Impact of IPA on SLA Warping

A while back we posted some interesting findings on the impact of IPA on dimensional accuracy for flexible resins. and how it’s incredibly important to dry them thoroughly (compressed air is very inadequate). I recommend reading that before continuing.

Today’s blog post focuses on some really impactful information that has changed our SLA post processing workflow and has greatly reduced and eliminated post cure warping of rigid performance resins.

Some background first…engineering grade SLA resins typically require a heated UV post cure in order to maximize mechanical properties. This is apparent when looking at datasheets from Formlabs. These materials also all absorb IPA, but at differing rates. IPA absorption seems to be correlated to rigidity or toughness. The tougher or more flexible the material, the more IPA it absorbs. While the more rigid the material, the less it absorbs. Below are some screenshots taken from Clear V5, Tough 2K, Durable, and Elastic 50A datasheets.

So, we know that IPA absorption can cause swelling and excessive dimensional inaccuracy on flexible resins, where the IPA absorption rate is high…but can it also affect other resins? Turns out the answer is yes. IPA absorption can affect rigid resin print accuracy just like it can with flexibles, but to a lesser degree. However, the bigger problem lies in the fact that absorbed IPA can greatly introduce warping during heated post cures.

Warpy warpy

Printing a perfect part only to have it warp during post cure really sucks. We’ve been there many times.

For the longest time, we operated with the understanding that:

1. Leaving supports on before curing was a must to avoid warping on certain geometries

2. Curing for 3x the duration without heat was the only way to ensure zero warping across all geometries

Neither of these offer a great solution because #1 means more post processing cleanup (supports are always easier and better to remove before post cure) and didn’t always eliminate warp. The problem with #2 was that mechanical properties lie in an unknown area between the “green” and the “cured” state.

I’m happy to say that once we introduced dehydrating into our workflow, we’ve now solved both of these problems across part geometries that historically were a problem.

A picture is worth a thousand words

This image shows, very clearly, how drying can impact warping. The support structures are a perfectly long and thin geometry to exacerbate warping issues for this test.

From right to left, the samples are as follows. All underwent a heated post cure with standard formlabs settings.

- IMmediately after drying with compressed air
- After 3 hours of air drying
- After 3 hours in a dehydrator

all three of these parts looked idential prior to post curing.

So let’s talk about what’s happening on a molecular level. Since the resin, composed of monomers and oligomers, hasn’t been fully crosslinked yet, IPA makes its way into the gaps and can get trapped. Not only this, but IPA can act as a plasticizer which makes the resin softer and more pliable. In extreme cases, this can lead to swelling of parts.

But how come parts can look visibly “normal” before heated post cure and then come out all warped? My guess here is that heating up the part causes the IPA to expand and evaporate, creating localized internal pressure between the monomers and oligomers in their softened, semi-crosslinked state. Adding this on top of the fact that absorbed IPA will soften the “green” state resin, a lot of internal stresses can occur here, introducing warp onto parts which eventually gets “locked-in” once the post cure is complete and the polymer is fully cross-linked.

Anecdotally this also explains why warping can vary wildly if you do not have a consistent drying workflow. There have been times in the past where we’ve had to post cure parts immediately after washing and others when the parts would sit for 24 hours. Because of the variation in process, it sometimes felt like warping would rear its ugly head at very unpredictable times. Drying parts takes all the guesswork out of this and ensures consistent, minimal warpage across many resin types and geometries.

Dehydrate

Using a food dehydrator or a filament dehydrator like this printdry unit is now part of our standard post processing workflow

For any soft resins like Tough 2K and beyond, we throw parts into a food dehydrator for 3 hours before post curing. The set temperature depends on your HDT, but we’ve generally been setting the dehydrator to around 40-45C with great success. A summary of our workflow is below:

1. Print parts

2. Wash parts according to Formlabs recommended wash times*

3. Let parts air dry or blow off with compressed air

4. Drop parts into a dehydrator at 40-45C for 3 hours**

5. Post cure parts according to Formlabs recommended cure times***

*Increasing wash time for tricky parts is now less of a concern because of the drying process
**While not as necessary for Standard or Rigid materials, there is no harm in doing so
*** If warping still occurs, it may be a result of HDT, material properties, geometry and other internal stresses - in these rare cases we will cure for 3x the duration without heat

Stay tuned for further in depth testing and a YouTube video in the future!
In the meantime, you can use commonly available dehydrators for 3D printing filament like these (affiliate links):

- PrintDry Filament Dryer
- Sunlu Filament Dryer
- Food Dehydrator