What Are Shells In 3D Printing?

The 3D printer produces some awesome results with shells. We take a look at how to design them and what factors affect their performance.


We’ll introduce you to two key aspects of 3D printing: shells and infill.

What Are 3D Printing Shells?

Shells are the outermost layer-thickness of your print, while infill is the percentage of space between the shells that’s printed with support material so that the shell has something to support it when it’s done printing.

The shells should be as thin as possible so that the inside of the object is hollow, while the infill percentage should be maximized to save material and make sure all the support material that was printed for the shell can be used.

Shells are also used in combination with fiber reinforced resin to create parts that exhibit the strength of polymers while having the benefits of higher glass transition temperature and superior impact resistance compared to traditional thermoset matrix materials.

The number of shells an object has can be modified during the slicing process, or can be set when exporting a model as an STL file.

Some Common Shell Settings

Some common settings for a 3D printed shell.  these are all things to think about when designing 3d printed phone cases and other items.

Shell Thickness

It is important to know the 3d printing shell thickness for different prints, but it doesn’t always need to be at the same thickness. Why? The thicker the shell, the better the 3D print will hold its shape. Objects with thin shells tend to be more fragile.

The ideal thickness of a shell depends on your goal, and how you wish to accomplish it. In general, higher quality object printing is achieved with a thicker shell. However, you will also pay a price for this quality in time and material cost. Printing one layer at a time may result in the same object thickness as printing multiple layers.

The thicker the shell, however, the stronger, more rigid and better looking the resulting object. For this reason you will often see hard to find print in which a 3D printing material is used with a very thin shell. There are also some people who like to print two different shell layers for different purposes. This allows for both strength and detail for one part.

Outer Shell Offset

The Outer Shell Offset is the outermost layer of a 3D printed object. It is typically a solid shell that follows the contours of an object’s shape. The Outer Shell Offset can be a separate object, (like the famous “Lattice” from MakerBot). It can also be built into the main part of a print.

In this case, the part-to-part gap between the outer shell and other layers of a printed object should be minimized to avoid an empty void.

The Outer Shell Offset should ideally be less than 50% in most cases. If you need to balance it out with the infill percentage, make sure you choose a lower Infill setting.

Inner Shell Offset

Inner shell offset is a 3d printing method used to create parts with complex geometry, which cannot be created by subtractive methods. Inner shells are first created by traditional printing methods and then the parts are assembled using laser sintering or fusion sintering.
Example: Finite Element Analysis (FEA) shows the stress in a 3D printed part.

Calculating Shell Thickness Requirements

The rule of thumb is that a wall must be thick enough that the nozzle line width (how wide the extruded filament is) will be no more than half of the wall thickness [1]. So if we have a printer with a layer height of 0.25 mm, and our nozzle has a line width of 0.4 mm, then wall thickness must be at least 0.25/0.4= 0.75 mm.

Shell Thickness Minimum And Maximum Measurements

Shell thickness measurements are the most important parts of a 3D printed object. The minimum shell thickness is the smallest possible thickness that will allow the printed object to be functional, and the maximum shell thickness is a safety measure that allows the part to be sturdy and secure.

These measurements are associated with different filament types:

  • Thin Filament: Requires a minimum shell thickness of 0.5 mm to prevent the wall from breaking. The maximum shell thickness for thin filament is 2 mm, because it cannot support more than that. However, increasing the shell thickness above 2 mm will not significantly increase the strength of the print. Therefore, we recommend a maximum of 2 mm for thin filament.
  • Warm Filament: Requires at least 0.5 mm of shell thickness for the walls to be strong enough to hold their shape and prevent breakage. The maximum shell thickness is 2 mm. Because of its flexibility, hot-air printers and FFF printers are able to print shells with a thicker wall than those needed for thin filament alone.
  • Wire Filament: Uses the same minimum and maximum shell thicknesses as thin filament, but also requires a small amount of infill to increase strength.
  • Hot-Air Filament: Requires a minimum shell thickness of 0.5 mm to prevent the wall from breaking. The maximum shell thickness is 2 mm, because it cannot support more than that. The high temperature of an air-jet nozzle may cause thin walls in printing if they are allowed to melt due to excess heat distortion (high temperature).
  • Fused Filament Fabrication (FFF) Filament: Requires a minimum shell thickness of 0.5 mm to prevent the wall from breaking. The maximum shell thickness is 3 mm.

The reason for these limitations is that all filaments need to be supported by the print medium in some way in order to be printed, so they cannot be more than the same diameter as that of the print medium itself.


In conclusion, it’s important to know how to design shells for your 3D prints. It’s also important to know the different settings which are involved with each type of 3D printer. It’s important to know the thickness requirements of your filament types. Knowing how to properly design thin or thick shells for your 3D prints will help you to create better prints, and produce a higher quality finished product.