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Surface defects of aluminum castings: causes and countermeasures of strains, cold shuts, flow marks, and bubbles

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  • Release time: 2026-11-07

Surface defects of aluminum castings: causes and countermeasures of strains, cold shuts, flow marks, and bubbles
Appearance defects directly affect customer acceptance, and many foreign trade orders are rejected due to poor appearance. Xinfeng Machinery disassembles the mechanism, mold end, and process end solutions one by one.
1、 Pulling (pulling flower)
Aluminum liquid adheres to the mold cavity and pulls the surface of the casting during demolding; Causes: aluminum adhesion, uneven polishing patterns, insufficient release agent, unbalanced ejection, and insufficient reverse draft angle. Countermeasures: Polish in the direction of demolding, increase demoulding, apply AlTiN coating, and adjust the synchronicity of ejection. A356 is more prone to strain.
2、 Cold insulation (cold joint)
The temperature at the front end of two streams of aluminum liquid is too low, and when they merge, they cannot fuse together, forming a gap; The characteristic is linear patterns, which can seriously penetrate and leak. Causes: Low aluminum liquid temperature, low mold temperature, insufficient filling speed, and too long material flow distance. Countermeasures: Increase mold temperature, increase sprue speed, adjust sprue to shorten the process, and increase overflow.
3、 Flow mark
The surface has wavy patterns, and the front edge of the material flow cools to form an oxide film, which is then covered by aluminum liquid; Different from cold insulation, there are no gaps, only the appearance pattern. Causes: Excessive spraying, mold temperature fluctuations, poor atomization. Countermeasures: Stabilize mold temperature, optimize spraying timing, and reduce single spraying volume.
4、 Surface bubbles (subcutaneous bubbles)
The epidermis bulges and shallow pores can be seen on X-ray; Release agent water vapor, aluminum liquid dissolved hydrogen, and gas trapped under the skin. Causes: Insufficient blowing, excessive hydrogen content in aluminum liquid, and water accumulation during spraying. Countermeasure: Strengthen blowing, degassing, and reduce spraying water volume.
5、 Defect prioritization
Firstly, distinguish whether it is a surface appearance defect or an internal hole; Cold insulation is a structural risk defect and is prohibited for pressure bearing components; Flow marks mostly only affect the appearance. Low pressure casting has a gentle flow rate, with fewer strains and flow marks, and cold insulation often occurs at the end of the filling process. Google's "aluminum die casting surface defects troubleshooting" is aimed at quality inspection and manufacturing processes.
FAQ
How to distinguish between cold insulation and flow marks with the naked eye?
There are gaps in the cold insulation, and knocking hard may cause cracking; Flow marks are just patterns, fused perfectly.
What is the minimum draft angle?
Ordinary aluminum alloy 0.5 ° -1 °, mirror polished, easy to stick aluminum surface with upper limit.
What is the reason why the strain is only in a fixed position?
The polishing at this location is poor, uncoated, and the rounded corners are too small.
Can flow marks be covered up by polishing castings?
Shallow layers are acceptable, but batch flow marks need to be cured from the mold and spray coating.
5. Is adding an overflow groove useful for cold insulation at the end?
Useful, take away low-temperature cold materials.
6. Will there be holes exposed after polishing subcutaneous bubbles?
Yes, if the surface is polished, holes will appear.
7. Can unbalanced ejection also cause strain?
The top pin is not synchronized, and the casting tilts and pulls the surface of the mold cavity.
8. Which is easier to cold isolate between ADC12 and A356?
A356 has better liquidity and less cold insulation under the same conditions.

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