To check if the parting line of the injection molding mold fit properly on each other inking is used.
This is done by applying inking-colour to one side of the mold (by spraying or brushing).
By closing and reopening the mold the colour is transferred to the other side of the mold.
Now it is easy to see where the mold fits exactly and where it does not.
This is an important information when it comes to subjects like short shot, flash formation or even multi-component molding.