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What are Shell and Infill in 3D Printing? - RP Space

What are Shell and Infill in 3D Printing?

Parts printed using FDM (Fused Deposition Modelling) 3D printing technology are usually not printed solid. This is because it would use a lot of material and a lot of time to print solid parts.

Instead, they are printed using shell and infill.

This means even though the outside of the 3D printed part looks like a solid model, inside the model is usually filled with patterns known as infill.

Infill of 3D Printed Parts
Infill of 3D Printed Parts

Infill is used to greatly reduce the amount of material and time needed to print a part, while not compromising too much of strength.

And as you may have guessed, the outer layers surrounding the infill is known as shell.

Shell and Infill
Original Model (Left) and Dissected Model (Right)

Shell includes both the walls (sides) and the top and bottom layers.

However, the difference between walls and top/bottom layers can sometimes be really hard to differentiate on angled and curved surfaces. Therefore to prevent confusion, the walls and top/bottom layers are usually called as one entity, known as shell.

 

Why Bother Shell and Infill?

Both the shell and infill of a 3D printed part can be tweaked using two settings known as shell thickness (for shell) and infill percentage (for infill).

These two settings are extremely crucial to the strength and density of your printed part.

Shell thickness and infill percentage are mainly used to increase/decrease the amount of material used to print the part, which will eventually increase/decrease the strength and the density of the printed part.

When these two settings and the types of materials are properly selected, you could get the desired strength and density out of your printed part.

 

Strength Don’t Always Increase That Much

Before we move on to explain the two settings, take note that even though increasing the shell thickness and infill percentage will increase the strength of the printed part, but the amount of strength increase is not the same in different directions.

This is because 3D printed parts are anisotropic, which means the part is stronger in certain direction, and weaker on other direction.

Part Strength for 3D Printed Part is Direction-Dependent
Part Strength for 3D Printed Part is Direction-Dependent
(Image by 3D Hubs)

This is because the strength on the weaker direction is determined more by the bonding between each layer.

Although increasing shell thickness and infill percentage will increasing the surface area where each layers bond together, the effect is not as great as on the stronger direction.

This means if your part is printed in an orientation where the load is applied on the weaker direction, increasing shell thickness and infill percentage will help, just not as much as if the load is applied on the stronger direction.

Therefore, increasing shell thickness and infill percentage will increase the strength of the printed part in all direction, just the effect will be greater on the stronger direction.

If you wish to gain the most strength out of the increase in material usage, take note of the orientation of the printed part to make sure you are getting the strength on the desired direction.

 

Shell Thickness

Shell thickness is, as the name suggests, the thickness of the shell.

The thickness of the shell is usually determined by layers of shells/perimeters (2 layers, 3 layers, etc.).

Each layer of shell is determined by the diameter of the nozzle, where 0.4mm is the most commonly used nozzle diameter.

So if the nozzle diameter is 0.4mm, one layer of shell would be 0.4mm thick, 2 layers would be 0.8mm thick, 3 layers would be 1.2mm thick and so on.

Shell Thickness: 0.8mm (Left), 1.2mm (Middle), 1.6mm (Right)
Shell Thickness: 0.8mm (Left), 1.2mm (Middle), 1.6mm (Right)

By increasing the shell thickness, the parts would be stronger and denser (heavier) due to the additional amount of material used.

However, additional material consumption means higher cost for the part and longer print time. Therefore, choosing the suitable shell thickness for your application would help you to achieve your objectives in a cost-effective manner.

 

How Thick Should I Use?

0.8mm shell thickness is sufficient for most parts which may experience low to moderate load. 0.8mm may sound like a small number, but together with the infill, 0.8mm shell thickness can perform well in most cases.

For applications where the parts which experience higher load or impact, 1.2mm shell thickness would be necessary and would greatly increase the strength of the part.

If more strength is needed, you could further increase the shell thickness, or increase the infill percentage, or increase both.

Take note that if you are communicating shell thickness with others based on layers of shell, difference nozzle sizes used on your 3D printer would affect the number of shell layers.

Smaller nozzle diameter would need more layers of shell, while bigger nozzle diameter would need less layers of shell.

 

Infill Percentage

Since infill is the pattern/structure printed inside the shell, infill percentage is the percentage of infill. A 10% infill percentage indicates 10% of the volume inside the shell is printed with infill, 20% infill percentage indicates 20% of the volume inside the shell, and so on.

Infill Percentage: 10% (Left), 20% (Middle), 30% (Right)
Infill Percentage: 10% (Left), 20% (Middle), 30% (Right)

Take note that for example, when 20% infill percentage is used, the amount of infill would not be exactly 20%.

This is because the slicing software would create the infill patterns in a way that will achieve a good part strength and print time.

Therefore, the actual infill percentage may be slightly lesser or more than the desired infill percentage.

However, the same rules apply, higher infill percentage results in more material used, and the printed part would be stronger and denser.

 

What Infill Percentage Should I Use?

20% infill percentage is sufficient for most parts which may experience low to moderate load. If you think your part may not experience any load, you could go lower, but in some cases, the top surface quality may suffer due to lack of material holding the top layers.

For applications where the parts which experience higher load or impact, an increase to 50% would help increase the strength.

If more strength is needed, we suggest you to increase the shell thickness instead.

This is because any further increase in infill percentage above 50% would not help the strength as much. The strength will still increase, but you are likely wasting more material (higher cost) for a very little increase in strength.

Therefore, if your part already has 50% infill percentage, increase the shell thickness instead to about 1.6mm if you wish to get more strength from your part.

 

Types of Infill Pattern

Infill pattern is, as the name suggests, the pattern of the infill. There are many infill patterns available depending on the slicing software used.

Each infill pattern has different functions, some for strength, some for print time, some for strength to weight ratio, some for strength in multiple directions and some for flexible prints.

There are a lot of infill patterns available, pictures below shows some example of infill patterns.

Infill Pattern: Grid (Left), Triangle (Middle), Tri-Hexagon (Right)
Infill Pattern: Grid (Left), Triangle (Middle), Tri-Hexagon (Right)
Infill Pattern: Cubic (Left), Gyroid (Middle), Concentric (Right)
Infill Pattern: Cubic (Left), Gyroid (Middle), Concentric (Right)

However, these infill patterns can be mainly categorized into three categories, 2D infill, 3D infill and flexible infill.

 

2D Infill: Grid, Triangle, Tri-Hexagon

3D Infill: Cubic, Gyroid

Flexible Infill: Concentric

 

2D infill is more suitable for parts which does not require much strength, such as display models.

This is because 2D infill are infill patterns which are only capable of holding load from the top of the print. If the load acts on the side of the print, 2D infill do not have as much strength.

Direction of Top Load and Side Load
Direction of Top Load and Side Load

3D infill on the other hand, are infill patterns which are capable of holding load both from the top and the side of the print. 3D infill is more suitable for functional parts where the load on the parts can be unpredictable.

Flexible infill are infill patterns which are mostly used to print flexible material. This is because flexible materials printed using other types of infill pattern may lose its flexibility. Therefore, flexible infill is when the flexibility of the printed part is required.

For our 3D printing service, we use 3D infill for all our prints to ensure the printed parts will be able to withstand load from multiple directions.

 

Conclusion

So, what is shell and infill in 3D printing?

Shell is the outer layer surrounding the 3D printed model, while infill is the patterns filled inside the shell. Infill is something we could not see on a 3D printed model unless we cut it apart.

3D printed parts are printed using shell and infill, instead of a solid part, because printing a solid part would use a lot of material and takes a long time to print.

Since 3D printing is a technology capable for producing complex geometries, that advantage is used to help reduce its print time and material consumption by utilizing infill.

 

Both the shell and infill of a 3D printed part can be tweaked using two settings known as shell thickness (for shell) and infill percentage (for infill).

By increasing the shell thickness and/or infill percentage, the strength and the density of the printed part will increase.

However, increasing the shell thickness and infill percentage will not always result in a proportional increase in strength.

Therefore, by optimizing the shell and infill of the printed parts, you could achieve the desired strength and density of the printed parts at a cost-effective manner.

 

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