news

What Defects Often Occur in Aluminum Alloy Die Casting?

2026-06-02 15:30

Aluminum alloy high-pressure die casting is a mature near-net-shape manufacturing process widely used in automotive parts, consumer electronics, new energy equipment and smart home appliances. However, affected by molten aluminum quality, mold design, die casting process parameters and operating environment, various unavoidable defects will appear during mass production. These defects will damage the appearance quality, dimensional accuracy, structural strength and air tightness of die-cast parts, increasing scrap rate and production cost. This article classifies and analyzes five most common die casting defects, including appearance defects, internal structure defects, deformation defects, filling defects and surface adhesion defects, explaining their causes, hazards and practical prevention solutions respectively.

1. Porosity and Blowholes: The Most Common Internal Hidden Defects

Porosity and blowholes are the most frequent internal defects of aluminum die casting, accounting for more than 40% of all defective products in actual production. Porosity refers to tiny scattered pinholes inside parts, while blowholes mean large-sized smooth round holes. Both defects are invisible from the outer surface, but will seriously reduce the compactness, tensile strength and air tightness of die-cast components. For sealing parts and pressure-bearing structural parts, internal pores will directly lead to air leakage and oil leakage, making the whole batch of products scrapped.
There are three major root causes of air pore defects. Firstly, excessive gas is mixed into molten aluminum during smelting and feeding. Moisture on aluminum ingots, release agent and lubricant will decompose hydrogen after contacting high-temperature molten aluminum, and the gas cannot be discharged completely before metal solidification. Secondly, unreasonable mold exhaust system is the key mold factor. Insufficient exhaust grooves, blocked exhaust channels and poor vacuum exhaust design make trapped air unable to escape the mold cavity during high-speed injection.
Thirdly, improper process parameters will worsen pore problems. Excessively fast injection speed wraps more air into the cavity, while too low mold temperature accelerates rapid surface solidification and locks internal gas. Practical prevention measures include fully degassing molten aluminum before production, optimizing mold exhaust structure, reasonably adjusting two-stage injection speed, and reducing residual moisture of release agent. Standardized process control can reduce porosity rate below 1% for qualified die casting production.

2. Cold Shut and Flow Marks: Typical Appearance Filling Defects

Cold shut and flow marks are typical surface filling defects, which directly damage the appearance integrity of die-cast parts and reduce local structural strength. Cold shut presents obvious linear welding lines on part surface, formed when two streams of molten aluminum meet but fail to fuse completely. Flow marks are uneven wave-shaped textures left on the surface after molten aluminum flows and cools down, which cannot be removed by simple post-deburring.
These two defects mainly happen on thin-wall parts, long-distance flow positions and product corners. The core cause is excessive temperature drop of molten aluminum during cavity filling. When molten aluminum flows through a long and narrow runner, the surface temperature drops rapidly before full fusion. Besides, low mold preheating temperature, insufficient injection pressure and unreasonable gate position will aggravate cold shut and flow marks. Compared with internal pores, such appearance defects are easy to inspect visually, but will affect subsequent surface treatment effects such as anodizing and powder coating, causing uneven color and poor coating adhesion.
Effective improvement solutions include raising mold preheating temperature and molten aluminum temperature appropriately, optimizing gate location to shorten molten metal flow distance, and increasing injection pressure to enhance fluidity. For parts with serious cold shut, local polishing can repair surface appearance, while optimizing front-end die casting process is the fundamental solution to avoid reoccurrence.

3. Shrinkage Cavity and Shrinkage Porosity: Volume Shrinkage Structural Defects

Different from gas-caused porosity, shrinkage cavity and shrinkage porosity are volume shrinkage defects generated during aluminum alloy solidification. After molten aluminum fills the mold cavity, volume contraction appears during cooling and solidification. If no enough molten metal supplements the shrinking position, irregular hollow holes will form inside thick wall positions, rib roots and material gathering areas. Shrinkage holes are concentrated large cavities, while shrinkage porosity is dense tiny irregular holes.
Such defects concentrate on thick-thin junction positions of die-cast parts. They will greatly reduce local mechanical strength, causing part cracking under vibration or pressure load. The biggest difference between shrinkage defects and air holes lies in hole shape: air holes are smooth round holes, while shrinkage holes have rough inner walls with irregular shapes. The main inducements include unreasonable part structure with sudden wall thickness change, insufficient holding pressure and short pressure holding time during die casting process.
Targeted optimization methods are carried out from product design and process adjustment. Designers should avoid partial thick material accumulation and adopt gradual wall thickness transition. During production, manufacturers increase final holding pressure and extend pressure holding time to compensate solidification shrinkage. Meanwhile, optimize cooling water channel layout to realize sequential solidification from thin wall to thick wall, ensuring sufficient molten aluminum supplement for shrinkage positions.

4. Warpage and Deformation: Dimensional Tolerance Out-of-Tolerance Defects

Warpage and deformation are common dimensional defects of aluminum die casting, which make finished parts exceed drawing tolerance range and fail assembly matching. Uneven cooling speed of different part sections leads to inconsistent internal shrinkage stress. After mold ejection, residual internal stress releases slowly, causing parts to bend, tilt and twist, especially for large thin-wall shells and asymmetric structural parts.
The primary causes cover three aspects. First, unreasonable cooling system design leads to unbalanced mold temperature distribution. Some positions cool fast while others cool slowly, producing uneven internal stress. Second, unreasonable ejection pin layout causes forced deformation during part ejection. Third, improper part structure design without enough reinforcing ribs will reduce overall rigidity and increase deformation risk. Deformed parts cannot match well with aluminum extrusion or other mating components, and large deformation parts can only be scrapped directly.
Common improvement measures include optimizing mold cooling pipeline to balance overall mold temperature, adjusting ejection pin quantity and position to realize uniform ejection force, designing reasonable reinforcing ribs to improve part rigidity, and adding shaping fixtures after part ejection for cooling and stress relief. For slightly deformed parts, manual calibration is available, while severe deformation needs mold modification and process adjustment fundamentally.
5. Sticking Die and Burrs: Mold-Related Surface Defects
Sticking die and burrs are surface defects closely related to mold maintenance and mold precision. Die sticking means partial aluminum alloy material adheres to mold cavity surface, causing surface scratch, material shortage and rough surface on castings. Burrs are extra thin aluminum sheets overflowing from mold parting surface, generated by tiny mold gaps under high injection pressure.
Die sticking mainly results from insufficient release agent spraying, rough mold cavity surface and mold surface oxidation. Long-term mass production will cause mold cavity wear and scratch, leading to frequent material sticking. Burrs are caused by mold clamping gap, mold surface damage, insufficient mold locking force and excessive injection pressure. Although burrs can be removed by post-deburring process, excessive burrs will increase post-processing workload and lower production efficiency.
Regular mold maintenance is the core solution. Factories need to polish mold cavity regularly to keep smooth surface, spray release agent evenly and quantitatively, and repair damaged parting surface timely. Besides, match proper mold locking force and injection pressure to reduce molten aluminum overflow. Good daily mold maintenance can effectively eliminate die sticking and burr defects, stabilizing batch product quality.

Conclusion

In general, common defects of aluminum alloy die casting are mainly divided into internal structure defects, appearance filling defects, shrinkage defects, dimensional deformation defects and mold-related surface defects. Most defects are caused by unreasonable mold design, unoptimized die casting process parameters, poor molten aluminum quality and insufficient mold maintenance. All die casting defects can be effectively controlled and reduced via structural optimization, process debugging and standardized mold maintenance, instead of being inherent unavoidable flaws of high-pressure die casting technology. Strict incoming material inspection, in-process quality monitoring and regular mold maintenance are essential to lower scrap rate and ensure stable batch production.


Related News

More >
Get the latest price? We'll respond as soon as possible(within 12 hours)
  • This field is required
  • This field is required
  • Required and valid email address
  • This field is required
  • This field is required