3D printing & additive manufacturing
Building a part by adding material layer upon layer — no mould, so it is unbeatable for prototypes, custom shapes and low volumes. This is the whole field in one place: how a print is made, the seven process families and their machines, what to print in, how to design for it, and the one property fact that catches everyone out.
On the numbers: bulk-polymer values link to their cited database entries. The print-specific figures — temperatures, resolutions, isotropy — are typical engineering ranges, because a printed part's real properties depend on the machine, the settings and the build orientation. That dependence is the whole point, not a footnote.
How a print is made
Every print, in every process, runs through the same pipeline — model to finished part.
1 · Model
Design the part in CAD, or download/scan a mesh. Additive lets you build geometry no mould could make — internal channels, lattices, one-piece assemblies — so design for the freedom, not around a mould.
2 · Export a mesh
Save the model as a triangle mesh (STL, or the richer 3MF/OBJ that carry colour and units). The mesh must be 'watertight' — a closed surface — or the slicer cannot tell inside from outside.
3 · Orient & support
Choose how the part lies on the plate. Orientation sets the strength direction, the surface quality and where supports are needed — these three usually pull against each other, so it is the key decision (see design for additive).
4 · Slice
Slicer software cuts the model into layers and generates the machine toolpath (G-code): perimeters, infill, supports, temperatures and speeds. This is where most print quality is won or lost.
5 · Print
The machine builds the part layer by layer — minutes for a small FDM part, many hours to days for a dense powder-bed build. Bed adhesion, temperature and cooling all matter.
6 · Post-process
Almost nothing comes off the machine finished: remove supports, wash and UV-cure resin, de-powder and sinter, then sand, smooth or dye. Post-processing is real work, not an afterthought.
The seven process families
ISO/ASTM 52900 sorts all additive manufacturing into seven families — melt a filament, cure a liquid, fuse a powder, jet a material, glue a powder, deposit molten metal, or bond sheets. Open any one for its machines, materials and trade-offs — or see how to choose the right one.
FDM / FFF
Melt a filament and lay it down road by road.
SLA / DLP / MSLA
Cure a liquid resin with light, one thin layer at a time.
SLS / MJF / DMLS
Fuse a bed of powder selectively, then spread the next layer on top.
PolyJet / MJ
Inkjet droplets of resin, cured on the fly by UV.
Binder jetting
Glue powder together with jetted binder, then sinter.
DED / WAAM
Melt metal as you deposit it, straight into the growing part.
LOM / UAM
Bond sheets together and cut each layer to shape.
The one thing to get right
A printed part is not the bulk polymer
Most 3D-printed parts are little layered composites: strong along the printed roads or within a fused layer, weaker across the layers. In FDM the bond between layers is formed by partial re-melting and never fully heals, so the build (Z) direction carries voids and weaker welds than the solid polymer around it.
Interlayer (Z) strength is typically only 30–70% of the in-plane strength — and lower with poor settings.
So orientation is a load-bearing design decision. Vat photopolymerization is far more isotropic because layers cross-link chemically (but the resin is often brittle); powder-bed fusion is the most isotropic of the polymer processes. This is exactly the gap between a datasheet number and the number a real part can hold — see design allowables.
See it for yourself — rotate this printed block, then pull it along the layers and across them:
What to print in
The common printing polymers, from the easiest (PLA) to the most demanding (PEEK, PEI). Each links to its cited bulk-property data; each process page lists the materials it runs.
→ Full materials guide — every filament with print temperatures, the resin families and the SLS powders, side by side.
Resins (vat, jetting) are thermoset photopolymers and are not bulk database materials. Browse the full cited materials database.
Designing for additive (DfAM)
Orientation first
Decide how the part lies on the plate before anything else — it sets the strength direction, the surface finish and where supports go. These three trade against each other, so orient for the load that matters most.
Design around the layers
A printed part is weakest between layers (the Z direction). Lay it down so tension and bending run along the layers, never across a critical joint — orientation is a load-bearing decision, not a cosmetic one.
Walls carry the load, not infill
The solid perimeters (walls) do most of the structural work; infill is a stiffness-vs-weight-vs-time trade whose effect on strength is far from linear. More than ~40% infill rarely pays off — add walls instead.
Respect the overhang angle
Most processes can bridge shallow overhangs but need supports past roughly 45° from vertical. Chamfer instead of overhang, add fillets at corners to cut stress risers, and keep walls a sensible multiple of the nozzle/beam width.
Mind shrinkage and warp
Semi-crystalline and high-temp polymers (ABS, PA, PC, PEEK) shrink as they cool and warp off the bed without a heated chamber. Sinter-based metal binder jetting shrinks 15–20% — the part is designed oversize and the shrink is predicted.
Know when not to print
Printing wins for prototypes, low volume, and complex or custom geometry no mould can make. It loses to injection moulding on cost per part at high volume, and it is the wrong call when you need fully isotropic, watertight, bulk-property performance.
Post-processing
Almost nothing comes off the machine finished. What happens after the print is real work.
Support removal
Break off or dissolve the sacrificial structures under overhangs. Soluble supports (a second material) leave a cleaner surface than snap-off ones.
Washing & UV cure (resin)
Vat/jetting parts are washed in IPA to strip uncured resin, then UV post-cured — a green part is under-strength until it is cured through.
De-powdering & sintering (powder)
Powder-bed parts are dug out of the cake and blasted clean; metal binder-jet parts are then de-bound and sintered, shrinking ~15–20%.
Sanding & polishing
Progressive grits knock down layer lines. Effective but labour-intensive on the visible faces.
Vapour smoothing
A controlled solvent vapour (acetone for ABS, others for nylon) melts the surface glassy-smooth without touching dimensions much.
Annealing
A gentle heat soak relieves stress and raises crystallinity (and heat resistance) in PLA, PEEK and nylon — at the cost of a little shrinkage.
Dyeing, painting & plating
SLS nylon dyes deep and evenly; parts can be primed and painted, or even electroplated, for colour and a sealed surface.
Glossary — the vocabulary
- Additive manufacturing (AM)
- The formal name for 3D printing: building a part by adding material layer upon layer, the opposite of subtractive machining.
- Layer height
- The thickness of each printed layer. Smaller means finer detail and smoother surfaces but longer prints.
- Slicer
- Software that cuts a 3D model into layers and writes the machine toolpath (G-code): perimeters, infill, supports, speeds and temperatures.
- STL / 3MF
- The file formats a slicer reads. STL is a bare triangle mesh; 3MF also carries colour, units and materials.
- Infill
- The internal lattice that fills a part's hollow interior — a density (%) and pattern trading strength and weight against time and material.
- Perimeter / wall / shell
- The solid outer loops of each layer. They carry most of the load, so wall count matters more than infill for strength.
- Support structure
- Sacrificial material printed under overhangs to hold them up, removed afterwards. Powder and sand processes need none — the bed supports the part.
- Raft / brim / skirt
- Bed-adhesion helpers: a raft is a base under the whole part, a brim a flat collar around it, a skirt a loose outline that primes the nozzle.
- Overhang & bridging
- An overhang leans out beyond the layer below; a bridge spans a gap. Past ~45° from vertical, most processes need support.
- Green part
- A just-printed part before its finishing step — uncured resin (vat) or un-sintered powder-and-binder (binder jetting) — fragile and under-strength until processed.
- Anisotropy
- Direction-dependent properties. Printed parts are typically weaker across the layers (Z) than within them (XY).
- Warping
- The corners of a part lifting off the bed as unevenly-cooling plastic shrinks — worst on ABS, PA and PC without a heated enclosure.
- Retraction & stringing
- Pulling filament back when the nozzle travels stops molten plastic oozing into thin 'strings' across gaps.
- Recoater
- The blade or roller that spreads each fresh layer of powder in a powder-bed or binder-jetting machine.
- Sintering
- Fusing powder into a solid below its melting point with heat; the finishing step that densifies metal binder-jet and de-bound parts (shrinking them ~15–20%).
New to all this? Start with the guided learning path, see where printing sits in the Make map, or browse the cited materials database. The anisotropy lesson generalises — see design allowables.
Which 3D printing process should I use?
For most functional prototypes and hobby work, FDM (filament) is cheapest and good enough. Choose resin (SLA/DLP) when you need fine detail and a smooth surface, powder bed fusion (SLS/MJF) when you need strong functional parts with no support marks, and a metal process only when the part genuinely has to be metal.
Why are 3D-printed parts weaker than moulded ones?
A printed part is a stack of welded layers rather than solid plastic, and the weld between layers is its weak point. Pulled along the layers it behaves roughly like the bulk material; pulled across them it can hold only about a third as much, because only those thin welds carry the load.
Which filament should I start with?
PLA. It prints easily, needs no enclosure, barely smells and holds its shape well. Its limit is heat — it softens around 60 °C, so it will sag in a hot car. Move to PETG when a part must survive being used, and to ABS or ASA when it must take real heat.
Are 3D-printed parts strong enough for real use?
Often yes, but only if you design for the process rather than for the datasheet. Orient the part so the layers run along the load, treat the through-thickness direction as the weak link, and confirm anything load-bearing by testing. The bulk material strength on a datasheet is not what your printed part will deliver.