A DMLS part density specification a supplier can actually meet has three parts on the same drawing note: a target relative density expressed as a percentage (for example ≥99.5%), the measurement method that number is defined against (Archimedes, metallographic cross-section, or CT), and an acceptance rule for the largest allowable single pore. A note that says only "parts to be fully dense" is unmeasurable and every supplier will interpret it differently. At Layer X, our Ahmedabad studio has shipped 2,000+ metal parts, and the drawings that print right the first time are the ones that pin down all three. This guide shows you how to write that callout.
Why "fully dense" is not a specification
DMLS parts are never 100.0% dense. Even a perfectly tuned process leaves some residual porosity — gas pores, lack-of-fusion voids, keyhole pores — at the sub-percent level. "Fully dense" is marketing language, not an engineering requirement. When it lands on a drawing, three things go wrong: the supplier cannot quote against it, cannot inspect against it, and cannot reject a part against it. The word that fixes this is relative density — the ratio of the part's measured density to the wrought bulk density of the same alloy, written as a percentage.
If you want to understand what actually drives that residual porosity — the parameter and powder mechanisms behind each void type — read our companion piece on the causes, detection and prevention of porosity in metal 3D printing. This post is the other half: how to turn that knowledge into a callout a supplier can meet and a QA team can verify.
The three numbers every density callout needs
A complete density note answers three questions. Miss any one and the requirement becomes ambiguous.
- What density, as a percentage? State a relative density minimum, e.g. "Relative density ≥99.5% of wrought Ti-6Al-4V." Naming the reference alloy matters because the denominator changes with material.
- Measured how? Archimedes weighing, polished metallographic cross-section, or X-ray computed tomography (CT). Each gives a different number for the same part — more on that below.
- Where, and what is the worst single pore allowed? Bulk average density can pass while one large void sits in a fatigue-critical fillet. Specify a maximum individual pore size and, if it matters, the region it applies to.
Choose the measurement method deliberately
The same part yields three different density figures depending on how you measure it, so the method is part of the specification, not an afterthought. The Archimedes buoyancy method is governed by ASTM B962; it is fast and cheap but gives a single bulk average and is blind to internal void distribution and surface-connected porosity. A polished cross-section counts pore area on one plane — good resolution, but destructive and only representative of that slice. CT, referenced by ASTM E1441 for computed tomography practice, is the only method that maps porosity in three dimensions and locates individual pores, but it is the slowest and most expensive.
| Method | Reference standard | What it tells you | Best used when |
|---|---|---|---|
| Archimedes buoyancy | ASTM B962 | Bulk average relative density (single %) | Batch density screening, non-critical parts |
| Metallographic cross-section | ASTM E3 preparation | Pore area fraction on one plane, pore morphology | Process qualification, sacrificial witness coupon |
| X-ray computed tomography | ASTM E1441 | 3D pore map, individual pore size and location | Fatigue-critical, aerospace, medical implants |
Do not silently demand CT on every part. It multiplies cost and lead time. Reserve it for the fatigue- and pressure-critical items, screen the rest by Archimedes, and say so explicitly on the drawing.
Sample this many parts, from these zones
Density is not uniform across a build plate or through a tall part. Powder spreading, gas flow and thermal history vary with position, so where you take the measurement changes the result. A specification that just says "measure density" invites the supplier to take the easiest, most favourable reading. Instead, tie the requirement to a witness coupon printed alongside your part in the same build, from the same powder lot — this is the standard way to get a destructive Archimedes or cross-section reading without cutting up the deliverable part.
- Frequency: one witness coupon per build, minimum, for qualified production; per-part CT only where called out as critical.
- Location: if wall-thickness or height varies, ask that the coupon reflect the thickest and thinnest sections — thin walls and downskin surfaces are where lack-of-fusion tends to concentrate.
- Powder traceability: tie acceptance to a specific powder lot. Every DMLS order at Layer X already ships with a material certificate carrying powder-lot traceability, so this costs you nothing to require.
Set an acceptance limit for the largest single pore
Average relative density and maximum individual pore size are two independent requirements, and fatigue life cares far more about the second. A part can read 99.7% dense on Archimedes and still contain one 0.4 mm lack-of-fusion void acting as a crack initiator. For dynamically loaded or pressure-containing parts, add a maximum pore-diameter callout alongside the density percentage, and state whether it applies everywhere or only in a defined critical region. CT is the method that can actually verify a maximum-pore-size clause; Archimedes cannot. If your note demands a maximum single pore size, it has implicitly demanded CT — make that consistent so the supplier can quote it correctly.
Say what happens after the print: HIP and heat treatment
Whether the density requirement applies before or after post-processing changes everything. Hot isostatic pressing (HIP) collapses internal gas porosity and can lift relative density well above the as-built figure, but it does nothing for surface-connected voids and it will shrink and slightly distort the part. If your drawing says "≥99.9% after HIP," that is a different — and more achievable — requirement than "≥99.9% as-built." State the condition. The same applies to stress relief and solution treatment, which change grain structure and residual stress but not bulk density; our guide to HIP, stress relief and annealing for metal AM parts walks through when each step belongs in the spec. Decide the sequence, then write density acceptance against the final delivered condition.
A drawing note you can copy
Here is the anatomy of a callout that removes ambiguity. Adapt the numbers to your alloy and criticality — do not lift a percentage you cannot justify against your load case.
| Element | Weak note (avoid) | Specifiable note (use) |
|---|---|---|
| Density target | "Fully dense" | "Relative density ≥99.5% of wrought [alloy]" |
| Method | (none stated) | "Verified by Archimedes per ASTM B962 on witness coupon" |
| Worst pore | (none stated) | "No single pore >0.2 mm in region A, by CT per ASTM E1441" |
| Condition | (ambiguous) | "Requirement applies after HIP and stress relief" |
| Sampling | (ambiguous) | "One coupon per build, same powder lot, lot cert supplied" |
Note that the standards you cite — ASTM B962 for Archimedes, ASTM E1441 for CT — and the AM terminology framework of ISO/ASTM 52900 for defining part conditions give both parties a shared, auditable vocabulary. That is what makes a note enforceable rather than aspirational.
How this ties to tolerance and inspection
Density is one clause on a drawing that also carries dimensional, geometric and surface requirements. Keep them consistent. If you are already specifying datums and true position, your density note should read in the same disciplined language — see our guide to GD&T on 3D-printed parts: tolerances, datums and inspection. Our DMLS metal 3D printing service holds ±0.1 mm on general geometry, with critical surfaces post-machined below ±0.05 mm, on a 250×250×325 mm build envelope across 316L SS, 17-4 PH SS, H13 tool steel, Ti-6Al-4V, Inconel 625 and Inconel 718. Every order ships with a CMM dimensional report, material certificate, and hardness test as standard; radiographic inspection and additional NDE are available on request for aerospace and defence work under our AS9100 Rev D and ISO 13485:2016 quality systems. A density callout written the way this guide describes drops straight into that documented flow — which is a large part of how we hold a 99.4% first-pass yield.
One more discipline pays off: keep the density note internally consistent. If you demand a maximum single-pore size, you have demanded CT — so do not also write "verify by Archimedes only," because Archimedes cannot see individual pores. If you write "≥99.9% after HIP," do not also forbid HIP elsewhere on the print. Contradictions inside a single specification are the most common reason a metal drawing bounces back with quote queries, and every round trip adds days. Read your density clause against your post-processing clause against your NDT clause before the drawing leaves your desk.
The short version: write a relative density percentage, not "fully dense." Name the method — Archimedes per ASTM B962 for screening, CT per ASTM E1441 where fatigue or pressure matters. Add a maximum single-pore limit for critical regions, and make the note demand CT if it does. Define the witness coupon, tie it to the powder lot, and state whether the requirement applies before or after HIP. Get those five things onto one note and any competent supplier can quote it, print it, and prove it — instead of guessing at what "fully dense" was supposed to mean.
Ready to quote a metal part? Upload your CAD file for a 24-hour quote and send your density and NDT requirements with it — we will confirm the achievable relative density, method and documentation for your alloy before you commit.