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I'm not an expert in this field.

My assumption is that there are plastics lighter and stronger than the materials used to build planes. So that would make plane lighter and thus would consume less fuel which pollutes less and so on.

Maybe it would be more maneuverable, go faster, be safer as you may be able to safely land it with a number of parachutes in case of engine failure for example.

If these are true, why are planes not built from such a lightweight plastic?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Sep 5 at 12:42
  • $\begingroup$ While there are plenty of answers explaining it very well let me say that Additive manufacturing (ADM) is starting to pick up quite strong in the aerospace industry for different parts of airplanes. ADM is basically 3D printed metal. $\endgroup$ – Ander Biguri Sep 18 at 12:07
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My assumption is that there are plastics lighter and stronger than the materials used to build planes.

That is not a correct assumption.

Typical 3D printer plastics have a best-case tensile strength of 45-50 MPa.
Aluminum 7075, a common aerospace alloy, has a tensile strength of 500-570 MPa.

After dividing by specific gravity, that's at a 1:4-1:5 ratio of specific strength in favor of metal. There is no significant application in which 3D printer plastics, the properties of which are driven by viscosity, adhesion, and other printing qualities, would offer better strength-to-weight than aerospace metals and fiber-reinforced composites.

Some amount of 3D-printed plastics will likely appear in cabin interiors, for low-volume parts or complex hollowed-out parts, not subjected to significant stresses. But load-bearing parts require high strength and high strength-to-weight ratio. If it's too heavy, it won't take off, and if the material's too weak, it won't stay in one piece.

So the brief answer is, they don't build airplanes from 3D printer plastic (or clay, or plaster, or thatch) because they want them to fly.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Sep 5 at 12:41
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Let's take ABS, which is an extremely common plastic used for 3D Printing. At a typical flight altitude, the exterior air temperature will be in the order of -51°C/-60°F. The lowest rated temperature for ABS is -20°C. I hope you can see why this alone might be a problem.

Wikipedia also says that ABS and PLA, which is the other major 3D printing plastic, are damaged by sunlight. Planes generally see a lot of sunlight.

Also, in regards to "you may be able to safely land it with a number of parachutes in case of engine fail", two quick points:

  1. although plastic is less dense than aluminium, it's not as if the plane is suddenly going to be that much lighter
  2. planes are decent at gliding in the unlikely situation of all engines failing

More information about why parachutes don't make sense can be found at Why don't big commercial planes have full aircraft parachutes?.

In case you weren't aware, the Boeing 787 fuselage is principally made up not of aluminium, but of a carbon fibre reinforced polymer, which is a material based off of plastic. So there is work going on to make sure planes out of different materials, but it's not particularly simple or straightforward.

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    $\begingroup$ 3D printed ABS is much weaker than machined or molded ABS. $\endgroup$ – Eric Shain Sep 2 at 2:12
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    $\begingroup$ @EricShain making the case for using it even weaker... $\endgroup$ – jwenting Sep 2 at 3:30
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    $\begingroup$ Why would you 3D print in the first place? It’s great for prototyping or small production runs, but for mass production it’s too slow and therefore expensive. $\endgroup$ – Michael Sep 2 at 7:09
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    $\begingroup$ @Michael the main use cases are complex geometries where AM allows to slowly produce a monopart where conventional techniques would require an assembly, for example rocket engine nozzles $\endgroup$ – AEhere supports Monica Sep 2 at 7:25
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    $\begingroup$ @Michael Aircraft manufacture is small-run. That's why 3D printing is commonly used in the industry for interiors, ducting, manufacturing jigs, etc. Just not for structural parts (yet). $\endgroup$ – Dan Hulme Sep 2 at 8:30
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There's a few things going on here.

You assume plastics are stronger and lighter than the metals and composites used for aircraft manufacture today. Therac's answer counters this assumption (spoiler: plastics are much weaker and less stiff), but there's a subtlety. In many applications, 3D printed plastics can be made stronger or stiffer for the same weight, because complex shapes (that can't be machined from metal) can be printed hollow. It's the same kind of advantage you get by replacing a solid beam with an I-beam or truss, but workable for smaller scales and more complex geometries. But the structural parts of an aircraft are typically the skin, which is made of thin sheets, and the ribs and other members, which are already made with trusses, I-beams, and other geometry optimizations, so it's harder to make up that difference.

Even though you wouldn't want to build the airframe or engines from plastic (whether 3D printed or manufactured by another technique), there are a lot of plastic parts in today's airliners, and many of these are 3D printed because it's cheaper for the size of production run than creating the tooling necessary to injection-mould them. 3D printing is also increasingly used for spares or modifications for end-of-life aircraft types where original parts are scarce or expensive. Longer-term, there's the potential to drastically reduce the need to distribute spare parts to all the places your aircraft are. Right now, if you run an airline and you fly into some regional airport, you have to trade off the risk of an aircraft being stranded there lacking some spare part vs the cost of keeping an inventory of spares on that site. This isn't just flight-critical spares: say all the toilet seats on your aircraft break, and you don't have any spares on-site, you might need to cancel flights and fly your aircraft empty to a "hub" where you have the spares, which is very costly.

Getting 3D-printed parts (or any other method of manufacture) certified is a long process, because they need to be safe, reproducible, and traceable. The printer needs to guarantee that every part is within tolerance, and each part needs to be traceable back to the original plastic shipment it was made from - so that the correct aircraft can be grounded, in the unlikely event that a bad batch makes it through inspection. Only certain materials can be used for interiors, because they need to be tested to ensure they don't release toxic smoke if there's a fire. This testing isn't to the level of "PEEK is fine", but "this particular brand of PEEK filament, made via this particular process in this factory, is fine".

Despite this long path to certification, the two biggest aircraft OEMs are shipping aircraft today with hundreds of 3D-printed components.

Although you specifically mention 3D printing with plastics, 3D printing with metals is a growing field. Some boutique automotive companies are using 3D-printed titanium engine parts, because they can print structures that aren't achievable with machining, to reduce weight. Although these techniques are less mature - getting the dimensions right is still a challenge - metal printing of non-structural aircraft parts is already starting to happen.

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As other answers mention, the strength of molded or printed plastics is an order of magnitude lower than that of typical aerospace metals. But also the stiffness is much lower. Compare Al 7075, which Therac's answer mentions, with Stratasys' certified Ultem 9085 thermoplastic. The aluminium has an elastic modulus around 70 GPa, while the Ultem's elastic modulus is 2–2.6 GPa (depending on how it's printed).

We once tried to build a wind tunnel model in a 3D-printer. Looked nice when it was finished. But when it was subjected to the loads in the tunnel, it warped out of shape horribly. The results were unusable. A 3D-printed wing constructed like a conventional wing today would not only break, but before doing so would warp and twist completely out of shape.

Another weakness is UV sensitivity. While metals can withstand years of intense solar radiation unharmed, the bonds in polymers suffer form the high energy of UV rays (PVC is an exception but has got an undeserved poor reputation), so any 3D-printed surface will decay in the open. Protective coatings are only of temporary help and add weight.

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    $\begingroup$ Was the structure ABS as well? I've had decent results using 3D printed ribs and panels over a composite structure. $\endgroup$ – AEhere supports Monica Sep 2 at 7:29
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    $\begingroup$ @AEhere: I think your composites provided the stiffness. The 3D model was from UV curing resin. If you restrict the printed parts to ribs and secondary structure, 3D printing is no problem. $\endgroup$ – Peter Kämpf Sep 2 at 11:53
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    $\begingroup$ Well, yeah, I specifically disregarded the aeroshell since it was an unknown quantity with the printing quality I had and sized the spar to match. I ran the numbers on an ABS spar and it would have been massive to hold up. Haven't worked with UV resin but from what i have read it is not too far removed from ABS or PLA. I guess my question should have been if the structure was not strong enough or if stuff like the leading edge bucked in due to dynamic pressure. $\endgroup$ – AEhere supports Monica Sep 2 at 12:03
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The plastics used for airplanes are a lot like reinforced concrete, where both high compression and tensile strength is needed. The plastic compound itself, polysester, vinylester, or most commonly, epoxy resin, provides the compression strength and stabilizes the fibre component, like the concrete, and the fibre component, either glass or carbon, provides most of the tensile strength, more or less like the rebar in concrete.

Like a reinforced concrete structure, the problem becomes one of how to or orient the fibre component so that the fibres can continuously carry the tensile loads. You can see right away that a resin compound by itself won't work for a high stress part; you have to have a tensile load bearing element embedded in the resin, and this load bearing element should be more or less continuous along the load path.

Random fibre segments in a resin matrix, like the chopped fibre fibreglass used in boats, won't do for something like a highly stressed beam. The fibres have to be continuous from end to end, again, a lot like a reinforced concrete beam. So this tends to rule out a process where a 3D printer could deposit resin and fibres at the same time.

It is possible to make certain aircraft parts from 3D printed plastic where the plastic by itself is replacing, say, an aluminum casting and the plastic resin is as strong, has the required hardness, and can handle the temperatures. Currently such parts would most likely be made by injection molding, 3D printing being so new. But you will certainly see lower stress casting-equivalent parts start to emerge in aviation by 3D printing, especially for low volume parts where the process is just crying for a viable application and a certifiable process. It's a conservative industry, so you have to give it time.

The challenge for now is how to make a resin matrix part, that needs high tensile strength, that can somehow be 3D printed with both the compression and tensile strength elements incorporated, and correctly oriented in the 3D print process. Not so easy.

What will probably happen in the next 10 years is someone will come up with a radical new plastic compound incorporating something like graphene in it that has all the desired properties in all directions and can be machined from a block or deposited and cured in a printing process. Then we'll have 3D printed wing spars, frames and skins.

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    $\begingroup$ Even the compression strength is mainly delivered by the fibers. The polymer matrix only helps to keep the fibers in place (prevent buckling) and transfer loads into and out of the fibers by shear. $\endgroup$ – Peter Kämpf Sep 2 at 5:09
  • $\begingroup$ Yes where the fibres are straight as with unidirectional glass roving as used for spar caps and pull-truded products like Graphlite. Graphlite gets its incredible compression strength from having the carbon fibres perfectly straight so the resin has an easy job of stabilizing them. But I would say that in a component made from glass or carbon cloth where the fibres are wavy, already partly buckled you might say, not so much. $\endgroup$ – John K Sep 2 at 15:36
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    $\begingroup$ "The fibres have to be continuous from end to end, again, a lot like a reinforced concrete beam. So this tends to rule out a process where a 3D printer could deposit resin and fibres at the same time." You can buy 3D printers that lay down continuous carbon, glass, or Kevlar fibres end-to-end inside their filament. $\endgroup$ – Dan Hulme Sep 3 at 7:38
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    $\begingroup$ Wow. Well I guess it's coming down to development of a suitable resin that can be deposited, and size? $\endgroup$ – John K Sep 3 at 11:45
  • $\begingroup$ Check out Markforged. They build FDM printers that can also lay carbon fibers in the structure. $\endgroup$ – edgerunner Sep 4 at 5:53
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A short answer is that there are 3D printed planes in the RC model world.

However, they are heavier and more fragile than traditional materials, so they aren't common. What they are good at is producing a complex shape with lots of detail without expensive tooling.

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Aviation is all about materials science

That is the only thing that kept Leonardo Da Vinci on the ground. He was on the right track; If he had access to fiberglass-epoxy and a Lycoming engine, he would have had no trouble building an airplane.

Even in 1800 when the wood and sail technology was coming along, the metallurgy wasn't good enough to make an engine light enough. Stevenson was showing Watt that metallurgy was good enough to build smaller, faster 20 horsepower steam engines that could fit in a room instead of a house, but "faster" was a relative word.

The Achilles' heel of 3-D printing is materials strength. That is why we don't 3-D print cylinder heads or hinges, and why it hasn't taken over the world.

3-D printing simply cannot accommodate aviaton-strength materials... Yet.

You could build an airplane, but to have enough strength to fly, it would be much too heavy to fly.

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    $\begingroup$ We actually do 3D print fuel nozzles for the world's most powerful commercial jet engine. GE also has a turboprop core that is constructed largely of 3D printed parts. Of course, to your answer's point, these are much fancier 3D printing materials than what you'll find in the average hobbyist's 3D printer. $\endgroup$ – reirab Sep 3 at 16:32
  • $\begingroup$ Your last two paragraphs are way out of date. There are aviation strength materials that can be 3D printed and I’m not just talking a few gimmick parts. There are major structural components being printed for aero engines currently in development and near production readiness. That technology exists and is extremely close to being production level. $\endgroup$ – Notts90 Sep 4 at 20:57
  • $\begingroup$ @Notts90 I don't disbelieve you, but keep in mind OP said "airplane" not "some components which lend themselves". How far out are we from 3-D printing a wing or fuselage? Or let's drop the difficulty a bit, how about a vertical stabilizer? I am not naysaying you, I expect there may be a real answer there, and I would even suggest expanding it into an answer. $\endgroup$ – Harper - Reinstate Monica Sep 4 at 22:19
  • $\begingroup$ @Harper In my head I stuck the word “components” On the end as it made more sense than having a giant 3D printer. A key benefit of 3D printing in engines is being able to make complex parts that have historically been assembled from various components in a single piece. Can be stronger for less weight. I don’t mean normal things that happen to be in an engine, I mean major structural components specific to a particular engine and I believe blades will be next too. I’d write my own answer but it’s be too antidotal. $\endgroup$ – Notts90 Sep 5 at 6:37
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If we were to drop "plastic", the answer would be yes, Boeing is using 3D printing in the 787 Dreamliner. These are not plastic however but titanium. Somewhat stronger than your run-of-the-mill ABS plastic. From https://aerospaceamerica.aiaa.org/departments/making-3d-printed-parts-for-boeing-787s/

33-centimeter-long titanium fittings that anchor the floor of the aft kitchen galley to the 787 airframe and bear structural stresses

this also mentions the fuel nozzles of the GEnx engines are 3D printed by fusing metal powder with lasers.

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They certainly do. My airplane is built 100% out of 3d- printed plastic. It's really handy. Everything is easily replaceable. Crashes are no longer any concern. I've saved a template of my cranium too, just in case it might someday be need to be replaced.

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