I have a Mars-like 3D printer, which so far worked very well with typical resins. Then I decided to print a mechanical part with Siraya Blu. I followed their recomendations of a "heavy" support, which means columns of a diameter of 1.2 mm minimum. The setting are 14 s (60 s) exposure, 60 mm/min (40 mm/min) lift/retract speeds, in parentheses the bottom layers. Temperature 25 °C, the printer is leveled.
The object would have an even, well-defined resolution everywhere if there weren't some large "empty patches" directly below the support platform and of the height of about 1 mm, that damage the support structure, which in turn warps the printed object. The photo shows the problem. Everything prints very well apart of the inside of the blue patch, where the flat part of the platform is perfectly empty, no columns, no platform fringe, the flat part itself prints as it should (and sticks well to the build plate). The stray fragment of a column is a left-over after cutting off the platform with a knife.
The patch has crisp borders - outside, the columns are well printed and precisely round, inside the patch there is just the flat support part, followed by the object warped because of the momentary lack of support (the support shows again a millimeter or so later as round islands, the black arrow points to one). The object's layers gradually self-correct and print with the same high quality everywhere, inside the blue patch or not. So I assume the FEP film is okay, as the empty patch has only about 1 mm height and above/below it the precision is very good, as seen within the cyan ellipse. Also, other resins (all much more fluid, Blu is very viscous for the extra toughness) print with no problems.
There are 2 or three similar empty patches in this print elsewhere, but this is the largest one. Any idea what can it be? Trapped air bubbles? I did several tests with different settings, the problem persists. I increased the exposure time to 18 s and cannot increase it even more as this resin has a tendency of a "remote polymerisation" of overhangs. Changing lift/retract speeds does not help or help minimally, but I didn't go below 40 mm/min. Siraya Tech's recommendations are verbose but say nothing about the retract speed anyway. If these are bubbles, why the flat part of the support platform prints everywhere? To see if it has something to do with the bottom layers I tried different numbers of them, but the problem always starts exactly below the flat part, and not few layers sooner or later.