5 to 400 kNTJ-125
Direct Compression Fixture
Flat, parallel platens large enough to cover the whole specimen. Flat crush is a whole-area test: any platen tilt loads one side of the board first and the flutes there collapse early.
SpecificationsTesting standard
ISO 3035
Corrugated fibreboard — Determination of flat crush resistance
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 3035 determines the flat crush resistance of corrugated fibreboard: a specimen is compressed between flat parallel platens perpendicular to its surface until the flutes collapse, and the maximum force is reported. It is a measure of the fluting medium and how well it was formed, not of the liners.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ISO
- Edition
- ISO 3035:2025
- Material
- Paper, board & corrugated
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen of corrugated fibreboard is cut to a specified area, conditioned in the standard atmosphere for paper and board, and placed centrally between two flat, parallel platens. The platens close at a constant specified rate, compressing the board perpendicular to its surface, and the force is recorded until the flute structure buckles and collapses. The maximum force reached is divided by the specimen area and reported as flat crush resistance. The peak is the whole result: once the flutes have folded the trace falls away, and nothing after that point describes how the board will behave in a stack.
What it measures, and why it matters
The standing strength of the flutes themselves. In this direction the load is carried almost entirely by the fluting medium and by the geometry the corrugator gave it, with the liners contributing very little — which is what makes flat crush one of the most direct process checks available in a corrugated plant. A low result points at the medium, the corrugating rolls, or the heat and moisture at the corrugator, and it does so before the board becomes boxes. It is also the property most obviously damaged in converting: board that has been crushed passing through a printer or a die-cutter loses flat crush first, and measuring it on finished blanks rather than on the sheet is one way of finding out whether the conversion line is doing that.
Specimen and what it must not have suffered
The specimen is the board itself, and the one thing that ruins it is the thing that happens most easily — being crushed before it reaches the platens.
- Material
- Corrugated fibreboard, single-faced or single-wall
- Not applicable to
- Double-wall and microflute boardOn those constructions the collapse is not a clearly observable end point, so the method does not claim to cover them.
- Specimen
- Cut to the specified area with clean, square edges
- Conditioning
- In the standard atmosphere for paper testing before the testFibreboard is hygroscopic and its strength moves with moisture content. An unconditioned specimen measures the store room, not the board.
- Number of specimens
- As specified, from across the sheet
- Handle by the edges, never stack cut specimens
- DakA thumb pressed on a flute or a pile of specimens left on a bench pre-crushes the very structure being measured, and the loss never shows up as an obvious defect.
Flat crush is destroyed by handling more easily than any other board property. A specimen that has been leaned on, stacked, or carried flat under a stack of others has already given up part of the result before the platens touch it.
Test speed
- Rate
- A constant rate of platen approach, as specified
- Reported
- Maximum force, and flat crush resistance as force per unit area
- Atmosphere
- The standard conditioning atmosphere for paper and board
- Zero the load with the platens just clear
- DakClosing the platens onto the specimen to find zero applies a small crush of exactly the kind the test is trying to measure.
Calculations
Flat crush resistance—
Maximum force divided by the specimen area
- maximum force
- the peak recorded before the flutes collapse, N
- area
- the loaded area of the specimen, in the units the report uses
Reported as a pressure, so a result from one specimen area is comparable with another.
What the peak means—
The point at which the flute structure buckles
After the peak the trace falls away as the flutes fold. The falling part carries no information about the board's performance in a stack.
Why the liners barely matter—
The flutes carry the load in this direction
Flat crush is dominated by the fluting medium and by how well the corrugator formed it. A heavy liner does not rescue a poorly formed flute.
How the test runs
- 01Take specimens from across the width of the sheet or web.
- 02Cut to the specified area with clean, square edges.
- 03Handle by the edges from this point on, and never stack them.
- 04Condition in the standard atmosphere for paper and board.
- 05Check that the platens are flat, clean and parallel.
- 06Place the specimen centrally so the whole area is covered.
- 07Zero the force with the platens just clear of the surface.
- 08Compress perpendicular to the board surface at the specified rate.
- 09Record the maximum force before the flutes collapse.
- 10Calculate the flat crush resistance as force per unit area.
- 11Report the mean, and note any specimen that showed pre-crushing.
Grips and fixtures for this method
Self-identifying
Load Cells
A low-range load cell. Flat crush resistance on single-wall board is measured in a few hundred newtons, and a cell sized for the frame rather than the specimen throws away the resolution the test needs.
SpecificationsWhat the report has to contain
- Reference to ISO 3035 and the edition
- Board construction, flute type and grammages
- Where in the sheet or web the specimens were taken
- Specimen area
- Conditioning atmosphere and duration
- Rate of platen approach
- Maximum force for each specimen
- Flat crush resistance as force per unit area
- Mean and the number of specimens
- Any specimen rejected for pre-crushing, and why
What the machine must be capable of
Low force, well resolved, between platens that stay parallel. Flat crush on single-wall board is commonly a few hundred newtons, so the load cell has to be chosen for the specimen rather than for the frame; a cell sized for a heavy compression machine simply cannot resolve the peak. The platens must be flat, clean and large enough to cover the whole specimen, and their parallelism decides whether the flutes collapse together or one edge goes first and takes the peak down with it.
What goes wrong in practice
Pre-crushed specimens, which is the dominant failure and almost never looks like one. Closing the platens onto the board to find the force zero, which applies a small crush of precisely the kind being measured. Testing unconditioned board, which puts the store room's humidity into the result. Reading a peak from the falling part of the trace after the flutes have folded. And quoting a flat crush figure where the specification asked for edge crush, which is a different loading direction answering a different question.
Flat crush, edge crush and box compression
| Flat crush (ISO 3035) | Edge crush (ISO 3037) | Box compression (ISO 12048) | |
|---|---|---|---|
| Loaded | Perpendicular to the board face | On the board edge, along the flutes | The finished box, top to bottom |
| Tells you about | The fluting medium and how it was formed | The board's stacking capability | The whole box, closures included |
| Specimen | A piece of board | A piece of board | A made-up box |
| Damaged by handling | Very easily | Easily, at the loaded edges | Less so |
These three answer different questions and are not substitutes. Flat crush says whether the corrugator formed good flutes; edge crush predicts stacking strength; box compression measures the actual box. A specification that names one is not satisfied by another.
Questions we are asked about this test
What is ISO 3035?
ISO 3035 is the international method for the flat crush resistance of corrugated fibreboard. A specimen is compressed between flat parallel platens perpendicular to its surface until the flutes collapse, and the maximum force is reported as a force per unit area. The current edition is ISO 3035:2025, which supersedes the withdrawn 2011 edition.
Why does it not apply to double-wall or microflute board?
Because the collapse is not a clearly observable end point on those constructions. On single-faced and single-wall board the flutes buckle together and the trace shows an unambiguous peak. Where two flute layers of different heights are stacked, or where the flutes are very fine, the load transfers between structures rather than peaking cleanly, and there is no single force that means what flat crush resistance is defined to mean.
What does flat crush actually tell you?
How well the fluting medium was formed and how well the corrugator ran. The flutes carry the load in this direction and the liners contribute very little, so a low flat crush figure points at the medium, the corrugating rolls, or the moisture and heat at the corrugator — not at the liner grammage. It is one of the most useful process checks in a corrugated plant for exactly that reason.
Why is handling such a problem?
Because the property being measured is the standing structure of the flutes, and a thumb pressed on a specimen, a stack of specimens left on a bench, or a sample carried under something else all crush that structure before the test. The damage rarely looks like damage. Cutting cleanly, handling by the edges and never stacking cut specimens are the difference between a real number and a low one nobody can explain.
How does it differ from edge crush?
They load the board in different directions and answer different questions. Flat crush presses perpendicular to the face and reports on the fluting medium; edge crush, under ISO 3037, loads the board on its edge along the flutes and is the property that correlates with stacking strength. Both are routinely run on the same board, and neither substitutes for the other on a specification.
What machine does it need?
A compression frame with flat, parallel platens covering the whole specimen and a load cell sized for the specimen rather than the frame — single-wall flat crush is commonly under a kilonewton, so a cell chosen for a heavy frame throws away resolution. Platen parallelism matters more than capacity: any tilt loads one edge first and collapses the flutes there before the rest.
Does moisture affect the result?
Yes, substantially. Fibreboard is hygroscopic and its stiffness and strength fall as moisture content rises, which is why the method requires conditioning in the standard atmosphere for testing paper and board. A specimen tested straight off a warm machine or out of a damp store measures the environment it came from rather than the board.
Running ISO 3035 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low — flat crush on single-wall board is commonly under 1 kN | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Gripping | Flat, parallel compression platens covering the whole specimen area | Our compression anvils, built to the specimen |
| Environment | Standard atmosphere for conditioning and testing paper and board | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.