How to Clean Laser Mirrors and Focal Lenses

OMTech Laser
·
24.August.2023

Have you ever tried to watch TV through smeared or dirty glasses? If you have, you will know how the image is distorted and how cleaning them returns it to a crystal clear image. You will also know that using the wrong cleaning materials can hurt or even destroy the glasses!  

Why should the CO2 Laser Engraver's Laser Mirrors and Focusing Lenses be Cleaned?

Your laser cutter uses mirrors and lenses to reflect and focus the laser beam onto the workpiece and is just as susceptible to distortion and degradation as the TV picture was — but unlike the TV picture, the laser beam is hot and powerful. Once you set up your laser engraver and start to laser cut various materials, the laser beam vaporises your workpiece, emitting airborne solids, smoke, and other gases into the air. These particles will land on the optics, and the next time you fire your laser, these contaminants can get burned onto the surface and damage them permanently.

As the laser beam strikes a mirror, if the mirror is perfectly clean then virtually all of the beam energy is reflected along a single path. However, if debris is present, it can cause the beam to fragment, weakening the beam. Not only will the weakened beam affect the quality of your cuts and engraves, but the mirror will heat up and absorb infrared energy from the beam. This can allow the dirt to embed itself onto the mirror or lens surface and eventually cause it to crack.

So you can see — keeping the mirrors and lenses perfectly clean is essential to maintain the performance of the laser cutter and extend the lifetime of your laser mirrors and laser focus lens.

What Causes Dirty Laser Mirrors and Focal Lenses?  

When cutting wood or acrylic, the material under the laser beam is vaporised. This then condenses as smoke and ideally is extracted from the laser through the ventilation fan. However, even with the best ventilation, smoke will work its way around the laser machine, and over time, the particles will adhere to all the surfaces, including the mirrors and lens.  

When engraving, part of the material is being vaporised, but also a part of it is being burnt or charred and this creates a lot more smoke and exacerbates the problem.  

When the laser is operating, the air is supplied to the nozzle and should create positive pressure inside it, so that air is forced out of the nozzle alongside the laser beam. As the material is cut or engraved, this airflow should prevent debris from the kerf (the area being cut) from entering back into the nozzle. However this again is not 100% perfect, and smoke or debris can be drawn back into the nozzle over time, contaminating the lower face of the lens.  

Something else to be aware of is that when aligning the mirrors and head, the use of tape across the mirrors or across the laser head can lead to contamination of the mirrors, or smoke residue on the upper surface of the lens.

Regular cleaning of the mirrors and lenses is essential. It's also important to keep your workspace clean and eliminate extra dust around the room. We perform a simple clean-in-place procedure at the end of each job and a deeper clean each week.

Laser Engraver Maintenance: Cleaning Laser Mirrors

What Should I Use to Clean my CO2 Laser Mirrors?

Clean your CO2 laser mirrors with single-use Lens Cleaning Paper Tissue soaked in isopropyl or denatured alcohol of 90% or higher. You can also use a lint-free microfiber cloth, but be sure it is very clean. Do not be tempted to use cheaper cleaning solutions as they can leave residues behind. Never use cleaners that contain ammonia on your laser mirrors or laser focus lenses.

How to Clean Laser Mirrors

At the end of each job, with the laser switched OFF, perform a wipe of each of the 3 mirrors using wet lens cleaning wipes. These are readily available for cleaning reading glasses and come in convenient, individual packets.

1. Ensure that you can easily reach each mirror by opening the access doors and then  open the wipe.

2. Fold the wipe in half and gently wipe Mirror 1 (the mirror at the end of the laser tube).

3. Fold the wipe in half again and wipe Mirror 2 clean. The mirror surfaces should dry in a few seconds and you can use a flashlight to check that they are clean. You may observe a yellow staining on the wipe — that is the smoke residue from cutting and engraving.

4. Cleaning mirror 3 is a little tricky since it is mounted inside the laser head. Take the folded wipe and wrap it around a Q-tip. Insert it into the head to clean mirror 3. Generally, we find that Mirror 3 is far less contaminated than the other mirrors, likely due to it being more enclosed.

5. Do not operate the laser for at least 15 minutes after cleaning to allow the moisture to completely evaporate.

Cleaning Mirrors - Weekly Maintenance 

At the end of each week, along with the cleaning and lubrication of the linear bearings and  axis slides, we perform a deep clean of the mirrors and lens.  

To perform this, ensure that you have plenty of access to the mirrors through the access doors and again, ensure that the laser is switched OFF when performing the clean. We also recommend wearing clean cotton or latex gloves to keep fingerprints off as well as to  protect your skin. 

You will need a mirror removal tool, or if you are careful, a pair of circlip pliers can be used.

The mirror is held in place by a threaded ring that can be unscrewed by inserting the tool or the plier tips into the two holes on the back of the mirror mount and turning counterclockwise. Be careful not to apply too much pressure as you do not want to upset your mirror alignment which is critical to the performance of the laser cutter.

Once you have unscrewed the retaining ring, take a piece of masking tape and gently place it on the back of the mirror. This will allow you to easily remove the mirror from its holder. 

All 3 mirrors can be removed this way ready for deep cleaning.

Instead of using the wipes, we clean the mirrors with a purpose-made lens cloth soaked in 100% isopropyl alcohol. Do not be tempted to use cheaper cleaning solutions as they can leave residues behind. Never use cleaners that contain ammonia on your laser mirrors or laser focus lenses.

Wipe each mirror quite firmly to ensure that all contamination is removed and then examine the surface carefully. Any pitting or damage means that the mirror should be replaced. Mirrors cannot be repaired and should be considered a consumable item, so keep a spare set or two in stock.  

Once all 3 mirrors have been cleaned and allowed to dry, they can be replaced into their holders and the retaining rings tightened. DO NOT OVERTIGHTEN - they only need to be tight enough to stop the mirror from moving — over-tightening can cause the mirror to break or distort.  

The mirrors can be replaced in any order as all 3 are identical.  

How to Clean the Laser Focus Lens

We clean the lenses during the weekly maintenance or if we notice any degradation of the  cutting performance of the laser. This may occur after a long engraving job or if you forget to switch on the air assist.

There are several types of laser heads available, some have the lens located in the nozzle,  and others have the lens located in a lens tube that is held in the head by a single screw. It  should be obvious which you have by first removing the nozzle. Loosening the screw  allows the lens tube to be removed. However, you may need to lower the bed of the Laser  cutter to allow it to be removed. If your lens is inside the nozzle then simply removing it  from the head allows you to access the retaining ring.

Once the lens tube has been removed, the lens itself is held in place with a similar  threaded retaining ring as the mirrors, except it is deeper inside the tube. You will need  a removal tool, or again, long circlip pliers may let you undo the ring. Under the ring, you  should find a silicon O ring and then, the lens.  

Invert the lens tube and remove the O ring and lens — be careful when handling the lens as it has special coatings that can easily scratch.  

Clean it in the same way as the mirrors, using the lens cloth and pure isopropyl alcohol,  and ensure that it is perfectly clean and not scratched. Damaged lenses should not be  used but discarded and replaced. As with the mirrors, the laser lens is a consumable item; always keep a spare of the same focal length and diameter.  

When replacing the lens, carefully drop it into the tube, ensuring that the ‘convex’ side of  the lens goes in first, followed by the O ring and finally the threaded ring. Tighten the  threaded ring down to ‘finger tight’ only — it is there to just ensure that the lens does not move.  

When the lens tube is reinserted into the head, the convex side will be facing upwards  when reassembled correctly. Remember ‘belly up’ to help!

How to Clean the Laser Tube Lens?

There is one more lens that will need your attention, although this is likely to only needed  every few months or if you have laser performance issues that cleaning of the mirrors and  focus lens does not resolve. 

At the firing end of the laser tube, a small lens is installed. It is quite deep inside the tube  and can only be inspected with a mirror. As it is deep inside the tube, it rarely becomes  contaminated, but built-up smoke in the laser tube chamber, for example if the  ventilation fan fails, can cause contamination and so we like to clean this every few  months just as a precaution.

Cleaning your Laser Tube Lens: Step-by-Strp Instructions

1.Ensure your laser machine is switched OFF.

2. Bend your Q-tip to a 90-degree angle. This will help you reach the lens inside the laser tube.

3. Dip one end of the Q-tip into the 100% isopropyl alcohol.

4. Gently slide the wet end of the Q-tip into the laser tube and wipe the lens a few times in a circular motion.

5. DO NOT be tempted to use Acetone as the end of some laser tubes is glued on during manufacture and acetone can damage the glue. DO NOT operate the laser for at least 15 minutes, as the lens must be completely dry.

Conclusion

Maintaining your laser cutter machine at optimum performance requires several systems to work together. The laser beam needs to be generated correctly in the tube before it bounces precisely across three perfectly clean laser mirrors and then through a perfectly clean laser focusing lens. If any aspect of the optics system is imperfect, the laser will not cut at 100% efficiency. Mirror and lens cleanliness not only affects the beam quality, but a build up of contamination can cause rapid failure of the component, resulting in the expense and inconvenience of replacing parts.

Always perform the quick cleaning procedure with pre-moistened lens wipes at least once every day you use your laser. In addition, a deep clean and inspection should be part of your regular maintenance schedule. These preventive maintenance steps will extend the lifetime of your laser mirrors and focus lens, and ensure your laser is always ready to perform at optimum efficiency.

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