That is interesting… maybe it does actually have some kind of sensor fitted because they knew we would try to do exactly what you are attempting to do… would definitely be interesting to find out the response from MarkForged… keep us updated.
Perhaps you really should try surreptitiously switching the labels on the filament spools
Carbon Fiber is too stiff to make the sharp turns that the machine can make with either fiber glass or Kevlar. Hence the reduced options in the Carbon Fiber menu.
I was a bit disappointed by this as well with my Mark One. Pretty sure the limitation is due to the brittle nature of carbon fiber. Dig into you composites text books. You will find that there is a minimum radius of curvature that you can bend a brittle fiber around before it breaks. It is a function of the individual fiber diameter, modulus, and strain capability of the fiber. Markforged does not really tell you what the exact fiber they use. So exact fiber modulus, fiber diameter, strain capability are hard to come by. Isotropic Fiber mode has to bend the fiber around a very sharp radii after each pass. The fibers would likely break at every pass and jam up the extruder. Looks like the Mark Two can run in Isotropic Fiber mode to put down discrete fiber orientations. I think the Mark Two keeps the chamber hotter which might help keep the tow bundle from breaking. They also put the fiber cutter much closer to the extruder so the minimum cut area is 15x smaller per their literature. The cutter location distance to the extruder on the Mark One dictates the minimum filament length that has to be put down. The Mark Two might be able to cut the fiber at each pass before it changes direction eliminating the need to bend the filament. Haven’t seen the Mark Two in action yet, so I am only hypothesizing.
Well said, this is by far the best comment on the thread. I would only add an extra tidbit. The Mark one has a distance of just over 600mm between the cutter and the extruder. I believe the distance is 45mm on the Mark two.
That 3DXMax stuff is basically Nylon loaded with carbon fiber dust. It won’t even come close to the structural performance (stiffness and strength) that the Mark One or Two can deliver with continuous fiber. There is actually a fiber cutter in the Mark One and Two to cut the fibers.
They are all pretty much carbon fibre dust… including the Markforged filament… by the time the filament has been heated up and forced out through the nozzle its just a ‘mush’ anyway so not actually sure what your point is here?
…I do have to add that you can always rely on you Yanks to try and knock us Brits off our Limey ‘high horses’… always makes me chuckle… bless you… you rarely succeed of course but don’t stop trying cos we love you ‘lot’ and always have done… trouble is in general we always seem to be a couple of years behind you though… :):)
The voids did not surprise me. I think you would find significant porosity in most any FDM part under the microscope. Would have been interesting to also see a more zoomed out view. Guessing there would be significantly more porosity between the courses for each fiber bundle. That level of porosity wouldn’t impact the performance all that much parallel to the fiber direction. 2-3% porosity is usually kind of a magic number where the matrix dominated properties (shear and transverse strength). Eyeballing those photos look to be less than 5% not bad for a process without an autoclave or press.
Failure at tabbed areas of the thicker coupons probably has more to do with how they chose and prepped the tab material. Coupon alignment in the load frame and the stress discontinuities where the tabs terminate will also cause low failures. Add to that grip damage and their higher variability in the 6 ply coupon probably has more to do with the test than with the material. Even on the low end the ply modulus and strength were significantly higher than say 6061-T651 (aluminium to you). Average modulus was about double 6061. The strength ranged from 370 to 520MPa. That is much higher than the yield strength of 6061 at 276MPa. You could still probably match the stiffness in laminate form with multiple ply angles. Strength might be a bit less in the laminate but that would really depend on the ply orientation stacking sequence. So their marketing material stating you can get stiffness similar to aluminum is still accurate. I am still holding out hope that Markforged can update the settings on the Mark One to allow multiple ply orientations with Carbon. I hope you were not under the illusion that printing with the Mark One/Two would give you isotropic performance like a metal. No composite can do that. The properties are always planar and highly tailorable.
As to structural guarantees for parts that I print on 3D HUBS, I make none. I do not offer structural analysis service. It’s up to the customer to proof test their own design. I just give advice on whether their design will be printable or not. 3D HUBS is a nice way to help offset the cost of a very expensive hobby.
