When Lasertec first began making laser-based systems for mold-form generation, they initially used a lamp-pumped solid-state laser. As the power of DPSS lasers increased, the company investigated all-solid-state technology. Some of the reasons for the switch to DPSS lasers were their small footprint, low-thermal load, superior pointing stability, high-wall-plug efficiency, and long maintenance intervals, but a major reason was the goal of improving the surface roughness and precision of the molds. This proved possible by switching to an end-pumped Nd:YVO4 laser. This specific type of laser emits all of its power in the TEM00 mode, with an output profile that is stable over many thousands of hours.
In contrast, the output of lamp-pumped lasers is typically multi-mode, and this mode-structure changes with time as the lamps age. The negative effects of higher-order-output modes on surface quality are illustrated in Figure 1. An end-pumped DPSS laser is also capable of excellent (1.5 percent RMS) pulse-to-pulse stability, which is another factor contributing to the 50 percent improvement in observed surface quality as well as improved contour accuracy (see Figure 2).
In addition, prior experience indicated that for a given laser power, superior surface quality and process control are obtained with a high-repetition rate and modest pulse energy. This is because it is better to remove a small amount of material with each pulse when surface quality is the prime consideration. A DPSS laser can be optimized for pulse-repetition rates as high as 100 kHz and can be operated at even higher rates, whereas lamp-pumped lasers were limited to 40 kHz. Therefore, the end-pumped DPSS laser is a more precise and stable tool for producing small features and filigree-type structures.
In order to determine what other characteristics the optimum DPSS source should possess for this application, the process was first investigated in an applications lab setting. This was accomplished using a flexible laser (Coherent PRISMA) rated at 15 W, which could be configured for different output performance characteristics, most notably pulse duration and pulse-repetition rate. Production lasers were then built to deliver the optimum pulse duration for this application.
In terms of beam delivery, with a laser output power of 15 W, the best results are obtained with a laser spot size of 30 to 40 μm. A repetition rate of 100 kHz then leads to a material removal rate of 0.1 mm3/minute. Depending on the part complexity and size, a typical mold for a model train component takes around 24 hours to produce at this ablation rate.
One final feature merits brief mention for completeness: With lamp-pumped lasers and slower pulsing rates, a mechanical shutter was used to switch the laser power off as the beam is scanned across the workpiece. With the 100 kHz DPSS laser, standard mechanical shutters are too slow to take advantage of the faster pulsing rate. Instead, the laser is equipped with an ultra-high-speed shutter, which enables the beam to be vector scanned at speeds in the 500-800 mm/s range, while maintaining maximum spatial resolution.