Run your finger over a part coming out of a metal 3D printing machine: it feels like fine sandpaper. The shape is there, the dimensions too, but the surface catches. That is normal; it is inherent to the process. And for a part that must rotate, slide, seal, or simply withstand fatigue, it cannot remain like that.
Finishing is often the neglected step in metal printing projects. Weeks are spent on design and on orienting the part on the build plate, then at the last moment you discover that the surface fails inspection. Mass finishing solves a large part of the problem. Not all of it. Here is what you can expect from it.
- Metal 3D printing: why finishing is essential
- Where the roughness of printed parts comes from
- Post-processing by Mass finishing
- Supports, internal channels: the limits
- Titanium, stainless steel, aluminium: what changes
- Frequently Asked Questions
- Equipping your workshop for post-processing
Metal 3D printing: why finishing is essential
In machining, the tool creates the surface. In additive manufacturing, the surface is a by-product: the laser melts the powder layer by layer, and what remains at the periphery was never intended to be attractive or functional.
Three very practical reasons
First, fatigue. Cracks almost always start at the surface, at a protrusion. A raw printed part has thousands of them. Next, sealing: a gasket placed on a granular seating surface leaks. Finally, cleanliness, which matters greatly in medical or food processing, where every recess retains residues.
Plan for it at the design stage
Finishing has a cost and constraints, so it is best to understand them before starting production. A surface that must be smoothed in a machine must be accessible to the media. A very thin wall must withstand the process. These are design questions, not workshop questions.
Where the roughness of printed parts comes from
Two phenomena add up, and they are not addressed in quite the same way.
Adhered powder grains
Around the melt pool, the powder heats up without fully melting. These grains remain welded to the wall. They are what create the granular feel, especially on downward-facing surfaces, those that “overhang” during printing.
The stair-step effect
Each layer is a few tens of microns thick. On an inclined face, you therefore see a succession of small steps. The closer the face is to horizontal, the longer and more visible the steps are. Orienting the part on the build plate reduces the phenomenon; it does not eliminate it.
What this changes for the part
It is not only about aesthetics. Roughness accelerates wear at contact points, reduces fatigue strength, and increases pressure losses in ducts. Hence the value of specifying an Ra value on the drawing and measuring it, as with any precision part. To give an order of magnitude, a mirror polish goes below 0.1 µm Ra.
Post-processing by Mass finishing
The principle does not change: parts are tumbled in a bowl with abrasive media and a compound. Friction removes the adhered grains, wears down the edges of the steps, and evens out the surface. All external faces are treated at the same time, without having to guide a tool.
Why it works well on these parts
Printed parts often have shapes you would not dare to machine: lattices, ribs, twisted curves. Polishing them by hand takes hours and the result depends on the operator. In a machine, the process is repeated from one batch to the next with the same parameters.
The principle in operation, on a Mass finishing machine:
Usually in two passes
A first aggressive pass to remove the powder and break down the steps. A second, with finer media, to bring the surface to the required level. On small parts, a high-energy centrifugal machine significantly shortens cycle times. If the target is a very low Ra, you enter the realm of superfinishing.
Supports, internal channels: the limits
We are often asked this question, so let us be clear: a Mass finishing machine does not remove printing supports. These structures are cut off or machined beforehand. The machine comes afterwards, to erase the marks they leave and smooth the rest.
In what order
- Depowdering, then stress-relief heat treatment if the material and the part require it.
- Support removal, by sawing, EDM, or machining.
- Mass finishing to remove support witness marks and smooth all accessible faces.
- Roughness measurement to validate the result.
Where something else is needed
Long, narrow internal channels, typically cooling circuits, are out of reach: the media do not circulate there. For these geometries, abrasive flow polishing or chemical polishing is used. Mass finishing also does not correct internal porosity or a shape defect. When in doubt, a trial on a real part is better than a long debate.
In short: if the media can touch the surface, Mass finishing will treat it. If they cannot get in, another process is required.
Titanium, stainless steel, aluminium: what changes in metal 3D printing
The three most commonly printed metals react differently in the bowl.
- Titanium. Widely used in medical and aerospace. It requires clean media reserved for it, to avoid embedding particles from other metals.
- Stainless steel. The easiest of the three. It accepts very fine surface finishes. Our remarks on Mass finishing of stainless steel also apply to printed parts.
- Aluminium. Soft, so it smooths quickly. It also marks quickly: softer media, shorter cycles.
The exact sequence is set by trial, depending on the alloy, the initial roughness and the target value.
Frequently asked questions about finishing metal 3D-printed parts
Can a metal 3D-printed part be polished by Mass finishing?
Yes, for all surfaces that the media can reach. The machine removes adhered powder grains, eliminates the stair-step effect and gives the batch a uniform appearance. Narrow internal channels remain out of reach.
Does Mass finishing remove printing supports?
No. Supports are cut off or machined beforehand. Mass finishing then removes the marks they leave and smooths the part.
What roughness can be achieved on a printed part?
It depends on the metal, the initial roughness and the sequence. The value is validated during machine setup by measuring Ra on real parts rather than relying on a theoretical value.
Which machine should be chosen for small printed parts?
For small parts, the high-energy centrifugal machine is often the most effective, as it significantly shortens cycle times. The final choice depends on the size, fragility and volume to be processed.
Equipping your workshop for post-processing
If metal 3D printing in your company is moving from prototype to small series, manual finishing quickly becomes the bottleneck. This is the time to size the equipment: part size, batch volume, materials and the target surface finish determine the type of machine and the media sequence.
The characteristics of each technology are detailed on our Mass finishing machines page, and the suitable media on the consumables page. For a specific project, our team can help you choose via the contact page.
From an as-built printed part to a controlled surface finish, in production.
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