FDM 3D Printing: How Infill Density Changes Strength

Reducing infill can save material and printing time, but it also changes how an FDM-printed part responds to a load. For designers, the practical question is how much tensile strength and stiffness are lost as the interior becomes less filled. A study by Adi Pandžić, PhD, tested three materials at four infill densities to examine that trade-off.

Key findings

• The study tested 60 specimens, with five replicates for each of 12 material-and-density combinations.
• Specimens were printed at 20%, 60%, 80%, and 100% infill, using a triangular infill pattern at a 45° infill angle.
• At 100% infill, cPLA recorded the highest tensile strength, 61.4 MPa, and the highest elastic modulus, 3.1 GPa.
• Tensile strength and elastic modulus decreased as infill density decreased for all three tested materials.
• At 20%, 60%, and 80% infill, tPLA had the highest tensile strength among the tested materials at each density.
• The paper reports that tPLA showed the greatest plasticity in the stress–strain diagrams.

Why infill density matters

Fused deposition modeling, or FDM, is an additive manufacturing process in which a 3D printer deposits melted filament layer by layer to build an object. Instead of filling the entire interior with solid material, a printed part can have an internal infill structure. Infill density describes how much of that interior is filled.

Lower infill density can reduce filament use and printing time. The paper identifies these as benefits of making products with an infill structure. But a part that uses less material is not automatically suitable for every load. Measuring how tensile strength and elastic modulus change helps engineers and product designers evaluate the consequences of choosing a less dense interior.

Tensile strength is the maximum tensile stress a material withstands during a test. Elastic modulus describes stiffness in the elastic range: a higher value means greater stress is needed to produce a given elastic deformation. These are not the only properties that determine how a printed part performs, but they are the mechanical measures examined in this study.

Materials and experimental method

The study tested three FDM materials: an antibacterial PLA nano-copper composite, called cPLA; Tough PLA, called tPLA; and ABS-X. The tensile specimens were designed according to ISO 527-2. For each material, specimens were made at four infill densities: 20%, 60%, 80%, and 100%. Five replicates for each of the 12 combinations gave a total of 60 tested specimens.

All specimens used a triangular infill pattern and a 45° infill angle. The cPLA specimens were printed on an Ultimaker S3 using a CC red print core with a 0.6 mm nozzle, intended for composite materials. The tPLA and ABS-X specimens used a standard AA core with a 0.4 mm nozzle. Layer height was 0.15 mm for all three materials. Cura used its normal PLA profile for cPLA and tPLA, and its normal ABS profile for ABS-X.

Other print settings differed by material. Printing speed was 70 mm/s for cPLA, 45 mm/s for tPLA, and 60 mm/s for ABS-X. Printing temperature was 200 °C, 215 °C, and 245 °C, respectively. Tensile testing was performed on a Shimadzu AGS-X universal testing machine with a 10 kN capacity, following ISO 527-2 at a strain rate of 5 mm/min. The paper reports average results for the five specimens in each condition.

Tensile strength and stiffness results

At 100% infill, tensile strength was 61.4 MPa for cPLA, 49.5 MPa for tPLA, and 38.6 MPa for ABS-X. The corresponding elastic modulus values were 3.1 GPa, 2.5 GPa, and 1.8 GPa. Thus, cPLA had the highest measured value for both properties at full infill, while ABS-X had the lowest elastic modulus.

At 80%, 60%, and 20% infill, cPLA tensile strength was 37.5 MPa, 32.7 MPa, and 26.7 MPa. The values for tPLA were 41.6 MPa, 38.9 MPa, and 35.4 MPa; for ABS-X they were 30.9 MPa, 29 MPa, and 26.9 MPa. The measurements show a downward trend as infill density decreased for each material. Among specimens below 100% infill, tPLA had the highest tensile strength at every tested density.

Elastic modulus also fell with decreasing infill. For cPLA, it was 2.5 GPa at 80%, 2.1 GPa at 60%, and 1.8 GPa at 20%. For tPLA the values were 2.2 GPa, 2.1 GPa, and 1.9 GPa, while ABS-X measured 1.6 GPa, 1.5 GPa, and 1.3 GPa. At 80% infill, cPLA had the highest modulus. At 60% and 20%, the paper’s conclusion reports equal values for cPLA and tPLA.

Table 4 gives percentage differences relative to specimens with 100% infill. For tensile strength, the listed reductions at 80%, 60%, and 20% are 48%, 61%, and 78% for cPLA; 17%, 24%, and 33% for tPLA; and 22%, 28%, and 35% for ABS-X. For elastic modulus, the reported differences are 21%, 38%, and 53% for cPLA; 12%, 17%, and 27% for tPLA; and 11%, 18%, and 32% for ABS-X. These percentages are presented as reported in Table 4, alongside the individual average measurements listed in Table 3.

What the findings mean in practice

For engineers and product designers, the main point is not that one infill density is best for every part. The results show that changing infill density changes the measured tensile strength and stiffness, and that the response also varies by material. When designing a part that will be pulled in tension, these measurements can help frame the trade-off between interior fill and mechanical properties. They are not universal specifications for all PLA or ABS filaments: they apply to the particular materials, printer, settings, and specimen design used in this study.

Students can use the paper as an example of a controlled experiment in additive manufacturing. The study varied infill density while keeping the infill pattern and angle the same, then tested tensile properties to a stated standard. It also illustrates why reporting print settings is important when comparing results. SLA is not part of the experiment: the tested specimens were produced with FDM.

The study did not test compression, bending, fatigue, impact, or the performance of finished products in real operating conditions. Its results therefore cannot establish how the same specimens would behave under those types of loading. In addition, only one infill pattern and one infill angle were examined, so the findings do not establish which infill shape is best across applications.

Limitations and next steps

The paper identifies infill pattern and infill angle relative to the load direction as areas for further study, along with the orientation of the infill pattern in relation to the load. Examining these factors would extend understanding of how internal geometry affects FDM-printed materials. The paper also points to developing material models for FEM/FEA analysis of elastic and plastic deformation.

The repeatability finding should be understood within the scope of this experiment. The study generally found no pronounced scatter in the stress–strain curves across the five replicates, while noting scatter in strain for tPLA. The paper relates the results to good repeatability when printing multiple pieces on an Ultimaker 3D printer. This is not evidence that every printer, material, or set of settings will produce the same repeatability.

Conclusion

The study demonstrates that infill density is relevant to tensile strength and elastic modulus in the three FDM materials tested. At full infill, cPLA had the highest measured values, while tPLA had the highest tensile strength among the specimens printed at 20%, 60%, and 80% infill. Lower infill density was accompanied by lower values for both measured properties.

Adi Pandžić, PhD, published the study in TEM Journal in 2021, Volume 10, Issue 3, pages 1195–1201. For practical use, its results provide a comparison of specific materials and print settings, not a substitute for testing a component under the conditions for which it is designed.

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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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