“Wet lock” is a performance requirement, not one coating name
Buyers use “wet lock,” “wet-strength” and “moisture-resistant” for several board and coating systems. A converter may change paper sizing, adhesives, liner grades, impregnation or surface treatment. Those constructions do not behave identically around cut edges, score lines, seams and drain openings. Define what the box must survive before choosing the treatment.
What the main tests measure
| Control | What it tells the buyer | What it does not prove | Reference to discuss |
|---|---|---|---|
| Water absorptiveness (Cobb) | Mass of water absorbed per unit area under the stated exposure conditions. Useful for controlling a sized liner or combined board. | Finished-box leakage, wet seam security or pallet stacking strength. | ISO 535:2023; TAPPI/ANSI T 441 |
| ECT | Edgewise compressive strength of a short corrugated-fiberboard column, perpendicular to the flutes. | Performance of slots, scores, joints, hand holes, pallet overhang or the product inside. | TAPPI/ANSI T 811 om-23 |
| Finished-box compression | Resistance of the converted container to external compressive force in a stated orientation. | Long-duration creep, vibration, repeated handling or a different humidity profile unless those are included. | TAPPI/ANSI T 804 om-24; ASTM D642-25 |
| Constant-load stacking | How a container responds to a sustained vertical load for a stated time and atmosphere. | Every shock, vibration and load shift in commercial transport. | ASTM D4577 or an agreed route-specific protocol |
| Packed pallet trial | Combined effect of product, coolant, liner, box, stacking pattern, stretch wrap, dwell time and handling. | A universal laboratory value transferable to a different crop or route. | Buyer-approved commercial protocol |
Why dry results cannot be copied into a wet route
Paper strength changes with moisture, and a box may fail first at a converted feature rather than in the board panel. A dry ECT or compression certificate is useful for incoming quality control, but it cannot predict a box exposed to meltwater for hours without a defined conditioning factor or wet test. Record which face contacted water, exposure time, temperature, relative humidity, sample preparation and time between conditioning and testing.
A six-stage approval plan
- Define the hazard: condensation, rain, hydrocooling, top ice, wet product, reefer dwell, or a combination.
- Lock the construction: internal dimensions, flute/ply, paper grades, adhesive, treatment, joints, vents, drain holes and liner.
- Set material controls: grammage, thickness and agreed water-absorption/strength tests on identified components.
- Test the converted box: dry and conditioned compression, using the orientation and deformation limit relevant to the pallet.
- Run a full-pack pallet simulation: actual product or representative load, coolant, wrap, pallet pattern, dwell and handling.
- Confirm commercial production: inspect production-run samples, not only a hand-made prototype.
Copyable RFQ checklist
- Product/crop, net weight, coolant type and quantity
- Internal dimensions and maximum external dimensions
- Board combination, flute/ply and paper grades—or performance targets if design is supplier-led
- Moisture treatment on each liner, medium, adhesive and exposed edge
- Cobb method, exposure time, tested face, units and maximum value
- ECT method, specimen preparation, units and minimum value
- Box-compression method, dry/wet conditioning, orientation, deformation limit and minimum value
- Vent/drain pattern, hand holes, score lines, manufacturer’s joint and liner/bag
- Pallet footprint, stack pattern, pallet height, overhang limit and wrap
- Temperature, humidity, wet exposure and dwell time before test
- Sampling frequency, release report and claim procedure
- Destination recovery/disposal requirements for the selected treatment
When to compare corrugated plastic
PP corrugated boxes deserve a parallel trial when the route repeatedly wets the package, the box must be washed, drain geometry is important, or a managed return loop exists. The PP sheet is not a complete performance guarantee: joints, closures, edges, print, pallet pattern, cleaning and realistic reuse loss still require validation. Compare cost per successful trip, not only carton price against plastic purchase price.
Frequently asked questions
Is Cobb the same as a waterproof test?
No. Cobb is a standardized measure of water absorptiveness under defined conditions. It does not establish that a complete box is waterproof.
Can ECT predict BCT?
ECT is one input to structural performance, but box dimensions, board thickness, flute, converting, scores, joints, openings and environment also matter. Use the finished-box test for the acceptance requirement that matters.
How many boxes should be tested?
Set a sampling plan based on risk, order size, process capability and the test method. The RFQ and purchase order should state both the number of specimens and how they are selected across the production run.
Build a wet-route carton test plan
Send Fuxing your crop, coolant, box size, pallet pattern and route conditions. We can quote a wet-strength carton and, where useful, a PP corrugated comparison sample.
Request a test-ready quotation →