Tuesday, September 17, 2013

Understanding Shells, Layer Height and Infill

Getting great products out of a 3D printer requires an understanding of the settings that will ultimately dictate how the object feels in your hand. Those settings are the number of shells, infill percentage and layer height which directly control the density, surface finish and durability of the final print.

Shells and Parameters

Examples ranging from 0% to 100% of 3d printed infillIf we think about each printed layer of an object as a two- dimensional drawing laid out on the X and Y axes, then the number of shells on the object refers to the number of times the outline of the drawing is retraced. If the printer only traces the outline once, it is said to have one shell, if it retraces the outline a second time then it is said to have two shells. They are called shells because they are the outer most layer of the object and ultimately the part of the object we see and interact with.

The more shells on an object, the stronger it is. However, adding shells will also increase the print time significantly. Shells are also referred to as perimeters in some software and documentation.

Rule-of-thumb: Use fewer shells when prototyping or printing decorative objects, use more shells when printing items that will be put under more stress.

Layer Height

The number of layers in a print, or the object’s resolution, is determined by the layer height setting. Layer height is measured in microns (one millionth of a meter).

High-res objects use many very thin layers to create a smooth object. With high-resolution printing it becomes difficult to see individual layers in the object because layers are printed as thin as a sheet of paper at a thickness of just 100 microns (0.1mm).

Low-res objects are made of fewer, thicker layers. These objects feel rough to the touch and contain layers that are more visible to the eye, like sediment or the rings of a tree.

Items intended for display purposes are typically printed in high resolution, while prototypes and everyday objects can usually be printed at lower resolutions and at faster speeds.

High-resolution objects take longer to produce than low resolution ones. This is because for every one layer of a low-resolution object, there could be 5 times as many layers in a high resolution object. Each of those layers is extra time spent printing, but they can make a substantial difference in visual quality.

Rule-of-thumb: Use a bigger layer height for prototyping and rapid production, use a smaller layer height for display objects and more-accurate tolerances

Infill

Infill is the material used to fill the empty space inside the shell of an object, it refers to the density. Infill is measured by percentage, so an object printed at 100% infill will be 100% solid. More infill will make an object stronger, heavier, and slower to build. Likewise, less infill is lighter and quicker to build.

A 3D printer can extrude infill in several patterns. Some slicing engines create a grid pattern while others will use hexagonal or other geometric patterns. Items printed for display purposes rarely need more than 10%-20% infill, but functioning mechanical parts and pieces that will take more abuse will need 75%-100% infill.

Rule-of-thumb: Use less infill on test objects and prototypes that wont be subjected much stress, use more infill on functional mechanical parts and objects that need to be durable.