Seaming is one station in a converting line, not a standalone process. Understanding what comes before and after it clarifies why certain parameters matter and why a seam that looks good at the seamer can still fail downstream.
This page walks through the converting sequence from the printed roll to finished tube stock, with emphasis on what the seaming operator controls and what is inherited from upstream or handed off downstream.
The Converting Line
A shrink sleeve converting line typically runs in this order:
Unwind – The printed film arrives as a flat roll. The print was applied earlier — usually by gravure or flexographic printing — and the roll has already been slit to the correct lay-flat width for the target container. The unwind station feeds the web into the line under controlled tension.
Seaming – The flat web is formed into a tube by overlapping one edge onto the other. A solvent applicator lays a thin film of seaming solvent onto the overlap zone — typically 3 mm wide — and a nip roller presses the two faces together. The solvent dissolves the polymer surface on both faces, the chains interdiffuse, and as the solvent evaporates the joint becomes a solvent weld. This station is covered in detail in Seamer Setup and Mechanical Faults.
Slitting – The seamed tube is slit to the finished sleeve width if the roll was printed as a multi-up layout. On single-up converting, this step may be omitted.
Rewind – The finished tube stock is wound onto cores. The tube is flattened as it winds, so the seam must be fully dry by this point. Residual solvent causes the layers to stick together — blocking.
What the Seaming Operator Controls
Four parameters are directly in the operator’s hands at the seaming station:
Solvent type. The choice of solvent class — determined by substrate identity and line speed. Class 1 through 3 for PVC and PETG, Class 4 for PET, Class 5 for polyolefin. This is typically set during qualification and does not change run to run unless the film changes.
Flow rate. The volume of solvent applied per unit time. The starting point is approximately 1.5 mL/min per 100 m/min of line speed, producing a liquid film roughly 5 microns thick on the overlap — about a tenth of the film thickness on a standard 40-50 micron sleeve. See Dosing for the detail.
Nip pressure. The force that presses the two overlap faces together while the solvent is active. This determines contact quality across the joint. See Seamer Setup for mechanical considerations.
Web speed. Line speed is usually set to match downstream requirements or throughput targets, but it directly affects the seaming process. Speed determines how long the solvent has to work between the applicator and the nip, and how much time remains for drying between the nip and the winder.
What Comes After Converting
The seamed tube stock goes to an application line — often a separate facility, sometimes weeks later.
Application. The tube stock is cut into individual sleeves and placed over containers by an applicator machine. The sleeve is larger than the container at this stage and sits loosely around it.
Shrinking. The sleeved containers pass through a shrink tunnel — either steam or hot air — that heats the film past its activation temperature. The oriented film relaxes and contracts, conforming tightly to the container shape. Typical shrink ratios in the transverse direction range from 40% to 78%, depending on the film type.
This is the most severe mechanical test the seam will face. The shrink forces act in the circumferential direction — perpendicular to a longitudinal seam — pulling the two sides of the seam apart. A marginal weld that held together during converting can open in the tunnel.
Why Seaming Quality Matters Downstream
A seam does not need to be “strong enough to survive converting.” It needs to survive the tunnel. The shrink tunnel applies sustained thermal and mechanical stress that exceeds anything the seam experiences on the converting line. A seam that peels apart at 50% of the force needed to tear the parent film may survive winding, shipping, and application — and then split when the sleeve tries to shrink around a container shoulder.
This is why seaming problems are sometimes not caught until the application line, long after the tube stock was converted. Inspection at the seamer catches obvious failures — open seams, solvent flooding, visible delamination — but marginal welds require testing.
Inspection Methods
Visual inspection catches gross defects: open seams, applicator misalignment, solvent flooding. It does not reliably detect marginal welds.
Peel testing pulls the seam apart under controlled conditions and measures the force required. A good solvent weld should show cohesive failure — the film tears near the seam rather than the seam peeling apart cleanly. A clean peel with no fibrillation on either surface indicates poor interdiffusion. This is the most informative routine test.
Dyne testing measures surface energy, which can confirm whether the solvent actually wetted the film surface. It is more diagnostic for incoming film qualification than for finished seam assessment.
Regular peel testing during the run — not just at startup — catches drift in pump output, nip pressure, or film properties before it becomes a reel of scrap at the application line.