The printing process was originally developed at the Institute of Technology (MIT) in the early 1990s in order to to print complex parts in industrial-grade materials.
ExOne obtained the exclusive licence for the binder jet method of additive manufacturing in 1996 and launched the first commercial binder jet 3D printer for metals, the RTS-300, in 1998. Launching their first sand 3D printer, the S15, in 2002, ExOne were acquired by Desktop Metal in 2021.
Here is a step-by-step guide to the binder jetting process:
Firstly, a recoating blade spreads a fine layer of the powder material across the build platform.
A carriage of inkjet nozzles, similar to those used for 2D desktop printers, travels over the bed and selectively releases droplets of binding agent to bond the powder particles together. With full colour binder jetting, coloured inks are also deposited alongside the bonding agents during this step. Each drop is around 80 μm in diameter, allowing a good resolution to be achieved.
Once the first layer is complete (based upon a CAD design), the build platform moves down and the surface is recoated with more powder.
Repeat steps two and three until the final part is complete.
Once the build is complete, the part needs to be left to cure and gain strength ready for use, after which pressurised air can be used to remove any excess powder.
Some materials will require a post-processing step to finish the part. Metal binder jetting parts need heat treating (sintering, for example) or infiltrating with a metal with a low melting point, such as bronze. This step is because some materials leave the printer in what is called a ‘green’ state, which means they have poor mechanical properties and may be brittle and highly porous. Sand casting parts do not typically need a post-processing step, but full-colour parts are infiltrated with acrylic to improve the vibrancy of the colours.
Almost all of the process parameters used in binder jetting are preset by machine manufacturers. This automation makes the process easy to use, but the typical layer height varies according to the material being used.
Full colour models typically use a 100 micron layer height while metal parts use a layer height of just 50 microns. Sand casting materials typically use a much higher layer height of between 200 and 400 microns.
Metals and ceramics are widely used materials in binder jetting applications, although it is also possible to use other powdered materials, like sand. In addition, polymers like ABS or PLA can also be used in binder jetting operations.
Metal alloys, including titanium, stainless steels and copper, are used regularly due to the their characteristics, which allow for the manufacture of strong yet light parts.
Outside of industrial applications, binder jetting type processes have even found their way into baking, with businesses like The Sugar Lab using 3D printing with granules of sugar and water to create complex culinary structures.
The primary advantage of binder jetting is that the process occurs at room temperature, meaning that the distortion of parts associated with thermal effects is not a problem. As a result, the build volume of binder jetting machines are among the largest of any of the 3D printing technologies. The largest machines (up to 2200 x 1200 x 600 mm) are generally used to produce sand casting moulds. Metal binder jetting systems are smaller (up to 800 x 500 x 400 mm) but still larger than DMSL / SLM systems, allowing for multiple parts to be built simultaneously.
Binder jetting also requires no support structures. Instead, the powder itself provides its own support as the build grows. Not only does this remove the need for post-processing to remove supports, but also allows for parts to be positioned so as to maximise build volume.
Binder jetting produces metal parts with low surface roughness (as low as Ra 3 μm if a bead-blasting step is employed) compared to DMLS/SLM (Ra 12-16 μm). Such low surface roughness is beneficial for parts with internal channels and geometries that can be difficult to post process.
Binder jetting is also faster and more cost-effective than many other additive manufacturing techniques, being able to build quickly using multiple print head nozzles or holes to create several parts at once.
Despite the advantages of binder jetting, there are also some challenges associated with the process.
The main problems associated with binder jetting are accuracy and tolerance, which can be difficult to predict as a result of part shrinkage during post processing steps. For example, metal parts can shrink by up to 2% for smaller items and by more than 3% for larger items as a result of infiltration. Sintering can cause average shrinkage of 20% and also lead to warping caused by friction between the furnace plate and the bottom surface of the part. The heat used in sintering can also soften the part and cause unsupported areas to deform under their own weight. While these problems can be compensated for in the build, non-uniform shrinkage can be more difficult to account for.
Binder jetting parts may also show poor mechanical properties as a result of internal porosity. This porosity can be reduced by sintering (producing 97% dense parts) or infiltration (90% dense parts), but it can leave voids that lead to crack initiation. As a result, fracture and fatigue strengths can be an issue.
Taking account of the advantages and disadvantages of the binder jetting process, it is clear that it is more suited to some applications than others.
Binder jetting is used for the production of full-colour prototypes, low-cost metal parts and for producing large sand casting cores and moulds. Because of the low cost and fast production times, the process is even used to make accessories for the film industry and used by mobile printing systems to produce replacement parts in the field for US Army troops. Binder jetting has also been used in the production of jewellery.
Sustainability covers a range of factors, but binder jetting certainly offers some environmental benefits when compared to other manufacturing methods.
Firstly, because binder jetting uses a wide range of powdered materials, it may be possible to source them locally to reduce logistics. As with all additive manufacturing methods, binder jetting has very low levels of material wastage and low energy use compared to conventional manufacturing methods. However, the speed and high volume production possible with binder jetting reduces the carbon footprint further.
A less obvious area that needs considering are any debinding and cleaning methods used, along with the cleaning fluids that are used for these steps. Common binding agents for metal parts, like carnauba, paraffin or special polyethylene waxes, need to be selectively removed from the part before sintering. This reduces the sintering time and can now be achieved using modern, sustainable debinding fluids. Furthermore, applying these fluids to vapour degreasing techniques offers a range of additional environmental advances, including reducing the amount of power or water required for the process, without compromising performance.
There are similarities between binder jetting and material jetting, in that they both lay particles onto a build plate to create a 3D object in layers. However, material jetting involves the depositing of droplets of photosensitive resin that are then hardened with an ultraviolet light, while binder jetting deposits layers of powdered materials that are bound together with a binding agent.
The accuracy of binder jetting depends on which materials are being used for the process and whether colour is being introduced into the part.
In addition, those materials that require post processing can shrink as a result (see ‘disadvantages’ above), although this shrinkage is often taken into account during the build stage.
For example, the dimensional accuracy of metal, full colour or ceramics/sands are:
Metal: ± 2% or 0.2 mm (down to ± 0.5% or ± 0.05)
Full-color: ± 0.3 mm
Sand: ± 0.3 mm
Binder jetting uses a binding agent deposited onto layers of powder. This agent acts as an adhesive to bind the powders together before more powder is added on top and bound to build up a part layer-by-layer.
The process doesn’t require the use of support structures but may require post processing, depending on the materials being used. In addition, coloured binders can be used to create full colour parts or prototypes.
Compatible with a range of materials, the exact manufacturing steps will differ accordingly. Metal parts will require sintering or annealing to properly bind the powder particles together, but sand moulds created with binder jetting will be ready for use right away. Although, in all cases, you will need to remove excess powder from the build before it is finished.
The advantages of binder jetting include less warping as it takes place at room temperature, as well as being less expensive than many other methods and able to deliver high-volume production.
However, parts tend to have only moderate mechanical properties and a high porosity, meaning that they may not be suitable for all requirements.