UV Curing Warping

I’ve read through the FormLabs whitepaper “How Mechanical Properties of Stereolithography 3D Prints are Affected by UV Curing.” It has really great information on UV light, time, and temperature needed to maximize the mechanical properties of the resins. However, one thing I found lacking was a quantification of the warp seen in parts.

Some of the parts we are printing suffer warping after UV curing them. Our UV cure box has a turntable to rotate parts and reflective surfaces to reflect the UV light. We have monitored the temperature inside the UV cure oven and it has not exceeded the recommended temperatures. Does FormLabs or anyone else have quantifiable data on how time, UV light exposure, and heat affect the warping in parts (say a sample plaque)?

How are you monitoring the temperatures inside of your box?

I just began testing a UV curing oven I’ve been building over the past few weeks and my first two prints are definitely warped a bit. The first one more so than the other, as it was a taller part and was closer to the heating elements. The second part was a flat platform intended to replace the clear platform that came with a solar powered turntable, and while the warping wasn’t as distinguished, there is one part of the platform that came out quite warped after the curing.

The reason I ask how you’re monitoring the temperatures is that when I have the chamber at 60 C the part itself is actually around 70-75 after 60 minutes. The White Paper document states “Post-cure temperature should be limited by the HDTs
of the resins, which means 60 ºC for Formlabs Standard Resins.” where HDT is “The heat deflection temperature or heat distortion temperature (HDT, HDTUL, or DTUL) is the temperature at which a polymer or plastic sample deforms under a specified load.” - Wikipedia

I could be wrong in assuming that we are experiencing similar issues, but I am curious if you have tested what temperature the part itself is after the curing cycle.

Try curing with the supports still in place. That helps.

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I am monitoring the temperature inside the UV curing oven with a thermocouple placed on the part. I do not believe that the temperature is causing the warping as the HDT of the high temp resin is 60 ºC and the temperatures we are reaching are about 38 ºC as we do not have a heating element in the UV curing box. In fact, I was wondering whether increasing the temperature would reduce warp by helping the material relax or whether this would make problems worse.

We have cured the parts with and without the support material still in place. While the supports help to seem to reduce the warp slightly, the amount of warp we are seeing on the parts is still unacceptable for our use.

Curing will complete any un-initialized photo-polymerization reactions which can cause warping, but it’s important to note that leaving parts out in the sun to cure would likely yield a similar result. So long as your cure chamber isn’t going beyond the Heat Deflection Temperature of the Resin, curing parts in a UV chamber won’t yield significantly different results in regards to warping as compared to passively curing parts in direct sunlight.

Warping is most impacted by printing orientation and thickness of features. Orienting parts at an angle will help to reduce internal forces. If possible, thickening long and thin geometry will also help to cut down on warping. Once all of the polymerization reactions in a part have been completed, it won’t warp further.

Our temperatures are well below the HDT of the high temp resin we are trying to cure. Will increasing the temperature during UV curing decrease the warping of the part by allowing the polymers to relax?

The parts we have been trying to cure were printed at an angle and were a fairly thick in cross section. We did not core out the parts. Despite this, we experienced significant warping across the parts.

Post a picture of the part, screenshotted from PreForm showing positioning and supports?

Unfortunately, I am unable to post a screenshot as the parts are proprietary. However, it appears that the warping occurs after UV curing. The parts do not seem to be deformed from the printing process.

Heat aids in the curing process but the photo-polymerization reaction that occurs is somewhat different from a typical polymer annealing process that you’d perform by heating a material near its glass transition temperature. Sticking to the recommended post-curing specs is best and increasing temperature might soften the material and worsen warping. If your designs are proprietary, you might get in touch with our support team so that we can help to troubleshoot further.

My experience with warping has been that orientation of the model on the build plate can matter to post-print cure warpage. If you can’t post pictures due to the proprietary nature of your project, you should definitely open a ticket with FL. They can view it under NDA and give you any necessary feedback.

I had the same experience, if you arrange a flat rectangular part parallel to the build platform, the part will be distorted much more after uv-curing compared to the same part which was arranged diagonal by the software.

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All our parts were printed at an angle to the build platform. The parts were too large to print parallel to the platform even if we were to try. Despite this, we still experienced warping after UV curing. Does the resolution of the print affect the warping during UV curing?

No, the he’s saying the more parallel, the worse the warping, which I agree with.

The fact that you printed at “an” angle doesn’t necessarily mean it’s the best angle. But it’s impossible to say anything specific because we can’t see what you’re printing…

The parts were printed at close to the largest maximum angle possible to the build platform as the parts were large and that was the only way they could fit. Because of this, I do not believe the print orientation was the largest contributing factor to the warping we saw after UV curing. As a general rule, we avoid printing our parts parallel due to best practice.

Could the build resolution have impacted the warping during UV curing? We printed the parts with the 0.05mm layer thickness setting.

What do you mean by maximum angle? For many geometries, a 45 degree angle from parallel is optimal to reduce warping but this is a generalization and highly geometry dependent.

Resolution is going to have little to no impact on warping. Warping forces for a given layer are proportional to the thickness of that layer so 200, 50 micron layers are going to warp similarly compared to 100, 100 micron layers.

Sorry I should have clarified that statement. By maximum angle, I meant the largest angle between the part and the platform which would still allow the part to fit within allowable print space. Since the parts were long, the parts were printed along the longest hypotenuse of the rectangular prism of the allowable build volume.

From what everyone has been saying it does not seem like there are many options to reduce the warping during UV curing. Our parts were printed at an angle to the platform (about 60 deg) and our temperatures during UV curing were below the HDT of the high temp resin. This article suggests that over-curing is better than under-curing so I’m not sure if this is a contributor. Regardless, without any data from FormLabs on variables which affect UV curing warping, any suggestion seems to be experimentation with our resin to solve an issue that is affecting many FormLabs customers.

Will increasing support density can help curing warping?
I tried some experiments with DLP ( same too SLA) machine, and it help me alot. <3 But not complete!

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We can give that a shot next time we run parts.

It probably will if you leave the print on the supports when you cure it. It probably won’t if you strip the supports before cure.