FAQ

Why lamination?

Lamination is a technology that allows for improvement of both the mechanical and optical properties of prints by application of optically transparent polymer foil under high pressure to one or both sides of the printed substrate.
Lamination serves to improve the properties of the printed substrate listed below:

  • Higher mechanical durability and toughness,
  • Better appearance of the printed document,
  • Higher resistance to abrasion, lower proclivity to corrosion from human skin,
  • Higher resistance of the print to UV radiation,
  • Impermeability against moisture.

Laminated sheets are easily workable in technology operations such as dry lamination, punch cutting, spot coating, etc.

What is the best way to laminate?

The result of laminating is subject to several major factors which also affect one another:

  • Selected lamination technology,
  • Laminating film,
  • Glue layer,
  • Laminating machine,
  • Printed matter (printed material, ink type used, printing method),
  • Single- or two-side lamination.

In consideration of the factors mentioned above, these are general principles for quality lamination:

Prior to lamination, the printed matter must be perfectly dry as the remaining solvents might react with the glue, which would lead to loss of strength in the affected locations.

High humidity delays the drying process. For example, at humidity levels 70-80%, lamination may not be performed until at least 24 hours after printing.

The adhesive power of the glue changes over time (it usually improves); therefore, any other finishing operations should be performed some time after lamination (the laminated material ought to be left to “rest”) for at least 1 hour.

Laminating film usually binds better with unprinted substrate than with the layer of ink. It is recommended that the laminating film should overlap into the print margin.

Laminating film should not be stretched during the process. The reason is that the film might have a tendency to relax: this could lead to wrinkling, loss of lamination, or other defects.

Cutting of laminated prints may cause local tension on the edges and subsequent loss of lamination (the effects of blunt blades, incorrect combination of film and substrate, rough handling). While cutting, it is recommended to leave the edge of the laminating film on the print as the film binds better with the substrate than with the printed area.

In single-face lamination, the tension generated between the film and substrate may result in loss of flatness (curling). Two-face lamination offers better protection of the printed material from the environment, especially from moisture and dirt.

Beware of dusty environments. The surface of the printed matter must be free of particles of the dusting media and dust. This is where the surface is not perfectly covered with the glue: this leads to the existence of air cavities that disperse the light; the phenomenon is known as silvering.

The selection of lamination conditions depends on the type and use of the printed matter; the selected method must be tested first.

The substrate for lamination should be smooth; the glue should only penetrate the top layer (i.e. not seep through the paper sheet structure to the untreated face).

Printing ink should be low in wax and should contain pigments that cannot react with the glue or the potential UV radiation.

Lamination of metallic prints tends to be complicated as the shape and material of metallic pigments leads to lower binding capacity of the glue layer.

How does the printed matter affect the quality of lamination?

Paper

Properties of the paper that affect the result of lamination:

  • Structure and surface properties of the paper sheet (smoothness or porosity),
  • Orientation of fibres,
  • Areal weight of the sheet,
  • Humidity,
  • Sheet size.

The smoothness or porosity of the lamination substrate affects the quality of the film’s binding to the paper (how easily the glue penetrates the paper structure) and the possibility of silvering. Refer to the structure and surface properties of the paper to adjust the pressure of the lamination process (depending on the paper pore size). For instance, use higher lamination process pressure in smooth and coated paper.

It is better to laminate paper sheets in a direction where the fibres of the paper are parallel to the lamination roller. It is not obligatory, however, according to our experience, this way prevents wrinkling much better.

The areal weight of the lamination substrate affects the likelihood of wrinkling and loss of sheet flatness (curling). In particular, sheets of low areal weight pose problems. Wrinkling may be eliminated by bending the sheet before the laminating roller using a bar. To prevent curling after single-face lamination, use a straightening bar to bend the laminated paper one more time.

If the sheets prior to lamination are high in moisture, the binding capacity might be reduced through catching of water vapour under the film. This generates an optical defect that is known as silvering. Absolute moisture of the laminated sheets higher than 10% leads to considerable reduction of paper strength, to waviness of the sheets and occurrence of wrinkles.

When laminating sheets of various sizes, the regularity of temperature on the laminating roller may vary depending on the temperature profile of the roller. When laminating sheets of small sizes, the true temperature of the lamination process might differ from the value recorded by the sensor as more heat is consumed in the centre of the roller than the edges where the temperature sensors are typically located.

Print Ink

Properties of the print ink that affect the result of lamination:

  • Type of printing ink (offset printing ink, digital print toner, special printing inks with specific optical and other effects, etc.) and the related composition of the ink (type of the binder structure and its presence in the printing ink, type of pigment),
  • Thickness of the ink layer (adhesive capacity of the printing ink),
  • Ink drying rate, method of curing.

The printing ink used and the properties of the ink layer are the key factors that affect the binding capacity of the lamination film or of the glue to the printed surface. Low binding capacity of the glue and ink layer may be caused by the reaction of solvents or other oils in the ink with the glue (the glue might lose its “stickiness”). The above is subject to the type of printing ink, i.e. it chemical composition, and to the thickness of the ink layer.

The ink thickness parameter is important especially when laminating printed matters made in digital printing equipment. When compared to the traditional offset printing method, the ink layer in the digital output is thicker and the ink does not penetrate the paper structure so deeply. In this case, it is important that the glue of the lamination film be applied continuously throughout the surface of the printed matter, especially in the vicinity of elevated print points.

If the printed ink has not dried completely, wrinkling may occur. The type of pigment used in the production of the printing ink affects the binding capacity of the glue and ink layers (the pigment structure in metallic inks deters the lamination process) or even changes the chromacity of the printed matter (some pigments may be heat-sensitive and the hot temperature of lamination might change the colour of the print).

Nature of Image

Properties of the printed image that affect the result of lamination:

  • Surface coverage of the substrate with ink (ratio of solid and unprinted areas on the sheet),
  • Rate of areas out of register (thickness of the ink layer),
  • Size of unprinted margins.

Laminating film binds better with unprinted substrate than with the layer of ink. The higher the area coverage with ink and the thickness of the ink layer, the lower the binding capacity of the laminating film will be.

The size of unprinted margins on the sheet is also very important. A small pocket of air exists in the location of the sheet margin under the laminating film; the pocket can be a source of complications if the margin is covered with ink and not clean substrate (full-format printing). The above is accelerated if the paper has high areal weight and the lamination process is fast. The roller pressure might extend an air pocket into a long band where the lamination has not glued.

Printing Method

The influence of the printing method on the result of lamination:

  • Finalisation of the printed matter upon extraction from the printing equipment (toner curing in digital printing equipment, dusting in offset printing, etc.).

Any surface treatment of the printed matter prior to lamination affects the final quality of lamination. Fixation oil on the surface of some digital prints, dust on offset prints, or even coating leads to lower binding capacity of the glue layer to the print and silvering occurs.

What is the thermal lamination process?

The lamination process creates a system of polymer layers: printed material – printing ink – glue layer – laminating film. The condition of good lamination quality is, above all, excellent adhesion of all individual layers and perfect coverage of the surface with liquid glue. The above suggests that the result of lamination is affected by these principal factors as well as the circumstances of the process as well as operations that have been previously performed in the printed matter.

Application of thermal laminating film onto a printed paper

What does the glue layer in thermal lamination film consist of?

The thermal lamination process uses hot-melt glues that are already incorporated into the manufactured film. Hot melt glues are thermoplastic materials which preserve their plasticity also in a glued connection. Under ambient temperature, they are solid and contain no solvents or water. Once heated, they liquefy and become sticky. In comparison to other types of glues, hot-melt glues have a short open time (time period after the application of the glue during which a quality connection can be established) as well as a short sealing time (the minimum time that is necessary to provide a good glued connection by pressing the glued materials together). While the glue is being applied onto the base film it is important to observe that the glue washes the surface of the polymer film, is flexible and clear, that it does not affect the flatness of the film and that the layer quality is uniform.

There is a wide range of hot-melt glues in existence (thermoplastic, rubber copolymers, polyurethanes); however, the glue with most frequent application in the printing industry is ethylene vinyl acetate (EVA).

Parameters of hot-melt glues:

  • Working temperature: 150-180 °C;
  • Viscosity of molten glue during application: 1000-10000 mPas;
  • Open time (gluing period): 2-60 s;
  • Sealing time (curing time): 0.5-15 s;
  • Softening point: 55-110 °C.

What heating systems are most popular in laminating machines?

Comparison of individual systems of the heating rollers provides a view of their efficiency and the properties that should be considered when selecting a lamination machine. Two types of heating systems for lamination rollers have become the most popular – electric and liquid circulation systems.

Electric Systems

Nearly all electric heating rollers used for laminating operate internal heating; this leads to temperatures higher within than on the outside of the roller. As electric heating rollers are typically controlled by temperature monitoring in the jacket near the surface, they are easier to control and respond more quickly than liquid circulation systems. Heat loss is limited primarily to loss by flow from the roller surface.

Compared to liquid circulation systems, electric systems use less complicated components. Downtimes caused by component failures are minimum and as no hot fluids or leaks with potential danger are present, the safety risks are lower as well.

Most electric heating systems use a resistance sensor (or a thermocouple fitted in the roller jacket) that is installed as close to the roller surface as possible. The signal enters the temperature control unit that supervises the amount of energy brought to the heating element. As the system has better thermal response, heat control is faster than in the liquid circulation systems; this reduces the temperature setting process only to the setting of the desired temperature.

Liquid Circulation Systems

Most liquid circulation systems use a tank and a heating element that increases the temperature of the liquid (oil or water), a control system, a heat-sensitive sensor (it is usually fitted near the tank and system controls), a pump, a rotary connector, and heat piping components for transportation of liquid to the roller.

Each of the components represents a maintenance point that requires regular inspection, repair, and on-hand storage to minimise downtime and safety issues caused by failure of the component given. Heating systems that use oil for heating of the roller carry yet another problem that lies in the disposal of toxic, carbonised oil that is the result of continuous heat recycling at high temperatures.

The heating roller of liquid circulation systems uses either a simple, single-wall design or, more often, a two-wall structure with spiralled partitioning to provide higher consistency of heat transfer across and around the roller. The temperature is usually read in the liquid tank, not in the roller; this arrangement means that the routine of temperature setting requires several trial-and-error stages. Liquid circulation systems are assumed to yield heat loss of 10–15% during transfer from the liquid tank to the roller.

Although most rollers specify the uniformity of heat distribution without load, heat distribution in the roller may vary greatly by the speed (heat load) or width of the film. The temperature of the roller surface will be similar to the inner temperature of the roller in locations outside the load (i.e. contact with the film) where heat load is low. Uniform distribution of temperature cannot be achieved without further measures.

What technology issues may be connected with heating of laminating roller?

An issue that is often present in all types of rollers with energy supply from within is roller overheating as the machine starts and stops. This problem can be quite easily resolved at machine startup by proper setting of the adaptation mode of the PI controller. A situation that is more complicated is overheating of a machine that has stopped from full-speed operation. This situation offers no easy solutions. The cause is the thermal resistance of the heating system roller-film for the heat flow in question. The resistance is based on the roller design and it results in a heat difference between the inside and outside of the heating roller; the difference is proportional to the heat flow. The required amount of heat flow depends on the preset roller temperature, lamination speed, heat conductivity of the paper, and on the thickness and other properties of the film. In situations close to the maximum heat flow the source of energy can generate, the difference between the temperatures on the inside and outside of the roller may be as much as 30 °C. Energy proportional to the heat difference accumulates in the roller wall; after the machine has stopped, the heat shows by increasing the temperature of roller surface by as much as one half of the difference. All manufacturers aim to minimise that resistance. Several sophisticated solutions exist. Komfi has decided to pursue the concept of minimising the roller wall thickness using a strength calculation for complex pressure load that acts on the roller. The approach also includes minimising the heat resistance between the heating element and roller by expanding wedges that provide excellent contact between the mentioned components.

An equally important problem of the lamination machine is the uniformity of surface temperature in the entire width of the roller for lamination of various width of film (sheet). The problem appears in all machines regardless of whether heat energy approaches the roller surface from within or from the outside. It is not difficult to achieve uniform distribution of temperature on the roller surface without load. Load causes the need to increase heat flow so that the required temperature is achieved at the location of the load. If the film covers only a part of the whole width of the heating roller, heat flow will cause the temperature of the uncovered part to rise although the temperature of the covered part remains constant. Heat conductivity of the roller will cause the higher temperature to expose the edges of the films and result in local overheating. Without any further measures, this phenomenon cannot be removed in any heating type.

Komfi has addressed the issue by separating the heating area into several zones. Each of the zones has its own heat sensor located in the roller wall in the middle of the respective heating zone; therefore, each zone uses a proprietary temperature control system. Machines for B2 size have three zones, machines for sizes B1 and B0 operate five heating zones.

Another possible solution is the use of a roller with high heat conductivity of the walls, e.g. heat pipes fitted in the roller walls.

What are the principal benefits of thermal lamination?

Economic Benefits

  • The result of lamination may be inspected immediately to minimise the scrap rate.
  • Machines using this technology are cheaper than machines for wet lamination.
  • The machine can easily be manned by operators without excellent levels of skill.
  • Stoppage caused by cleaning and maintenance is kept at a minimum.
  • The price of expandable materials (thermal films) continues to decrease in the long term.

Technological Benefits

  • The time between printing and laminating is short.
  • Higher quality of production without direct influence by the operator is achieved repeatedly.
  • Prints do not need to be cleaned from printer’s dust prior to lamination.
  • Prints may be laminated on both faces immediately without waiting for the glue to dry.
  • Thermal lamination technology is suitable for processing digital prints.

Flexibility

  • Prints can be further processed almost immediately after lamination.
  • The times required for transition from one production batch to another are minimised.
  • High flexibility of production may be achieved in line with the general trend of reducing the costs of printing orders.

What factors affect the quality of lamination over digital print?

Although lamination of prints prepared in digital printing equipment is subject to the general principles of good lamination, the paramount role in this system belongs to the printing ink used as well as the lamination film.

Interaction between the printing ink and glue may improve as well as worsen the lamination process and the final appearance of the product. In order to clarify the interaction between the inks and glues used in lamination, it is necessary to first classify the glues by their chemical structure and reaction type. Each glue type hardens in a different type of reaction; the application of the glue depends on the nature of the materials being joined and on the required properties of the connection. It is also due to the high number of manufacturers that the formulae of printing inks show a higher degree of diversity than the glues in relation to the printing method, required properties of the print, and the mechanism of curing/drying of the ink layer. In spite of the complexity and high diversity of both systems, potential reactions between the glue and printing ink may be characterised as follows:

  • Residual solvents either in the printing ink or glue may have a negative effect on the quality of lamination. For example, the presence of solvent in an insufficiently dried layer of ink in the print may cause the glue to reduce its binding capacity to the substrate or the adhesive rate in the connection. The result is a lower heat and chemical stability of the laminated product. Furthermore, some lamination films may absorb solvents, which will be demonstrated by defects in the laminated areas.
  • Mutual solubility of ingredients found in printing ink and glue may also hamper the good lamination process. It is possible that the resins and other compounds of the glue may be soluble in certain chemicals that are present in the ink. Direct contact of the ink and glue may subsequently smudge the ink of the print.
  • Interaction between the ink and glue may be caused by considerable shearing forces that occur during laminating. In systems where the initial mechanical strength of the connection is low, the mechanical force may lead to defective lamination. Mutual displacement of the bodies being glued leads to mechanical reaction of the ink and glue or to a change in their positions. Therefore, shearing forces may, in some systems, promote interactions that are already present in those systems.

Digital printing most often uses dry toners, inks, or liquid toners. Digital printing technology, as indicated above, does not transfer ink to paper by pressure acting from the roller. If present at all, as in xerography, pressure is not generated until the toner is being fixed to the printed substrate. When compared to the traditional offset printing method, thickness of the ink layer in the digital output is higher and the ink does not penetrate the paper structure so deeply. As such, it is important that the glue of the lamination film be applied continuously throughout the surface of the printed matter, especially in the vicinity of elevated print points. Apart from the ink layer thickness, an important factor of lamination is also the composition of the printing ink in terms of solvent content or the final oil fixation of the ink on the substrate. This is the reason why adhesion of the layer of printing ink and glue is probably the most problematic part of lamination of digital prints.

The lamination process must necessarily use a film that corresponds to the type of printing ink and print method used. The lamination machine itself is also important as well as the parameters of the process, such as the pressure induced by the laminator and selected lamination temperature. The selection of the lamination machine for processing digital prints ought to consider smaller units (in respect to the typical formats of prints prepared in digital printing equipment as well as the quantity of prints to be laminated). The setup of the laminator in the beginning of the run must, especially when processing digital prints, be selected with the aim of scrapping as few sheets as possible. That is why most prints prepared in digital printing equipment are laminated in smaller machines with manual feeding of sheets. Considering their quantity, sheets made in offset printing machines are typically laminated in fully automated lamination machines. The setup of this type of laminator is easier as there are plenty of starting sheets supplied in the case of offset printing.



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