Also known as “Additive Manufacturing,” 3D printing describes the process of building a part up additively, layer by layer, from a CAD file to create a new part. There are many different 3D Printing techniques, each with particular strengths and weaknesses. Below is a summary overview of some of the most common 3D printing processes, roughly in order of application intensity:
Sheet lamination is the process of building up a part in paper. For each layer, a sheet of paper is brought onto the build tray and adhesive is applied to the paper. The outline of the layer is cut into the paper with a laser or a blade and then a new sheet is brought in so that the process can repeat. The process does not automatically discard the unused portions of the sheet of paper. As a result, when the printing process is complete, the part requires significant post-processing effort to remove excess material. This process is great for full color prints of visual mock ups that are not terribly complex, have internal structures, or expected to undergo very much stress.
Binder Jetting is a process whereby an adhesive (and ink) is selectively dispensed into one layer of powder (typically gypsum) at a time to bind the particles together into a part design. The end result is effectively a plaster part. Because this process requires no heat source, it can be done fairly affordably. This can work great for display models, but is not great for functional parts. A benefit of binder jetting is that printing is possible in both full color and without as part of the printing process. Most 3D Printing processes are limited to one or two color options at most. Color Jet Printing (CJP) from 3D Systems is the best known Binder Jetting technology.
Example surface finish for the Binder Jetting Process. This part is roughly 4 inches long by 1 inch wide.
Material Jetting technology borrows concepts from paper inkjet printing to create parts. Instead of jetting drops of ink onto paper, material jetting 3D Printers jet layers of curable liquid photopolymer onto a build tray. The resulting parts are highly accurate, with a glossy, smooth surface finish. Part designs also do not require support columns, as the printer uses two print heads – one that dispenses the part material, and one that dispenses filler material that is easily dissipated after the production run is complete. Parts made using this process generally lack the durability of thermoplastic parts, and their UV curable nature makes them susceptible to brittleness over time with extended exposure to sunlight. Material Jetting parts are similar to parts made by the Vat Photopolymerization process in this effect. PolyJet from Stratasys and MJP from 3D Systems are the best known branded versions of this process. Please see the videos below for a closer look at these material jetting processes.
Example surface finish of a Material Jetting process
Vat Photopolymerization is an additive manufacturing process which employs a vat of liquid ultraviolet curable photopolymer resin, a moving build platform, and an ultraviolet laser to build parts' layers one at a time. For each layer, an ultraviolet light source selectively cures those portions it wants solid – either as part of the final design or a support for it – then the part descends further into the resin pool allowing for the next layer to be cured. The original 3D Printing technology, stereolithography, belongs to this process family, although there are a number of others that have appeared in recent years as original stereolithography patents expired. Vat photopolymerization machines are typically able to produce highly accurate parts, however those parts are generally subject to rapid wear given the nature of resin as a material. Also, support pillars are commonly required as part of the printing process, and given that they are printed in the final part material, removing them can be a time and labor intensive process, as well as messy. Popular branded technologies in this family include 3D Systems’ stereolithography, Prodways MOVINGLight, and Envisiontec’s Digital Light Processing (DLP) and 3SP (Spin, Scan, Selectively Photocure process). The following link provides a useful visual representation of a stereolithography printer in action (please fast forward to the 1:30 mark if short for time):
Powder Bed Fusion is the process of using a roller or recoater blade to move a thin layer of very small plastic beads (typically 20-60 microns in diameter) onto a platform, using a laser to selectively melt those parts of the layer that are desired to be solid, then repeating this process until a cake of loose powder with embedded solid parts remain. Plastic Powder Bed Fusion parts do not require supports as the loose powder from the layers below provides sufficient support for what’s being sintered above. This allows for virtually unlimited design flexibility, constrained only by the size of the build box. Moreover, the material typically used – Nylon – is quite durable. With that said, its durability is not that of metal printed parts and the breadth of material options is limited relative to extrusion, the next thermoplastic process we’ll discuss. The most commonly known Plastic Powder Bed Fusion process is Selective Laser Sintering (SLS) from 3D Systems. Please see this video for greater insight on the process:
Material extrusion is an additive manufacturing technology commonly used for modeling, prototyping, and production applications. However, unlike the processes outlined previously, extrusion doesn’t utilize UV curing or powder fusion to create parts. Instead, it operates something like a hot glue gun on a gantry system. A plastic filament is unwound from a coil and run through an extrusion nozzle that melts and dispenses it onto a platform, where it then hardens. By extruding this material cleverly according to the information provided to the machine by the CAD design, it gradually builds up a part. Extrusion parts are generally strong relative to other 3D Printing processes, and there are some production grade plastics that make extrusion a common choice for functional parts. At the same time, the accuracy achievable by a heated extrusion nozzle is limited relative to the other technologies, the stepped surface quality of an extrusion part is undesirable for those seeking smoothness comparable to an injection molded part (which typically requires a resin process). Lastly, extrusion parts typically require post-processing to remove supports with the possibility of finishing since extrusion is not known to meet tolerance as well as other techniques. The original Material Extrusion technology is Fused Deposition Modeling (FDM) from Stratasys, but since original patents expired in recent years, hundreds of Fused Filament Fabrication (FFF) technologies leveraging FDM intellectual property have come to the market. Please follow this video link to see an FDM printer in action:
Example surface finish for the FFF process.
A process similar to Plastic Powder Bed Fusion, Metal Powder Bed Fusion also utilizes a focused energy source to selectively melt layers of powdered metal beads. With that said, there are key differences as well. First, metal printing requires the use either an inert gas (if melting with a laser) or vacuum (if melting with an electron beam) environment. Second, metal printing processes can create a great deal of residual stress in the parts created. This, in turn, typically requires the inclusion of metal supports to help a part maintain its shape throughout the printing and post-processing steps. There are a significant number of companies with well known metal powder bed fusion processes, most notably EOS’ Direct Metal Laser Sintering (DMLS), SLM Solutions Selective Laser Melting (SLM), Concept Laser’s LaserCUSING, and Arcam AB’s Electron Beam Melting (EBM). Please see pictures and videos below of laser and electron beam melting Powder Bed Fusion processes:
A heat sink printed in inconel 625 superalloy using a laser powderbed melting process
Binder Jetting with Metal Infiltration is a similar process to binder jetting with gypsum powder outlined above, but with an extra couple steps. Layers of metal powder are selectively bonded together with a binding agent and partially cured, creating a very brittle part. That part is then put into a kiln, where the metal powder can sinter together. Next, liquid metal is fed into the sintered part to create a fully dense final part which reflects a mix of the original powder and the metal that infiltrated its pores. In this sense, it bears some resemblance to casting and many of its typical applications are in areas where complex casting is commonplace. This process can be more cost effective than its metal cousins for certain applications, especially those applications that do not require complete metal purity.
CNC Machining describes a rapid subtractive manufacturing process whereby CAD data is converted to a Computer Aided Machining (CAM) file, and the device then uses one or more traditional manufacturing processes (lathing, milling) as guided by the CAM file to produce the part design. This video shows Mori Seiki’s CNC Machines in use by Andretti Motorsports:
Molding and Casting is all about starting with an original object (a “pattern”) that you want to make copies of. Molding entails taking the pattern, then encasing it within a contained with molding material (typically silicone for plastic parts or a sand-based mixture for metal parts). You can then remove the original pattern a number of different ways to then create an empty space – basically the negative image of your pattern. To cast parts, you then pour liquid material into the now vacated space within the mold, and when it hardens, you’ve got a copy of your original part.
This process is commonly used in tandem with 3D Printing, as it is sometimes more cost effective to simply use 3D Printing to create a mold pattern rather than mass producing parts on a 3D Printer.
Note: the definitions of “molding” and “casting” are a bit different depending on who you talk with – but for our purposes, we’ll call the “mold” the object that the part is created in and the part that comes out of it the “cast” or “cast part.”
Metal Casting (large scale)
Injection Molding describes a process whereby a typically metal tool is created with negative spaces based on a pattern, and then melted pellets of plastic or a combination of plastic and metal are injected into the tool to create cast parts.
Injection molding overview
In some instances, there can also be a post-injection process of curing the cast parts in a kiln.
Injection Molding isn’t “rapid manufacturing” in the truest sense, as it requires this timely process of creating a tool manually using a CAD design file, then then executing injection “shots” to create parts. It is, however, a logical next step for companies when they are ready to mass produce a design that they’ve 3D Printed. Depending on a part’s geometry, the crossover point on a per unit basis typically falls between 100 and 10,000 to justify injection molding as the more cost effective production option. Additionally, Injection Molding also offers a broader range of material options than 3D Printing.