If you’re new to the wonderful world of 3D printing, then may we be the first to offer you a warm welcome. You’re going to have lots of fun.
The immediate challenge newcomers face with 3D printing technology is distinguishing between the different processes and materials available.
What’s the difference between types of 3D printing like FDM and SLS, for example? Or SLS and DLP? Or EBM and DMLS?
It can be pretty confusing. With so many different acronyms, you’d be forgiven for mistaking a type of 3D printing for a genre of dance music.
The first thing to understand is that 3D printing is actually an umbrella term that encompasses a group of 3D printing processes.
The ISO/ASTM 52900 standard, which was created in 2015, aims to standardize all terminology and classify each of the different types of 3D printer.
In total, seven different categories of additive manufacturing processes have been identified and established. These seven 3D printing processes brought forth ten different types of 3D printing technology that 3D printers use today.
To learn more about these technologies — including design rules for 3D printing and how to find the best 3D printing materials — we recommend you pick up a copy of The 3D Printing Handbook from 3D Hubs, available now from all good bookshops.
Material extrusion is a 3D printing process where a filament of solid thermoplastic material is pushed through a heated nozzle, melting it in the process. The printer deposits the material on a build platform along a predetermined path, where the filament cools and solidifies to form a solid object.
Material Extrusion devices are the most commonly available — and the cheapest — types of 3D printing technology in the world. You might be familiar with them as Fused Deposition Modeling, or FDM. They are also sometimes referred to as Fused Filament Fabrication, or FFF.
The way it works is that a spool of filament is loaded into the 3D printer and fed through to a printer nozzle in the extrusion head. The printer nozzle is heated to a desired temperature, whereupon a motor pushes the filament through the heated nozzle, causing it to melt.
The printer then moves the extrusion head along specified coordinates, laying down the molten material onto the build plate where it cools down and solidifies.
Once a layer is complete, the printer proceeds to lay down another layer. This process of printing cross-sections is repeated, building layer-upon-layer, until the object is fully formed.
Depending on the geometry of the object, it is sometimes necessary to add support structures, for example if a model has steep overhanging parts.
Vat Polymerization is a 3D printing process where a photo-polymer resin in a vat is selectively cured by a light source. The two most common forms of Vat Polymerization are SLA (Stereolithography) and DLP (Digital Light Processing).
The fundamental difference between these types of 3D printing technology is the light source they use to cure the resin. SLA printers use a point laser, in contrast to the voxel approach used by a DLP printer.
SLA holds the historical distinction of being the world’s first 3D printing technology. Stereolithography was invented by Chuck Hull in 1986, who filed a patent on the technology and founded the company 3D Systems to commercialize it.
An SLA printer uses mirrors, known as galvanometers or galvos, with one positioned on the X-axis and another on the Y-axis. These galvos rapidly aim a laser beam across a vat of resin, selectively curing and solidifying a cross-section of the object inside this build area, building it up layer by layer.
Most SLA printers use a solid state laser to cure parts. The disadvantage to these types of 3D printing technology using a point laser is that it can take longer to trace the cross-section of an object when compared to DLP.
Looking at Digital Light Processing machines, these types of 3D printing technology are almost the same as SLA. The key difference is that DLP uses a digital light projector to flash a single image of each layer all at once (or multiple flashes for larger parts).
Because the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular blocks called voxels.
DLP can achieve faster print times compared to SLA. That’s because an entire layer is exposed all at once, rather than tracing the cross-sectional area with the point of a laser.
Light is projected onto the resin using light-emitting diode (LED) screens or a UV light source (lamp) that is directed to the build surface by a Digital Micromirror Device (DMD).
A DMD is an array of micro-mirrors that control where light is projected and generate the light-pattern on the build surface.
Powder Bed Fusion is a 3D printing process where a thermal energy source will selectively induce fusion between powder particles inside a build area to create a solid object.
Many Powder Bed Fusion devices also employ a mechanism for applying and smoothing powder simultaneous to an object being fabricated, so that the final item is encased and supported in unused powder.
Creating an object with Powder Bed Fusion technology and polymer powder is generally known as Selective Laser Sintering (SLS). As industrial patents expire, these types of 3D printing technology are becoming increasingly common and lower cost.
First, a bin of polymer powder is heated to a temperature just below the polymer’s melting point. Next, a recoating blade or wiper deposits a very thin layer of the powdered material — typically 0.1 mm thick — onto a build platform.
A CO2 laser beam then begins to scan the surface. The laser will selectively sinter the powder and solidify a cross-section of the object. Just like SLA, the laser is focused on to the correct location by a pair of galvos.
When the entire cross-section is scanned, the build platform will move down one layer thickness in height. The recoating blade deposits a fresh layer of powder on top of the recently scanned layer, and the laser will sinter the next cross-section of the object onto the previously solidified cross-sections.
These steps are repeated until all objects are fully manufactured. Powder which hasn’t been sintered remains in place to support the object that has, which eliminates the need for support structures.
Material Jetting is a 3D printing process where droplets of material are selectively deposited and cured on a build plate. Using photopolymers or wax droplets that cure when exposed to light, objects are built up one layer at a time.
The nature of the Material Jetting process allows for different materials to be printed in the same object. One application for this technique is to fabricate support structures from a different material to the model being produced.
Material Jetting (MJ) works in a similar way to a standard inkjet printer. The key difference is that, instead of printing a single layer of ink, multiple layers are built upon each other to create a solid part.
The print head jets hundreds of tiny droplets of photopolymer and then cures/solidifies them using an ultraviolet (UV) light. After one layer has been deposited and cured, the build platform is lowered down one layer thickness and the process is repeated to build up a 3D object.
MJ is different from other types of 3D printing technology that deposit, sinter or cure build material using point-wise deposition. Instead of using a single point to follow a path which outlines the cross-sectional area of a layer, MJ machines deposit build material in a rapid, line-wise fashion.
The advantage of line-wise deposition is that MJ printers are able to fabricate multiple objects in a single line with no impact on build speed. So long as models are correctly arranged, and the space within each build line is optimized, MJ is able to produce parts at a speedier pace than other types of 3D printer.
Objects made with MJ require support, which are printed simultaneously during the build from a dissolvable material that’s removed during the post-processing stage. MJ is one of the only types of 3D printing technology to offer objects made from multi-material printing and full-color.
Drop on Demand (DOD) is a type of 3D printing technology that uses a pair of ink jets. One deposits the build materials, which is typically a wax-like material. The second is used for dissolvable support material. As with typical types of 3D printing technology, DOD printers follow a predetermined path to jet material in a point-wise deposition, creating the cross-sectional area of an object layer-by-layer.
DOD printers also use a fly-cutter that skims the build area after each layer is created, ensuring a perfectly flat surface before commencing the next layer. DOD printers are usually used to create patterns suitable for lost-wax casting or investment casting, and other mold-making applications.
Binder Jetting is a 3D printing process where a liquid bonding agent selectively binds regions of a powder bed.
Binder Jetting is a similar 3D printing technology to SLS, with the requirement for an initial layer of powder on the build platform. But unlike SLS, which uses a laser to sinter powder, Binder Jetting moves a print head over the powder surface depositing binder droplets which are typically 80 microns in diameter. These droplets bind the powder particles together to produce each layer of the object.
Once a layer has been printed, the powder bed is lowered and a new layer of powder is spread over the recently printed layer. This process is repeated until a complete object is formed.
The object is then left in the powder to cure and gain strength. Afterwards, the object is removed from the powder bed and any unbound powder is removed using compressed air.
With Sand Binder Jetting devices, these are low-cost types of 3D printing technology for producing parts from sand, e.g. sandstone or gypsum.
For full color models, objects are fabricated using a plaster-based or PMMA powder in conjunction with a liquid binding agent. The printhead first jets the binding agent, while a secondary print head jets in color, allowing full color models to be printed.
Once parts have fully cured they are removed from the loose unbonded powder and cleaned. To enhance mechanical properties, parts are often exposed to an infiltrant material.
There are a large number of infiltrants available, each resulting in different properties. Coatings can also be added to improve the vibrancy of colors.
Binder Jetting is also useful for the production of sand cast molds and cores. The cores and molds are generally printed with sand, although artificial sand (silica) can be used for special applications.
After printing, the cores and molds are removed from the build area and cleaned to remove any loose sand. The molds are typically immediately ready for casting. After casting, the mold is broken apart and the final metal component removed.
The big advantage of producing sand casting cores and molds with Binder Jetting is the large, complex geometries the process is able to produce at relatively low-cost. Plus, the process is quite easy to integrate into existing manufacturing or foundry process without disruption.
Binder Jetting can also be used for the fabrication of metal objects. Metal powder is bound using a polyer binding agent. Producing metal objects using Binder Jetting allows for the production of complex geometries well beyond the capabilities of conventional manufacturing techniques.
Functional metal objects can only be produced via a secondary process like infiltration or sintering, however. The cost and quality of the end result generally defines which secondary process is the most appropriate for a certain application. Without these additional steps, a part made with metal Binder Jetting will have poor mechanical properties.
The infiltration secondary process works as follows: initially metal powder particles are bound together using a binding agent to form a “green state” object. Once the objects have fully cured, they are removed from the loose powder and placed in a furnace, where the binder is burnt out. This leaves the object at around 60% density with voids throughout.
Next, bronze is used to infiltrate the voids via capillary action, resulting in an object with around 90% density and greater strength. However, objects made with metal Binder Jetting generally have lower mechanical properties than metal parts made with Powder Bed Fusion.
The sintering secondary process can be applied where metal parts are made without infiltration. After printing is complete, green state objects are cured in an oven. Next, they’re sintered in a furnace to a high density of around 97%. However, non-uniform shrinkage can be an issue during sintering and should be accounted for at the design stage.
Metal Powder Bed Fusion is a 3D printing process which produces solid objects, using a thermal source to induce fusion betwen metal powder particles one layer at a time.
Most Powder Bed Fusion technologies employ mechanisms for adding powder as the object is being constructed, resulting in the final component being encased in the metal powder. The main variations in metal Powder Bed Fusion technologies come from the use of different energy sources; lasers or electron beams.
Both Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM) produce objects in a similar fashion to SLS. The main difference is that these types of 3D printing technology are applied to the production of metal parts.
DMLS does not melt the powder but instead heats it to a point so that it can fuse together on a molecular level. SLM uses the laser to achieve a full melt of the metal powder forming a homogeneous part. This results in a part that has a single melting temperature (something not produced with an alloy).
This is the main difference between DMLS and SLM; the former produces parts from metal alloys, while the latter form single element materials, such as titanium.
Unlike SLS, the DMLS and SLM processes require structural support, in order to limit the possibility of any distortion that may occur (despite the fact that the surrounding powder provides physical support).
DMLS/SLM parts are at risk of warping due to the residual stresses produced during printing, because of the high temperatures. Parts are also typically heat-treated after printing, while still attached to the build plate, to relieve any stresses in the parts after printing.
Distinct from other Powder Bed Fusion techniques, Electron Beam Melting (EBM) uses a high energy beam, or electrons, to induce fusion between the particles of metal powder.
A focused electron beam scans across a thin layer of powder, causing localized melting and solidification over a specific cross-sectional area. These areas are built up to create a solid object.
Compared to SLM and DMLS types of 3D printing technology, EBM generally has a superior build speed because of its higher energy density. However, things like minimum feature size, powder particle size, layer thickness and surface finish are typically larger.
Also important to note is that EBM parts are fabricated in a vacuum, and the process can only be used with conductive materials.
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