I noticed that the side of my prints that faces the build platform comes out slightly thicker, as if “swollen”. See the picture, this is the same model printed in two different orientations. On the left, a print whose side facing the camera was facing the build platform. On the right, a print whose right-side was facing the build platform.
Another observation: after removing from the printer, the print appears to be covered with a layer of resin which is quite dense, it’s hard to remove even with IPA from the spray bottle unless I spray really hard. I usually spray IPA to remove what I can, then I do a manual wash in IPA, followed by curing in FormCure.
What is the reason for this effect and can I avoid it? Could be that the resin is too old? It’s been sitting in the printer and only rarely used for printing for like 9 months. Would printing at an angle to the build platform help?
For the part on the left, it looks like the affected surface was oriented parallel to the build platform on supports. This effect is due to increased peel forces exerted on the part as well as deformation that happens when peeling a large surface with relatively small contact points. As such, printing at an angle would help; in my experience 20-30 degrees from horizontal is ideal. I would recommend looking over Model orientation best practices for SLA printing. Aside from that, the layer of resin you mentioned could potentially be due to material settling if the resin was not thoroughly shaken up prior to printing.
