In order to ensure an efficient process and excellent and consistent quality of the printed product, the temperature and humidity in places designed for the storage and printing of paper and cardboard needs to be controlled appropriately.
The hygroscopic nature of paper
Paper is a hygroscopic material that tends to reach a condition of equilibrium with the surrounding environment in terms of moisture content.
Swings in ambient temperature and humidity with respect to equilibrium values can therefore cause dimensional variations and loss of substrate flatness
If paper is exposed to an environment with excessive moisture, the outer parts of the sheets quickly absorb moisture and
increase in length, causing corrugation (more frequently in summer, in places without air-conditioning).
If the air is too dry in the storage environment, the paper releases part of its moisture from the edges to the centre. The result is a sheet with the edges curled upwards.
In addition to variation in sheet size and loss of flatness, the mechanical properties of the substrate itself may vary. Fibre shrinkage, for example, means greater stiffness, which may affect runnability, resulting in increased tears, breakages and creases on the finished products.
Relative humidity and static electricity
If the air is too dry, unwanted static electricity may also form, another cause of rejects due to improper paper feed and consequent inaccuracies in the printing process.
Humidity also affects runnability, i.e. the workability of the substrate, as well as its ink absorption properties, in other words how well and how quickly it absorbs the ink or attracts the toner.
Using a humidification system that keeps ambient conditions at 55-60% RH (offset printing) or 50-55% RH (digital printing) thus brings significant savings in terms of time and production and operating costs.
Due to static electricity, the sheets of paper tend to adhere to one another, creating difficulties in feeding the printing machinery.
Dimensional variations and loss of substrate flatness cause misfit on entering the machinery and in the subsequent stages. This results in machine stoppages or printing defects.
Dimensional variations and loss of substrate flatness may also cause misregister, a defect whereby the subsequent printing units do not release the ink according to the position defined by the screens, creating an image that is blurry and without defined edges.
In the presence of high static electricity, small droplets of ink may deviate from their required path. These can also block the nozzles. The result is an irregular and “blurry” print due to the formation of halos.
The table below also shows some print defects also caused by uncontrolled ambient humidity.