Making An Object Hollow, My First Print on Form 1+


First some background. I started at Formlabs last Monday, and while I have extensive experience with FDM 3d Printing, stereolithography (SLA) is a dark art to me. There are LASERS and GOO! Now I’m willing to embrace the idea of a magic box that makes things, but I’d like to know more about how it works and the things it can do.

I figured my options are either to pester the Formlabs engineering staff, or to come to the forums and learn the answers for myself. Also, since I didn’t go to MIT, the engineers can be a bit intimidating.

I’ve always wanted to experiment with thin walled hollow objects, which aren’t really doable on FDM machines.

I started from an older model of mine, the Tachikoma.

The Tachikoma isn’t originally my creation, its a model from the SketchUp 3d Warehouse that I extensively reworked several years ago.

To make the Tachikoma a thin shelled object I used ‘Hollow’ function in Meshmixer.

I used a 1mm wall thickness and a high mesh density and accuracy. I added drain holes on the bottoms of the legs, ends of the arms, and back of the abdomen, places where they won’t be obvious. All of the drain holes are 1mm in diameter.

In PreForm I rotated the Tachikoma backward 70 degrees so that the drain holes in the abdomen are at the lowest point.

The intent is that these two holes will act as vents, letting air into the model so excess resin can escape.

I also turned on Internal Supports to keep the shell from collapsing in on itself.

There won’t be any way to remove these internal supports, but If the Tachikoma was printed in opaque resin they wouldn’t be visible. For this print I am using clear, so I can see if the resin and isopropyl alcohol drain properly.

The final print worked amazingly well.

There where no draining issues, and all the details came out nice and crisp. The print is a bit soft, it flexes when you squeeze it. From what I’ve read, post-curing may help eliminate that by further hardening the resin.

I hope you found my experiment as entertaining as I did.



I would love to print it myself actually!
Which resolution did you use?

Awesome print Michael!

I printed at the standard 0.1mm resolution. I didn’t want to get to wild the first time around.

wonderful! IS the model share-able?

I added the thin-shell version of the model the the Thingiverse page. You can download it here

Great job. Sounds like you have the fundamentals down well. Just a couple comments. 1mm thick walls are a little on the thin side, they will warp and may as the object cures more over days / weeks start to crack. 2mm is better in practice. Also the internal supports are probably not necessary, if you turn them off you can scan through the layers looking for unsupported areas if there aren’t any you are probably fine, the form1+ is really good at overhangs.


thanks. I think also 2mm is probably better against warping.

This Model would be quite interesting to print in separate but assembled form. This was done with the Crab Brian Chan creates in this Formlabs video.

You might even leave channels to run stiff wire through and make the model pose-able.

I agree; 2mm wall thickness is my default setting for shelling parts while keeping them strong.

The guideline I made, Designing for the Form 1, has wall thickness descriptions as such:

• Strong: 2 mm
• Flexible: 1 mm
• Delicate: 0.5 mm


This is a very valuable post and discussion! I would really like to see more of these. I think Formlabs could benefit greatly from a series based on prints like this (I know I would!).


Absolutely. I agree with @Nick_Eide. It is great to see FormLabs staff printing and sharing. It is a whole different side of FormLabs than we typically see here. While the typical post is professional, it is usually defensive. I can’t even express how surprised and impressed I was to see a printing project on the forum that came from the staff.


I’ve continued to mess with thin walled prints. The general consensus seems to be for slightly thicker shell, so this time I made the walls 1.5mm thick. As to @RocusHalbasch 's advice, I turned off internal supports on these models.

There are some sections that look like they need a little support from the inside. With the supports already computed, I enabled internal supports and went into ‘edit selected’ mode.

And this is where I got stuck for a while. How do I put supports on the inside of the model if I can’t see inside it?

It turns out the answer is non-obvious.

On the right side of the Preform window is a slider that lets you scrub up and down through layers. In ‘edit selected’ mode, the behavior of the slider reverses, and you can see from the selected layer up, instead of the selected layer down.

With the model sliced open like this I can rotate around to see inside the inside and place internal supports as needed.

This took me an embarrassingly long time to figure out.

The original non-hollow model is available for download here

Nicely done Sir. The slider in supports mode interface definitely gets poor scores in the discoverability and principle of least astonishment categories. How did it come out? Also how did you handle drainage? The reason for the second question is I’ve noticed a lot of new users get this wrong for a while and run into blowouts and don’t understand why so stepping through how you handle that and check for it would also probably be an awesome thing to share with people.

Lastly this is something you pick up as you use the printer over time and really analyze a lot of prints, but it is worth mentioning here. The red coloring can not always be trusted. It will sometimes not appear when parts of your print are not supported, and other times will appear even though supports are adequate, it’s useful to help you get most cases done quick, but it’s no substitute for thinking about and analyzing the print yourself.

The part came out great!

I added 4 drain holes on the underside of the base, and a single drain hole in the top. My operating theory is more based on getting the IPA to flow through the inside of the model while keeping drain holes on visible surfaces to a minimum. In general I’m trying to err on the side of caution by adding lots of holes

A discussion of drain holes would be awesome. It seems like something that is really non-intuitive for a new users, especially users not use to producing physical prototypes.

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There are also specific additional considerations for platform up SLA machines life the Form1+. This in a nutshell boils down to for every region of your model that has it’s own closed off region of hollow space at some point in the print you need a drain hole positioned at or near the hollow portin nearest to the platform. This is something I have described my method for working out but I’ve not seen anyone step through visually. Currently I don’t have time to do so either.

I’d love for preform to have a way to add drain holes! I tried hollowing with meshmixer which adds the holes but it keeps mixing up the mesh. I hollow out with Geomagic and it works amazingly well but I haven’t figured out how to add drain holes. I’ve been drilling holes in the base of my models to drain the trapped resin out and to prevent cracking.

That looks fantastic! Thanks for the detailed walk through of your process.


@Michael_Curry, Huh, don’t be too embarrassed, I never did figure that out. That’s neat.

@Michael_Curry , another great print!

From your pictures I would guess that you printed it an 0.1mm layer heights.

@JasonLivingston gives some valuable guidelines for the relationship between thickness & strength.I was wondering if there are any relationship between wall thickness and print layer heights we should also be consider when designing or using Preform.