Digital Alloys’ Guide to Metal Additive Manufacturing - Part 10
Metal Binder Jetting
July 11th, 2019
Binder Jetting is a category of metal additive manufacturing (AM) that utilizes liquid binder, metal powder, and sintering to print high-resolution parts. Sintered parts can have limits on size, density or geometry but despite this, Binder Jetting has garnered a lot of attention due to its potential advantages in scalability and production cost. This post provides an overview of Binder Jetting, digs deeper into its advantages and disadvantages, and highlights applications for which it is best suited.
Companies that have commercialized or announced their development of Binder Jetting technology are highlighted in the chart below. ExOne and Digital Metal are shipping commercial Binder Jetting systems (ExOne has by far the most systems in the field) and also offer printed parts as a service, as does 3DEO. Desktop Metal, HP, Stratasys, GE, and 3DEO are not yet shipping a commercially available Binder Jetting printer.
Definition of Binder Jetting – ASTM F2792:
“An additive manufacturing process in which a liquid bonding agent is selectively deposited to join powder materials”
How Binder Jetting Works
Binder Jetting Diagram:
As with any manufacturing method, understanding the core process provides insight into its fundamental strengths and challenges. Three key elements of the Binder Jetting process influence design, quality, speed, and economics:
1) The Printing Process
In Binder Jetting, a layer of fine metal powder is first spread across the build plate with a roller. An inkjet printhead then selectively deposits binder onto the powder, binding the layer together. After each layer is created, the build plate shifts down and the next layer of powder is spread on top of the previous (typical layer thickness is 50-100 microns). This process repeats until a full bed of parts is created within a volume of powder. Parts in the as-printed state are referred to as “green” parts because they require post-processing (including de-binding and sintering) before they become strong and useful.
Below is an accelerated Binder Jetting video from HP (https://www.youtube.com/watch?v=Igq8gQuXfR4).
The Binder Jetting print process may appear relatively simple, but it is a sensitive process and there are a number of factors that can wreak havoc on quality, especially at high print speeds. Sophisticated technology is required to ensure these key variables are well managed:
1) Consistent powder distribution
2) High powder packing density
3) Precise binder deposition
4) Even binder saturation
5) Sufficient binder drying
Precisely spreading and compacting fine metal powders is not trivial; many precise processes are required to accomplish this operation quickly and repeatably. Furthermore, ensuring a good layer of powder is only half of the battle; the inkjet printheads used in binder jetting must precisely deliver thousands of tiny droplets (10-80 picoliters) of binder per second. At print head speeds up to 1m/s and even faster droplet velocities, the droplets must accurately impact the powder without splattering. In addition, inkjets are notorious for clogging, especially with the high viscosity binders used in Binder Jetting. As a result, a combination of frequent cleaning operations and redundancy of jets are required. Once the binder droplets are successfully deposited, their complete saturation through the layer depends on the consistent distribution and packing density of the powder. Finally, the binder must be mostly dry before the next layer can be successfully rolled on top. All of these elements must be tightly controlled to achieve high-quality “green” parts - one reason industry experts are skeptical of some OEM’s extreme print speed claims.
Binder Jetting OEMs advertise a wide range of print speeds. Digital Metal and 3DEO, who target smaller, higher resolution parts, publish speeds around 100 cc/hr whereas Desktop Metal claims “up to 12,000 cc/hr”. In Binder Jetting, this metric can be misleading. First, “up to” may assume unreasonable layer thickness and 100% packing density of parts in the build volume (i.e., printing a single solid block). In practice, even a highly optimized build is less than 20% part packing density (see build packing representation below). Second, the metric is cc/hr of “green” parts. As we will explain in more detail later, the final part size is only about 60% of the “green” part volume. Maximum speed will be dependent on a customer’s specific application and quality requirements.
2) The Materials
Binder Jetting utilizes two main input materials: metal powders and liquid polymer binders.
Binder Jetting uses the same fine metal powders used by the metal injection molding (MIM) industry (MIM is a conventional process where the “green” binder+powder parts are produced by injection molding). These fine metal powders have particle sizes of approximately 5-45 microns. Powders in the small end of this range make it difficult to flow and spread the material in the printing process but are required to achieve higher densities. Fine metal powder must also be handled carefully because of the risk of inhalation or explosion.
The liquid polymer binders used in Binder Jetting can be thought of as a glue with a water or solvent-based component and a polymer-based component. The binders are proprietary to most OEMs, who optimize them for a few factors: compatibility with the OEM’s unique jetting process, ability to bind powder with minimal binder, and effective evaporation of the water or solvent-based component in low heat (during printing or in oven) and of the polymer-based component in high heat (furnace). Everything from wax to latex chemistries is used as the binding component. The water or solvent-based component is included to carry the binder and allow it to flow through the print nozzles and wick or penetrate down into the powder. Binder chemistry is one of the areas where OEMs differentiate their technology solutions.
3) Post Processing
The post-processing of binder-jet printed parts has three goals: extracting the weak “green” parts from the powder bed and de-powder them without damage, removing all the binder (de-binding) from the green part, and sintering the part to shrink it to an acceptable density and geometric accuracy. The post-processing Binder Jet parts can be more important than the printing process itself.
The “green” printed part is volumetrically 25-50% binder and air (see the microscopic cross section, above). The “green” part is a brittle sponge-like material. It is very weak and useless until the binder is effectively removed and the part sintered to a much higher density. Therein lies the challenge of post-processing in Binder Jetting. Here is a detailed outline of the workflow:
1) The full powder bed build is removed from the machine and placed in an oven to evaporate the water or solvent-based component of the binder and crosslink the polymer-component to increase strength (some printers perform this step in the machine). This typically takes at least a few hours, roughly proportional to the print time.
2) The parts are carefully extracted from the powder bed and the remaining powder that is not contaminated by the binder or other agents is recycled (as much as possible, more on this in the economics section, below).
3) Any overhanging features that are susceptible to warping during sintering must be supported. This is typically done in one of three ways:
1. Supports are printed with the part. Some printers print a ceramic layer between the supports and the final part, making it easy to separate them after printing. Printing supports results in wasted material, but it is usually the most efficient process at low volumes.
2. Supports (aka “setters”) are separately manufactured out of a ceramic material. Setters are more expensive and time-consuming to produce, but they are reusable so they can save time and material cost in higher volume production. The setters have to be carefully designed to support the part during sintering to compensate for part shrinkage and shifting from thermal stresses.
3. The printed green part geometry is compensated to account for the distortion. Software tools are being developed to better simulate and predict this effect and then adjust the part geometry accordingly. This is not a trivial solution and only makes sense for specific geometries.
4) The parts are arranged with supports/setters on racks and placed in a furnace with an inert atmosphere. The furnace first performs a de-binding cycle where it burns off the polymer component of the binder. The de-binding temperature is typically in the 200-600 C range. All the binder must be completely removed from the part or the residual carbon in the binder will negatively impact the sintering process and compromise final part properties. De-binding is a slow process because the binder must evaporate through the tiny porous material structure. If too much heat and energy are applied, the metal particle matrix is disturbed, and the final part quality adversely impacted. The binder is removed at about 1 cm/hr from the outside surface in, so thicker parts can take days to de-bind. Once de-binding is complete, the part is referred to as “brown”.
5) The furnace performs a second, sintering cycle on the “brown part” at approximately 80% of the metal’s melting temperature (1200-1400 degrees C for stainless steels). Sintering slowly shrinks and densifies the part to 93-99% density. The large range in advertised Binder Jetting densities is due to the high variability caused by different types of material, sintering parameters, furnace quality, powder particle sizes, green part packing density and uniformity, and part geometry. Just as with de-binding, the sintering process can be time-consuming, especially for larger, thicker parts.
The Binder Jetting process is unique from almost every other metal printing process in that significant heat is not applied during printing. This makes it possible to print at high speeds and avoid the residual stress issues that plague Powder Bed Fusion and Directed Energy Deposition printing processes (see our relevant blog posts on Design Rules for Metal AM & Energy Consumption in Metal AM). Shifting the application of heat to the sintering step makes it easier to manage thermal stresses because sintering temperatures are lower than full melt temperatures required in most metal AM processes and the heat is applied more uniformly. However, this does not completely eliminate the challenges of temperature gradients and the resulting residual stresses. In the furnace, thinner sections of the part will heat and sinter faster than thicker sections introducing stresses into sections of the part where thickness varies. The cooling cycle after the part is sintered further magnifies this effect. These thermal gradients and stresses can warp and damage parts and may create non-homogenous grain structures that affect material properties.
Managing and compensating for a large amount of shrinkage that occurs in the sintering stage is one of Binder Jetting’s biggest challenges. Parts shrink in the furnace by 30-40% volumetrically, 15-20% linearly. If the part is small and has a uniform wall thickness, then shrinkage is fairly predictable. However, large parts of varying thickness can create a very complex problem for geometry. Sintering shrinkage severely limits the types of geometry and applications for which Binder Jetting is useful. See more on this below under ‘Design’ and ‘Applications’.
After all of Binder Jetting’s unique post-processing steps are completed, the sintered parts usually still require the other post-processing steps germane to metal AM: HIP, finish machining, etc. Please see our post on Process Steps in the Metal AM Workflow. It is worth noting that 3DEO has a hybrid process that performs CNC machining during the printing process. This improves surface finish and can potentially eliminate fixturing and machining as a secondary operation.
Metal Binder Jetting requires fine spherical powders with particle sizes of approximately 5-45 microns. It is able to utilize the same metal feedstocks used by the Metal Injection Molding (MIM) industry. This is a key advantage over Powder Bed Fusion (PBF) because the feedstock is about half the cost, more metals are available, and there is an established global supply chain.
The Binder Jetting process, just like MIM, is in theory compatible with any metal that can be sintered. However, most Binder Jetting machines are only printing a few grades of stainless steel: 316L, 304L, and 17-4. ExOne, the most mature binder jetting company, advertises materials in development but it is not clear how soon they could be available (from their website): Inconel 718, Inconel 625, M2 Tool Steel, H11 Tool Steel, 4140 Steel, 420 Steel, Cobalt Chrome, Copper, Tungsten Carbide-Cobalt, Tungsten Heavy Alloy. Digital Metal, a Binder Jetting company based in Sweden is the only OEM advertising that they can print Titanium.
Please note: there is a category of metal Binder Jetting that involves infiltrating the “brown” part with bronze at lower temperatures than conventional sintering. There are very few production applications for this, so it is not the focus of this post.
Binder Jetting is great for small, complex parts of a uniform wall thickness like those from HP shown above. Binder Jetting design criteria is best summarized in terms of size, geometry, and resolution:
Size: The general guideline is that parts should be smaller than a tennis ball, less than 1 cm wall thickness and under 200g. ExOnes’ website states “Preferred part size < 50 x 50 x 25 mm “. 3DEO’s specifies “up to a 1-inch cube”. This is not to say that larger, thicker parts are not possible with Binder Jetting, but these are usually not attractive when one considers materials strength, quality, economics, and throughput.
Geometry: Designing for Binder Jetting is mostly an exercise in optimizing the de-binding and sintering steps. Thin and uniform wall thickness is the most simple and important rule in this case. 30-40% volumetric shrinkage in post-processing means that parts must be printed proportionally larger than the final geometry to compensate. Varying wall thicknesses make it very hard to accurately predict the correct geometry to compensate for the way that the part shrinks and shifts during sintering. Sophisticated software is being developed by MIM and Binder Jetting OEMs to attempt to do this but there is still a large capability gap.
Binder Jetting has a unique advantage in that it does not require support structures during the printing process or for the part to be anchored to a build plate. That said, certain features will have to be supported during sintering, so large overhangs cannot be ignored. Effective design of the supports or “setters” for the sintering can require a lot of experience.
Resolution: Binder Jetting systems range from 400x400 to 1200x1200 droplets/dots per inch (dpi). The high resolution is attractive from a design perspective because it can produce small, fine features like lattices and small gear teeth. One caveat is that since parts are very weak in their green state, any thin structures or features can easily break while removing them from the powder bed.
For more comprehensive design guidelines, it is helpful to look again to MIM which shares the same de-bind and sintering process limitations. Below is an example of a design matrix created for MIM:
Speed & Economics
The chart below illustrates how Binder Jetting compares to other metal AM technologies in printing, material, and post-processing costs for an example Titanium part production program. You can read more on this study in our post: Economics of Metal Additive Manufacturing.
The largest cost drivers for Binder Jetting are equipment and feedstock materials. The equipment requirements include the printer, a specialized sintering furnace, a de-powdering station, an oven, post-processing equipment, and all the facility infrastructure needed to safely store and handle fine metal powders.
Production Binder Jetting printers are less expensive than most other industrial metal printers, ranging from $400K to $800K. However, the high-quality furnaces required for de-binding and sintering can actually be the largest cost item, usually over $600k. The rest of the equipment brings the total system cost to $1.2 - 1.6M.
At the high speeds claimed by some OEMs, the de-binding and sintering steps may cause a bottleneck in a single printer & furnace setup (assuming a batch vs continuous furnace). In the case of needing multiple furnaces per printer, this has significant negative consequences on the unit economics. If a single furnace can, in fact, keep up and high throughput is achieved, then the material feedstocks are likely the dominant cost driver.
Binder Jetting has key economic advantages over other metal powder bed printing processes (Powder Bed Fusion) not just in print speed but in material feedstocks too. First, it is relatively efficient in its use of metal powder. All the binder and bound powder that is not in the final part becomes waste but remaining unused powder (80-95%) can be recycled. Most OEMs have said they will allow customers to purchase MIM powders directly from suppliers but will require binder to be purchased through them (similar to most commercial 2d printer, paper, ink models). OEMs have not publicized much on the cost of binders. The commodity MIM powders accessible to the Binder Jetting industry are much lower cost than the specialized high-quality powders required in PBF.
Industries & Applications
The strengths and weaknesses of the Binder Jetting process determine the applications where it is most useful. Because Binder Jetting has similarities to the conventional MIM, we can look to that market to illuminate potential industries and applications for Binder Jetting. See the below chart for industries and applications where MIM is used today.
There are three primary drivers for choosing metal AM over conventional manufacturing: improved product designs, time savings, and production cost reduction (please see our blog post on The Business Value of Metal AM).
Binder Jetting has fewer design constraints than MIM which allows certain industries to produce optimized, high-value designs. In addition, MIM tooling is relatively expensive making the process uneconomical for low volume production. Binder Jetting requires minimal tooling, making it more cost-effective low volume applications that are not practical for MIM. Binder Jetting can also offer speed to market benefits by reducing the tooling lead time.
Binder Jetting solves many of the problems of PBF, the most widely used metal AM process today; namely its speed, cost, and scalability. However, it introduces its own set of challenges including part density, strength, size, thickness, and accuracy. This makes it applicable for new applications that metal AM has largely unaddressed.
For low to medium volume programs with small, thin, and complex part geometries, Binder Jetting will make an impact across a number of industries: automotive, medical, industrial and consumer products. But this will take time. MIM took a couple of decades to identify and validate a large number of production applications, creating a ~$3 Billion market today. Binder Jetting will be able to eat a small slice of this pie but also grow the market over time with new, lower volume opportunities where MIM is not a great solution.
There is no shortage of well-funded companies working to advance Binder Jetting. ExOne, Digital Metal, and 3DEO were first to market - so, at this point in time, they likely have the most refined processes and best understanding of how to meet the industry’s production requirements. Newer players such as Desktop Metal, GE, HP, and Stratasys are bringing large amounts of capital and technology expertise to the table (e.g. HP has been developing inkjet technologies for decades) but their Binder Jetting solutions are still in their infancy. Most Binder Jetting OEMs acknowledge the need for a strong ecosystem and have partnered with some of the leading MIM manufacturers and powder suppliers to accelerate their development and go-to-market. The companies which can first develop and demonstrate robust manufacturing solutions for real, valuable applications will win.
This table of Binder Jetting OEM’s and advertised printer or service capabilities may be useful if you wish to determine which are best aligned to your application needs:
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We would like to acknowledge and thank the following individuals that contributed to this post:
Andrew Klein, Director of R&D – ExOne
Mark Reibel, Sales Manager - ExOne
Richard Huff, Senior Staff Engineer - GE Additive
John Barnes - Barnes Group Advisors
Joris Peels, Editor in Chief – 3DPrint.com
Matt Sands, CEO - 3DEO
Lance Kallman, VP BD - 3DEO
VP Business Development
Digital Alloys is committed to providing the technology and expertise manufacturers need to use metal additive manufacturing in production — enabling them to save time, shrink costs, and produce valuable new product.