Let it shine the easy way: Laser polishing

Jörg Schwartz, joerg.schwartz@photonics.com

Polishing by hand is not a very popular job – unless you own a classic car and give it a shine over the weekend. But in many industries, polished surfaces are a very necessary evil; for example, medical implants, various metallic products and optics all require precise polishing.

In molding and toolmaking for industrial production, polishing often is the last step in making the important master that is used to make car or machine parts, medical instruments, or even pills or food. The polished surface should be free of contamination and should not create friction or adhesion.

No doubt, there is a wide range of mechanical polishing machines available to do this; however, in practice they can be applied to relatively simple surface geometries only. Whenever the shape is more sophisticated and the parts hard to reach – as is often the case with injection molds for plastic parts – manual polishing has been the only option.

Now, however, lasers may revolutionize how these polishing tasks are performed. Unlike the classic process, which removes material via grinding and polishing, the laser process involves melting the surface of the medium to a depth of about 50 to 100 µm. Surface tension makes the liquid metal flow evenly and solidify smoothly.

Laser polishing is described by Dr. Edgar Willenborg, a researcher at Fraunhofer Institute for Laser Technology (ILT), in his award-winning 2005 dissertation. In it, he asserts that manual metal polishing is an art, not just a skill: “Polishing is not only a very labor-intensive task. The quality also depends to a huge degree on the skills and experience of the polisher – making it almost an art rather than a technology.”

Free-form surfaces such as this mold require high-quality polishing to ensure seamless manufacturing and good surface quality of the mass product. To date, this has been a manual job, but now lasers can help.

As for the laser method of polishing metal, it has one thing in common with the traditional manual art: It requires several stages. “This is because metals are a little too liquid,” Willenborg explained. “Amorphous materials like glass are more viscous and also won’t create crystallization defects when changing phase.”

In the first step, the laser is set to melt to a depth of about 100 µm; during the following stages, the penetration depth gradually is reduced. The result is a surface roughness (Ra) of about 50 nm, an adequate level for many applications. The high end of metal polishing, however, remains a human domain, with an experienced polisher reaching as little as 5 nm.

The melting depth can be determined by adjusting various parameters, such as the laser output power, the speed the beam travels along the surface and the length of the pulses. In fact, the laser is operated in continuous-wave mode for the rough preprocessing stage before it is switched to pulsed operation for the fine work. For metals, typically diode-pumped solid-state lasers (Nd:YAG or Nd:YVO₄) are used.

This process also can be used for materials other than metals, including glass and plastic optics, and for functions other than polishing. “Molds are only one application, and we are working on a whole range of laser polishing applications at ILT,” Willenborg stressed.

For glass and plastic optics, CO₂ lasers are used because they absorb better and can simplify another tough job: polishing aspheric lenses.