3D Printing: Which Infill Pattern Makes PLA and PC Stronger?

In FDM 3D printing, the internal geometry of a part affects more than how much filament it uses: it can also influence how the part behaves under load. Designers and engineers therefore need to know whether changing the infill pattern can change tensile performance when other settings are held constant. A study by Adi Pandžić, PhD, compares seven patterns in PLA, Tough PLA and PC.

Key findings

• The study tested seven infill patterns at 60% density and a 45° angle: Cubic Subdivision, Cubic, Grid, Octet, Quarter Cubic, Triangles and Tri-Hexagon.
• For PLA, Octet had the highest tensile strength at 45.57 MPa, closely followed by Grid at 45.53 MPa. Quarter Cubic was lowest at 38.40 MPa.
• For Tough PLA, Grid produced the highest tensile strength, 40.10 MPa, while Quarter Cubic produced the lowest, 36.40 MPa.
• For PC, Octet reached the highest tensile strength at 50.03 MPa, and Cubic Subdivision the lowest at 47.20 MPa.
• The highest reported elastic modulus was 2.40 GPa for PLA with Octet. PC had the lowest reported value, 1.80 GPa, with Cubic Subdivision.
• The paper concludes that infill pattern affects tensile properties, while noting results are specific to the materials and printing conditions tested.

Why infill geometry matters

Additive manufacturing builds an object layer by layer from a digital model. In FDM, or fused deposition modelling, a 3D printer heats filament and deposits it through a nozzle. Many printed parts are not solid throughout: infill is the internal structure enclosed by the outer walls. Slicing software lets users select its pattern and density.

Compared with a solid part, infill can reduce material use, product weight and print time. But the way this internal structure is arranged can also affect the part’s response to load. Tensile strength is the maximum tensile stress a material withstands during a test before it fails. Elastic modulus describes stiffness in the elastic range: a higher value generally indicates less deformation under the same stress.

This paper focuses on one aspect of infill design: pattern. The study compared PLA, Tough PLA and PC while keeping infill density and angle the same. That makes it easier to compare the patterns within the experiment, but it does not establish one pattern as best for every part, material or service condition.

Materials and test method

The study used black Ultimaker filaments: PLA, Tough PLA and PC, or polycarbonate. For each material, the study examined seven patterns available in Cura 4.8 and identified by the manufacturer as “strong”: Cubic Subdivision (CS), Cubic (CU), Grid (G), Octet (OC), Quarter Cubic (QC), Triangles (T) and Tri-Hexagon (TH).

Dog-bone tensile specimens were modelled in Solidworks according to ISO 527-2 and prepared for printing in Cura 4.8. Three replicas were printed for every material-pattern combination, for a total of 63 specimens. Printing used Ultimaker S5 and S3 3D printers. The paper reports that the same printer and the same build-plate position were used to reduce the influence of external factors.

The settings included a 0.15 mm layer height, a 0.4 mm nozzle, 60% infill density and a 45° infill angle. Printing temperatures were 200 °C for PLA, 215 °C for Tough PLA and 280 °C for PC. Print speeds were 70 mm/s for PLA, 45 mm/s for Tough PLA and 50 mm/s for PC. Infill print speed was 50 mm/s for all three materials.

Tensile testing followed ISO 527-2 on a Shimadzu AGS-X machine with a maximum force of 10 kN. Tests were run at 5 mm/min and room temperature. Results were recorded and statistically analysed in Excel. The paper does not specify the statistical test or p-values, so its reported averages should be read as comparative experimental results, not as evidence of a particular level of statistical significance.

Results across the three materials

For PLA, tensile strength ranged from 38.40 MPa with Quarter Cubic to 45.57 MPa with Octet. Grid was close to the top at 45.53 MPa. Elastic modulus ranged from 2.10 GPa with Quarter Cubic to 2.40 GPa with Octet. In its conclusion, the paper reports that PLA tensile strength varied by up to 17% depending on pattern, and elastic modulus by up to 9%.

For Tough PLA, Grid gave the highest tensile strength, 40.10 MPa, while Quarter Cubic gave the lowest, 36.40 MPa. Elastic modulus was 2.10 GPa for six patterns and 2.00 GPa for Quarter Cubic. The paper reports tensile-strength variation of up to 10%; the Quarter Cubic modulus was up to 5% lower than the 2.10 GPa value.

For PC, tensile strength ranged from 47.20 MPa with Cubic Subdivision to 50.03 MPa with Octet. Elastic modulus was 1.90 GPa for six patterns and 1.80 GPa with Cubic Subdivision. The paper reports PC tensile-strength variation of up to 6%, and an elastic modulus up to 5% lower for Cubic Subdivision.

Across the tested patterns, PC had the highest tensile strength and Tough PLA the lowest. PLA showed the highest elastic-modulus values, while PC showed the lowest. These are findings for the specific black Ultimaker filaments and settings in this experiment, not universal values for every product made from these polymers.

What the results mean for designers and engineers

For engineers, product designers and users of 3D printing, the practical message is that infill selection can affect tensile strength. It should not be chosen only for appearance or because it is a convenient slicer default. In this test, Grid produced the highest tensile strength for Tough PLA, while Octet performed best by that measure for PLA and PC. Quarter Cubic had the lowest tensile strength for all three materials.

The appropriate choice still depends on the part’s requirements. If stiffness matters, elastic modulus is relevant too: Octet gave the highest value for PLA in this experiment, while Tough PLA and PC showed mostly consistent modulus values across patterns. Students can use the study as an example of isolating one design variable by keeping infill density and angle constant.

The results do not establish a best material or pattern for medical, military, educational or other applications. Selecting a print for a specific load requires testing relevant to the actual part and its use, including other loading modes where appropriate. Nor can these FDM results be directly applied to SLA: that printing technology and process were not examined in this study.

Study limits and future work

The experiment covered three materials, seven patterns, one infill density (60%) and one angle (45°). It did not compare different densities or angles, or filaments from other manufacturers and colours. The paper notes that manufacturer, colour and print settings can affect mechanical properties, so the results should not be transferred to other filaments, machines or parts without verification.

Each material-pattern combination had three replicas, and testing was limited to tensile strength and elastic modulus. The paper recommends future work on infill density and angle, examining multiple design parameters and their interactions, and relating mechanical performance to print time and material consumption. The paper also recommends studying compressive and flexural properties.

Conclusion

The study shows that infill pattern can change the tensile results of FDM-printed parts even when density is 60% and angle is 45°. Under the tested conditions, Octet produced the highest tensile strength for PLA and PC, while Grid did so for Tough PLA; Quarter Cubic was lowest for all three materials by that measure. For product development, the central lesson is to select infill according to required properties rather than assume one pattern will perform best with every material.

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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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