Voids are a somewhat common defective occurrence in injection molding applications. They present as “air pockets” in transparent molding but can also be present in colored or opaque molding applications.
Voids tend to appear on thicker part surfaces and can occur when the outside layer of the part cools off and solidifies faster than the internal which can in turn cause a void.
There are several factors that can come into play when voids are discovered and they can be caused by either machine (process), mold, or material issues.
The most common cause of voids would be the process and there are some adjustments you can make to better fill the part out and reduce your risk of void defects. Most common suggestions would be to increase the shot volume, increase injection pressure, increase screw forward time, reduce melt temperature, or adjust the injection speed.
The mold could also be a cause of the void as well. In those cases, these areas should be considered. Is the gate that is being used too small for the part design? Is the gate positioned in the incorrect place for the part? Is the runner incorrectly designed or sized? The mold temperature could also be too low or inadequate venting in the mold.
The last area to consider is the material. Voids can sometimes be caused by wet material with excessive moisture. If this is a possibility, then you may want to dry the material for a longer period of time to expel all excessive moisture.