Weak and Open Seams: A Diagnostic Walkthrough

The seam is weak or open. Production is scrapping. The first impulse is almost always to blame the solvent — switch to something more aggressive, increase the flow, call the supplier.

That impulse is usually wrong.

Most seam failures are dosing failures. Of the remainder, most are drying-speed mismatches. A genuine solvency problem — where the solvent class itself is wrong for the substrate — is the least common root cause, but it is the one that gets changed first.

This page walks through a diagnostic sequence that starts with the most likely causes and works toward the least likely. The order matters: changing solvency class before confirming the flow rate is correct will often make things worse.

Step 1: Confirm the substrate

Before touching anything on the seamer, verify what film is actually on the machine.

PVC and PETG dissolve readily in the solvent classes designed for them (Classes 1 through 3). PET requires a dedicated chemistry (Class 4) that will not work on PVC or PETG. Polyolefins — PP and PE — require a fundamentally different bond mechanism (Class 5) and will not respond to any of the other classes at all.

What to check:

• The film spec sheet or purchase order. Do not rely on what was loaded last shift.
• If the converter has recently switched suppliers or film grades, even within the same polymer family, confirm the grade. Copolymer ratios, surface treatments, and coatings can all affect solvent response.
• If the film is a coextruded or laminated structure, the seam-side layer is what matters. The bulk substrate may be PET, but a PETG skin layer changes the solvent requirement.

Symptoms that point here:

• Solvent sits on the surface and does not wet the film — wrong chemistry entirely.
• Seam peels cleanly with no sign of surface disruption on either face — solvent never dissolved the polymer.
• A solvent that worked last week on “the same film” no longer works — the film changed, not the solvent.

If the substrate is confirmed and correct for the solvent class in use, move to Step 2.

Step 2: Check the flow rate

This is where most problems live. The correct solvent dose for shrink sleeve seaming is small — roughly a tenth of the film’s own thickness, laid down as a liquid film across the overlap width. At a 3 mm overlap on 40-50 µm film, that is approximately 5 µm of liquid. The starting-point flow rate is about 1.5 mL/min per 100 m/min of line speed.

Small deviations in either direction cause distinct failures:

• Too little solvent: The weld is shallow. The seam may hold under light handling but fail under shrink forces in the tunnel. The seam faces, when peeled apart, show little or no fibrillation — the surfaces look almost untouched.
• Too much solvent: The weld is too deep. The solvent dissolves through a significant fraction of the film thickness, thinning and weakening the overlap zone. Visible distortion, print damage at the seam edge, and blocking on the reel are all signs of over-solvation. See Over-Solvation for the full picture.

What to check:

• Actual flow rate vs. target. Pumps drift. Tubing ages. Nozzles partially clog.
• Whether line speed has changed since the flow rate was last set. A speed increase without a proportional flow increase starves the seam.
• Overlap width. A wider overlap spreads the same volume thinner; a narrower overlap concentrates it.

For the quantitative detail — the calculation, the assumptions, and how to measure — see Solvent Dosing: Flow Rate, Overlap, and Film Thickness.

Symptoms that point here:

• Seam quality varies along the reel (inconsistent pump delivery).
• Seam improved when line speed was reduced (dosing was marginal at higher speed).
• Seam worsened after an operator increased the flow rate (over-solvation).

If dosing is confirmed correct, move to Step 3.

Step 3: Match drying speed to line speed

The solvent must stay liquid long enough for polymer chains on both faces to interdiffuse — this is what creates the weld. But it must be gone before the tube winds onto itself, or the layers will stick together.

At 200 m/min the web moves 3.3 metres every second. The window between the nip roller and the winder is measured in fractions of a second. If the solvent evaporates too fast, the chains never interdiffuse and the seam is weak despite correct dosing. If it evaporates too slowly, the seam blocks on the reel.

What to check:

• Web path length from nip to winder.
• Ambient temperature and humidity. Cold, humid conditions slow evaporation.
• Whether line speed has changed. A faster line shortens the drying window and the interdiffusion window simultaneously.

Symptoms that point here:

• Seam is weak but the overlap zone looks clean and dry — solvent evaporated before interdiffusion completed. The dose was correct but the drying was too fast.
• Seam blocks on the reel — solvent is still present at the winder. The dose may be correct but the drying is too slow for this line speed. See Blocking.
• Problem appeared when the ambient temperature dropped or humidity rose.

For the full discussion of evaporation rate, class selection, and environmental factors, see Drying Speed vs Line Speed.

Step 4: Consider changing solvency class

Only after confirming that the substrate is correct, the flow rate is right, and the drying speed is matched to the line should you consider changing the solvent class.

And when you do, the change is as often downward as upward. A Class 3 (high solvency) solvent on a film that only needs Class 1 (general purpose) can cause over-solvation at flow rates that would be correct for a less aggressive chemistry. Moving to a milder class and keeping the same flow rate can produce a better seam — not because solvency was the problem, but because the excess solvency was masking a dosing issue.

When a class change is genuinely warranted:

• The substrate requires it (PET vs. PVC/PETG, or a polyolefin).
• Dosing and drying are confirmed correct, but the seam still lacks depth — the solvent is not dissolving deeply enough in the available contact time.
• Line speed has increased to the point where a faster-evaporating class is needed to avoid blocking, but the faster class in the same solvency tier does not wet the film adequately.

Diagnostic summary

Outside this framework

Some seam problems fall outside the substrate-dosing-drying-class sequence:

• Moisture pickup. Some solvent chemistries are hygroscopic. In humid environments they absorb atmospheric moisture, which can cause blushing (a hazy, whitened seam) and reduced weld strength. This is a solvent storage and handling issue, not a dosing issue.
• Ink compatibility at the overlap. If the print layout does not reserve the overlap zone, the solvent must dissolve through an ink layer before reaching the substrate. Some ink systems resist this; others are disrupted by it. Print reserve — leaving the seam area unprinted — eliminates the variable.
• Residual solvent as a taint risk. On food and beverage packaging, residual solvent in the seam can be a regulatory concern even if the seam itself is strong. This is a drying-speed and solvent-selection issue with consequences beyond seam quality.