Solutions for:
Blisters
Blisters show up as a swelling on the surface of the casting, typically 1 to 20 mm in diameter and 0.5 to 5 mm in height.
If they appear on a visible surface of the casting, they almost always have to be rejected. They cannot be re-worked or recovered and the only action is to remelt them.
Blisters are caused by gas becoming trapped inside the casting at the time the cavity is filled. The most likely source of the gas is the air which was initially present in the cavity, runners and shot sleeve.
When the plunger comes to the end of its stroke, it compresses the metal, and any gas trapped inside it, to a very high pressure – typically 20 to 70 MPa. This causes the gas to be compressed to very small bubbles, trapped inside the casting at high pressure.
Blisters form in two main circumstances.
Ejection: If the casting has very thin wall sections, the die surface temperature is very high and the casting is still very hot, then blisters can form soon after ejection.
This is because the strength of the metal alloy surrounding the blister is very low at elevated temperatures. The pressure in the trapped gas is therefore able to force the surface skin apart and blow it up like a balloon.
Secondary Heating: Blisters can appear when castings are subsequently heated up in a secondary process such as:
- Powder coating
- Baking
- Heat treatment
- Testing
- Welding
The secondary heating reduces the tensile strength of the alloy and the energy stored in the trapped gas enables it to expand, creating a blister on the surface of the casting.
HotFlo! engineers can assist you in eliminating blisters with actions such as:
- Venting analysis and modification
- Changes to gates and runners
- Changes to die thermal control
- Vacuum Die Casting
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