High temp resin cracks

I’m having a hard time getting functional parts with the high temp resin.

After printing the parts were cleaned in IPA for 5 minutes, then cured for 2 days. All discolored nicely. I tried UV curing for 2 hours and one day with the same results.

Some parts have been in an oven at 150C not assembled.
Others were assembled in the application after curing and have seen a temperature cycle between -25C and 140C. Looking at the test data from the product, they seem to have cracked between 120C and 140C.

Why do they crack this fast? I’m still far from the 289C spec.

I cant see from your photo, have you another part inside the high temp resin, could that part be expanding and causing the cracking?

The high temp material is relatively brittle (as shown on the material data). With such a small cross section of material you will likely see the part fail regardless of temperature simply because of the material properties.

5 minutes in IPA for a part this thin will weaken the material. IPA is only used to get excess resin off the part before post cure. Use a paint brush to get the excess resin off and you should be able to get away with a minute or less in the IPA.

You also may need to reduce the diameter of the barbed area if the breakage continues. High temp just isn’t going to take the same loads as an injection molded high temp plastic.

It’s important to leave parts to dry for an hour or two after removing them from IPA. Soaking in IPA does swell your part and post-curing before it returns to its original form can weaken it resulting in the cracks you’re seeing. You might try giving your parts a little longer to soak in IPA and you might let them dry off for 2 hours prior to post curing. We also do see a transition to a much more orange color (similar to DentalSG though a bit less dark) after fully post-curing.

Also, it can take quite a while for IPA to completely evaporate off the surface of a model. For however long that takes, the print area that’s still wet is effectively still “in” the IPA. I print parts with internal passages where IPA can remain for a really long time so I’ve taken to post curing under water for everything. I use my UV Curing “Jar”, which I can fill with water because the LEDs are on the outside. IPA dissolves very quickly in water, so this gets all the IPA out of every nook and cranny fairly quickly. It doesn’t help the internal curing much I suspect (though I use warm water). But the surface cure (no longer tacky feeling) completes much, much faster under water, with the absence of Oxygen (which impedes curing).

There is an o-ring inside that is being compressed. But I have seen this with parts that have not been assembled as well.

This is a 14mm diameter part and cross section is at least 1mm. Without putting them in a temperature chamber, they are very strong.

Formlabs recommends a maximum of 10 minutes so I’m still well below that.

The problem is that the cracks are everywhere. Not just on weak pieces, they start at random.

What I’ve done so far is:

  1. 5 minutes of IPA soak -> no cracking
  2. 2 hours to 24 hours of post curing under 3Watt 405nm UV LED at 8cm distance -> no cracking, parts turned yellow-ish
  3. temperature chamber cycle from 23 to -25 to 140 (in this order)

I’ll try a new batch now, while letting them dry longer after a 10 minute soak before post curing. Thanks!

The first suspicion I have is the curing - they should look much more yellow/orange. The thermal cycling is second suspicion, since I’ve seen very similar cracks in parts after subjecting them to high temperature and cooling again. The expansion and the following contraction crack the parts. High temp is not supposed to expand like that when at an elevated temperature though, which is why the curing is first suspicion.

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I’ve made and used exhaust tips with high temp so I can confirm that the material can take high temperatures. I’ve run above the advertised temp for short periods without major issues.

We only saw cracks if there were stress risers on the clamping surface or fit issues that caused the material to bend excessively.

The IPA rinse that Formlabs recommended above is just another way to get a dry part before post cure. You can shorten the IPA rinse and the dry time or increase the IPA rinse and dry time. As long as the part doesn’t have IPA soaked into the resin when you post cure you’re fine.

The thermal cycling is really slow. From 140C to 23C takes about an hour in the chamber. But it is certainly not the cooling as I have seen cracked parts at 130C while still inside the oven(and only see 23C as start temperature).

Thanks for all the hints, I’ve just printed new ones and will leave them in the dark for a few hours to dry from the IPA bath and let them cure for three full days in the sun.

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A UV curing chamber may be necessary to properly post-cure these. The color in your images is a fair bit lighter than we typically see of fully post-cured High Temp Resin. I’ve added an image of what we typically see of cured High Temp. These are the curing boxes we use in the office and they get the job done in an hour.

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After having to explain to my colleagues that I really didn’t buy the nail drying chamber for the weekends, it seems to have worked! Indeed about one hour for it to discolor and the failed parts also discolored a lot. So I guess this was the issue. Going to test them out now with another temperature cycle :slight_smile:

Thanks!

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I can now confirm this was the solution!

Parts just came out of the oven: 14mm parts with 12mm silicone tubing over it(so compressing it), an o-ring and press-fit features and tested in vacuum(-98kPa rel.) survived 140C without any cracks.

Thank you! Makes the customer really happy :smiley:

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I hadn’t actually noticed this earlier but to stress the importance of post-curing High Temp; the HDT in a ‘green’ state is 55.9C @ 0.45MPa and jumps up to 289C @ 0.45MPa after being post-cured. Glad everything is working well for you now!

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