The patents for Binder Jetting are as old as the ones from Laser Beam Powder Bed Fusion. However, in recent years, the technology is getting more attention due to several new players in the field who claim, Binder Jetting might enable large volume metal Additive Manufacturing production. In this report section, you learn everything about the background and state of the art of this technology.
Binder Jetting is a powder based Additive Manufacturing technology in which a liquid polymer binder phase is selectively deposited onto the powder bed joining the metal particles and forming a green body.
The metal powder is applied to a build platform in a typical layer thickness of 50 µm to 75 µm. Subsequently a modified 2D print head ejects a binder liquid selectively into the powder bed. Depending on machine technology a hardening or curing process of the binder phase is performed in parallel for each layer and/or at the end of the whole build. During the in-situ curing process a heat source is used to solidify the binder and form a solid polymer – metal powder composite. Afterwards the build platform moves downward by the amount of one layer thickness and a new layer of powder is applied. Again, the liquid binder is deposited and hardened in the required regions of the next layer to form the green body. This process is repeated until the complete part is printed. After the complete printing process is finished the parts have to be removed from the “powder cake” meaning the surrounding loose but densified powder. To improve the removal of the excess powder from the green body often brushes or a blasting gun with air pressure are used.
To create a dense metal part the 3D printed green body has to be post-processed in a debinding and sintering process. Similar to the metal injection molding process BJT parts are placed in a high temperature furnace, where the binder is burnt out and the remaining metal particles are sintered together. The sintering results in densification of the 3D printed green body to a metal part with high densities of 97 % to 99,5%, dependent of the material.
Due to the low volume of binder in Binder Jetting green parts, the debinding process is usually performed in the sintering oven. For this purpose, the sintering temperature curve will remain at a lower temperature level to evaporate the binder before the temperature is increased to start the sintering phase.
After the binder is removed, the part is sintered to increase strength and structural integrity. The part is heated close to the melting temperature. The temperature is just high enough to initiate a neck formation at the contact points between the metal particles. In the first sintering stage, channels remain between the necks. In the intermediate stage of sintering, the packing density of the part increases and particles merge. In the final stage, the pore size decreases further and the second level binder decomposes completely.
During the sintering process the part shrinks in all directions of about 16 to 20 %. In z-direction shrinkage is larger than the other two directions due to the influence of gravity. Sinter supports used to prevent undesired deformation have to be removed in a subsequent step.
Binder jetting was developed and patented at the Massachusetts Institute of Technology (MIT) in 1993. The machine technology was adapted from normal inkjet printers. The company Z CORPORATION (since 2012 3D SYSTEMS) obtained a license for the process in 1995. In the beginning only polymer powders or sand was used in Binder Jetting processes in order to fabricate prototypes or sandcasting forms. The early adoption of this technology for prototyping applications made Zprinter one of the most popular commercial 3D printer lines available. Later, several companies such as EXONE, HEWLETT PACKARD, DESKTOP METAL, DIGITL METAL and 3DEO obtained licenses and started to develop their own printing systems for metal powders.
EXONE and DIGITAL METAL have been selling metal BJT systems for many years. However, the market penetration of the machines of this technology is very low compared to other processes such as LB-PBF. Since DESKTOP METAL and HP announced their planned entry into the BJT market with newly developed single pass systems and claims of productivity gains of up to 1000x, BJT gained a lot more attention. First single pass commercial systems are available for beta customers since late 2018, early 2019. On this wave of excitement additional machine manufacturers such as GE have been announcing the development of their own BJT printing systems as well.
Binder Jetting is still waiting for the wider industrial break through. Although smaller systems from DIGITAL METAL and EXONE are used in first serial production scenarios, the overall installed base of Binder Jetting systems is still very small. However, AMPOWER expects Binder Jetting to quickly rise in the Maturity Index over the next two to three years. Especially the announcements of HP and DESKTOP METAL partnering with major players in the powder metallurgy industry shows, that the bottle neck of the sintering process is targeted and potentially solved soon. The release of large production systems of multiple machine OEMs in 2020 shows, that the target application is beyond the typical PBF part complexity and batch size, but rather trying to cannibalize metal injection molding and powder metallurgy applications.
AMPOWER has a proprietary system to calculate the technology readiness level of an Additive Manufacturing technology based on two indices. The first index assesses the maturity of a technology (technology maturity index). It is rated by a number between 1 (basic research has been done) and 5 (established full-scale production technology). The second index estimates in how far a technology is established on the market (industrialization maturity index). It is measured on the basis of several weighted parameters. Both indices are important factors for estimation of the success of a technology.
Binder Jetting is especially suited for small part sizes in the range between 5 to 50 mm. Due to the debinding and sintering process and the current state of knowledge, thinner wall thicknesses are preferred. With a surface roughness of about Ra of 5 µm, surface quality is high compared to other metal Additive Manufacturing technologies.
Since the bonding between the polymer ink and metal powder during the 3D printing process takes place without the melting of the metal component, no dimensional distortions occur due to thermal effects. Additionally, the surrounding powder bed supports overhangs and complex geometries. Therefore, no additional support structures are required for the printing process. The dimensional accuracy of printed green bodies is very high.
However, the elevated temperatures and shrinkage during the sintering process require deep knowledge on process specific design. Poorly designed or insufficiently supported green bodies show large distortion and cracking during the sinter process. The shrinkage can be around 20% per direction. While the green body printing process does not require supports, such stabilizing structures are sometimes required for the sintering to avoid undesired deformation.
It is expected that Binder Jetting will replace low volume, high cost metal injection molding parts. These parts typically have applications in automotive, medical or tooling industry. Low volume jewelry and consumer goods are also potential applications for BJT.
The only prerequisites of materials for BJT are the sinterability of the alloy and the availability as a powder. Since the manufacturing process does not rely on a welding process such as PBF, many alloy groups with inherent difficulties in AM, such as high carbon steels and carbides, become a possible option for BJT. However, so far, the most commonly available alloys are stainless steels and tools steels such as 1.4404/316L or 1.4542/17-4 PH. Furthermore, Nickel and Cobalt-based alloys, Copper and titanium alloys are available as well. Cemented carbides are currently under development. Due to general difficulties of sintering aluminum, aluminum and its alloys are not being processed so far. However, the development of aluminum alloys with better sinterability is under development, too.
BJT typically use very fine-grained powder feedstocks with grainsizes from 5 µm to 20 µm. At the moment mainly spherical powders are used, however many system suppliers claim to be able to process non spherical particles. This would significantly drive down the costs of the feedstock in the future.
The material properties of parts fabricated by Binder Jetting in 316L excel compared to the values required for steel bars according to ASTM A276 (Standard Specification for Stainless Steel Bars and Shapes). Typically, the material density is around 99 % and therefore compares to high end MIM applications. The pore size and distribution are mainly depending on the debinding and sintering process applied, but typically results in small round pores below 20 µm in diameter. The microstructure shows twinned austenitic grains with grain sizes between 50 to 100 µm. The printed parts exhibit hardness close to the requirements for MIM alloys defined in ISO 22068. Values for yield strength, ultimate tensile strength and elongation exceed the required properties defined for MIM parts in 316L. Fatigue properties of BJT parts is topic of numerous R&D activities, as there is yet little information available.
Even and round pores typical for sintered materials. Density between 98-99 %. Comparable to high end MIM properties.
Larger and uneven pores be due to green part failues
Typical round gas pores due to sintering.
Last data update: 26. March 2021
Published: 19. November 2019
Source: AMPOWER
Source ASTM values: ASTM A276 Standard Specification for Stainless Steel Bars and Shapes