3D Printing: Do Carbon Fibers Strengthen PLA and PET-G?

Choosing a filament for FDM 3D printing involves more than finding the material with the highest stiffness. A printed part can resist elastic deformation better yet withstand less tensile stress or absorb less energy before failure. In a study by Adi Pandžić, PhD, standard PLA and PET-G were compared with carbon-fiber-reinforced versions to examine how the reinforcement affected tensile behavior.

Key findings

• In the tested specimens, carbon fibers increased elastic modulus by 50% for PLA and 100% for PET-G, as reported in the paper’s conclusion.
• At the same time, ultimate tensile strength decreased by 27% for PLA and 12% for PET-G.
• Yield strength fell by 21% for PLA and 7% for PET-G; strain decreased by 65% and 45%, respectively.
• Toughness decreased by 68% for PLA and 50% for PET-G after carbon fibers were added.
• Print orientation had different effects across materials: changes in ultimate tensile and yield strength reached up to 10% for CFPLA and up to 20% for PET-G and Carbon-P.

Why this question matters

Fused deposition modeling, or FDM, makes parts by extruding thermoplastic filament through a nozzle and depositing material layer by layer. It is a widely used additive manufacturing process because it is relatively inexpensive and straightforward to apply. However, a printed part’s mechanical behavior depends on both the material and the printing conditions. The paper describes FDM applications in education, prototyping, engineering, the automotive and aerospace industries, and other fields.

Carbon fibers are added to a polymer matrix to change its mechanical properties. It is important to distinguish stiffness from strength. Elastic modulus describes a material’s resistance to elastic deformation, while ultimate tensile strength is the maximum tensile stress a specimen withstands. Yield strength marks the onset of permanent deformation. A higher elastic modulus therefore does not automatically mean that a part can withstand a greater tensile load or absorb more energy before breaking.

Materials and experimental method

The paper tested four filaments: PLA, CFPLA, PET-G and Carbon-P. CFPLA is a carbon-fiber-reinforced PLA, while Carbon-P is a PET-G-based filament with carbon fibers. Specimens were printed under the same general conditions, with some settings adjusted for the reinforced filaments. Dogbone specimens were designed according to ISO 527-2. For each material, two specimens were printed at each of four print orientations: 0°, 15°, 30° and 45°.

An Ultimaker 2+ was used for printing, and Cura 4.0.0 generated the G-code. Because carbon-fiber materials are abrasive, the setup used a 0.5 mm steel nozzle. Layer height was 0.15 mm, wall thickness was 0.7 mm, and infill density was 100% for all four materials. Print speed was 60 mm/s for PLA, CFPLA and Carbon-P, and 45 mm/s for PET-G. Printing temperatures were 200 °C for PLA, 210 °C for CFPLA, 240 °C for PET-G and 230 °C for Carbon-P. The build plate was set to 60 °C for PLA, CFPLA and Carbon-P, and 70 °C for PET-G. Fan speed was 100% for PLA, PET-G and Carbon-P, and 20% for CFPLA.

Tensile testing followed ISO 527-2 on a Shimadzu AGS-X machine with a 100 kN capacity, at a test speed of 1 mm/min. The measured properties included elastic modulus, ultimate tensile strength, yield strength, strain and toughness. Infill is the internal structure of a printed part; these specimens were printed with full infill, so the findings refer to that configuration rather than every possible infill setting.

What the results show

The clearest benefit associated with carbon-fiber reinforcement was a higher elastic modulus. At 0°, PLA measured 2517.89 MPa, compared with 4546.54 MPa for CFPLA. PET-G measured 1594.45 MPa, while Carbon-P measured 3085.00 MPa. The highest reported modulus was 4546.54 MPa for CFPLA at 0°. Carbon-P reached its highest value, 4267.22 MPa, at 45°. Its modulus rose from 3085.00 MPa at 0° to 4267.22 MPa at 45°.

Higher stiffness did not translate into higher ultimate tensile strength. At 0°, PLA reached 37.06 MPa and CFPLA 29.96 MPa; PET-G measured 30.45 MPa and Carbon-P 31.88 MPa. The highest ultimate tensile strength in the reported data was 38.53 MPa for PLA at 15°. PET-G reached its maximum of 36.15 MPa at 45°. The lowest displayed value was 26.20 MPa for CFPLA at 15°.

Yield strength provides another reason to look beyond a single property. At 0°, it was 31.68 MPa for PLA and 28.17 MPa for CFPLA, compared with 24.76 MPa for PET-G and 24.77 MPa for Carbon-P. Strain at 0° was 6.67% for PLA and 1.17% for CFPLA; PET-G measured 3.08%, and Carbon-P 1.99%. Overall, the paper reports lower strain and toughness in the reinforced materials. Toughness—the energy absorbed up to failure—decreased by 68% for PLA and 50% for PET-G in the comparison.

Practical implications for engineers and designers

The right choice depends on the part’s intended function. If increased stiffness is the priority, CFPLA and Carbon-P may be candidates based on these tests. If the design requires higher tensile strength, greater deformation before failure or higher toughness, the results show that adding carbon fibers is not automatically an improvement. Filament selection should therefore consider the required performance rather than relying only on a product description or one value from a datasheet.

Print orientation also matters. The paper found no significant effect of orientation on ultimate tensile and yield strength for PLA. For CFPLA, the effect reached up to 10%; for PET-G and Carbon-P, it reached up to 20%. Material comparisons should therefore record the print orientation. Where possible, prototype and test specimens should reflect the way the final part will be printed.

For students and researchers, the study offers an example of a controlled comparison using one printer, standardized specimen geometry, stated process parameters and multiple orientations. Its findings should not be treated as universal values for every printer or filament: the work tested specific materials using specific printing settings.

Limitations and next steps

The study used two specimens for each material and orientation combination. The paper recommends using as many specimens as possible in future work to obtain more accurate results. The conclusions are limited to tensile properties, the four filaments examined and the printing conditions used. The paper also identifies bending, hardness and twisting as properties for future investigation.

Conclusion

This comparison highlights a trade-off that can accompany carbon-fiber filament in FDM 3D printing: higher elastic modulus alongside lower tensile and other measured properties. In this experiment, the modulus increased by 50% for PLA and 100% for PET-G, while ultimate tensile strength, yield strength, strain and toughness decreased. The paper was published in 2019 in the Proceedings of the 30th DAAAM International Symposium on Intelligent Manufacturing and Automation, with Adi Pandžić, PhD, as its author. For material selection, the practical lesson is to match the filament to the part’s requirements rather than assuming that fiber reinforcement improves every property.

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Author: Adi Pandžić, PhD in Mechanical Engineering

This article summarises a published research paper; please refer to and cite the original paper. About the author

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