Resin pooling on top side of parts adding 0.5 mm to height

This was my first Form3 print and I am very pleased overall with the results, however, the problem I am having is that resin pools on the top of parts where supports attach, and to the top side of horizontally printed holes. It seems that this is especially where features are closer to the build platform. The accumulation is 0.52 mm per 1 cm^2 top surface area. The accumulation is not washed within the recommended 10 minute new 99% IPA bath in the FormWash and then is cured in the FormCure fixing the dimensions.

I suspect that the reason is that there is insufficient IPA circulation around the supports but was nervous about IPA absorption so I didn’t try a longer bath. I plan to try the following:

  • Tilt and gently shake print surface after printing before washing
  • Space print pieces further apart on the build platform (there were 19 about 6 mm apart)
  • Fill IPA to the top line rather than between the lines
  • Double the formWash time

Has anyone who cares about the vertical dimension solved this issue?
Are there other suggestions that could help with this?
How long can I wash before IPA absorption becomes a problem?

I am currently working on a robotics project using Gray, FormLabs FLGPGR04. The print resolution was set to 25 um because geometries and clearances are tight.

Otherwise, I was very impressed with the print quality. I print process monitors along with the parts to validate FormLabs design rules and verify density. The FormLabs vertical wire, supported wall, and hole diameter design rules were not achieved. However, the density monitor (a 1 cc cube) measured 10.00 mm x 10.00 mm x 10.52 mm height (due to pooling). I was not expecting the horizontal dimensions to be so precise with a 25 um resolution and 85 um laser spot size. Also, different parts mate exactly with no seams evident. On filing the 1 cc cube to exact vertical dimensions I measured a density of 1.16 gm/cc (within the FormLabs published range - 1.15 to 1.20 gm/cc).

Generally, 25um is not the best choice if that is the criteria. Stick with 50um.

I’ve only printed < 20 times on my Form 3 (learning how resin printing works as well), and I also see this on every single surface that is parallel to the build platform, facing the build platform.

I suspect it is because, for more viscious resins anyway, resin getting stuck on the horizontal platforms, and getting cured by stray laser light during printing, and not during post processing.

The only solution I’ve seen so far is to avoid having horizontal surfaces altogether during print (allowing resin to flow back into the tank), or creating drain holes for the same purpose.

I’m not sure if optics come into play here as well to reduce the stray light…

Thanks for the ideas. I will try the 50 um print fantasy2 suggested.

Although I wondered about the transmittance of light by the resin up beyond the active print plane, I didn’t think of it when I wrote my message. So thanks for the idea, kelvie. If indeed resin pooling is the problem and it is being cured in the print process, It may be that by using a 50 um vertical step size, the transmittance will be halved. But then again, since I don’t know the details of how the Form3 is designed maybe the laser power is doubled to cure a 50 um layer thickness and there will be no improvement. I have noticed that even parts at a surface angle of about 10° from vertical still have this issue so the angle could be important. To address that question, I will make a process monitor with various top-surface angles.

This is not caused by ‘pooling’, ie uncured resin trapped on the platform side of the build. Rather it is a side effect of the SLA mechanism.

If the model has a surface parallel to the build platform, the first full layer to be printed, even with a 0.1mm layer height, will be a fragile and a relatively soft film. While it is likely to survive the peel forces, when pushed down into the resin for the second layer the film will bow towards the platform and trap a layer of resin that is thicker between the supports than the nominal layer thickness. This will be repeated with subsequent layers until the model is thick enough and stiff enough to counteract the pressure of being pushed into the resin. The result is a surface where the supports are at low points and the surface seems to bulge between them.

This is exactly why Formlabs recommend that models are built with an angle to the build platform in both the x & y directions.


@billb here is exactly right, but jumping in to just reiterate that this is correct! Also I would even recommend using 100 microns rather than 50. Your print will go faster, and you really shouldn’t notice any significant issues with details(unless you have some pretty specific characteristics of your model). :slight_smile:

I’ll also mention that the more pictures you can post(that you’re comfortable posting) the easier it is for everyone here to provide useful feedback!

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Thank you @billb. Your comment rings true. My first impression of the motion of the platform down into the resin was that it was pretty violent for the viscosity of the resin. I am doing a print at 50 um with a process monitor with various angles relative to the build platform and different heights. We will see how that comes out.

Of course all layers are parallel to the build platform when they print - even if the model doesn’t have a parallel surface. So they would all be subject to the minimum design rule overhang which is 5 mm and if the layer height is 25 um, the minimum print angle with respect to the platform without support would be 0.3° with 5 mm support spacing (according to the design rules). There is a 10° minimum unsupported angle mentioned in the design rules but all surfaces I printed were supported so I didn’t worry about them and PreForm didn’t raise any warnings. I would suggest that FormLabs should update the design rules to reflect these bowing constraints. I have noticed weakness in thin films in my design rule validation test articles.

I am interested in the bending properties of the materials immediately after laser curing during print if anyone has that information for various resins, especially the Grey resin I am using. That would help to develop process constraints beyond FormLabs design rules while waiting for design rule updates.

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There is a knob in the software to adjust Z-Compression. You can read up on it here. I resolved my similar issue by including a raft instead of printing directly off the platform, and adjusting the orientation of the part.