Elastic Resin Prints Shrinkage/oversize


#1

Hello,

I’m printing arteries models based on CT scans of patients in order to simulate the anatomy and to determine if this patient can be implanted, and if so, with witch size of device.
For this purpose i’m using the Elastic resin that is more or less simulating the vessel.

The thing is, i’m not sure what is the offset of the print, I will print several cylinders and measure them to see what the difference from the designed model but I wonder if someone already did it and can shear the results.
Our model wall thickness is normally 1.5mm.

Thanks!


#2

For various reasons, including the compliance of the material, it can bet difficult to get the same kind of dimensional accuracy in Elastic resin as with other Formlabs resins.
See
https://support.formlabs.com/s/article/Using-Elastic-Resin?language=en_US if you haven’t already.

In contrast with other resins, Elastic does seem to print slightly larger than specified in the model/mesh, and this may be a combination of how the resin cures, and with the resin absorbing some IPA during the washing process and swelling as a result. For example, another user on these forums today is reporting their parts are about 1% larger than expected.

If you’re going to be printing blood vessels (like tubes) from Elastic, then your test parts should probably be cylindrical tubes with your normal wall thickness, not solid cylinders.

1.5mm walls are not too thin for elastic, but the thinner the part, the higher your surface-area-to-volume ratio is, and the more likely that IPA absorption in the wash will give you distortion problems. If you run into that kind of issue, you can try washing for a shorter amount of time and/or letting your parts dry out longer after washing before post-curing,

Good luck!


#3

Thanks for information, I will test it and upload the results in the following days. I do have problem with transparency and sticky models even after washing, I solved it with (as you said) about 2 hours of self drying and about 3 hours in the curing flipping the model from time to time as it was relatively large. Took the entire space of the build platform and the height of the machine.


#4

Another trick for reducing stickiness that hasn’t made it into Formlabs official recommendations for elastic is to post-cure the part underwater - that is, put it in a clear container of warm water large enough to submerge it, and put the whole thing in the Form Cure. If the part is too big for that to work well in the Form Cure, another option is to brush a layer of mineral oil all over the part before post-curing. In both cases, the idea is to reduce the amount of oxygen getting to the surface of the part.
Oxygen inhibits the curing reaction, and incomplete curing is what leads to the stickiness. Covering the part up with something transparent like a water bath or a layer of mineral oil might not eliminate oxygen completely, but it can help. Other similar solutions could include curing it inside a bag filled with nitrogen or CO2 gas, if that’s something you have access to.


#5

Yes I tried it out with the water, working well with a method I’m using. The problem is that if I place the model in water after the two bath’s + drying, the model is completely opaque. If i’m curing for 1 hour and then placing it inside a water at 60 degrees C for another hour or 2 then it keep it partly transparent.

Didn’t try the oil yet.


#6

Its looks like there is a problem with the machine, contact Formlabs support as the optic test came out bad.

This is what happens to my print and resin LT tank:

This is what sent:

This is what I got:


#7

Your optics test in the other thread does look like you’ve got some contamination that can be cleaned, and hopefully you can fix that with instructions from Support.

For this model in particular, even with perfectly clean optics, I would advise manually orienting it to keep as much of it as close to the build platform as possible, even if you have to add a hole or two to avoid cupping.

Any print in any resin gets a little riskier as it gets taller, but it’s especially true with Elastic, since the building part is often displaced by the peel cycle and has to come back to its origin to be ready for the next layer, and this effect gets larger the taller the print gets.

Your tank should be fine when you filter that resin, but it will be a good idea to drain all the resin into a separate container as thoroughly as you can to inspect the tank film for any distortions or blemishes in the clear area.


#8

Hey @Ike,

Thanks for your reply.
You are completely right, this is not the best positioning of the part. I came to this location as I need the internal part on the straight section (right side of the picture where there is a small horizontal tube) to be with less support as possible as this is the area we are working with. Theretofore I had to place it this way.

I will give a closer look on the LT as you advised, I’m waiting for the 400 micron filter to arrive as the 190 micron is just too dense for it.

Tomorrow I hope to get the answer from the support so I’ll keep you posted with the answer and the results.

Thanks!


#9

Hello,
I am attempting to recreate similar but more basic vascular components centered around Fontan procedures using elastic resin. Would you mind sharing the results of the different wall thicknesses of elastic resin you tested as to which thickness most accurately represents the vessels with regards to elasticity? Will the standard 1.5 mm thickness you stated suffice or is there a better and more accurate wall thickness I should use?

Thank you so much for your help


#10

Hey,

As my machine is disabled right now due to a very poor prints, I haven’t got the chance to print the gauges to determine the accuracy of the machine yet. But from my experience till now, 1.5mm is quite good for a vessels of 25-35mm diameter. I’m printing the ascending aorta, aortic arch and descending aorta.

If you go for smaller vascular vessels as Fontan procedure performed in children, I will go for 1 mm wall thickness, it will be more prone to holes due to catheters and tool that might be in use but will be more realistic in flexibility and softness.

the 1.5mm is quite robust and not that flexible.

I will upload the results of accuracy once I will fix the machine, hopefully tomorrow.

Please keep me posted on the 1 mm print if you go for it, I would like to hear how it went.


#11

Thank you, your response is extremely helpful. I am essentially going to print the intersection of the of the RPA, LPA, SVC, and the Fontan. All of which range from about 12-20mm in diameter so I would imagine the 1mm thickness may be more accurate.

I will surely share the results of the 1mm print along with my thoughts on its accuracy in truly representing a vessel with regards to elasticity and response to flow.

Hope you get your machine up and running accurately as I know first hand they can occasionally be a pain.

Thanks again.


#12

Ok, the machine mirrors cleaned up and finally got the Aorta model that I wish for.
The resin located in the LT tank changed the color to light purple and was with tons of debris, I disposed the resin from the LT tank and cleaned the tank.

Send to a print and we are back on te road again.

Thanks for the help @Ike!


#13

I’ve also checked the diameter of the printing it order to characterize the oversize of the printer. I’ve printed 3 rings 10mm height , 1.5mm wall thickness with diameter of 19.82mm, 20.76mm, 33.77mm. After optical measurements, the oversize is about 4-5%. The parts are larger then the printed model.


#14

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