Coating of paper material

Federle and Keller (Federle and Keller, 1992a; Federle and Keller, 1992b) mentioned in their studies that there is no difference between different paper materials, when they are cut with laser

beam. But they especially mentioned that if paper material was coated, there was severe colouring of cut kerf and formation of smoke was slight depending on quality of coating. When cutting parameters were changed and fume extraction was proper there was adequate cutting quality.

Ainsworth (Ainsworth, 1978) mentioned in his study that unburned particles left from laser cutting of coated paper materials had to be somehow removed from paper surface.

Figure 22.5. Laser cutting limits with different dry matter content on 175 g m^-2 CTMP samples (Malmberg et al., 2006).

Figure 22.6. Laser cutting limits with different dry matter content on 175 g m^-2 pine pulp samples (Malmberg et al., 2006).

Figure 22.7. Laser cutting limits with different dry matter content on 175 g m^-2 birch pulp samples (Malmberg et al., 2006).

Ramsay and Richardson (Ramsay and Richardson, 1992) stated in their study that because of problems in removing large amounts of clay (kaolin) remains after laser cutting, whole laser cutting process of coated paper materials was not successful.

Rämö and Malmberg et al. (Rämö, 2004; Malmberg et al., 2005; Malmberg et al., 2006) studied effect of paper material coating to success of laser cutting. Figure 22.8 and 22.9 illustrates the effect of coating to the dependency of laser power and cutting speed, when uncoated, calcium carbonate and clay coated boards were cut with laser beam. Figure 22.10 shows the same effect, when boards with carbonate and clay blended coatings were laser cut (Malmberg et al., 2005).

As it can be seen from figure 22.8-22.10, it was found that laser power was linearly dependent on cutting speed, when uncoated and CaCO3 coated boards were laser cut with laser power values of 110-1500 W. With increase of laser power also the cutting speed increased. When cutting of coated board was done with higher laser power values than 800-1500 W, dependence between laser power and cutting speed was strongly nonlinear. This laser power value is called in this study as power limit. In this range increase of laser power meant decrease in cutting speed or constant cutting speed. The exact laser power value where the linear behaviour changed to nonlinear depended on coating and amount of coating. This was probably due to high light scattering and refraction ability of coating pigments (Malmberg et al., 2005).

It is known that CaCO3 decomposes in temperature of 825˚C to calcium oxide CaO and carbon dioxide CO2. This reaction is endothermic and this way consumes energy of laser beam. It is also known that CaO formed in decomposition reaction of CaCO3 absorbs well far infrared light. CaCO3

also absorbs far infrared light but this absorption is slight, only about 6.4 % (Rämö, 2004).

Figure 22.8. Effect of laser power on cutting speed, when uncoated board and calcium carbonate coated boards were cut with laser beam (Malmberg et al., 2005).

Figure 22.9. Effect of laser power on cutting speed, when uncoated and clay coated boards were cut with laser beam (Malmberg et al., 2005).

Figure 22.10. Effect of laser power on cutting speed, when uncoated and boards with CaCO3

and clay mixture coatings were cut with laser beam (Malmberg et al., 2005).

Higher grammage of coating in board samples caused the nonlinear behaviour to begin with lower laser power values. Increased amount of coating caused stronger scattering and absorption of laser light due to larger amount of matter (Rämö, 2004).

Nonlinear dependence between laser power and cutting speed also produced broader kerf widths.

This was also with boards with high CaCO3-coating amount. It can be that scattering properties of coating pigments and blue flame caused intensity of energy to distribute to wider area and this was noticed as broader kerfs. Also higher amount of pigment particles meant more scattering and that resulted broader kerfs (Malmberg et al., 2005).

When cutting was done from bottom side (uncoated side) of CaCO3-coatedsamples laser power depended linearly on cutting speed, but in certain laser power limit value it turned to be non-linear.

This laser power limit value was higher, when cutting from bottom side than when cutting from top side. In some cases, no turning to non-linear behaviour occurred. No overall conclusion of when and why this phenomenon occurred could draw. Cutting kerf width also stayed constant, when bottom side cutting was done in linear zone, otherwise kerf width increased. Cutting from uncoated side of board could be a solution, when trying to avoid the blue flame and non-linear behaviour of laser cutting of boards (Malmberg et al., 2005).

The cut kerf with uncoated and CaCO3-coated board was V-shaped. The kerf was broader from the side which was in contact with laser beam. The top of the cutting kerf was always wider than bottom. This may be related to the laser beam caustics (laser beam is diverging outside of focal point), and to the material thickness which was large compared to beam caustics (Malmberg et al., 2005).

It was noticed that laser power depended also linearly on cutting speed, when uncoated and clay coated boards were laser cut with power range of 110-1300 W. When cutting of coated board was done with higher laser power values than 1300-1700 W, dependence between laser power and cutting speed was strongly nonlinear. This power limit was higher with clay than with calcium carbonate (Malmberg et al., 2005).

When cutting clay coated board with high laser power values cutting speeds were reduced as it happened with calcium carbonate coated samples. However, this reduce was slighter with clay than with calcium carbonate. Clay as a mineral pigment scattered and refracted laser light (Malmberg et al., 2005).

Increase in grammage of clay coating decreased the cutting speed compared to uncoated board. This same non-linear behaviour occurred also with CaCO3-coated boards. This was due to higher amount of matter which had to be evaporated (Malmberg et al., 2005).

Crystal water of clay leaves clay structure in temperature of 450˚C and this reaction changes the structure of clay. This reaction consumed laser energy so the reaction was endothermic.

Decomposition also changed the structure of pigment and optical properties of it (Rämö, 2004).

When cutting clay coated samples from bottom side of sample, laser power depended linearly on cutting speed but in certain laser power limit value it turned to be non-linear. This happened also with calcium carbonate coated boards. This power limit was remarkably higher than when cutting from top side (Malmberg et al., 2005).

Also some board samples coated with mixtures of CaCO3/clay were cut with laser beam. The nonlinear dependence between laser power and cutting speed occurred also in these tests. It was also noticed that these curves followed the cutting curve of CaCO3 coated samples. It could be so that calcium carbonate as a coating pigment was the dominant pigment, when thinking of decrease

in cutting speed. This decrease of cutting speed was more significant with coating mixtures than with separate pigments. This can partly explain by the fact that CaCO3 has little higher refraction index than clay. It can be partly explained by the fact that both pigment types together catalyzed decomposition (Malmberg et al., 2005).

Blue flame observed with CaCO3 coated boards occurred also with clay coated board. It appears that blue flame phenomenon as non-linear dependence between laser power and cutting speed emerge, when mineral pigment coated boards are laser cut (Rämö, 2004; Malmberg et al., 2005;

Malmberg et al., 2006). This blue flame phenomenon is discussed later in this literature review.

IV PHENOMENA OCCURING DURING INTERACTION OF LASER BEAM AND PAPER MATERIAL