Creating a Functional Action Figure Buck

Another extremely busy month, another last-minute article to maintain my streak of posting at least once a month. But this is one I’m really excited to talk about. Last weekend, I was able to combine what I’ve learned about resin casting and 3D printing to create a function buck system for 4-inch scale, 5POA (five points of articulation) action figure that is specifically designed to be cast in flexible resin (Specialty Resin’s FlexIt90 specifically). Just a disclaimer here before I get into it: I didn’t take quite as many process photos as one might be used to seeing in one of these articles, mostly because I wasn’t sure the process would work until it all came together in the very end. Also, if you don’t know what a buck or buck system is, I recommend watching ToyGuru’s video on the subject. But basically, it’s a blank template for action figures with a common body type.

Aside from a few examples like my glyos figures or the odd He-Man bootleg that had just the right pegs and sockets, creating functional and easily reproducible articulated action figures has been a constant struggle for me and was the main thing that derailed Project OAF. Unsightly gaps between pivot points and leg joints that popped out of their sockets after attempting to move them were just a few of the bugbears I’ve encountered in my experimentation. That is, until I started to become more comfortable with 3D printing and creating my own (albeit limited at best) models.

I started off by downloading a blank male action figure model that I found on sale on CGtrader, one patterned after those 3.75” Marvel Legends Retro figures (despite the default head for the male figure looking like Superman and the size being closer to 4” than 3.75” but maybe that was to account for material shrinkage). I downloaded the female one as well, but that will be a project for another day. I printed out a test figure or two in basic PLA filament on my Prusa MK3S+ using the default joints that came with the 3D models and while the joints were as loose and brittle as one would expect for a 3D printed action figure (though I admit I probably only printed them with a 10-15% infill), I definitely saw the potential here.

So I began messing around in TinkerCAD, a free browser-based program from Autodesk. I’ve used Microsoft 3D Builder in the past, but I find Tinkercad to be a lot more user friendly (especially for a beginner like me). The first order of business was to create a peg-style pivot joint that would be common throughout the body and it was as simple as taking a sphere shape and mashing it together with a cone or paraboloid.

Creating the socket in which these custom peg joints would be inserted and lock into place was a little more complicated but it was hardly rocket science. I merely copy and pasted the joint, sized it up to be slightly larger than said joint, and selecting TinkerCAD’s “hole” option to turn it into a negative/cavity that could be aligned with and merged into the parts where the sockets would be located (neck, arms, and legs). I would also use the hold feature to chop off the existing joints on the model to make way for my custom joints and sockets. Also note that you can upload existing 3D model files in TinkerCAD like I did as long as the file size isn't too big. This was a godsend since I would otherwise have to add the joints and sockets to the existing files in Microsoft 3D Builder. Which is doable, but way more cumbersome.

This was where the bulk of the trial and error came into play as I played with different tolerances to find the best fit. A socket has to be tight enough to keep the limbs from falling off as you rotate them but there also has to be enough room in there to allow the joints to rotate in the first place. I think the one I eventually settled on was making the sockets about 0.10 mm bigger than the joints but it took a few attempts to work my way down to that.

Something that saved me a lot of time when adding sockets to the limbs was only doing one arm or leg, copying it, and them mirroring it in TinkerCAD (I could have also done that in my slicer program). This prevented me accidentally making one arm or leg socket slightly higher or lower than the other. It was a bit tricky to make sure the sockets on the limbs and head lined up with pegs I put on the torso, since I was relying mostly on TinkerCAD’s alignment and measurement features with a touch of blind luck.

Here are all the parts from my first attempt in my PrusaSlicer program prior to adding supports and exporting the Gcode file (i.e. the print file that is actually read by the printer):

And here’s said first attempt printed in translucent yellow PLA. The articulation worked well enough but I knew some adjustments needed to be made. The neck peg ended up being too small and the arms too loose. I knew that wouldn’t fly in a flexible resin casting so back to TinkerCAD I went.

Ultimately, I enlarged the neck and arm pegs on the torso and settled on the aforementioned +0.10mm socket tolerances for the arms. Once again, I deleted one of the arms, readjusted the socket on the remaining one, and then copied and mirrored it. Whatever I had set for the legs was fine and didn’t need further adjustment.

My next printing attempt was made in flesh-colored PLA+ filament with a 40% infill (possibly more) and a 0.15mm layer height.

Since, I say again, the joints were designed to be cast in flexible resin and I needed to use my heat gun to get the limbs to pop on. This was true with the previous print as well. But once the PLA+ figure was assembled, the articulation worked exactly how I wanted to it. The looseness in the arms was gone and the neck was much tighter and easier to attach than before.

I was so chuffed with the results that I scaled up the model and made a 5.5”-6” scale version. But again, that was just a one-off experiment for now and a project for another day in case I ever want to make a set of molds for early-to-mid 90’s style 5POA action figures.

While the PLA+ masters were a success, the real test would be to see how the articulation fared when cast in FlexIt90 resin. But before that could happen, I would have to print out another set of parts in PLA+ filament and create a set of silicone molds. Here are some mold prep pics wherein I glue air vents and pour spout funnels to the master parts. I molded them using the same process I outlined in my comprehensive article on my silicone molding and casting techniques. The only thing I did differently here was pre-fill the sockets with a tiny amount of silicone rubber and allowing it to cure prior to pouring the rest of the limb molds. I was more worried about the sockets not molding correctly than I was with the potential for delamination between the joint and the rest of the mold (though the idea of either one occurring was pretty harrowing). Thankfully, all my fears were unfounded and I was left with a nice little set of one-part cut molds.

Now it was time for the main event. Once the silicone molds were cured, cut, and the masters (with vents and sprues) extracted, I taped up the molds and injected them with some FlexIt90 flexible resin that I dyed a translucent green just for fun (because what good is an action figure test shot that isn’t cast in fun colors?). I placed the molds in my pressure pot and let the resin cure under 55-60psi for the requisite 90 minutes before demolding them. Here they are pictured below.

FlexIt90 resin is such that even after its fully cured and demolded, it’s still somewhat soft and if you assemble something like this action figure too soon, you risk the joints twisting off in the sockets when you rotate them. Instead, the move is to wait about 6-12 hours (ideally 24 hours) to reach its full 90A shore hardness before popping the limbs onto their pegs. I think I waited about 8 hours to start assembling my test casting because I was so giddy with anticipation. And when I did, the articulation worked like a charm! The joints were nice and tight, nothing became loose or threatened to pop off after repeated movement, and the limbs and head were flush with the torso like a proper action figure would be (and not like my previous crappy attempts).

Here’s a quick video I made of the articulation test to show you exactly what I mean:

Now that I knew the buck body was fully functional, the last thing I needed to do was produce some copies cast in Ultracal 90 gypsum plaster. Casting parts in gypsum plaster or one of its many alternatives (Permastone, PerfectCast, Hydrocal, Resincrete, etc) allows me to easily sand down the surfaces and add my own sculpting with polymer clay because the stone/plaster material is incredibly heat resistant and won’t melt in the oven when I bake the polymer clay. But that’s part of a whole ‘nother process that I’ll hopefully be able to divulge in the very near future when I begin work on my first original figure using these molds.

I’m not really used to my weird crafting experiments bearing fruit and I still have yet to see if there will be anything radically different happening when I make new molds of the plaster-based “sculpting master” parts and subsequently cast those in flexible resin, but I have a really good feeling about this new buck system that I’ve created. Worst case scenario, I can cast the parts in hard resin and sculpt onto them with epoxy clay and mold those without much trouble, but I would prefer not to have to do that. We shall see…