The register between colors is a fundamental aspect for the quality of the printed product. But what is the correct register tolerance and how to evaluate it?
ISO 12647-6 connects the register tolerance to the print linescreen. The higher the linescreen, the greater the expected accuracy on the printed product. According to this norm, register tolerance shall be within 2÷linescreen, and it should be within 1÷linescreen. For example: at 120 lpi (48 l/cm) it shall be ≤ 420 µm and it should be ≤ 210 µm.
Atif Doc04, Check-Up procedure for flexographic presses (Rev. 04-2013), suggests different levels of acceptability: good if ≤ 50 µm, sufficient if ≤ 100 µm, insufficient if > 100 µm.
How would you evaluate it? With a microscope, featuring sufficiently accurate measurement capabilities, the register is measured by checking some printed samples that are representative of a print run for the printing condition under examination (different substrates could produce different register values).
In any case, the register tolerance is a characteristic of the printing system that must be assessed and managed: a 100 µm register cannot be requested from a printing system that has 500 µm accuracy. The register tolerance must therefore be assessed during the system check-up and fingerprint operations and shall be shared and agreed between all parties involved in production workflow: printers can keep their production under control, prepress has precious information about necessary trap values and customer can have better and clear expectations of the final result.