Additive manufacturing

When Dev Patel's team needed fixtures to hold the KG-40 housing on the coordinate measuring machine, a set of drill guides for the cross holes, and a prototype breather body to check fit before cutting the injection mould, none of them went to the machine shop. They were printed overnight on the plant's polymer printers. Later in the launch, a bracket for the robot's welding torch that kept cracking was redesigned with lattice ribs and printed in stainless steel by a service bureau. Additive manufacturing, often called 3D printing, builds parts layer by layer directly from a digital model. It has grown from a prototyping curiosity into a set of production processes, and every manufacturing engineer now needs to know when it makes sense and when it does not.

From model to part

Every additive process follows the same digital chain. A 3D CAD model is exported, usually as a triangulated surface mesh or, increasingly, in richer formats that carry colour, material and lattice data. Build preparation software orients the part, adds support structures where needed, and slices the model into layers, typically 0.02 to 0.4 mm thick, generating the path the machine will follow in each layer. After printing, the part goes through post-processing: removing it from the build plate, removing supports, cleaning, curing or heat treating, and often machining critical surfaces. Post-processing is frequently half or more of the total time and cost, a fact that early enthusiasm tended to overlook.

Building in layers has consequences. Sloped and curved surfaces show a staircase effect, with a step height equal to the layer thickness; finer layers reduce it but lengthen the build. Properties are often weaker between layers than within them, so orientation affects strength. Overhangs beyond about 45° from vertical usually need supports, which cost material and time and leave marks when removed. Designers learn to orient parts to minimize supports on critical surfaces and to keep important loads in the strong directions.

The process families

An international standard groups additive processes into seven families, and the names are worth knowing because they describe how each one works.

Material extrusion and vat photopolymerization

Material extrusion, often known by a trade name for fused filament fabrication, is the most familiar process: a heated nozzle traces each layer with a bead of molten thermoplastic. Machines range from desktop printers to large-format systems. Parts are relatively coarse and anisotropic, but materials are cheap, the machines are simple and reinforced filaments with chopped carbon fibre give stiff, useful fixtures. The KG-40 measuring fixtures are printed in carbon-filled nylon at 0.2 mm layers with 40 percent infill, enough stiffness for gauging at a fraction of the cost and lead time of machined aluminum fixtures.

Vat photopolymerization cures a liquid resin with ultraviolet light, either tracing each layer with a laser (stereolithography) or projecting a whole layer at once from a digital light projector or masked LCD screen. It gives fine detail and smooth surfaces, ideal for prototypes, dental aligner moulds and investment casting patterns. Resins can be brittle and may degrade with UV and heat, though engineering resins have improved greatly.

Metal powder bed fusion

Laser powder bed fusion is the leading process for metal parts. A recoater spreads a powder layer 20 to 60 µm thick over the build plate, inside a chamber filled with argon or nitrogen. One or more lasers, each a few hundred watts, scan the cross-section, melting the powder into tiny tracks that solidify within milliseconds. The plate lowers, a new layer is spread, and the process repeats thousands of times. Electron beam melting works similarly in vacuum with a preheated bed, giving lower residual stress in titanium parts.

The extremely fast cooling gives fine microstructures and good strength, often matching or exceeding cast and wrought properties after heat treatment. The steep thermal gradients also build up large residual stresses, which can warp parts or crack them off their supports, so parts are usually stress relieved before removal from the plate. Porosity from lack of fusion or trapped gas, rough as-built surfaces (Ra 6 to 20 µm) and anisotropy must be managed. Critical parts are often hot isostatically pressed, held at high temperature and gas pressure to close internal pores, and then machined on functional surfaces.

Build time depends on the volume melted and on the number of layers, since each recoat takes several seconds whether the layer contains a tiny section or a large one. Tall parts are therefore slow even if they are small in volume, and machines are filled with as many parts as will fit to share the recoating time.

# Powder bed fusion build time and cost for the stainless torch bracket
import math

V_part, V_supp, height = 38.0, 9.0, 85.0      # cm^3, cm^3, mm
melt_rate = 12.0                               # cm^3 per hour (one laser)
layer, recoat = 0.040, 8.0                     # mm, s per layer
rate, powder, post = 95.0, 0.35, 60.0          # $/h machine, $/cm^3 powder, $ post-processing per part
machined = 160.0                               # $ for a conventionally machined bracket (no lattice)

def per_part(batch, melt=melt_rate):
    layers = math.ceil(height / layer)
    t_recoat = layers * recoat / 3600
    t_melt = batch * (V_part + V_supp) / melt
    t_total = t_melt + t_recoat
    cost = (t_total * rate) / batch + (V_part + V_supp) * powder + post
    return layers, t_total, cost

print("Batch  build time (h)  cost per bracket ($)")
for b in (1, 2, 4, 8, 12):
    layers, t, c = per_part(b)
    print(f"{b:5d}  {t:14.1f}  {c:20.0f}")
print(f"(layers per build: {layers}; machined bracket for comparison: {machined:.0f} $)")

layers, t, c = per_part(8, melt=24.0)
print(f"\nTwo lasers (24 cm^3/h), batch of 8: {t:.1f} h, {c:.0f} $ per bracket")

print("\nLayer thickness (mm)  layers  recoat time (h)")
for lt in (0.02, 0.03, 0.04, 0.06):
    n = math.ceil(height / lt)
    print(f"{lt:20.2f}  {n:6d}  {n * recoat / 3600:15.1f}")

Batch  build time (h)  cost per bracket ($)
    1             8.6                   897
    2            12.6                   673
    4            20.4                   561
    8            36.1                   505
   12            51.7                   486
(layers per build: 2125; machined bracket for comparison: 160 $)

Two lasers (24 cm^3/h), batch of 8: 20.4 h, 319 $ per bracket

Layer thickness (mm)  layers  recoat time (h)
                0.02    4250              9.4
                0.03    2834              6.3
                0.04    2125              4.7
                0.06    1417              3.1

Binder jetting and directed energy deposition

Binder jetting spreads powder like powder bed fusion but, instead of melting it, jets a liquid binder to glue each cross-section. The fragile green part is cured, removed from the loose powder and then sintered in a furnace, where it shrinks by 15 to 20 percent as it densifies. Because no laser is involved, binder jetting is much faster and suits higher-volume metal parts, though shrinkage must be predicted carefully. Binder-jetted sand is already a production process in foundries: printed sand moulds and cores allow complex castings without patterns. The KG-40 housing's first prototype castings were poured into printed sand moulds, weeks before the production pattern was finished.

Directed energy deposition melts wire or powder fed into a focused laser, electron beam or welding arc, building material bead by bead on a substrate. It can build large parts quickly, add features to existing parts and repair worn or damaged components such as turbine blades, moulds and shafts. Wire-arc additive manufacturing, using robotic arc welding, can build metre-scale parts at several kilograms per hour, finished by machining. Hybrid machines combine deposition and milling in one setup.

Designing for additive manufacturing

Additive processes reward designs that conventional processes cannot make. Lattices and gyroid infills give stiffness with little material. Topology optimization software removes material wherever stress is low, producing organic shapes that use material efficiently. Internal channels can follow any path, giving conformal cooling in mould inserts and the cooling passage in the torch bracket. Several parts of an assembly can be consolidated into one printed part, removing fasteners and joints. At the same time, designers must respect the processes' rules: minimum wall thicknesses, maximum unsupported overhangs, escape holes for trapped powder or resin, allowance for machining stock on precision surfaces and orientation for strength.

When additive makes sense

Additive manufacturing is rarely the cheapest way to make a simple part in quantity. It makes sense when the part is complex enough that conventional processes struggle, when quantities are small so tooling cannot be amortized, when lead time matters more than unit cost, when consolidation or lightweighting pays off over the product's life, or when customization is the point, as for dental aligners, hearing aids and orthopaedic implants matched to each patient. Spare parts on demand, especially for old equipment whose tooling has been lost, are a growing use. For Kestrel Drive Systems, the payoff is clearest in tooling: fixtures, gauges, drill guides, robot grippers and printed sand cores have cut weeks from the KG-40 launch.

Quality assurance for printed production parts is still maturing. Properties depend on machine, parameters, powder condition, orientation and post-processing, so production parts need qualified processes, test specimens printed with each build, and inspection by X-ray computed tomography where internal defects matter. Fine metal powders are also a safety concern: they can be inhaled, and some, such as aluminum and titanium, can ignite or explode as dust clouds, so powder handling needs inert gas, grounding, dust control and protective equipment.