I have printed some tensile test bars following the ASTM D638 type IV dimensions. The bars come out of the printer just fine. I have printed them vertically. After IPA wash and UV cure the parts start to deform, forming a curve. The resin is Grey V4, it hasn’t expired yet. The curve seems to have a larger curvature when UV cure time increases. But even 5min per side is enough to alter the geometry. Have you ever come to an issue like this?
Can you share a screenshot of your Preform file? Are you curing with supports on?
I bet they are curving/warping during curing.
This is a problem that I encounter often when using heat during the UV cure, especially on thin, long models.
If you’re letting your part dry properly after IPA washes, there isn’t really a solution to this other than curing without heat, which may alter mechanical properties slightly.
@hoolito I’m printing them directly attached on the platform (with the cross section aligned parallel), so I don’t use supports.
@leonhart88 Yes, you are correct. After printing the bars are straight, even after 2-3 days. So these are heat - related stresses that cause the part to bend. That’s interesting. I always thought that heat would help to release residual stresses, but this is quite the opposite. It seems that shrinkage is the parameter to blame. Maybe if the part was cured using UV from both sides would minimize the issue. Thanks for your help.
Then that’s the reason probably. The supports are the scaffolding needed to hold shape during curing… put parts inside the cure when it’s at room temp en let them warm up gradually with the Cure
I’ve actually tried several things with UV curing, including different orientations, with supports on/off and gradual heating during cure.
Different orientations do help, as the internal stressed will be in different directions but it’s hard to know for different geometries.
Gradual heating, or lower heating (Form Cure only goes down to 40C) doesn’t really help either. SLA resins have such low HDTs that my hypothesis is that the internal stresses are enough to cause warping.
Emperically I’ve found that turning off heat but increasing UV cure time by 2X or 3X seems adequate for my uses. Mechanical properties might not be exactly the same as what’s in the datasheet though and I don’t think anyone has done a significant test/study on this (although there might be papers on this and I know that heat will accelerate and cure more of the model, so it’s logical that properties may take a hit).
For all intensive qualitative purposes, what I’m doing has been working fine for me.
I suspect that even if you help this up vertically and had more even UV, it would still warp. Something you could try?
Then if it’s really important, may be annealing might help relieve internal stresses prior to curing?
Maybe - I never got around to doing any further testing. All I know for certain is that heat, even at 40C can still cause warping. I suspect this may happen even without UV.
In the past, I’ve heated parts up in a fixture in order to straighten them out and reduce warp. Parts definitely get soft quickly because of the low HDTs that photo polymers tend to exhibit. Once they cool in the fixture, they straighten out permanently (or for as long as is reasonably “permanent” for SLA parts out in the wild).
I think I can give it a try and check the results. I’ll post my findings.
It seems that the specific geometry type is one of the limitations of SLA. I’m currently using a method similar to the one you mentioned with the fixture with good results. Another solution could be the use of thin fins in order to increase the cross section rigidity and remove them after cure. The applicability depends on the part design and it adds another post processing step which is not easy to accomplish.
Are you post-curing these in a Form Cure, or using another cure method?
I’ve been handling hundreds (literally) of printed ASTM D638 Type IV bars across a large number of resins in the past few months, including many of Grey V4, and we have not had a significant warping issue like your photo depicts. We cure them flat on the Form Cure turntable for half the recommended cure time, flip them over, and then cure them for the rest of the recommended time.
(Also, it’s a little hard to tell from the photo but that looks like an ASTM D638 Type I bar to me, is the cross-section of the gauge length 12.7x3.2mm or 6x3.2mm?)
Yeah model geometry can have vastly different outcomes.
You should try curing without heat for 2-3X longer and see if you get any warp. From my experiments, warp should be eliminated.
I’d love it if someone actually quantified the change in mechanical properties in doing so though.
More often than not, part tolerance and accuracy is the most important requirement so it’s a tradeoff I have to live with unfortunately (unless I make a fixture like described before, but that’s a pain)
I had a similar issue printing some sprue test connectors for a prototype injection mold that we are going to run. I didn’t have a good cure method in place, and therefor was holding a 405 nm light about 5 cm above the print for 20 minutes, then turned the print. They came out like your print above.
I then “built” my cure station proper, and did another print. The major difference was having the lights further away, a temp controller environment, and I left the prints on the support structures. They came out flawless.
When I print anything long and flat, I typically will leave the print on the support structure for my double wash and cure. No problems since. Wish I took some pictures now …
It really depends on geometry and resin though. I’ve left support structures on and still encountered warp for thin walled injection molded prototypes, and even in some thicker parts and wasn’t able to eliminate it unless I turned the heater off.
Nice catch! They are not ASTM but ISO 527-2 test coupons.
I use an old UV curing station for nails that I had bought since I got form1+, so I do not use form cure.
Do you print them horizontally on the platform or vertically?
Maybe inside iced water? I will try it out.
Maybe the use of supports could minimize the deformation since I have printed them vertically, without supports.
Whoops, sorry for the delay. I print them completely vertically, with no supports. Careful tapping with a sharp scraper removes them from the build platform and then I cure them flat (horizontally) for equal amounts of time on each side, in a Form Cure unit.