Additive Manufacturing — Engineering Decision Guide

Use FDM/FFF for cheap, fast functional plastic parts and jigs. Use SLA/DLP resin for fine detail and smooth surfaces (patterns, dental, models) accepting brittleness/UV aging. Use SLS/MJF (powder polymer) for strong, isotropic, support-free functional nylon parts and small production runs. Use metal (DMLS/SLM) for end-use metal parts with complex geometry, and binder jet for higher-volume metal/sand at lower cost (with sinter shrinkage to manage).

AM processes trade resolution, strength, material, support burden, and cost. FDM lays down molten filament — cheapest and toughest-per-dollar but layered and anisotropic. Resin cures a photopolymer for the finest detail but the parts are brittle and degrade in UV. Powder-bed polymer (SLS/MJF) self-supports in the powder, giving strong, near-isotropic nylon parts with no support scarring. Metal PBF builds dense functional metal but is expensive and needs supports and stress relief; binder jet skips the melt (less residual stress) but must be sintered, which shrinks the part.

FDM where a smooth, watertight, or isotropic part is needed disappoints. Resin for a load-bearing or outdoor part cracks and yellows. Metal PBF for a part machining could make cheaply wastes money. Binder jet without accounting for sinter shrinkage misses size.

Orient the part so loads run along (not across) the layers, critical surfaces and holes face up or vertical (not down-facing on supports), and overhangs beyond ~45° get supports or are re-oriented away. Minimize support on cosmetic/critical faces, and place the part to balance strength, surface finish, accuracy, and build time — these usually conflict, so pick the one that governs.

Layer-based parts are anisotropic — weakest across the layer bonds (Z), so a load pulling layers apart fails at a fraction of the in-plane strength. Orientation also sets which surfaces are smooth (up-facing/vertical) versus stair-stepped or scarred by supports (down-facing), and drives how much support material and time the build needs. Because strength, finish, accuracy, and speed pull in different directions, orientation is the single most consequential AM setup decision.

A part oriented with the load across the layers snaps along a layer line. Critical faces built downward come out rough and support-scarred. Ignoring overhang limits causes sagging and failed surfaces. Optimizing only for build time can sacrifice the strength the part needed.

FDM: PLA for easy prototypes, PETG for tougher/food-adjacent, ABS/ASA for heat and outdoor (ASA for UV), nylon for wear/living hinges, PC for high heat/strength, carbon-fiber-filled for stiffness. Resin: standard for models, tough/durable for snap-fits, high-temp for tooling. SLS/MJF: nylon 12 general, glass/carbon-filled for stiffness. Metal: match the alloy to service (AlSi10Mg light, 17-4/316L stainless, Ti-6Al-4V aerospace/medical, Inconel high-heat).

AM material choice follows the same strength/heat/chemical/UV logic as any material selection, plus printability. PLA prints easily but softens in a hot car; ABS/ASA/PC take heat but are harder to print (warping); nylon is tough but absorbs moisture. On the metal side the alloy families behave much as their wrought counterparts, tuned for the powder-bed process. Match the property that governs the part, then confirm the material actually prints reliably on the chosen process.

PLA in a heat or load application creeps and softens. An unfilled polymer where stiffness governs deflects. Skipping UV-stable material outdoors yellows and embrittles it. A metal alloy mismatched to the service corrodes or lacks strength.

Use AM for low volume, complex/organic geometry (internal channels, lattices, consolidated assemblies), fast iteration and lead time, and mass-customized parts. Use conventional (machining, molding, casting) once volume, tight tolerance, surface finish, or certified material properties dominate — injection molding for high-volume plastics, machining for tight-tolerance metal. Watch the cost/volume crossover: AM is flat-cost per part, molding/machining amortize tooling over volume.

AM's cost is roughly flat per part (no tooling), so it wins at low volume and for geometry conventional methods can't make in one piece — but it doesn't get cheaper with scale. Injection molding has high tooling cost amortized across thousands of parts, so its per-part cost plunges at volume. The crossover point is where the decision flips: below it, AM; above it, tooling-based methods. Geometry complexity shifts the line — a part impossible to machine or mold keeps AM competitive at higher volumes.

AM for a high-volume simple part costs far more than molding it. Machining an organic lattice AM could build in one piece is impossible or absurdly expensive. Ignoring the crossover — sticking with AM as volume grows — quietly bleeds margin.

Corners lift off the bed and the part curls (warping), layers split apart (delamination), or the first layer won't stick and the print fails at the start. Worst on large, high-temp materials (ABS, nylon, PC) and thin flat parts.

Thermal contraction as the material cools unevenly (the physics behind warping), a cold or unheated bed/chamber, poor first-layer adhesion (leveling, dirty/wrong surface), drafts and fast cooling on high-temp polymers, and too-low nozzle/chamber temperature causing weak inter-layer bonding.

1) Heat the bed (and use an enclosure/heated chamber for ABS/nylon/PC) to slow cooling. 2) Get the first layer right — level the bed, correct nozzle height, adhesion aid (brim/raft, adhesive, correct surface). 3) Control the environment — no drafts, appropriate part cooling (little to none for high-temp materials). 4) Raise nozzle/chamber temp into range for strong layer bonding. 5) Design out the risk — add fillets, reduce large flat footprints, avoid thin unsupported spans.

Parts come out over/undersize, holes shrink, tight-fit features don't fit, or the part is strong one way and snaps along a layer line the other way. Surfaces show stair-stepping or support scars needing cleanup.

Material shrinkage on cooling (and sinter shrinkage on binder jet), the layer-line anisotropy that makes Z-strength a fraction of XY, process resolution limits, and expecting as-printed tolerances/finish that AM can't hold on critical features.

1) Design critical fits with post-machining stock and machine them after printing — don't expect molded/machined tolerances as-printed. 2) Account for shrinkage/scale factors (and sinter shrink on binder jet) in the model. 3) Orient so the load runs along the layers, or use a near-isotropic process (SLS/MJF) for strength-critical parts. 4) Plan post-processing — support removal, sanding/vapor smoothing, heat treat/HIP for metal, sealing for watertightness. 5) Ream/drill critical holes rather than printing them to size.