Dissemination of IT for the Promotion of Materials Science (DoITPoMS)


Material Extrusion

Material extrusion AM methods operate under the principal of using a nozzle to extrude and eject hot plastic onto a build surface. There are two slightly varying methods of material extrusion which are Fused Deposition Modelling (FDM) and Arburg Plastic Freeforming (APF) (translated from the German name - Arburg Kunststoff Freiformen).

Fused Deposition Modelling (FDM)

FDM is what most people imagine as 3D printing due to being the most common type of additive manufacturing, mainly due to its low cost making it accessible to industry and hobbyists. It also has a relatively large selection of materials available to use, as numerous thermoplastics can be printed using FDM. The most common plastics used are ABS and PLA.

FDM uses a spool of plastic that feeds a thread of plastic (known as a filament) through a nozzle. The nozzle is on a print head which has the operation of mechanically forcing the filament through the print head in a cold section, then heating and melting the plastic in a hot section before extruding the plastic through the nozzle. Now melted and extruded, the plastic is printed directly onto the surface of a build in a continuous stream. The print nozzle moves in the x-y plane to control where the plastic is placed. Once the layer is completed, the platform the object is on can move downwards in the z direction by a small amount for the next layer to be built.

As the deposited plastic is hot, it is able to fuse to layers below, softening and binding to the surface of the previous layer. This leads to anisotropic properties as this fuse is weaker than the extruded thread of material, meaning printed objects are generally weaker in the z-direction. For more on anisotropy in FDM, see the properties of FDM section

Arburg Plastic Freeforming (APF)

APF works similarly to FDM with a few notable differences. The first of which is that the plastic supplied is in pellets. These melted pellets are then forced along to a nozzle using a rotating screw, similar to injection moulding. Once in the nozzle, the nozzle periodically opens and closes, letting out individual droplets of melted plastic onto the build surface.. The platform being printed on can move in the xyz directions to control where the droplets are placed. Droplet size can be controlled by changing the nozzle diameter.

APF and FDM usually print onto a hot surface (the exact temperature may depend on the plastic being used) to reduce warping (plastic can warp due to shrinkage on rapid cooling), and give better adhesion between the print and print bed.

Both process need support structures, so post processing mostly involves the removal of these structures. Some more advanced printers are able to print multiple plastics at once so support structures can be printed in a material, such as PVA, that can be easily removed (usually using water). Further post processing can involve smoothing the surface of the object as especially FDM can give poor surface finish, however surface finish can vary quite a lot between printer models.

Benefits and Limitations

As mentioned earlier, material extrusion (or specifically FDM) printing is cheap and widely accessible. The common plastics used are also cheap meaning FDM is ideal for amateurs.

Also mentioned earlier, material extrusion has many plastics available to it, including polymers infused with other materials. This enables more choice over properties, with each polymer having its own pros and cons when used to print.

On the other hand, material extrusion can suffer from lower resolution and worse quality prints than photopolymerisation or PBF processes. Surface finishes are bumpy with usually easily visible layer lines. This can lead to more time post processing to achieve a smooth surface.

Binding between layers can be poor, especially for some polymers. To explain what this means for the print, read the Properties of FDM section.

Print failure can also be quite common. This is a problem for all AM methods, but FDM is susceptible to quite a few potential faults in printing e.g. fast heating then cooling of plastic can lead to thermal contraction as the object is printed, resulting in the print warping. For more on problems that can occur during printing, you can read this article: https://all3dp.com/1/common-3d-printing-problems-troubleshooting-3d-printer-issues/.