Infill is a characteristic unique to the FDM process. In FDM, an extruder nozzle deposits material along the toolpath, fusing it to previous layers to build a 3-dimensional body. Thus, it is called Fused Deposition Modeling.
In this process, the exterior surface is printed with several contour paths to ensure dimensionality. The interior of the volume can be printed as “solid,” but it does not necessarily have to be.
The pattern printed inside the contours is referred to as “infill.”
Types of Infill
While there are a theoretically infinite number of infill configurations, we offer three standard infills that provide options for balancing weight, strength, and cost.
Solid infill
Highest strength, closest to full density. This is equivalent to Stratasys' Solid build style.
Method: The extruder deposits alternating diagonal patterns such that the deposition lines connect and press together, creating a near-solid.
Solid is 100% density, right?
Not exactly. There may be some very small voids in between layers where the infill lines meet, but the part will behave as a solid part does. As a result, FDM will not be water-tight as built.
Light infill
Weight reduction with some strength reduction. This infill is equivalent to Stratasys Sparse Double Dense build style.
Note: Light infill may trap liquid in air gaps. Sometimes, this trapped liquid does not dry out during post-processing and may leak during shipping and handling. Ultralight or solid infill is recommended for large volumes.
Method: A grid pattern is deposited in every layer.
Ultralight infill
For form retention, when the part's strength isn’t an issue. This infill is equivalent to Stratasys Sparse build style.
For Prototyping PLA, which is run on desktop 3D printers (Prusa MK3S/MK4 or Bambu Lab X1C), an ultralight infill of 20% is used per their build setup program (PrusaSlicer or BambuStudio).
Method: A diagonal lined infill pattern. Direction alternates layer by layer, creating an ultralight grid infill.
Common questions
What percent infill is this?
The final percent infill by volume will vary based on the geometry of the parts. Every part will have a set number of “cap” layers on the top and bottom (default 4) and 2-3 contour layers based on part size. Because of this, the surface area to volume ratio will cause the total density of the part to vary.
A thin, flat plate can be 100% filled despite having ultralight infill selected if it is thinner than the sum of the top and bottom cap layers.
A large cube may have a very low density due to the high volume to surface area ratio.
Because of this, the percent density of parts varies considerably based on part geometry.
For example:
Lookup Table of Infill % Via GrabCAD Print
GrabCAD Print is a Stratasys software used to prepare files for Fortus FDM machines, the primary FDM equipment used at Xometry. Sometimes, a % can be found based on different infill options. The reference table below can help clarify %, but the information above this section is better for understanding the infill context since geometry plays a significant role.
These are for reference only but demonstrate that infill varies depending on the material chosen. For example, ASA has good bridging characteristics for FDM and can have more sparse infill under "ultralight" than ABS-M30.
Where can I learn more about FDM?
For more information on infill options and the FDM 3D printing process, check out our FDM Design Guide!
Visit our 3D printing services page.
