Digital foam 3D printing: how lattice liners absorb impact

• 1
  What digital foam 3D printing actually builds
• 2
  How a printed lattice absorbs impact energy
• 3
  Where digital foam 3D printing stops making sense
• 4
  Resin choice and the numbers behind it
• 5
  From impact map to finished part
• 6
  Digital foam 3D printing vs the alternatives

Digital foam 3D printing is the catch-all name for printing an elastomeric lattice in place of a solid foam pad. The helmet liner that made the term well known, the one paired with EOS equipment and the Bauer REAKT helmet, replaces a slab of foam with a structure whose stiffness is designed cell by cell.

The build is additive. A print head lays down a thermoplastic polyurethane, or a similar elastomer, in a repeating open-cell pattern. Because the pattern is generated in CAD, the designer controls wall thickness, cell size, and strut angle at every point on the part. A solid foam slab has one density. A lattice can have ten.

That is the whole idea. Foam is cheap and light, but it is uniform. A lattice is still light, and it can be soft where the skull is fragile and firm where the shell needs support. The trade is process cost and design effort.

So digital foam 3D printing is not a new material. It is a new way to place material where it earns its keep.

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  Open-cell latticeStruts and nodes printed in a repeating pattern, not a blown foam.
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  Voxel-level controlCell size and wall thickness vary across the part in one build.
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  Elastomer onlyTPU and similar soft polymers. Not for rigid structural loads.

Energy absorption in a lattice happens in three stages. First the struts bend elastically and give the part its initial softness. Then cells buckle locally, which is where most of the impact energy goes. Finally the structure densifies as cell walls touch, and stiffness climbs fast.

That third stage is the design limit. Once the lattice densifies, it transmits force instead of absorbing it. So the useful working range of a liner sits between the first buckle and full densification. Engineers size the cell walls so a 5 J impact stays inside that window.

Geometry drives the curve more than the resin does. A 2 mm cell with 0.4 mm walls buckles at a very different load than a 5 mm cell with the same wall. Change the angle of the struts and the plateau gets longer or shorter.

This is why digital foam 3D printing is attractive for helmets. The designer can tune each zone against a measured impact map instead of picking one foam density and living with it. In practice we see the same logic applied to shin guards, shoulder pads, and bike saddles.

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  Elastic stageStruts bend. Low force, fully reversible.
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  Plateau stageCells buckle. Most impact energy is absorbed here.
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  DensificationWalls touch. Force rises sharply. Design limit.

Print time is the first wall. A helmet-sized liner with a fine cell structure can run many hours per part. If the annual volume is in the tens of thousands, the economics rarely work against molded foam. The lattice wins on performance and on parts that would need three or four foam densities glued together.

The second wall is load direction. A printed lattice is anisotropic in a way that surprises people. Struts printed in the build plane carry load differently from struts printed along the Z axis. Drop a liner on its edge and the failure mode changes. Orientation must be fixed before the print file is released.

The third wall is surface feel. A lattice is open. Skin contact, sweat, and cleaning all become design problems. Most commercial liners add a fabric cover or a thin film layer, which changes the impact curve slightly and has to be tested with the cover on.

If your part is a flat pad, a simple wedge, or a shim, a machined foam block is faster and cheaper. Reach for digital foam 3D printing when the stiffness has to change across the part.

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  High volumeAbove roughly 10,000 pcs a year, molding usually wins.
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  Print orientationZ-axis struts buckle earlier. Fix orientation first.
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  Open surfacePlan for a cover. Test the assembly, not just the lattice.

Most digital foam parts are printed in TPU with a Shore hardness between 80A and 95A, or in a photopolymer elastomer with comparable rebound. Harder grades hold shape better under repeated load. Softer grades feel better against skin but creep more in a warm car.

Rebound matters as much as hardness. A resin that returns 90 percent of its deformation will survive thousands of hits. One that returns 70 percent will pack down and go flat. Ask for hysteresis data, not just Shore A.

Wall thickness is the practical lever. On a typical machine, a 0.3 mm to 0.5 mm strut prints cleanly. Below 0.25 mm the strut can fail mid-build. Above 0.8 mm the lattice starts to behave like solid foam and you lose most of the tuning benefit.

For the rigid parts around the liner, the shell, brackets, and adjustment hardware, we machine them. Aluminum 6061-T6 and 7075 hold up well, and 5-axis work keeps the mounting bosses and the shell radius in one setup. That is where our own shop spends most of its time on this kind of project.

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  Shore 80A–95ATypical band for impact liners in TPU.
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  0.3–0.5 mm strutsReliable print window on most elastomer printers.
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  Rebound over hardnessLow hysteresis means the pad stays useful longer.

The workflow starts with data, not CAD. You need a map of where the body contacts the part and how much force each zone sees. For a helmet that comes from drop tests and head-form data. For a shin guard it comes from field measurements.

Once the zones are set, the lattice is generated from a base cell and graded across the surface. Each zone gets a target stiffness, and the generator solves for wall thickness and cell size. This is where simulation pays off. Run a compression curve on the graded lattice before you print anything.

Then print a coupon, not the whole part. A 40 mm cube of the lattice compresses in minutes on a universal test rig and tells you whether the plateau sits where you predicted. Iterate on the coupon until the curve matches.

Only then print the full liner. Expect two or three coupon rounds. Skipping them is the most common way teams burn a week of machine time.

The rigid hardware that clamps the liner, the shell, the clips, the adjustment dial, still comes off a CNC. We hold ±0.005 mm on those features and inspect 100 percent before shipment, so the soft part and the hard part fit on the first assembly.

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  Map firstZone stiffness targets come from impact data.
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  Coupon before part40 mm cube, compression test, two or three rounds.
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  Machine the hard partsShells and clips stay in aluminum on 5-axis.