Infill in 3D Printing: Definition, Main Parts, and Different Types

3D printing infill refers to the internal structure of a 3D printed part. This internal structure can be produced using many different shapes. The purpose of infill is to optimise part weight, strength, and printing time. Many different infill patterns exist.

What is infill in 3D printing?

“Infill” in 3D printing refers to the internal patterns found inside most 3D printed parts. Parts made by some manufacturing processes, like injection moulding, must be made either completely solid or completely hollow. Fused deposition modelling (FDM) 3D printed parts, on the other hand, can be made with a variety of structural patterns that partially fill the space inside the outer printed walls. 

What is the purpose of infill in 3D printing?

The purpose of infill in 3D printing is to save both printing time and material by creating a lattice structure inside of a 3D printed part. Printing fully dense parts is often unnecessary and is just a waste of material. The infill can be strategically placed to provide strength where in-service loads on the part are the highest. A higher infill density means a greater percentage of infill.

What are the main parts of infill in 3D printing?

A typical 3D printed part will have an outer shell with a predefined thickness. The infill structure is completely enclosed by this outer shell and is not visible when the part print is complete. The infill will be evenly distributed throughout the part. 3D printing infill is normally printed at higher speeds than the outer shell to save time. The infill print lattice is also not as thick as the shell. The many different infill patterns each have their own advantages and disadvantages.

What are the different types of infill in 3D printing?

There are many different types of 3D printer infill. All achieve the same purpose of reducing print time and material usage. Below are some of the 3D printing infill patterns in use today:

• Line
• Concentric
• Gyroid
• Grid
• Octet
• Lightening
• Triangular
• Tri-Hexagon
• Cubic
• Cross

Line infill pattern

Line infill consists of multiple parallel lines per layer. Each layer crosses over the previous one at a 90-degree angle. Unlike other patterns, the lines pattern don’t cross over each other on the same layer. This strengthens the part in two dimensions. Line infill is marginally quicker than grid and triangular patterns.

Concentric infill pattern

The concentric 3D printing infill is one of the fastest infill patterns to print, and it uses the least material. However, this comes at the cost of reduced part strength. The concentric pattern is not as strong as the other infill types, especially when loaded in the x or y-directions.

Gyroid infill pattern

The gyroid 3D printing infill pattern creates alternating wavy lines or curves. This infill pattern takes longer to print than the others. However, the unique gyroid internal structure allows for almost isotropic mechanical properties. Prints are still weaker on the z-axis but this pattern improves the x and y-axis shear strength. The gyroid infill pattern works well with flexible materials.

Grid infill pattern

The grid is one of the most common infill types. The grid pattern lays down plastic in a cubic grid pattern that crosses over itself at 90-degree angles. This infill pattern is ideally used for prints with large, flat surfaces. Gridded infill patterns can cause nozzle clogging since the lines criss-cross over each other in the same layer.

Octet infill pattern

The octet 3D printing infill pattern creates tetrahedral (pyramid-like) volumes inside the part. It is best for parts with large horizontal surfaces. Smaller gaps between the infill walls are possible due to the tapering nature of the pyramid-like volumes. This allows for shorter span distances between infill walls and a lesser chance for the material to sag. Improved top surfaces can therefore be achieved without having to increase the infill density.

Lightning infill pattern

This unique infill type provides the fastest possible print time at the expense of part strength. Supports are added in a lightning bolt structure and only placed where necessary. Parts are shells except where support is needed for horizontal features or internal overhangs.

Triangular infill pattern

The triangular 3D printing infill pattern lays down plastic in a triangular grid that crosses over itself at 60-degree angles. This type of infill pattern is best used on parts with large, flat surfaces, and performs similarly to the grid pattern. The triangular infill can cause the nozzle to clog, as its lines criss-cross each other in the same layer.

Tri-Hexagon infill pattern

The tri-hexagon is the strongest infill pattern. Like the grid and triangular infill types, it will cross over itself to create a hexagonal pattern interspersed with triangles. Due to the crossing of lines in the same layer, this type of infill tends to clog the print nozzle.

Cubic infill pattern

The cubic infill type creates cubic volumes inside the part. It does this by producing a layered pattern similar to a triangular infill. However, it offsets each subsequent layer to build enclosed cube-shaped volumes in the part. The cubic volumes are oriented with the cube balanced on one corner.

Cross infill pattern 

The cross pattern creates multiple cross shapes as infill. This 3D printing infill pattern is ideal for flexible part shapes as it allows for the part to bend and twist. It is not very useful for more rigid plastics, however. A 3D version of the cross is also available to add additional strength.

What does the ideal value of the infill percentage depend upon?

The main determining factor for infill percentage is the type of application for which the part is destined to be used. Prototypes and hobbyist creations rarely need more than 20% infill. Functional parts that will be exposed to mechanical stress loads will typically require infill percentages of 50% or more.

What is a good infill density?

In most cases, an infill density between 20 and 50% is ideal. Less than 20% results in flimsy parts whereas more than 50% begin to take too much printing time and use too much material.

What is the recommended infill percentage for other simple exposure objects?

For objects that will not see any significant mechanical load, a fill percentage of 20% is sufficient. It must be noted that the geometry of the part may require a different infill percentage. For example, a flat, horizontal surface may need a denser infill percentage to ensure that the top layer does not sag due to a lack of support. 

How much infill is required to achieve maximum tensile strength?

A higher infill density will ensure a better tensile strength for the part. It must be noted that infill percentage is not the only factor that determines tensile strength. For example, filament material and print orientation play a major role. FDM parts, in particular, are anisotropic. They are weaker along the z-axis due to interlayer bonding weakness. 

If an FDM part is loaded in the z-direction, the dominant factor controlling its tensile strength will be the interlayer bonding quality. Infill percentage will have little effect in this case.

How to choose the best type of infill

The steps in choosing the best type of infill are:

1. Check the part requirements. Calculate the infill percentage needed.
2. Assess your resources, including the available material and the time you can spend per part.
3. Shortlist the infill patterns that match your requirements and resources.
4. Select the best infill pattern for your part.

Choosing the best infill pattern is often a trial-and-error process. It always pays off to do a trial print on a small part to figure out what the best settings are. Sticking with the simplest patterns, like a grid or line pattern, will normally yield good results.

Is infill important for 3D printing?

Yes, infill is an important consideration in 3D printing. Without infill, some parts may not be printable at all due to unsupported surfaces in the design. Infill adds extra strength where needed to strike a balance between the print time and the material requirements. Some parts can be printed without any infill. 

However, these are typically hollow shells with open tops (e.g., vases) and cannot be used for structural applications. 

Summary

This article discussed the use of infill in 3D printing, the common types of infill patterns that exist, and how to select a pattern for use.

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