3D printing contributes to metal replacement and weight reduction
A process-level look at how printed polymer and metal parts replace steel and aluminium, where the mass actually comes out, and the load cases where they should not. Written for design engineers and buyers who need to judge a part before committing to a process route.
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Where the mass actually comes out
Weight loss in a printed part rarely comes from the material alone. It comes from deleting material that a conventional process forces you to keep. A milled bracket needs constant wall thickness because the cutter needs clearance and the blank must survive clamping. A printed bracket only needs material where the load path runs.
That difference is structural, not cosmetic. On a typical housing, the load arrives at three bosses and leaves through two flanges. Everything between them is dead weight in aluminium, and it stays there because removing it would mean a second setup on a 3-axis mill.
Printing lets you taper ribs, hollow out bosses, and thin the web to 1.2–2.0 mm in glass-filled PA. The result is often 30–50% lighter than the machined aluminium version of the same envelope, with stiffness held by geometry rather than section thickness.
The limit is deflection under peak load, not the print itself. If the part has to survive a 6 g shock without touching a neighbouring board, run the FEA on the printed geometry before you cut metal from the BOM.
Lattice and hollow sections for metal replacement and weight reduction
A lattice is not a decoration. It is a way to move stiffness away from the neutral axis, where it does nothing, and into the outer skin where bending stress peaks. For a panel in bending, a 0.8 mm skin over a 6 mm octet lattice can match the stiffness of a 4 mm solid plate at roughly a third of the mass.
Lattice works best when the load is distributed and the part is stiffened in two directions. It works badly when the load is a single point contact, because the strut directly under the bolt sees almost all of the force and the rest of the lattice just adds print time.
Strut diameter below 0.6 mm is usually a mistake in FDM and often in SLM too. The strut prints, but it prints oval, and the oval strut is weaker than the round one your simulation assumed. Keep the thinnest strut at 0.8 mm or above and let the cell size grow instead.
Trapped powder is the other trap. Every closed lattice cell in a metal print holds powder that you cannot reach. Leave a 2 mm drain hole per cell, or design open-cell geometry that a bead blast can reach.
What polymer can and cannot replace
Glass-filled PA and PEEK carry real load. A 30% glass-filled PA bracket at 3 mm wall typically lands within 20–30% of a 6061-T6 aluminium bracket of the same shape in stiffness, at about half the weight. That gap closes further when the aluminium part was over-thick for machinability, which is common.
The failure modes differ, and this matters more than the numbers. Aluminium yields and then holds. Glass-filled PA creeps under a sustained load at 80 °C, and it loses stiffness fast above its glass transition. A bracket that sees 40 N once is fine. The same bracket under 40 N for 2,000 hours is a different question.
PEEK and PEI (Ultem) push the temperature ceiling to roughly 150 °C and 170 °C. They cost far more per kilogram, so they earn their place only where a metal part also needed insulation, corrosion resistance, or a non-magnetic housing.
Carbon fibre reinforced grades are stiff but brittle. They are a poor choice for anything that gets dropped or over-torqued, and a good choice for a stiffening rib or a jig that never sees impact.
When printed metal beats printed plastic
Some parts cannot go to polymer at all: exhaust-adjacent brackets, parts that carry current, or anything that must survive 200 °C in service. For those, laser powder bed fusion in AlSi10Mg or Ti-6Al-4V removes weight through topology optimisation rather than material substitution.
A topology-optimised AlSi10Mg bracket commonly comes out 35–45% lighter than the machined 6061 version, because the optimiser is free to remove material that a 3-axis cutter could never reach. The saving is in the geometry, not the alloy.
Two costs follow. Surface finish off the printer is rough, near Ra 10–15 μm, so any mating face needs machining. And the as-built surface has notches that cut fatigue life, so a load-bearing metal print usually needs a stress relief and a machined interface.
Ti-6Al-4V prints well but is expensive per kilogram, and support removal on thin features is slow. For a one-off, printed titanium can still beat machined titanium because you skip the tooling and the roughing passes.
Moving a printed design to CNC without losing the savings
The usual path is print, test, then machine the validated shape. The mistake is to hand the printed file straight to the mill. The printed part has overhangs, thin ribs, and a lattice that a cutter cannot follow, and the shop will thicken everything to make it machinable.
The right hand-off keeps the load path and drops the printability features. Fill the lattice with a ribbed pocket, keep the tapered boss, raise the minimum wall to about 1.5 mm in aluminium, and add a 0.5 mm corner radius so the cutter can clear. The part gets slightly heavier and much cheaper per unit.
At GreatLight we run the DFM check on both versions side by side. If the printed geometry only works in plastic, we say so. If it can be machined in 6061-T6 at ±0.005 mm and Ra 0.8–1.6 μm, we quote the machined version, which is what most production runs actually need.
For quantities above a few hundred, the machined or die-cast route usually wins on cost per part. Printing wins for the first article, the low-volume variant, and the shape that no cutter can reach.
Process choice by load case and volume
Match the process to what the part actually has to do.
| Part condition | Printed polymer | Printed metal | CNC metal |
|---|---|---|---|
| Static load, room temperature | Good, 30–50% lighter | Good, topology optimised | Good, heavy but stiff |
| Sustained load above 80 °C | Creep risk | Good to 300 °C+ | Good, grade dependent |
| Low volume, 1–50 parts | Best cost per part | High cost per part | Setup cost dominant |
| Volume above 500 parts | Unit cost stays high | Slow build rate | Lowest unit cost |
| Mating faces at ±0.005 mm | Needs post-machining | Needs post-machining | Achieved as machined |
| Internal channels and lattices | Straightforward | Straightforward | Not possible |
The trade-off in one line
Choose printing when the part is complex, low volume, or heat-limited; choose CNC when the part needs ±0.005 mm faces, sustained load at temperature, or a unit cost that survives production. Many projects need both, printed first and machined for the run.
Questions engineers ask next
How much weight can a printed part actually save over aluminium?
For a glass-filled PA bracket replacing 6061-T6 of the same envelope, 30–50% is a realistic range, and most of it comes from thinner walls and a tapered rib layout rather than the density difference.
For printed AlSi10Mg with topology optimisation, 35–45% is typical against the machined aluminium original. Both numbers depend on the load case, so treat them as a starting point for FEA, not a guarantee.
At what temperature does printed plastic stop being an option?
Standard PA and ABS lose most of their stiffness well before 100 °C. PEEK and PEI hold useful properties to roughly 150 °C and 170 °C, but cost several times more per kilogram.
If the part sits near an exhaust manifold, a motor winding, or a battery pack vent, printed metal or machined metal is the safer route.
Does a printed design have to be redesigned before CNC machining?
Yes, if it contains lattices, overhangs, or ribs thinner than about 1.5 mm in aluminium. A cutter cannot follow those features, and the shop will thicken them anyway.
Keep the load path, fill the lattice with a ribbed pocket, and add a 0.5 mm corner radius. The part gains a little mass and loses a lot of cost per unit.
Can printed parts hit a tolerance like ±0.005 mm?
Not as printed. Polymer printing typically holds ±0.1 mm at best, and metal printing lands around ±0.05 mm before post-processing.
Critical faces are machined after printing. At GreatLight, CNC work reaches ±0.005 mm and Ra 0.8–1.6 μm, so a hybrid print-then-machine route is common for functional prototypes.
Is there a minimum order quantity for either route?
No minimum order quantity. One prototype and a 10,000-part run both go through the same quoting process.
Quotation and DFM analysis come back within 12 hours, and production can start within 24 hours of approval.
How do you handle confidential designs?
Uploads are secure and confidential, and an NDA is available on request before any file changes hands.
Inspection covers raw material check, in-process monitoring, and final inspection, with reports on request. Every part is inspected before shipment.
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Upload the STEP file and we will tell you whether printing, CNC, or a print-then-machine route gives the better part for your volume.
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