COPPER SURFACE – ROUGH/WAVINESS

Although copper is not the actual printing surface, its roughness and waviness have a large impact on printing because of epitaxy:

A bad copper surface yields a bad chrome surface. For this reason roughness and waviness are normally measured in copper.
The desired roughness of a cylinder depends on the finishing process and the printing substrate. A polished cylinder has a surface roughness that follows a random path with a relatively large standard deviation when plotted out, whereas cylinders finished with a diamond cut show a less dispersed pattern.

This means that the roughness graph of a polished cylinder is more dispersed with steeper peaks and valleys.

The targeted roughness for polished cylinders ranges between 0,5 and 0,6 Rz; for cut cylinders, 0,4 and 0,5 Rz. Coated stock, transparent, or reflecting substrates are more sensitive than uncoated stock to the roughness of the cylinder surface. They require lower roughness as well as a polished cylinder roughness. Roughness exceeding the target range produces ink transfer in non-printing areas.

Surfaces that are too smooth yield too little doctor blade lubrication, which causes permanent chrome wear.

Roughness describes the short cycles of unevenness. Waviness describes larger-scale unevenness. Waviness is measured in axial direction on the cylinder face. If the waviness is too high, the cylinder face will have valleys to which the doctor blade will not be able to adjust, thus creating unintended ink transfer. The target value for waviness is to have less than 1 micron over a measuring length of 15 mm.Roughness and waviness can both be measured on the same instrument.

The instrument drags a diamond probe across the cylinder surface and records the relief. The waviness (long cycle) and roughness (short cycle) components are statistically separated and printed out or displayed.Roughness measurements are denominated in Rt, Rz and Ra. They are defined in the drawing to the left.

Ra is the arithmetical average value of all absolute distances of the roughness profile from the center line within the measuring length.Rz is the average maximum peak to valley of five consecutive sampling lengths within the measuring length.Rt is the maximum peak-to-valley height within the measuring length.
It follows from these definitions that the relation Rt >= Rz >= Ra is always true. For the gravure cylinder, Rz and Ra are useful measurements. Ra averages all measurements and does not have any discriminating value in separating rejects from acceptable cylinders