Hollowing Out Models #3DPrinting

3D printing

3D printing is becoming big news and we’ve started investigating it ourselves here in Brown Bag Films with a view to printing out our own models. We're very excited by it and the possibilities it brings!

As an animation studio, you've got a room full of geeks and we’ve obviously all been creating our models for some time, but now there are tools available which allow us to create models really quickly and efficiently, and complimentary online services which allow us to get our models printed and brought into the real world with minimal effort - ready for painting up, without the need for the usual rigmarole of keeping our models tidy and worrying about topology, unwrapping etc. Pretty sweet, huh? Not quite the Star Trek replicator machine, but a step closer, and let's be honest, that's pretty darn cool.

So, we’ve done some research and tests and would like to share what we’ve learned with you. It can be difficult to get decent tutorials on many aspects of 3D printing, so hopefully with this blog, we can have an open question and answer type forum, where we can share our progress as much as possible with everyone interested, get feedback, and hopefully also show you our results as we learn. So if you’re interested in making your own 3D prints, just follow along these tutorials on the blog, and as we learn, you will too! We'd love to get your feedback also, so if you have any suggestions/tips, etc, let's share the knowledge!


Hollowing out a model = making cheaper 3D prints.

We’ll be covering our results in a set of tutorials over the coming weeks and we thought we'd start off with a quick tutorial on hollowing out models, a difficult thing to find information about on the web, and one that took us some searching and testing before we had reliable results. What we’ve come up with seems to work consistently, so while it may not be the ONLY way to do this, it’s certainly effective enough and has worked for us so far. Please feel free to suggest alternative methods, we'd love to hear about them!

In instances where support materials are required and the costs are similar to the main material chosen, hollowing out models becomes unnecessary. However, for most models, hollowing out provides for significant cost savings as you are not paying for material inside the model which you don’t need. This can be done using either Meshlab or ZBrush, amongst others. We’ve chosen to use ZBrush, as we're creating our models in ZBrush, though I’m sure you can find ways in other software too, and as stated, Meshlab is free and does the job too.

For ZBrush, the basic process is as follows:

  • Make sure your model is a Dynamesh model. Dynamesh will recalculate the distribution of poilygons across a model's surface to distribute them evenly every time Dynamesh is invoked (by Ctrl + dragging anywhere on the canvas while Dynamesh is active). You want to ensure you have enough resolution that when you reDynamesh it, the model will retain the necessary amount of detail. You can do this by simply selecting your subtool, clicking the Dynamesh button in the Geometry rollout, and Ctrl + dragging on the canvas to update the model. Once you release the mouse, Dynameshing will happen! If you’re happy with the resolution the first time you do it, you’re fine. If not, undo, increase the Dynamesh resolution slider and try again until you are happy. It really is that easy. Honest.
  • Once you are happy, hit ‘b’ for Brush, then ‘i’ for Insert brushes, and select the Insert Cylinder brush (you can select any insert brush, I just find cylindrical holes the best, because, well, holes in my world tend to be round and I'm used to that now - nothing stopping you having a square hole, or even an ear shaped one though). Now hold down Alt and drag over your model where you want to insert the hole. ZBrush will mask out your mesh, and show an inverted cylinder drawn on that area.
  • Now simply clear your mask by Ctrl + dragging on the canvas. What the…? Didn't I just say Ctrl + dragging causes Dynamesh to kick in? Am I trying to make a monkey out of you? Well, no. Not if you have something masked. In that case, Ctrl + dragging on the canvas will tell ZBrush to clear the mask first. Only if there is no mask and you Ctrl + drag, will ZBrush perform a Dynamesh on the model (presuming of course Dynamesh is enabled). So basically we have to clear the mask first in order to do a reDynamesh operation (which is essentially what we're going to do when we hit the Create Shell button later on - I know, spoiler alert, right?).
  • In the Dynamesh menu, change your Thickness to the thickness you want your hollowed out walls to be and then simply hit Create Shell when you're happy (this value will change depending on the model size, so just undo and try again with different values until you get the one you want). ZBrush will redynamesh your model now with a hole in it, and create a wall using the thickness value you chose. If the wall isn’t perfect in size you may experience problems of the shell not being perfect, so simply undo and redo, changing the size until you get the desired results. Your mesh is now hollow and A LOT cheaper to print.
  • One thing to look out for is the length of the cylinder you poked through your mesh. If it's not long enough, the shell won't be created. You have to make sure the cylinder is inserted far enough into your model to be at least the thickness of the shell you want to create.
  • Most 3D printing services online have limitations as to the file size you can upload and the amount of polygons allowed for your model. Luckily, ZBrush has another tool built in which looks after that for you. It’s called Decimation Master, and it does an amazing job at reducing your polygon count, even allowing you to specify the exact amount of polys thatyou need for your final result. To use it, select the subtool you are using and from the ZPlugins menu, choose Decimation Master. Hit “Pre-process Current” to have ZBrush calculate what it needs to calculate, and once done, you can choose either the percentage of the original model poly count that you want to keep, or even specify the amount (in thousands) directly using the sliders available. Once you’ve made your selection, just hit “Decimate Current”, and BAM!, ZBrush will reduce your polycount to that number, with minimal loss of detail. It doesn't actually make a BAM! sound, but it gives spectacular results, often reducing models from several millions of polygons down to hundreds of thousands instead, with no discernible loss of detail. Pretty frickin' sweet.
  • Once done, Export your subtool from the Tool menu using the Export button as normal.

We've made a short video to explain the technique which you can see here, so please, watch and feel free to leave feedback/suggestions here on the blog.

As good as ZBrush is at creating models for 3D print, you do however usually need to fix mesh problems before printing, as there invariably will be some problems which are going to be difficult to see or fix in ZBrush. To do that, we use Netfabb studio, a free open source piece of software available on the web (all hail the Mighty InterWeb!).

We’ll do another video tutorial on that soon, so bookmark the blog now and remember to come back to check it out. For an online tutorial on how to do this, please see this link on the Shapeways website.



Sean Forsyth

Sean Forsyth is an Art Director


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