Paper cockle

The resin acts as a barrier to prevent the ink components from penetrating into the base paper, ehminating paper "cockle" and adverse interactions between the paper addenda and the imaging dye. 

As water penetrates into paper, it swells fibers, and also disrupts hydrogen bonds that hold paper fibers together. Over a short ( 0.1-10 sec) timeframe, this can lead to paper cockle, a localized warping of the original flat sheet (see Fig. 6). The simple way to fight cockle is to put less water on the sheet. Over longer (1 minute... 

K) Drying newly made paper on high speed rolls in a paper mill leaves stresses in the paper which are released when the paper is later wet, as in a flood, and redried. This is probably one major source of cockling (4). 

Cockling—Was the surface of the paper uniformly dried Were irregularities slight or extensive ... 

Cockling of paper results mainly from imperfect sheet formation. The difference in felt tension may also cause cockling and uneven drying. Uniformity of moisture content and basis weight in CM direction prevents cockling. 

If a pop-up is to be decorated, be careful when using water-based media. Water makes most papers and cards "cockle" that is. warp and ripple. Not only does this look unsightly, but it makes a pop-up difficuU to shut. So. use gouache, watercolour and poster paints with care, testing them out first. Even broad marker pens and fell-tipped pens can cockle paper. One solution is to use only those papers and cards designed to take water, such as watercolour paper. 

Coloured pencils, pastels, charcoal and pens and pencils arc much gentler and generally will not cockle paper, though ihc> will need a good fixative (a cheap hair spray makes an ine.xpensh c alternative to c.vpensivc... 

Cockling A waviness acquired by paper that has dried without tension. 

Furthermore, the moisture content of paper impacts the dimensional stability of paper and board products. The physical dimensions of materials made of paper are sensitive to the moisture content as well as the history of its change. Most paper materials expand with moisture content due to the swelling of the fibers. This swelling is anisotropic and is predominant in the radial direction of the fibers. As a consequence of non-uniform fiber distributions inside them, paper sheets tend to curl and deform on a small scale locally. The local deformation is termed as cockle. Hence, knowledge of the interaction of moisture with paper will help in producing more dimensionally stable products [7]. 

A typical application of a floater dryer on a sack paper unit is shown in Figure 35.19. As natural shrinkage occurs principally between 50 and 85% dryness, the floater dryer is positioned between the predryer and the after-dryer in order to produce high-stretch paper. The after-dryer is used to remove the cockles and to give a smooth surface to the paper. If desired, a smoother surface can be obtained by calendering the paper. 

First, ink-jet papers must be smooth. They must have sufficient and uniform porosity, with small pores, in order to absorb the solvent quickly and to counteract the spreading tendency. Dimensional stability is also important to avoid cockling and curling. Typical uses for digitally printed products are manuals, price Ksts, and various direct mail materials, but also low-volume paperbacks and hard[Pg.455]

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