I am going to replace mirrors in my K40.

Lasers
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1

I am going to replace mirrors in my K40. I found the following mirrors available:
20X3mm Mo Molybdenum Reflection Mirror For CO2 Laser Cutter Engraver: http://www.banggood.com/20X3mm-Mo-Molybdenum-Reflection-Mirror-For-CO2-Laser-Cutter-Engraver-p-966219.html?utm_campaign=android-share&utm_source=androidApp&android_share=1&_branch_match_id=261042582605385078
and
20X3mm Si Silicon Reflection Mirror For CO2 Laser Cutter Engraver: http://www.banggood.com/20X3mm-Si-Silicon-Reflection-Mirror-For-CO2-Laser-Cutter-Engraver-p-966218.html?utm_campaign=android-share&utm_source=androidApp&android_share=1&_branch_match_id=261042582605385078

Which one is better to equip the K40?

2

The MO mirrors are better quality and have a higher reflective index for infrared light. So if you can afford it, go with the MO mirrors. And be carefull of the way they word the ad, 20X3 does not mead 3 20mm mirrors, it means the mirror is 20mm diameterX3mm thick.

3

Thanks @funinthefalls ​, I ordered 3 pcs. and it looks Molybdenum mirrors are less expensive than Silicone.

4

The Si mirrors on Banggood are not real SI plated mirrors. They are SI substrate, gold plated. If you look at lightobjects, the quality is from worse to best GoldPlated (What Bangood is calling SI), SI plated, and MO. So Banggood is trying to pass off a cheap mirror as the more expensive one. May just be a language barrier that made their mistake, generally they are very fair and honest.