What is the crack defect in die casting?

Rongbao.com/aluminium-alloy-die-casting/cast-housing">Die casting is a sophisticated manufacturing process that provides exceptional dimensional accuracy and surface finish for metal components. This process involves injecting molten metal under high pressure into reusable steel molds, known as dies. Upon solidification, the formed components are ejected from the die, resulting in precisely engineered parts suitable for various industries, including automotive, aerospace, medical equipment, and electronics.

Despite its numerous advantages, it presents certain challenges that can affect product quality and production efficiency. Among these challenges, crack defects represent one of the most serious issues manufacturers encounter, as they can significantly compromise the structural integrity and functionality of cast components. Understanding the causes and solutions for crack defects is essential for maintaining high-quality standards in operations.

Ejector Rods: Unevenly Distributed 

One of the most common causes of crack defects in die casting relates to the ejection system. Ejector rods (also known as ejector pins) are critical components of the die casting tool that facilitate the removal of solidified parts from the die cavity. When these ejector rods are poorly designed, insufficient in number, or unevenly distributed across the casting surface, they create localized stress concentrations during part ejection.

The ejection phase is particularly critical in the die casting process because the metal has already solidified and developed considerable strength. Unlike plastic materials that maintain some flexibility, metal castings can only withstand limited deformation before cracking. When ejector forces are concentrated in specific areas rather than distributed evenly across the part, the localized stresses can exceed the material's tensile strength, resulting in crack formation.

Several factors can contribute to uneven ejection force distribution:

First, inadequate design consideration during the die development phase often leads to suboptimal ejector rod placement. Engineers must analyze the part geometry carefully to determine the appropriate number and positioning of ejector rods based on the component's features, wall thickness variations, and potential stress points.

Second, thermal gradients within the casting can cause different sections to cool and shrink at varying rates, creating internal stresses. If ejector rod placement does not account for these thermal considerations, the ejection force may work against natural shrinkage patterns, further increasing the risk of crack formation.

Third, complex part geometries with deep pockets, undercuts, or varying wall thicknesses present unique challenges for ejector system design. Such features can significantly affect how forces are distributed during ejection and may require specialized ejection mechanisms beyond standard ejector rods.

Solution: Increase the number of ejector rods and adjust their distribution positions to balance the force of the casting ejection

The most effective approach to addressing this problem involves comprehensive ejector system redesign. By increasing the number of ejector rods, the ejection force is distributed across a larger area of the casting, reducing the pressure at any single point. This distribution minimizes the risk of localized stress concentrations that could lead to crack formation.

Furthermore, the strategic positioning of ejector rods based on casting geometry, material properties, and thermal behavior ensures more uniform force application. Modern simulation tools can help engineers predict stress distribution during ejection and optimize rod placement accordingly. For complex parts, a combination of primary and secondary ejector systems may be necessary to achieve balanced force distribution.

In addition to ejector rod quantity and placement, the size and design of the ejector rod tips merit consideration. Larger diameter tips distribute force over a wider area, while specially designed profiles can better accommodate particular geometrical features. For thin-walled sections or delicate features, custom ejector configurations such as blade ejectors or sleeve ejectors may provide more appropriate force distribution than conventional cylindrical pins.

Ejector Rod Fixing Plate: Tilted 

While proper ejector rod distribution is crucial, even the most optimally designed ejection system can fail if the mechanical components supporting it are not functioning correctly. The ejector rod fixing plate (or ejector retainer plate) is a critical element that holds all ejector rods in place and ensures they move in unison during the ejection stroke. When this plate becomes tilted or misaligned during operation, it creates an uneven ejection scenario that can lead to catastrophic part defects.

A tilted ejector plate causes some ejector rods to engage with the solidified casting before others, creating a sequential rather than simultaneous ejection process. This condition forces the casting to pivot on the initial contact points, generating tensile stresses on one side and potentially compressive stresses on the opposite side. The resulting force imbalance can easily exceed the material's mechanical limits, particularly for thin-walled aluminum alloy components.

Several factors can contribute to ejector plate tilting:

First, mechanical wear of guide bushings or pins that maintain plate alignment is a common cause of tilting issues. Over thousands of production cycles, these components experience gradual wear that can introduce play into the system.

Second, inadequate maintenance of the die casting machine, particularly the ejection system, can lead to progressive misalignment. Without regular inspection and adjustment, small deviations can compound over time.

Third, thermal expansion of die components during production can cause dimensional changes that affect ejector plate alignment. If the thermal management system is insufficient or if the die design does not properly account for thermal expansion, misalignment may occur specifically during production runs.

Solution: Adjust and reinstall the ejector rod fixing plate

Addressing ejector plate tilting requires both immediate corrective action and preventive maintenance strategies. The immediate solution involves disassembling the ejection system, inspecting all components for wear or damage, and reinstalling the ejector rod fixing plate with proper alignment. This process should include verification of parallel alignment using precision measurement tools such as dial indicators or digital levels.

During reinstallation, technicians should ensure that all guide pins and bushings are in good condition and properly lubricated. The ejector plate mounting bolts should be tightened following a specified sequence and torque specification to prevent uneven loading that could induce misalignment.

For long-term prevention, implementing a regular maintenance schedule for the die casting tool is essential. This schedule should include periodic checks of ejector system alignment, particularly after high-volume production runs. Some advanced die casting operations incorporate sensors that can detect misalignment during production, allowing for immediate intervention before defective parts are produced.

Additionally, upgrading to more robust ejector system designs can provide greater resistance to misalignment. Features such as additional guide pins, self-aligning mechanisms, or hydraulic ejection systems with balanced force distribution may offer improved reliability compared to conventional mechanical ejector systems.

Casting Wall: Too Thin 

Beyond ejector system issues, die casting crack defects can also result from fundamental design and process parameters. One significant factor is inadequate wall thickness in the casting design. While thin walls are often desirable for weight reduction and material savings, excessively thin sections can create conditions conducive to crack formation, particularly during the solidification and cooling phases of the die casting process.

When metal solidifies, it naturally undergoes volumetric shrinkage. For aluminum alloys commonly used in die casting, this shrinkage typically ranges from 4% to 8% depending on the specific alloy composition. In adequately designed components with uniform wall thickness, this shrinkage occurs relatively evenly across the part. However, in components with excessively thin sections, several problems can emerge.

First, thin sections cool and solidify more rapidly than thicker sections, creating temperature gradients across the part. These gradients lead to differential shrinkage rates, generating internal stresses that can cause warping or cracking. The problem is exacerbated when thin sections are adjacent to significantly thicker areas, creating sharp transitions in cooling rates.

Second, thin-walled sections have less structural strength during the ejection phase. Even with properly designed ejection systems, these delicate areas may lack sufficient rigidity to withstand the mechanical forces applied during part removal. The combination of ejection stress and residual solidification stress can easily exceed the material's strength limits, resulting in crack formation.

Third, material flow patterns during the filling phase can create weaknesses in thin-walled sections. If the molten metal cools prematurely while flowing through these areas, cold flow defects or flow lines may form, creating potential stress concentration points that are prone to cracking.

Solution: Optimize wall thickness and design features

Addressing thin wall-related crack defects requires a comprehensive approach that begins at the design phase. While maintaining minimum wall thickness is often a design requirement, several strategies can minimize the risk of cracking:

First, implementing gradual transitions between thick and thin sections helps reduce stress concentration and allows for more uniform cooling. Design guidelines typically recommend transition ratios of 3:1 or less between adjacent wall thicknesses.

Second, adding strategic ribs or gussets to thin-walled sections can significantly increase structural integrity without substantially increasing overall material volume. These features provide mechanical reinforcement during the ejection phase while also helping to distribute stresses more evenly.

Third, optimizing gating and runner systems to ensure proper filling patterns and minimize turbulence in thin-walled sections can improve material integrity. Computational fluid dynamics (CFD) simulations can help identify potential flow issues before finalizing the die design.

Finally, adjusting process parameters such as melt temperature, injection pressure, and cooling strategy can help compensate for the challenges associated with thin-walled sections. Higher injection pressures and carefully controlled cooling rates can improve material consolidation and reduce the risk of premature solidification in thin areas.

Conclusion

Preventive approaches such as optimized ejector rod distribution, regular maintenance of ejection systems, and thoughtful component design with appropriate wall thickness can significantly reduce the incidence of crack defects. When combined with modern simulation tools and process monitoring capabilities, these strategies enable operations to achieve exceptional quality and consistency.

For more information about addressing crack defects in your die casting applications or to discuss your specific manufacturing needs, please contact our technical team at selinazhou@xianrongbao.com or steve.zhou@263.net. Our experts are ready to assist you in achieving optimal quality and performance for your aluminum alloy components.

References

• Society of Manufacturing Engineers. (2023). Die Casting Handbook.
• North American Die Casting Association. (2024). NADCA Product Specification Standards for Die Castings.
• International Journal of Metalcasting. (2023). Recent Advances in Die Casting Technology.
• American Foundry Society. (2024). Aluminum Casting Technology.