[Florin_Andrei]

> The strength of high-quality carbon fibre comes from having many pieces of fibre woven together over the surface omnidirectionally.

No, not at all.

The strength of CFRP (carbon fiber reinforced polymer) comes from having very long strands of carbon fiber, ideally as long as the whole piece, held together by some polymer matrix such as epoxy resin. It also comes from having a very high fiber-to-epoxy ratio in the final product, which is usually achieved by squeezing out all the extra epoxy using vacuum while curing.

The length of the fibers, and the minimal amount of epoxy, is what makes CF strong. Short fibers, and having too much epoxy matrix, weakens the composite.

There is a kind of extruded CF where short CF fibers are mixed with epoxy matrix and extruded in the desired shape. This is strong compared to ordinary plastic, but not as strong as long-fiber CF.

What you're describing is woven CF, and it's not the strongest there is. The strongest CF pieces are made of unidirectional CF, where all fibers are oriented in the direction of the main effort. E.g., CF tubes are strongest when they are made of unidirectional CF, with fibers as long as the whole tube.

Woven CF is a good compromise in that it's reasonably strong in several directions in the plane of the CF cloth. Also, for CF tubes, an outer layer of woven CF gives it a bit more resistance to splintering.

Another way to achieve multi-directional strength is by laying unidirectional cloth alternatively in different directions. The piece will gain strength from each unidirectional layer in a specific direction.

Most people recognize the classic "CF look" only when the top layer is woven. Unidirectional CF cloth has a different look. This is why many CF items are made of unidirectional cloth, with a single woven layer on top.

If this CF 3D printer can lay long-fiber CF, and can achieve a very high fiber-to-epoxy ratio in the final product, then chances are the pieces produced this way will be strong.

[Timmmmbob]

> The strongest CF pieces are made of unidirectional CF, where all fibers are oriented in the direction of the main effort.

You're confusing issues even further! To anyone who wants to actually understand this:

Carbon fibre doesn't have a single "strength" value since it is virtually always anisotropic - its strength varies massively depending on which direction you stress it in.

GP is correct in that the carbon fibre weave with the best minimum strength is the 3D woven stuff which is very fancy and difficult to make, and not what this printer makes.

The most common carbon fibre is 2D woven cloth which is laminated together like plywood. It is strong in the directions of the fibres but can very easily delaminate (the layers become unstuck). It's a pretty big problem for things like the Boeing Dreamliner because the delaminations can be under the surface and impossible to see.

CFRP tubes are often made with the fibres all running along the axis of the tube, but it is then extremely weak in the circumferential direction and will tend to split like bamboo.

[marvin]

Delaminated Dreamliner sounds like a maintenance person's worst nightmare. Wouldn't you have to replace the entire part? Where "part" is wing, rudder or fuselage.

[a-priori]

Replacing a defective part is no big deal, just incorporate an inspection of the part into a regular maintenance window every X cycles (a 'cycle' is usually one flight) and model its cost as an amortized per-cycle cost based on its mean time to failure.

The problem is when it's difficult to know whether a part is defective. Something like subsurface delamination might cause visible bubbling or warping, but if it's deep enough then it may only be apparent on an ultrasound or X-ray scan. That sort of scan might be more expensive than just replacing the part regularly, and shipping it off to a factory to be inspected and refurbished.

That is a maintenance technician's worst nightmare: an expensive and bulky part that fails in invisible ways, requiring either regular replacement or time-consuming inspection with expensive equipment.

[na85]

Delaminated Dreamliner sounds like a Fedora or Ubuntu release codename.

[sbierwagen]

From the product page: http://markforged.com/ (This HN submission is blogspam)

  The incredible strength of carbon fiber comes from the 
  long, continuous strands that carry load down the entire 
  part. This is why space shuttles, rockets, and Formula 1 
  cars are constructed from continuous strand carbon.  And 
  it’s how we print.  Don’t settle for plastic with a dash of 
  chopped carbon fill. Longer is stronger.
  -
  7mm x 3mm x 100mm.  3D Printed beam is packed with tens of 
  thousands of full length, continuous carbon fiber strands

Conventional thermoplastic FDM printing has a lot of design constraints: no overhangs, voids, etc. Looking at this, it seems like it takes those design constraints, and adds some more: the fibre is aligned in the direction of the print head! This is going to take some really tricky modelling work to get it to print what you want, with the direction of strength actually oriented in the direction you want.

And how do you even cure it? Is it just carbon fibre in a thermoplastic matrix?

[Retric]

It depends on the load, for a bullet poof vest you need to have woven threads, for a cable it's less of an issue but weaving still helps once the threads start breaking.