Like any manufacturing method, die casting is not free from defects. One of the common types of defect encountered in die casting is the blister defect. It is a formation of bubble-like small cavities or blisters on the surface of a die-cast part. They typically appear as raised, rounded protrusions on the surface of the casting.
Here, the article explores the causes, effects, and ways to prevent/rectify the defect during a die casting process.
Causes of Blister Defect
Blisters mainly occur due to the presence of trapped gases or moisture within the molten metal. However, it should not be confused with gas porosity. The pores are larger in case of blisters. It may be caused due to various factors such as,
1.Trapped Gas:
Gases entrapped within molten metal is one of the primary cause of blister defect in die casting. The trapped gases push up the soft metal surface due to solidification pressure.
2.Inadequate Venting:
If proper mold design and venting systems are not ensured, gases can remain trapped within the die cavity leading to blisters.
3.Alloy Composition:
Thecomposition of the alloy used for die casting can contribute to more gas release. Certain alloying elements may react with other elements and may be more prone to gas release.
4.High Temperature:
Too high molten metal or die temperature can lead to the formation of blisters. It can cause vapor pressure to build up within the casting and break down lubricants on the die surface to form gases.
5.Excessive Lubricant:
Excessive lubricants can also contribute to the defect. If excess lubricant is left on the die surface before closing, it will vaporize inside the die cavity and release gases.
Effects of Blister Defect
Blister defects can severely affect the functionality and aesthetics of die-cast parts.
1.Surface Imperfections:
Blister defect causes the formation of unsightly raised spots on the casting surface. Hence compromising the visual appearance of the casting.
2.Inaccurate Dimensions:
Blisters can cause a variation in the exterior dimension of a casting. Such parts may fail to meet the required specifications.
3.Reduced Mechanical Properties:
The formation of blisters can impact the mechanical properties such as durability and strength of a part.
4.Weakened Structural Integrity:
The defect can compromise the structural integrity of a casting and make it susceptible to failure under stress.
Prevention of Blister Defect
The following preventive measures may be employed to prevent the possibility of Blisters from occurring in die casting.
| 1 | Improve Venting | Proper mold design ensures adequate venting systems that allow trapped gases to escape. |
| 2 | Preheat & Dry Materials | Molten metal can be preheated to eliminate moisture present in it and degassing agents can minimize the presence of gases. |
| 3 | Temperature Control | Maintaining precise temperature controlfor the material and mold cavity can prevent the formation of gas and reduce the chance for defects to occur. |
| 4 | Process Control | Properly monitoring the die casting process such as injection speed, pouring speed, clamping force, etc. should be closely monitored. |
| 5 | Minimizing use of Lubricant | ExcessLubricants should be avoided as they can serve as a primary source of moisture leading to blisters. Use in adequate amounts, and allow time for residual lubricant to evaporate before closing the mold. |
| 6 | Optimizing Alloy Composition | The alloy composition also affects the gas formation during casting, so careful consideration should be given to the composition. |
| 7 | Quality Control | Employ sufficient quality control measures and inspection equipment to detect blisters early on during the production process. |
Rectifying Blister Defect
There are some techniques used by die casting manufacturers to rectify blister defect once it has occurred.
| 1 | Welding and Filling | Blister defects can be corrected by welding to fill the pores. |
| 2 | Heat Treatment | Some forms of blisters can be effectively reduced or eliminated by heat treatment of the die-cast component. |
| 3 | Reworking | Parts may need to be reworked. But If the blisters are too severe, the part may need to be scrapped and recast. |
In conclusion, the blister defect can be a challenging obstacle in die casting. It can cause surface imperfections, and dimensional inaccuracy and reduce structural integrity. However, a good manufacturer can identify and address the root causes to prevent and mitigate the blister defect.