Standard Paper Tests Identify Creasing Properties

Standard test procedures for coated fine papers to identify their clinching and creasing properties have resulted from work by paper firm Sappi with Zwick Roell's Competence Centre of paper and board.

The company also worked in association with the Paper Technology Foundation Munich (PTS).

Sappi's products are used in a variety of applications, including books to wine labels, as well as magazines, brochures, catalogues and calendars.

When printed sheets are subjected to clinching and creasing, they experience strong mechanical stresses and in unfavourable conditions the paper's surface may burst, or the fibre structure may break.

The human eye is able to detect such quality breeches on printed surfaces, so such defects must be avoided, said Zwick.

Premium coated papers are sensitive to burst effects in the creasing area.

Correct creasing is of paramount importance to avoid such complaints during paper processing in printing shops, or in book binding.

To investigate the clinching and creasing properties of paper, different tests are carried out in a paper laboratory, which simulate the creasing action, but a significantly lower speed is used compared to the clinching or creasing in modern production lines, added Zwick.

If lower creasing speed is applied, paper is stressed much more than in the production conditions, which means that by using slow laboratory creasing instruments, the paper is damaged much earlier than in production and so the data produced regarding superficial bursts or web breaks tends to be exaggerated.

Laboratory die cutters provide a high working speed, which is comparable to the creasing machines used in production and could compensate this disadvantage.

However, they are expensive, large and noisy and are not usually found in the paper laboratory.

Zwick explained that as testing machines are frequently used in the paper industry, the idea was to develop a more suitable crease testing system using a modern computerised materials testing machine capable of generating sufficient acceleration and speed.

Having developed the testing system the next step was to correlate the test result data from the laboratory and transfer that to the creasing punches in the production line.

That process allowed Sappi to optimise the creasing process on the final product.

The creasing tool has a modular design, which means that the creasing die and creasing blades can be exchanged with high precision, allowing the use of the optimal combination of blade and die.

The creasing process operates according to a sequence that involves creasing parameters that are fixed (die and blade width, creasing depth, creasing speed) and, after the start of the test, the blade moves towards the specimen and in to the die, then continues up to the pre-set creasing depth.

The paper is then creased with the creasing force and blade travel monitored and stored by the system.

The creasing blade then moves back in to its initial position.

The results of the new test system are compared with a visual check according to DIN 55437 part two, a measurement of the bending moment and calculation of the clinching factor according to DIN 55437 part three and a surface profile measurement, which distinguishes between correct and bad creasing.

The advantages of the new method compared to conventional creasing are claimed to be that the required creasing depth can be pre-set in Zwick's Testxpert software and can be set to an accuracy of 2um to 5um.

The company added that the test results can be correlated and transferred to creasing punches in a production line and that the method is suitable for process control and for product development.

has now released this as a .

Back to top