More solvent does not make a stronger seam. Past a point, it makes a weaker one. This is counterintuitive, which is why over-solvation is one of the most persistent problems on converting lines — operators see a weak seam, increase the flow, get a worse seam, and increase it again.
What happens inside the overlap
A correctly dosed seam lays down roughly 5 µm of liquid solvent across a 3 mm overlap on a 40-50 µm film — about a tenth of the film’s thickness. The solvent dissolves and swells a few microns of polymer on each face. Chains from both faces interdiffuse across the interface. The solvent evaporates. The chains re-entangle. The result is a weld that is, at the molecular level, continuous polymer — no adhesive, no third material.
Now double the flow rate. The liquid film is 10 µm — a fifth of the film thickness instead of a tenth. The solvent dissolves deeper into each face, reaching 8-10 µm into a 40 µm film. At the overlap, where both faces are dissolving simultaneously, the total dissolved zone can represent 40-50% of the combined film thickness in that region.
The overlap zone thins. It distorts. It loses the mechanical integrity that the rest of the film retains. The weld itself may have excellent interdiffusion — the chains are thoroughly entangled — but the film around the weld has been compromised. The seam is weaker because the film is weaker.
The dosing math
The numbers from Dosing, restated to make the over-solvation case:
| Flow scenario | Liquid film (3 mm overlap) | Fraction of 40 µm film |
|---|---|---|
| Correct (1.5 mL/min at 100 m/min) | ~5 µm | ~12% |
| 50% over | ~7.5 µm | ~19% |
| Double | ~10 µm | ~25% |
| Triple | ~15 µm | ~37% |
At double the correct dose, each face of the overlap is being dissolved through roughly a quarter of its thickness. The weld zone is no longer a surface bond — it is a structural compromise.
Symptoms
Visible film distortion at the seam. The overlap zone is thinner than the surrounding film and may show wrinkling, waviness, or a visible channel. Hold the tube up to a light — if the seam is noticeably more transparent than the rest of the film, the solvent has thinned it.
Print smearing or damage at the overlap edge. Excess solvent migrates beyond the intended overlap zone and dissolves or disrupts ink layers. This shows up as a blurred or discoloured band along the seam edge, extending into the printed area.
Blocking on the reel. More solvent takes longer to evaporate. Even if the web path provides enough drying time for the correct dose, an over-dose may still be active when the tube reaches the winder.
Wrinkling. The dissolved polymer shrinks slightly as the solvent evaporates, creating localized stress that manifests as wrinkles or pucker along the seam.
Seam gets worse when flow rate is increased. This is the defining symptom. If an operator raises the flow rate in response to a weak seam and the seam degrades further, the problem was already over-solvation, not under-solvation.
The “more is better” trap
This is the cycle:
- Seam is weak. Operator increases flow rate.
- Seam gets worse — or blocking appears. Operator assumes the solvent is not aggressive enough.
- Operator switches to a higher-solvency class (e.g., Class 1 to Class 3) at the same elevated flow rate.
- Over-solvation worsens. Film distortion becomes visible. Blocking increases.
- Operator concludes the solvent is defective.
The fix at Step 1 was to decrease the flow rate, not increase it. Or, if the seam was genuinely weak, to check dosing uniformity (a clogged nozzle can produce a weak seam at a flow rate that reads correct on the pump), nip pressure, or whether the line speed had changed.
Diagnosis
Reduce the flow rate by 15-20% and evaluate the seam. If the seam improves, you were over-solvated. This is the simplest and most reliable test. It costs a few metres of film and a few minutes.
Inspect the overlap zone against a light. Thinning is visible. On printed sleeves, compare the overlap transparency to an unprinted reference area of the same film.
Check for solvent migration beyond the overlap. If the solvent wetting pattern on the film is wider than the intended overlap, the flow rate is too high or the nozzle is misaligned.
Review the history. Over-solvation often develops incrementally — a small flow increase to fix a minor issue, another small increase next shift, another the shift after. The flow rate on the machine may bear little relationship to the original setpoint.
Correcting over-solvation
- Return the flow rate to the calculated starting point: 1.5 mL/min per 100 m/min at 3 mm overlap. See Dosing for the full calculation.
- If a higher-solvency class was introduced during the troubleshooting cycle, consider stepping back to the original class at the correct flow rate.
- Verify pump calibration. If the pump has been adjusted repeatedly, confirm that the displayed setpoint matches actual output.
- Document the correct flow rate for the current substrate, line speed, and overlap. Over-solvation recurs because the correct setting is not recorded and operators adjust by feel.