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Using Lasers for Cleaning

Surface cleaning through traditional methods

Surface Cleaning is one of the important steps in the processing of materials. Interestingly surface cleaning by mechanical or chemical processes (e.g. media blasting and dry ice blasting) is prevalent in most of the industries.  These methods are complex, time consuming and involves a lot of labour. In addition, these methods will produce waste, have precision issues and can be dangerous for environments.

Surface cleaning through Lasers

Laser based surface cleaning, on the other hand, is contactless processing method in which no chemical or mechanical force is involved. Cleaning operations have long been a difficult application for industrial lasers because of the significant variation of target materials, often worn and weathered by the time removal becomes necessary.  Cleaning operations have long been a difficult application for industrial lasers because of the significant variation of target materials, often worn and weathered by the time removal becomes necessary. The parametric flexibility of the newest laser sources is equal to the task, allowing parameters to be “dialed in” to the susceptibility of the target, and allowing for sequential steps toward the end goal.  And due to negligible waste generation after processing, laser cleaning is regarded as environment friendly technique. In the traditional methods like media blasting and dry ice blasting, surface (substrate) damage is inevitable.  Coating removal chemicals ultimately reacts with the surface of substrate and cause irreversible damage. Whereas laser cleaning provides very high precision with good reproducibility. Laser cleaning has high potential in future industries as they offer these advantages at lower cost as compared to conventional methods. Due to increased attention, laser hardware’s are getting continuous upgrade and differnt sorts of laser sources appear on the laser cleaning market.

Fiber lasers with high reliability, stability, and flexibility have become the best choice as a laser cleaning beam source.

Laser cleaning uses a focused laser beam to rapidly vaporize or strip the contaminants on a material’s surface. Compared with other traditional physical or chemical cleaning methods, laser cleaning is non-contact, with no consumables or pollution, high precision, and small residual damage. This process is an ideal choice for the new generation of industrial cleaning technology.  Furthermore, fiber lasers with high reliability, stability, and flexibility have become the best choice as a laser cleaning beam source. Two types of fiber lasers, continuous-wave (CW) fiber lasers and pulsed fiber lasers, occupy the market for macromaterial processing and precise material processing. For emerging laser cleaning applications, there are questions about whether the CW fiber laser or pulsed fiber laser should be used. 

Two types of fiber lasers,

Continuous-Wave (CW) fiber lasers and Pulsed fiber lasers,

occupy the market for material surface cleaning.

Continious Wave Laser

CW-Laser

High Economical Power

Lowest Surface Quality

Pulsed Wave Laser

Q-Switch Laser

Economical Medium Power

Medium Surface Quality

Pulsed Wave Laser

MOPA Laser

High Performance & Precision

Superior Surface quality

Laser light interaction with contamination on the surface to clean

It is important to note that the mechanisms of light-matter interaction are not the same for all laser cleaning applications. Effective cleaning may include a combination of ablation, evaporation, melting, and melt ejection by the creation of vapour pressure. Generally speaking, the efficacy of a cleaning process is dependent on a number of parameters. In the ideal situation, the laser light should be well absorbed in the layer to be cleaned, but below the relevant process thresholds for the substrate material. In this way, the process can be made to be self-limiting such that over-processing will not cause significant damage to the substrate. This can be achieved by optimising the laser wavelength or by adjusting beam parameters such as the pulse energy, pulse duration (peak power), and beam profile (energy and peak power density). Key performance indicators for laser cleaning often include surface quality, edge quality, and process speed.

For laser cleaning applications, the question is whether the CW fiber laser or a pulsed fiber laser should be used.

When to use a CW fiber laser or a pulsed fiber laser .

Here, we compare the laser cleaning applications of CW fiber lasers and pulsed fiber lasers, and analyze their respective characteristics and applicable application scenarios. 

We use three models of lasers to clean different kinds of materials: one material to clean is aluminum alloy with white paint (around 20 µm) and another another test material is carbon steel with white paint (around 40 µm). The cleaning performance is achieved by adjusting the pulse width, the pulse repeat frequency, and the scanning speed.

With a CW fiber laser, the slower the scanning speed is, the greater the damage to the substrate. However, when the speed is higher than the threshold, a faster speed will cause insufficient cleaning. Thus when using the CW fiber laser to perform laser cleaning, it’s critical to choose the right scanning speed.

For pulsed fiber lasers, a laser with lower frequency is more likely to hurt the substrate during the cleaning process, and a laser with a narrow pulse width (around 100 ns or smaller) could clean the paint more easily. It’s essential to balance the heat between cleaning the paint and melting the substrate (heat effect). A pulsed fiber laser with a master oscillator power amplifier (MOPA) structure offers the advantage of precise heat control, which is a critical point during the cleaning process. A Q-Switch laser source used for the better pricing, can never reach the precision in heat control like a MOPA laser source, with the result of more surface damage!

The cleaning performance of Mopa is far better than a Q-Switch laser and uncompareable with a CW laser source.  The materials are darker after cleaning by a CW fiber laser when compared with a Q-Switch or MOPA pulsed fiber laser. Improper heating by the CW-laser will result in the substrate metal melting during the cleaning process, which is not acceptable especially in the module cleaning industry. The metal on the substrate surface melts during the CW fiber laser cleaning process, even though the paint is removed. However, when cleaning by the Q-switch pulsed fiber laser, the damage to surface of the substrate is small and the surface has minimal damage. When cleaning with a MOPA  fiber laser the damage tot the surface of the substrate is almost non-exisitng and the surface is smoother and not damaged at all.

The damage brought by MOPA fiber laser cleaning is very small, and the roughness value is close or even lower (better) than the original surface (the laser cleaned some dust on the original surface, too). While cleaned by CW lasers, the roughness value will be 1.5X more than the original surface.

 

Another advantage brought by the MOPA fiber laser is the very high cleaning efficiency this besides the perfect finished surface..

  •  When cleaning the dust and dirt on the aluminum alloy, the cleaning efficiency of the MOPA pulsed fiber laser is 2.77 m2/h, which is 7.7X the cleaning efficiency by a CW fiber laser (0.36 m2/h).
  • When cleaning the dust on the carbon steel, the cleaning efficiency of the MOPA pulsed fiber laser is 1.06 m2/h, which is 3.5X the cleaning efficiency of the CW fiber laser (0.3 m2/h).

In conclusion, dust could be removed by both the MOPA pulsed fiber laser and CW fiber laser. Using the same average output power, the cleaning efficiency of the MOPA pulsed fiber laser is quicker than the efficiency of the CW fiber laser. In the meantime, precise heat control between cleaning and melting produces good cleaning performance, without damaging the substrate.

However, the cost of a CW fiber laser is lower, which compensates for the disadvantage of the cleaning efficiency by increasing the average output power. However, it will cause a heat effect, which will hurt the substrate.

Thus, different cleaning applications will require different laser models. For precise cleaning such as mold cleaning, it is better to choose the MOPA pulsed fiber laser. For some large steel structures, pipes, etc., due to their large volume, fast heat dissipation, and low requirement for substrate damage, the CW fiber laser will be a good choice. 

Pulse Tune Technology (MOPA technology) or chirped pulse amplification Technology

MOPA laser sources or chirped pulse amplification

MOPA principle laser source PulseTune technology provides the ability to select waveforms, offering multiple pulse widths from 2ns up to 500ns. Each pulse waveform is optimised for maximumpeak power and pulse energy at its preferred operationalfrequency. This gives users of the MOPA series greater control of pulse conditions over the full pulse repetition range.

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