I’ve been doing a lot of work on a diy flare solution following on from these theads : Form1+ laser flare issues illustrated - pics and video Laser Flare & the effects on your print and the original thread when I became aware of the flare issue: Is the Y galvo mirror supposed to be transparent?
First up - I’ve finally had some success, and I think now I have a good understanding of what’s going on - this post is going to be long, and even then I won’t be covering everything I’ve learned over the last month or two, but to start with here’s two comparison pics of the tall cross print test - showing the flare side (rear) with solution in place, and without…
flare solution in place - rear “flare” side shown - notice lack of flaking
bare laser, flare solution removed - same vat, same resin, only change is to slightly move position of parts to avoid clouded pdms from previous print, and filtered resin.
As I covered here Form1+ laser flare issues illustrated - pics and video and here Form1+ laser flare issues illustrated - pics and video the laser beam is not collimated - instead it has a focal length of approximately 30cm so that it converges to the small spot as it hits the build surface. See this picture from the above link showing the laser beam profile as it changes over distance:
Now - initially I thought this meant we had to discard the simple choke/aperture/iris approach - since the flare “grows” out from the spot over distance, so it wouldn’t be as simple as cutting it off at the laser barrel opening.
However - after a LOT of experimentation - I’ve come to the conclusion that extending the beam path with a mirror and refocusing lenses is not a viable solution. I’ll go over this much more complex approach and why it failed in a follow up post - it’s a long story, so I won’t bore those only interested in the solution in this post.
I’m now quite confident that the flare issue is caused the final cylindrical lens being off-centre in the laser - I hope this diagram illustrates the problem effectively. It is crude and much simplified, but hopefully catches the main point.
In fact - having looked inside my laser again after all this experimentation I believe it’s actually clear that the lens is off-center in pics I shared earlier:
and now annotated to pick out the edges of the cylindrical lens
So - a solution? well I think in honour of @JoshK’s pioneering effort I’ll call it a choke, because it’s not an aperture or an iris, and it operates like an old car choke (albeit in reverse, push, instead of pull).
and here it is - un-installed as I’m running another test print at the moment. I’ll post again about porosity later once I can take the laser out and take some pictures of the magnified spot profile
The silicone ring slides into the laser barrel opening, and the choke slides between it and the barrel threads. It’s adjusted while running the laser spot test once the laser is installed back in the F1, by pushing the choke inwards with a fingernail until the flare is visibly reduced. I’ll post a video of this installation process once I’m done with the current tests without the choke, so it’ll probably be tomorrow. Here’s an STL file of the mold for the silicone ring: https://drive.google.com/file/d/0Bxi9QnsGaF7EWWRFNFdnNVZleGM/view?usp=sharing
edit - here’s the youtube install link from post 24 below https://www.youtube.com/watch?v=z7AiD0EpBzY
and here’s a before after comparison of the laser spots