Two bales can carry the same grade code, the same brightness and the same CSF, and still behave differently on the same line. The difference is usually moisture — and it is the variable most buyers never put in the purchase specification.
What moisture actually does in the hammer mill
Fluff pulp is defibrated mechanically. The energy required to separate fibres rises sharply as moisture climbs, because water plasticises the fibre wall and lets fibres bend instead of breaking apart at the middle lamella.
- Below 6%: fibre becomes brittle. You get more fines and airborne dust, and sheet integrity in the forming head suffers.
- 7–8%: the usual sweet spot for hammer-mill defibration. Energy per tonne is stable and dust is manageable.
- Above 9%: defibration energy climbs, output per hour drops, and wet lumps start appearing in the forming air stream.
Why the number drifts
Moisture is not fixed at the mill gate. A bale shipped at 7.5% can arrive at 9% after four weeks in a container crossing the tropics. The pulp is hygroscopic: it equilibrates with ambient relative humidity, and the equilibrium point moves roughly 1 percentage point for every 15% change in RH.
Three things accelerate the drift:
- Bales loaded warm from the production line, then cooled in a sealed container — condensation forms inside the wrap.
- Storage without pallet clearance, so the bottom layer wicks moisture from the floor.
- Wrap damage in transit, which turns a slow equilibration into a local wet patch.
How to control it without a lab
You do not need a moisture analyser on every bale. You need a repeatable sample point and a fast instrument.
- Sample from the middle of the bale, not the surface. Surface moisture reads 1–2 points lower than core moisture.
- Use a halogen moisture analyser at 105 °C. A reading takes six to eight minutes and is accurate enough for line decisions.
- Test one bale per pallet on arrival, and log the result against the supplier lot number. After three months you will see which lots drift.
What to write into the specification
A moisture clause should specify both the target and the test method, because the method changes the number. ISO 287 (oven method) and a halogen analyser will disagree by up to half a point on the same sample.
A workable clause looks like this: Moisture 7.5% ± 0.8%, tested per ISO 287 on core samples, measured within 72 hours of arrival. That gives the mill a clear target and gives you a clear basis for a claim.
The commercial point
Moisture is water you paid pulp prices for. At 8% moisture, a 1,000 tonne order contains 80 tonnes of water. Every point you remove from the delivered moisture is a point of fibre you actually get to convert — and it shows up in cost per absorbed gram long before it shows up in any quality report.