Hengshi (Langfang) Precision Machinery Manufacturing Co., Ltd.

Aluminum is common when low weight is the main priority. Stainless steel, titanium and nickel alloys may be selected when temperature, fire resistance, corrosion or brazing requirements become more demanding.

Metal honeycomb is often described as a lightweight core material. That description is correct, but incomplete.

A metal honeycomb is more accurately understood as a controlled cellular structure. It divides a large space into many small, repeated cells or passages. By changing the cell size, core depth, foil thickness, material and joining method, engineers can control how the structure interacts with:

• mechanical loads;
• air and liquid flow;
• sound waves;
• electromagnetic energy;
• leakage paths; and
• high-temperature rubbing surfaces.

This is why metal honeycomb is not limited to sandwich panels. Related cellular geometries are also used in wind tunnels, gas turbines, aircraft acoustic liners, shielded rooms, MRI facilities and industrial ventilation systems.

The industry exists because one basic geometry can solve several difficult engineering problems.

What Is a Metal Honeycomb Structure?

A metal honeycomb consists of thin metal walls arranged into repeated cells.

The cells are commonly hexagonal, but other geometries are possible, including:

• rectangular cells;
• square cells;
• triangular cells;
• corrugated channels;
• circular passages; and
• application-specific cellular patterns.

The word honeycomb describes the repeated cellular arrangement rather than one mandatory cell shape.

A complete metal honeycomb component may include:

• the cellular core;
• a surrounding frame;
• face sheets or perforated sheets;
• mounting flanges;
• brazed, welded, soldered or bonded joints; and
• protective plating or coating.

Depending on the application, the honeycomb may act as a structural core, flow conditioner, acoustic cavity, rubbing seal surface or electromagnetic waveguide array.

Why Use Cells Instead of Solid Metal?

A solid block of metal can provide strength, conductivity and temperature resistance, but it is often heavier than necessary and may block airflow, sound or other required functions.

A cellular structure places material only where it is useful. Depending on the design, this may provide:

• high stiffness with relatively low mass;
• a large open area for airflow;
• repeated channels with controlled dimensions;
• a large internal surface area;
• stable support for thin face sheets; and
• predictable cavities for acoustic or electromagnetic design.

In a structural sandwich panel, the core separates two face sheets and helps them resist bending. The principle is similar to an I-beam: separating the load-carrying surfaces increases bending stiffness without filling the entire section with solid material.

Metallic honeycomb sandwich structures have therefore been used in demanding aerospace structures where low mass, stiffness and temperature capability are important.[1]

However, structural efficiency is only one reason the metal honeycomb industry exists.

Function 1: Lightweight Structural Support

When bonded, brazed or welded between face sheets, a metal honeycomb core becomes the core of a sandwich panel.

The face sheets carry much of the tensile and compressive stress, while the honeycomb core:

• maintains the distance between the skins;
• transfers shear loads;
• supports the skins against local buckling; and
• adds thickness without the mass of a solid plate.

Typical applications include:

• aircraft and spacecraft panels;
• train and marine panels;
• machine covers;
• platforms and optical tables; and
• heat-resistant sandwich structures.

Aluminum is common when low weight is the main priority. Stainless steel, titanium and nickel alloys may be selected when temperature, fire resistance, corrosion or brazing requirements become more demanding.

Function 2: Flow Straightening

A flow entering a duct or test section may contain swirl, cross-flow and large-scale velocity variations.

A custom metallic honeycomb flow straightener divides that flow into many narrow passages. The cell walls restrict lateral movement and encourage the velocity inside each passage to align with the cell axis.

Published wind-tunnel research shows that honeycomb is particularly effective at reducing lateral flow fluctuations. Screens perform a different but complementary function, which is why low-turbulence facilities frequently combine honeycomb, screens and a contraction section.[2][3]

A honeycomb straightener does not automatically turn every turbulent flow into laminar flow. Its more accurate functions are to:

• reduce swirl;
• suppress transverse velocity components;
• improve flow direction;
• improve velocity uniformity; and
• divide large flow structures into smaller passages.

Applications include:

• wind tunnels;
• water tunnels;
• fan inlets;
• ventilation systems;
• test benches;
• flow-measurement systems; and
• industrial process ducts.

Function 3: Acoustic Treatment

In a conventional acoustic liner, a perforated face sheet is placed over a honeycomb core and a rigid backing sheet. Related metal honeycomb noise-reduction panels use controlled cellular cavities as part of the acoustic structure.

The perforated face sheet contributes acoustic resistance, while the honeycomb cavities contribute acoustic reactance. Together, they can form tuned acoustic elements that absorb sound energy within a designed frequency range.[4]

The honeycomb core is useful because it:

• creates cavities with controlled depth;
• separates adjacent acoustic chambers;
• supports the perforated face sheet;
• withstands airflow and vibration; and
• can be manufactured from heat-resistant metals.

This architecture is widely associated with aircraft-engine nacelles and ducts, but related concepts may also be used in industrial silencers, exhaust systems and fan-noise treatment.

Acoustic performance cannot be determined by the honeycomb alone. It also depends on:

• face-sheet hole diameter;
• open-area ratio;
• core depth;
• cell geometry;
• backing configuration;
• airflow speed;
• sound pressure level; and
• target frequency range.

Function 4: Turbine Honeycomb Seals

Gas turbines, compressors and other turbomachinery require small clearances between rotating and stationary components.

Large clearances increase leakage. Extremely small clearances increase the risk of rubbing damage.

A honeycomb-faced seal provides a cellular surface against which a rotating seal tooth or blade tip may operate. The structure influences leakage, cavity flow, heat generation and rubbing behaviour.

Research on labyrinth seals with honeycomb facings has investigated how the cellular surface changes the flow field and leakage behaviour compared with smooth stator surfaces.[5]

Materials may include:

• stainless steel;
• nickel-based superalloys;
• Hastelloy;
• Inconel; and
• Haynes alloys.

These components commonly require controlled cell dimensions, foil thickness, height, roundness and high-temperature brazing quality.

Function 5: EMI and RFI Shielding Vents

Electronic equipment needs ventilation, but every ventilation opening can also become a path for electromagnetic energy.

An EMI shielded honeycomb vent divides one large opening into many small metallic passages. Each passage behaves as a waveguide. When properly designed below its cutoff frequency, the passage attenuates electromagnetic energy while still allowing air to move through the panel.

This creates a practical compromise:

Air can pass through the enclosure, while unwanted electromagnetic energy is attenuated.

The final shielding performance depends on more than the honeycomb core. Important variables include:

• cell size;
• cell depth;
• core conductivity;
• plating;
• electrical bonding between cells;
• frame construction;
• contact between the vent and enclosure;
• corrosion;
• frequency range; and
• installation quality.

For this reason, shielding effectiveness should be confirmed by testing rather than estimated from cell dimensions alone.

Why Use Metal Instead of Paper, Plastic or Composite Honeycomb?

Different materials are suitable for different environments.

Metal honeycomb is preferred when the application requires one or more of the following:

• electrical conductivity;
• high-temperature resistance;
• fire resistance;
• vacuum brazing or welding;
• corrosion resistance;
• erosion resistance;
• controlled rubbing behaviour;
• low outgassing; or
• long-term dimensional stability.

Material	Typical reason for selection
Aluminum	Low density and good structural efficiency
SS304	General corrosion resistance and weldability
SS316L	Improved corrosion resistance
SS321	Elevated-temperature applications
Copper or brass	Electrical conductivity and solderability
Nickel alloys	High temperature and corrosion resistance
Carbon steel	Strength and cost-sensitive industrial use

Material selection should be based on the operating environment, not only the purchase price of the core.

How Is Metal Honeycomb Manufactured?

A typical manufacturing process includes the following stages.

1. Material Selection

Metal foil is selected according to temperature, corrosion, conductivity and joining requirements.

2. Foil Forming

The foil is corrugated or otherwise formed to create the required cell geometry.

3. Layer Assembly

Formed layers are positioned to create repeated cells.

4. Joining

The layers may be resistance welded, laser welded, brazed, soldered or bonded.

5. Cutting and Shaping

The core is cut into rectangular, circular, annular, curved or custom profiles.

6. Frame Integration

The core may be installed into a metal frame or combined with face sheets.

7. Surface Treatment

Nickel plating, tin plating, passivation, painting or other treatments may be applied.

8. Inspection

Dimensions, cell uniformity, flatness, joint integrity and material documentation are checked according to the project requirements.

The joining method is critical. A glued aluminum core, a resistance-welded stainless-steel straightener and a vacuum-brazed nickel-alloy turbine seal may all be called honeycomb, but they are not interchangeable products.

The Main Design Variables

Metal honeycomb performance is controlled by a group of connected variables:

• cell size;
• core depth;
• foil thickness;
• open-area ratio;
• material;
• cell geometry;
• joining process; and
• frame and interface design.

Changing one variable usually affects several performance characteristics.

For example:

• smaller cells may improve flow-direction control but increase pressure loss;
• greater depth may improve directional control or electromagnetic attenuation but increase weight and resistance;
• thicker foil may improve strength but reduce open area;
• a more conductive material may improve electrical performance but increase cost; and
• a stronger joining process may improve reliability but require more complex manufacturing.

There is therefore no universally “best” honeycomb specification.

The correct design is the one that balances the requirements of the complete system.

Manufacturing Perspective

HengShi manufactures customized metal honeycomb cores, stainless-steel honeycomb cores, flow straighteners, EMI/RFI honeycomb vents, acoustic honeycomb panels and honeycomb seals.

Available options include stainless steel, copper, brass, carbon steel and selected nickel-based alloys. Depending on the application, structures can be resistance welded, laser welded or vacuum brazed.

Typical customization variables include:

• cell size;
• foil thickness;
• core depth;
• overall dimensions;
• circular or rectangular shape;
• frame construction; and
• surface treatment.

Engineering evaluation should begin with the operating conditions rather than only a drawing of the outside dimensions.

Information Required for an Engineering Evaluation

To evaluate a metal honeycomb component, please provide:

• application;
• required material;
• overall dimensions;
• cell size;
• core depth;
• foil thickness;
• operating temperature;
• pressure or airflow conditions;
• corrosion environment;
• joining requirement;
• required test standard;
• drawing; and
• annual quantity.

When the exact honeycomb specification is not yet defined, the operating conditions can be used as the starting point.

Conclusion

Metal honeycomb exists because repeated thin-walled cells provide functions that solid metal cannot provide efficiently.

The same structural concept can:

• increase stiffness without the mass of a solid plate;
• straighten airflow;
• create acoustic cavities;
• control leakage and rubbing surfaces; and
• provide ventilation with electromagnetic attenuation.

The value of a metal honeycomb is therefore not simply its hexagonal appearance.

Its value comes from matching geometry, material and manufacturing process to a specific engineering problem.

Related Products

• Metal Honeycomb Core
• Stainless Steel Honeycomb Core
• Metal Honeycomb Flow Straightener
• EMI Shielded Honeycomb Vent
• Honeycomb Seal
• Honeycomb Noise-Reduction Panel

References

1. NASA, Thermographic Inspection of Metallic Honeycomb Sandwich Structures, 1998.
2. J. Scheiman, Comparison of Experimental and Theoretical Turbulence Reduction Characteristics for Screens, Honeycomb, and Honeycomb-Screen Combinations, NASA Technical Paper 1958, 1981.
3. G. Johl, M. Passmore and P. Render, “Design Methodology and Performance of an Indraft Wind Tunnel”, The Aeronautical Journal, Vol. 108, 2004, pp. 465–473.
4. R. E. Kraft, J. Yu and H. W. Kwan, Acoustic Treatment Design Scaling Methods: Phase II Final Report, NASA, 2003.
5. V. Schramm, K. Willenborg, S. Kim and S. Wittig, “Influence of a Honeycomb Facing on the Flow Through a Stepped Labyrinth Seal”, Journal of Engineering for Gas Turbines and Power, Vol. 124, No. 1, 2002, pp. 140–146.

Request an Engineering Evaluation

HengShi manufactures customized metallic honeycomb structures for flow control, EMI shielding, acoustic treatment and turbine sealing.

Please contact the HengShi Engineering Team and send your drawing, material, cell size, core depth, operating conditions and required quantity for an engineering review.