The gas can be from trapped air, hydrogen dissolved in aluminium alloys, moisture from water based die lubricants or steam from cracked cooling lines.
Air is present in the cavity before the shot. It can easily be trapped as the metal starts to fill the cavity. The air is then compressed as more and more metal streams into the cavity and the pressure rises. When the cavity is full it becomes dispersed as small spheres of high pressure air. The swirling flow can cause them to become elongated.
HotFlo! has a number of solutions for gas porosity including: changing the runner and gating system, improving the venting and adding vacuum to the die.
The use of Delta Gates has been very successful with some castings. Contact us for more information.