The two biggest determining factors of molding sink marks are product design and processing with the assumption that the tool is functioning correctly. With that being said there is a fine line between the two. You can over compensate for poor product design with molding parameters, and vise versa. If you over pack a part you can begin to effect it dimensionally or ride on the fringe of flashing your parting line. You can very easily make a part that is too big by trying to pack out molding sink marks that are caused by poor product design. The other down fall of over packing is a small processing window.