Resin-Printed Gears Versus PLA: Which Is Tougher?

When it comes to making gearboxes, 3D printing has the benefit that it lets you whip up whatever strange gears you might need without a whole lot of hunting around at obscure gear suppliers. This is particularly good for those outside the limited radius served by McMaster Carr. When it came to 3D printed gears though, [Michael Rechtin] wondered whether PLA or resin-printed gears performed better, and decided to investigate.

The subject of the test is a 3D-printed compound planetary gearbox, designed for a NEMA-17 motor with an 80:1 reduction. The FDM printer was a Creality CR10S, while the Creality LD02-H was on resin duty.

The assembled gearboxes were tested by using a 100 mm arm to press against a 20 kg load cell so that their performance could be measured accurately. By multiplying the force applied to the load cell by the  length of the arm, the torque output from the gearbox can be calculated. A rig was set up with each gearbox pushing on the load cell in turn, with a closed-loop controller ensuring the gearbox is loaded up to the stall torque of the stepper motor before letting the other motor take over.

The resin gearbox failed relatively quickly, and dissection indicated some of the internal gears had failed. A reprint with stronger resin was done, and the test begun again. This time, each gearbox lasted over 500 cycles without issues, but the resin gearbox failed shortly after, wiggling about before jamming up for good. Opening the gearbox led to broken teeth and powdered resin falling out. Meanwhile, the PLA gearbox showed very little wear despite the repetitive test.

Thus, if you’re looking to put serious loads through your 3D printed gearbox, you probably want to go with PLA or another FDM material rather than resin. This result is unsurprising, as a general rule of thumb is that resin prints are more brittle than their FDM counterparts. Of course, the exact plastic or resin you print with will vary this result however, so don’t take it as a hard-and-fast rule, more of a general guide. Video after the break.

[Thanks to Zane Atkins for the tip!]

32 thoughts on “Resin-Printed Gears Versus PLA: Which Is Tougher?

  1. What kind of resin? We use some pretty solid/flexible stuff like Pro410 which we utilize a lot of and nothing has failed yet in the real world application for over a year. Several other resins are even more capable, just not perhaps as cheap as those tested. ;)

      1. not my circus, not my monkeys, but… i find this kind of frustrating that the details are in youtube. i wanted to know some characteristics, such as the name, of the resins tested. and all i get is this youtube video. i guess that’s fine but it makes it less useful as an engineering resource…they’re putting the vicarious enjoyment of the methodology ahead of the material properties result. *shrug*

        that is to say, i testing is useful but don’t think this is the shining example of that utility.

        1. The video is below 10 minutes and not even fully monetized. Also this has nothing to do with intelligence. Where is he reaching the most of his audience, globally speaking? Right.

      2. From what I’ve read from people who works with resin 3d printing, all cheap, hobby-grade resins have so many additives to make them easy to use that they became very brittle, and that there are high-grade resins that could be up to the task. But again, that’s info that I acquired from people who claims to work in the industry, take it with a grain of salt.

        1. I’ve been fiddling with resin printing for about a year and in my experience the quality of the resin varies wildly.

          The inexpensive resins are very brittle and remind me of resin castings like Garden Gnomes, Christmas ornaments or those Hummel figurines. You can get insanely high detailed fine features but the prints shatter like sugar crystals. The unpainted parts also “oxidize” over time forming a patch powdery surface finish.

          The mid grade “tough” resins are similar to acrylic or lexan. They’re tough but you can still snap them. I’ve used this stuff to make GoPro cases and mounts that have held up really well.

          The price of the mid grade tough resin is two to three times the cost of the cheap stuff, so I’ve actually taken to mixing them in a 50:50 ratio. This is a neat feature of resin printing is you can customize the material to your needs. My 50:50 blend works out really well for custom cases, it’s tough enough. I’ve used this blend for Raspberry Pi module and custom keyboards with great success.

          1. Hows your mix handle oil and water? Most UV resins seem to love sucking one or other (maybe both) up and getting soft, goopy or deformed.

            Which is for me the biggest problem to anything mechanical with resin prints, yes the UV resins tend towards brittle (even the ‘tough’ ones) but you can design around that, its hard to design around them taking on odd shapes all on their own just because they were exposed to extra moist air, peoples grubby little fingers or the lubrication needed for the rest of the mechanisms.

            (I’m sure there must be some great resins for those jobs out there but the cost of experimenting when the data sheets are often not all that helpful it is somewhat off putting, and as it seems those problems only manifest after a fairly long time frame its not easy to test either.)

      3. I don’t mind articles like this, but it is frustrating that they didn’t include the type of resin specifically made for this purpose. We know that clear resin is hard and brittle; thats why all these companies also put out “tough” resin we well. Comparing just the brittle resin to PLA doesn’t really help in determining which process to use, it just presents a false choice.

  2. Excellent! I can’t help but think that resin might be a good option for building injection molds since it is a higher resolution and less prone to layering artifacts. You could injection mold parts if you wanted and then just keep the molds on file.

    I guess what I’m saying here is that injection molding seems like the next logical step to making mechanically stable components.

    1. Plastic has shit heat conductivity, which means long cycles. You can make very short run injection mold inserts out of plastic, but why wouldn’t you just CNC cut aluminum. It cuts like butter and lasts more then 3 shots. When anodized it’s hard enough to stand up to fiber filled plastic. If you can’t cut it on a 3 axis, you can’t get the part out of the mold either.

      Seriously, you’d be lucky to get one good shot out of plastic mold inserts. Likely break the thing with short shots as the machine starts up. Once up to temp, it would open gaps due to expansion vs. ejectors etc.

      1. Hobby SLA 3D printers with cheap resin produce molds that can withstand 50 to 100 cycles on a arbor style injection molder.
        Thats enough for short runs and test pieces before going to aluminium.

        Industrial, read electrohydraulic, injection machines can be tricky to be regulated to a low enough pressure to be used that often.

    1. PLA won, a fact that really surprised me until I read other comments about hobby grade resin being brittle.

      Agreed, it was a good video and I would put it on par with Thomas Sanladerer’s testing videos.

    1. PLA is hard, inelastic, has low deformation properties, is abrasion resistant, etc. ABS is less wear resistant, softer, has a higher elongation at break and virtually no natural surface lubricity. Nylon is even softer, has a lower heat deflection temperature, migrates under screw head pressure, but has more surface lubricity than PLA. Those properties make ABS and Nylon terrible stand-ins for something like this; at least the 3D printed versions. Injection Molded Nylon with some sort of stiffener (glass) would work pretty well, but printed Nylon generally sucks pretty hard for this kind of thing.

        1. A keyword search “3d printed gears strength” without the quotes on YouTube includes at least one about that. The channel CNC Kitchen has some excellent, properly done strength tests for various materials, printing methods/temps, and various attempts at strengthening completed prints, but not in most cases specifically about gears.

    2. To echo other reply, PLA is among the strongest thermoplastics available to hobbyists. It just has a low deformation temperature. If temperature is in the usable range though, you almost can’t do better than PLA.

      1. It also has horrible wear when used in gears, and is prone to snapping off teeth if loaded too much. especially if it gets wet. Hard nylon gears will just bend the teeth and skip a few, then keep going as if nothing happened, as long as it is printed correctly

        1. And that right there is a highly important point – steel,brass,nylon,abs (etc) all make great gears, but they don’t all make great gears for every use, sometimes its rust resistance so steels are largely ruled out, some times its making sure the gear is the wear part as the body its running in is much harder to make, and sometimes you just want that elasticity to cope with the overloaded condition (at least a few times up to a point) without lasting failure.

          No one material is always the right choice for gears, PLA for me is almost nevera good choice as it softens at such low temp, but then its cheap and stiff will be good for many things for it, and its annealable to improve that temperature resistance (though that is a great deal of work to do without warping – so probably better material choices can be made to skip this step).

  3. “Resin” is a huge category of materials that have a very wide range of properties. We use a variety of engineering-grade resins that aren’t brittle and have very high impact strength. But you dont even need to look at commercial level SLA machines to find resins that aren’t crap quality. Some examples: the formlabs standard resin has a tensile strength of 65MPa and elongation at break of 6%, which is equivalent to PLA.

    TLDR, choose the right material for the job. If you really needed to print a gear, you’d want something with higher lubricity and fracture toughness. A material like the FL durable resin typically won’t fail by fracturing, instead you’ll just get to a yield point.

    If I needed to make a custom gear profile in plastic, I’d print an SLA mold and cast it in a hard urethane.

  4. Please correct me if I am wrong, but what he is measuring is tensile strength and elasticity and we already know that pla has a higher tensile than most COTS resins based on the technical data sheets provided by the manufacturer. And I while I respect the scientific method, I think we could have predicted this out come based on known info.

    I think this test would have been more comparable if done vs ABS since the resin he is using shares more mechanical properties with ABS than PLA.

    1. Well, I am surprised that the PLA one survived. Yes, PLA is tough, but it also fails pretty rapidly when it gets warm. Which in my experience is the prime cause of failure in PLA gears, the teeth get hot and start to deform (at something like 45C already)

        1. I made a 2 stage 80:1 gear reduction out of PLA, for a force feedback steering wheel. It’s mainly stalled out most of the time, so it doesn’t get hot. It worked out pretty well. It has a relatively small sun gear, which is the highest stress part. Due to the overall constraints of the design, it only had 3 or so walls of plastic connecting it to the first stage. My first one failed by shearing off after 20 minutes of playing Dirt Rally. The second one I heat treated in packed sand at 210C, and it’s held up well.

          I’ve since changed the gearing to 40:1, which resulted in a larger sun gear that doesn’t have the thin wall limitation. It feels like a better ratio and will make my wheel better overall (I have plenty of motor torque, so the lower ratio will halve the affect of motor friction.) Unfortunately my first set of gears seem to be skipping teeth, so I haven’t gotten the wheel usable again and I need to tweak some tolerances and print another batch.

          Early on in this hobby project, I experimented with a 2 stage cycloidal gear reduction. I got it to spin, but it was way too high friction to be back driven. I spun my prototype up to high speed, and it melted itself to failure after about 30 seconds! :-)

          The goal of the wheel project itself was a full size steering wheel optimized for playing Euro Truck Simulator 2.

  5. touch of silicone grease for lube might help. Ideal involute spur gears are rolling friction, but they are made to incredibly close tolerances and even then a bit of sliding friction occurs at the contact point.

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