Introduction
Binder Jetting has multiple capabilities in diverse production of functional metal parts, full-color prototypes, figurines and large sand casting. We will introduce the mechanics, characteristics and key benefit of Binder Jetting.
Binder Jetting Process
Binder Jetting create parts with adhesive layer-by-layer by powder articles. The recoater will spread a thin powder layer on the bed of platform, then print heads will deposit adhesive agents selectively on powder, bind them together to form a layer. In full-color prototypes, the color agents is jetted on powder in this step. The build platform will move down in a layer thick and repeat this process until the final part is complete.
Binder Jetting apply different materials depending on applications, form metal to plaster or PMMA. In order to satisfy full-color prototypes and high-temperature functions requirement.
Binder Jetting Design
Binder Jetting provides great design freedom, all manufacturing process are at low temperature. So there is no errors connected with thermal effects. In addition, there is no support structures, parts are fully encapsulated and supported by surrounding powder particles.
After printing process, the parts are fragile in green state, post process like sintering or infiltration will create the final parts. This will add restrictions in design, as thin features in the green state during handling.
Print Features
| Feature | Recommended minimum value |
| Wall thickness | We recommend minimum wall thickness is 2.0 mm. This will reduce damage in removing and handling process. |
| Unsupported walls | Unsupported walls have higher risk of damage during handling, this wall thickness should be thicker than 3.0 mm. |
| Embossed & Engraved details | Embossed and engraved details must be at least 0.5mm below or above surface for visible requirement. |
| Unsupported edges | Unsupported edges should be less than 2.0 mm, in order to avoid breaking in green state. |
| Fillets | All fillets should be at least 1.0 mm radius, this design should be used in any possible edges. In order to avoid damaging in green state. |
| Hole size | The minimum diameter should be larger than 1.5 mm. |
| Escape holes | For hollow sections, the escape holes diameter must be at least 5.0 mm, the escape holes number should be at least 2. |
| Feature size | Although this process can produce very fine details, the brittle parts in green state is an big issue. We recommend the minimum features size is 2.0 mm. |
Green State
In Binder Jetting, the parts in green state are fragile. In reason of they are comprised of sand or powder particles. It is difficult to apply post-processing once parts are too fragile, designers should follow guild-line to ensure the parts are strong enough in handling and post-processing.
If the features or details are too fragile, we should add extra bracing structures to improve parts stiffness, than remove them after infiltration or sintering. However, it will add cost and time.
Infiltration and Sintering
Binder Jetting apply secondary processes like infiltration or sintering to produce functional metal parts. Full-color sandstone parts also can be enhanced by infiltration to increase mechanical properties.
Infiltration: After Binder Jetting, the green parts are placed into furnaces, the binder will be burnt out, in this process, the parts are approximately 60% porous. Then infiltrate bronze into voids by capillary action, finally get low porosity with good strength.
Sintering: After Binder Jetting, the green parts are placed into high temperature furnaces, the binder is burnt out and the metal particles are sintered together, finally get low porosity of 3%.
Infiltration and sintering in metal Binder Jetting will give rise to shrinkage and dimensional reduction, the sintering effect is more prominent. Shrinkage is highly dependent on part geometries, non-uniform shrinkage is an issue, this should be considered in design stage.
Binder jetting parts after infiltration or sintering will achieve density of 90% and 97%, there are still some internal porosity. This means their mechanical properties are inferior than bulk material or other 3D printing technologies like DMLS.
Full Color Printing
Binder Jetting can print full-color parts by jetting ink and binding agent on the plaster or PMMA powder. After printing, the parts should be coated with super glue to improve part strength and enhance color vibrancy. The second epoxy layer will be added to further improve strength and color appearance. With these extra steps, the parts are also very brittle and not suitable for functional parts.
Binder Jetting Materials
We summarize the common used materials with characteristics.
| Material | Description | Characteristics |
| Full Color Sandstone | Realistic, high definition, full-color concept models and prototypes | Full color non-functional modelsVery brittle |
| Silica Sand | Industrial casting molds and cores | Very high thermal resistanceExcellent for sand casting applications |
| Stainless steel(bronze infiltrated) | Low yield strength for easy machining and polishing | Good mechanical propertiesCan be machined10% internal porosity |
| Stainless steel(sintered) | High tensile strength and temperature resistance, high corrosion resistance | Good mechanical propertiesHigh corrosion resistance3% internal porosity |
| Inconel alloy(sintered) | Nickel-chromium supper-alloy with excellent mechanical properties | Excellent mechanical propertiesGood temperature resistanceHigh chemical resistance |
| Tungsten carbide(sintered) | For high wear-resistant abrasive, carbide cutting tools | Very high hardness |
Post Processing
In reason of the nature of Binder Jetting, there are limited number of post processing method.
Metal Binder Jetting parts after infiltration and sintering have 3 normal post processes, sanding, polishing and machining, in order to improve parts appearance and dimensional accuracy. Ohter common metal finishes also can be applied to theses parts.
Sand casting cores and molds require thermal post-processing to improve strength.
Full-color models need general dipping or coating to improve mechanical properties.
Clear lacquer also can be coated to improve wear resistance and get smooth surfaces.