Recommendations to printers/converters
The correct handling of paperboard is very important for achieving the best results in printing, converting and use. To ensure this Holmen Iggesund recommends the following procedures to printers, converters and users.
The paperboard is identified by the order number and the pallet or reel number. It is essential that these numbers are recorded to provide identity and traceability throughout printing, conversion and use, or that you keep the labels until the job is finished. In the case of doubt about the performance of the paperboard or its compliance with specifications, it is a requirement to provide this information and relevant samples to Holmen Iggesund.
The moisture-proof wrapping must not be removed until the board has attained the temperature of the environment concerned (printing room etc):
|Pallet or reel weight (kg)||Temperature differences board – printing room
(print room temp. about 20°C)
|400||2 days||2 days||3 days|
|800||2 days||3 days||4 days|
|1200||2 days||4 days||5 days|
When cold paperboard is exposed to a warm environment the air adjacent to the board can be cooled below its dew point (point of condensation) and this moisture is then absorbed by the board. The time for temperature equilibrium to be established varies depending on the temperature difference and the weight of the board (pallet or reel). Therefore the wrapper should never be removed before the board has reached the temperature in the printing room.
Recommended climate in the printing/converting room to prevent curl and/or misregister is 45–60 % relative humidity and a temperature of 20–23°C.
The moisture content of paperboard can rise as a result of its exposure to air at high relative humidities, during periods of wet weather. It can also occur as a result of the condensation effect in cold weather discussed above. The result of moisture absorption can be waviness at the edges of the sheets and a general tendency for upcurl i.e. curl towards the print surface. If wavy sheets of board are printed creases may occur at the back edges of the sheets as they pass through the offset press, misregister may occur, together with press misfeeding, wasted sheets and loss of output and efficiency.
A paperboard which has a high moisture content also becomes tough and difficult to cut. The moisture content of paperboard can fall as a result of its exposure to air at low relative humidity. This can occur during very dry weather and also occur in centrally heated buildings when the external air is very dry and very cold. A paperboard which is too dry may also become wavy, develop downcurl i.e. curl towards reverse side of the board and become brittle. These features can cause misfeeding and the board will be difficult to crease satisfactorily.
Avoid rubbing or touching the surface of extruded/ laminated board because the corona treatment is easily damaged.
Paperboard in sheet form should be rewrapped with moisture-proof material after printing. Rewrapping is particularly important in order to ensure good register when board is printed in two or more passes through the printing press. It is also important to wrap in this way to achieve good register between the print and the next process e.g. cutting and creasing, guillotining or bookbinding. Paperboard products should be wrapped in moisture resistant material after conversion and prior to shipment to the customer (end-user) or to further conversion operations.