains considerably hotter, it can negatively affect the casting process and result in formation of sinks. Certain treatments can heat the entire surfaces, including the hot as well as the cold spots.
The above actions are primarily applicable during casting upon recognizing the probable sink formation. However, some pre-casting countermeasures can eliminate the chances of heat sinks.
⦁ Uniform Wall Thickness
Maintaining an equal wall thickness forces the last solidifying spots to spread more uniformly. It dissipates the extreme heat concentration from the shrinkage point to ensure a balance in temperatures.
⦁ Preferred Thickness
It is suggested to keep a wall thickness of 2mm to 6mm to achieve optimal performance. RIB signs and pillars within casting will also require some adjustments. Keeping the ratio of top surfaces to RIB signs/pillars within 0.7 is preferable.
⦁ Placement of Fountain
Putting a bubbler or fountain directly under the sink-prone spots can help the cooling. It effectively allows the concentrated heat to escape from the spots without affecting the adjacent high-temp surface layers.
⦁ No Shrinkage Method
Bubble placement remains the most effective way to eradicate any shrinkage-based depressions. The most advantageous part concerns its near-unanimous applicability as the process of casting becomes irrelevant in this treatment.
Temperature balance is the ultimate key to avoiding heat sinks under the casting surfaces. The dendritic structure surrounding the shrinkage porosity exerts the visible symptoms. So, proper supervision of the process at all stages can help identify and take action instantly.