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I have designed and built a top-down SLA 3D printer that uses a 405 nm 500 mW laser along with some galvanometers. I filled the vat with glycerin and then topped it off with about 2.5 cm of resin. I designed a wiper system for creating an even layer of resin quickly even on layers with large surface area. In my tests I have been dunking the build plate into the resin and then raising it to wipe the excess off to get a quick even layer (I raise it slightly above the level of the resin to be safe).

The problem I am having is that after the wiper wipes and the laser cures the layer, after a couple of layers the wiper will begin physically hitting the top of the part. The part continues to get higher and higher than the wiper level, which doesn't make sense because the wiper wipes all the excess off, and I am printing above the level of the resin so no extra resin is flowing onto the layer from the sides. This eventually stalls the wiper motor and the print fails.

I have a theory that the resin's surface tension might be pulling the layer of resin to the edges of the layer, causing the layer of resin to "bulge" up a little which would explain why the layer is curing higher than the wiper level. If this is the case does anyone know of any remedies to this? Do I have to heat the resin to lower its surface tension maybe?

Have any of you guys ever had this issue? Does anyone have any other theories? I am really stumped here and can't wait to start printing some cool models but I cannot continue experimentation until this is solved.

Thanks so much in advance for any help!

(I should mention I am using Anycubic Plant-Based Eco UV Resin in translucent green)

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  • $\begingroup$ Welcome, and before theorizing about resin properties, it's better to rule out mechanical or programmatic error. can you post a setup or photo? Is the wiper a hard or a soft part, like a rubber lip (which can give the machine a safety margin in movement)? Did you check your move commands and firmware, as in, might your Z-motor be ordered to move into the wrong direction or have its movement inverted? $\endgroup$ – Trish Oct 6 '20 at 8:42
  • $\begingroup$ Here is an image of the printer: drive.google.com/file/d/11b63leqxwlFiEx8XvBRE04Hknt8rmKo1/… $\endgroup$ – Porter Morgan Oct 6 '20 at 23:45
  • $\begingroup$ The wiper is a hard wiper. The build plate goes down 4 mm, then up 3.92 mm in order to create a 60 micron layer height. I have watched the printer do this for many layers without the laser on and I can verify that this works mechanically and programmatically. $\endgroup$ – Porter Morgan Oct 6 '20 at 23:48
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Let's look at the problem mathematically:

  • The bed has a 0 that puts the surface some distance $d_0$ above the surface of the resin, the wiper does put the first layer on that height.
  • after curing, the printer moves down distance $d_d$ to coat the top surface.
  • afterwards, the bed lowers back up to $d_0+d_l$ - a thickness of 1 layer lower, or in other terms, it moves $-(d_d-d_l)$

Now, how is it implemented in your G-code? check that!

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  • $\begingroup$ This is precisely the algorithm I used. To be specific, the build plate goes down 4 mm, then up 3.92 mm in order to create a 60 micron layer height. I have watched the printer do this for a bunch of layers without the laser on and I can verify that it does this properly, mechanically and programmatically. $\endgroup$ – Porter Morgan Oct 6 '20 at 23:52
  • $\begingroup$ then, we need a video of dry operation $\endgroup$ – Trish Oct 7 '20 at 7:42
  • $\begingroup$ Wouldn't down 4, then up 3.92 leave an 80 micron layer instead of 60? $\endgroup$ – Davo Nov 5 '20 at 19:41
  • $\begingroup$ @PorterMorgan right, is there a calculation error? $\endgroup$ – Trish Nov 5 '20 at 19:51

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