CMT (concora medium test)
The compressive strength test for corrugating medium. How the concora fluter works, what CMT values mean, and why flat crush resistance is the right metric for fluting grades.
CMT — the concora medium test — measures the flat crush resistance of corrugating medium. It is the primary quality specification for fluting grades because it directly tests the property that determines how well corrugated board resists vertical compression: the strength of the corrugated wave after forming.
What CMT measures and why it matters
Corrugating medium (fluting) serves one structural purpose: to form a wave that transmits vertical load through the corrugated arches. When the corrugated board is compressed from the top — by a stacked box, by a warehouse racking load, by an over-filled pallet — the flute arches are what carry that load before the board collapses.
CMT measures the peak force required to crush a strip of that corrugated wave. A higher CMT value means the medium produces stiffer, more load-resistant flutes. All other things equal, higher CMT medium produces higher-ECT corrugated board.
The critical feature of CMT is the pre-corrugation step: the medium is tested after being corrugated in the lab apparatus, not as a flat sheet. This is why CMT is a better predictor of corrugated board performance than ring crush test (RCT) or tensile measurements on the flat medium — it accounts for how the fibre behaves under the actual forming and bonding conditions.
The concora fluter apparatus
The standard apparatus (TAPPI T809) consists of:
- Corrugating rolls — a pair of matched single-faced fluted rolls (C-flute geometry in TAPPI T809; some standards use different flute specifications) heated to approximately 175 °C
- A set of pressure rolls — to corrugate the test strip uniformly
- Compression platens — parallel flat plates on a testing machine for the crush phase
The procedure: a 152 mm × 32 mm test strip is conditioned (23 °C / 50% RH, TAPPI T402), then passed through the heated corrugating rolls to form the flute profile. The corrugated strip is immediately transferred to the compression platens and crushed at a constant crosshead speed. Peak force in Newtons is the CMT result.
Multiple strips are tested (typically 5–10) and the mean and coefficient of variation reported. CVs above 10–12% indicate inconsistency in the medium's formation, which is itself a quality flag.
Typical CMT values
Values vary with testing standard (TAPPI T809, ISO 7263, and various national equivalents give slightly different results due to flute geometry differences in the lab corrugator).
| Medium type | Basis weight (g/m²) | Typical CMT (N) — TAPPI T809 | |---|---|---| | Recycled OCC medium (Asia/Europe) | 100–115 | 180–250 | | Recycled OCC medium, premium | 115–130 | 250–320 | | Semichemical medium (NSSC) | 115–130 | 290–380 | | Virgin kraft medium (uncommon) | 100–120 | 350–450+ |
The OCC-based fluting grades above are typical of the global market. Semichemical medium commands a premium because the higher lignin content (NSSC retains more lignin than kraft) produces a stiffer, higher-CMT product. The trend in most markets has been toward improved recycled medium quality rather than premium semichemical, driven by cost and the improving quality of OCC secondary fibre.
CMT vs other compression tests
CMT is specific to corrugating medium and tests after corrugation. It is not directly comparable to ring crush test (RCT) on flat strips.
RCT (ring crush test, ISO 12192) tests a flat strip bent into a ring and compressed edge-on. RCT is used for both liner and medium. For medium, RCT correlates with CMT but the relationship is not linear — fibre type, refining intensity, and sheet formation all affect the ratio between RCT and CMT. Mills typically report both.
SCT (short span compression test, ISO 9895) is increasingly used as the primary compression metric because it tests very short column lengths (0.7 mm) that eliminate buckling effects and give the best correlation with in-plane compressive strength. SCT is measured on flat paper and requires no corrugation step, making it faster and less variable than CMT. Some buyers have shifted to specifying SCT of the incoming medium and modelling ECT performance from there.
ECT (edge crush test, ISO 3037) tests the finished corrugated board, not the individual components. CMT contributes to ECT but so does liner RCT/SCT and the quality of the glue bond. See ECT.
WPI editorial note: A medium with excellent CMT can produce poor ECT board if the glue line is poorly applied (excess adhesive crushes the flute tips; insufficient adhesive produces bond failures). When investigating a board compression problem, always check both the component CMT/RCT results and the glue bond integrity before adjusting medium specification.