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Connection 120

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Residual strength of heatset papers

Test of the residual strength of heatset papers after drying and fold

Fogra HOT

In heatset offset printing a paper web is guided through the printing machine with high velocity. It first passes the printing units, before reaching the heated air dryer, where the high-boiling mineral oils are evaporated from the inks. As the paper thereby reaches temperatures clearly above 100 °C, it loses most of its humidity and gets brittle.

In combination with the embrittlement of the paper the mechanical stresses during folding lead to a considerable reduction of the strength properties. A residual strength which is too low can cause breaking of the fold. If this is observed during the print run, the printer can only counteract by decreasing the printing speed or the air temperatur of the dryer. But in most cases the problem is noticed too late, when single pages of the completed print product fall off the saddle stich or the thread sewing.

The generated damage can be immense. It is therefore recommended to firmly fix the residual strength in the conditions of supply. Whether the promised quality has been met, can be checked objectively in eventual cases of damage.

Test procedure

The residual strength is determined according to the Fogra method that is mentioned in DIN 19306-2. For this a test line is utilized which consists of a laboratory dryer (Fogra-HOT), a folding instrument (Fogra-FI) and a tensile strength measurement device. In the dryer the paper samples are applid to hot air similar to heatset machines. Afterwards the samples are fold in a definite way. Finally, with the tensile strength measurement device the force is measured that is necessary to break the fold.

As characteristic value for the residual strength acts the mean tensile strength of 20 paper samples. On the basis of experience from numerous expertise and research projects it can be allocated to a critical, a mean and an uncritical range. In the critical range problems during the machine fold, the print finishing or the usage of the completed products are likely. In the mean range paper-based as well as process-based faults can occur.

The test usually is performed with a fold along the grain direction, because this is the weak point of the paper. But as needed, a fold perpendicular to the fold also can be examined.

Prices

Test of one paper grade with fold along the grain direction: € 181.50
Fogra members: € 132.45



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