Hi,
I’m printing mainly thin flat shells on my Form3 and I have seen many times exactly the same effect. I can tell you that it’s not old resin, not bubbles and not low temperature. What I found is a process I call “parasitic curing of residual resin” - it occurs when I do printing of flat surfaces oriented close to horizontal, which are facing to the build plate (the side not facing to the build plate is not affected by this effect). It’s parasitic resin curing - when the resin stays for longer on detail surface and when printing is in progress, the diffused UV light from the laser spot cures partially the residual resin in vicinity. Having supports in the area worsen the effect as supports have capilar effects and slow down the resin flowing out (resin is trapped for longer).
The scenario of the effect is like this:
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During the printing, if the resin can’t flow out from the area and stays longer “unwashed” from new resin from the tank, it becomes “sensibilised” and with the time cured by the parasitic UV light. Once the resin is semi-cured its viscosity decreases and becomes more difficult to flow out from the area and the process accelerates
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Large horizontal areas facing to build plate can form structures similar to “vane” and hold for longer the resin. I noticed that if the area has supports in it, or ends with a row of supports - the effect increases (capilar effects hold for longer the resin trapped in the area and extend the parasitic UV light exposure time).
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You may noticed that the artefacts have orientation which follows the path of rein flow out, areas that prevent flow out get covered with thicker layer of parasitically cured resin (parasitic cured resin layer is thicker behind the supports, in “resin flow out” direction).
Fortunately there is a simple solution for the effect - changing the detail orientation and trying to minimise the supports in the area can completely remove it. For example if you print the same detail and the affected surface has 60-70 deg angle to the build plate (detail is closer to vertical position) you may not see any problems on the surface. Well, as Form3 print time depends significantly on the height of the detail, changing the detail orientation will make the printing much longer (maybe 2 - 3x longer).
It would be good if the software can detect these “vane” conditions just like “caps” and unsupported areas and warn the user. Users are tempted to speed up the printing and they try to orient big flat areas parallel to build plate to decrease the total time, unfortunately this is affecting dramatically the print quality in some places because of the described effect.