Vat Flexing PDMS vs FEP


I would ask Mark to weigh in on this. I have been using my Moai 120 machines for about 18 months now full time for production. I have two and they are set up identically with PDMS vats. I have my machines levelled properly and generally have good success with prints. The one challenge I have is when I print full build volumes of small parts. The ones around the outside flex the bottoms of the vats and bow the middle so t middle prints fail between the slow and fast layers, no matter how many slow layers I set. This is better with a heater because the resin is thinner and doesn’t act as a cushion on the peripheral print bases. I understand this is a design issue that presents itself particularly when printing a lot of small parts and isn’t a design flaw so much. I would switch to FEP because the springy nature of PDMS is eliminated. The question I have is- how thick and stiff is the new FEP bottom plate, is it thicker than the PDMS vat bottom, and stiff enough to not have this problem present?



Hi Jeff,

The nature and benefit of FEP is that it flexes and bows to allow the part to peel easily. If the problem you are facing is not having a stiff base, then FEP will not be the solution.

Here is a pretty good article describing how FEP vats work.

The new Form three using a different peel technology which might be something that you could look into.

Perhaps someone in your local area with a FEP setup can do a test print for you.



I’m not interested in Form3. I have two Moai for my production shop and I want to nail down how to convert these two machines so my prints are predictable. I also prefer the way FEP works but I need to know how the acrylic bottom sheet is thicker than the PDMS ones which I think are too thin. Also I don’t like pdms



both pdms vat and fep vat has same thickness at 3mm.

if you are concerned about resin not getting back in time, then use a large pm reset value to 60 or even high to allow resin to flow back. (hence it is part of the recommended fep setting)

The updated advanced acrylic plate for both Moai and 200 further reduces the peel force so that should perform even better

The settings for fep vat initially back in Summer 2018 was not as optimized but with optimization and now advanced plate, many users run it for production and some prefer it over pdms not just for cost but also for clean flat finishes.



The problem isn’t flow. The problem is on large prints the 3mm material flexes down in the middle and fails prints.



we don’t have that happening on Moai for either fep vat or pdms vat. on Moai 200 it was a problem during development and we switched to glasses. I don’t doubt your experience but we actually never get report on this. we had vat that was bad from the get go but not ones that would concave down.



You had it during development of the Moai 200 so you went to glass…I would bet you haven’t tested out a build on a 120 that does what I do. I fill up the build volume every time with small flat pads. This is a worst case scenario for the down pressure from the outer prints and would require an extremely stiff vat bottom to prevent. This photo illustrates the surface that can trap a thin layer of resin and flex it down, failing what is in the middle. It only has to flex it down and keep it down by .001" to fail it.



here is dxf for that piece if you want to cut glass on your own

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FEP installed. It’s so much better than PDMS. Leveling is more critical but I level +/- .001" weekly anyway. I have a file for vat flanges that fixes the slop in the vat mounting and adds thumbscrews.