The application and its demands
Carton and case sealing closes the flaps of a box after filling, normally on an automatic or semi-automatic packing line. The adhesive has to wet the board quickly, build enough strength to hold the flaps shut while the pack travels down the line, and then keep the box closed through stacking, palletising, transport and storage. Most sealing problems appear at one of two moments: on the line, when the flaps spring back, or later in the supply chain, when the seal opens under load or climate stress.
It is a common packaging challenge, so it is worth designing your checks around it from the start. [1]
Most sealing is done with hot melt adhesives for packaging, which set quickly on cooling and suit fast lines. Water-based adhesives are also used where the process and board allow, and the choice should be made against your line speed, board type and end-of-life requirements rather than by habit.
Material combinations to consider
List every surface the adhesive will touch before asking a supplier for a recommendation. The same adhesive can behave very differently on two boards that look alike.
- Virgin or recycled corrugated board, and whether the grade changes between deliveries.
- Flaps that are printed, varnished, coated or wax-treated at the sealing zone.
- Laminated or film-faced board, and any flap area where the surface is not plain paper.
- Board that has absorbed moisture in storage or been chilled before sealing.
- Cartons made by different converters for the same product.
Recycled content matters for two reasons. Recycled fibre is the main raw material for new corrugated packaging, at a share reported by the European corrugated board federation, so board quality can vary from batch to batch and the adhesive needs some margin. [2]
Second, recyclability is now part of the specification discussion. Whether a pack can be recycled is determined by what it is made of and how it is designed, and also by the way it is collected, sorted and handed over for reprocessing. [3]
Trade press coverage of the corrugated industry's recyclability guidelines indicates that cold set, curable or water-soluble adhesives are preferred over hot melt. If your customers or retailers ask for recyclable packaging, ask them which guideline they apply and ask your supplier how the adhesive performs against it. [3]
Process factors that decide the result
Many seal failures are process failures rather than adhesive failures. Review these in order before changing the product.
Timing between application and compression. Preset happens when the bead of hot melt has already set before the surfaces are pressed together, so no real bond forms. If the bead is cold on arrival at the compression section, the flaps may look closed but will not hold. [1]
Application temperature. An incorrect adhesive application temperature is one recognised cause of preset, so check the actual temperature at the nozzle rather than the set point on the melter, and ask the supplier for the working range for the grade. [1]
Compression. Poor compression is a cause of pop opens in its own right, separate from preset. Check that the compression section holds the flaps closed long enough and with even contact across the full seal. [1]
Bead position and coverage. Make sure there is glue on both sides of the case or carton rather than on a single flap, and note where the bead sits in relation to the edge of the flap. Worn nozzles, partial blockages and changes in carton squareness all shift the bead from its intended position. [1]
Environment. Cold warehouses, hot trailers and humid storage all change how a seal behaves. Tell the supplier where the packs will travel and be stored, since the right adhesive grade depends on it.
How to test a carton seal
Test on the line with production board and at normal line speed, not only in the laboratory. Open sealed cartons by hand shortly after sealing and again after conditioning, and examine the bond area rather than only noting whether it held.
Fibre tear is the first thing to look at. When you pull a good seal apart, the board itself should tear. Ask whether fibre tear is partial or missing, because that points to a bond that is weaker than the board. [1]
The appearance of the failed bead also tells you what went wrong. Signs of cold failure include a compressed bead, atypical or slight fibre tear and fracture lines in the adhesive. [1]
Heat failure looks different, with inconsistent bead compression, bubbles in the adhesive and stringing between the two substrates. [1]
| What you see | Likely direction to investigate |
|---|---|
| Flattened bead, little fibre tear, cracked adhesive | Cold failure: review temperature, set speed and board temperature |
| Bubbles, stringing, uneven compression | Heat failure: review melter and nozzle temperature and compression |
| Glue on one flap only | Bead placement, nozzle condition, carton geometry |
| Flaps spring open before the compression section ends | Compression length and pressure, preset |
For heat resistance, ask the supplier whether the adhesive has been evaluated with a recognised test for heat-fail temperature in shear. One standard test method measures the temperature at which hot-melt-bonded specimens delaminate under a static shear load. [4]
Keep a record of each trial: board supplier, adhesive batch, nozzle and melter settings, line speed and storage conditions. Without that record, a later failure cannot be traced to a cause.
Common mistakes
- Raising the temperature to fix a weak seal without checking that the bead is not setting too early or overheating.
- Judging a seal only by whether the carton stays shut in the first minute, instead of opening samples and reading the bond.
- Testing on a clean sample board when production uses printed, coated or recycled board.
- Ignoring worn nozzles, bead position and compression equipment while changing adhesive grades.
- Specifying the adhesive without telling the supplier about storage, transport and refrigeration conditions.
- Treating recyclability as a late add-on, rather than raising it when the adhesive is first selected.
For a broader view of the product families used across production, see the overview of industrial adhesives. For sealing in particular, compare the options in hot melt and in water-based systems against your own line and board.