Honeycomb Board Edge Crush FAQ: Stacking, Humidity and Safety Factors

Edge crush is the material input to a stacking calculation, and the calculation is what decides how many boxes a pallet can carry. These questions cover the units, the conditioning, the geometry, the humidity derating and the creep factor that most specifications leave out.

What is edge crush strength?

The force a strip of board carries when compressed on its edge, per unit of length, reported in kilonewtons per metre. It is measured on a short strip held in a fixture and crushed between platens at a controlled rate, and it stands in for the material's contribution to the failure of a box wall, which buckles locally and then crushes the paper.

Why is a value without its condition meaningless?

Because paper is hygroscopic and its stiffness falls as it takes up moisture. A board conditioned at 50 percent relative humidity and one at 85 percent can differ by 20 to 30 percent in edge crush, and they look identical: same thickness, same weight, same print. A number quoted without a temperature and humidity cannot be compared with the condition in a container.

What units and orientation should be stated?

Kilonewtons per metre, with the sample orientation relative to the cell axis. Honeycomb is anisotropic: a strip cut along the cell axis and one cut across it give different values, so a single number describes one direction. Ask for both, and specify that the sample excluded any manufacturing joint, because a strip through a splice tests low.

How is compression strength estimated?

By an empirical relation: a constant multiplied by edge crush, multiplied by the square root of the product of box perimeter and board thickness. The structure matters more than the exact constant, because it shows that strength scales linearly with material and sub-linearly with panel size. A larger box is not proportionally stronger.

What does a worked example look like?

A board at 6.5 kilonewtons per metre, 10 millimetres thick, in a box of 1,200 millimetres perimeter, gives roughly 2,700 newtons with a constant near 5.9. Divide by a safety factor of three for a dry warehouse and the allowance is 900 newtons per box, which is nine 10 kilogram boxes above the first. Derate for humidity first and the answer falls by a quarter.

What safety factor should be used?

Three for a controlled warehouse in a temperate climate, five for a container, a tropical warehouse or an unknown storage time. The factor covers humidity, handling, stacking misalignment and the difference between a test sample and a production box. A factor of two is adequate only where every one of those variables is controlled, which is rare in export.

Why is humidity the largest variable?

Because water softens the fiber network, and warm air carries more of it. A container in the tropics can reach 85 percent relative humidity at 45 degrees, and at that condition a paper honeycomb loses a substantial part of its edge crush. The board still measures to thickness and still looks right, so the loss is invisible until the bottom layer deforms.

How should humidity be handled in a specification?

Specify edge crush at a stated condition, and ask for a value or a derating curve at 85 percent relative humidity. If neither is available, apply a conservative derating of 25 percent before the safety factor, and say so in the stack calculation so the assumption is visible to whoever reviews it later.

What is creep and why does it matter?

Paper deforms slowly under a sustained load, so the strength in a short test is not the load a box holds for a month. The convention is a creep factor of 1.5 to 2.0 applied to the allowable load for storage of several weeks. Applied to a 900 newton allowance, the working load becomes 450 to 600 newtons, or four to six boxes rather than nine.

Where does the failure actually happen?

At the bottom layer, which has carried the load longest and absorbed the most creep, and at corners, where the panel is least supported. This is why a container can arrive with the lower packs crushed and the cartons intact: the damage is a gradual deformation rather than an impact, and no drop test would have found it.

Should the box be tested or the board?

Both, for different questions. A box compression test on three conditioned samples answers whether the design works, including score lines, joints and print. A strip test per lot answers whether the material arrived as specified and is fast enough for routine control. Keep the conditioning record with either result.

How long should samples be conditioned?

At least 24 hours at the destination condition before a compression test, and longer for a thick board. A sample tested straight from a dry store gives a number that describes the store rather than the market. Record the temperature, the humidity and the time, because a result without them cannot be compared with a second result from another month.

How does cell direction affect the design?

The cell walls carry load more efficiently along one axis, so the stronger direction should be placed vertically in the box wall. This is a design decision that costs nothing and is routinely missed when board is specified by thickness and weight alone. Ask which direction the reported value refers to and orient the blank accordingly.

What about the manufacturing joint?

A spliced or glued board has a joint that is not representative of the panel, and a strip that includes it tests low. Sample away from joints and state where the sample came from. A joint placed in the middle of a wall under load is a weak line, so the joint position belongs in the box drawing as well as in the test report.

What should a buyer ask a supplier?

The edge crush value with units, the test standard, the conditioning, 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 a humidity derating curve exists. Five answers separate a tested product from a described sheet.

Stack pattern is part of the calculation and is usually left to the person loading the pallet. A column stack carries the load directly down the corners and uses the board efficiently, and it is unstable because the columns lean independently. An interlocked stack is stable and forces the load through the panels rather than the corners, which reduces the allowable load by twenty to thirty percent. If the calculation assumed a column stack and the warehouse interlocks, the number on the paper describes a different pallet from the one on the floor.

Pallet overhang is the other silent deduction. A pack that overhangs the pallet by twenty millimetres has unsupported corners, and the corner is where a honeycomb panel is least able to carry load. Keep the pack footprint inside the pallet deck, and treat any overhang as a design change rather than a handling detail. A stack that fails at the bottom corners after a sea voyage has usually been asked to carry a load its corners never received.

What belongs in the specification?

Edge crush at a stated condition in both directions, thickness with tolerance, cell direction relative to the load, joint position, the compression estimate with its inputs, the safety and creep factors, the conditioning for the acceptance test, and the storage condition assumed. The last item is the one that turns a material specification into a stacking plan.