Relative Humidity and Printability

The processes underlying these technologies are quite different, but they all share one need: to control humidity in the processing and storage environments.

Storing or printing paper in a place where humidity is outside of the specified range can lead to a significant COST item, due to lost productivity (machinery stoppages) and rejects (print defects or low quality products).

What are the reasons for these problems?

Firstly, dimensional variations and the loss of substrate flatness.

Paper is a hygroscopic material that, when its moisture content is not in equilibrium with the humidity of the surrounding air, gives up or absorbs moisture, thus varying its dimensions. For example, for a 16 x A4 print sheet, a 10% variation in ΔRHroom can create a 2 mm difference!

In order to properly feed the printing machinery and allow the suction bars to pick up the print substrates correctly, the latter must be positioned perfectly flat. Any dimensional variations may, however, compromise correct positioning, and therefore paper feed.

In addition, printing defects due to MISREGISTER may occur, meaning the printing units do not release ink in the position defined by the screens, creating an image that is blurry and without defined edges.

As reported by ASHRAE in ASHRAE 2015 APPLICATION A20 printing plants, printing problems caused by paper expansion and contraction can be avoided by controlling the moisture content throughout the manufacture and printing of paper.

A reliable humidity control system is therefore necessary to ensure process continuity and product quality.

Secondly, ambient humidity can affect the paper's ink receptivity, in other words the ability of the substrate to accept and stabilise the ink received from the printing plate.

Humidity, in fact, is one of the factors that determines ink drying times. The ink needs to dry quickly to avoid any halos or irregularities. High paper moisture content makes it more difficult to absorb the ink that remains on the surface, affecting colour uniformity and intensity.

Keeping humidity below 60% RH and at temperatures between 20-25°C ensures favourable conditions for drying. Higher humidity can in fact cause drying times to be up to three times longer than in standard conditions.

What about digital laser printing?

In this case, humidity variations can change the rigidity of the substrate, i.e. its ability to accept and stabilise the toner, resulting in uneven printing, faded areas, or cracks in full areas.