The residence time is calculated based on cavity volume, hot runner volume and cycle time.

In other words, it is the calculation of the plastic remaining at high temp from the time of plasticisation to the time being injected into mould.

If the plastic stays at molten and high temp for too long, it will lead to risk of thermal degradation.

The calculation is  [( hot runner volume/ cavity volume)+ 1]* cycle time.

For the +1 means in the barrel, there will be melt volume needed for a cycle.