Laser power vs. cutting speed

Federle and Keller (Federle and Keller, 1992a; Federle and Keller, 1992b) cut offset-printing paper (grammage 80 g m^-2) with CO2 laser and figure 21.11 represent results of that cutting study.

Figure 21.10. Effect of cutting gas pressure to cutting speed, when uncoated and carbonate coated boards were cut with laser beam (Rämö, 2004).

It was noticed that laser power and cutting speed depend linearly on each other. When laser power increases, parameter range for good cutting quality is increased. In this study it was also noticed that, when cutting was performed with higher laser power than required the width of cut kerf was increased, carbonisation of kerf was stronger and amount of fumes released during cutting increased. It was concluded that, when excess laser power is used in laser cutting of paper materials suction of fumes and dust released during cutting is very important (Federle and Keller, 1992a;

Federle and Keller, 1992b).

Figure 21.11. Laser power vs. cutting speed, when offset-paper (grammage 80 g m^-2) was cut with CO2 laser (Federle and Keller, 1992a).

Ramsay and Richardson (Ramsay and Richardson, 1992) noticed also that for good quality cutting it was very important to remove all dust and fumes released during laser cutting of paper material.

They also concluded that laser power and cutting speed has linear dependence. In this study it was found out that, when cutting speed was increased but laser power remained constant, cutting quality was destroyed: cut kerf became yellow, was uneven and out-sticking fibres occurred. It was also noticed that if paper web was fluttering under laser beam and focal plane position was then changing, this had negative effect on cutting quality. This is why support of paper web under laser beam was important (Ramsay and Richardson, 1992).

Juselius (Juselius, 1974) found out that dependence between laser power and cutting speed, when paper materials are cut with laser beam, is linear.

Cutting trials of laser cutting of paper materials by Hovikorpi et al. (Hovikorpi et al., 2004b) were performed to establish relation between laser power and cutting speed. Focus position was in the middle of the sample. Nozzle hole diameter was 0.2 mm and stand-off distance 0.1 mm. Nozzle stand-off distance is represented in figure 21.6. Nitrogen was used as cutting gas at 5 bar pressure.

Limit value for cutting speed was defined. Cutting limit was the speed in which the cutting kerf was continuous from start to end without any fibre bonding. Cutting trials with pulps are illustrated in figures 87-89 (Hovikorpi et al., 2004b).

As figures 21.12-21.14 show, the effect of laser power on the cutting speed is linear. Increase in grammage increased also required laser power in order to reach same speed. Birch pulp was possible to cut with higher speeds than pine and CTMP pulps with the same laser power. Difference between partial cutting speed and cutting speed limits is bigger with pine than birch and CTMP pulps. Difference between pine and birch can be caused by the higher absorption of birch pulp. The difference can also be caused by the longer fibres of pine. Difference between CTMP and the other pulps is probably related to fact that board made from CTMP is much thicker at the same

grammage. This leads to a situation that in CTMP cutting lower power densities are used because material is thick compared to beam caustics. This leaded also wider cutting kerfs in CTMP versus other pulps (Hovikorpi et al., 2004b).

Figure 21.12. Laser power versus cutting speed for CTMP sample (Hovikorpi et al., 2004b).

Figure 21.13. Laser power versus cutting speed for pine pulp sample (Hovikorpi et al., 2004b).

Figure 21.14. Laser power versus cutting speed for birch pulp sample (Hovikorpi et al., 2004b).

It was found out also that the cut edge quality and kerf widths were stable for each grammage and pulp types produced with parameter combinations on cutting limit (laser power/cutting speed). Top of the cutting kerf was always wider than root. This is partly related to the laser beam caustics (focal point was set to the middle of sample) and partly to the fact that beam is interacting with upper part of the sample longer than with root part (Hovikorpi et al., 2004b).

It was also noted that kerf width increases, in optimal cutting limit, if grammage and thickness increase. This behaviour is also relating to the fact that laser beam is diverging outside of focal point. Kerf width is an average value of top and root kerf widths produced with each parameter combinations of cutting limit (laser power/cutting speed) (Hovikorpi et al., 2004b).

In cutting trials there was no colorization or carbonization of cut edges with pine and birch pulps.

Some colour change was noticed with CTMP. This is related to the fact that CTMP includes all wood components like lignin, which is known to cause yellowing very easily in any conditions. In birch and pine samples laser cut edge was fused and sealed. With CTMP kerf edge was also fused, but not totally sealed, some individual fibre ends can be seen (Hovikorpi et al., 2004b).

Archer et al. (Archer et al., 2005) noticed in their study of laser cutting of office paper (grammage 80 g/m²) with diode laser (wavelength of laser beam 810 nm) that cutting threshold is found to increase linearly with the laser power for a large enough power. Results are reported on figure 21.15, for a spot diameter of 30 μm, and for both black marker ink and invisible ink.

Figure 21.15. Results of laser interaction for different laser powers and speeds, on paper inked using black marker ink (squares) or invisible IR absorbing ink (triangles). Full symbols indicate a complete cut, and hollow symbols indicate a partial cut. The lines represent the asymptotic dependence of maximum cutting speed as a function of laser power (Archer et al., 2005).

As figure 21.15 represents, when value of the speed is higher or when value of the power is lower, the paper is not completely cut. For speeds up to 20 % higher than the threshold speed which is necessary to obtain a complete cut, easily-tearable lines are obtained. For speeds 20 % to 100 % higher than the threshold speed, easy-folding lines are obtained (Archer et al., 2005).