Rubbing Endurance
Laboratory testing equipment of this category evaluates the resistance of printed paperboard to the transfer of colorant under dry or wet friction. The crockmeter utilizes a standardized weighted finger that moves in a straight path to rub the test specimen. This motion helps to simulate the stresses encountered during folding and shipping.
Friction Mechanism
Linear abrasion arises when a weighted arm travels along a fixed track across the printed surface. The arm exerts a constant downward force of nine newtons through an acrylic finger covered with a standard white cotton cloth. Rubbing occurs over a path of one hundred millimetres, and the rate of travel is one stroke per second.
This action continues for a set number of cycles to transfer any unstable ink or coating from the board onto the dry or wet cotton cloth. Each cycle consists of one forward and one backward movement of the mechanical slider, which operates via a manual crank or an electric motor.
Reciprocal Stroke
Standardized evaluation of the resulting color transfer requires a spectrophotometric measurement of the rub cloth. The test does not apply to deeply textured surfaces where the finger cannot maintain uniform contact across the board. Heavy boards with extreme corrugation can cause the arm to bounce and distort the friction profile.
Consistent humidity remains necessary because moisture softens the paperboard fibers and accelerates ink rub-off.
