Edge Crush in Honeycomb Board: The Strength That Decides a Crate
Edge crush is the strength of a honeycomb panel at its cut edge, and it is the property that decides whether a crate holds its stack. A panel that performs well in bending can crush at the edge where the crate frame bears on it, because the cut edge exposes open cells and carries load through very little material. Specify the edge treatment, the support at the frame and an edge crush test on a cut sample, not only a flat compression test.
Scenario
A crate maker switches to a heavier honeycomb panel to carry a higher stack. The panel is stronger on a flat compression test. The crates still fail, and the failure is a crushed edge under the top frame. The heavier panel has a larger cell and a thinner wall, so the edge carries less than the lighter panel it replaced. The specification compared thickness and compression, and the failure was at a feature that neither number describes.
Pain Point
Honeycomb board is purchased by thickness and by a compression figure, and both describe the panel face. The edge is a different condition: the cells are open, the load is applied to the cell walls in line with the axis, and the effective bearing area is only the wall thickness multiplied by the length. A panel with large cells has less wall area per metre of edge, so its edge crush can be lower even when its flat compression is higher. The cube rule is that thickness buys bending and cell size buys edge strength, and a specification that mixes the two buys the wrong board. For the fuller picture, see honeycomb packaging case studies.
Solution
Specify the edge and the support.
Cell size: state the cell size in millimetres, because it sets the number of walls per metre of edge.
Paper grade: state the basis weight and the grade of the liner and the cell wall.
Edge crush test: cut a sample with a defined edge and crush it with the load along the cell axis, at a stated humidity.
Edge treatment: fill or cap the edge with a denser material, or use a solid insert where the frame bears, so the load lands on material rather than on open cells.
Frame detail: make the frame bear on a continuous member rather than on a point, and keep fasteners away from the cut edge by at least one cell.
| Specification | What it controls | Test |
|---|---|---|
| Cell size | Walls per metre of edge | Cell count on a section |
| Paper grade and weight | Wall stiffness | Basis weight check |
| Edge crush | Load at the cut edge | Crush test on a cut sample |
| Edge treatment | Bearing area | Section through the edge |
| Frame detail | Load distribution | Assembly inspection |
| Humidity at test | Fiber softening | Conditioned test report |
Result
A crate maker that added an edge crush test to the specification found the heavier panel was weaker at the edge than the panel it replaced. The supplier offered a panel with a smaller cell and a heavier wall, which carried more at the edge at a similar thickness. The frame detail changed to a continuous member with a denser insert at the bearing points, and the crates stopped crushing. The panel weight rose slightly, the damage rate fell, and the specification now names the cell size and the edge crush rather than the thickness alone.
Why the Cut Edge Is the Weakest Line
At the cut edge the cell walls end, and the load passes through the wall thickness only. A panel that is strong in the face is strong because the faces carry tension and compression and the cells carry shear; at the edge, the faces are no longer continuous and the cells are open. A crate frame that bears on that line applies a concentrated load to a small area, and the cells buckle. The fix is either to increase the wall area at the edge, by using a smaller cell or a heavier paper, or to cap the edge so the load is spread across a solid material.
Edge Treatment Options
A filled edge, where the open cells are closed with adhesive or a resin, restores some bearing area and adds cost. A capped edge, where a denser strip is glued along the edge, gives a defined bearing surface and is easier to control in production. A solid insert at the frame point is the simplest and most local solution, and it is the one to try first because it does not change the panel. Each option has a weight and a cost, and the choice should follow the load path rather than the availability of a material.
Fasteners and the Edge
A fastener placed close to the cut edge splits the cell wall and loosens under vibration. Keep fasteners at least one cell width from the edge, and use a washer or a plate to spread the load. In a paper crate, the joint is often the first thing to fail, and the joint detail is usually drawn after the panel is chosen. Draw the joint before the panel, because the joint decides which panel property matters.
Humidity Again
Edge crush, like every paper property, falls as the moisture rises. A panel tested in a dry room overstates its performance in a humid container, and the edge is the first place to soften because the cells are exposed. Condition the sample at the route humidity, and seal or cap the edge on a wet route so that water does not enter the open cells. A humidity indicator in the crate shows whether the pack held, and that record is what a claim needs.
Writing the Specification
State the cell size, the paper grade and basis weight, the edge crush at a stated humidity, the edge treatment, the frame bearing detail, the fastener spacing from the edge and the test method for each. Add a first-article report with a section through the edge and a crush result, and a retained sample. A specification that names thickness and compression has described a panel; the crate needs a description of its weakest line.
The Joint Is Part of the Edge
A crate joint transfers load from one panel to another, and the transfer happens at the edge. A joint that is glued and stapled across a continuous member shares the load, while a joint that relies on fasteners through open cells concentrates it on a few walls. Draw the joint and the edge together, specify the fastener spacing, and test the assembly rather than the panel. Most crate failures that are reported as panel failures are joint failures at an edge, and the repair is a detail change rather than a heavier board.
Bottom Line
Edge crush decides whether a crate holds its stack, and it depends on cell size, paper weight and edge treatment rather than on thickness. Test a cut sample at the route humidity, spread the frame load, keep fasteners away from the edge and cap or fill the edge on a wet route. The strongest panel in the face can be the weakest one at the line where the load arrives.
ISPM 15 sets the treatment and marking requirements for wood packaging material made from raw wood in international trade. A honeycomb board crate is not wood, so it carries no ISPM 15 heat-treatment or IPPC mark obligation, which removes one fumigation step and one border hold-up from the export chain.
The Confederation of European Paper Industries publishes recyclability guidelines for paper-based packaging, which set out how coatings, laminates and other materials combined with paper and board in a honeycomb board must be designed so they do not hamper the recycling process.
Title 21, Part 176 of the U.S. Code of Federal Regulations lists the substances and the conditions permitted for paper and paperboard components that come into contact with food, which is the requirement a honeycomb board tray has to meet before it can be sold as a food-contact article.