Lesson 8.6ShapeChapter 8 · Additive manufacturing

Material jetting (PolyJet)

Jets droplets of resin and cures them with UV light (material jetting)

Material jetting works like an inkjet printer, but its “ink” is a liquid plastic that hardens under ultraviolet (UV) light. Thousands of thin layers of droplets stack up into a solid object. PolyJet, the best-known version, makes prototypes that look and feel like real products: colored, clear and soft-touch in one print.

Photo: Varinex, CC BY-SA 4.0, via Wikimedia Commons

PolyJet printer · side view, 5 stepsThe print block is shown mid-pass; circled numbers match the steps below.

How it works

Jet the droplets. The print block sweeps across the build tray. Its printheads (inkjet heads with hundreds of nozzles) fire droplets of photopolymer (a resin that hardens under light) where the layer needs material. Different heads can fire different resins in the same pass.
Level the layer. A roller skims the wet droplets flat, so every layer has exactly the same thickness.
Cure it with UV light. UV lamps ride on both ends of the block. They flood the fresh layer with ultraviolet light, which hardens it almost instantly.
Lower and repeat. The tray drops by one layer, about 0.016–0.03 mm, and the block makes its next pass. A part needs thousands of passes, so even small parts take hours.
Wash off the support. Under overhangs, other nozzles jetted a soft gel support (temporary material that holds up the part). After printing, you blast it away with a water jet or soak it off.

Key facts

Materials: Acrylic-based photopolymers: rigid, rubber-like, transparent and full-color resins, plus a gel support. Two resins can be blended drop by drop for in-between stiffness
Typical parts: Look-and-feel prototypes, soft-grip handles, clear housings, anatomical and dental models
Part size: From a few millimeters to about 0.5 m on most machines; the largest reach about 1 m
Precision: About ±0.1 mm on small features, up to about ±0.2 mm across a large part
Surface finish: Very smooth with barely visible layers; glossy or matte depending on where support touched
Batch size it suits: One-offs and small batches, roughly 1 to 100 parts
Tooling and machine cost: No tooling. Printers cost from tens of thousands to several hundred thousand dollars; resin costs far more per kilogram than FDM filament
Speed: A palm-sized part takes a few hours; a full tray usually runs overnight

Strengths and limits

Strengths

Rigid, rubbery, clear and colored materials in one print
Very fine detail and smooth surfaces straight off the printer
No tooling: a changed design is just a new file
Many small parts can share one tray and one run

Limits

Parts are weaker and more brittle than molded plastics
Resins yellow and turn brittle in sunlight and with age
Printers and resins are expensive, and support gel is wasted material
Not suited to load-bearing or hot parts

Where it's used

Consumer productsLook-and-feel prototypes of phones, remotes and toothbrushes with soft grips
MedicalAnatomical models for planning surgery, with soft and hard tissues in different materials
DentalDental models and surgical guides
AutomotiveDashboard and interior prototypes with real colors and textures
ManufacturingShort-run injection mold inserts, jigs and fixtures
Fashion and designEyewear and footwear prototypes; prints made directly onto fabric

How to recognize it

Several materials in one seamless piece: a rigid body turns rubbery or transparent with no joint or glue line.
Very smooth surfaces; layer lines are hard to see, even on curves.
Glossy and matte zones on the same part: the matte areas are where gel support touched it.
Full-color parts with printed textures, labels or color gradients.

Compare with

Stereolithography8.2also hardens liquid resin with light, but cures it in a vat; jetting places every droplet, so it can mix materials and colors.Binder jetting8.5also uses inkjet heads, but they jet glue onto a bed of powder instead of jetting the part material itself.FDM8.1much cheaper and gives tougher plastic parts, but with coarse visible layers and usually one or two materials.

Watch it

Stratasys · The maker of PolyJet explains droplets, UV curing and support

Protolabs · An animation of the printheads, UV lamps and gel support at work

TriMech · Real PolyJet machines and the kinds of parts they make

Proto3000 · Close-ups of clear, soft and full-color printed parts

See real photos

Wikipedia: 3D printing processes — overview of every printing family, including material jetting
Wikipedia: Objet Geometries — the company that invented PolyJet and its printer lines
Wikimedia Commons photo search

Remember

Material jetting builds parts from droplets of liquid resin, hardened layer by layer with UV light.
Its superpower is mixing rigid, rubbery, clear and colored materials in one print.
Use it for realistic prototypes and models, not for strong, long-lasting parts.