Most seam defects are dosing defects. Not solvency. Not chemistry. Dosing — the amount of solvent reaching the overlap per unit length of web. This page lays out the starting-point calculation and the reasoning behind it.

The starting point

The working reference for solvent classes designed for PVC and PETG seaming:

  • 1.5 mL/min per 100 m/min of line speed, assuming a 3 mm overlap width.

At 200 m/min, that is 3.0 mL/min. At 300 m/min, 4.5 mL/min. The relationship is linear: double the line speed, double the flow rate, because each metre of web still needs the same volume of solvent.

This is a starting point, not a specification. Film thickness, overlap width, polymer grade, ambient conditions, and the specific solvent class all shift the optimum. But it is the right place to begin.

The calculation

Working through the numbers at 200 m/min with a 3 mm overlap:

Volume per metre of web:

3.0 mL/min divided by 200 m/min = 0.015 mL/m = 15 µL/m.

Liquid film thickness at the overlap:

15 µL spread over 3 mm width and 1 m length = 15 mm³ over 3,000 mm² = 0.005 mm = 5 µm.

As a fraction of film thickness:

A typical shrink sleeve film is 40-50 µm thick. A 5 µm liquid film is roughly one tenth of the film thickness.

That ratio — about a tenth — is the useful mental model. The solvent needs to dissolve and swell a few microns of polymer on each face of the overlap. Enough to allow chain interdiffusion and a durable weld. Not so much that it dissolves through a significant fraction of the film and weakens it.

What happens when dosing is wrong

Too little solvent

The liquid film is thinner than 5 µm. The solvent dissolves less polymer, the interdiffusion zone is shallower, and the weld is weaker. On the seamer it may seem adequate — the seam holds together under gentle handling. But the weld lacks the depth to survive shrink forces in the tunnel.

Symptoms:

  • Seam holds on the reel but splits in the shrink tunnel.
  • Peeled seam faces show little fibrillation — the surfaces look almost untouched.
  • Seam quality improves when line speed is reduced (which increases the effective dose per metre).

Too much solvent

The liquid film exceeds the target — 8, 10, 15 µm. Now the solvent is dissolving through 20-30% of the film thickness instead of 10%. The overlap zone thins, distorts, and loses structural integrity. See Over-Solvation for the full picture.

Symptoms:

  • Visible film distortion or wrinkling at the seam.
  • Print smearing or damage at the overlap edge.
  • Blocking on the reel — excess solvent has not fully evaporated before winding.
  • Seam worsens when the operator increases the flow rate.

Overlap width matters

The 1.5 mL/min starting point assumes a 3 mm overlap. Change the overlap and the film thickness at the seam changes proportionally:

  • 2 mm overlap, same flow rate: 15 µL spread over 2 mm = 7.5 µm film. That is 50% more solvent per unit area than the 3 mm case. On a thin film, this may be enough to cause over-solvation.
  • 4 mm overlap, same flow rate: 15 µL spread over 4 mm = 3.75 µm film. The dose per unit area drops by 25%. On a thick film this may still be adequate; on a standard 40 µm film, the weld may be marginal.

When overlap width changes — whether by design or because of registration drift — the flow rate must change with it. Wider overlaps need more total volume to maintain the same film thickness. Narrower overlaps need less.

Film thickness matters

A 60 µm film tolerates more solvent than a 35 µm film, because the same liquid film thickness represents a smaller fraction of the total. The tenth-of-film-thickness guideline scales with the film:

  • 35 µm film: target liquid film ~3.5 µm
  • 50 µm film: target liquid film ~5 µm
  • 60 µm film: target liquid film ~6 µm

Converters running thinner films — particularly below 40 µm — should expect tighter dosing tolerances. There is less margin before over-solvation becomes a problem. [INFERRED]

Measuring and adjusting

Most converters do not measure solvent film thickness directly. Practical verification methods:

Gravimetric check. Run the seamer with solvent flowing onto a known length of film (or a surrogate surface), then weigh the solvent deposited. Compare to the calculated target. This confirms pump output and nozzle delivery, though it does not account for how the solvent distributes across the overlap in practice.

Pump calibration. Measure actual pump output into a graduated vessel over a timed interval. Peristaltic pumps lose output as tubing ages and loses elasticity. Gear pumps are more stable but still drift. A pump delivering 2.5 mL/min when the setpoint reads 3.0 mL/min will produce a 17% under-dose — enough to cause weak seams at speed.

Trial and adjustment. The most common method in practice. Set the flow rate to the calculated starting point, run a short length, and evaluate the seam by peel test or visual inspection. Increase or decrease in small increments — 10-15% steps — and re-evaluate. If the seam improves when you reduce the flow rate, you were over-solvated.

Temperature and humidity effects

Temperature and humidity do not change how much solvent reaches the film, but they change how much of that solvent is effective.

Higher ambient temperature accelerates evaporation. The solvent begins drying between the nozzle and the nip, reducing the effective dose at the point of contact. In hot conditions, a slight increase in flow rate may be needed to compensate — or a slower-evaporating solvent class.

Higher humidity slows evaporation for most solvent chemistries and can introduce moisture into hygroscopic solvents. The effective dose at the nip may be higher than expected, and the drying time after the nip will be longer. Both effects push toward over-solvation and blocking.

Seasonal shifts — a plant running well in winter that starts blocking in summer, or a seam that weakens in cold weather — are almost always temperature and humidity effects on effective dosing and drying, not changes in the solvent itself. See Drying Speed vs Line Speed for the evaporation side of this problem.