The "Wohlers" column is authored by Terry Wohlers for Time Compression.
This column was published in the January/February 2010 issue. 


Additive Manufacturing 101: Part I

By Terry Wohlers, President, Wohlers Associates

The importance of additive manufacturing (AM) is certainly growing, and many people are being confronted with the tech that they may not have even a basic understanding about. So we’re going to start a bit of a tutorial on AM systems, giving you information about the technology and the applications they’re best suited for.

It’s important to have an understanding of the term “additive manufacturing.” ASTM International Committee F42 on Additive Manufacturing Technologies, an industry-led standards group, defines it as the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. Synonyms are additive fabrication, additive processes, additive techniques, additive layer manufacturing, layer manufacturing, and freeform fabrication. 

It’s important to note that AM includes all applications of the technology, including modeling, prototyping, pattern-making, tool-making, and the production of end-use parts in volumes of one to thousands or more. It isn’t just about prototyping.

Nearly all AM systems work in the same basic way, starting with a 3D computer model. In the world of product development, a CAD solid-model is used to drive the process. Using software that ships with the AM system, the CAD model is sliced into a stack of very thin, horizontal cross sections. Each slice is typically in the range of 0.1-mm (0.004-in.) thick, but can be much thinner or thicker. These slices are transformed into material as the AM system produces each layer, one adhering to the next. When complete, the physical model looks similar to the digital version. It is possible to scale the model up or down, depending on the intended application and the maximum build volume of the AM system. Parts and assemblies that exceed the size of the build volume of the system but need to be built at full scale are often produced in sections and then brought together with glue and/or fasteners to produce relatively large structures. For example, a full-scale landing gear assembly for a large jet aircraft was produced using an AM process, even though the AM machine itself was much smaller than the assembly. 

Some AM systems produce parts in thermoplastics, a type of plastic that, by definition, becomes molten liquid when heated to a high enough temperature. Examples are polyamide, polypropylene, polystyrene, and ABS—plastics that are used in everyday products that we buy and use at home and work. 

Fused deposition modeling (FDM) from Stratasys (stratasys.com) is a process that uses thermoplastics in filament form housed on a spool. The filament is fed through a deposition head and tip that heats and melts the plastic. A motion system moves the head in the X and Y axes. The part being built rests on a platform that moves downward at the end of the application of each layer. The distance it moves down is the thickness of the next layer. The build process is complete when the top-most layer of the part is deposited. Most systems, including FDM, can produce multiple parts at one time.

Parts with overhanging features require material underneath them to support the features as they are being produced. Some systems use the same material for both the part and the support structure, while others use a second material. Stratasys offers FDM systems with a soluble support material, making it easier to remove after the part is built.

Other than removing the support material, a completed FDM part does not require any post treatment, which is also true with some other AM systems. If a fine finish is required, it is possible to sand and polish the parts and even apply paint or another type of coating. Most FDM parts, however, are not post-processed in this way, partly because they are more difficult to finish to a fine surface compared to some other AM processes. Consequently, FDM is not used as much for master patterns (e.g., to make silicone rubber molds) or for presentation models with highly finished and painted surfaces.

Among the FDM materials available are ABS, polycarbonate, PC/ABS, polyphenylsulfone, and ULTEM 9085. FDM parts are used most commonly for early concept models, as well as for fit and function testing of a design. Examples are prototypes for power tools, motor sports, industrial machinery, computer accessories, automobiles, and aircraft. FDM is also being used increasingly for the building of jigs, fixtures, drill guides, and other types of manufacturing and assembly tools. Some companies are using it for short production runs of plastic parts that would otherwise be machined or injection molded.

Other FDM-like extrusion-based systems are available. The most established industrial systems, other than those from Stratasys, are those from Beijing Yinhua Laser Rapid Prototypes Making and Mould Technology Co. Ltd. (rpyinhua.com/english.htm). The small company has been producing “FDM clones” for years and is beginning to gain some traction. However, to put it into perspective, Stratasys has sold an estimated 37 times more systems than Beijing Yinhua.

Most of the open-source systems that have been sold to date use an extrusion process and thermoplastic material. Examples are the RepRap-based systems from Makerbot Industries and Bits from Bytes (England). KoBa Industries has had some success in selling the Fab@Home open-source system developed at Cornell University. Unlike FDM and the systems from Makerbot and Bits from Bytes, Fab@Home systems use a syringe filled with a material rather than a filament on a spool. See my column in the November/December 2009 issue of Time Compression (“3D Printing Goes Open-Source”; timecompression.com/articles/3d-printing-goes-open-source) for more details.

The next column will continue where this one left off. It will cover laser sintering and other systems that produce plastic parts.