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What Are the Common Defects of Zinc Alloy Die Casting Parts?

2026-06-04 15:30

Zinc alloy high-pressure die casting is a mainstream precision metal forming process widely used in hardware accessories, electronic structural parts, automotive small components and decorative hardware. Compared with aluminum alloy die casting, zinc alloy die casting parts feature lower melting temperature, better fluidity, higher dimensional accuracy and superior surface finish, making them suitable for thin-wall and intricate precision parts. However, affected by zinc alloy material characteristics, mold design, die casting process parameters and post-treatment operation, various typical defects frequently occur during mass production. These defects damage appearance effect, structural strength and anti-corrosion performance of finished products. This article elaborates five most frequent defects of zinc die castings, including causes, hazard impacts and targeted prevention solutions respectively.

1. Oxidation and White Rust: Unique Corrosion Defects for Zinc Die Castings

Oxidation and white rust are the most distinctive defects exclusive to zinc alloy die casting parts, rarely appearing on aluminum die castings. White rust refers to white powdery corrosion spots generated on the part surface after long-term exposure to humid air, rainy environment or coastal salt spray conditions. Zinc alloy has active chemical properties, and it is easy to react with water vapor and oxygen in the air to produce zinc hydroxide and zinc carbonate attachments, forming obvious white rust stains. In severe cases, white rust will further evolve into red rust, causing permanent surface damage and scrapping decorative parts directly.
Three core factors lead to white rust defects. Firstly, residual moisture inside raw zinc ingots and molten zinc will increase internal hydrogen content, accelerating surface electrochemical corrosion. Secondly, incomplete cleaning after die casting leaves residual release agent and cutting fluid on part surface, destroying the original passive protective film. Thirdly, unqualified or incomplete surface treatment is the main inducement. If electroplating, passivation or chromate sealing layers are too thin or have cracks, external moisture will penetrate the coating and erode the zinc substrate rapidly.
Effective preventive measures include full degassing and drying of zinc liquid before production, complete ultrasonic cleaning before surface treatment, and adopting double-layer passivation sealing process. For outdoor-used zinc die castings, thick nickel electroplating layer can completely isolate air and moisture, fundamentally solving white rust corrosion problems and extending service life by more than 3 times.

2. Shrinkage Cavity and Porosity: Internal Structural Hidden Defects

Similar to aluminum die castings, porosity and shrinkage cavity are also common internal defects for zinc alloy die castings, but they have different formation mechanisms. Zinc alloy has larger solidification shrinkage rate than aluminum alloy. During rapid cooling after high-pressure filling, molten zinc shrinks sharply in a short time. Without sufficient feeding pressure, irregular shrinkage holes will form at thick material positions, rib roots and hot joints of parts. Gas porosity is mainly caused by trapped air in mold cavity and gas decomposed from release agent.
The hazards of internal defects are obvious. Tiny internal pores will reduce part compactness and structural toughness. Unlike aluminum parts, zinc alloy parts are more prone to fracture under slight impact due to internal hidden holes. For threaded columns and assembly structural surfaces, internal porosity will cause thread sliding and assembly failure. Besides, pores will destroy the uniformity of subsequent electroplating coating, leading to blistering and peeling of electroplated layers in later use.
Optimization solutions are formulated from process and mold aspects. Manufacturers appropriately increase holding pressure and extend pressure holding time to compensate solidification shrinkage of zinc liquid. Optimize mold exhaust grooves and add vacuum exhaust system to discharge trapped air thoroughly. Meanwhile, avoid excessive local thick wall design in product development to reduce concentrated shrinkage risks. Strict molten zinc degassing treatment can control internal pore rate below 1% for qualified zinc die castings.

3. Cold Shut and Flow Lines: Surface Appearance Defects

Cold shut and flow lines are intuitive surface appearance defects affecting the decoration performance of zinc alloy die casting parts. Zinc alloy has excellent fluidity, but unreasonable process parameters will still cause poor metal fusion. Cold shut presents visible linear welding lines on part surface, formed when two streams of molten zinc meet but cannot fuse completely. Flow lines are wavy uneven textures left on the surface after molten zinc flows and cools down, which cannot be removed by simple polishing.
Different from aluminum die castings, zinc die castings have lower melting point, so low mold temperature is the primary cause of cold shut defects. If the mold preheating temperature is lower than 180℃, molten zinc will cool down instantly after touching the mold cavity surface, resulting in poor fluidity and incomplete fusion. In addition, unreasonable gate position leads to overlong molten metal filling path, and insufficient injection speed aggravates surface flow marks. These appearance defects will lead to uneven electroplating color and poor surface gloss, failing high-standard appearance inspection requirements.
Targeted improvements include raising mold preheating temperature to 200℃-240℃ stably, optimizing runner and gate structure to shorten filling distance, and matching medium and high injection speed suitable for zinc alloy characteristics. Compared with aluminum parts, zinc die castings can eliminate cold shut defects more easily by adjusting mold temperature, thanks to its superior metal fluidity.

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

Warpage and dimensional deformation are frequent problems for thin-wall zinc alloy die casting parts. Zinc alloy has larger thermal expansion coefficient than aluminum alloy, bringing more obvious residual internal stress after mold ejection. Uneven cooling speed between thin walls and thick ribs causes inconsistent stress release, resulting in part bending, twisting and size deviation. This defect is especially prominent in large thin-wall zinc decorative shells and long strip hardware parts.
The main inducing reasons cover three aspects. First, unbalanced mold cooling water channel layout causes uneven mold surface temperature, forming differential cooling stress. Second, unreasonable ejection pin layout leads to unbalanced ejection force, forcing parts to deform during demolding. Third, improper part structure with too thin wall thickness and lack of reinforcing ribs reduces overall rigidity. Slightly deformed parts cannot match assembly dimensions, while severely deformed parts can only be scrapped directly.
Common improvement methods include optimizing cooling pipeline to realize synchronous cooling of the whole mold, adjusting ejection pin quantity and distribution to ensure uniform demolding force, and adding reasonable reinforcing ribs to improve part rigidity. Besides, manufacturers adopt fixture shaping and natural stress relief after part ejection to reduce residual internal stress effectively, ensuring dimensional tolerance meets drawing requirements stably.
5. Die Sticking and Burrs: Mold Matching Surface Defects
Die sticking and burrs are surface defects closely related to mold maintenance and mold clamping precision. Zinc alloy has strong adhesion to steel mold cavities, so die sticking happens more easily than aluminum die castings. Die sticking means partial zinc material adheres to mold cavity surface, causing surface scratches, material shortage and rough surface on finished parts. Long-term die sticking will also damage mold cavity surface and increase subsequent defective rate continuously.
Burrs are thin overflow zinc sheets generated on mold parting surfaces. Zinc alloy has good fluidity, so molten zinc is easier to overflow from tiny mold gaps under high injection pressure. Insufficient mold locking force, worn mold parting surface and mismatched injection pressure will worsen burr problems. Although burrs can be removed by post-deburring process, excessive burrs increase manual processing time and raise production cost.
Regular mold maintenance is the core solution. Workers need to polish mold cavity regularly to reduce surface roughness and lower adhesion force between molten zinc and mold steel. Spray release agent evenly and quantitatively to avoid insufficient local demolding lubrication. Timely repair worn parting surfaces and match reasonable mold locking force to eliminate molten metal overflow. Standard daily mold maintenance can greatly reduce die sticking and burr defects in zinc die casting mass production.

Conclusion

In conclusion, the five common defects of zinc alloy die casting parts are white rust corrosion, internal porosity and shrinkage cavity, surface cold shut, dimensional warpage as well as die sticking and burrs. Most defects are caused by zinc alloy's unique physical and chemical properties, unreasonable die casting process parameters, poor mold maintenance and incomplete surface protection. Different from aluminum die castings, zinc die castings are more prone to white rust corrosion and die sticking, but have better fluidity with fewer filling defects. By optimizing process parameters, improving mold structure and completing qualified anti-corrosion surface treatment, manufacturers can effectively control all defects and improve the yield of zinc alloy die castings


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