A shrink sleeve starts as polymer pellets and ends as a printed label conforming to a container. Six steps span that distance. Each imposes constraints on the steps that follow — and seaming, the step this site focuses on, inherits from everything upstream and determines what is possible downstream.

Step 1: Film Extrusion

Polymer pellets — PETG, PET, PVC, or polyolefin — are melted and extruded into a flat film. The critical operation during extrusion is orientation: the film is stretched, predominantly in the transverse direction (TD), and then cooled, locking the polymer chains in a strained configuration. When reheated in the shrink tunnel, the chains relax and the film contracts.

The degree of orientation determines the shrink ratio. Typical TD shrink ratios range from 40% to 78% depending on the polymer and grade. Higher orientation means tighter conformance to complex shapes, but also more internal stress that the seam must resist during shrinking.

Film thickness for shrink sleeves is typically 40 to 50 microns. This is the substrate that all downstream steps — printing, seaming, application — must work with. For more detail on how film type affects seaming, see the film comparison page.

Step 2: Printing

The oriented film is printed, usually by gravure or flexographic printing. Registration marks, colour bars, and eye marks for downstream cutting are printed alongside the graphics.

A detail that matters for seaming: the print is reverse-printed. The ink goes on what will become the inside surface of the finished sleeve, protecting the graphics from abrasion. The seam is formed on the outside surface — the unprinted side — unless the print layout does not reserve the overlap zone. If the overlap zone is printed, the solvent must dissolve through the ink layer before reaching the polymer. Most well-designed print layouts reserve the overlap zone — an unprinted strip along one edge — to eliminate this variable.

Step 3: Seaming

This is the step that converts a flat printed web into a tube. One edge of the web is overlapped onto the other — typically by about 3 mm — and a seaming solvent is applied to the overlap zone.

The solvent dissolves and swells a few microns of polymer on both mating faces. Polymer chains from each face interdiffuse across the interface. The solvent then evaporates, and the chains re-entangle into a continuous network. The result is a solvent weld — a bond that, when done correctly, is as strong as the parent film.

Dosing is precise. A common starting point is 1.5 mL/min per 100 m/min of line speed, producing a liquid film approximately 5 microns thick across a 3 mm overlap. That is roughly a tenth of the film’s own thickness. Over-dosing floods the interface, extends drying time, and risks defects. Under-dosing produces a shallow weld that may fail in the shrink tunnel.

The solvent must be matched to the substrate. PVC and PETG seam with general-purpose solvents (Classes 1 through 3). PET requires a dedicated chemistry (Class 4). Polyolefin cannot be solvent-welded by conventional means and requires alternative approaches (Class 5).

For the mechanical side of seaming — nip pressure, web tension, applicator alignment — see Seamer Setup.

Step 4: Slitting and Rewinding

The seamed tube is flattened and wound onto cores. If the original roll was printed as a multi-up layout (multiple sleeves side by side), the tube is slit to the individual sleeve width at this stage.

The seam must be fully dry by the time the tube is wound. Residual solvent causes the flattened layers to bond to each other — blocking. Faster lines or slower-drying solvents require either more web path distance or a move to a faster-evaporating solvent class.

The finished tube stock is the converter’s product. It is shipped to the brand owner or contract packer for application.

Step 5: Application

At the packaging line, an applicator machine opens the flattened tube, cuts it into individual sleeves, and places each sleeve over a container. At this point the sleeve is loose — larger than the container circumference. Seam quality affects feeding, opening, and cutting: a stiff or over-thick seam can cause jams, and a weak seam can split during opening.

Step 6: Shrinking

The sleeved containers pass through a shrink tunnel. Steam tunnels and hot-air tunnels are the two main types. Steam provides more uniform heat transfer and is dominant for high-speed beverage lines. Hot air is used for heat-sensitive products or where steam contact is undesirable.

The tunnel heats the film past its activation temperature. The oriented polymer chains relax, and the film contracts in the transverse direction, pulling tight against the container. The film conforms to every contour — shoulders, waists, recesses, embossing.

This is the most demanding test of seam quality. The shrink forces act perpendicular to a longitudinal seam, pulling the overlap apart. A seam that survived converting, shipping, and application may split in the tunnel if the weld depth was insufficient. See Tunnel Splitting for the diagnostics.

Common tunnel defects — fisheyes, uneven shrink, label shift — most often trace to film orientation or tunnel temperature profiles, not seaming. But when the seam is the problem, it is visible: the seam line shows through the shrunk sleeve, or the seam opens during shrinking.