The Engineering Analysis Of Plastic-Dissolving Lubricant

Over the years, E3D has made a name for themselves as a manufacturer of very high-quality hotends for 3D printers and other printer ephemera. One of their more successful products is the Titan Extruder, a compact extruder for 3D printers that is mostly injection-molded plastic. The front piece of the Titan is a block of molded polycarbonate, a plastic that simply shouldn’t fail in its normal application of holding a few gears and bearings together. However, a few months back, reports of cracked polycarbonate started streaming in. This shouldn’t have happened, and necessitated a deep dive into the failure analysis of these extruders. Lucky for us, E3D is very good at doing engineering teardowns. The results of the BearingGate investigation are out, and it’s a lesson we can all learn from.

The first evidence of a problem with the Titan extruders came from users who reported cracking in the polycarbonate case where the bearing sits. The first suspect was incorrectly manufactured polycarbonate, perhaps an extruder that wasn’t purged, or an incorrect resin formulation during manufacturing. A few whacks with a hammer of each production run ruled out that possibility, so suspicion turned to the bearing itself.

After a few tests with various bearings, the culprit was found: in some of the bearings, the lubricant mixed with the polycarbonate to create a plastic-degrading toxic mixture. These results were verified by simply putting a piece of polycarbonate and the lubricant in a plastic bag. This test resulted in some seriously messed up plastic. Only some of the bearings E3D used caused this problem, a lesson for everyone to keep track of your supply chain and keep records of what parts went into products when.

The short-term fix for this problem is to replace the bearing in the Titan with IGUS solid polymer bushings. These bushings don’t need lubricant, and therefore are incapable of killing the polycarbonate shell. There are downsides to this solution, namely that the bushings need to be manufactured, and cause a slight increase in friction reducing the capability of the ‘pancake’ steppers E3D is using with this extruder.

The long-term solution for this problem is to move back to proper bearings, but changing the formulation of the polycarbonate part to something more chemical resistant. E3D settled on a polymer called Tritan from Eastman, a plastic with similar mechanical properties, but one that is much more chemically resistant. This does require a bit more up-front work than machining out a few bearings, but once E3D gets their Tritan parts in production, they will be able to move back to proper bearings with the right lubrication.

While this isn’t a story of exploding smartphones or other disastrous engineering failures, it is a great example of how your entire supply chain goes into making a product, and how one small change can ruin an entire product. This is real engineering right here, and we’re glad E3D finally figured out what was going on with those broken Titan extruders.

31 thoughts on “The Engineering Analysis Of Plastic-Dissolving Lubricant

  1. It’s stories like this that always terrify me when considering making the jump from benchtop hacks to releasing an actual product. There are so many ways to fail that are nearly impossible to control for that a motivated amateur would have to be crazy to jump into actual production without substantial capital reserves to deal with any issues down the road.

  2. another common cause of cracks in plastic is loctite. people use it on metal threaded inserts for vibration proofing but it can drip onto surrounding plastic bosses and the solvent in it will attack the plastic.

      1. We found exactly the same thing – something in loctite attacks polycarbonite. We designed a part with a cam profile machined in it, originally in aluminum. The aluminum was not durable enough for this part and would wear very fast, especially once the anodizing was knocked off. The replacement parts made from polycarbonite performed considerably better but we discovered that all forms of oils had to be avoided. Cutting oil in the CNC machine was bad so we changed to an air blast for cooling and chip evacuation. Tapping oil caused problems in the threaded hole so we changed to tapping in a water bath. And likewise, red loctite caused cracking around the threaded hole so we started using jam nuts instead. These changes resulted in parts that would last years instead of weeks.

  3. There is a lot of information about this kind of weakness of polymers available. I think the best search for this on google is “environmental stress cracking”. All this information might not help you much, because you might not be able to predict what kind of chemicals your product will be exposed to. Sometimes you might not believe that the chemicals will get in where they can hurt your product. And these “chemicals” can be something as ordinary as the grease you have on your fingers after eating a burger. As far as I know, the problem for polycarbonate is usually oils/greases. For acrylic (PMMA), you should be careful with alcohols. Remember to only clean the build platform with alcohol, if your printer has acrylic frame!
    In any case, the problem is a combination of mechanical stress and the chemicals. The mechanical stress can originate from fasteners, or in this case the bearing, causing stress/deformation on the plastic part. But stress can also be internal stress originating from the manufacturing process.

      1. I guess I tried to say too much in one single sentence…. Yes, keep the alcohol away from the 3D printer acrylic frame. In particular the area around the t-shaped cutouts with the nuts. The acrylic plates as such are not immediately damaged by alcohol. It is only a problem when there is also mechanical stress. And that is how it is with Environmental Stress Cracking: A combination of stress and chemicals.
        But the build platform should be cleaned with alcohol every now and then. At least that is what is recommended be the supplier of the ‘spidersheet’ I have on my build platform. And I’m told that acetone will damage the surface.

  4. “The long-term solution for this problem is to move back to proper bearings, but changing the formulation of the polycarbonate part to something more chemical resistant.”

    Why couldn’t the long-term solution just be to do everything the same, but always use a “plastic-safe” lubricant in 100% of the bearings?

    1. Supply chain problems, as mentioned in their own assessment on their site.
      I assume being 100% guaranteed to only receive bearings with the completely plastic safe grease turns out to be more trouble than using slightly more expensive but tougher plastic.

  5. Why not a brass bushing? Slow rotational speed, low rotational mass, low radial load and virtually zero axial load. It should be no problem for brass, and the stepper will have plenty of torque to overcome what tiny amount of friction the brass might have. It seems to me that the designer used ball bearings just to be able to say that it had ball bearings, rather than for good engineering reasons. Sometimes the simple solution is a good solution, and the flashy more complex solution has subtly hidden pitfalls.

    1. How about reading the article? They *are* using igus bushings in the short term until they can get the new tritan plates manufactured. But it’s a difficult part to machine, and has higher friction than the bearing, so their smallest motor doesn’t work with it.

  6. I’ve had similar problems with polycarbonate. I rebuilt an entire Lulzbot out of polycarbonate (had to be high temperature), only to have to replace everything with Ultem parts as the polycarbonate parts started stress cracking from the heat and various oils over a few months.

  7. Why not just use a different plastic instead of polycarbonate? Delrin or nylon, perhaps? Whole lot easier to use a plastic that isn’t wisely known to be susceptible to stress cracks.

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