Tips & Tricks: Corners on Wall Calendars Upraised

Error Definition and Effect
One can observe a slight curvature on wall calendars in many cases. This condition is generally acceptable to clients and buyers, above all when the curvature occurs symmetrically to the longer or shorter side. However, in flagrant cases the calendar pages curve so strikingly that the appearance becomes unacceptable. The pages may curve symmetrically, either on the front or the flipside. This means that the curvature runs parallel to the grain. There is also a type of curvature with upraised corners, which looks similar to the "dog-eared" pages of a book. In these cases, the aesthetic appearance is affected particularly negatively.

Causes and Remedies
One-sided coated paper grades tend to curve, since the ability to swell and shrink of the front and reverse side may differ depending on climactic conditions. The effect of printer's ink and dampening solution in the case of the more common front-side printing may additionally reduce flatness. The effect can be counteracted with costly prestretching. As a rule, the upraised corners are related to diagonal bracing, which is caused by suboptimal fiber orientation. In one instance, a printer has no way of reducing this effect.

Case History
After finishing the calendars and hanging them out in different rooms, it was discov- ered that corners were visibly upraised. It was suspected that the source of the problem involved some defect in the paper. On the other hand, the paper manufacturer was of the opinion that the selection of an incorrect running direction or the printing process itself caused the problem. In order to establish the cause, the disputed calendars and unprinted dummies were placed at FOGRA's disposal. It emerged that the calendars showed normal flatness in rooms with high humidity, whereas when stored in a dry climate, it became evident that the corners were clearly upraised (illustration No. 1).

1. Test of Fiber Orientation
Since suspicion fell on diagonal bracing, measurements of tensile strength were made on samples from the calendar pages, which were cut out at an angle of +30 degrees and -30 degrees. In a paper with an ideal fiber orientation, no large differences in tensile strength between the left and the right angles should manifest themselves. However, in the current case, it emerged that strips with +30 degrees generally showed lower values than samples with -30-degree angles. These measurements strengthened the suspicion of the unfavorable fiber orientation of the paper.

2. Investigation of Flatness on Unprinted Samples
Samples sized 10 × 10 centimeters (4 × 4 inch) were cut out from the same delivery of unprinted hand specimens and stored in a heating furnace for five minutes at 40 de- grees Celsius (104 degrees Fahren- heit). The result, as had been the case in the disputed calendars, the corners became clearly upraised (illustration No. 2). This test demonstrated that the delivered paper showed diagonal spanning regardless of the printing process, and was consequently responsible for the problem.