The tube comes off the seamer looking fine. The seam is holding. Then, at the next station — or the next day — the reel will not unwind cleanly. Layers stick together. Film tears. The reel is scrap.

Blocking is residual solvent. The seaming solvent has not fully evaporated before the tube winds onto itself, and the remaining liquid welds adjacent layers together the same way it welded the seam. The mechanism is identical — solvent dissolves polymer, surfaces contact under winding tension, chains interdiffuse. Except this time, it is happening where it should not.

Why it happens

The solvent has a job with two deadlines. It must stay liquid long enough to create the seam weld — polymer dissolution and chain interdiffusion require the solvent to be present and active. But it must be fully evaporated before the tube reaches the winder, or it will bond layers that should remain separate.

Blocking occurs when the drying window is too short for the solvent’s evaporation rate. The most common triggers:

Line speed increased without other changes. Faster web speed shortens the time between nip and winder. A solvent that dried adequately at 150 m/min may still be active at 200 m/min. The dosing may have been adjusted for the new speed, but the drying path was not.

Web path is too short. The physical distance from the seaming nip to the winder determines the available drying time. On compact converting lines or retrofitted seamers, this path may be marginal even at moderate speeds.

Ambient conditions changed. Higher humidity slows evaporation. Lower temperature slows it further. A line that runs clean in a climate-controlled plant may block during a humid summer week or when the heating is off overnight. These are the problems that appear and disappear with the weather.

Flow rate is too high. More solvent takes longer to evaporate. If the flow rate exceeds what the seam requires — see Over-Solvation — the excess has nowhere to go except into the wound reel.

Diagnostic questions

When blocking appears, work through these in order:

  1. Has line speed changed recently? Even a 10-15% increase shortens the drying window meaningfully at high speeds.
  2. What is the web path length from nip to winder? Measure it. Calculate the transit time at current line speed. At 200 m/min (3.3 m/s), a 2-metre web path gives about 0.6 seconds of drying time.
  3. What are the ambient conditions? Temperature and relative humidity at the seamer, not at the plant office thermostat.
  4. Has the flow rate been adjusted recently? Especially upward — an operator responding to a weak-seam complaint by adding solvent may have solved one problem and created another.
  5. Is the blocking uniform across the reel width, or concentrated at the seam? Uniform blocking suggests a general residual-solvent issue. Blocking only at the seam overlap suggests localized over-dosing or solvent migration.

Fixes

Reduce flow rate. If the seam is over-solvated, reducing the flow rate fixes both the blocking and the seam quality simultaneously. Try reducing by 10-15% and evaluating both the seam strength and the blocking. If the seam holds and the blocking diminishes, you were over-dosing. See Dosing for the quantitative framework.

Increase web path. Where the machine allows it, adding idler rollers or rerouting the web to lengthen the nip-to-winder path gives the solvent more time to evaporate. This is the most reliable fix because it does not require changing the solvent or the dose.

Move to a faster-evaporating solvent class. A Class 2 (high speed) solvent has the same solvency profile as a Class 1 (general purpose) but evaporates faster. If the current class is drying too slowly for the line speed and web path, switching to a faster-evaporating class within the same solvency tier can resolve blocking without affecting seam quality. See Drying Speed vs Line Speed for the tradeoffs.

Address ambient conditions. In plants without climate control, seasonal blocking can sometimes be managed by adjusting flow rate or class selection for the conditions. Warmer, drier air around the seamer — even localized air movement — accelerates evaporation.

Blocking vs. over-solvation

Blocking and over-solvation are related but not identical. Over-solvation means too much solvent is reaching the overlap, dissolving the film too deeply, and weakening the seam zone. Blocking means solvent — whether the right amount or too much — has not evaporated before winding.

Over-solvation often causes blocking, because the excess solvent takes longer to dry. But blocking can also occur at correct dosing if the drying conditions are inadequate. The distinction matters because the fixes are different: over-solvation is solved by reducing the flow rate; blocking at correct dosing is solved by increasing drying time or switching to a faster-evaporating class.

Residual solvent and food packaging

On food and beverage sleeves, blocking is not just a production defect — it is a signal that residual solvent levels may exceed what regulatory frameworks permit. If the solvent has not evaporated enough to prevent blocking, it has not evaporated enough to meet residual-solvent limits for food-contact applications. The root cause and the fix are the same, but the consequences extend beyond reel quality.