Common Defects of Traditional High-pressure Die-casting Castings

High-pressure die casting is a high-speed and high-pressure forming process. Its advantages lie in high efficiency and high precision. However, precisely because of the process characteristics, it is prone to some typical defects.

I. Hole-type defects

1.Gas Porosity

• Characteristics:The holes have smooth walls and are round or oval in shape. They are usually located just below the surface of the casting or internally, and are exposed after machining.

• Cause of occurrence:

(1)Air Entrapment:During the injection process, air in the mold cavity and gases volatilized from the die coat fail to be expelled in time through the vents and overflow wells, becoming trapped by the high-speed advancing molten metal.

(2)Excessive or Rapidly Volatilizing Die Coat:Spraying too much release agent or insufficient volatilization leads to excessive gas generation when encountering the high-temperature molten metal.

(3)Poor Mold Venting:Insufficient depth, blockage, or improper placement of vent slots.

• Improvement measures:

(1)Optimize the gating system and injection process: Reduce the slow shot speed to minimize turbulence; optimize the high-speed switchover point.

(2)Improve mold ventilation: Increase, clean, and rationally arrange vent slots and overflow wells.

(3)Control spraying: Precisely control the amount and concentration of the release agent, ensuring it is thoroughly dried.

2. Shrinkage Porosity/Cavities

• Characteristics:The holes have rough, uneven walls, appearing dark gray or with a dendritic crystal structure. They often occur in thicker sections, hot spots, or areas that solidify last.

• Cause of occurrence:

(1)Insufficient Feeding:During solidification, the metal volume shrinks. However, due to the rapid solidification of die castings and the typical lack of feeding risers like in gravity casting, thick sections cannot receive adequate liquid metal replenishment.

(2)Improper Process Parameters:Insufficient intensification pressure leads to inadequate final feeding pressure; excessively high pouring temperature increases the shrinkage amount.

• Improvement measures:

(1)Optimize product and mold design: Avoid drastic changes in wall thickness, and add cooling water channels or point cooling needles to accelerate cooling at thick-walled areas.

(2)Adjust process parameters: Appropriately increase the injection specific pressure to enhance the feeding effect; Reduce the pouring temperature under the premise of ensuring filling.

II. Surface defects

1.Flow Lines/Cold Flakes

• Characteristics:Smooth, irregular streaks or seams appear on the casting surface. This is caused by the incomplete fusion of two metal streams meeting.

• Cause of occurrence:

(1)Molten metal temperature too low or mold temperature too low, leading to poor fluidity at the flow front.

(2)Filling speed too slow, causing the leading edge of the molten metal to cool prematurely.

(3)Excessive release agent, which lowers the temperature of the flow front.

• Improvement measures:Appropriately increase the molten metal and mold temperature; increase the injection speed; optimize spraying.

2.Scratches/Sticking to the mold

• Characteristics:Scratches, roughness and even defects appear on the surface of the casting along the demolding direction.

• Cause of occurrence:

(1)The draft Angle of the mold is insufficient.

(2)The surface of the mold is poorly polished or has upside down.

(3)The mold temperature was too high, causing the alloy to weld together with the mold steel.

(4)Unbalanced ejection or unreasonable setting of the ejector rod.

• Improvement measures:Ensure sufficient draft Angle and good surface finish of the mold; Strengthen mold cooling; Use special anti-sticking mold coatings or carry out surface treatment of the mold (such as nitriding).

III. Dimension and shape defects

1.Warping/Deformation

• Characteristics:The casting's geometry does not conform to the drawing, exhibiting bending or twisting.

• Cause of occurrence:

(1)Internal Stress:Stresses generated by non-uniform cooling and contraction of different parts of the casting.

(2)Improper Ejection:Uneven placement or force of ejector pins, causing deformation during ejection.

(3)Poor Structural Rigidity of the Casting,such as large, thin-walled flat parts.

• Improvement measures:Optimize the ejection system layout; add stiffening ribs to increase rigidity; adjust mold opening and ejection times to allow sufficient cooling of the casting within the mold; use correction fixtures.

2.Dimensional Deviation

• Characteristics:Unstable casting dimensions, exceeding tolerance ranges.

• Cause of occurrence:

(1)Mold Temperature Fluctuation:Unstable mold temperature leads to variations in shrinkage rate.

(2)Process Parameter Fluctuation:Such as variations in injection speed, pressure, time, etc.

(3)Mold Wear or Deformation.

• Improvement measures:Use mold temperature controllers to stabilize mold temperature; strictly control the stability of process parameters; regularly inspect and maintain the mold.

IV. Internal defects

1.Inclusions

• Characteristics:The casting contains non-metallic impurities, such as release agent residue, oxides, etc.

• Cause of occurrence:

(1)Insufficient Molten Aluminum Refining:Oxides generated during melting are not completely removed.

(2)Release Agent Residue:Excessive release agent is entrapped in the molten metal.

• Improvement measures:Enhance slag removal and refining during the melting process; control the amount of release agent used.

V. Crack-like defects

1.Hot Tearing

• Characteristics:Cracks occurring in the final stages of solidification or just after solidification due to hindered contraction. The crack surface has an oxidized color.

• Cause of occurrence:

(1)Inherent High Hot Brittleness of the Alloy(e.g., high-purity aluminum alloys with excessively low iron content).

(2)Sharp corners or excessive wall thickness variations in the part design,creating stress concentrations.

(3)Excessive Mold Restraint, hindering contraction.

• Improvement measures:Adjust the alloy composition; optimize product design to avoid stress concentrations; appropriately reduce mold restraint.

VI. Conculsion

High-pressure die casting is a systematic engineering process. Defects are often the result of multiple factors acting together. Solving defect problems requires systematically analyzing the four key links: alloy material, mold design, casting process, and production operations, to identify the root cause and implement effective countermeasures.