After grammage and thickness, rigidity is the most important property to keep in mind when choosing paperboard. As far as functional features are concerned, rigidity is probably even the key factor if the chosen board is intended for packaging.
The rigidity allows paperboard to be used for a wide range of purposes, in the graphic and packaging fields. The demand for rigidity is constant throughout the chain, from shipping to shelf placement for the end user. Without rigidity, paperboard could not fulfil its main function, i.e. to protect the contents of the packaging.
What is flexural rigidity?
Rigidity is the resistance of paperboard to bending produced by the application of a given force.
Rigidity can also be defined as the limit of the force that must be developed on the sample of a material, to make it flex over a distance or according to a certain angle. This definition applies to most methods used for rigidity measurement.
There are two types of rigidity:
- MD rigidity, i.e. Machine Direction, grain direction;
- CD rigidity, i.e. Cross Direction, dimension at right angles to the machine direction.
Paperboard, in fact, is an anisotropic material, i.e. a material whose properties vary depending on the “direction”, that is, from the orientation of the fibers in the fiber direction (MD) and in the counterfiber direction (CD).
During the board manufacturing process, the fibers are aligned parallel to the conveyor direction of the tape. This means that the board is increasingly rigid and resistant in that direction.
The direction of the machine, therefore, describes a right angle to the width of the tape. In terms of creasing attitude, creasing transverse to the direction of machine is preferable to one parallel to the direction of machine.
In general, large boxes should have greater fiber rigidity, while smaller boxes should have greater counterfiber rigidity.
Some benchmarks for choosing the correct paperboard for the purpose:
Depending on the packaging to be realized, the choice of raw material is essential for the final yield and for the enhancement of its content.
In our sector the choice between the various types of board available is certainly not easy, we are faced with a wide choice. Each type of board is marked by a finishing that determines its visual appearance, and a grammage that establishes its density. It is not always true that an higher grammage corresponds to greater thickness and rigidity, this varies according to the composition of the board: a recycled board compared to a virgin fiber of the same grammage, will have lower thickness and rigidity.
The rigidity of paperboard is of primary importance to determine the level of machinability on packaging lines. An proper degree of rigidity also ensures that the package withstands the various stresses during distribution, exposure and use. The rigidity of the paperboard, which must also be commensurate with the size and weight of the contents, is obviously not the only element to influence the overall performance of the case.
In case, for example, of packaging for frozen products, packaged and distributed in the presence of high relative humidity, the quality of the components and the treatments of the board also determine to a significant extent the overall performance of the packaging.
The choice of the type of board for a given application is therefore taken among many possibilities.
Which are the “Pronto” products with higher rigidity?
The paperboard among the ProntoPack line with the greatest rigidity is Pronto Cote, which with a grammage of 245 gr / m2 shows a CD rigidity of 9.9 mNm and an MD rigidity of 24.7 mNm.
Another product with high rigidity is Metsa Board Classic FBB, which for a grammage of 250 reports a CD rigidity of 9.7 mNm and an MD rigidity of 19.2.
If you have doubts about the most suitable paperboard for your packaging, do not hesitate to ask us for advice: we are specialists in board!
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