The seaming solvent has two jobs that work against each other. It must stay liquid long enough for polymer chains on both overlap faces to interdiffuse and form a weld. Then it must evaporate completely before the tube winds onto itself. The window between these two deadlines is narrow, and it shrinks as line speed increases.

The timing problem

At 200 m/min, the web moves 3.3 metres every second. If the distance from the seaming nip to the winder is 2 metres, the tube has about 0.6 seconds of drying time. At 300 m/min, that same 2-metre path gives 0.4 seconds.

The solvent must do its work — dissolve, swell, enable interdiffusion — during the fraction of a second it is in the nip and immediately after. Then it must be substantially gone within the remaining transit to the winder. If either deadline is missed, the result is a defective seam.

Too-fast drying

The solvent evaporates before the polymer chains have time to interdiffuse across the interface. The surface was dissolved — briefly — but the chains did not entangle before the solvent was gone. The seam looks clean. The overlap zone shows no distortion, no print damage, no residual tackiness. It simply is not strong.

This is deceptive because the seam has all the visual signs of a correct process. The solvent was delivered, it wetted the film, it evaporated. The only thing missing is the bond.

Symptoms:

  • Weak seam with a clean, undistorted overlap zone.
  • Peeled faces show minimal fibrillation — the surfaces separated without significant material transfer.
  • Seam quality improves at lower line speed (longer contact time before evaporation).
  • Problem is worse in hot, dry ambient conditions.

Fixes:

  • Switch to a slower-evaporating solvent class within the same solvency tier. A Class 1 (general purpose, standard drying) instead of a Class 2 (high speed, fast drying) gives the same solvency with a longer working window.
  • Increase flow rate slightly to compensate for pre-nip evaporation. If the solvent is partially drying between the nozzle and the nip, more volume at the nozzle means more active solvent at the point of contact. But this is a workaround, not a fix — it adds solvent to compensate for evaporation, which raises the risk of blocking at the winder end.
  • Reduce the air movement around the seamer. Forced air intended for downstream drying can accelerate evaporation at the nip if it reaches the seaming zone.

Too-slow drying

The solvent is still present — still active — when the tube reaches the winder. The wet overlap bonds to the adjacent layer on the reel. This is blocking, and it is the most visible consequence of a drying-speed mismatch.

Less visible but equally important: residual solvent trapped in the wound reel continues to work on the polymer. The weld zone may over-dissolve after the fact, thinning the film at the overlap even if the initial dose was correct. And on food and beverage packaging, residual solvent is a regulatory concern.

Symptoms:

  • Layers stick together when the reel is unwound.
  • Solvent odour from the wound reel.
  • Seam zone shows thinning or distortion that was not visible at the seamer — the residual solvent continued dissolving after winding.

Fixes:

  • Switch to a faster-evaporating class in the same solvency tier. Class 2 (high speed) instead of Class 1 (general purpose).
  • Increase the web path between nip and winder. Additional idler rollers, a longer drying tunnel, or a rerouted web path all add drying time.
  • Reduce flow rate, if the current rate exceeds what the seam requires. See Dosing.
  • Increase air movement or temperature in the drying zone between nip and winder.

How class choice affects drying

Within the standard solvent classes for PVC and PETG:

  • Class 1 (general purpose) has moderate solvency and standard drying. It is the default for moderate line speeds with adequate web path.
  • Class 2 (high speed) has comparable solvency but faster evaporation. It is designed for high-speed lines where the nip-to-winder transit is short.
  • Class 3 (high solvency) is more aggressive and dries fast. It solves solvency problems but can introduce drying mismatches on lines where a less aggressive class would be adequate.

The choice between Class 1 and Class 2 is primarily a drying-speed decision, not a solvency decision. If the seam quality is good but blocking occurs, moving from Class 1 to Class 2 changes the evaporation rate without changing the dissolution behaviour. [INFERRED]

For PET (Class 4) and polyolefins (Class 5), the drying behaviour is specific to those chemistries and does not map directly onto the Class 1/2 distinction.

Environmental factors

Temperature. Higher ambient temperature accelerates evaporation. A line tuned for a 20 C plant will behave differently at 30 C — the solvent dries faster, the working window shortens, and the seam may weaken from insufficient interdiffusion time. Conversely, a cold plant slows evaporation and may introduce blocking.

Humidity. Some seaming solvent chemistries are hygroscopic — they absorb moisture from the air. Absorbed water does not contribute to polymer dissolution but does affect evaporation rate and can cause blushing (a hazy, whitened seam). High humidity slows the evaporation of the solvent itself and introduces a contaminant. Low humidity accelerates drying.

Air movement. Forced air in the converting area — from HVAC, tunnel exhaust, or dedicated drying systems — can accelerate evaporation at the seaming station. This is helpful if blocking is the problem; harmful if the seam is already drying too fast.

When to move classes vs. when to adjust the machine

A class change is warranted when the drying speed of the current class is fundamentally mismatched with the line speed and web path. If you are running a Class 1 solvent at 300 m/min with a short web path, no amount of flow adjustment will fix the blocking — the chemistry simply dries too slowly for the available time. Move to Class 2.

A machine adjustment is warranted when the current class should work but conditions have shifted. If a Class 2 solvent that ran clean all winter starts causing weak seams in summer, the solvent has not changed — the ambient temperature has. Adjusting flow rate, web path, or air movement is the correct response, not switching to a Class 1 that may then block when winter returns.

The diagnostic sequence in Weak and Open Seams applies here: confirm the substrate, verify dosing, then evaluate whether the drying-speed mismatch calls for a class change or an environmental adjustment.