Key Takeaways

  • What is infill? Infill is the material that fills the interior of a 3D printed object, providing strength, weight, and durability.
  • How to choose infill percentage? Infill percentage depends on the purpose and function of the object. Higher percentage means more strength, but also more filament and print time. Lower percentage means less strength, but also less filament and print time.
  • How to choose infill pattern? Infill pattern affects the strength, flexibility, and appearance of the object. Different patterns have different advantages and disadvantages. For example, grid is fast and simple, honeycomb is strong and light, and cubic is a good compromise.
  • How to use variable infill density? Variable infill density allows you to customize the infill settings for different parts of the object. This can help you optimize the strength, cost, and print time of your object. You can use different tools in different slicers to achieve variable infill density.

Infill density is the amount of filament printed inside the object, and directly relates to the strength, weight, and printing duration of your print.

Most FDM objects are not printed as solid objects, because that would use a ton of filament and take quite a long time to print.

On the other hand, a totally hollow print would be impractical, as it would easily fail under the stress of normal usage.

Infill 3D printing is a compromise between these two positions.

In this guide, I’ll explain the methods I use to choose the right infill percentage and patterns for my projects to maximize strength and cost savings.

We’ll step through mastering variable infill density across all the main slicer software options, and go over the most common infill troubleshooting issues so your 3D prints are as effective as possible.

How to Choose Infill Percentage in 3D Printing

3D Printer infill patterns, or the internal structure of an object, are a necessary part of printing some 3D objects, especially those that require a measure of strength or sturdiness. That being said, infill is also something of a pain.

If you want your print job to succeed, you’re likely to have to use a solid shell with at least a modicum of infill.

Infill percentage density
Here’s your Fill Density, 3D printer style.

The main goal of 3D printing infills is to endow parts and models with certain properties – strength, durability, weight, flexibility, and more. But, infill also plays a major role in reducing print times and cutting down on material usage through the creation of an internal lattice structure inside the 3D print.

The greater the percentage of infill, the higher the density of the part.

As such, infill is a question of balance. A solid, dense part is often overkill and typically wastes filament.

Instead, the aim is to be strategic and provide a structure that delivers strength where needed, typically load bearing parts of the print, while keeping print speeds high and print times down.

Compared to traditional manufacturing techniques, infill is rather unique. Manufacturing methods like injection molding and subtractive manufacturing work in extremes – parts either hollow or completely solid with no middle ground between the two.

In contrast, 3D printing infills allows you to fill the part or model as you please by fine tuning the internal structure with a choice of almost any pattern or density to suit the needs of the project.

Basic Object Sections

In general, a routine FDM object consists of four sections. The design criteria of each of these sections can be individually altered so that an optimized design is achieved. The sections are:

  • Shell – The outside walls of an object, typically built up vertically along the z-axis.
  • Bottom layers – A part of the shell comprised of an outside wall of an object, initially attached to the build plate.
  • Top layers – A part of the shell comprised of an outside wall of an object, facing upwards. Usually the last part of an object to print.
  • Infill – The material that comprises the interior of the object between the shell or walls.

Infill Shapes

There are several infill shapes typically available in most slicer programs. For example, Cura’s infill patterns include Honeycomb and Triangles among others.

Which one is right for you depends on what type of object you’re planning on creating and the 3D printing infill strength you require. 

Here are some examples of Cura infill patterns. Some are novelty (like the cross, not pictured) and take longer, whereas others are faster. Honeycomb infill Cura is not currently available. 

Cura Infill Pattern

Grid / Rectangular is a good default, fast option. Concentric is 2nd best to wave for flexible prints to keep it soft. Triangle is the strongest Cura infill pattern apart from Honeycomb.

Cubic is a great compromise between strength, print speed, and keeping the model light via low filament usage.

Infill PatternDescriptionRecommend Prints
RectangularThis standard infill pattern provides a reasonable amount of rigidity in all directions. It is also one of the easiest infill patterns to print, requiring a minimum amount of bridging on the part of your print head.Standard prints
TriangularAppropriate when strength is required in the direction of the shell. However, it takes longer to print.Strong standard prints
Wave or WiggleAs the name implies, a waveform infill pattern. Particularly useful when an object needs to be able to twist or compress. This is great to use for flexible materials. Flexible prints
3D Honeycomb InfillOne of the more popular infill patterns. Provides greater overall strength in all directions than a rectangular pattern, with very little increase in print time. It is generally considered the most commonly used, strongest infill pattern. Standard prints
LinesConsists of multiple parallel lines per layer. Each layer crosses over the previous one at a 90-degree angle. This strengthens the part in two dimensions.Decorative prints, figurines, and models
GyroidCreates 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.Functional and mechanical prints
OctetCreates tetrahedral (pyramid-like) volumes inside the part. It is best for parts with large horizontal surfaces.Functional and mechanical prints
ConcentricOne of the fastest infill patterns to print, and it uses the least material. However, this comes at the cost of reduced part strength.Flexible prints
LightningProvides the fastest possible print time at the expense of part strength. Supports are added in a lightning bolt structure and only placed where necessary.Decorative prints, figurines, and models
Tri-HexagonThe strongest infill pattern. Like the grid and triangular infill types, it will cross over itself to create a hexagonal pattern interspersed with triangles.Functional and mechanical prints
CrossCreates 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.Flexible prints
Cross 3DSimilar to the cross pattern, except that it removes the vertical rigidity for added flexibility.Flexible prints
Cubic Based on a series of cubes piled on top of one another at an angle for added strength.Strong standard and functional prints
Cubic SubdivisionSimilar to Cubic, except that it’s optimized to use less filament by printing cubes of different sizes while still maintaining strength.Strong standard and functional prints
Quarter CubicPieces together tetrahedrons for strength suitable for load-bearing parts, especially tall and thin printsStrong standard and functional prints

Art Infill

Aside from reinforcing prints and delivering specific properties, infill is increasingly used in creative ways in what’s called art infill. The basic idea is to leverage infill patterns to produce striking artistic effects.

Typically, the bottom and top layers of a print are deleted from the print to expose the internal structure of the print so that infill patterns are visible. Art infill is increasingly popular among artists but also for the creation of bespoke jewelry pieces.

Cura Infill Overlap

Overlap is the amount the edges of your infill is printed into the outer walls of your print. If the overlap is too great, you’ll end up with the infill forcing through the walls – which isn’t a pretty sight.

The default is 10%, which gives room for good consistent adhesion between the infill and walls, without the infill coming through. 

The default in most cases is sufficient, but should you want to change it:

Infill Overlap is hidden by default (as are most options in Cura 2.x/3.x)

To enable Cura Infill Overlap:

  1. Click on the little cogwheel next to infill when you mouse over it.
  2. Then check the box for infill overlap in the window that pops up.
  3. Default is 10% of LineWidth – i.e. 0.04mm for a 0.4mm nozzle.

Using Shell Thickness to Reduce Infill Percentage

The shell of an object consists of layers on the outside of an object. In many designs, the shell is often the first area that is printed in any layer. This means that shell thickness is intimately tied to infill amount and percentage.

When you increase the shell thickness of an object, you are also increasing its strength. This means that the object becomes sturdier and more capable of handling stress without the need for increasing the 3D printing infill density.

The majority of slicer programs will allow you to adjust the density of shell thickness in specific areas of the object, thereby offering localized strength where it is needed most.

Shell thickness is usually measured in print nozzle diameters. If you do decide to slightly increase shell thickness to reduce infill amounts, make sure that the thickness specified in your design is a multiple of your nozzle diameter.

This will help reduce voiding in your walls, bottom and top layers.

It really helps to use good quality filament when printing, especially if you’re looking to maximize strength while cutting back on material used. This is where high-quality filament comes into their own, your prints will be stronger, but with lower (or no) infill, you can use less material and save more time.

You may even save money with fewer failed prints or unusable parts. 

Low top layer count
Here’s an example of low top layer count, making the infill visible from underneath. Less infill normally means you’ll need a thicker layer count to compensate, especially on larger area prints.

It should be noted that there are some drawbacks to this approach. Any post-printing finishing process, such as sanding or annealing, will reduce shell thickness and directly affect strength.

This can be offset by increasing shell thickness even further. However, every increase in shell thickness will drive up print costs and time. So, at some point, increasing shell thickness to reduce or eliminate infill amounts becomes a losing proposition.

Experimenting with your designs and slicer settings will help you determine if this approach is right for your particular circumstances.

3D Printer Infill Percentage and Overall Object Strength

To understand infill, think about the doors in your home. Very few doors that are mass-produced are made of solid wood. The cost is simply too prohibitive. The majority of doors available commercially have a wooden or metal outside surface built around a core consisting of a lower density material.

This allows the door to be produced quickly in large volumes while remaining affordable.

So, to an FDM object. The typical FDM design consists of a solid outer surface (the shell) which is built around a lower density infill. As was the case with doors, this arrangement allows the object to be printed as quickly as possible at a reasonable cost.

The majority of slicer programs have a default infill setting somewhere between 18% and 20%. For many designs and objects, this default density is perfectly acceptable. However, when it comes to infill percentage, there is no hard and fast rule that fits all scenarios.

An 18% to 20% infill percentage may work fine for a prototype object where strength takes a backseat to form or shape. However, that same infill percentage will be completely inadequate for an object that has been designed to hold weight, like a bracket.

In general, the strength of an FDM object is directly tied to the infill percentage used during printing. For example, a part utilizing 50% infill is approximately 25% stronger than a part that utilizes 25% infill.

However, the amount of strength gained by increasing infill percentage does not increase linearly. For example, increasing infill percentage from 50% to 75% only results in an additional strength increase of 10%.

In addition to increasing overall object strength, infill percentage is also critical to object feature strength. For example, consider a two-piece object designed to connect together using an integral attachment feature like a snap-fit.

A snap-fit connector is usually designed as a cantilever. This means that its weakest point will be the small area attaching it to the main body of the object.

At a low infill percentage, the internal density of the cantilever is insufficient to withstand the stress of connection. As a result, it will snap off at its connecting point.

Increasing the infill percentage will increase the density of the connection, with a corresponding increase in strength.

The same situation holds true where a multi-part object is designed to be assembled using screws or bolts. Using a low percentage of infill usually will result in a weak connection, due to the fact that the bolt or screw is more likely to gain insufficient purchase or miss the infill altogether when density is low.

Again, if you’re looking to maximize strength, using a higher quality filament with a stronger pure based resin (higher grade and without filler that cheaper makes can use) with better layer to layer adhesion will build more strength into your prints. 

Variable Infill Density

While preset infill settings and densities are great for the majority of projects, you can further personalize thanks to variable infill density. Put simply, variable infill density allows for different infill densities and patterns within the same part or model. 

Variable infill density is especially useful for functional parts, where different parts of the print require different levels of strength and durability. For example, load bearing support on specific parts of a print, contact points on mechanical parts subject to substantial friction, or adding extra density – and therefore weight – to a decorative model or figurine so that it stands upright.

Conversely, you can dial in a lower density for areas that don’t serve a mechanical purpose or don’t need added strength.

With clever use of variable infill density you also waste less filament and reduce print times by only using denser and more complex patterns where they are needed.

Variable Infill Density in Cura

Unfortunately, Cura doesn’t feature a specialized variable infill tool, so it’s about workarounds and the most popular option is to use the support blocker feature. Though a little complicated, it does work to tweak the infill density at various points on a part.

Load a model in Cura, then tap the Support Blocker button in the left-hand toolbar. This will create a Support Blocker, either cube shaped or a custom shape. You can modify the shape later on so don’t worry too much about where it lands on the model right now.

Click on the model, then click on the Per Model Settings option in the left-hand toolbar – it’s just above the Support Blocker button. Select Modify Settings for Overlaps as the mesh type. Click on Select Settings. Type Infill Density into the filter search bar at the top, then tick the box next to Infill Density. Tap close.

Back in the mesh settings window on the left, dial in the infill density you want for the area the cube covers. For example, you could crank it up to 80% for a load bearing part, and take it down to 10-20% for a purely cosmetic area.

Click on the model, then move and alter the shape of the Support Blocker to match the part of the print where you want a different density to the global setting. We recommend creating a plane that exceeds the size of the print, so that you’re defining parameters for layers. This makes it much easier to work with than small cubes and ensures you are covering the full layers on that part of the print.

You can then repeat the process for all the points of the print you want to modify. Slice and print as usual from here. You can get an overview of your changes after slicing the model by heading over to the preview tab then cycling down through the layers to view the different density settings as defined by the Support Blockers.

Variable Infill Density in Simplify3D

In Simplify3D, you can use the Variable Settings wizard to set variable infill. Head to tools, then select the Variable Settings Wizard. Move the red plane to the area of the print you want to modify, then tap Add Location. You can repeat this for the areas of the print you want to tweak. When you’re ready, tap Split Process. 

In the processes window, select the area you want to edit, then change the infill settings as needed. Repeat for all areas. Remember to tap save. Alternatively, you can use Process Grouping to edit several areas at the same time. 

Once you’re done, click on Prepare For Print to bring up a new window. Select all the processes then tick continuous printing, then tap Ok. Simplify3D will then slice your part, taking into account the variable area settings you dialed in previously. 

Once it’s done, you’ll get an overall preview of your print. By clicking Current Process, the preview will breakdown you different areas using several colors to get a better sense of what the variable infill settings are located. When you’re ready, fire off the print. 

For a more comprehensive walkthrough, check out Simplify3D’s dedicated guide.

Variable Infill Density in PrusaSlicer

PrusaSlicer allows you to alter settings for specific areas of a print using several tools – height range modifier and model modifier mesh.

Height Range Modifier

Load a model into Prusa Slicer. Right-click on the model and select ‘height range modifier’ from the drop down menu. A new ‘height ranges’ window will pop up on the right where you can define areas based on layers. For example, layer 2 to layer 30, or layer 20 to layer 40. 

Select the intervals you want to tweak, then tap the gear symbol. Here you can define a custom infill, layers, perimeters, support material, extrusion width, speed, layer height input, and more.

Model Modifier Mesh

Load a model into PrusaSlicer, then right click on it and choose ‘Add Modifier’. From here, you can select a preset modifier type (cube, sphere, slab, cylinder) or tap load to load in a custom mesh shape pulled from, for example, a different 3D model. Try and choose a modifier that best fits the shape of the area you want to tweak.

From here, right-click on the modifier to change the infill, layers, perimeters, support material, speed, extrusion width, and more. You can then place the modifier over the part of your print you want to alter, signaling to PrusaSlice that you want custom infill settings for that specific place. Repeat as needed for the rest of the print, then slice and print as you usually would.

For more insight into how modifiers work to offer variable infill settings, check out the dedicated guide on the PrusaSlicer website.

Support Infill Percentage

Much the same way you may want to increase infill in areas that are higher stress, it’s often wise to reduce support infill percentage as much as you can get away with. For very small supports often 0% will be fine.

This allows you to save filament and keep printing speeds lean. 

Infill Problems

infill_problems

This common printing issue, shown in the image above, is often assumed to be a problem with your infill settings. However, this issue is actually a simple case of under extrusion – if a very extreme example. 

Because the infill wall widths are often printed much thinner than the outer walls of your print, under extrusion issues nearly always become more obvious with infill, even if the thicker printed outer walls appear fine at first. 

If you’re getting spongy infill problems, then you may need to look at sorting out your under extrusion issue first. 

Other issues, such at the infill not touching or binding fully with the outer walls of your print (put simply, gaps between infill and outer walls), could be caused by incorrect slicer settings, if not already a symptom of under extrusion mentioned above.

To remedy this, you’ll want to ensure your ‘infill overlap’ settings in your slicer are set correctly. Often the cause is that they’re not set initially, or set to ‘0’.

Experiment with incrementally higher values (start at around 10% and usually don’t exceed 50%) with your specific print until the problem is solved. 

Summary

In the end, when thinking about infill, you want to remember the unique relationship between strength, cost and print time. Every increase in an object’s strength comes with a corresponding increase in printing cost and time.

The secret to a successful use of infill is to find the sweet spot where sufficient strength is obtained for an object’s designed purpose, with both cost and time being kept within acceptable parameters.