A lot of quality problems may be encountered in die casting production, which are caused by various reasons. The quality problems in production shall be reasonably identified. The root causes shall be found before it is possible to put forward the corresponding feasible improvement measures, so as to continuously improve the quality of castings.
The quality problems, defect characteristics, causes including improvement measures in die casting are described as follows.
In the die casting forming process, some locations of the part are not filled completely, which is referred to as undercasting. When the location has serious undercasting, the shape of castings can be considered as non-conformance to the requirements of the drawings. Usually, it is allowed to have undercasting in parts.
Causes of undercasting:
1) Poor conditions for filling; the location of undercasting has irregular set metal
In case the pressure is insufficient, the forefront of fluid metal solidifies prematurely, which causes angle, deep concave, thin wall (even thinner than average wall thickness) and cylindrical holes in the wall.
- Low mold temperature
- Alloy injected at low temperature
- Poor location of injection gate, forming large flow resistance
2) Gas blocking results in smooth surface but irregular shape at undercasting locations
- Gas accumulation in locations difficult to make relieving system,
- When the molten metal is flowing, it has severe turbulence and includes gases
3) Mold cavity with residue
- Inappropriate paint dosage or spraying method cause local coating deposition
- Large splicing clearance in molded parts or large sliding fitting clearance allows in metal during filling. After the casting is released, the metal is not completely taken out but forms chips in the clearance. When such metal chips (their thickness is same as clearance) protrude out the mold surface, and in this case, the protruding part can form the wall thickness of casting, which cause the castings produced later to have penetrating groove (for wall thickness). That penetrating groove is a special form of undercasting, which is commonly seen in the deep spliced cavity.
- Insufficient material injected (including excessively thin surplus material section).
- On vertical die casting machines, during injection, the downward movement of lower punch doesn’t open the nozzle orifice wide enough, which causes a series of poor filling conditions.
The base material of casting is destroyed or disconnected to form a narrow gap in the form of irregular line. Under the action of external forces, it has the tendency to develop. This kind of defect is called crack. On the die casting parts, cracks are not allowed to exist.
Causes of cracks:
1. Structure and shape of casting
- Abrupt transformation at connection between thick wall and thin wall of castings
- Insufficient trounded corners on castings
- Insufficient locations on castings for ejector rods, which causes uneven distribution of ejector rods
- Thoughtless casting design, which results in rupture due to shrinkage stress.
2. Poor surface quality and unstable fastening of the molded parts
- Concave on formed surface along the direction of mold releasing, which cause tears on casting
- Failure to eliminate the processing traces at the root of convex molded surface
- The molded parts has deviation in fastening, which impedes mold releasing.
- Improper placement of mold ejection components (location or number)
- The ejection mechanism has deviation, which brings about unbalanced force on casting
- Unreasonable connection between mold ejection mechanism and hydraulic ejection device, deviated or uncoordinated.
- Inconsistent lengths of ejection rods when the ejection pins are ejecting, inconsistent lengths of hydraulic ejection rods.
4. Alloy composition
1) Zinc alloy
A. High contents of harmful impurities like lead, tin and cadmium
B. Insufficient purity
2) Aluminum alloy
A. High iron content, acicular ferrite increased
B. High silicon content in the aluminum silicon alloy
C. High magnesium content in the aluminum magnesium alloy
D. High contents of other impurities increasing brittleness
3) Magnesium alloy
4)High contents of aluminum and silicon
5) Alloy melting quality
A. The melting temperature is too high, resulting in segregation
B. The heating time is too long, resulting coarse grains
C. Too many oxide inclusions
6) Improper operation
A. Too long stay in mold, especially for alloys of high brittleness, such as magnesium alloy
B. Improper coating quantity, resulting in deposits
7) Poor filling, incomplete metal base fusion, insufficient strength after setting, especially prone to locations far from injection gate.
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