Die Casting – Bubbles on the gate line

CAUSE: This is usually caused by the gate area getting too hot. As a result, the actual gate freezes after the runner or casting next to the gate.

Shrinkage usually is localized in this last area to freeze and hence bubbles expand and shrinkage pores form. When the runner is trimmed off it leaves a bubble on the gate line. This defect normally occurs with thick castings which have a narrow section of die steel in between the casting and the runner. This thin section is heated by a combination of the runner, the casting and the large amount of metal flowing through the gate.

SOLUTION: The best option is to

1. Design the region between the runner and the casting to have more die steel and
2. Cool the gate area.
3. Keep the gate as thin as possible