Overcoming Z-Axis Strength Limitations in 3D Printed Composite Parts

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Printed parts have a long and notorious history of being significantly weaker than their production counterparts. New developments in processes and materials now allow printed parts to meet or exceed parts constructed from more traditional techniques. Composite parts with continuous fiber reinforcement created on a Markforged printer can have a higher strength to weight ratio than metal parts, but not in all directions. Fused Deposition Modeling (FDM) involves extruding plastic through a nozzle layer by layer. The bond between two layers is weaker than the material strength within each individual layer. If the print bed is considered the XY plane of the part, the Z-axis would be considered the height of the printed part. Tensile strength is weaker along the Z-axis, especially in taller printed parts.

3D printing X Y and Z axis
Clevis design with post stress fasteners

Weakness in tension is not unique to printed parts, concrete is exceptionally strong in compression but not in tension. To work around this limitation a combination of concrete and steel is typically used to achieve the necessary final material properties. Continuous fiber reinforcement in composite parts is analogous to rebar steel in concrete. The embedded fiber is melted into the Nylon base material, and just like rebar becomes an inseparable part of the structure of the part.

Concrete placement with internal steel rebar reinforcement

Fiber can be embedded in the XY orientation quickly and easily during the print on a Markforged printer, creating an inexpensive tensile reinforcement that gives composite parts their superior strength in the print plane.

Onyx printed part with internal fiber reinforcement
Onyx printed part with internal fiber reinforcement

For even stronger and stiffer parts, concrete can be post-stressed. Post-stressed designs incorporate channels for rods or cables to apply compression to the structure after it is formed. With applied compression the resulting design loses any weakness in tension and even stabilizes the structure. Post-stressed concrete is a stronger but more labor-intensive method of reinforcement.

post stressed concrete installation
Post stressed concrete installation

Since fiber cannot currently be printed out of the print plane, a post stressed reinforcement strategy can be employed using simple fasteners. Parts should be oriented to allow the embedded fiber to carry the primary load whenever possible. This post stress strategy is perfect for parts with loads that change directions, such as this clevis. Tension on the clevis pin will try to split the layers apart, but the fasteners will offset the tension to strengthen the design.

Unsupported posts are another perfect application for this approach. The Composite Design Guide recommends no posts higher than 5x the diameter to avoid a print failure, but a post that tall will not be capable of resisting significant side loads.

composite design guidelines for unsupported posts
Composite Design Guidelines for Unsupported Posts
Maximum post size can be extended or strengthened by integrating a fastener

To maximize load-carrying capacity, add fiber reinforcement layers directly under both the nut and the bolt heads. Choose a fastener with a large enough head to prevent pull-out or use washers to spread out the preload.

For a clean finished look add a counterbore to the top and add a recess for the nut at the bottom. The cavity for the nut can even be made internal and inserted during the print if access or outer shape is a concern.

Hex and Square nuts embedded at a paused layer

Additive manufacturing is shaking up the design world and enabling new ideas to become viable. Existing design concepts such as post stressed reinforcements offer an elegant solution when coupled with the flexibility of 3D printing. To learn more about Markforged printers or to partner with a company that can truly add value to this amazing equipment, contact MLC CAD Systems today.

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