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Binder Jetting

Binder jetting is a rapid prototyping and additive manufacturing technology for making three-dimensional objects from computer models or CAD files. Binder jetting is one of the seven types of additive manufacturing processes according to ASTM standards.

Process

As in many other additive manufacturing processes, the desired part is created by sequentially printing layer by layer. Binder jetting specifically relates to the process of strategically depositing a liquid binder onto a bed of powdered material. For each pass of the printhead, the bed of powder is lowered and a new layer of dry powder is added atop the previous layer. The printhead then deposits the liquid binder onto the new layer of powder to create a single layer of the part. This process is repeated until the entire part has been created. The printing technology is in essence very similar to that of conventional 2D inkjet printers that leverage drop-on-demand technology.

Sketch of 3D printer using binder jetting technology.

Despite sharing essentially the same printing process, there are two main types of binder jetting technology, separated by material properties: silica and ceramic binder jetting and metal powder binder jetting. Recently, additional variations such as saw dust binder jetting have emerged.

A main difference between the two types of binder jetting, as it relates to the printing process, is the ability to inject color while printing to create multicolor models using silica, ceramic and saw dust. This is not possible when printing with metal materials.

3D selfie in 1-20 scale leveraging binder jetting technology with color.

Post Printing Operations

Parts created using binder jet technology are unique to parts created using other 3D printing methods, as there is substantial work left to complete after the conclusion of the printing process.

Freshly printed parts are called "green", borrowing the term from ceramics to indicate they are only weakly bound by the binder and have yet to be cured. Green parts are fragile and can break easily.

ExOne binder jet printer depositing liquid binder onto a bed of metal powder.

It is required to cure the green parts to remove them from the print bed. During this intermediate step, parts are hardened in a kiln for several hours around 200°C. This process increases the part strength and allows the parts to be safely removed from the print bed without damage. Parts that have been cured are called "brown". For some printing methods, such as printing using saw dust, this is the last process to produce a finished part. However, for most other material types, there are additional process required. At this stage, brown parts are still highly porous, significantly degrading mechanical performance. To combat this, parts can be sintered or infiltrated with liquid metal.

Before sintering, all loose powder must be removed from the part's features. Any leftover powder will fuse to the part, causing unwanted deformities. Major challenges to removing excess powder include inhalation of fine particulates and risk of explosion. For these reasons, the de-powder process generally uses specialized equipment to contain loose, airborne powder particles. For some systems, up to 99% of unused powder can be recycled for future prints.

In the sintering process, the part is placed in a kiln and heated to just below the material's melting point. This process creates bonds on the molecular level which decreases the porosity of the part, resulting in increased part strength. However, sintering can cause parts to non-homogenously shrink based on the part geometry. Thus, for complex parts, the amount of shrinkage can be difficult to predict. Companies have developed software that predicts how parts may be affected by this phenomena and will automatically modify the part geometry to combat the effect. Additionally, the binder is evaporated from the part during the sintering process.

In some cases, to further increase part density, liquid metal infiltration can be used to fill the remaining voids left by the evaporated binding agent.

The final process to complete a part varies greatly depending on the application but could include machining, polishing or plating, among other processes to complete the printed part.

Materials

Due to the nature of the printing process, the materials used must be in a fine powdered form. This limits the materials eligible for use, however, there is ongoing research to develop new materials. Currently, it is possible to print with the following materials:

Materials Used In Binder Jetting
Gold
Silver
Titanium (Ti64)
Tungsten Carbide Cobalt
S7 Tool Steel
Austenitic Stainless Steel
420 Stainless Steel
304L Stainless Steel
316L Stainless Steel
17-4PH Stainless Steel
Inconel 718
Inconel 625
H13 Tool Steel
D2 Tool Steel
Copper
Aluminum 6061
Sawdust
Sand
Acrylic
Sugar

Note: This list is not totally inclusive and there exist other materials that are compatible with binder jetting technology.

Fine metal powder, similar to that used in binder jet printing.

Binder jetting technology has advanced such that many modern printers have the ability to print using the same family of materials. It is important to note that generally for every material, there is generally a unique binder that has been designed and engineered specially to be used with that material.

Advantages and Disadvantages to Binder Jetting

Because it is not possible for other methods to use organic materials, ceramics or silicas, this section focuses on comparing metal binder jetting to other 3D printing methods, such as selective laser sintering (SLS) and direct energy deposition (DED).

Advantages

1. Material variety
   1. The compatible materials vary significantly, including many metals and ceramics, as well as organic materials such as sugar and wood dust. For this reason, binder jetting technology is appealing, as in many cases, the same system can be used for multiple material types. To use some materials, binder jetting is the only viable method.
2. Room temperature printing
   1. Due to the printing method, there is no heat injected into the powder during the binder jetting process, unlike many other printing methods. The advantage is that no internal stress builds up when the part is printed.
3. No support structures
   1. The surrounding, unused powder in the build volume provides support for future layers. This feature enables the creation of very complex structures with overhangs and freeform geometries. Additionally, the entire build volume can be used. To take advantage of the speed of binder jet printing, it is important to efficiently fill the build volume with parts (comparable to bin packing).
   2. Because parts can be printed on previous layers of powder, it is possible to stack parts on top of each other to maximize the build volume.
4. Fast printing
   1. Compared to other printing methods, binder jetting is significantly faster. For example, Desktop Metal's Production System P-50 prints up to 12,000 cm^3 per hour.
5. Build volume
   1. The effective usable build space is on average among the largest of any of the 3D printing methods. Printers for sand mold casting top out around 2200 x 1200 x 600 mm and metal printers range around 800 x 500 x 400 mm.
6. Surface finish
   1. Typically, binder jetting produces a better surface finish compared to SLS or DED. Binder jetted parts generally have a surface roughness around Ra 6 μm, after post processing. In comparison, SLS parts have a surface roughness of around Ra 12-16 μm. This is beneficial for parts with critical internal features where post processing is either very difficult or not possible.
7. Batch production
   1. Multiple versions of a part can be printed at the same time in the same build volume, allowing for important flexibility and versatility. Part specific fixturing is unnecessary in binder jetting.
8. High part accuracy
   1. Metal printers boast a high printer accuracy of 1,200 dpi (dots per inch) with layer thicknesses as small as 50 µm allowing for precise parts off the printer.
9. High isentropic strength for metal parts
   1. After metal parts are sintered, they effectively become homogenous which enables high strength in all directions. This is not the case for other printing methods, where parts generally have a weak axis, forcing designers to modify parts to accommodate for this drawback.

Disadvantages

While there are many advantages to binder jetting, there are notable disadvantages.

1. Post printing operations for metal parts
   1. Binder jetting does not create a finished product after printing. Parts must be cured, depowered and sintered, which requires specialized equipment and can take multiple days to complete.
   2. The post processing operations are generally where geometric inaccuracies are introduced. Parts can shrink up to 20% in size during sintering.
2. High equipment and employee costs
   1. Binder jetting setups are generally more expensive than other printing methods, largely due to the additional equipment required: curing machine, industrial kiln, and a dedicated depowder machine with ventilation. Generally, additional employees are required to operate the auxiliary equipment, increasing overhead.
3. Weaker parts
   1. Due to the granular nature of the powder, there are small voids that exist within the finished part. This effect can be mitigated by sintering and infiltrating. Sintered parts are around 95-99% dense, depending on the system used and material properties. As a result, these parts are weaker than their SLS printed counterparts.