Evacuated voids formed inside thick sections when shrinkage cannot be compensated. The transfer of holding pressure and the sizing of the gate are decisive.
A vacuum void is a void forming inside the thick sections of a part because the volume that shrinks as the material cools cannot be fed from outside. It arises from the same physical cause as a sink mark; the difference is that the surface is strong and does not collapse inward, and the void stays inside the part. In transparent materials it is visible to the eye; in opaque materials it is only noticed in a cross-section.
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|---|
| Sink Marks | A sink mark is a surface depression visible from outside. With a vacuum void the surface is smooth and the void stays inside. The strength of the surface layer determines which one will form. |
| Air Bubbles | An air bubble is air coming in from outside and its position varies. A vacuum void is always at the center of the thick section. |
If the gate freezes early, however much you raise the pressure the material cannot get in. Determine the effective holding time by weighing the part: the moment the weight stops increasing, the gate has frozen.
Yes, and it is the most effective one. Balancing the wall thickness, coring out thick areas and positioning the gate close to the thick section eliminate the vacuum void at its source.