Optimal Shell and Infill Parameters for FDM

Jack Lie CNC machining expert

Specialize in CNC milling , CNC turning, 3D printing, urethane casting, rapid tooling, injection molding, metal casting,


Most FDM parts are not fully solid, this will reduce total amounts of materials and printing time, finally result to economical printing cost. In order to optimize the FDM printing process, solid shells and addition with infill will play an important role on final parts quality, appearance and functions. We will summarize the difference between shells and infill and how to optimize shells and infill in FDM design.

FDM Layout

As standard FDM process can be divided into 4 sections, all these section parameters can be adjusted to optimize final design.

Shells: this is the walls in printing process, which are exposed to the model outside area.

Bottom layers: this is a type of shell, which is facing the build plate in the model.

Top layers:this is another type of shell, which is facing upward and toward the nozzle in the model. Normally has the best surface finish.

Infill: this is internal structure in print model.


In FDM, shells are the first area to print per layer, there are several design considerations for FDM process:

  1. High shells thickness can increase shells strength, this can result to more robust printing without infill materials usage increase. As most shell thickness can be adjust to create high stress area with high density.
  2. Post-processing methods like sanding or chemical smoothing will reduce shell thickness, so we need to increase shell thickness as necessary.
  3. Shells number will determine the amount of time and material in printing process, and finally determine the overall part cost.
  4. Shell thickness should be specific number of nozzle diameters. We should design shell thickness of multiple size of nozzle diameter, in order to avoid void in printing process.


Most FDM parts have default 18% to 20% infill to create typical low density core, this is the perfect for FDM applications with more affordable and effective operations.

Infill Percentage

Infill percentage determines the printed parts final strength, parts with 50% infill is typical 25% stronger than that of 25% infill, while infill percentage increase form 50% to 75% will only increase strength by 10%.

Final printed parts application will determine the optimal infill percentage. In FDM prototyping, low infill percentage can be used for form appearance to reduce cost and time significantly, while high infill percentage can be used for bracket to perform experience loading. Our standard infill percentage is 20%, which is the default parameter in most printer and acceptable for most application.

Infill Strength

Infill plays a critical role in the way of connection between extended or protruding features and models. In snap-fit connections, the base of this connection is normal a weak point. Especially in a low infill density of 20%, the cantilever is easier to break as only short extruded clip connect to the body by a small cross-section area. Increasing infill percentage will result to more cantilever connect to body area, finally increase the connection strength.

Screwing, Tapping, Bolting

Once FDM parts requires further post-processing like drilling and screwing, infill percentage will be a important factor. Normally, the drill holes will go through the top and bottom layers, and miss the low density infill in models. This will give rise to a weak connection, so higher infill percentage is desired at minimum of 50%.

Connection with clearance holes and bolts are more suitable for low infill percentage, the shells, walls and infill will provide excellent compressible strength for parts anchoring.

Infill geometry

In standard FDM, infill geometry are in forms of angled hatch or honeycomb shape, we list the most common infill shape as following:

Rectangular: this is the standard infill pattern for FDM process, it has high strength in all directions with reasonable time. It only requires the least amount of bridging across in infill pattern.

Triangular or diagonal: this is normally used for strength requirement of wall directions, triangular will spend longer time to produce.

Wiggle: this structure will create model to be soft, to twist or to compress. This is a excellent choice for soft rubbery material or soft nylon.

Honeycomb:this is the most popular infill geometry, it is quick to print and very strong, and has strength in all directions.