The intent of this post is to provide the reader with an overview of today’s 3D printing landscape.
- Material Extrusion (FDM): Material is selectively dispensed through a nozzle or orifice
- Sheet Lamination (LOM, UAM): Sheets of material are bonded and formed layer-by-layer
- Vat Polymerization (SLA & DLP): Liquid photopolymer in a vat is selectively cured by UV light
- Powder Bed Fusion (SLS, DMLS & SLM): A high-energy source selectively fuses powder particles
- Material Jetting (MJ): Droplets of material are selectively deposited and cured
- Binder Jetting (BJ): Liquid bonding agent selectively binds regions of a powder bed
- Direct Energy Deposition (LENS, LBMD): A high-energy source fuses material as it is deposited
The following section will introduce you to the most used types of manufacturing, and its respective cons and pros.
Fuel Deposition Modeling (FDM)
In FDM, a spool of filament is loaded into the printer and then fed to the extrusion head, which is equipped with a heated nozzle. Once the nozzle reaches the desired temperature, a motor drives the filament through it, melting it.
The printer moves the extrusion head, laying down melted material at precise locations, where it cools and solidifies (like a very precise hot-glue gun). When a layer is finished, the build platform moves down and the process repeats until the part is complete.
After printing, the part is usually ready to use but it might require some post-processing, such as removal of the support strucures or surface smoothing.
FDM is the most cost-effective way of producing custom thermoplastic parts and prototypes. It also has the shortest lead times – as fast as next-day-delivery – due to the high availability of the technology. A wide range of thermoplastic materials is available for FDM, suitable for both prototyping and some functional applications.
As of limitations, FDM has the lowest dimensional accuracy and resolution compared to the other 3D printing technologies. FDM parts are likely to have visible layer lines, so post-processing is often required for a smooth surface finish. Additionally, the layer adhesion mechanism makes FDM parts inherently anisotropic. This means that they will be weaker in one direction and are generally unsuitable for critical applications.
- Low-cost prototyping
- Fast turn-around (less than 24 h)
- Functional applications (non-critical load)
Limited dimensional accuracy
Visible layer lines (can be post-processed)
- Anisotropic mechanical properties
Popular FDM Materials:
Stereolithography & Digital Light Processing (SLA & DLP)
SLA and DLP are similar processes that both use an UV light source to solidify liquid resin in a vat layer-by-layer. SLA uses a single-point laser to cure the resin, while DLP uses a digital light projector to flash a single image of each layer all at once.
After printing, the part needs to be cleaned from the resin and exposed to a UV source to improve its strength. Next, the support structures are removed and, if a high quality surface finish is required, additional steps are carried out.
SLA/DLP can produce parts with very high dimensional accuracy, enhance details and a very smooth surface finish ideal that are ideal for visual prototypes. A large range of speciality materials, such as clear, flexible, castable and materials taylored for specific industrial applications, are also available.
SLA/DLP parts are more brittle than FDM parts, so they are not best suited for functional prototypes. Also, SLA parts must not be used outdoors, as their mechanical properties and color fades off when they are exposed to UV radiation from the sun. Support structures are always required in SLA/DLP which may leave small blemishes in the surfaces they come in contact with that need extra post-processing to remove.
High accuracy & intricate details
Smooth surface ideal for visual prototypes
- Large range of specialty materials
Produces relatively brittle parts
Degrade with exposure to sunlight
Removal of support marks required
Popular SLA and DLP Materials:
Selective Laser Sintering (SLS)
The SLS process begins with heating up a bin of polymer powder to a temperature just below the melting point of the material. A recoating blade or roller then deposits a very thin layer of powder – typically 0.1 mm thick – onto the build platform.
A CO2 laser scans the surface of the powder bed and selectively sinters the particles, binding them together. When the entire cross-section is scanned, the building platform moves down one layer and the process repeats. The result is a bin filled with parts surrounded by unsintered powder.
After printing, the bin needs to cool before the parts are removed from the unsintered powder and cleaned. Some additional steps can then be employed to improve their visual appearance, such as polishing or dying.
SLS parts have very good, almost-isotropic mechanical properties, so they are ideal for functional parts and prototypes. Since no support structures are required (the unsintered powder acts as support), designs with very complex geometries can be easily manufactured. SLS is also excellent for small-to-medium batch production (up to 100 parts), since the bin can be filled throughout its volume and multiple parts can be printed at a single production run.
SLS printers are usually high-end industrial systems. This limits the availability of the technology and increases its cost and turn-around times (compared to FDM or SLA, for example). SLS parts have a naturally grainy surface and some internal porosity. If a smooth surface or watertightness is required, additional post-processing steps are needed. Beware that large flat surfaces and small holes need special attention, as they are susceptible to thermal warping and oversintering.
Ideal for functional prototypes
- Complex Geometries – no support needed
Small batch production capabilities
Higher cost than FDM or SLA
Slower turn-around due to batch production
Grainy surface & internal porosity
Popular SLS Materials:
- Carbon Filled
- Glass Filled
- PA 11
Material Jetting (PolyJet)
Material Jetting works in a similar way to standard inkjet printing. However, instead of printing a single layer of ink on a piece of paper, multiple layers of material are deposited upon each other to create a solid part.
Multiple print heads jet hundreds of tiny droplets of photopolymer onto the build platform, which are then solidified (cured) by the UV light source. After a layer is complete, the build platform moves down one layer and the process repeats.
Support structures are always required in Material Jetting. A water-soluble material is used as support that can be easily dissolved during post-processing and that is printed at the same time as the structural material.
Material Jetting is the most precise 3D printing technology (with SLA/DLP being a close second). It is one of the few 3D printing processes that offers multi-material and full-color printing capabilities. Material Jetted parts have a very smooth surface – comparable to injection molding – and very high dimensional accuracy, making them ideal for realistic prototypes and parts that need an excellent visual appearance.
Material Jetting is one of the most expensive 3D printing processes and this high cost may make it financially unviable for some applications. Moreover, parts produced with Material Jetting are not best suited for functional applications. Like SLA/DLP, the materials used with this process are thermosets, so the produced parts tend to be brittle. They are also photosensitive and their properties will degrade over time with exposure to sunlight.
High accuracy and very fine details
- Injection molding-like finish
Multi-material and full-color capabilities
Mechanical properties degrade over time
Produces relatively briddle parts.
Popular Polyjet Materials:
- Digital ABS
Direct Metal Laser Sintering & Selective Laser Melting (DMLS & SLM)
Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) produce parts in a similar way to SLS: a laser source selectively bonds together powder particles layer-by-layer. The main difference, of course, is that DMLS and SLM produce parts out of metal.
The difference between the DMLS and SLM processes is subtle: SLM achieves a full melt of the powder particles, while DMLS heats the metal particles to a point that they fuse together on a molecular level instead.
Support structures are always required in DMLS and SLM to minimize the distortion caused by the high temperatures required to fuse the metal particles. After printing, the metal supports need to be removed either manually or through CNC machining. Machining can also be employed to improve the accuracy of critical features (e.g. holes). Finally, the parts are thermally treated to eliminate any residual stresses.
DMLS/SLM is ideal for manufacturing metal parts with complex geometries that traditional manufacturing methods cannot produce. DMLS/SLM parts can be (and should be) topology optimized to maximize their performance while minimizing their weight and amount of material used. DMLS/SLM parts have excellent physical properties, often surpassing the strength of the rough metal. Many metal alloys that are difficult to process with other technologies, such as metal superalloys, are available in DMLS/SLM.
The costs associated with DMLS/SLM 3D printing are high: parts produced with this processes typically cost between $5.000 and $25.000. For this reason, DMLS/SLM should only be used to manufacture parts that cannot be produced with any other method. Moreover, the build size of modern metal 3D printing systems is limited, as the required precise manufacturing conditions are difficult to maintain for bigger build volumes.
Highly complex, optimized metal parts
- parts with excellent material properties
Ideal for high-end engineering applications
Specialized CAD softwared needed
Limited build volume
Popular Polyjet Materials:
- Stainless steel
Binder Jetting is a flexible technology with diverse applications, ranging from low-cost metal 3D printing, to full-color prototyping and large sand casting mold production.
In Binder Jetting, a thin layer of powder particles (metal, acrylic or sandstone) is first deposited onto the build platform. Then droplets of adhesive are ejected by a inkjet printhead to selectively bind the powder particles together and build a part layer-by-layer.
After the print is complete, the part is removed from the powder and cleaned. At this stage it is very brittle and additional post-processing is required. For metal parts this involves thermal sintering (similar to Metal Injection Molding) or infiltration with a low melting-point metal (for example, bronze), while full-color parts are infiltrated with cyanoacrylate adhesive.
Binder Jetting can produce metal parts and full-color prototypes at a fraction of the cost of DMLS/SLM or Material Jetting respectively. Very large sandstone parts can also be manufactured with Binder Jetting, as the process is not limited by thermal effects (for example, warping). Since no support structures are needed during printing, metal Binder Jetting parts can have very complex geometries and, like SLS, low-to-medium batch production is possible by filling up the whole build volume.
Metal Binder Jetting parts have lower mechanical properties than the bulk material though, due to their porosity. Due to the special post-processing requirements of Binder Jetting, special design restrictions apply. Very small details, for example, cannot be printed, as the parts are very brittle out of the printer and may break. Metal parts might also deform during the sintering or infiltration step if not supported properly.
Low cost batch prodution of metal parts
- Full color prototyping in acrilic or sand
Very large printing capabilities in sand
Inferior material properties to DMLS/SLM
Design restriction due to post-processing
Fine details may not be printable
Popular Polyjet Materials:
- Stainless steel
The chart below provides the best way to choose the main design requirements, such as what purpose will the part hold, or if the part is desired to be very detailed.
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