In the wide array of additive-manufacturing technologies (AM) available today, binder jetting stands on its own for the ability to combine the flexibility and part complexity typical of AM processes with a productivity rate unmatched by other AM technologies.
Developed from an MIT patent in the early 1990s, binder jetting (BJT) is a powder-bed-based process where, contrary to powder-bed-fusion technologies such as SLM and EBM, the material is compacted through the selective deposition of a liquid binder instead of melting. That means binder jetting is able to process a wide array of materials, including ceramics and polymers, even though the spotlight in the last few years has been on the BJT production of metal parts. To achieve their final strength and density, metal BJT-printed parts then rely on sintering inside a furnace (Fig. 1).
This article will go through the BJT workflow, compare it to other 3D-printing and powder-metallurgy processes, discuss its advantages and talk about application fields of the technology.
What’s Binder Jetting?
Binder jetting (BJT) basically consists in the selective deposition of a liquid binder through a printing head, following the “slice” (i.e., the 2D section) of a CAD file. The build platform then lowers itself, and a fresh layer of powder is spread by the recoater. The process is repeated layer after layer until the build is completed (Fig. 2).
The printing head contains hundreds of nozzles, meaning that a high volume of binder can be deposited rapidly but still maintain a high accuracy and resolution of the parts. Usually, BJT parts do not need specific supports since the workpiece is already supported by the loose powder inside the building volume.
As already mentioned, BJT is a “cold process” because no melting of the material is involved. For this reason, there’s no need for preheating of the powder bed to avoid cracking or distortion of the part even though heat can be applied after the processing of a layer to accelerate the drying of the binder. After printing, the entire powder bed is cured at low temperature (around 392°F/200°C) to further dry and compact the part while still surrounded by the loose powder.
Fig. 1. TAV mim5 debinding and sintering furnace // All graphics provided by the author, except where noted.
Fig. 2. Workflow of the binder-jetting production cycle
For its characteristics, this process could be applied virtually to any metallic powder even though some requirements in term of powder flowability and particle size distribution are present to ensure a homogeneous spreading of the powder, good wettability with the binder, good surface roughness and sufficiently high final density of the part. At the moment, the most common metals for metal BJT production are austenitic 316L stainless steel and martensitic 17-4 PH stainless steel. Other qualified materi-als available from machine producers include 304L stainless steel, Inconel 625 and Inconel 718, H13, D2 and M2 tool steel, Ti6Al4V titanium alloy and copper.
After the curing step (Fig. 3), the part needs to go through an infiltration or sintering cycle to achieve its final mechanical properties. Infiltration consists of densifying the part through the introduction of a different material that does not deteriorate the property of the base metal. Steels are typically infiltrated using copper, which penetrates inside the open pores to obtain a good final density.
The only way to obtain a tough and dense single-alloy metal part is through sintering (Fig. 4), or keep-ing the part at an adequate temperature for an adequate time to promote atomic diffusion between the powder particles and fuse them together.
Fig. 3. Watch parts in the green state, printed with an ExOne X1 25PRO BJT machine (courtesy of Cor.Sa 3D)
Fig. 4. Sintered watch parts, printed with an ExOne X1 25PRO BJT machine and sintered in a TAV mim3 vacuum furnace (courtesy of Cor.Sa 3D)
Transforming Good-Looking Parts into Strong Parts by Sintering
Vacuum sintering furnaces are usually the go-to choice for sintering of BJT parts thanks to their ability to provide bright and shiny parts, the tight process control and the possibility to work with different debinding and sintering atmospheres.
Sintering cycles for BJT parts include low-temperature soaks to burn off all the cured binder residuals. For the organic liquid binders typically used in BJT, evaporation starts at around 392°F (200°C) and is completed before the furnace temperature reaches 932°F (500°C) even though this temperature interval can be slightly affected by the actual binder composition and debinding atmosphere. Proper debinding parameters are critical to avoid cracking, deformations or the formation of carbon-based binder residual, which will lead to carbon diffusion inside the part hindering the corrosion resistance and worsening the final part properties. For high-volume production, it is also possible to carry out this step in a dedicated debinding furnace (Fig. 5), which will increase productivity and facilitate the cleaning process.
In any case, whether the choice is to use separated furnaces for debinding and sintering or to carry out the entire process in a single-step debinding and sintering furnace, it is important that the furnace is equipped with a proper trapping system to prevent the vaporized binder from reaching the vacuum pumps.
Fig. 5. TAV’s D3 debinding furnace
Sintering parameters (i.e., time and temperature) should be fitted to the part material and size. The sintering cycle should also include intermediate dwellings and different heating rates to maximize temperature homogeneity on the part while minimizing cycle time. In general, most metal injection molding (MIM) best practices regarding sintering can be transferred to BJT even though effects such as shrinkage and deformation must be kept under consideration even more carefully for BJT parts because this technology can produce larger parts with a more complex geometry. Moreover, the different production method doesn’t make use of external pressure during the forming stage, leading to parts with a higher tendency to lose shape during sintering.
Sintering of BJT parts in vacuum furnaces can be carried with different atmospheres, including vacuum, nitrogen or argon, nitrogen/hydrogen or argon/hydrogen mixtures, and pure hydrogen (both at low pressure below 10 mbar and in overpressure slightly above 1 bar). The choice of sintering atmosphere should be fitted to the material and to the desired part properties.
Both graphite and all-metal vacuum sintering furnaces are suitable. The choice should be made, again, keeping into consideration the part material and the final applications. All-metal vacuum sintering furnaces are the go-to choice for oxygen-sensitive materials (such as titanium), allowing for more material versatility.
On the other hand, investment and operating costs are in favor of graphite vacuum sintering furnaces. Moreover, vacuum sintering furnaces usually include a multi-level retort, such as TAV’s MIM-Box (Fig. 6), to maximize loading capacity inside the useful volume. Similarly, MIM-Box is also available in both graphite and all-metal configurations.
Fig. 6. The hot zone and MIM-Box of TAV’s mim5 debinding and sintering furnace
The Current Binder-Jetting Industry
If we only consider metal parts production, there are currently only a few manufacturers of BJT ma-chines: ExOne, Digital Metal and Desktop Metal. HP and GE Additive have also been developing BJT systems that are expected to be released in the near future. The peculiarities of BJT make it a competitive technology compared to other processes both inside and outside the world of additive manufacturing.
The layer-wise nature of BJT allows it to have the same design freedom typical of powder-bed-fusion (PBF) processes. Compared to PBF processes, the cost per part when operating at production rate is certainly lower. Moreover, being a “cold” process, BJT doesn’t induce residual stresses inside the part that require further heat treatment, as is the case with PBF. On the other hand, optimization of the BJT production process requires not only good knowledge of the printing phase but also of the debinding and sintering post-processing.
Currently, the number of BJT systems installed worldwide is comparatively much lower than PBF systems even though the technology has been around for more than two decades. The larger diffusion of PBF systems also means that many studies are available on those technologies, allowing for a deeper understanding of the process and post-process parameters. Finally, the presence of published standards for PBF processes has certainly helped the growth of on-field applications for PBF parts in the aerospace, automotive and biomedical industries, while BJT parts applications are still mostly limited to consumer products.
MIM is undoubtedly the most common powder-metallurgy process for the production of small to medium-sized components. MIM can produce parts in a wide array of metal alloys with very good resolution of the features and surface finishing. All of the metal alloys that are available today for BJT have been part of the MIM industry for quite some time. However, MIM has some limitation on the design complexity related to the impossibility to mold certain features (inside cavities, spirals, sharp edges, etc.). Moreover, the latest-generation BJT machines can produce parts that are quite larger than typical MIM-produced parts even though size constraints for both technologies are more related to the available equipment and sintering than the process itself. Cost per part for low-volume production is unquestionably in favor of BJT, which allows for unlimited product flexibility at virtually no cost. For high-volume production, however, costs are still in favor of MIM. The break-even point for cost production between the two technologies can be found between 10,000-30,000 produced parts, depending on part characteristics.
An interesting alternative to BJT is material jetting (MJT), where droplets containing both the base material and the binder are selectively jetted altogether. Metal MJT is a new technology that has only been patented and commercialized by Israel-based company XJet through its NanoParticle Jetting technology. The main advantages of MJT are related to the ultra-thin layer thickness and the subsequent high part resolution thanks to the nanoscale dimensions of the particles that are being jetted and the ease of handling the feedstock, which is exempt from any particular safety requirement.
Binder jetting (BJT) has unquestionably evolved from a promising technology, mostly limited to proto-typing, to a serious alternative for both conventional subtractive manufacturing and additive manufacturing. It has to be expected that a deeper knowledge of the design for BJT, the process parameters and the optimization of sintering cycles will spread among the industry.
In the meantime, relying on experts such as TAV Vacuum Furnaces will certainly accelerate the learning curve of new BJT adopters, helping them make the most out of the technology.
TAV VACUUM FURNACES would like to thank Cor.Sa 3D (https://www.corsa3d.it) for its contribution to this article.
For more information: Giorgio Valsecchi is a research-and-development engineer working on optimizing vacuum heat treatments such as brazing, sintering and heat treatment of additive-manufactured components. He can be reached at email@example.com.
All graphics provided by the author, except where noted.
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