Hello
I see different types of lenses available on !Aliexpress:
Maybe some other types as well.
I’m thinking of changing my lens (18mm width, 50.8mm focus length). What kind of lens would you advise? Any good product on Aliexpress?
Thanks again for your help.
CVD (chemical vapor deposition) ZnSe is a bit more expensive than PVD (physical vapor deposition) ZnSe, but its transmittance is a bit higher (something like 99.8% vs 99.5%), which is why it doesn’t get as hot and why it can handle more power. A reasonable maximum for PVD is around 80 W.
Apart from ZnSe (zinc selenide), you can also use GaAs (gallium arsenide) and Ge (germanium) lenses with a CO2 laser, but those two are much less common.
With an 80+ W laser and a suitable head, a 2.5" GaAs lens can be a good addition for cutting somewhat thicker sheet materials. Compared to ZnSe, its transmittance is a bit lower, but there is less spherical aberration. With enough power, you can overcome that handicap and reap the benefits. There is one caveat: It doesn’t work with beam combiners. It’s opaque to visible light.
Ge only works with low-power engraving machines, but their transmittance is lower than ZnSe. I don’t really see why anyone would ever use them. You can get a good 12 mm ZnSe lens for a K40 for just $10.
There are two types of geometry: planoconvex (flat bottom) and meniscus (concave bottom). Meniscus lenses produce a slightly crisper spot, but they somehow have slightly worse cutting performance. The amount of detail you can reproduce with a planoconvex lens is high enough - even for anodized aluminium. While a bit more cutting power is always welcome. So, this trade-off isn’t really that appealing for typical use.