Honeycomb Board Edge Crush: The Number Behind a Stacking Claim
A stacking claim is a calculation, not a material property, and it starts with edge crush. Ask for the edge crush strength in kilonewtons per metre, the board thickness in millimetres, and the box perimeter, then compute the compression strength and divide by a safety factor of at least three, five for a humid warehouse or a thirty day storage. A honeycomb board that tests well in a laboratory at 23 degrees and 50 percent humidity can lose a quarter of its strength in a container in the tropics, and the pallet that collapses is the one whose stack was designed from the laboratory number. For the fuller picture, see packaging industry news.
Scenario
A furniture parts supplier switches from corrugated to honeycomb paper board for a line of flat-packed cabinet doors. The new packs are cheaper, lighter and pass the drop test. The supplier stacks them six high in a warehouse and ships a container to a distributor in a humid region.
The container arrives with the bottom three packs crushed at the corners, but the cartons themselves are intact: no drop damage, no water marks, no punctures. The distributor's inspector measures the board, finds it within the drawing tolerance, and the supplier concludes that the damage happened in handling.
What nobody checked was the storage condition. The board was specified at 23 degrees and 50 percent relative humidity, and the container reached 45 degrees and 85 percent humidity in transit. At that condition a paper honeycomb loses a substantial part of its edge crush, because the fiber network softens as it takes up moisture. The stack was designed with a safety factor of two against the laboratory value, which was adequate in the warehouse and not adequate in the container.
What Edge Crush Actually Measures
Edge crush strength, usually reported as ECT in kilonewtons per metre, is the force a strip of board carries when it is compressed on its edge, per unit of length. It is measured on a short strip, typically 25 by 100 millimetres, held in a fixture and crushed between platens at a controlled rate.
The test is a proxy for what happens in a box wall: the wall is a panel loaded on its edge by the weight above it, and the failure mode is local buckling of the panel followed by crushing of the paper. ECT captures the material's contribution to that failure, and the geometry contributes through the box dimensions.
Two properties of the test matter for a buyer. It is expressed per unit length, so a value is meaningless without the sample orientation relative to the flutes or the honeycomb cell direction. And it is a conditioned test, so a value is meaningless without the temperature and humidity at which it was measured.
The Stacking Calculation
The common engineering estimate for a box's compression strength is empirical and takes the form of a constant multiplied by the edge crush, multiplied by the square root of the product of the perimeter and the thickness. The constant varies by source and by board construction, and the useful discipline is not the exact constant but the structure of the equation: strength scales with edge crush linearly, and with panel size sub-linearly.
| Input | Unit | What it decides |
|---|---|---|
| Edge crush (ECT) | kN per metre | Material contribution, measured conditioned |
| Board thickness | mm | Panel stiffness, enters under the root |
| Box perimeter | mm | Panel size, enters under the root |
| Safety factor | dimensionless | Storage time, humidity, handling |
| Creep allowance | dimensionless | Load duration over weeks |
| Humidity derating | percent | Strength loss at the storage condition |
A worked example makes the arithmetic concrete. A board with an edge crush of 6.5 kilonewtons per metre, a thickness of 10 millimetres and a box perimeter of 1,200 millimetres gives a compression estimate of roughly 2,700 newtons with a constant of about 5.9. Divide by a safety factor of three for a dry warehouse and the allowable stack load is 900 newtons per box, which is nine boxes of 10 kilograms stacked above the first. In a humid container, derate the edge crush by 25 percent first and then divide, and the answer falls to about 675 newtons, or six boxes.
Why Humidity Is the Largest Single Variable
Paper is a hygroscopic material, and its stiffness falls as moisture content rises. A honeycomb board conditioned at 50 percent relative humidity and one conditioned at 85 percent can differ by 20 to 30 percent in edge crush, and the difference is not visible: the board looks the same, measures the same thickness and prints the same.
The effect is compounded by temperature, because warm air holds more moisture, and by time, because a stack under load creeps. A pallet that would stand for a week at a controlled condition can fail after four weeks in a warehouse, and the failure is at the bottom layer, which is the one that has carried the load longest.
The practical response has three parts. Specify the edge crush at a stated condition rather than as a single number. Ask for the derating curve or at least a value at 85 percent humidity. And use a safety factor that reflects the destination: three for a controlled warehouse in a temperate climate, five for a container or a tropical warehouse, and higher where the storage time is unknown.
Sample Orientation and the Cell Direction
Honeycomb board is anisotropic. The paper walls of the cells carry load in one direction more efficiently than in the other, and a strip cut parallel to the cell axis and a strip cut across it give different values. A supplier that reports one number has reported one direction.
Ask which direction the value refers to, and ask for both. The box design should then place the stronger direction vertically, which is a design decision that costs nothing and is routinely missed when the board is specified by thickness and weight alone.
The same applies to the manufacturing joint. A board that is spliced or glued to reach a size has a joint that is not representative of the panel, and a strip that includes the joint will test low. Sample away from joints and state where the sample was taken.
Creep and the Thirty Day Number
A stack is a long term load, and paper creeps under a sustained load. The compression strength measured in a short test is not the load a box can hold for a month; it is the load at which the box fails in a few minutes.
The engineering convention is a creep factor applied to the allowable load, typically 1.5 to 2.0 for storage of several weeks. Applied to the example above, a 900 newton allowance becomes 450 to 600 newtons, which is four to six boxes rather than nine.
That is the number that decides a warehouse layout, and it is usually the number that is missing from a specification. A buyer who asks for compression strength and gets a short term test value has a number that describes a laboratory event rather than a storage plan.
What to Test, and on What
Test the finished box, not only the board, when the stakes justify it. A box compression test on three samples, conditioned at the destination humidity for 24 hours, gives a direct answer that includes the manufacturing effects: the score lines, the joints, the print and the handling.
For routine control, edge crush on a strip per lot is enough, and it is fast. The two tests answer different questions: edge crush tells you whether the material arrived as specified, and box compression tells you whether the design works.
Keep the samples and the conditioning record. A compression result without a conditioning record is a number without a condition, and it cannot be compared with a second result from a different month.
What to Ask a Supplier
The edge crush value with its unit, the test standard, the conditioning temperature and humidity, the sample orientation and whether the joint was excluded. The thickness with a tolerance. The recommended safety factor for the stated storage condition. And whether the supplier has a derating curve for humidity.
A supplier that answers all five has tested the product for the way it will be used. One that answers with a thickness and a weight has described a sheet, and the stack will be designed by whoever is standing in the warehouse.
Bottom Line
A stacking claim is a calculation with three inputs and two derating factors, and edge crush is the input that is most often quoted without its condition. Ask for edge crush in kilonewtons per metre with the humidity it was measured at, compute the compression strength from the geometry, divide by a safety factor for the destination and by a creep factor for the storage time, and check the cell direction against the load. The pallet that crushes in a container was usually designed from a laboratory number that was true on the day it was measured.
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