How Does 3D Printing Work? Building Objects a Layer at a Time
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Every method of making a solid shape before 1984 either removed material, by cutting, drilling and grinding, or forced it into a mould. A 3D printer does neither; it builds the shape up, depositing or solidifying material one thin layer at a time from a computer model, so that a hollow chess piece, a hip implant shaped for one patient or a rocket engine with cooling channels no drill could reach can be made without a mould or a tool. The technique was patented in 1984, spent twenty years as an expensive way of making prototypes, and is now in schools, dental surgeries and aircraft.
From model to layers
Every 3D print starts as a digital model, drawn in design software or scanned from a real object, and the first step is to slice it: software cuts the model into horizontal layers, typically between a twentieth and a quarter of a millimetre thick, and works out for each layer the path the printer must trace or the pattern it must expose. The printer then builds the layers in order from the bottom, each one fusing to the one below, and the object rises from the build plate over minutes or hours. Overhangs are the difficulty, since a layer cannot be laid on air, so the slicer adds temporary supports that are cut or dissolved away afterwards, and the skill of the designer is partly in shaping the object so that it needs as few as possible.
The main methods
Additive manufacturing, the industry's name for it, covers half a dozen distinct processes:
- •Fused deposition: a plastic filament is melted and squeezed through a moving nozzle like a hot-glue gun, tracing each layer; the cheapest and commonest method, in nearly every hobby and school printer
- •Stereolithography: a vat of liquid resin is cured by ultraviolet light, a laser or a projected image hardening one layer at a time; the original 1984 method, giving fine detail for dental models, jewellery and figurines
- •Powder bed fusion: a laser or electron beam melts a pattern in a bed of powder, a new layer of powder is spread, and the part grows inside the loose powder, which supports it; used for nylon and, at higher power, for titanium, steel and aluminium parts
- •Binder jetting: an inkjet head prints glue into a powder bed, layer by layer, and the part is baked afterwards; fast and used for sand casting moulds and metal parts
- •Material jetting: droplets of resin sprayed and cured, in several materials and colours at once, for full-colour models
What it is good for
The technique wins where the shape is complex, the quantity is small or the object is one of a kind. A moulded part costs tens of thousands in tooling before the first one is made and pennies thereafter; a printed part costs the same for the first and the thousandth, so for a run of ten it is far cheaper and for a run of a million far dearer. That makes it the tool for prototypes, which is where it started; for medical parts, since a hip cup, a dental crown or a hearing aid shell is different for every patient and the aid industry switched almost entirely to printing within two years; for spare parts that would otherwise sit in a warehouse for decades; and for shapes that cannot be made any other way, such as the lattice structures that make a metal part light and strong, and the internal channels of a rocket engine's cooling jacket. The GE fuel nozzle for jet engines, printed as one piece where it had been twenty welded parts, was the case that persuaded aerospace, and a rocket company in California prints entire engines.
What it is not
It is slow, hours per part against seconds for a moulding, and the layer structure leaves a surface that shows the layers and a part that is weaker along them than across them, so printed parts are often finished by machining and tested carefully before they carry a load. The materials are limited, since not everything can be melted, cured or fused, and the metal machines cost hundreds of thousands and need argon, powder handling and post-processing that add up to more than the printer. The prophecy of the 2010s, that everyone would print their own goods at home and factories would empty, did not arrive, for the same reason people do not bake their own bread: it is possible and mostly not worth it. What arrived instead is a manufacturing method alongside the others, chosen for the parts it suits.
Where it is going
Printers now lay down concrete for houses, with walls extruded by a gantry in a day; they print in living cells, building scaffolds of cartilage and skin for research and, in early trials, for grafts; they print food, chocolate mostly; and on the International Space Station a printer has made tools that were emailed up as files, which is the model for any long space voyage. The bioprinted organ, the cheap printed house and the printed jet engine are at different distances, and the cheap desktop machine that made the fuss is now a tool like a drill, in a few million homes and most engineering departments, making the thing that was needed by tomorrow.
The takeaway
3D printing builds a solid object from a digital model by slicing it into thin layers and adding material one layer at a time, by extruding melted plastic, curing liquid resin with light, fusing powder with a laser, or jetting glue into powder. It costs the same for one part as for a hundred, so it wins for prototypes, one-off medical parts, spares and shapes that cannot be moulded or machined, and loses to conventional methods on speed, surface and strength for anything made by the million.