A void occurs during cooling while the part is inside or outside the mold, usually in thick sections. In thick sections of a part, the center cools slowly and the polymer shrinks more, pulling away from itself to form a bubble. If you run the mold hotter and the bubble goes away, but you have a sink as a result, that is an indication your bubble was a void. Voids and sinks are signs of internal stress and are warning signs that the part may not perform as expected.
Insufficient plastic is the main reason for sinks or voids, so packing more material into the cavity is recommended. Make sure you have a consistent cushion and are not bottoming the screw so that you can properly pack out the part. Higher hold pressures or longer hold time may help, but many times the gate is freezing before you can adequately pack out the center of the nominal wall.
To solve voids or sinks, try slow fill rates, use of gas counterpressure, or increasing backpressure. Ensure the runner or gate is not freezing too early and that longer hold times will allow more packing during the second stage. If the gate is freezing off too soon, a slight opening of the gate may be all that is needed, as a small change in diameter results in a longer gate-seal time. Also try reducing melt temperature, if possible.
Other approaches to eliminating voids or sinks is to thin down the nominal wall. Thicker is not always stronger in plastic parts. Thick nominal walls should be redesigned to be thinner with ribs for strength. This will save plastic and cycle time.
Core out the thick section, if possible. Changing the gate location to fill thicker areas in the mold first may allow more polymer into the part before the gate freezes. You may also try raising the mold temperature and/or ejecting the part sooner, which can prevent voids by allowing the outside walls to collapse during cooling, though that may result in sinks.