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:
Laminated sheets are easily workable in technology operations such as dry lamination, punch cutting, spot coating, etc.
What are the factors affecting lamination?
The result of laminating is subject to several major factors which also affect one another:
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 or more.
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:
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:
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). So the ink supplier should be intimated about the sealing type while placing order for ink.
Nature of Image
Properties of the printed image that affect the result of lamination:
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 (bubble) 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.
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:
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
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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. Some manufacturers 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.
Some manufacturers 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
Technological Benefits
Flexibility
What causes laminating bubbles?
Few issues look worse than when bubbles appear under the film. Bubbling can occur for many reasons, the most common of which are listed below.
Tension
Lack of supply roll tension can cause the film to not be taut as it enters the nib. This insufficient tension causes bubbles when air gets trapped between the film and image. Too little supply roll tension will cause bubbles before it will cause wrinkles in the film. Insufficient tension easily is fixed by adjusting the tension of the feed spool to the proper specifications for the type of film being used. For instance, thinner film needs less tension.
Speed
Feeding too quickly causes bubbles if the item is pushed into the nib faster than the speed of the laminator. The solution to this problem is too obvious to mention.
Heat
“Hot spots” are a key problem when laminating inkjet prints. Inkjet printers dump a lot of ink onto the print and a rushed operator may try to laminate the image before the ink is completely dry. The problem can be exacerbated by the fact that, if a laminator is allowed to sit for extended periods of time without the rolls turning, there can be varying degrees of temperature on different parts of the rolls. This creates hot spots. When running a laminator at 85 degree C, the point where the rolls sit together easily can become a hot spot of +95 degree C. When a moist inkjet print encounters one of these hot spots, the ink easily can boil, creating bubbles. Among the solutions to heat-related bubbling is not to let the rolls sit idle for extended periods of time, to speed up the laminator so less heat gets into the printer or to work with a temperature about max 40 degree C. Always use the appropriate pressure.
Silvering
Silvering is caused by the capture of tiny bubbles of air, which look like hazy or reflective areas in the lamination. It may go unseen over light areas of an image, but will be more obvious over dark or black areas. Silvering usually is caused by too low an operating temperature and most often is seen when a machine is not given time to warm up or if the film temperature drops too low. When a large machine is warmed up without the motor on, one side of each laminating roll remains relatively cool. When lamination is begun, a pattern of repeated silvering can be seen each time the cool side of a roller is applied to the lamination. “Cool spots” are areas that aren’t hot enough to melt the adhesive. If bands of silvering alternate with bands of properly adhered areas, it means one part of the roller was too cool.
Be sure to pay attention to any defect which repeats. This is typical of debris, damage or low temperature on a section of the roller. Bubbles also can be caused if the adhesive doesn’t bond properly to the image. It can look like a sheen on the image. Bonding problems also can be caused by inadequate heat, inadequate or uneven pressure or too much speed. Inadequate pressure can cause silvering if the film isn’t pressed onto the image hard enough, preventing the adhesive from properly bonding.
What causes wrinkles during lamination?
The film will wrinkle if there is not enough supply roll tension. Every laminator needs a certain amount of resistance in the unwinding of the supply rolls so that the film lays flat as it goes into the nib of the laminating rollers. Thinner film needs less tension, thicker film needs more. Also, respect the laminating temperature of 40°C to avoid wrinkling due to damp materials during the laminating process. The moisture content of the printed substrates must be the lowest.
Uneven film thickness due to extrusion process, board waviness and overlap in higher gsm are few more factors causing wrinkles.
What are “orange peel” wrinkles?
The industry term “orange peel” refers to very fine wrinkles or waves in the laminate. This is a common condition resulting from too much heat. Simply adjust the temperature to a lower setting. NOTE: A quick fix can be to speed up the laminator. As the film runs at greater speeds, the actual film temperature is reduced. Some orange peel may occur when laminating prints with a lot of solvent residues or oil-based prints.
Why does waving/blistering/dimpling occur?
These occur from the natural moisture in the item making it swell during printing. When too much heat is used, the moisture is dried out, but the areas that swell prevent the item from lying flat. They can also be caused by too little tension and uneven pressure.
How does pressure affect lamination?
Users should start with as little tension as possible and then add more as needed rather than starting with a lot and loosening up. Some of the most common problems with laminated prints due to the wrong amount of tension or pressure are as follows:
Infeed waves are mars in the finished product that result from the original image being somehow distorted before it went in. For instance, if there is insufficient tension on the image as it is going in, it could end up crooked or folded as it enters the nib of the rolls. When feeding an image into the laminator, make sure it is parallel to the roll. Once it gets pulled in, grab the back of the image and hold it taut so any wrinkles get smoothed out before they enter the nib.
Print curl occurs when an image is unrolled and wants to roll itself back up again. A little bit of this is normal, but if the print doesn’t lay flat after some gentle smoothing, it is a problem. Print curl happens when one side of an encapsulated image changes size out of proportion to the other side, causing the print to curl towards the smaller side. This can be caused if the user puts a heavy laminate on the top side and something thin and cheap on the back, but doesn’t adjust the tension to allow for the different dimensional stabilities of both types of film. For instance, if the user applies the same tension to both the top and bottom film, the bottom film will likely stretch more than the top since it is weaker. As the image cools, the bottom film will tighten up a little more than the top film since it was stretched farther, causing the image to curl towards the back.
Boat wake is an undesirable ripple pattern in a “V” shape on a finished print. Boat wake is caused by several things. For example, when trying to encapsulate something thick, the edges of the film hanging off the sides of the print will not get as much pressure, so they’ll drag behind, causing the ripple pattern. “Reverse boat wake” can be caused when too much pressure is applied to the sides of the roller, forcing the middle up and causing the film to drag behind the film on the sides.
Ripples occur when too much pressure is exerted on the item when it goes through the rolls on the way out of the laminator while the film is still hot. To prevent this, make sure the pressure is at the proper ration of speed for the pull and front rolls. Make sure to work with a low temperature without exceeding 40-45 degree C.
How can static electricity build-up be handled?
Static electricity is an issue that may arise and is impossible to prevent. However, it easily can be kept under control by taking a few precautions.
Removing or neutralizing static electricity by induction is the oldest and simplest method. Tinsel is the most common tool for this application. However, tinsel oftentimes is misused and, therefore, sometimes not successful. When using tinsel, it must have a metal core. Tinsel with a string (non-conductive) core will not function properly. Secondly, the tinsel must be well grounded electrically, stretched tight and placed ¼ of an inch from the material to be neutralized. There also must be “free air space” under the material to be neutralized, directly under or over the spot where the tinsel is placed. If done properly, the tinsel will reduce static electricity on both sides of the static-laden material.
In addition, grounding all plant machinery and related equipment is most important. Besides the safety factor, a grounded machine will help drain off extremely high charges of static electricity from partial conductors.
Static electricity is an issue when laminating lighter weight films since they have a tendency to curl back towards the rear rolls. The film then can begin to wrap around the rear rolls. To alleviate this problem, put the laminator in reverse and gently pull the wrapped film off the rear rolls. To prevent wrap around, laminate all documents using a continuous feed as the weight will hold the laminations away from the rear rolls or leave a few inches of extra film as the weight will not allow the static to pull the film back into the rolls.
What causes repeating marks?
Repeating marks occur when there is something on the roll, usually a piece of paper or a scrap of film. If the defect is adhesive residues, simply clean the rolls.
What does the Technical data sheet of lamination film talks about?
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