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the previously added material, like laying down a new layer of LEGO bricks on top of the existing layer.
As with any building project, the first step to build the object is to create a design and generate building plans. In the case of AM, the design is done on a computer by drawing a 3D representation of the object. A wide variety of commercial tools are available to do this, and the goal is to create a computer file that the printer can under- stand. Because AM is mostly done in a layer-by-layer approach, the 3D object one wishes to create must be broken down into 2D cross sections, known as slices. For example, to build a solid cube, the plans would consist of identical slices of squares. To build a sphere, each slice would be a different sized circle. The job of the printer is to fabricate the object in successive 2D slices until a 3D object is constructed. Three common AM approaches are shown in Figure 2.
In the first example, the base material is in powder form, with grains having dimensions on the order of tens of microns. The process begins by spreading the powder to create a thin layer on the print bed, which is the “ground” for the object. One such printing technique, known as laser powder bed fusion (LPBF), then fuses grains together using a laser that heats the powder beyond its melting point. Only the powder that should be part of the final object is melted, and the rest of the powder remains in powder form. If the first layer is a circle, the laser will fuse powder together within the circle, leaving the rest of the powder unaltered. Another layer is then created by spreading powder from a stock basin using a rolling cylinder called the recoater, and the powder is again fused together at locations required to create the object. This process repeats, sometimes thou- sands of times, until the object is complete. This method is extremely versatile in that the base material can be a metal like steel or a polymer like nylon. Figure 2a shows an exam- ple of LPBF including two powder beds, one for the built part and one for the stock powder.
In the second example the base material is in liquid or resin form, and it is cured/solidified in a layer-by-layer manner using a light source such as a laser or projector, a process called photopolymerization. The most common AM techniques based on photopolymerization is stereo- lithography (SLA), which is used to print plastic objects ranging in size from a few nanometers to centimeters. Figure 2b illustrates a common approach where the “new”
build material is added to the bottom of the part as it is moved in the positive z-direction by a gantry. In this configuration, light is projected through a transparent window onto a thin layer of the uncured resin directly below the part.
The third method, material extrusion, is likely the most familiar to the reader. In this approach, the base material is a thin strip of plastic called a filament that is deposited along a predefined path that makes up the layer of the object being created. The nozzle deposits the filament by melting the base material, like how a glue gun converts a glue stick to a semiliquid substance that can be deposited at will. The filament is used to trace out each 2D shape on top of the previous one to create the 3D structure.
Material extrusion approaches can print a variety of materials and have the added advantage of being able to print parts made from two or more materials or even functional materials such as piezoelectrics (Chen, 2020). A common filament-based AM approach is fused depo- sition modeling (FDM), which is shown in Figure 2c. FDM employs the heated nozzle mounted on a gantry to extrude semisolid filaments like plastics, metals, or com- posite materials. This type of printer is by far the most accessible in terms of cost, with base models starting at a few hundred dollars. For this reason, and the fact that they are easy to use by beginners and have small foot- prints of less than half a meter in each direction, these are the most common type of printers available.
It is worth noting that each of these methods has advan- tages and disadvantages, the details of which are beyond the scope of this article. As a short summary, powder- based methods are very expensive but produce objects with good mechanical strength and desirable attributes such as smooth surfaces. Liquid-based photopolymeriza- tion methods can be messy and cannot be used to build objects larger than a few centimeters. However, small objects, such as those used in many high-precision engi- neering applications, are readily fabricated using this approach. Filament-based methods are low cost and easy to use, but the finished products have rough surfaces and have poor mechanical strength.
Musical Acoustics
Access to musical instruments takes on a new twist when AM is incorporated. In some cases, by using existing
Spring 2022 • Acoustics Today 51