Reinforced FDM System

Onyx Pro (Gen 2 - No WiFi) | Markforged

TEAM Lab Equipment: FDM 3D Printing

Overview of Reinforced FDM Printing:

Reinforced FDM 3D Printing is an extension of standard FDM printing (see documentation on FDM
printing for details). The primary differences being:

  1. The primary build material is a composite consisting of chopped carbon embedded in the bulk plastic (Nylon), to give us “Onyx” in place of PLA, ABS, etc.
  2. (Optional) continuous bands of fiber filament (typically carbon fiber) embedded in the above “bulk” printer material.

It’s printing resolution is generally also somewhat better than standard FDM; Thinner layers, and
higher-precision machine mechanics generally yield a +/- 100 micron accuracy.

Vs. FDM

Reinforced FDM is more-or-less the same as FDM printing, with the addition of continuous bands of
fiber filament for increased stiffness and strength. Therefore, FDM printing is an advanced version of
FDM printing.

Vs. SLA

In comparison to SLA printing, FDM parts are quite limited in shape complexity, but are typically
stronger and less expensive.

Vs. Polyjet

In comparison to Polyjet printing, FDM parts are far stronger, and far cheaper, but are also significantly
less accurate. Again, FDM parts are also far more limited in shape complexity when compared to Polyjet
printing.

Aesthetically, these prints tend to appear very “finished” looking (layers nearly invisible) when
compared to standard FDM printing, due to their very thin layers, and the machine’s high-precision
movement mechanics. Coating these parts with special brush-on epoxy coatings (XTC-3D) will also likely further enhance this property.

Materials:

Our reinforced FDM printer uses a proprietary material known as “Onyx” as its primary build material.
In addition to this material, we can add secondary bands of continuous fibers.

Primary Build Material:
Onyx – A Nylon with embedded chopped carbon-fiber. Proprietary to Markforged, Inc.
Reinforcement:
Optional strands of 0.12mm diameter filament:

  • Carbon Fiber
  • Kevlar
  • High Strength High Temperature HSHT Fiberglass
  • Fiberglass

Techniques:

Designing for Reinforced FDM Printing:

As with standard FDM printing, overhanging features often necessitate the existence support structure
(made of build-material) which must often be mechanically removed. In this case, expect occasional
print defects in these locations (rough finish, occasionally drooping, etc). Through careful design, often
times support structure can be avoided (ideal scenario). Support structures are covered in more detail
below.

Hollow Prints - One important consideration for FDM prints – parts derived from this technology are
rarely solid; instead, they are generally created in a series of “shells” extending from the outside surface
inward, coupled with an “infill” to provide bulk-support. This process is achieved within the
printing/slicing software, not within CAD.

2shell50%infill

 

Figure 1: Zoomed Depiction of 3D Print Cross-Section

Reinforcement Bands – To increase stiffness, and strength of parts, we often have the option of adding
strands of reinforcement fiber, planar to the build-axis during printing. As carbon-fiber is an expensive
resource, this action is typically limited to ~8 total layers, distributed across a part; however, more can be
added as necessary. Note that minimum wall thicknesses must be maintained in order to “fit” traces of
fiber reinforcement.

infill
shell

Adding Threads – Threads are generally not directly printed onto parts, but are instead added as a
secondary process by way of tapping. Exceptions may occur where two bodies are designed to
couple/decouple by way of coarse threads. Other (superior) techniques for adding threads involve using
threaded inserts (usually brass), or capturing a nut; both of these techniques result in much stronger
threads than a printed part alone!
For more details, see: https://markforged.com/resources/videos/the-strongest-thread

threads


Distortion:

Due to thermal contraction, all FDM prints are prone to distortion from their intended geometry. The
following parameters are major factors in this process:

Aspect ratio – As a part becomes long, relative to its width, thermal distortion is more likely (bowing).
Occasionally, this distortion can become so bad that prints fail due to the nozzle knocking against the
print, causing it to detach from the built platform. It’s generally recommended to limit prints to a 5:1
length to width ratio (or smaller).

aspectratio1

 

Figure 2: Intended Part with Large Aspect Ratio (via Markforged)

figure3 distortion

 

Figure 3 Resultant Part is Warped (via Markforged)

Wall Thickness/Shell Count – While thick walls result in stronger parts, too many “shells” can result in
print defects that lead to poor surface finish, or warping. Typically, we use 2 shells for light-duty prints,
3 for heavy-duty, and 4 for instances where holes may be tapped (threads added). Note: Actual wall
thickness (dimension) is a multiple of the Nozzle diameter (typically 0.4mm), aka 1 “shell” thickness.