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  • Charlie Kocur

What is 3D Printer Infill and When Should it be Used?

Intro to Infill

What the average person might not realize about 3D printed objects is that they are not always hollow. In fact, I would say most 3D prints have some form of interior structure. This structure is called infill, and it's extremely important to the 3D printing process.


Infill is what gets printed on the interior of a 3D model. While it can't be seen, infill's purpose is to add strength as well as support the top of the model. Infill does not have to be created as part of the 3D model itself but instead is generated by slicer software (the tool that creates instructions for the printer to follow). While not all 3D models require infill, you will likely find yourself using it often. Because of this, it's important to understand what 3D printed infill is, why it's important, and how to use it.


Before I did deeper research on 3D printing, I had no idea what infill was or what its purpose was. I'm going to share with you the things I've learned about infill so you don't make the same mistakes I did. It's my hope that with this knowledge, you will have more successful prints and you maybe even will save some time and money once you become an infill expert.


 

What is infill?

As briefly mentioned, infill is an interior structure within a 3D printed model. It is printed using the same plastic and method as the rest of the model. It's basically a pattern that fills the inside of a 3D printed model and it can't be seen from the outside.


The infill structure is generated as part of the slicing process. The first step in 3D printing, after you've found a model to print, is to load the 3D file into what's called a slicer tool. This software creates a set of instructions for the 3D printer to use. Within these slicer tools, there are infill options that can be set. I'll discuss some of these options in more detail further along in this article.


First though, here is a very simple example of infill. In the below images are the same exact model of a cube shown within the Cura slicer software. On the left, the cube is empty because there is no infill. on the right, the cube has an interior filled with a grid pattern. This is the infill pattern generated within the software.



Once the settings are set, the slicer software will generate a pattern within the 3D model based on the settings and it will become part of the instructions for the printer. The interior infill pattern is printed along with the rest of your 3D model. As far as the printer is concerned, the infill was just a part of the 3D model all along.


 

Why use infill?

Including infill in a 3D print takes significantly more time and material compared to just printing the exterior shell of the model. If it costs more time, material, and money; why would you use infill?

The simple answer is, you would possibly be spending even more time and money re-printing the model from scratch if you didn't use infill at all! There are many 3D models that would simply fail entirely or would just be low-quality without infill. Instead of the failed print time and plastic going to waste, infill will help make prints successful the first time.

Infill serves a couple of key purposes for making a successful, good quality 3D print. Without infill you may get a model that has poor quality, or that may not even print at all!


Infill Purpose One: Strength

This one may be kind of obvious, but it's worth talking about. Having an interior structure throughout the print ties the entire thing together and makes it stronger.

To demonstrate, I printed two versions of a simple cube. Once with and one without infill. I stopped the print before it was finished so the top of the cube wasn't printed. That way, the inside could be seen. Not only does this show what the infill pattern looks like, but it also demonstrates how infill gives prints their strength.


No Infill = A reduction in strength.

As you can see in this below example; if the cube without infill is squeezed, the sides bend inwards and the entire structure flexes. Since there is nothing to press against on the interior, the entire model just bends.




Infill Added = Strong!!!

In this example, I'm squeezing even harder than before, but the model with infill doesn't bend or deform at all. It remains rigid and keeps its shape. This interior structure connects all sides of the model and creates a lot of strength.




Strong is good... most of the time.

A general rule of thumb is that the more infill you include, the stronger the part is. This is true all the way up to 100% infill, which means the print is completely solid and filled entirely with plastic. With that said, there is a balancing act because infill uses additional material and takes significantly more time. You want to make sure to only use enough infill to create a functional print.


For example, if I have a model that I'm printing with the intent of having it just to sit on my desk as a statue, I'll probably go for as little infill as possible. My goal is to use some infill as necessary for the model to complete, but not so much that I wasted extra time and material beyond what was needed.


An example of this is this Walt Disney bust I printed. amazingly, this was done with absolutely no infill! Since it just sits on my desk, I knew it didn't have to be strong at all since it wasn't supporting any weight, and wouldn't be touched once I had set it in place.



On the other hand, if I'm creating a plastic piece that needs to be more functional and serve a purpose, I will take the extra time to increase the infill because I know it will make for a stronger part.


An example of a functional piece is this bracket that I created for my dishwasher. I designed this piece to work in a pretty rough environment with heat and moisture. Because of that, I used a high infill to make it extremely solid.



The purple bracket was printed with a very high infill value to maintain strength.

There are always trade-offs but as you learn about 3D printing and gain more experience knowing where to set your infill for strength will become second nature.



 

Infill Reason Two: Support Structure

If you are familiar with the concept of overhangs then this section might be obvious to you. One of the main purposes of infill is to create an interior support structure. This structure not only gives the overall print strength, but it also builds a framework so the printer can successfully print at high interior angles.

The Opposite of Overhangs

This is where familiarity with overhangs comes into play. Infill solves the problem of what are essentially interior overhangs. A simple way to picture it is by visualizing a pyramid, and a cube.


The pyramid gradually gets narrower as it gets towards the tip. With this shape, each layer that prints shifts over slightly and allows the printer to print at an inward angle that slowly builds up. Because each layer is able to print on top of the previous layer, there is support all the way up.


With a cube however, once the sides are printed on top of each other printer would have to attempt to print in midair to put the top on the box. Because there is no support structure below, it's likely that the plastic will droop down.


Because a 3D printer uses hot plastic, it needs time to cool. Giving the hot plastic something to rest on gives it the time it needs to cool and maintain shape. If there is nothing for the plastic to rest on, it can deform due to gravity and temperature fluctuations.


To show this, I printed two of the exact same cube. One with, and one without infill.

On the left, the top of the cube looks solid, with a nice flat top. On the right, the cube has a deformed and bubbly top because there was no interior support to help hold up the plastic while it cooled.




A cube with a totally flat top is a extreme example but you can see how using infill comes in handy. Not only is it providing strength overall because it connects all sides of the model, but it also provides support for the printer to put plastic on top of so the plastic has time to cool and doesn't droop out of place.


 

When to Use Infill

Since using too much infill for a print can be a waste of both time and money, it's important to know the situations where infill should, or shouldn't, be used. Over time and with experience, home 3D print enthusiasts will quickly be able to determine if a print requires infill or not. It can easily be done just by looking at the print. As you learn how to 3D print, make sure to pay close attention to your failures and learn from those mistakes.


Reasons to use infill:

Do use infill if the part needs to be strong.

As mentioned earlier in the article, I've printed parts before that required extra strength. Examples include weight-bearing brackets, or items that need to survive a while in a tough environment like the dishwasher bracket I mentioned earlier.

Because these prints are being put to good use, it's worth the extra time and material to print them strong the first time. Otherwise, it's likely you'd end up using even more material re-printing a broken piece! If parts are going to be put to everyday or practical use, it's most likely you'll want a high infill percentage (depending on the use of course!).

What is "high" when it comes to infill? Personally, I'd consider anything 20% or above a pretty solid piece. For very small parts like the dishwasher bracket, I printed it at 100%. Since the piece was so small and going to be under a fair bit of stress, I figured I would make it as solid as possible. For a larger piece, like a miniature windmill prototype I created, I printed at about 25% infill. That was probably overkill, but as I mentioned, I wanted to make sure this held-up to some tough use. It did the job great!

As a side note, if you want to check out the windmill model I created, you can download it here.



Do use infill if the print will likely fail or suffer poor quality without it.

As described earlier in this article. Some prints might require infill just to finish successfully. Even if a print finishes, you might lose quality by not supporting internal overhangs with infill acting as a support structure.

If you have a model with complicated geometry at the top, or severe interior overhangs that your printer wouldn't be able to print over; make sure to at least use some infill to provide enough support for the plastic to cool before getting warped by gravity or temperature changes.

The need for this to support upper layers may depend a little on how well your printer is tuned, if you have a fan that cools the filament, the filament you're using, etc. But if you find that the tops of models droop down or have a poor quality compared to the rest of the print, it's possible that turning up the infill percentage slightly will help!

When not to use infill:

Don't use infill for parts that don't need strength.

A lot of 3D printing for the home ends up being some kind of decoration. Be it a replica of a favorite video game character, a cool art piece you've designed, or even a stand to hold something small; all of these things don't really require a ton of strength. Because of this, it's a major time saver to use very little or no infill at all.

For show pieces like this, the absolute minimum should be used for infill. Many of my desktop trinkets have been printed with 0% infill, and they work just great! That's right, whenever possible, I try my best to print models that are completely hollow. I can't even begin to count the amount of time I've saved by keeping the inside empty!


 

How to use infill.

3D models do not arrive to you with infill already included as part of the design. In fact, infill is generated within the slicer software as part of setting-up the model for printing. Slicer software generates instructions for the 3D printer to follow, and creating these settings is one of the last steps before pressing "start" on the printer.


Within Ultimaker Cura (my slicer software of choice), there is simply a section that says "Infill". To add infill to a print, simply put a number in to the field marked "Infill Density". Unless you need serious strength, I generally set my infill to 8%-10%. This tends to be plenty for the print to get the support it needs.



There are many additional options in Cura that are available to you for infill settings. These are the ones I use most often though:

  • Infill Pattern: This defines the type of infill the slicer software will generate. For examples of this, see the next section of this article called "Types of infill".

  • Gradual Infill Steps: This is a slightly advanced feature, but learning it could save you some time in the long run. Basically, this field reduces the amount of infill at the bottom, and gradually adds more as the print gets near the top. That way, the top of the print has an underlying support structure exactly where it's needed while using as little plastic as possible elsewhere.


 

Types of infill.

While infill is pretty simple to understand, there are multiple options available for how it's created. These options include different patterns like grids or circles; and the density can be set as well.

Within the slicing software I use (Ultimaker Cura), there are several options for the style of infill that gets generated. For my examples below, I've gone through the latest version of Cura and set up some examples of each pattern using the cube from earlier in the article. Generally, I use a basic cube grid and it's worked perfectly for me so far. That said, it's good to know your options!

Cura breaks these down in to a few categories, which I've included below:

  • Strong 2D Infill: This is the most basic and easy to understand. It's also what I use 99% of the time so it's effective! These types are god for regular prints that need some strength or interior support.

  • Quick 2D Infill: Similar to the Strong 2D infill; Quick 2D Infills provide support, but are not as strong as the Strong 2D infills.

  • 3D Infill: These are unique in that they provide 360 degree strength throughout the model. 2D infills do provide some overall strength, but still creates some weak points, especially on the sides. 3D infills create a pattern that is equal in all directions providing equal strength all around.

  • 3D Flexible Infills: These are interesting and what I have the least experience with. 3D Flexible infills give the model some support and strength, but also provides a little flexibility for the model.

Strong 2D Infill Patterns:

Cube: (For ease of use, I recomend you start here, it's what I use most of the time with great success!).



Triangles: Similar to cube, but a different, triangular pattern.



Tri-Hexagon: Yet another variation of cube pattern. This is a combination of hexagons and small triangles.



Quick 2D Infill Patterns:

Lines: While this looks just like small cubes, they're actually lines that alternate direction as the model is created. This is the quickest infill pattern and while not as strong, is a great option for interior support.



Zig-Zag: A variant of the lines, these print in a single, diagonal direction.



3D Infill Patterns:

A quick note on the rest of these patterns. Since they are 3D patterns, they're difficult to capture in a 2D image. I set them at an angle to show them the best I can.

Cubic & Cubic (subdivision): These infill patterns create small cubes inside the print, providing strength in all directions. According to Cura, the (subdivision) version of Cubic 3D pattern saves material vs. the regular Cubic style.



Octet: A variation of cubes within the 3D model. Again, providing strength in all directions.


Quarter Cubic: The final set of Strong 3D infill, again providing all-around strength throughout the model.



3D Flexible Infill Patterns:

Concentric & Concentric 3D: The first set of "flexible" infill patterns. These don't run from edge to edge and connect to the sides like the other patterns, but instead print within the model entirely.



Cross & Cross 3D: These are by far the most unique infill patterns. Similarly to the concentric pattern, since this design doesn't completely interconnect, it allows for things to bend a bit. no to mention, it just kind of looks cool!



How much longer does infill take to print?

This answer can vary wildly based on the printer settings, the model you are making, how high of a quality print you want, etc.

Just for an example though, I will cover a few various settings with the cube I printed earlier in this article. here are side-by-side comparisons of time vs. infill percentage while maintaining all other settings. You will have to play around for yourself to see what your machine is capable of, but this is a good starting point!

Below I created two main scenarios.

  • Lower Quality (0.2mm layer height)

  • Higher Quality (0.1mm layer height)

The following settings were the same across both scenarios, the only change made was adding infill percentage. This way we can easily see how infill alone changes the print time. For both Lower Quality and Higher Quality prints:

  • Wall Thickness: 0.6mm

  • Top/Bottom Thickness: 0.6mm

  • Print Speed: 40mm/s

To be consistent, no settings other than the infill was adjusted on each print. Based on that info, the Cura software will give an estimate of how long the print will take. Now for the results!


Layer Height (Quality)

Infill %

Est. Print Time

Time Increase % (vs.0% infill)

0.2

0%

1h 12min

n/a

0.2

10%

2h 37min

+118%

0.2

25%

4h 3min

+237.5%

0.1

0%

2h 25min

n/a

0.1

10%

5h 13min

+116%

0.1

25%

8h 4min

+234%


As is shown in the last column, infill adds significant print time because the printer has so much more material it needs to lay down. Not to mention, the use of material is much higher as well.

Using lots of infill when it's not necessary will mean that you'll have to be ordering filament replacements more often. Over a few years of 3D printing, this can add up to a lot of extra money!


 

Conclusion

Mastering infill settings is an important part of not just printing efficiently, but also ensuring your prints complete successfully! The goal of this overview was to teach beginners about how infill can be used, when to use it, and why you would want to use it.