The seam looks fine on the reel. It survives slitting, if there is a slitting step. It survives application onto the container. Then the sleeve enters the shrink tunnel and the seam opens.

This is a different failure mode from a seam that is weak or open at the seamer. A seam that splits in the tunnel had enough strength to survive handling — but not enough to survive shrink.

Why the tunnel is the hardest test

Shrink forces are significantly higher than handling forces. When the sleeve enters the tunnel, heat activates the orientation built into the film during manufacture. The film wants to return to its pre-stretched dimensions. On a typical PVC or PETG shrink sleeve, the primary shrink direction is circumferential — around the container. The seam runs parallel to this direction, so in principle the shrink forces act along the seam, not across it.

In practice, the forces are not perfectly aligned. Differential shrink rates across the sleeve width, variations in tunnel temperature profile, and the geometry of the container all introduce peel and shear components at the seam. A shallow weld that held under gentle tension fails under these complex, high-magnitude forces.

Cause 1: Insufficient weld depth

The most common cause. The solvent dissolved and swelled the polymer surface, chains interdiffused to some degree, and the seam formed — but the weld zone is too shallow. The interdiffusion depth was not enough to resist the stresses imposed during shrink.

This is usually a dosing problem:

  • Flow rate too low. The liquid film at the overlap was thinner than the target ~5 µm (at standard 3 mm overlap on 40-50 µm film). Less solvent means less dissolution, shallower interdiffusion, and a weaker weld.
  • Effective dose reduced by conditions. The flow rate may be set correctly, but high ambient temperature or a long open path between nozzle and nip allowed partial evaporation before contact. The solvent that reached the overlap was less than what left the nozzle.

It can also be a drying speed problem — if the solvent evaporated too quickly after the nip, the chains did not have enough time to interdiffuse even though the initial dose was correct.

Cause 2: Nip pressure

The nip roller presses the two faces of the overlap together while the solvent is active. Insufficient nip pressure means the dissolved surfaces do not make full, intimate contact. Interdiffusion requires the swollen polymer chains on both faces to be within molecular reach of each other. If there is a gap — even microns of gap — the weld is incomplete.

Check:

  • Nip roller pressure and uniformity across the web width.
  • Roller condition — worn, flat, or contaminated rollers reduce effective pressure.
  • Web tension upstream of the nip. Inconsistent tension causes inconsistent contact.

Cause 3: Shrink orientation mismatch

Less common, but worth considering when dosing and nip pressure are confirmed correct.

Shrink sleeve films are oriented primarily in the transverse (circumferential) direction, with minimal machine-direction shrink. If the seam overlap is positioned or oriented such that the shrink forces act to peel the seam open rather than shear it along its length, even a good weld can fail.

This is a sleeve design and registration issue, not a seaming issue. But it shows up as a seam failure in the tunnel, so it enters the diagnostic.

Diagnosis

Peel the failed seam and examine the faces. This is the single most informative step.

  • If the faces look clean and undisturbed — little evidence of surface disruption — the weld was too shallow. The solvent did not dissolve deeply enough. Increase flow rate or verify that the solvent class has adequate solvency for the substrate.
  • If the faces show fibrillation, tearing, or substrate damage — material transferred from one face to the other — the weld was strong enough in that zone, but coverage was incomplete. Look for dosing uniformity issues, nip pressure variation, or partial clogging of the solvent nozzle.
  • If the split follows the seam edge rather than running through the middle of the overlap, the failure is at the boundary of the wetted zone. The solvent is not reaching the full overlap width. Check nozzle alignment and overlap registration.

Check whether the problem is speed-dependent. Run a short batch at reduced line speed without changing the flow rate. If the seam survives the tunnel, the effective dose per metre was too low at the higher speed — confirm with the calculation in Dosing.

When it is not a tunnel problem

If the seam is splitting before the tunnel — during slitting, during application, or on the reel — the problem is more fundamental than tunnel-splitting. The seam lacks basic handling strength, and the diagnostic should start at the beginning: Weak and Open Seams.