Laminating Short-Run Print: Film, Heat and Curl Explained

Lamination is often the last step before a print job leaves the building, and it is also the step where a good job can still go wrong. A cover that curls, a film that lifts at the corners or a cloudy patch across a dark solid are all problems that show up after the printing is already paid for. Most of them come down to four things: the film, the heat, the pressure and the time the sheets were given to settle before they were laminated.

Cold, heat-assist and thermal: three different processes

The word “thermal” gets used loosely. In its laminating guide, the print federation FESPA separates three adhesive types. Cold adhesive is pressure-sensitive and is used over media that is likely to be heat-sensitive, such as vehicle wrap vinyl. Heat-assist films run with at least one heated roller at around 60°C and are used for general single-sided lamination onto paper and board. Thermal, in the strict sense, refers to double-sided encapsulation running at about 130°C, although the term is sometimes applied to any heated process.

For commercial print such as covers, menus and cartons, the common choice is a single-sided heat-activated film. Knowing which of the three processes a film was designed for is the first check before a roll goes on the machine.

Choosing the film material

The base material decides how the laminated sheet behaves afterwards:

• BOPP (biaxially oriented polypropylene) is polypropylene that has been stretched in two directions, which improves strength, water resistance and sometimes clarity. Film manufacturer Polyplex describes its polypropylene thermal films as suited to commercial work that needs one-side lamination, in thicknesses from 22 to 50 microns, with leaflets, posters, menu cards and carton boxes as typical applications.
• Polyester (PET) is often used for encapsulation, where thicker film, typically 75 to 250 microns, is applied to both sides.
• PVC appears in clear films and in white media for decals, and is classed as monomeric or polymeric, with monomeric being less flexible and having a shorter outdoor life.

Finish is a separate decision. Beyond gloss and matte there are silk, embossed and textured films, metallics, and soft touch films that give a very matt appearance with a velvet feel.

Why digital prints are harder to laminate

Toner-based prints are a known difficulty. Laminating supplier Skandacor explains that toner is typically a mixture of wax, silicone and oil, which makes it very hard to stick to, and that traditional laminates adhere poorly to digital prints because of toner and fuser oils. Films with a different, higher-tack adhesive are made for this purpose, and manufacturers list digital grades separately from their standard ranges.

Waiting helps as well. The same supplier recommends letting prints cure before lamination so that fuser oil can dissipate: at least 24 hours for offset prints and at least 12 hours for digital prints. A standard film may still work on digital output if temperature, speed and pressure are tuned carefully, but that is something to prove on test sheets, not on the live job.

Heat, pressure and speed work together

When film will not stick, the adjustments are simple and all related: increase pressure, raise the temperature so the adhesive melts properly, or reduce speed so the adhesive has time to activate. Paper condition matters too, and a relative humidity of around 50% for the stock is the usual target.

This is where the machine itself starts to matter. A sheet-fed automatic laminator built for short-run digital and offset work keeps these variables stable from the first sheet to the last. The SF-390A listed by Digital Finishing Systems in Athens, for example, is specified with a temperature range of 80 to 130°C, hydraulic pressure control, a maximum speed of 15 metres per minute, air suction feeding and automatic sheet separation, for films of 20 to 45 microns on paper from 130 to 400 gsm. Whatever the model, the point is the same: settings that can be read off a screen and repeated are easier to control than settings judged by feel.

Dealing with curl on single-sided work

Curl is the classic complaint with one-side lamination, because film on one face pulls against bare paper on the other. Skandacor’s troubleshooting advice separates two cases:

• End-to-end curl: engage the decurler bar or roller further, reduce the supply tension on the film, lower the roller pressure and bring the temperature down.
• Side-to-side curl: reduce supply tension, keep the stock at around 50% relative humidity and use short grain paper.

Film choice plays a part. Polyester is thicker and stiffer, so it tends to curl sheets when applied to one side only, while flexible polypropylene or nylon films are the recommended alternatives for single-sided jobs.

Checks before the job is delivered

Two quick checks save reprints. The first is patience with silvering: small trapped air bubbles can give a cloudy look straight off the machine, and FESPA notes that they will often disappear over 48 hours or so. The second is a pull test. A few hours after laminating, cut an “X” through the film only on a spare sheet and try to peel it back. If the film lifts cleanly, adhesion is not good enough and the settings or the film need to change.

It is also worth being honest with customers about end of life. Once film is laminated to printed media, the adhesive, the ink and the substrate all have to be treated differently, which complicates recycling. For work with a short life, a lighter finish may be the better answer.

A short routine that prevents most problems

Match the film to the process and to the print technology, give the sheets time to cure, run test sheets while adjusting heat, pressure and speed together, correct curl at the source and test adhesion before trimming. None of it is complicated, but skipping any one step is usually how a finished job ends up back on the press.

Sources

• FESPA, The Ultimate Laminating Guide: Films and applications. https://fespa.com/en/news-media/the-ultimate-laminating-guide-films-and-applications
• Polyplex, Polypropylene Thermal Lamination. https://www.polyplex.com/saralam/polypropylene-thermal-lamination
• Skandacor, Mastering Single-Sided Lamination. https://www.skandacor.com/blog/news-knowledge-1/mastering-single-sided-lamination-39
• Skandacor, UltraGrip vs Regular Laminate Films. https://www.skandacor.com/blog/news-knowledge-1/ultragrip-vs-regular-laminate-films-33

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