Prolific maker and product designer [Eric Strebel] has years of experience making reusable mold boxes for silicone and resin casting. He’s always used 3/4″ plywood before, but it comes with some problems such as inaccuracy, screws that eventually slip out, and no room at all for expansion. Now [Eric] has decided to devise a modular mold box system that’s so awesome, it’s even stack-able. Check out the design and build process in the video after the break.
[Eric] took advantage of additive manufacturing and made fancy trapezoidal walls with recessed bits that allow for the magic that this modular system hinges on — a handful of M6 socket cap screws and matching nuts for tensioning. Once the prints were ready, [Eric] pounded the nuts captive into the walls and marked fill lines every 10mm. As usual, [Eric]’s video comes with bonus nuggets of knowledge, like his use of a simple card scraper to clean up prints, smooth the sides, and chamfer all the edges.
Liquid two-part resins that cure into a solid are normally used for casting, and [Cuddleburrito] also found them useful to add strength and rigidity to 3D printed pillar supports. In this case, the supports are a frame for some arcade-style buttons, which must stand up to a lot of forceful mashing. Casting the part entirely out of a tough resin would require a mold, and it turns out that filling a 3D print with resin gets comparable benefits while making it easy to embed fastener hardware, if done right.
Filling the inside of an object with some kind of epoxy or resin to reinforce it isn’t a new idea, but [Cuddleburrito] learned how a few small design considerations can lead to less messy and more successful results. The first is that resin can be poured with screws in place without any worry of trapping the screws in the resin, if done correctly. As long as only the threads of the screw are in the resin, they can be backed out after the resin has cured. Embedding nuts into the resin to act as fasteners becomes a much easier task when one can simply pour resin with both nut and screw in place, and remove the screw afterwards. A thin layer of a lubricant on the threads to act as a release may help, but [Cuddleburrito] didn’t seem to need any.
The second thing learned was that, for a pillar that needs a cap and embedded nut on both ends, it can be tricky to fill the object’s void with the perfect amount of required resin before capping it off. On [Cuddleburrito]’s first attempt, he underfilled and there wasn’t enough resin to capture the nut on the top lid of the pillar he was making. The way around this was to offset the nut on a riser, and design in either a witness hole or an overflow relief. A small drain hole or a safe area for runoff allows for filling things right up without an uncontrolled mess in the case of overfilling.
Something worth keeping in mind when experimenting in this area is that in general the faster a resin cures, the more it heats up in the process. It may be tempting to use something like 5 minute epoxy in a pinch, but the heat released from any nontrivial amount of it risks deforming a thin-walled 3D print in the process. For cases where resin would be overkill and the fasteners are small, don’t forget we covered the best ways to add fasteners directly to 3D printed parts.
[Tommy] shares a simple 3D printing design tip that will be self-evident to some, but a bit of a revelation to others: the concept of a sacrificial bridge to avoid awkward support structures. In the picture shown, the black 3D print has small bridges and each bridge has a hole. The purpose of these bits is to hold a hex nut captive in the area under the bridge; a bolt goes in through the round hole in the top.
Readers familiar with 3D printing will see right away that printing the bridges might be a problem. When a printer gets to the first layer of the bridge, it will be trying to lay filament in empty space. By itself this is not usually a problem as long as a bridge is short, flat, and featureless. Unfortunately this bridge has a hole in it, and that hole means the printer will be trying to draw circles in mid-air, rather than simply stretching filament point-to-point across a gap. One solution would be to add a small amount of support structure, but that just moves the problem. Removing small supports from enclosed spaces can be a real hassle.
To solve this [Tommy] added what he calls a “sacrificial bridge”, shown as blue in the CAD image. He essentially gives the hole a flat bottom, so that the printer first lays down a thin but solid bridge as a foundation. Then, the portion with the round hole is printed on top of that. With this small design change, the print becomes much more reliable with no support structure required.
There is a bit of post-work involved since each hole needs to be drilled out to punch through the thin sacrificial bridge underneath, but it definitely beats digging out little bits of support structure instead.