L&W PPS Flex Tester is used to measure the roughness of coated and glossy printing paper. The instrument is specially intended for measuring the very smooth paper grades. The method is based on the same basic principles as the classic PPS method. The definition of surface roughness and the measurement of air leakage are much the same.

The measurement of compressibility

A wide range of contact pressures can be chosen to suit modern printing methods, such as flexo and offset. By measuring surface roughness at two predetermined contact pressures, the surface roughness at a third chosen contact pressure can be estimated. This gives great flexibility when assessing the paper’s printing properties, because the effect of linear pressure in different printing presses can be simulated. Air pressure can be set with great precision, thereby delivering repeatable measurement conditions.

Compressibility is reported as the reduction of the surface roughness when the contact pressure is doubled. The measurement of compressibility gives a good estimate of the material’s surface compressibility and can be used to compare the performance of different paper grades.

Linear measurement head with long measuring land

In the PPS Flex method a linear measurement head is used, which gives a long measurement land. The test piece is pressed against the measurement head with the help of a rubber membrane. This gives uniform contact pressure, independent of the paper’s stiffness. The contact pressure can also be varied with excellent repeatability over a very wide range.

The measurement is fast, 5–10 seconds per measurement point, which is unique when measuring the very smooth paper grades.

The measuring land, which presses against the test piece, is 330 mm long, giving a measurement range of 0.4–4.0 µm. The measuring land is composed of a number of 51 µm thick metal shims that are separated with similar interlayers that control the flow of air. All metal shims are composites of a small square measurement head. To avoid being direction dependent the measurement is done at an angle of 45° to the sheet of paper’s machine direction.

Evenly distributed contact pressure

The test piece is pressed against the linear measuring land by the aid of a flexible rubber membrane controlled by adjustable air pressure. Measurement errors are avoided, such as those given by the classic hard and soft backing, because the rubber membrane has low stiffness and gives a uniform, repeatable contact pressure over the entire contact area.

Stiff material, e.g. paperboard, achieves good contact with the measuring land. In that way the risk of high surface roughness values, which can occur because of the test piece’s stiffness, can be avoided. c