Most commonly picking of paper is defined as the damage of the paper surface during the printing operation. At the time the printing form is lifted off the paper the ink is exerting a certain force on the paper. This force is increasing with an increase in the viscosity and tack of the ink and the printing speed. When this force exceeds a certain value, the surface of the paper will be damaged. This test can be used to determine the delamination of paperboard as well. The pick velocity is defined as the velocity at which picking starts in this test method (this is not the velocity on the printing press in practice); the pick resistance is characterized by the product of pick velocity in m/s and viscosity in Pa.s of the pick test oil used.
This product is also called the VVP (Viscosity Velocity Product), which has a constant value for a certain paper or paper board. Using the VVP it is possible to compare the test results of different papers and paperboards obtained with different grades of pick test oil under certain conditions. Also it is possible to eliminate differences in temperature within certain limits. The determination of the pick velocity and the pick resistance is one of the most widely used tests performed on the IGT printability testers
Using the IGT-printability tester a print is made on the paper to be tested with pick test oil at an increasing speed. The first damaging of the print is observed and from a table the speed where picking begins is read. The VVP is calculated as the product of the speed where picking begins and the viscosity of the pick test oil used.
Method of operation
• It is recommended to execute the test in the standard atmosphere; to most standards it is 23.0 ± 1.0 °C (73.4 ± 1.8 °F) and 50 ± 2% rh.
• For the operation of the AIC2-5T2000, Global Standard Tester, High Speed Inking Unit and ink pipette follow the instructions of the manuals, IGT information leaflet W100 and the displays accurately.
• Handle the samples carefully