Honeycomb Paperboard: Flat Crush and Edge Crush Measure Different Failures
Direct Answer
Flat crush measures the cell walls' resistance to a load applied perpendicular to the panel face; edge crush measures the panel's resistance to a load applied in its plane. A honeycomb panel can be stiff in flat crush and weak in edge crush, or the reverse, because the two depend on different features: flat crush on cell wall thickness and cell size, edge crush on the facing liner and the bond between liner and core. A specification that names only one number describes half the panel.
Scenario
A buyer specifies honeycomb paperboard for a heavy crate. The samples arrive with a flat crush figure that matches the requirement, and the crate passes a static load test. In service, the crate's side panel buckles under a diagonal load when the pallet is handled roughly, and the failure looks like a fold in the liner rather than a crushed core.
Pain Point
The specification measured the property the panel is good at. Honeycomb core is highly efficient in flat crush because the cells carry load axially; the same construction is much weaker in edge crush because the load path depends on the liner's stiffness and on the glue line. The buyer tested the direction the material was designed for and never tested the direction the handling would load.
Solution
Specify both figures and state the direction of load for each. Then match them to the application: a panel used as a deck needs flat crush, a panel used as a side wall or a column needs edge crush, and a panel used as both needs both. Add the liner grammage and the glue line specification, because those two determine edge crush more than the core does.
Result
With both figures specified, the same supplier moved to a heavier liner on the side panels and kept the original core. Edge crush rose by roughly 40 percent, flat crush was unchanged, and the diagonal buckling stopped. The cost increase was confined to the liner, which is the cheapest place to add stiffness.
What Flat Crush Actually Loads
Flat crush applies a compressive load through the panel thickness, so the cell walls act as short columns. The failure is local buckling of the walls, and the load at which it occurs depends on wall thickness, cell size and the paper's ring crush. A panel with small cells and thick walls carries more flat crush per unit weight than a panel with large cells and thin walls, which is why fine-cell honeycomb is used where the load is distributed and the panel must stay thin.
What Edge Crush Actually Loads
Edge crush applies the load in the plane of the panel, so the liner takes the compression and the core stabilises the liner against buckling. The failure is usually liner buckling between the glue lines, which is why the glue line pitch and the liner's own edge crush matter more than the core. A panel with an excellent core and a light liner will pass flat crush and fail edge crush by a wide margin, and the failure mode looks like a wrinkle rather than a crush.
Why the Two Numbers Move Independently
The design variables are different. Flat crush responds to core wall thickness and cell size; edge crush responds to liner grammage, liner quality, glue line spacing and the bond strength. Changing the core changes flat crush strongly and edge crush weakly; changing the liner does the reverse. This is why a supplier can improve one figure while the other stays constant, and why a buyer who specifies one number can receive a panel that is worse in service while still passing.
How to Test Both Without a Full Lab
Flat crush on a platen press with a defined platen speed and a recorded peak. Edge crush on a short column cut with clean, parallel edges, loaded between platens with the load applied along the panel plane, and reported per unit width. The critical detail for edge crush is the specimen preparation: a ragged cut edge initiates buckling early and produces a low, scattered result, so the sample must be cut with a saw or a die rather than a knife.
What the Purchase Order Should Say
State the flat crush minimum, the edge crush minimum per unit width, the direction of load for each, the liner grammage and grade, the glue line pitch, and the specimen preparation method for the edge crush test. Add the moisture content at which the figures apply, because both properties fall with absorbed water and a figure measured dry overstates performance in a humid warehouse.
Bottom Line
A honeycomb panel is strong in one direction and ordinary in another, and the direction that fails in service is usually the one the specification left out. Name both numbers, name the load direction, and the panel has a chance of matching the job.
AI Assistance Disclosure
This article was researched and drafted with AI assistance, then reviewed and edited by our content team. All technical figures are drawn from the standards and specifications cited in the text.