ECT vs Bursting Strength: How to Spec Carton Board Grade

Two quotes are open on your desk. The incumbent says "3-ply, 200# burst". The new Shenzhen supplier says "3-ply, 32 ECT". A carrier's equivalence table says those are the same board, the new price is 11% lower, and you are about to sign. The ECT vs bursting strength comparison is one of the most common places a corrugated buyer loses strength without noticing, because the two numbers measure different failure modes and the table that links them was written to settle carrier liability, not to describe physics.

This is what each test actually does to a piece of board, how to convert between the units your two suppliers are using, and how to write a board grade that a laboratory report can later settle an argument about.

Two tests, two failure modes

Mullen burst

The Mullen test clamps a piece of combined board over a circular opening and pushes a rubber diaphragm through it with hydraulic pressure until the board ruptures. The result is the pressure at rupture: pounds per square inch in the US convention, kilopascals under ISO. The methods are TAPPI T810 for combined corrugated board and ISO 2759; 1 psi is 6.895 kPa, so a 200 psi board is about 1,379 kPa.

Burst is dominated by the liners — their tensile strength and their stretch. It tells you how well the board resists something being driven through the face of the box: a forklift tine, a corner of the pallet below, rough manual handling, a dropped parcel landing on an edge. It tells you almost nothing about whether the box will hold up a stack.

Edge crush

The edge crush test stands a small specimen on end, flutes vertical, and compresses it between two platens until the edge collapses. The result is force per unit width: pounds per inch in the US, kilonewtons per metre under ISO. This is a direct measurement of the board's ability to act as a column, which is exactly the property that determines stacking.

There is more than one edge crush method, and they do not give the same number:

• TAPPI T811 waxes the specimen edges and compresses between rigid platens. Waxing stiffens the loaded edges and prevents local crushing at the platen contact.
• TAPPI T839 uses a clamped, necked-down specimen with no wax.
• ISO 3037 uses an unwaxed specimen with a rigid support.

An unwaxed method can read lower than a waxed one on the same board, and the difference is comfortably large enough to move a board across a grade boundary. A test report that says "ECT 5.6 kN/m" without naming the method is not a specification; it is a number.

The conversion nobody writes down

1 lb/in = 0.1751 kN/m. That is the whole bridge between a US carton spec and a European or Chinese one. Useful values:

If a Chinese quote gives you 5.6 kN/m and your US drawing says 32 ECT, those are the same target. If the quote gives 4.5 kN/m, you have quietly dropped to a 26 ECT board.

The equivalence table, and why it is not physics

The board grades US buyers recite — 200#, 275#, 350# — come from carrier rules: Item 222 of the National Motor Freight Classification and Rule 41 of the Uniform Freight Classification. Those rules originally specified board by bursting strength and by minimum combined weight of facings, and later allowed an alternate route based on edge crush. The box maker's certificate printed on the bottom flap is a declaration against one of those two routes.

The canonical single-wall equivalence looks like this:

Confirm the current figures against the published NMFC and UFC text before you put any of them in a purchase order, because carrier rules are revised and the weight and size limits attached to each row are the part most likely to have moved.

Here is the part that matters commercially. That table is an acceptance equivalence — it says a carrier will treat either board as meeting its tariff for a given package weight. It is not a claim that a 32 ECT board and a 200# burst board are the same material. They are usually not. Meeting 32 ECT takes considerably less fibre than meeting 200 psi burst, because burst is driven by liner tensile strength while edge crush is driven by the compressive stiffness of the whole structure, which stiffer, cheaper, higher-yield papers can deliver. The migration from burst-declared to ECT-declared board across the 1990s was, in large part, a lightweighting exercise conducted entirely inside the rules.

So the practical rule:

• If the quote says "200# / 32 ECT" as a dual declaration, the supplier is claiming both, and a test report should show both.
• If the quote says "32 ECT, equivalent to 200#", the supplier is claiming edge crush only. The burst figure is a translation, not a promise. Your board will very likely have lighter liners and less puncture resistance than the board you are replacing.

That distinction is worth writing into the specification in one sentence, because it is the difference between a compliant substitution and a silent downgrade.

Grammage: what a US grade hides and a European spec states

US board shorthand uses the pound sign for two unrelated quantities, and this catches people every week.

• In "200# board", the number is bursting strength in pounds per square inch.
• In "42# liner", the number is basis weight in pounds per 1,000 square feet.

They are not the same units, the same axis, or the same thing. To convert a liner basis weight to the grammage your Chinese or European supplier will quote, multiply by 4.8824:

A European or Chinese board specification skips the shorthand and names every layer directly: for example a 175 g/m² kraft top liner, a 120 g/m² semi-chemical medium and a 140 g/m² test liner inside, on C-flute. That is more information than "200#" carries, and it is the form you want on the drawing regardless of which market you buy in, because it is the only form that survives a supplier change. Stating the board that way is the level of detail a corrugated and rigid packaging sourcing plan needs to be built on — and the flute letter belongs in the same line, because grammage without flute geometry still does not define a board.

One further detail buyers rarely ask for: both TAPPI and ISO require conditioning before testing — TAPPI T402 and ISO 187 both specify 23 °C and 50% relative humidity. Every ECT and burst figure you will ever be quoted is a 50% RH number. That is not the humidity inside a container crossing the tropics, and the gap between the two is where most stacking failures actually live.

Writing a board spec a lab report can settle

Most carton disputes are unresolvable because the specification was a phrase rather than a test. Six lines fix that.

1. Flute and construction. Ply count and flute letter, e.g. "single wall, C-flute".
2. Grammage of every layer in g/m², named in order: top liner / medium / inner liner.
3. The strength property you are actually buying, with units and a method: e.g. "ECT ≥ 5.6 kN/m, tested to ISO 3037" or "ECT ≥ 32 lb/in, tested to TAPPI T811". Name one primary property. If you need both edge crush and burst, say so explicitly and give a target for each.
4. Conditioning: "specimens conditioned to ISO 187 / TAPPI T402, 23 °C, 50% RH".
5. How it is verified: sample size, frequency (per board run, per production lot, per shipment), and who tests. Decide up front whether a factory-issued report is acceptable or whether a third-party report is required, and if third-party, name the scope — a report on a board sample is not a report on your carton.
6. Tolerance and remedy. A single minimum with no tolerance band invites a dispute over one low specimen. State the acceptance rule, e.g. a lot mean at or above target with no individual specimen more than a stated percentage below.

Line 5 is where most of the value sits. Board strength is one of the few packaging properties that can be verified independently and cheaply, which makes it a good candidate for pre-shipment inspection rather than a leap of faith. It also means you should read any certificate you are handed carefully: a quality-management certificate and a materials test report answer different questions, and what a China-issued certificate does and does not cover is worth understanding before you treat one as evidence of board grade.

Where to point these questions at a supplier

The published CMH listing for Jindong Packaging, a Shenzhen paper-packaging manufacturer founded in 2007, says the plant produces its own board on automatic corrugated lines rather than buying in sheets, then prints, slots, creases, glues and stitches on the same site, and works ODM on a full-material basis — sourcing the board and materials itself and manufacturing to a buyer's drawing or sample. That structure is relevant to a board-grade conversation, because a plant that runs its own corrugator can change liner and medium grammages to hit a target rather than passing through whatever a sheet plant produced.

What the listing does not state is which edge crush or burst grades are routinely produced, which liner and medium grammages are stocked, or whether an edge crush and burst tester sits on the floor. Those are questions for the quotation, not assumptions — and the same applies to any certificate or third-party report a supplier references: ask for the document, check what it was issued against, and check the date.

Common questions

If my supplier only quotes ECT, is my box weaker?

Not necessarily weaker at stacking — ECT is the better predictor of that. It is likely weaker at puncture and rough handling, because a board built to hit an edge crush target economically tends to use stiffer, lighter liners. If your damage history is punctured faces and torn corners rather than crushed bottom rows, specify a burst or puncture target as well.

Which test should I put in the specification?

Match the test to the failure mode. Stacking and warehouse storage: edge crush. Single-parcel shipping, drops, forklift damage and rough handling: burst, and consider a puncture target (TAPPI T803) as well. Heavy industrial exports usually need both, stated separately.

Can a factory test edge crush itself?

Yes, and many do — the equipment is not exotic. The question to settle is whether you accept a factory-issued report or require a third-party one, and at what frequency. Whichever you choose, require the report to name the method and the conditioning, and to identify the production lot it was drawn from.

What to ask your board supplier next

Send these with the drawing, and keep the answers filed with the quote:

1. State the board as ply + flute + grammage of every layer in g/m² for the exact board being quoted.
2. Which strength property is being declared — edge crush, burst, or both — and what is the target value with units?
3. Which test method and which conditioning standard produced any figure you are quoted?
4. Is there an edge crush and burst tester on site, or is testing outsourced? If outsourced, to whom, and how often?
5. Send a recent test report for a comparable board, and note whether it covers the board only or a finished carton.
6. What is the acceptance rule if a lot tests below target — rework, replacement, or price adjustment?

Jindong Packaging is one place to run that list: a CMH-listed Shenzhen maker of heavy-duty and export corrugated cartons, working to drawing or sample from 1,000 pieces with sampling quoted at about seven days and bulk at about fifteen working days on T/T terms. The answers to questions 2, 3 and 4 are the ones that separate a supplier who controls its board from one who resells it, whoever you are asking.