Pressure-sensitive label stock is a laminated material built from three visible parts: a face stock that carries the printed design, an adhesive layer, and a siliconized release liner. Because the adhesive is already part of the construction, the label needs no glue pot, no water, and no drying time at the applicator. A roll can run through an automatic labeler on a packaging line, or a single label can be peeled and pressed on by hand. That construction also explains why label work behaves differently from general commercial printing. Printing, die cutting, waste stripping, and rewinding usually take place in one pass on a label press, with a raw roll entering at one end and finished labels leaving at the other.
Seven Functional Layers in Label Stock
A label material looks like three layers, but quality control treats it as seven, because each one can fail on press or in service.
- Surface coating. A thin coating changes the face stock, raising surface energy, adjusting color, or adding protection so the stock accepts ink and holds it through handling. It is used mainly on non-absorbent faces such as foil, metallized paper, and film.
- Face stock. The printable layer that also receives the adhesive on its reverse side. Paper, film, laminated foil, textile, thin metal sheet, and rubber all appear here. The choice follows the application and the printing method, and the stock must survive die cutting, waste stripping, slitting, perforating, and automatic application.
- Primer. Applied to the back of the face stock to stop adhesive migrating into it, to reduce show-through, to anchor the adhesive, and to block plasticizer from a film face reaching the adhesive and weakening the bond.
- Adhesive. The link between label and substrate. Permanent and removable grades cover most work, and the chemistry must match both the face stock and the surface being labeled.
- Silicone release coating. A low-surface-energy layer on the liner that lets the adhesive release cleanly without tearing the face stock or building up peel force over time.
- Liner. Carries the silicone, protects the adhesive, and supports the face stock through die cutting, waste stripping, and labeler dispensing. Liner stiffness and caliper affect how well a label releases at high line speed.
- Back coating or back printing. A protective coating on the liner's reverse side stops adhesive from sticking to the liner after stripping and rewinding. It also supports multi-layer label constructions, and back printing can carry a manufacturer mark or a security pattern.
How Label Materials Are Classified
Five classification systems are in common use, and a label specification usually combines several of them:
- By adhesive behavior: permanent or removable.
- By adhesive application method: hot melt, solvent, or emulsion.
- By adhesive chemistry: rubber-based or acrylic.
- By liner: opaque, semi-transparent, or transparent.
- By face stock: paper, film, or specialty material.
The face stock split matters most on press, because paper and film demand different handling from the unwind through to the die.
Paper Label Printing and Converting
The process route depends on the material and the label's end use. A label press is a multi-station machine, and the converter arranges the stations to suit the job. A typical sequence runs as follows.
- Unwinding. Intermittent machines used with platen and flat-bed cylinder presses run slowly and need no tension adjustment, but they also lack web guidance, so the roll must be wound squarely for register. Continuous rotary presses need powered tension control and a web guide, because tension changes as the roll diameter falls.
- Hot foil stamping. Flat-bed stamping suits slower presses. Rotary stamping is fast and continuous but is used less often, with printed metallic ink replacing foil on many jobs.
- Printing. Flat-bed printing suits small areas and simple images such as price labels and barcodes. Flat-bed cylinder presses handle solid areas and simple halftone color. Rotary presses handle the widest range and are the usual choice for high-quality halftone label work. Adding UV drying to any of the three allows film stock to be printed.
- Foiling before or after printing. Foil first on a press without UV drying, using conventional inks, limits the design because the foil sits under the ink. Printing first and foiling afterward on a UV-equipped press lets the foil follow any design and is the better route for demanding work.
- Varnishing. A flood varnish over the ink protects the print, resists moisture, and raises gloss. It can replace lamination when UV cured.
- Lamination. A film applied to paper or film protects the ink, adds strength, and gives the image depth. Lamination without a liner is now common because it costs less, and clear film is more widely used than matte.
- Perforating. Used mainly for computer-printed labels, either as drive holes at both edges or as a center hole for locating and driving the label in a price gun.
- Die cutting. Label dies cut through the face stock and stop at the liner. Flat dies are cheap and quick to make, suit short runs, and are used by most label shops. Rotary dies run faster and hold tighter tolerances on long runs, but tooling costs more and takes longer to arrive.
- Sheeting or rewinding. The finished web is cut into sheets or wound back into rolls, depending on how the label will be applied.
Sheet-Fed versus Roll-Fed Work
Sheet-fed label printing is dominated by offset, which accounts for roughly 95 percent of the work, with letterpress, screen, and digital taking the rest. Each operation runs on a separate machine, so output per hour is low and waste is higher, but print quality is strong. A four-color offset label generally looks better than the same job from a label press. The drawback is format: a sheet cannot be rewound, so these labels are applied by hand and cannot run on an automatic labeler. Sheet-fed work suits large labels and poster-sized printed pieces as much as it suits labels.
Roll-fed work is the opposite trade. Letterpress dominates at roughly 97 percent, with screen, offset, and flexo taking small shares. Every operation happens on one machine, so output is high and unit cost is low. The common letterpress label press handles simple spot color and line work well but cannot match sheet-fed offset for halftone quality. Its advantage is the rewound roll, which feeds labelers, barcode printers, and weighing equipment. Roll-fed production remains the main route for pressure-sensitive labels worldwide.
Film Labels: Surface Treatment Comes First
Film offers properties paper cannot match, and film printing is the direction the label industry continues to move. The structure is the same as paper, with one difference: the film surface must be treated before printing.
Ink forms an image on paper by penetration, evaporation, film formation, and curing. On dense film there is no penetration, so the ink can only evaporate, form a film, and cure. Adhesion therefore depends on the film's surface energy being higher than the ink's surface tension. Three treatments are used: corona discharge, which raises surface energy and is normally done at the film plant but decays over time, so the level must be checked before printing and some presses carry an inline treater; chemical treatment, which alters the surface for ink or thermal transfer; and top coating, which adds a receptive layer.
Printing Methods for Film Labels
- Conventional resin ink letterpress. The most common method, but slow to dry. Sheets are either cut and racked, which takes floor space and invites setoff, or laminated while the ink is still wet, which risks trapping solvent and failing the bond.
- UV letterpress. Good quality at high speed. Because many letterpress machines lack UV units, film work often requires a press upgrade before it can run.
- Solvent flexo. Strong ink adhesion and a forgiving process, since solvent inks have low surface tension. Environmental and health rules are pushing it out.
- Water-based flexo. Low cost, clean, and capable of high quality, but it demands film surface energy of at least 40 dynes and tight control of ink pH and viscosity.
- UV flexo, UV screen, and UV offset. UV flexo relaxes the surface energy requirement at higher cost. UV screen handles heavy ink films on sheet or roll. UV offset is the only practical way to offset-print film, since conventional offset inks cannot dry on a non-absorbent surface.
- Gravure and combination printing. Gravure gives the best image quality but is rare for labels because post-press handling is awkward. Combination presses, such as letterpress with screen or offset with letterpress, put several methods on one image to reach the intended visual result.
Drying and Finishing on Film
Film inks dry by oxidation and polymerization, solvent evaporation, water evaporation, or UV curing. Hot air, infrared, and UV lamps do the work. Whatever the method, the ink must be fully dry without altering the film's physical properties, the adhesive's performance, or the liner. Dryer settings, web speed, additive choice, and film grade all interact, and an additive that speeds drying can dull the ink or weaken its bond to the film.
Film finishing follows the same route as paper. Hot foil stamping, lamination, perforating, folding, cutting, rewinding, waste stripping, and die cutting all apply.
Checks Before a Label Run
Four checks prevent most label press problems. Verify the film's dyne level immediately before printing, since corona treatment fades in storage. Confirm that the die cuts through the face stock and stops at the liner, with no nicks and no cut-through. Run a tape test on the first printed length to confirm ink adhesion on film. Check roll direction, core size, and wind tension against what the labeler expects, because a correct label on a wrongly wound roll still stops the packaging line.
