PLA 3D Printing: Which Infill Gives Better Tensile Strength?

In FDM 3D printing, the strength of a part depends on more than the material selected. Its internal infill can affect how much filament is used, how long a print takes and how the part responds to load. In a study by Adi Pandžić, PhD, these trade-offs were examined for PLA, a thermoplastic widely used in prototyping and functional part production.

Key findings

• The study tested 13 infill patterns and 130 PLA specimens, with results covering infill densities from 10% to 100%.
• Increasing infill density from 10% to 90% increased both ultimate tensile strength and yield strength for every pattern tested.
• Among the tested patterns, 90% concentric infill produced the highest ultimate tensile strength and yield strength.
• The 100% infill specimen had an ultimate tensile strength of 42.15 MPa and a yield strength of 36.40 MPa.
• Compared with 100% infill, the paper reports that 90% concentric infill reduced ultimate tensile strength by 15% and yield strength by 20%, while reducing print time by 30%.

Why infill pattern and density matter

Fused deposition modeling, or FDM, creates a part by depositing thermoplastic filament in successive layers. Infill is the internal structure inside the part; infill density is the specified degree to which that interior is filled. The infill pattern describes the geometry of the deposited structure, such as lines, a grid or concentric paths.

These settings matter because they change how much material is placed inside a specimen and the path followed by the printer. A designer therefore has more to consider than maximum strength versus minimum material use. Print duration also matters, as does whether the resulting part meets the demands of its intended use. Those considerations apply to product development, educational projects and functional prototypes.

How the study was carried out

The study used PLA filament from 3D Republika with a stated diameter of 2.85 mm and an Ultimaker 2+ 3D printer. The dogbone-shaped tensile specimen was designed according to ISO 527-2. Toolpaths were prepared in Cura 4.0.0 using its predefined “Normal” profile.

The specimens were printed under the same conditions. The paper lists a 0.4 mm nozzle, 0.15 mm layer height, 0.7 mm wall thickness, print speed of 60 mm/s, printing temperature of 200 °C and build-plate temperature of 60 °C. Specimens were printed flat, with a 45° raster angle. The study aimed to limit other influences by using the same material and colour, printing parameters and position on the printer bed.

The 13 infill patterns were Grid, Lines, Triangles, Tri-Hexagon, Cubic, Cubic Subdivision, Octet, Quarter Cubic, Concentric, Zig Zag, Cross, Cross 3D and Gyroid. Density was varied for each pattern, and the paper reports a total of 130 printed specimens. Tensile testing used a calibrated Shimadzu AGS-X machine with a 100 kN load cell and a test speed of 1 mm/min. The measured properties were ultimate tensile strength and yield strength. The study also recorded estimated printing time and material consumption per specimen.

What the results show

For every pattern tested, increasing infill density from 10% to 90% was accompanied by an increase in ultimate tensile strength and yield strength. The size of the change varied by pattern. For concentric infill, for example, the reported increases were 96% for ultimate tensile strength and 100% for yield strength. Cross infill showed smaller increases of 7% and 14%, respectively.

At 90% density, concentric infill had the highest strength results among the patterns compared. The paper reports that, relative to a fully filled specimen, this configuration reduced ultimate tensile strength by 15% and yield strength by 20%, while cutting print time by 30%. These figures describe the specific material, printer, specimen geometry and settings used in this experiment; they should not be treated as guaranteed results for every FDM printer or PLA filament.

The study reports an ultimate tensile strength of 42.15 MPa and a yield strength of 36.40 MPa for PLA at 100% infill. The print-time and material-consumption data also generally rise as density increases, although values vary by pattern. For Grid infill, for example, estimated print time increases from 46 minutes at 10% density to 93 minutes at 100%, while estimated material use increases from 7 g to 12 g.

What the findings mean in practice

For engineers and product designers, the findings show why infill pattern and density can be considered together rather than judging strength by infill percentage alone. In this test series, 90% concentric infill offered a shorter print time and lower material use than 100% infill, with measured differences in strength. Whether that trade-off is suitable depends on the loads and performance requirements of the actual part.

For students and researchers, the paper provides an example of comparing FDM process settings under controlled conditions. It demonstrates why an infill pattern should be reported alongside density, and why specimen geometry, print orientation and test speed need to be specified. The results should not be directly transferred to different materials or printing orientations without further testing.

Limitations and next steps

The research examined tensile properties in one direction and used one print orientation. The paper identifies different print orientations, testing in three directions and additional mechanical properties as areas for further investigation. These include bending, compression and hardness. Such work could build a broader picture of FDM material behaviour and inform further finite element analysis.

Conclusion

The study finds that both infill pattern and infill density influence the tensile strength and yield strength of FDM-printed PLA specimens. In this experiment, 90% concentric infill gave the highest results among the tested patterns, while 100% infill provided the reference values of 42.15 MPa and 36.40 MPa. The findings can inform infill choices for prototypes and parts, but the selected settings should be validated for the material, printer and loading conditions of the intended application.

Publication details

“Effect of Infill Type and Density on Tensile Properties of PLA Material for FDM Process” was written by Adi Pandžić, PhD. It was published in the Proceedings of the 30th DAAAM International Symposium on Intelligent Manufacturing and Automation in 2019, pages 545–554. DOI: 10.2507/30th.daaam.proceedings.074.

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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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