Binder Jetting
BJBJ3DPBinder JetHP Metal JetDigital Part MaterialisationNo high-energy beam during printing. A liquid binder (aqueous polymer or solvent-based resin) is deposited by piezoelectric inkjet printheads. Energy input occurs only in post-processing: thermal cure at 60–200°C (resistive oven) and high-temperature sintering at 1200–1400°C (furnace, resistive or inductive heating). No residual stresses from thermal gradients during the additive phase.
How it works
A powder layer (typically 50–100 µm for metals, 250–400 µm for sand casting cores) is spread across the build platform by a counter-rotating roller or blade. Piezoelectric inkjet printheads traverse the powder bed and deposit droplets of liquid binder (typically 10–80 pL per droplet) at the precise locations corresponding to the part cross-section. Binder saturation — the ratio of binder volume to inter-particle void volume — is the primary print parameter controlling green part strength. After the complete layer is deposited, the platform steps down and the cycle repeats. When the build is complete, the green part is encased in loose, unbound powder that supports it structurally — no dedicated support structures are required. The part is extracted from the powder bed, excess powder is removed by brushing or air blasting, and the green body undergoes a two-stage thermal cycle: a low-temperature binder cure (60–200°C) to harden the binder polymer and increase handling strength, followed by high-temperature sintering (1200–1400°C for stainless steels, in hydrogen or vacuum atmosphere) in which the binder burns out and inter-particle diffusion closes porosity. Linear shrinkage of 15–17% during sintering must be compensated in the CAD model by applying a uniform or anisotropic scale factor. HIP can be applied after sintering to close residual pores and reach >99.5% relative density for critical applications.
Parameter envelopes (2 material–machine combinations)
Layer thickness
50–100 µm
ExOne Innovent / Desktop Metal X-series. Binder: furan resin or aqueous polymer. Sintering: 1380°C/3h in hydrogen atmosphere. Linear shrinkage ~15–17% must be compensated in CAD scaling. Post-sinter density >97% for structural applications.
Layer thickness
50–100 µm
Preheat
60 °C
17-4PH BJ requires sintering at ~1260–1300°C. Phase mix depends on sintering atmosphere (H₂ → more martensite; N₂ → more austenite). Must apply Condition A + aging cycle after sintering for target properties. Shrinkage ~15–17% linear.
Defect modes (5)
Sintering Distortion (Gravity Sag)
Cause
At sintering temperature (1200–1400°C for metals), the part approaches its solidus and loses creep resistance. Large flat sections, thin cantilevers, or long overhanging spans sag under gravity over the 2–6 hour sintering dwell. Distortion magnitude is proportional to span length and inversely proportional to section modulus.
Indicator
Post-sinter dimensional inspection (CMM or 3D scan) reveals out-of-flatness on large horizontal surfaces. Camber in long, slender features. Dimensional deviation that correlates with gravity direction rather than shrinkage direction.
Prevention
Orient parts so critical surfaces face upward or are self-supporting. Use ceramic setter plates and sintering fixtures to support flat faces. Limit unsupported horizontal spans to <40–50 mm without setters. Simulate sintering distortion with dedicated FEA tools (e.g., Netfabb, Amphyon sintering modules) and pre-compensate geometry.
Detection
- CMM dimensional inspection
- 3D scanning vs. CAD nominal
- go/no-go gauging of critical bores
Incomplete Sintering / Trapped Porosity
Cause
Insufficient sintering temperature, too-short dwell time, or excessive heating rate prevents full densification. Binder residue (carbon, oxide) at particle surfaces forms inclusion layers that inhibit inter-particle diffusion and pin pore closure. Porosity >3% indicates under-sintering.
Indicator
Low Archimedes density measurement. Cross-section metallography showing remnant porosity or dark oxide/carbon inclusions at prior particle boundaries. Reduced tensile strength and elongation versus expected sintered values.
Prevention
Follow validated sintering cycle for each alloy and part geometry: ramp rate <5°C/min through binder burnout zone (300–600°C) to allow complete pyrolysis, full dwell at peak sintering temperature (minimum 2h at 1380°C for 316L). Use reducing atmosphere (H₂ or H₂/N₂ mix) to reduce surface oxides and promote diffusion bonding. Qualify sintering furnace with thermocouple surveys to confirm temperature uniformity.
Detection
- Archimedes density / water displacement
- metallographic cross-section and image analysis
- X-ray CT
Shrinkage Anisotropy
Cause
Layer-by-layer powder spreading imparts a preferred in-plane compaction in the XY directions relative to Z. During sintering, XY shrinkage is typically 1–2% lower than Z shrinkage because particles are more tightly packed laterally. Different binder saturation levels in the XY plane vs. through-thickness also contribute. The result is non-uniform scale factors: applying a single isotropic compensation factor produces parts that are within tolerance in XY but over-shrink in Z (or vice versa).
Indicator
Post-sinter dimensional inspection reveals consistent systematic dimensional deviation that differs between XY and Z axes. For example, a cylindrical part is within tolerance in diameter but short in height.
Prevention
Measure XY and Z shrinkage independently for each material-machine-sintering combination using calibration bars. Apply axis-specific scale compensation in the CAD model or in slicing software. Re-qualify if sintering furnace, batch size, powder lot, or sintering cycle is changed.
Detection
- CMM measurement of calibration artefacts after sintering
- 3D scanning vs. nominal
Green Part Cracking
Cause
The green part is held together by binder only (green strength typically 1–5 MPa — far below sintered strength). Thin walls (<2 mm), sharp internal corners, long unsupported spans, or aggressive depowdering operations can cause brittle fracture of the green body before it reaches the sintering furnace. Rapid thermal ramp during binder cure can also cause differential thermal expansion cracking.
Indicator
Visible cracks or missing sections on green parts after extraction from the powder bed or during handling. Cracks are often hairline in thin-walled sections and may not be obvious until the part is illuminated at an angle. Cracks that survive to sintering appear as planar porosity or surface steps in the final part.
Prevention
Design minimum wall thickness ≥2 mm (3 mm preferred for unsupported spans). Avoid sharp re-entrant corners — use radii ≥0.5 mm. Handle green parts with soft gloves and support large parts from below. Apply slow ramp rate during binder cure (≤1°C/min through 100–150°C moisture evaporation zone). For fragile geometries, cure in a vibration-free environment.
Detection
- Visual and tactile inspection of green parts under oblique lighting
- liquid dye penetrant on cured-but-unsintered parts
- post-sinter CMM for evidence of structural collapse
Binder Bleeding / Dimensional Inaccuracy
Cause
Liquid binder droplets impact the powder bed and wick laterally into adjacent unselected powder via capillary action before setting. The extent of bleeding depends on droplet size, binder viscosity, powder particle size and wettability, and binder saturation setting. Bleeding enlarges printed features beyond their design intent, reducing clearances and enlarging hole diameters. Fine features (<1.5 mm) and tight holes are most affected.
Indicator
As-built dimensions (pre-sinter) are consistently larger than nominal by 0.05–0.3 mm on external features. Holes and slots are systematically undersized. Tolerance stackup across multiple features shows consistent positive bias.
Prevention
Apply negative dimensional compensation (shrink outer contours, expand inner holes) in the slicing software based on empirical bleed offset measurements for the specific material-binder-machine combination. Use higher-viscosity binder or reduce binder saturation for fine features. Specify minimum hole diameter ≥1.5 mm to avoid complete closure from bleeding. Validate with a geometric calibration artefact before production runs.
Detection
- CMM or optical measurement of calibration artefacts
- pin gauge inspection of holes
- 3D scanning vs. CAD nominal