All users of 3D printers know that 3D printed parts have two distinct zones, the shell and the fill (infill). The correct parameterization of these two values influences the mechanical resistance, the finish, the printing time and the cost. The shell are the outer walls of the piece in which both the layers in contact with the printing base and the top layers that attribute the final surface finish are included.
In this article we will focus on the inside part of the piece, the infill. Lamination programsn (Cura3D, Simplify3D, etc) allow us to choose the shape and percentage of infill we want. Next we cite the most important possible configurations.
Infill percentage in 3D printing
The infill the amount of material that occupies the internal part of the piece. Normally, rolling programs allow the percentage of material to be modified from 0% (hollow part) to 100% (totally solid part). Always speaking of the same configuration of layer height and width of the housing, the ideal value of the infill percentage depends on the final application of the piece in question. The most used percentage, which many lamination programs use as standard data, is 20%. With this percentage you can get pieces with medium / high strength, low weight and a very efficient printing time, which transforms into parts with a good resistance / cost ratio.
For non-functional prototypes, models and other objects of simple exposure the recommended infill is 10%. With such a low percentage, the long printing times of complex figures or objects that do not need resistance to any type of stress are reduced. On the contrary, every user is clear that in order to achieve maximum tensile strength they must make their pieces at 100% infill, but this implies higher costs, both in terms of time and material and that the pieces are heavier. Outside of the values already mentioned, we recommend to study each case in detail according to the resistance / printing time taking into account that from 25% to 50% of infill the resistance is increased by 25% and from 50% to 70% of infill resistance is increased by only 10%.
Image 1: Different infill percentages
Our recommendation is to use a 10% fill for figures and objects that do not support loads, 20% for pieces of normal use with medium / low loads, 60% in case you need to make final pieces with a medium resistance and valid to be perforated or screwed, and finally 100% to achieve maximum strength of the material. We want to emphasize that the manufacturing orientation of the parts affects the final resistance, that is, the orientation matters as much as the infill percentage.
Depending on the lamination software used there are different types of infill, but the four most used (Rectangular, Triangular or Diagonal, Wiggle and Honeycomb) appear in all (Simplify3D, Cura3D, Slic3r, etc).
By default, these softwares use rectangular infill, which is logical for having a strong structure in all directions and relatively fast when making perpendicular layers on top of each other.
The triangular infill is applied, as in the structures of everyday life, to achieve maximum resistance in the direction of the walls, this is due to the decomposition at half of the force applied by being at 45° the lines that form each layer.
On the contrary, if our need is to obtain a piece that is as flexible, as compressible and soft as possible, the best infill is the Wiggle. This zigzag infill increases the rebound force and provides sufficient support to ensure the total coating of the top of the piece.
Honeycomb infill (Tri-hexagonal)
Finally, relying on wise nature, the honeycomb infill (bee panel) in hexagonal shape is widely used as a core to give strength to parts made of carbon fiber and other types of fibers. In 3D printing FDM / FFF offers the pieces a great resistance in all directions, greater than the rectangular infill, but with longer printing time.
In our view, rectangular filling is strong enough for 90% of 3D printing applications, the remaining 10% being cases where a very specific filling should be used.
Our recommendations for choosing the infill
The correct configuration of the filling is a very important step to obtain the desired resistance in the parts made by FDM / FFF 3D printing. Our recommendation is to use the rectangular infill with a 10% density for non-functional parts, models or prototypes, 20% infill for parts with normal use subjected to low / medium loads and 60% for elements that have to withstand high loads. The type of infill and percentage values must also be adjusted to each type of 3D printer, and, if the material to be used is rigid (PLA, ABS, PETG, Nylon, etc) or flexible (Filaflex TPE or TPU) to get the best possible result.