The most common term for additive fabrication is rapid prototyping. The term "rapid" is used because additive processes are performed much faster than conventional manufacturing processes. The fabrication of a single part may only take a couple hours, or can take a few days depending on the part size and the process. However, processes that require custom tooling, such as a mold, to be designed and built may require several weeks. Subtractive processes, such as machining, can offer more comparable production times, but those times can increase substantially for highly complex parts. The term "prototyping" is used because these additive processes were initially used only to fabricate prototypes. However, with the improvement of additive technologies, these processes are becoming increasingly capable of high-volume production manufacturing.
Several different additive fabrication processes are commercially available or are currently being developed. Each process may use different materials and different techniques for building the layers of a part. However, each process employs the same basic steps, listed below.
- Liquid based processes: These additive technologies typically use photocurable polymer resins and cure selected portions of the resin to form each part layer. The most common liquid-based additive process is Stereolithography (SLA), which was the first commercially available additive process. Parts produced using this technology offer high accuracy and an appearance similar to molded parts. However, photocurable polymers offer somewhat poor mechanical properties which may worsen over time. Other liquid-based processes include Jetted Photopolymer, which may use a single jet or multiple jets.
- Powder based processes: In powder based processes, such as Selective Laser Sintering (SLS), a selected portion of powdered material is melted or sintered to form each part layer. The use of powdered material enables parts to be fabricated using polymers, metals, or ceramics. Also, the mechanical properties of these parts are better and more stable than a photocured polymer part. Other powder-based processes include Direct Metal Laser Sintering (DMLS) and Three Dimensional Printing (3DP).
- Solid based processes: Solid based processes use a variety of solid, non powder, materials and each process differs in how it builds the layers of a part. Most solid-based processes use sheet stacking methods, in which very thin sheets of material are layered on top of one another and the shape of the layer is cut out. The most common sheet stacking process is Laminated Object Manufacturing (LOM), which uses thin sheets of paper, but other processes may use polymers or metal sheets. Other solid based processes use solid strands of polymer, not sheets, such as Fused Deposition Modeling (FDM) which extrudes and deposits the polymer into layers.
Fused Deposition Modeling (FDM):
Selective Laser Sintering (SLS):
Direct Metal Laser Sintering (DMLS):
Laminated Object Manufacturing (LOM):
The 3D printing process begins with the powder supply being raised by a piston and a leveling roller distributing a thin layer of powder to the top of the build chamber. A multi-channel ink-jet print head then deposits a liquid adhesive to targeted regions of the powder bed. These regions of powder are bonded together by the adhesive and form one layer of the part. The remaining free standing powder supports the part during the build. After a layer is built, the build platform is lowered and a new layer of powder added, leveled, and the printing repeated. After the part is completed, the loose supporting powder can be brushed away and the part removed.
Addictive MAnufacturing Advantages:
- Speed: As described above, these "rapid" processes have short build times. Also, because no custom tooling must be developed, the lead time in receiving parts is greatly reduced.
- Part complexity: Because no tooling is required, complex surfaces and internal features can be created directly when building the part. Also, the complexity of a part has little effect on build times, as opposed to other manufacturing processes. In molding and casting processes, part complexity may not affect the cycle times, but can require several weeks to be spent on creating the mold. In machining, complex features directly affect the cycle time and may even require more expensive equipment or fixtures.
- Material types: Additive fabrication processes are able to produce parts in plastics, metals, ceramics, composites, and even paper with properties similar to wood. Furthermore, some processes can build parts from multiple materials and distribute the material based on the location in the part.
- Low-volume production: Other more conventional processes are not very cost effective for low-volume productions because of high initial costs due to custom tooling and lengthy setup times. Additive fabrication requires minimal setup and builds a part directly from the CAD model, allowing for low per-part costs for low-volume productions.