For SLS nylon parts, vapour smoothing is the better route when you need a sealed, near-injection-moulded cosmetic finish and can accept a small dimensional gain, while media tumbling is the cheaper, more predictable route when you only need to knock back the raw powder texture without changing the part's fit. That is the short answer. At Layer X, we run both processes on PA12, PA12-GF and PA12-CF parts printed to our ±0.2mm SLS tolerance, and the right choice almost always comes down to whether the part has to hold liquid or gas, how it will look on a shelf, and how many microns you can afford to lose off a mating face. This guide breaks down the two dominant finishing routes side by side so you can specify the correct one before you order.
Why raw SLS nylon needs finishing at all
Straight off the powder bed, PA12 has a characteristic matte, slightly granular surface — roughly the texture of fine sandpaper. That texture is not a defect; it is the fused boundary of individual sintered particles, and it is why SLS gives you isotropic strength and support-free geometry in the first place. But the same open, porous skin has three practical consequences: it is not watertight, it holds dirt and skin oils, and it scatters light so colours look chalky. Bead blasting, which every SLS part gets as standard to clear loose powder, removes the debris but leaves that porosity intact. Vapour smoothing and media tumbling are the two established ways to go further — and they attack the problem from opposite directions.
What vapour smoothing actually does
Vapour smoothing is a chemical process. The part is suspended in a sealed chamber and exposed to a controlled solvent vapour that briefly liquefies the outermost few microns of the nylon skin. Surface tension then pulls that reflowed layer smooth before it re-solidifies, closing the open porosity and leaving a semi-gloss, sealed surface. Because it is a reflow rather than an abrasion, it reaches into recesses, internal channels and lattice structures that no mechanical media can touch — a decisive advantage for the complex geometry SLS is chosen for in the first place.
The trade-off is that vapour smoothing is additive to dimensions in a subtle way: the reflowed material does not disappear, it redistributes, so sharp external edges soften and very fine embossed detail below about 0.3mm can round over or fill. For most functional and cosmetic parts this is invisible; for a part carrying crisp text or a knife-edge sealing lip, it needs to be designed for. The physics of solvent smoothing of polymers is well documented in the additive-manufacturing literature, and the ASTM F42 committee on additive manufacturing treats surface finish as a reportable part characteristic precisely because processes like this change it measurably.
What media tumbling actually does
Media tumbling (also called vibratory or barrel finishing) is purely mechanical. Parts are loaded into a bowl or trough with a loose abrasive media — ceramic, plastic or organic chips — and the whole mass is vibrated or rotated for a set cycle. The media rubs against every exposed face, knocking down the peaks of the powder texture and rounding external edges. The result is a uniformly matte, de-burred, pleasant-to-handle surface that is measurably smoother than as-sintered but still open and porous at the microscopic level.
Tumbling is abrasive, so it is subtractive: it removes a thin, fairly even layer of material across all reachable surfaces. It is excellent at consistent batch work — you can finish dozens of small parts in one cycle at a fraction of the per-part cost of a chemical process — but it has two blind spots. It cannot reach deep internal channels, blind bores or the inside of a lattice, and aggressive cycles can round edges more than a tightly toleranced assembly wants. It also does nothing for porosity: a tumbled part is smoother to the touch but no more watertight than a bead-blasted one.
Side-by-side: cost, sealing, cosmetics, dimensions
Here is the direct comparison our engineers use when advising on a finish for PA12 parts. Costs are relative to a standard bead-blasted SLS part; we quote the exact figure per part on your specific geometry within our 24-hour turnaround.
| Attribute | Vapour smoothing | Media tumbling |
|---|---|---|
| Mechanism | Chemical reflow of surface skin | Mechanical abrasion by loose media |
| Relative added cost | Higher (per-part, batch chamber) | Lower (bulk cycle, many parts at once) |
| Sealing / watertightness | Closes porosity — sealed, splash and airflow resistant | No change — remains porous |
| Cosmetic result | Semi-gloss, deep colour, near-moulded look | Uniform matte, clean, de-burred |
| Dimensional effect | Additive redistribution; softens edges & fine detail below ~0.3mm | Subtractive; removes thin even layer, rounds external edges |
| Internal channels & lattices | Reaches everywhere vapour flows | Cannot reach — surfaces media cannot touch stay raw |
| Best for | Cosmetic housings, fluid/air parts, complex internal geometry | Batch functional parts, jigs, handles, general de-burring |
The sealing question — where the two routes truly diverge
If your part has to hold, channel or resist a liquid or gas, this is the deciding factor and it is not close. Media tumbling improves how a part feels but leaves the sintered skin's interconnected porosity intact, so a tumbled PA12 manifold or splash housing will still wick fluid through its wall over time. Vapour smoothing closes that porosity, which is why we specify it for fluidic manifolds, connector housings, ducting and any enclosure with an ingress-protection requirement. For applications that need a fully sealed, pressure-holding wall we sometimes combine a finish with an impregnation or coating step — the trade-offs there are covered in our guide to watertight 3D prints and sealing. As a rule: if the datasheet mentions IP ratings, leak rates or fluid contact, start with vapour smoothing.
The dimensional question — protecting your tolerances
Both processes touch the surface, so both interact with the ±0.2mm tolerance we hold on SLS parts, but in opposite directions. Tumbling removes material, so a heavily tumbled mating diameter can drift toward the lower end of tolerance — we account for this by masking or lightly finishing critical faces, or by tuning cycle time. Vapour smoothing adds a redistributed layer and softens edges, so it is the wrong choice for a part whose function depends on a sharp corner, a fine thread, or crisp sub-0.3mm embossed detail. The practical workflow is to identify the two or three faces that actually carry tolerance, tell us at quote stage, and let us protect them — the same design-for-finishing thinking we set out in our broader post-processing and surface-finishing guide. Where a part needs both a cosmetic exterior and a dimensionally untouched bore, we selectively finish rather than treating the whole part.
Cosmetics: shelf appeal versus honest matte
Vapour smoothing gives PA12 a semi-gloss finish that deepens dyed colours and reads as a moulded consumer product — the reason it is our default for enclosures, wearables and anything a customer will hold and judge. Media tumbling produces an even, professional matte that looks deliberately industrial; it is ideal for jigs, fixtures, internal brackets and functional hardware where a glossy finish would look out of place and add cost for no benefit. Neither is objectively better — they signal different things, and the right one follows the product, not the process.
How we choose at Layer X
Our decision path on an SLS nylon part is short. Does it contact fluid, air or need sealing? Vapour smoothing. Is it a cosmetic part a customer will see and touch? Vapour smoothing. Is it a batch of functional parts where feel and de-burring matter but appearance and sealing do not? Media tumbling, quoted at the lower per-part cost. Does it have critical toleranced faces? We flag them and finish selectively either way. Every SLS order — whether it ends in a smoothed housing or a tumbled fixture — is managed under our ISO 9001:2015 quality system, with the finish specification recorded on the job so it is repeatable across reorders and batch production of 10 to 500 identical parts. For an overview of where each choice sits in the wider process landscape, our SLS nylon 3D printing service page lists materials, tolerances and lead times in full.
The one habit that saves the most money is specifying the finish at quote stage, not after the part is printed — it is far cheaper to plan a sharp edge or a masked bore in than to rework it later. Tell us what the part must do, which faces carry tolerance, and whether it will ever meet a fluid, and we will recommend vapour smoothing, media tumbling, or a selective combination of both, with the cost of each spelled out against your geometry rather than a generic price list. That single conversation up front is what turns a good SLS part into a finished product that reorders identically batch after batch.
Upload your CAD file for a 24-hour quote and we will advise the right SLS nylon finish for your part — sealing, cosmetics, tolerances and cost, laid out side by side.