The short answer
The edge crush test stands a small piece of board on edge and presses until it folds. The result is force per inch of width, which is where a rating like 32 ECT comes from.
Five published methods measure it, and the standards say the numbers may not match. So the figure only means something next to the method behind it.
Edge crush is the number the corrugated trade quotes most and explains least. It sets the grade on a box maker’s certificate and decides which board a buyer pays for.
Ask which test produced it and the ground shifts. The trade discusses the edge crush test as though one procedure existed. There are at least five, and they use different specimens.
This reference covers the machine, the edge preparation, the five methods, the specimen size, and what to ask when a figure arrives.
What the Machine Actually Does
The test is a compression test on a short column. A small piece of board stands on edge between two flat platens, flutes running up and down, along the axis the load travels.
The platens then close at a controlled speed. Guide blocks hold the piece square at the start, then come away before it gives.
The machine records the peak force, and dividing it by the width of the piece gives force per unit width. North America reports pounds per lineal inch, and most of the world reports kilonewtons per meter.
The TAPPI method covers single, double and triple wall alike, and the figure belongs to the combined sheet. Where it then lands is a separate question, and the ECT grade chart covers the grades in common use.
Why the Edge Has to Be Prepared
Cut a rectangle of board, stand it on edge, and it rarely fails the way the test intends. The cut edge crushes locally first, so the board folds below the real strength of the material.
Testing people call this the edge effect. Every method answers it, and the waxed edge standard says so in its own introduction: wax the edges so edge effects do not influence the result.
Wax is the oldest answer: each loading edge goes into molten paraffin to a depth of 6 millimeters, and stays until the wax creeps above the dipped zone. A quick blot on preheated towelling follows, then the piece returns to condition.
Stiffening the edge moves failure inward, and the rule for a valid result follows from that. A test counts for nothing if either liner fails inside the waxed area.
The second answer removes material rather than adding it. Cutting the piece narrower across its middle makes the middle the weakest part, so the board fails there and the edges never decide. That specimen gives the test its hourglass shape.
The third answer holds the edges mechanically, clamping instead of waxing. One accredited laboratory describes edge crush as pressing an hourglass shaped piece on its edge, which says which specimen that lab reaches for.
Each method answers the same weakness, which is why the figure travels with its method.
The Five Methods Behind One Number
Three TAPPI methods and two ISO methods all report edge crush, as separate documents.
| Method | Edge treatment | Specimen | Status |
|---|---|---|---|
| TAPPI T 811 om-23 | Waxed edges | Short column | ANSI approved |
| TAPPI T 838 cm-21 | Necked down | Hourglass | Classical method |
| TAPPI T 839 om-24 | Clamped | Short column | ANSI approved |
| ISO 3037 | None | 100 × 25 mm | Unwaxed |
| ISO 13821 | Waxed edges | Set by flute | Waxed |
One line deserves a second look: T 838 is a classical method, and the only TAPPI procedure here without approval as an American National Standard.
The ISO pair puts the split in its titles. One is the unwaxed edge method and the other the waxed edge method, so the distinction sits in the standards.
The waxed method also explains its own appeal. It tracks the top-to-bottom compression strength of the finished package, and among the methods giving the highest values it runs the simplest.
The reference on ECT vs Mullen covers the other rating a certificate can carry.
The Specimen Is Smaller Than People Think
The unwaxed method cuts a piece 100 millimeters long across the flutes and 25 millimeters high along them. Width may not vary by more than a tenth of a millimeter.
The waxed method sets height by flute instead. A- and X-flute, plus every double and triple wall grade, get a 50.8 millimeter square. C-flute drops to 38.1 millimeters high, and B-flute to 32.
Specimen height therefore tracks the board. That is one more reason to know the corrugated flute types behind a figure, because a 32 millimeter piece and a 50.8 millimeter piece are different tests.
One piece is never enough either, because both ISO methods call for at least ten valid results, and testing continues until ten valid ones exist. Invalid attempts get discarded rather than averaged in.
What counts as invalid is specific: the unwaxed method rejects any piece that buckles or tips, and a waxed result fails if either liner gives way inside the band.
Speed matters as well: the unwaxed method runs the platens at 12.5 millimeters per minute, and notes that labs elsewhere run 10. It then says plainly that the rate can change results, and requires the report to state it.
Reading an ECT Figure You Have Been Given
All of which turns one number into three questions. Which method, at what rate, conditioned to what atmosphere.
Conditioning is not a detail, since fiber strength moves with moisture, so every method sends samples to a standard atmosphere first. TAPPI publishes a separate standard defining those atmospheres, and the same ones govern corrugated basis weight.
Inside one method the standards do promise consistency. The short column test offers a choice of constant strain rate or constant load rate, and states that any combination of its procedures agrees within the stated precision.
Across methods they promise the opposite. The waxed edge standard says the methods may not give the same numerical results, which is as close to a warning as a standards body prints.
So the practical reading is short: treat a figure as comparable only to one from the same method, and ask the supplier which standard the laboratory ran.
Then get it onto the specification beside the number. Edge crush describes the board, while the box compression test measures the container that board becomes.
Common Questions
What does the edge crush test measure?
The force a board edge carries before it folds, with the load running along the flutes. Dividing by the width of the piece gives pounds per lineal inch, or kilonewtons per meter.
Why are the specimen edges dipped in wax?
An untreated cut edge crushes locally before the board reaches its real strength, which the standards call an edge effect. Paraffin to a depth of 6 millimeters stiffens the edge and moves failure inward. A liner failing inside the waxed band voids the result.
Why is the test piece sometimes hourglass shaped?
That is the necked-down specimen, the alternative to waxing. Narrowing the piece across its middle makes the middle the weakest section, so the loaded edges never decide.
Do all the methods give the same number?
No. The waxed edge standard states in its own introduction that the methods may not give the same numerical results. The unwaxed method adds that even the platen speed can change the figure, and requires the report to state it.
How many pieces does one ECT figure need?
At least ten valid results. Both ISO methods require testing to continue until ten valid ones exist, since pieces that buckle, tip or fail in the waxed area get discarded. One reading is not an ECT value.
How to weigh this page. The designations, revision dates and figures come from the standards bodies themselves, from TAPPI’s own published index and from the ISO documents defining the waxed and unwaxed routes. Two things are missing on purpose: no source reachable here sizes how far the methods diverge, and the paraffin temperature was not legible. Where a figure governs a purchase, read the method your supplier names and ask for the laboratory report.
Key takeaways
- Edge crush presses a short column of board on edge until it folds, then reports peak force per inch of width.
- A plain cut edge fails early, so the methods stabilize it by waxing the edges, necking the piece down, or clamping it.
- Three TAPPI methods and two ISO methods all report edge crush, and T 838 alone carries no approval as an American National Standard.
- Specimen size differs by method and by flute, from a 100 by 25 millimeter unwaxed piece to a 32 millimeter B-flute waxed one. At least ten valid results make a figure, and pieces that buckle or tip get discarded.
- The standards promise consistency inside one method and state that different methods may not agree, so a figure travels with its method or means little.