Resin & Powder Printing

Resin printing belongs to the vat photopolymer family. The machine holds a tank of liquid photopolymer resin that hardens - cures - wherever UV light strikes it. A build plate dips into the vat, light selectively cures the first layer against it, the plate lifts by one layer, and the next is cured beneath. The part grows upside-down, hanging from the plate.

The two common types differ only in the light source. SLA (stereolithography, the original 3D printing process from the 1980s) traces each layer with a fine UV laser. DLP and its cheap cousin masked SLA (mSLA) flash a whole layer at once using a projector or an LCD screen, which makes them fast regardless of how much detail a layer contains.

What you get is the finest surface and detail of any accessible process - layers can be tens of microns thin, and features far crisper than any nozzle can draw. That makes resin the choice for jewellery, dental and hearing-aid parts, miniatures, fine architectural models and master patterns for casting. The trade-offs are real: standard resins are more brittle than FDM plastics, parts and their supports need careful placement, and every print needs post-processing.

There is also a growing palette of specialist resins that softens the brittleness objection: tough and ABS-like resins for functional parts, flexible resins, castable resins that burn out cleanly for lost-wax metal casting, and high-temperature resins for moulds and fixtures. They cost more and can be fussier to cure, but they widen resin well beyond display models. And because DLP and mSLA machines flash a whole layer at once, a densely packed plate of many small parts prints in the same time as a single one - so resin is quietly excellent for small production runs of detailed items, not just one-offs.

A resin print does not come off the machine finished - it comes off wet and only partly cured. Two steps always follow. First, washing: the part is rinsed, usually in isopropyl alcohol (or water for washable resins), to remove sticky uncured resin. Then post-curing: it sits under UV light for a set time to fully harden and reach its final strength. Skip either and the part stays tacky, weak and dimensionally unstable. This wash-and-cure step is part of the workflow, not an optional extra - budget time and kit for it.

Safety is not negotiable with resin. Liquid photopolymer and its vapour are irritants and sensitisers: they can cause skin allergies and irritation, so you wear nitrile gloves and eye protection, work in a ventilated space, never let it touch skin, and dispose of waste resin and alcohol properly - do not pour it down a drain. This is a process to learn under supervision in a lab that has the extraction and disposal set up, not to improvise on a kitchen table. Treated with respect it is safe and rewarding; treated carelessly it causes real harm.

Powder bed fusion works completely differently, and its great trick is that it needs no support structures. The machine spreads a wafer-thin layer of fine polymer powder across the build area, and a laser or heat source fuses the grains exactly where the part should be. A recoater sweeps the next layer of powder on top, and the process repeats. Crucially, the surrounding unfused powder holds the part up - so overhangs, undercuts, interlocking and fully enclosed shapes print freely, and you can pack many parts densely into one build.

SLS (selective laser sintering) uses a laser to sinter nylon (PA12 is the standard) into tough, slightly flexible, isotropic parts - far more uniformly strong than layered FDM. MJF (HP Multi Jet Fusion) achieves similar results by jetting a fusing agent and then flashing the bed with heat, and tends to be fast and consistent for batches. Both give a characteristic grainy, matte grey surface and genuinely functional mechanical properties - living hinges, snap-fits and enclosures that actually work. These are industrial machines, usually accessed through a print bureau rather than owned, but they are the accessible route to strong, complex end-use plastic parts.

The one real discipline powder adds is de-powdering and reuse: finished parts come out buried in a cake of warm powder that must be brushed and blasted off, and the unfused powder is partly recycled into the next build with a proportion of fresh material. Fully enclosed hollows therefore need an escape hole, or powder stays trapped inside forever - a design rule worth remembering. Handled in a proper facility with extraction and the right protective gear, none of this is a burden; it is simply why powder lives in bureaux rather than on desktops.

Powder bed fusion extends to metal. Processes like DMLS/SLM (direct metal laser sintering / selective laser melting) fully melt fine metal powder - titanium, stainless steel, aluminium, cobalt-chrome - layer by layer with a powerful laser, producing dense, strong, genuinely engineering-grade parts. This is how aerospace brackets, medical implants and complex tooling with internal cooling channels are made. It is expensive, needs heat treatment and machining to finish, and lives firmly in specialist bureaux - but it is real, load-bearing metal, not a model. A cheaper cousin, binder jetting, glues metal powder into a fragile green part that is then sintered in a furnace, trading some density for lower cost on suitable geometries.

So how do you choose? Reason from what the part must be. Need fine detail or a smooth master pattern? Resin (SLA/DLP). Need a strong, complex, support-free functional part in plastic? Powder (SLS/MJF). Need real end-use metal? Metal powder (DMLS) via a bureau. Need it cheap, quick or large, and detail is secondary? FDM from the last lesson. Cost climbs roughly in that order, and rarely do detail, strength and low cost all arrive together - the maker names the one that matters most for this part and picks accordingly.

These processes reshape how you get parts made, so it pays to understand the economics before you commit. Resin printers are now genuinely affordable desktop machines, so SLA and DLP are within reach of a well-equipped fablab or a keen individual - the real cost there is the consumables, the wash-and-cure kit, and the safety setup, not the printer. SLS, MJF and every metal process, by contrast, are industrial systems with powder handling, controlled atmospheres and heavy post-processing; almost nobody outside a factory or dedicated lab owns one. The normal route is a print bureau: you upload a model, choose material and finish, and receive finished parts in the post. That is not a compromise - it is how professionals get powder and metal parts, and learning to use a bureau well is a genuine skill.

Briefing a bureau is where your judgement shows. State the material precisely (PA12 nylon, 316L stainless, Ti-6Al-4V titanium), the quantity, the tolerance you actually need on the dimensions that matter, the finish (as-printed, bead-blasted, dyed, polished), and flag any critical features. Ask about minimum wall thickness and drain holes for hollow parts, because powder and resin can be trapped inside sealed cavities. A vague brief gets a vague, sometimes unusable part; a precise one gets exactly what you designed. Cost scales with volume, material and the height of the build (machine time is roughly proportional to how tall the packed job is), so nesting several small parts into one order is often far cheaper per piece - the same batching logic you met with laser nesting, now in three dimensions.