I had this how-to story posted over at the good folks of 3DHubs but I thought I'd share it with you as well, never hurts to share now does it? ;-)
This friend of mine is a big "Aliens"-movie fan. He's a skillful airbrush-artist and really wanted to have this movie prop, this particular "Egg" (that has been in different shapes and sizes throughout the whole series I might add!). I made a sweet deal with him: I would get free Airbrush tutoring and he would get this egg movie prop. So here's the thought- and creation process so you can try and make something similar.
I like to work with Zbrush, works really good for 3D modelling and printing. You will see some technical mumbojumbo coming by that has to do with certain functions in the software ;-) As always, feel free to ask any questions.
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Step 1: Preparation & Design
I had a 3D Printer, it was a UP! Mini with a whopping 12x12x12cm build volume and 200 microns printing resolution. I wanted to give something bigger than that so that meant I had cut things up and assemble it afterwards. I decided I would print it in White ABS and not Natural ABS because that looks a bit more transparant. My friend was going to airbrush it, so bright white is always good as a foundation since I wasn't sure if he was going to use a primer or not. More about painting later on.
After giving it some thought I gathered lots of reference photo's and found out there's a lot out there regarding this "Egg". I decided to go with something that would be cool to make and sort of unique but still recognizable, well, at least for the fans out there ;-)
In Zbrush I blocked out the shape pretty quick, it's not exactly an egg ofcourse but the whole thing is simple by design as you can see. The cool thing in Zbrush is that you can work in symmetry-mode(s) and quicly get a good shape. In overall, the whole thing took me maybe a few hours worth of designing and cutting it up about an hour or so. That's why planning ahead is important with designing for 3D printing.
Getting the texture onto the egg was done in Zbrush also through the usage of some Alpha-stencils. I use a Wacom Intuos3 myself which is excellent when working with Zbrush or Photoshop, I can highly recommend it (or any other Wacom-tablet)
- Photo: The bright green photorender is the final version of the model I made
- Photo: The more blue version was one of the iterations.
Step 2: Pre-Printing | Creating Keys and Holes
When the modelling part is done there's still more to do! Here's my take on creating some so-called "keys" onto the parts and also creating the fitting "holes". It's a technique that's widely used with toy-designers or sculpters. It makes sure objects fit together at the right place without messing up. On the photo you can see the keys and holes, circled bright green.
Splitting up the model
I merge all sub-tools (separate objects) into one. With Dynamesh I can turn it all into one continuous mesh. So now I have a mesh that has everything fused together. With this new mesh, I make sure it's Dynameshed (it should be ;-) ) and now I can slice it up one of the slicer brushes. You just draw a "line" where you want the cut and it will divide the object into 2 separate polygroups. For this model I had to make a few slices so I would have 8 separate pieces and they would all be there own object. After slicing I separate them into sub-tools, since they have their own polygroup that's is very easy with the click of a button.
Now I will Dynamesh each piece so it will have a nice continuous mesh and holes and errors will be filled up with new polygons. After this I will create the keys and holes. Optional technique: depends on the model a bit, for example a small figurine or something, or a model that you don't need to slice up. I make a copy of this unified mesh, I use the Zremesher function on it. This will create a low-resolution mesh but with even polygon distribution, makes a really nice mesh.
Then I will reproject the details of the copy onto this new mesh and divided it up until I have the desired detail back.
Creating Keys and Holes
After I have the pieces all separate there's going to be a bit of tedious work ;-) It's a simple process of adding and subtracting objects. First, I will plan out how I want the objects to connect to each other. I have made an Insert Brush which consists of just one cube that's a bit tapered toward it's top. Doing it like this will sort of guide it in place when printed. With the selfmade Insert Brush, I will create a cube onto the flat surface on the desired side. I Dynamesh it again so it will be part of the whole mesh.
Now I make a copy of this object because using the following technique will have it merged and if something goes wrong I can simply try it again, fast ;-) In the sub-tool pallette, you set to add or subtract. I choose the object with the key, to subtract with model above it which doesn't have a key or hole. This way you will have the perfect hole for the key onto your mesh. What's actually even better is creating a key where you want it, split this key off and duplicate it.
Now you can scale the copy of the key a little bit bigger and use this version to subtract a hole onto the other mesh. Now you will have a slightly bigger hole since printing with plastic isn't an exact science. Lots of factors will make the hole (or key) a fraction bigger or smaller. This way it will always fit and when you glue it it will never come off ;-)
Step 3: Printing
The "Egg" itself consists of 8 big parts and the bottom circle around it is divided in 4 parts. For the "Egg"-parts I made so-called "keys" and "hole's" so that once I printed everything, it would all fit just one way. Everything would line up exactly as on my screen. Ofcourse with big prints come great respons...uhm warping ;-)
I tried various options within my software that comes with the printer, and I thought it would save a lot if I would print it as a big shell. I could do that, having every piece of the egg, lay flat on it's "flat side" but since I needed keys and holes in there, I couldn't do this. I choose a hollow printing fill (similar to that 20% I see everywhere in other software). I made sure I calibrated my printingbed after each 2 prints or so, just to be sure it would be ok. I printed this at maximum resolution, 200 microns. This White ABS was just incredible I think...came almost like plaster-like!
Although I get excellent prints of this little printer, I still had the smallest bit of warping sometimes, just can't help it. It didn't matter. The whole thing was going to be glued up and I would fill in the cracks with some Gesso (which takes forever to dry I think...probably would use something else next time). It took me a few days to get everything printed but it was worth it! It almost pained me to give it away..
After I gave the model to my friend, he was really impressed! In the course of several months he would go over the model again, sanding it off very lightly, with very fine sandingpaper. Also use a Dremel-tool for polishing and sanding it a bit here and there. After he was satisfied he would go on and paint it. The pictures you see here are the last ones I have and he's still busy with it. I really have to mention he's a patient guy and really takes his time, it's more of a side-project for him.
My friend was going to airbrush it. For those who don't know what kind of paint can be used; to be safe you can go with Acrylic paint. My friend used a solvent-based paint and that works just as fine apparently! You have to be careful though, some solvents can be disastrous to plastics, so always check first!
He did a lot of sanding first on the freshly printed model. Using a very fine sandpaper he went over the model numerous of times. Eventually he even used a Dremel polising tool for sanding down or polishing the edges of the separate parts..
After that he would make a light brown base-coat and spend ages airbrushing the scales with a more dark brown color. He would go back and forth so you will get this nice extra "depth" onto the model. Eventually he would build it up all very slowly.
Hope you like my take on the infamous "egg" from the movie. It was a lot of fun to make and even better to have it in your hands when printed and fully assembled! You can see my lovely youngster holding up the assembled version
Because this is an awesome site, I've included the STL-files in a ZIP so you can check it out yourselfs and mess around with it ;-) Have fun!