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Die Casting Defect Series - Shrinkage (Shrinkage Porosity)
2025-09-03
One of the common defects of die castings is shrinkage cavities (shrinkage, porosity). The die casting alloy liquid shrinks in volume during the cooling and solidification process. Due to insufficient compensation of the internal alloy liquid, holes are formed. The inner surface of the hole appears dark, irregular in shape, and rough.
Shrinkage cavities and shrinkage porosity can be distinguished based on the size and density of the defects. Large and concentrated ones are shrinkage cavities, while small, dispersed, honeycomb-shaped and non-dense holes are shrinkage porosity.
Based on past experience, these dense shrinkage areas are often the most prone to leakage in the later stages of the market, especially near heated areas such as waterways. During actual use, repeated thermal shock causes the dense small holes to expand and penetrate, forming leakage channels.
Defect characteristics:
Shrinkage defects inside castings require dissection and visual inspection or NDT inspection.
Shrinkage cavities typically occur in thick walls, hot spots, hot zones, areas with higher mold temperatures, and where the alloy solidifies last. While their depth and size are difficult to determine visually, they are most likely to occur in thick walls and hot spots due to insufficient shrinkage compensation. Shrinkage cavities are often found in areas prone to gas porosity, resulting in die castings often containing a mixture of gas porosity and shrinkage cavities. A loose fracture is characterized by a coarse, loose structure.
There are two types of porosity: shrinkage porosity and gas porosityand the formation mechanisms of the two types of porosity are different.
① Shrinkage porosity occurs when liquid metal crystallizes and solidifies, shrinking in volume. This creates cavities between dendrite branches because there's no liquid metal to compensate for the shrinkage. Shrinkage porosity is typically very small and difficult to avoid from a casting technical perspective.
②. The mechanism of gas porosity is that the gas and hydrogen content in the melt is high, and the gas is hidden in the gaps between dendrites. During crystallization, the dendrite branches overlap each other to form a skeleton. The residual melt and gas between the crystal skeletons are divided into small molten pools that are not connected to each other by dendrites and branches. The gas between the branches and the hydrogen released during solidification cannot escape and gather. After crystallization, the position occupied by these gases becomes a cavity. This cavity is the gas porosity formed by gas, which leads to the formation of macroscopic and microscopic porosity.
The loose distribution pattern is that it is generally more in the center of the round casting and the tail of the filling, and in the flat casting it is mostly distributed at or near the center of the wide surface, and the thick wall and hot spot appear within half the wall thickness from the center.
③. Core surface shrinkage cavity. The front end of the casting core rod and the core is a hot spot. The metal solidification speed here is slow and it becomes the last part to solidify. When the molten metal can’t be replenished, shrinkage cavity or internal shrinkage cavity will occur.
Cause:
(1) The alloy pouring temperature is too high, or the metal liquid is melted or kept warm for too long.
(2) The injection pressure ratio is too low, or the pressure increase is not timely enough, and the shrinkage is not sufficient.
(3) The casting is structurally uneven, with large variations in wall thickness, thick walls or hot spots and a large number of thickened areas. In particular, the casting is too thick at the ingate, or the mold temperature at the ingate is too high, causing the alloy liquid to cool and solidify slowly, resulting in a large number of shrinkage cavities formed inside.
(4) The filling degree of the pressure chamber is too small, the material cake is too thin, and the shrinkage compensation is ultimately ineffective.
(5) The local temperature of the mold is too high.
(6) The gate is too thick or the gate is improperly positioned, and there are shrinkage holes in the gate and its vicinity.
(7) If the gate is too thin, the gate will solidify prematurely, affecting the pressure increase and shrinkage compensation.
Countermeasures:
(1) Comply with alloy smelting specifications to avoid overheating of alloy liquid for too long.
(2) Under the premise of ensuring that the casting does not produce cold shut or undercasting, the pouring temperature of the alloy liquid can be reduced as much as possible.
(3) Appropriately increase the boost pressure ratio for the final shrinkage compensation. Using the local extrusion function can effectively solve the shrinkage cavity.
(4) Improve the casting structure, eliminate thick walls and hot spots where metal accumulates, make the casting wall thickness uniform, allow a slow transition between thick and thin walls, and reduce metal accumulation in areas with large cross-sectional changes. Hollow structures or insert designs can be adopted, and water cooling of the mold at these locations can be increased.
(5) Use alloys with smaller volume shrinkage and linear shrinkage, or adjust the alloy liquid to reduce its shrinkage. Or modify the alloy to reduce the tendency of shrinkage cavity formation.
(6) Appropriately improve the pouring system to better transmit pressure. Thicken the pouring system, reduce the thickness difference between the primary and secondary horizontal (vertical) runners, and increase the fillet radius at the runner corners.
(7) Select appropriate casting pressure and increase the casting pressure appropriately.
(8) Extend the holding time until the gate is completely solidified.
The implementation of all improvement measures must pass process verification and effectiveness, and must ensure product molding requirements, appearance standards and performance requirements.