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Why does paper twist and cockle?

Developing and supplying innovative, high-tech measurement systems and services for reliable quality control and trouble-shooting in the pulp and paper industry has been AMBERTEC's speciality since the mid-1980s. The company's current product range extends from equipment, such as the AMBERTEC® Surface Orientation Profiler and the AMBERTEC® Beta Formation Tester, to on-line damage prevention monitoring and high-speed digital imaging equipment.

Take a closer look at the appearance of some of the newspapers on your local news stand on your way to work, and you may well ask yourself why the edges of the papers often seem curled. And when you get to work, and the paper jams in your copier – again, why is that?

Fast forward to your lunch break in the park, and you might notice that the pages in the paperback that you have been reading have seemed rumpled ever since you forgot the book in the bathroom a couple of days ago. Again, why?

A common explanation

The cause of all these phenomena lies in the structure of paper, according to paper scientists. The difference in the fibre orientation angle between the top and bottom surfaces of a sheet is one of the major causes for diagonal curl (twist), as well as for the cockling of a sheet, whenever the moisture content of the paper changes.

Paper machine manufacturers have only learned to control fibre orientation in an effective manner quite recently, however. As a result, measuring surface fibre orientation has become a very interesting topic.

A new solution to an old problem

Until recently, fibre orientation testers have not been able to distinguish between the top and bottom surfaces of a sheet – and the data on the average orientation through a sheet that they have provided has been insufficient to help solve the paper curl phenomenon.

AMBERTEC has developed an instrument capable of measuring the surface fibre orientation on both sides of the web simultaneously, and providing the results from a single cross-directional profile strip in less than half a minute.

The reliability of the AMBERTEC® Beta Formation Tester has made it a de facto standard method for modern paper formation quality analysis, and it is now used in major paper mills in 26 countries.

Known as the AMBERTEC® Surface Orientation Profiler, the unit can quickly reveal those parts of the web that are most likely to curl or cockle in end-use.

The AMBERTEC® Surface Orientation Profiler does this by taking 250 orientation angle and intensity measurements a second from both sides of the web, and then calculating cross-directional surface orientation profiles. The results are displayed on the unit's screen, and are stored in a database, enabling them to be forwarded automatically to printers, mill data networks, or other recipients via e-mail.

Thanks to its use of the AME (Ambertec Measuring Environment) operating system, the Surface Fibre Orientation Tester is extremely easy to use. It is also very compact.

Operation of the touch-screen display, and using AME, can be learned in just a few minutes, while measuring and analysing a 10-meter sample strip takes just 20 seconds.

A de facto standard

Another issue related to paper cockling lies in formation, and small variations in local basis weight.

Many modern paper grades include a variety of fibre and filler components, coating pigments, and various surface treatments that change a paper's optical behaviour and visual appearance. As a result, traditional visual inspection or optical formation measurement cannot provide a full understanding of a paper's structure – and cannot be relied on as the only method of characterising paper uniformity.

The AMBERTEC® Beta Formation Tester overcomes these problems, and provides a highly reliable way of measuring paper and board formation.

It is now available in two different versions: one for papers up to approximately 300 g/m2 basis weight, and one for measuring heavy-weight grades up to 700 g/m2.

The reliability of the AMBERTEC® Beta Formation Tester has made it a de facto standard method for modern paper formation quality analysis, and it is now used in major paper mills in 26 countries around the world.

The AMBERTEC® Surface Orientation Profiler is very compact and extremely easy to use. Operation of the touch-screen display, and the AMBERTERC AME operating system, can be learned in just a few minutes. Measuring and analysing a 10-meter sample strip takes just 20 seconds.
> Antero Komppa
(Published in High Technology Finland )