hi everyone, I’ve been working on a compliant mechanism design and I was wondering if there is a specific resin from the Formlabs (probably from the Engineering line of resins) that you would suggest/have tested with compliant parts. From the specs I’ve been reading on the website, I’m between Grey Pro, Tough 1500, Tough 2000 or Durable, probably leaning towards Tough 1500. The stress the mechanism is supposed to get is mostly small-angle bending, normal temperatures, repeated use. Something in the logic of mechanisms developed at BYU.
Would love to hear any experience you may have with this! I would also be really happy if @Formlabs support staff can jump into this thread too! 
I suggest trying tough 1500 first and then durable if 1500 isn’t flexible enough.
Tough 2000 and grey pro are probably too stiff, but depends on your geometry and compliance required.
The easiest way to think about it is Durable simulates HDPE, tough 1500 simulates PP and tough 2000 simulates abs.
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For that I would use Draft. It is strong but kind of flexible.
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I definitely would not use draft. Draft has elongation and stiffness similar to standard resins.
The best thing to do is look at the datasheet for mechanical properties. If you want compliant and flexible parts, you need parts that are less stiff and have higher elongation.
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Well, I have had great results printing BYU mechanisms using PLA (not formlabs of course).
IMHO Draft is similar to PLA. I would definitely use first draft and see what happens. But hey, this is just an opinion and could be wrong. At the end, there is only one way to know for sure. 
It really depends on your design - any plastic can be “compliant” if your wall thickness is thin enough. PLA is quite stiff in comparison to ABS. People in the 3D printing world often misunderstand what the terms “strong”, “tough” and “flexible” mean from an engineering perspective.
All I’m saying is if you look at the mechanical properties, that will tell you a lot more information than anecdotal experience and according to the mech properties, Draft is stiffer and has less elongation than Tough 1500.
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@artexmg, @leonhart88 thank you both for your feedback! I’ll go with Tough 1500, as it seems more appropriate, plus, I don’t have any Draft resin sitting around at the moment. 
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You would honestly probably be better off with a fused deposition print - ABS works really well for compliant mechanisms and the carbon filled nylons work more like aluminum. I would expect your part to work OK at first and then break down pretty fast. One thing to try would be putting a cheap nylon bushing into an oversized pivot hole before the final cure - the part will shrink down around the nylon and give you a more durable connection if it doesn’t crack during the cure.
FDM is only reliable for compliant mechanisms if the direction of bend is along the layer lines and not against since FDM is anisotropic. It can work, and I’ve done many in FDM, but you need to be aware of these things.
Generally speaking, I would rather use Tough 1500 for a nice compliant mechanism than FDM purely because of the anisotropic nature
My two cents are to agree that Tough 1500 is probably a good place to start for printed compliant mechanisms, but that it wouldn’t be surprising to wind up finding that another resin works a little better, depending on whether the desired mechanism would work better with more stiffness, more elongation, or what.
I’ve printed compliant parts (if not full mechanisms) in almost every Formlabs resin. Some compliant mechanisms, including textbook examples like a cross-strap hinge flexure, lend themselves more to printing than others.
thank you everyone for your 2 cents! how about curing time? have you experimented with shortening the curing time so that the parts come out a bit more flexible?