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In this article we’ll explain 3D printing infills, how they work in FDM 3D printing, how to set the correct percentage to optimise strength-to-weight ratios. We’ll also cover most troubleshooting issues surrounding rapid prototyping with infills.
In Fused Deposition Modelling (FDM), 3D printing infill refers to the internal structure printed within outer walls of a part. The selection of a 3D printing infill pattern has a significant impact on mechanical performance, material consumption, and print duration. Engineers use different 3D printing infill types depending on whether strength, weight reduction, or flexibility is the primary design goal.
See UK government overview of additive manufacturing and 3D printing
Infill or filling is one of the most important variables when 3D printing a part, this term refers to the internal structure printed inside of an object. Its density, patterns & orientation are all factors comprising how a part is made up effecting its strength and functionality. It determines how reliable & efficient it is to print flat, horizontal faces over an empty space.
This area of a 3D print is the outside wall or surface of a part that’s typically built up on the z-axis. The golden rule for defining shell thickness is to ensure that it is a multiple of the nozzle diameter, to prevent gaps / voids between the walls leading to weaker parts. Shells can be a useful variable when defining infills, strength can be added to parts by increasing this outside wall thickness and can help to reduce the infill density. However this is a balancing act and for smaller components can lead to increased material, print times & costs.
This is a great tool to utilise for parts that are being post-processed (sanding, polishing or chemical smoothing). It is mandatory that this variable is increased to reduce the amount of surface penetration from these processes.
Understanding how 3D printing infill affects top and bottom layers is also crucial. For example, sparse infill may result in poor surface quality on horizontal planes unless compensated with adequate top layers.
Bottom section of a part, typically attached to the build plate. This variable won’t effect part strength or functionality too much, however its recommended to adjust this value so that the surface is fully filled in.
Upward section of a part, typically the last section of a component to be printed. This variable is important to ensure the infill is bridged and filled in and sometimes is a higher value than the bottom layers for certain materials.
Common 3D printing infill geometries include grid, lines, triangles, cubic, gyroid, concentric, and octet patterns. Each infill type presents distinct mechanical characteristics. For instance, gyroid provides near-isotropic strength, making it suitable for structural applications. In contrast, lines or grid infills offer faster print times but lower mechanical resilience.
The standard go-to infill type for 3D printer, as it provides a reasonable amount of rigidity in all directions without compromising print times. Biggest advantage of this infill type is that it requires minimal bridging.
Appropriate when strength is required in the direction of the outside wall / shell. This infill type takes longer to print, however is better suited for printing taller parts such a pillars or lithopanes.
Waveform or ripple style pattern, that’s well suited for parts that need to twist or compress, This infill type is best suited for flexible materials and is slower to print than rectangular & triangular patterns.
This is the most popular infill pattern in 3D printing, as is provides the greatest strength in all directions due to its out-of-plane shear properties. The only downside to this infill type is the increased print times.
3D printing infill density is another critical parameter. A higher density enhances part strength but increases weight and print time. Densities of 15–25% are standard for visual or prototype parts, while 50% or more is used when mechanical integrity is essential. Optimizing 3D printing infill density and pattern allows engineers to tailor part behavior for functional requirements while conserving material.
At SGD we use a standard infill of 20% (light). Users can upgrade their infill to four different levels; ultra light (10%), light (20%), medium (50%) or solid (100%).
For parts that are being screwed, tapped or bolted in anyway we recommend a base infill of 50% for maximum strength. Increase in shell thickness & infill density will result in better compressible strength which provides for better anchoring.
In summary, 3D printing infill is not merely a filler—it’s a key design and engineering variable in FDM. Selecting the appropriate pattern and density ensures optimal performance, cost-efficiency, and structural behavior in printed components.
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