The key to unlocking strength in continuous fiber 3D printing is by understanding where you can leverage strands or panels such that they distribute the loading forces in tension or bending, as we covered in the Physics of 3D Printing. Below, we share a few different methods for identifying and placing fiber within your part to provide strength where you need it.

How to think about reinforcing with continuous fibers

Designing for fiber reinforcement is similar to designing for basic 3D printing, with added thought toward strength needs.

1. Identify loading conditions

Take a look at your design. Where will it undergo bending forces? Tensile forces? Compressive forces? If you’re not sure, think about how forces will transmit through other parts — draw a diagram if you need to! This will help you to make an informed decision about your fiber routing strategy.

2. Determine print orientation

In which directions are the largest loads traveling in on your part? You want your part to be oriented such that these forces largely travel in plane with the print bed to load fibers in bending or tension. If you have many large forces spanning multiple axes, you may want to consider modifying the design or splitting it up into a few parts.

3. Determine reinforcement areas

Based on loading conditions, what surfaces or segments need to be strengthened? With that in mind, think about what types of reinforcement you will need to implement in those areas.

4. Balance fiber panels

‍If only one side of a part is reinforced, it may be prone to warping due to an uneven sandwich panel, which happens when one face is strengthened and the other isn’t, or if one face has a vastly different cross-section than another. If one layer group of your part is reinforced, balance the sandwich panel by reinforcing an equivalent layer group on the furthest substantial Z layer with a similar cross section.

5. Confirm fiber pathing

Does fiber fit and travel through the areas you need it to? Can you trace continuous strands of fiber that route along the load paths and “brace” against the force? If not, you may need to adjust the fiber settings or modify features so that fiber runs through the places you need it to. Remember that fiber groups require at least four plastic roof and floor layers to print, so any faces that need reinforcement must start four layers offset from the closest roof or floor.

Basic reinforcement strategy: Shelling

Described below is a basic strategy for reinforcing a printed part. This strategy will ensure your part is generally strong and resistant to bending and impact forces on any axis. As described earlier, it’s more important to reinforce the extremes of your part than the core, so we are going to walk you through how to “shell” a part for efficient strength all around.

Shelling is a surefire way to reinforce any 3D printed part with continuous fiber, because it protects from most loading conditions.

1. Isotropic panels on furthest substantial Z layers

To maximize bending strength, create a sandwich panel with 2-4 layers of isotropic fiber on the top and bottom planes of the part, excluding any small surface extrusions. The fiber layers should start above the four “floor” layers or end below the four “roof” layers of a given horizontal surface.

2. Isotropic panels on intermediary large geometry changes

Add 2-4 layers of isotropic fiber below or above any surfaces that dictate large changes in part geometry, again accounting for four “roof” and “floor” layers.

3. Inner hole reinforcement for Z-axis bolt holes

Reinforce Z axis bolt holes with two rings of concentric fiber. Use "inner holes only" if you don’t need side load reinforcement, or use "all walls" to encompass Step 4. This will distribute the compressive force applied by the bolt and creates a composite “sleeve” to resist any off-axis torsional loads the bolt experiences.

4. Outer-wall reinforcement for any side loads

To maximize bending strength about the Z axis and reinforce against side loads, reinforce the outer walls of the part with two rings of concentric fiber. Use "outer shell only" if you have no Z-axis bolt holes, or use "all walls" to encompass Step 3. This will also reinforce any holes with axes on the XY plane.

Specialized Reinforcement Strategies

If you need to address more specific loading conditions, you can employ different tactics to strengthen specific areas, reinforce certain part sections, or control fiber placement. Below are some unique additional strategies you can implement inside your part.

Fiber panel striping

Use fiber striping to get increased bending strength. You can also stripe your parts to balance out uneven sandwich panels.

For increased bending strength on the XY plane, you can add “stripes” of ISOTROPIC FIBER across multiple Z layers. This is most effective with thicker parts that have a fairly consistent or symmetric cross section, because fiber striping creates multiple superimposed sandwich panels to further reinforce a part in bending.

Directing fiber with ribs

Designing ribs into your 3D printed parts will force the fibers to follow certain load paths.

You can route fiber in specific directions with reinforced ribs or cutouts that follow load paths from forces applied to your part. You can force the fiber to follow these load paths by applying concentric fiber to reinforce around the cutouts or walls.

Fiber angles can be used to reinforce in specific directions.

You can use the "fiber angles" tool to route the “zig-zag” of isotropic fibers in a certain direction to better align with the forces being applied to your part. The default setting rotates the fill pattern by 45 degrees each layer, but you can change that by putting a specific angle or pattern of angles in the "fiber angles" dialog box for any layer, any group of layers, or across the entire part.

Achieving Z-Axis Strength

You can circumvent issues with z-axis strength in 3D printing by adding bolts through your part.

Clever design and reinforcement strategies allow you to achieve greater strength on multiple axes. Running a bolt through your part with isotropic fiber reinforcing the compressed surfaces can strengthen the part and prevent shear or tensile forces from splitting the part on layer lines. You can reinforce the area around the bolt with "inner holes only" concentric fiber so that any of those forces distribute to the fiber in the form of bending forces.

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