Zinc Die Casting: Die Casting Process and Practice of Automotive Door Lock Housing-Case Studies

Zinc die casting technology is widely used in automotive lightweight components, electronic components and other fields due to its advantages of excellent fluidity, high forming precision, controllable cost and balanced mechanical properties. As a core component of the vehicle body safety system, the automotive door lock housing undertakes the key functions of locking the door, transmitting opening and closing force, and ensuring driving safety, with strict requirements on dimensional accuracy, structural strength, wear resistance and mass production capacity. The material characteristics of zinc alloy and the high efficiency of die casting process can perfectly meet the production needs of door lock housings. Taking the front door lock housing of a passenger car as the target product, this article elaborates on the application details, technical points and practical value of zinc die casting technology, providing a reference for the production of similar zinc die cast components.

I. Product Characteristics and Compatibility with Zinc Die Casting Technology

The front door lock housing of the passenger car in this case has an irregular cavity structure, with an overall size of 180mm×120mm×85mm, an average wall thickness of 2.5mm and a minimum local wall thickness of 1.2mm. It integrates complex structures such as a dead bolt mounting groove, connecting rod hole and motor mounting seat inside, and 12 bolt fixing holes need to be reserved. The coaxiality tolerance of the hole position is ≤0.03mm, which directly affects the assembly accuracy and opening/closing smoothness of the door lock and door. The product must withstand repeated impact and vibration during door opening and closing, requiring a tensile strength of ≥280MPa and a Brinell hardness of ≥90HB. Meanwhile, it must have excellent wear resistance and corrosion resistance to adapt to complex automotive working conditions.

Zinc die casting technology is highly compatible with the product requirements: Zamak 3 zinc alloy is selected as the raw material, which is mainly composed of zinc, aluminum and magnesium, with excellent fluidity. It can easily fill small wall thicknesses and complex cavities, resulting in castings with good compactness and no obvious shrinkage or air hole defects. Zamak 3 zinc alloy has balanced hardness and strength, and its corrosion resistance is significantly improved after passivation treatment, which can resist erosion by the humid environment inside the door and its service life can reach the whole vehicle life cycle. The forming precision of zinc die casting reaches IT7-IT9 grade, and the surface roughness Ra≤1.2μm. The castings can meet assembly requirements without extensive subsequent processing, greatly reducing the production chain cost. The single-mold forming cycle is only 45-60 seconds, adapting to the mass production demand of 100,000 units per year for a single passenger car model, and the production efficiency is far higher than that of traditional machining processes.

II. Key Design Points of Zinc Die Casting Mold for Door Lock Housing

The mold is the core guarantee for zinc die casting forming. It needs to be combined with the complex structure of the door lock housing, taking into account forming precision, exhaust efficiency, demolding smoothness and mold life. The overall mold adopts a two-plate structure suitable for horizontal cold chamber die casting machines, and the mold material is H13 hot work die steel. After quenching and tempering treatment and nitriding treatment, its high-temperature wear resistance and thermal fatigue resistance are improved. The core design points are as follows:

Parting surface and cavity design: The parting surface is set along the maximum contour line of the housing, avoiding key precision areas such as the dead bolt mounting groove and connecting rod hole. A stepped parting structure is adopted to reduce the impact of parting surface dislocation on product precision. The cavity adopts an integral structure, and the internal core is accurately designed according to the internal cavity and mounting seat structure of the housing. The core surface is rounded (R1mm) to avoid casting cracks caused by stress concentration. For high-precision structures such as connecting rod holes, an insert design is adopted. The insert is made of SKD61 material, which is precision ground and then embedded into the cavity to ensure that the hole position dimensional tolerance is controlled within ±0.02mm, and it is convenient for replacement after wear in the later period.

Gating and exhaust system: A side gate + spreader cone gating system is adopted, with a main sprue diameter of 12mm. The spreader cone is designed to be streamlined to guide the zinc alloy liquid to evenly distribute to all areas of the cavity, avoiding core deformation or casting defects caused by excessive local impact. The exhaust system is evenly arranged with exhaust grooves along the cavity end, mounting seat root and parting surface, with a groove width of 0.15-0.25mm and a depth of 0.03-0.08mm. Meanwhile, an exhaust pin is set at the highest point of the cavity to quickly discharge air and zinc alloy liquid volatiles in the cavity, controlling the casting air hole defect rate below 0.3%.

Cooling and ejection system: The cooling system adopts a zoned water channel design, fitting the outer wall of the cavity and the inside of the core, with a water channel spacing of 8-10mm. The water temperature is controlled at 25-35℃ by a constant temperature cooling machine to ensure uniform temperature in all areas of the mold and avoid casting deformation and shrinkage marks caused by uneven cooling. The ejection system adopts a combined structure of ejector pins and ejector sleeves. Ejector pins are evenly distributed on the non-appearance surface and inner wall of the housing cavity, and ejector sleeves correspond to the positions of connecting rod holes, ensuring uniform distribution of ejection force and avoiding deformation, scratches or ejection marks during casting demolding.

III. Zinc Die Casting Manufacturing Process and Quality Control

The zinc die casting of the door lock housing follows the full-process control of "raw material pretreatment - die casting forming - post-processing - quality inspection" to ensure that product performance and precision meet the standards. The specific process is as follows:

Raw material pretreatment and die casting forming: Zamak 3 zinc alloy ingots are melted at high temperature (410-430℃). Refining agent is added during the melting process to remove impurities and gases, and nitrogen is introduced for protection to prevent zinc alloy oxidation. A 1600kN horizontal cold chamber die casting machine is used for forming, with the following core parameters set: injection pressure 60-80MPa, injection speed 2-4m/s, mold temperature 100-120℃, holding pressure time 1-2s, and cooling time 20-30s. During die casting, the zinc alloy liquid temperature, injection pressure and mold temperature are monitored in real time through an automatic control system, and parameters are accurately adjusted to avoid casting defects caused by parameter fluctuations.

Post-processing procedures: After demolding, the gate, riser and flash are first removed by automatic equipment, and then the surface burrs and residual impurities of the housing are cleaned by a combination of light grinding with a grinding wheel and high-pressure air blowing to ensure a smooth surface without protrusions. The castings are passivated using a chromate passivation process to form a dense passivation film on the surface, improving corrosion resistance and meeting the requirements of the automotive industry salt spray test (neutral salt spray test for 48 hours without rusting). For key structures such as connecting rod holes and bolt holes, precision reaming is performed to further improve hole position accuracy and surface finish.

Quality inspection: Appearance inspection combines visual inspection and optical projector to check for surface defects such as air holes, cracks, shrinkage marks and flash on castings, with the defect qualification rate controlled above 99.7%. Dimensional inspection uses a coordinate measuring machine to fully inspect 10 key dimensions including the length, width and height of the housing, hole spacing and coaxiality, with all dimensional deviations controlled within the allowable range. For mechanical performance sampling, 5 samples are selected per batch, and both tensile strength and hardness meet the design requirements through tensile tests and hardness tests. For assembly testing, 10 finished products are selected and assembled with other door lock components, achieving smooth opening and closing without jamming, and the locking force meets the standard.

IV. Application Value and Practical Effects

After the zinc die casting door lock housing is put into production, the single-shift output reaches 1,200 pieces, which is more than 4 times higher than that of traditional sand casting, and the unit product production cost is reduced by 30%, successfully adapting to the mass production demand of 150,000 passenger cars per year. Compared with cast iron door lock housings, the product weight is reduced by 55%, contributing to vehicle body lightweight and reducing fuel consumption by 0.15L/100km per vehicle, conforming to the trend of automotive energy conservation and emission reduction.

Actual vehicle mounting verification shows that the zinc die casting door lock housing has high assembly precision and smooth opening and closing. After 100,000 opening and closing cycles, there is no deformation or increased wear, and the locking force remains stable, meeting the requirements of automotive safety performance. Meanwhile, the recyclability rate of zinc alloy materials is over 95%, with low pollutant emissions during production, conforming to the concept of green manufacturing.

Conclusion

With its core advantages of high efficiency, high precision and low cost, zinc die casting technology perfectly meets the complex forming and mass production needs of automotive door lock housings. Through scientific raw material selection, mold design, parameter regulation and full-process quality control, the high-performance forming of door lock housings is successfully realized, providing a practical technical solution for improving the quality and efficiency of automotive safety components. With the upgrading of the automotive industry towards lightweight, green and intelligent development, zinc die casting technology will be widely applied in more components such as automotive door locks, door and window hinges, and transmission forks, continuously promoting technological iteration and cost optimization in the automotive manufacturing industry.