RC10Nick

iTrader: (9)

Hi everyone! I've just started experimenting with casting rigid urethane resins into 3D printed molds to make my own custom plastic parts and I wanted to make a thread to share what I've done and hopefully inspire others to give this a try. A lot of what I'm doing with these molds is drawing on the things I learned from my DIY tire thread so check that out if you've got the time.

A few weeks ago I bought a trial kit of Smooth-Cast 65D. It's a 2 part urethane resin that cures into a rigid plastic. This stuff is seriously impressive - I cast 1/4" thick sample and it's plenty rigid but also surprisingly tough. I clamped the test piece in my bench vice and gave it a few really good smacks with a hammer to see if it would break. It survived with only slight marring from being clamped in the vice. I'm pretty sure I'll be able to make a-arms out of this stuff.

Anyway, here's my first test part. I started small and simple - an insert I designed to go into a TLR 22 series VHA hub. This specific insert is designed to remove 3° of toe for running on carpet. I designed the mold so I can inject resin from the top, added internal piping so the resin enters the part from the bottom, and included a vent at the top to let air escape. I also included a cutout for an M3 screw inside the mold to create the hole for the hinge pin. I sprayed the mold and the screw with urethane mold release and didn't have any issues getting the part out of the mold. The insert came out a tiny bit on the small side, and the drill I used to clean out the hole was a bit too big, but those are easy problems to fix on the next attempt and honestly it's not much sloppier of a fit than a worn out insert so it's not even a huge deal anyway. Overall I'm pretty excited with how well this worked and even more excited for all the possible parts I can now make.

Anyway, here's the pics:

smirkracing

iTrader: (19)

This is totally awesome. Can you talk about pros and cons of this approach vs simply 3d printing for the part? For the part you selected here (an insert) it seems to me 3d printing could work.


Suspension arms might be a different story.

RC10Nick

iTrader: (9)

Thanks!

I have printed this exact part before out of PLA, but it quickly develops excessive amounts of slop. I've gotten around this by just having a bunch printed up and replacing them every race day, but that's not an ideal solution. Besides, this specific part wasn't chosen because it needed to be strong, this part was chosen because it was small and has a very simple geometry that lent itself to being cast in a rigid mold tool.

The advantage of urethane resins is their superior physical properties. Urethane resins are as stiff and impact resistant as injection molded plastics, and importantly, they are equally stiff and strong in ALL directions. Like I said in my initial post, I took a hammer to a cured sample of the resin and couldn't break it. There's no way I could print anything near as durable. That's the real advantage of this stuff. You can make things like suspension arms and hubs that are as tough as factory injection molded parts that you could never hope to print.

Another minor advantage of this stuff is speed. This resin cures enough to de-mold parts in about 20 minutes which, depending on the part's size, can be significantly faster than printing it. And it also frees up my printer for other jobs.

belewis01

iTrader: (5)

Thanks for posting - been looking at different ways to make custom parts, and as you know 3D print is often limited for strength. Will be looking into this for sure.

Brandon11m

RC10Nick any updates on this? Have you tried this out on any other parts like a arms to see how durability compares to molded parts? I'm interested in trying this myself for my custom 2wd buggy I've been racing to replace some of the 3D printed parts on it that tend to break more than I'd like. And you did this with just a printed mold, only post treatment was some mold release? No need for a vacuum chamber or anything to avoid bubbles in the part?

RC10Nick

iTrader: (9)

Nah, I haven't done anything with this since my last post. I was having some dimensional accuracy issues with my parts which led me down a rabbit hole of completely re-aligning my machine. Then I started working on another project, then another project... I'm sure you know how that goes.

I will say the resin I used was impressively strong - I popped out the cured disc left in the bottom of my mixing cup and went at it with a hammer. The thickness of the disc varied, it was about 6mm at the edges and more like 4mm in the middle. It took an impressive amount of effort with the hammer to get it to break.

I'm not sure how relevant this process would be anymore - new printers on the market are fully enclosed with built in chamber heaters which would allow you to get much closer to fully isotropic prints. I suspect those printers might be capable of producing nylon parts strong enough for real use in an RC car. I'd probably explore that route before doing more resin casting experimenting.

Brandon11m

Oh yeah it can definitely be easy to get distracted away from one project and into multiple others. I've been printing all my parts using polycarbonate using my Prusa MK3S with a custom enclosure over it but still have occasional durability issues. I think nylon is a bit too soft and flexible to use for like the a arms on my car unless I were to go to a carbon fiber filled nylon but those usually don't have as good of impact resistance as non-filled and are much more expensive and prone to moisture so I just stick with PC. I don't think FDM 3D prints will ever be isotropic or very close to it, or to get to that point you'd need such a hot enclosure it would negatively affect print quality of overhangs and reduce the lifespan of any electronics within the enclosure. I think I'll order some resin myself to give this a go at least with A arms and then maybe move on from there to other parts if the arms turn out well and can hold up.