Stronger 3D Prints — Glue Or Carbon Fiber?

[CNCKitchen], like many others, is looking to make strong 3D prints. Using a high tech PLA bio copolyester compound, he printed a bunch of hooks in two different orientations. He used several different types of glue including epoxy and superglue. You can see the video of his results, below.

In addition to the glue, he used epoxy and bulk carbon fiber, again, in two different orientations. After several days of curing, he was ready to test.

Untreated parts managed about 53 kg or not quite 25 kg, depending on their orientation. The thin superglue part got up to 58 kg. Viscous superglue didn’t do much better than the thin glue. Since the epoxy cracked before the plastic, there wasn’t much difference and in one orientation it was even weaker than the reference part.

You can get carbon fiber enhanced PLA, but you get short fibers. [Stefan] glued long pieces of carbon fiber exactly where he wanted them using epoxy. This method did provide some benefits.

Unfortunately, at least with the plastic used, none of the results were amazing. The carbon fiber technique bears more investigation, but even so, the results were — so far — not astonishing. However, this is a great application of the scientific method. Intuitively, adding some glue ought to make parts better, right? Testing like this shows that it doesn’t in this particular circumstance.

By the same token, you’d think getting superglue in your eyes would be a life-changing event. Apparently, not so much, although we still don’t recommend it. If you want to know more about glue — maybe more than you want to know — we deconstructed glue awhile ago.

31 thoughts on “Stronger 3D Prints — Glue Or Carbon Fiber?

    1. As long as you printed with reduced flow, to create lots of micro voids. I see that making super strong parts. That is definitely a good idea to try out. A very unique way to 3d print a composite material. Could even use carbon fiber based filament and vacuum impregnate epoxy resin to make a psuedo carbon fiber part of normally impossible dimensions.

  1. I dont know why the author concludes that the results were lackluster- the part clearly failed in a new location, with no damage to the reinforced section. This suggests (as he states in the video) that the carbon fibre likely improved strength significantly, but his test setup was not able to quantify it. Also, did I miss it, or did he not do a test with CF reinforcing on a part printed laying down? Why is this the only one missing when it would be the most interesting result…

    1. I’m pretty sure that was the one you’re talking about. The one printed laying down had the CF strip glued vertically, while the one printed standing up had the CF strip wrapped around.

  2. This does bring up an interesting question, what if you had an extruder head where you could inject something into the stream just as the plastic left the tip? This would have the benefit of glue in every layer. Of it would totally screw up the plastic adhesion.

    The glue would have to be UV curable otherwise the thing would plug up between every print job, and I suppose you’d need some significant pressure in the glue system such that the plastic didn’t back up the glue tube.


    1. Not quite what you’re asking for, but take a look at Markforged printers. They use two nozzles, one for the build material (primarily “Onyx” which is a nylon + chopped carbon fiber composite) and a second nozzle that puts down continuous strands of carbon fiber (or glass fiber, or kevlar).

      1. How about 1 nozzle to put down the plastic and the other to pour out resin (UV or??) ? The resin only gets added every few layers and drips down into voids. Akin to the plastic being the form and rebar with the resin being concrete.

    1. We do they’re called resin printers ;)

      Most fdm printers will leave gaps in a print. Air is a poor structural element. It’s not beyond reason that adding a polymer (glue) in the air gaps might add strength. But that needs to be tested, like cnc kitchen did.

      Homeopathy’s situation differs because its adherents are unwilling to accept the experimental results.

  3. Simple coating or soaking aside, if you’re going to turn it into a multistage process, you may as well just print a negative mould and lay fibre and epoxy into it from the get go.

      1. I was thinking that magnetic particles could be adhered to fibers, with a whopping electromagnet in the printer. The fibers would be aligned the same way regardless of how they came out of the nozzle, then the colling fan would cool the extrusion and lock fibers in place, but not before they scraped up against the adjacent layer, adding cross-extrusion sturctural components.

    1. It’s really a sad tale of our time that we can no longer communicate effectively in the modes that really made civilisation kick off. Written word.

      Not only is this a decline in the ability of people to communicate using text alone but also read and comprehend text alone without extravagant videos to tell the story.

      Videos are great for many things but it’s become our only tool even when it’s a terrible idea. Mostly videos greatly devalue the time of the recipient of information because we need DEM CLICKS.

    2. Agreed. I usually skip the videos on HaD posts; people doing Youtube videos seem more interested in getting attention than altruistically sharing their knowledge.
      It is nice that HaD writers like Al spend their time watching the videos and then writing a summary so that we don’t have to endure the video.

  4. So the marketing behind epoxy has kind of ruined everyone’s perception of what it can bond – but epoxy actually does not meaningfully form chemical bonds with most everyday plastics (acrylic, polyethylene, polypropylene, polystyrene, ABS [depends on adhesion promoters/ABS blend]). Often times it is just so stiff that minor noncovalent interactions and surface texture make it seem to “stick,” but under any meaningful load it will break. It is much better for metals and maybe bonds where there is very high bond surface area for the required load, or where there is a mechanical lock into the epoxy itself.

    PLA has very few to no real sites that can react with standard epoxy. Instead, acrylate adhesives typically work much better on most plastics. This includes superglue (cyanoacrylate), of course, which explains the noticeable strength increase, and two part acrylate adhesives (usually marketed as “plastic bonder”). It smells a heck of a lot worse (that’s usually how you can tell it’s not epoxy), and unfortunately is higher viscosity, but it will actually allow carbon fiber reinforcements to properly reinforce PLA. For some plastics, some primer is necessary, but this adhesive system can actually covalently bond most plastics (though like acrylic, the bulk acrylate polymer doesn’t have great impact resistance), unlike epoxy.

    Again, epoxy DOES NOT BOND most plastics. It’s the equivalent of using a rigid hot melt adhesive in many cases.

    1. Picking a mixing cup or board for epoxy: “I’ll just use this plastic tub or sheet, it will easily crack off when dry if I want to use it again…”
      Picking a glue for plastics: “Epoxy, no thought required!”

  5. I can apreciate the effort that [CNCkitchen] puts in his many video’s of testing strength of 3D printed parts, even though I’m not very much interested in 3D printing myself because of general lack of strength and surface quality. I’m more into old fashinoned CNC-ing of metal parts.

    Some remarks though:
    If you want to add real strength with a bit of carbon (or glass) fibre, then you have to apply it to at least several cm beyond the area where stress is introduced. In this case that means all around the right side of the test part. Even with only 10% of the fibres used in the video that will give a significant difference.

    The easiest way to add more strength (except from using different filament) is probably to add some hollows into the test part and insert an all-thread with some nuts at the end, which take up all the tension, and the plastic only is loaded in compression.

    Combining the 2 above:
    Adding some channels into the test piece in which some carbon fibres can be inserted and glued.
    The way the test piece is done in the video is nice for demonstrations, but too much work for regular printing.
    How about printing slight indents in the side of the test piece with some hooks at the top and bottom. A thin section of carbon (or other) fibre can than be easily wound between the “hooks” and held in place untill the epoxy dries.

  6. If you think that’s cool, this will really blow your mind… there have been sightings of 3D printers in the wild that lay down polymers cross linked with lignin, and, this is the coolest bit, the feedstock is CO2 and water!!

  7. Carbon fiber is one material I have no desire to mess with. Even fiberglass I won’t use at home if I don’t have to, despite it being one of my very favorite materials.

    I’m looking for a reason to try Kevlar reinforced plastic though. The fibers are a lot less stabby, at least at a macro scale. I should read up on if they’re truly safe though.

    1. Kevlar is a pain. Carbon fiber makes nasty dust, but it’s way easier to work with as the fibers are more fragile.. Kevlar is difficult to cut, and a hell to sand if you aim to a nice finish, not a furry one.. I would use Kevlar only where I’m sure I don’t have to touch it again for finishing.

      1. Thanks for the tips!!

        I’m a big fan of separating the aesthetic and functional parts, so a nice finish isn’t much of a concern, It’s a lot easier to just design a nice case than to try to make parts that work well and look good at the same time.

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