FDM 3D Printing: Which Infill Makes PETG, ASA and PLA-Strong Stronger?

Choosing a filament is only part of designing an FDM 3D-printed component. The internal infill can reduce material use and printing time, but it also affects how a part responds to tensile loading. In a 2022 study by Adi Pandžić, PhD, PETG, ASA and PLA-Strongman were tested at five infill densities to compare their tensile properties.

Key findings

• The study tested 75 specimens: five replicates for each of three materials at five infill densities.
• At 100% infill, PETG recorded the highest tensile strength, 49.84 MPa; PLA-Strongman recorded 31.62 MPa.
• Tensile strength increased as infill density rose for all three materials.
• Elastic modulus also increased with infill density; among specimens with infill, PLA-Strongman generally had the highest values.
• At 100% infill, elongation at break was 6.88% for PETG, 6.10% for ASA and 5.94% for PLA-Strongman.
• ASA had the highest tensile strength at break at 100% infill, measuring 29.70 MPa.

Why infill density matters

Fused Deposition Modeling, or FDM, builds a part layer by layer by depositing heated filament through a nozzle. Unlike a solid interior, an infill structure occupies only part of the component’s interior. Infill density describes how much of that region is filled with material. The study used a triangular infill pattern and compared specimens at 20%, 40%, 60%, 80% and 100% density.

For engineers and product designers, infill is a design variable with competing considerations. Lower density can reduce filament use and part mass, while denser infill may improve measured tensile strength and elastic modulus. The best setting depends on the material and the property that matters for the intended design. The results therefore do not amount to a universal recommendation that one density is best for every FDM part.

Materials and test setup

Adi Pandžić examined PETG, ASA and PLA-Strongman, abbreviated as PLAs in the paper. All three filaments were from the same manufacturer, AzureFilm, and had a diameter of 2.85 mm. PETG and ASA specimens were black, while the PLA-Strongman specimens were white. The paper notes that material colour can affect mechanical properties, and includes the colours in its description of the test materials.

The specimen model was designed according to ISO 527-2. Specimens were printed on an Ultimaker S3 FDM 3D printer, with G-code and print settings prepared using Cura. Layer height was 0.15 mm, and the infill pattern was triangular. Printing temperature was 240 °C for PETG and ASA, and 200 °C for PLA-Strongman. Build plate temperatures were 85 °C for PETG, 80 °C for ASA and 60 °C for PLA-Strongman. Print speed was 60 mm/s for PETG and ASA and 70 mm/s for PLA-Strongman. Fan speeds were 20%, 10% and 100%, respectively.

Five replicates were printed for each material and infill density, giving 75 specimens in total. Tensile tests were conducted on a Shimadzu AGS-X universal testing machine with a maximum load of 10 kN. Testing followed ISO 527-1 at a speed of 5 mm/min. The study recorded tensile strength, yield strength, tensile strength at break, elastic modulus and deformation at break.

What the results show

Tensile strength followed the clearest pattern: it rose as infill density increased for all three materials. PETG increased from 35.88 MPa at 20% infill to 49.84 MPa at 100%. ASA rose from 29.56 MPa to 37.70 MPa, while PLA-Strongman increased from 24.96 MPa to 31.62 MPa. PETG recorded the highest tensile strength at every tested density, and PLA-Strongman the lowest.

Comparing 20% with 100% infill, the paper reports that PETG tensile strength increased by up to 39%. Based on the values in the results table, the increase was up to 28% for ASA and up to 27% for PLA-Strongman. When considering only the 20% to 80% range, the reported increases were up to 14% for PETG, 8% for ASA and 14% for PLA-Strongman. ASA did not follow the expected steady pattern at the lower densities: its 20% result exceeded those at 40% and 60%. The difference was no greater than 6.5%, and the paper recommends repeating the ASA test at 20% infill.

Yield strength did not rise in a similarly consistent way. At 100% infill, it measured 38.80 MPa for PETG, 31.34 MPa for ASA and 26.54 MPa for PLA-Strongman. Elastic modulus, by contrast, increased with infill density. At 100%, the values were 2.10 GPa for PETG, 1.98 GPa for ASA and 2.10 GPa for PLA-Strongman. Among specimens with infill, PLA-Strongman generally showed the highest elastic modulus.

Tensile strength at break followed a different pattern from tensile strength. At 100% infill, ASA had the highest value, 29.70 MPa, followed by PETG at 25.13 MPa and PLA-Strongman at 24.83 MPa. PETG’s value at 100% was lower than its result at 80% infill, which was 33.11 MPa. Elongation at break at 100% infill was 6.88% for PETG, 6.10% for ASA and 5.94% for PLA-Strongman. The paper reports substantially higher deformation at break in the fully filled specimens than in specimens with infill structures.

What this means for design and research

When selecting a material and print setting for a functional component, it helps to distinguish between the properties being targeted. In this test, PETG gave the highest tensile strength, and its strength increased as infill density rose. For tensile strength at break, ASA gave the highest result at 100% infill. Elastic modulus and elongation at break add useful information, but they are not interchangeable with a single strength measurement.

For students and researchers, the paper is a useful example of comparing several polymers while varying a print parameter. It also illustrates why a tensile test should be interpreted through multiple results: yield strength, strength at break and deformation do not necessarily change in step with tensile strength. These measurements provide a comparison under the study’s particular conditions. They do not guarantee the performance of every FDM-printed part or replace testing against the requirements of a real application.

Limitations and next steps

The study covered three filaments from one manufacturer, one FDM printer, one infill pattern and five replicates per material-density combination. The paper does not report print orientation details, so its results should not automatically be applied to parts printed in different orientations or with different settings. The research focused on tensile properties; the paper suggests examining other mechanical properties in future work. The paper also recommends repeating the ASA test at 20% infill because its result differed unexpectedly from the 40% and 60% results.

Conclusion

The study shows that infill density affects the tensile behaviour of FDM-printed specimens, but the response depends on both the material and the property measured. PETG reached the highest tensile strength at 100% infill, 49.84 MPa, while ASA had the highest tensile strength at break at that density. The findings can inform an initial comparison of PETG, ASA and PLA-Strongman for product development, while design decisions still need to account for the component’s specific requirements and be validated under its intended conditions.

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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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