Of all the questions and issues surrounding the lamination of large format digital graphics, film selection is one of the toughest. What follows is a basic outline of the things you need to know in order to make better decisions when choosing films. The first thing to realize is that film is not a commodity, and films of the same thickness and appearance can often behave very differently. Price alone is not a good basis for film buying decisions either. Unless you know from experience that it will work on your media, buying the lowest price film can be a losing proposition. The cost of reprinting and refinishing the pieces ruined by use of the wrong film is much higher than the extra cost of buying the right film at the start. Laminating films can be broken into two broad categories — hot, or thermal films, and cold, or pressure-sensitive films.
THE THERMAL FILM SANDWICH
All laminating films are constructed in layers, like a sandwich. Knowing what’s in the sandwich of materials that go into a film will help you make better film selection decisions. Understanding which combination of materials will work best for a particular job is essential to successful lamination.
In thermal films, the sandwich includes a layer of base film and a layer of adhesive, connected by a very thin layer of primer to insure bond strength. The base film is usually polyester, and the adhesive layer is usually polyethylene with an outer layer of more aggressive and lower-melt plastics (such as EVAs, EEAs, EMAs and other additives). Hot films can be generally classified as low-melt (about 210 to 260 degrees Fahrenheit) or very low-melt (about 170 to 210 degrees).
A decade ago low-melt films were less common, and many 3-, 5-, and 10-mil thermal films had to be run at temperatures of 260 to 280 degrees Fahrenheit. These films used monopolymer adhesives, with no low-melt additives. Monopolymer adhesives are still in common use today in cheap 1.5-mil film for the school market. This type of 1.5-mil film is run at about 310 degrees. Most thermal laminating of digital output will use 3- and 5-mil films. Fewer applications will require heavier films, or allow the use of thinner films.
THERMAL FILMS ARE NOT ALL THE SAME
Adhesive aside, thermal films of the same thickness are not the same. It’s important to know the proportion or combination of film base and film adhesive. In a 5-mil film, for example, the base can be from one to three mils of polyester. The remainder of the film thickness is adhesive.
This combination is expressed as a number such as 3-2 or 32, where the sum of the two digits is the thickness of the film. The first digit is the number of mils of polyester, and the second digit is the number of mils of adhesive. So a 3-2 film is a 5-mil film with three mils of polyester and two mils of polyethylene and additives.
The cheapest 5-mil films usually have only one mil of polyester (1-4), sometimes less. When a thicker film has too little polyester, it tends to stretch too much during application. Stretching causes unwanted curl, waviness, or other problems. Look for a 1-2 combination in 3-mil films (less-expensive 3-mil films have less than a half-mil of polyester), a 2-3 or 3-2 combination in 5-mil films, and at least a 4-6 combination for 10-mil films (several other combinations are available).
One of the factors that make a better film more expensive is the higher polyester content. Polyester is more costly than polyethylene. Since higher quality films generally contain more polyester, and higher quality polyester, it follows they will be priced higher. Film prices are trending higher now because film is a petrochemical product.
Since polyester is a lot stiffer than polyethylene, a wide-format laminate with more polyester will need less support in a frame or on the lattice of a trade show booth.
If the job at hand calls for the most possible rigidity from lamination, a 7-3 combination 10-mil film would be in order. (An 8-2 is possible, but may be a special order.) More rigidity than that is probably going to require mounting.
CLEAR ON CLARITY
Another important characteristic of any laminating film is clarity. Obviously, the finished laminate should be clear, with no coloration, cloudiness, or contaminants. Clarity is a function of the quality of the polyester purchased for the film base, and a function of how the adhesive is formulated. Lower cost films are usually not as clear because they contain less polyester and/or lower quality polyester and adhesive.
Although a very large majority of thermal films for the digital market are made with a polyester base, some other base materials are employed. The use of vinyl in thermal films has probably seen the most growth. Because vinyl tends to stretch a lot with heat, these films generally include a polyester liner on top of the vinyl to prevent stretching, or they use a very low-temperature adhesive that could be described as a heat-activated pressure sensitive. When a liner is used, it is removed some time after lamination, perhaps at the time of display.
Other thermal film base materials, like polyethylene, are quite common, but not so much in digital graphics as in other segments of the graphics market. Polycarbonate, nylon and other unusual thermal bases will be seldom used in digital applications except in unique applications.
Summary of thermal film factors
• right film base and adhesive for the job
• ratio of base to adhesive layer
• application temperature
• bond strength on target media
TOUGH COLD FILM CHOICES
More often than with thermal film, you’ll want help with the selection of pressure-sensitive films. Cold film choices are harder because there are so many more of them —more base materials in common use, many more variables in the adhesives, and the additional choice of material for the release liner that is usually part of the cold film sandwich.
While wide-format thermal films are mostly polyester based, PSA films are made with vinyl, polyester, polycarbonate, polypropylene and many other materials. While vinyl is the most common base for cold films, there are many different types of vinyl that can be employed. These and the other variations in cold films make them more adaptable to different applications, but also make the film selection process much more complicated.
Most cold laminating films have a release liner because PSA adhesive is tacky at room temperature and would otherwise bond to the layer of film underneath it on the roll. The release liner also ensures a smooth adhesive surface, important to preventing air bubbles.
Polyester is used for the release liners in many high-end cold films or transfer adhesives because it leaves the smoothest surface behind when it is removed. Other often-used liner materials are polypropylene, polyethylene-coated paper or clay-coated paper. These are less costly, but generally not quite as smooth. There are an increasing number of linerless cold films on the market, and they are often applied with some heat to help the adhesive flow. The surface of the base film may be treated to allow the adhesive to release evenly.
Another complex decision in the selection of a cold film or adhesive is the type of adhesive to be used. Pressure-sensitive adhesives (PSA) may be acrylic or latex-based, and it is important to know the initial tack and the permanence of the adhesive. Tack is the strength with which the adhesive grabs a surface. If an adhesive is permanent, it cannot be removed without damaging the surface where it was applied. An adhesive can have a fairly high initial tack and be removable. Vinyl for vehicle graphics is a good example. A film or adhesive can also be repositionable, but permanent after curing. Curing time can vary, but is seldom more than a few hours. So an adhesive can be repositionable and permanent, or high tack and removable. A knowledgeable film vendor or mentor may often be needed to help select the right adhesive for a particular job.
For laminating and face-mounting applications, the clarity of the adhesive is very important. PSAs can be water-clear. If the target graphic is textured, the shear, or ability of the adhesive to flow, is important. An adhesive that flows more easily is less likely to capture air in the texture of the media.
COLD VS. THERMAL
Wide format cold films tend to be thinner than hot films, and the use of cold films in the 5 to 10-mil range is unusual except for special applications. Cold films don’t generally use the same descriptive conventions as thermal. The ratio of base to adhesive is less of an issue. A cold film with a 1-mil base and a 1.5-mil adhesive layer may be described as a 1-mil film by one person and a 2.5-mil film by another, so be sure to ask specific questions in your selection process.
Summary of Cold Film Selection Factors
• base film and adhesive performance matched to the application
• thickness of base and adhesive
• degree of initial adhesive tack, degree of permanence
• clarity of base and adhesive
• release liner material
For those who have not operated a laminator before, or for those who have only done hot lamination, there is an adjustment to be made when cold laminating, because it is generally a one-sided process. While most thermal laminators are made to apply film to both sides of an item, and allow encapsulation, most cold lamination is a one-sided process.
When laminating single-sided, hot or cold, if items are not as wide as the web of film, or if items are not overlapped or fed close together, exposed adhesive can hang up on the bottom roller and cause a wrap-around. Once out of the back of the machine, exposed adhesive may also stick to itself or to other images. The different techniques required to avoid wasting time and materials in this way are one of the factors that form preferences for hot or cold laminating.
Since quite a bit of digital output could be laminated either hot or cold, choices are sometimes made based on preference or on available machines or materials. In most applications though, either hot or cold will offer a logical advantage. Some applications cannot be done cold in a practical way, and some cannot be done hot.
The biggest differences between hot and cold films are in cost and adaptability. Cold films tend to be quite a bit more expensive, even in some cases when comparing thick hot films to thinner cold films. Thermal laminating films are typically one-third or one-fourth the cost of same-thickness cold films, and they offer the true encapsulation that cold films cannot. On the other hand, PSA films will adhere to a much wider range of materials, and they are available in a far greater variety of base film materials and adhesive formulations, making them adaptable to more types of applications.
Cost of materials dictates that the basic rule for making the choice between hot and cold laminating is to use hot film whenever it is compatible with media being finished. Cold film is required for any media that is too heat sensitive to subject to a thermal process. It is required with some inkjet inks and papers because they won’t bond to thermal adhesives. Thermal film with its usual stiff polyester base is not good for work that needs flexibility.
Typical applications for thermal film:
• offset and other press graphics printed on paper
• inkjet graphics printed on paper
• items that need to be encapsulated
• color copier output printed on paper
Typical applications for cold films:
• graphics printed on vinyl, polyester and other plastics
• graphics that cannot tolerate heat (with wax-based inks, for example)
• art or presentation work that needs the utmost clarity
• inkjet graphics
RIGHT FILM FOR THE JOB
The common denominator of all film selection is matching the film to the job. Both hot and cold films, for example, offer products specifically designed for prolonged exposure to sunlight and moisture. When finishing for outdoors, make sure to use films designed for the required outdoor duration. The type of primer, adhesive, and base film used must all conform to outdoor requirements. Most plastics need additional UV screening for assured long-term outdoor use.
Whether they are applied hot or cold, not all vinyls, polyesters, and other base films are the same, and not all are as fit for outdoor use as others. Just about any thermally encapsulated image will hold up for a short time outdoors, but that doesn’t mean all thermal films are outdoor solutions for longer terms. In all applications, adhesives have to be suited to the target ink and substrate combination and the need for flexibility or rigidity has to be considered.
A new trend in films is the greater availability of products with color and with special finishes, including leather, canvas, linen, patterns, and more variations of matte and satin finishes. Another welcome trend from film makers is better labeling of film, including information about matching components. With a reputable vendor’s assurance of a match between the ink and substrate combination and the film, waste losses and frustrations can be greatly reduced.
A laminating mentor can be tremendously helpful in preventing finishing problems, and one of the best sources of such a mentor is a reputable vendor. Not all sales or support people are knowledgeable enough to be good mentors, but it’s worth the time it may take to find one who is. If the information in this article generates questions that a salesperson can’t answer, ask for the names of others in the company who could help. Keep going up the ladder, or into another company, until you find a person who will consistently be available to provide the reliable information you need. Then reward that company with your business.
A final point needs to be made about film selection — you can’t select the right film if you don’t have the right laminator to run it.
Most big color shops will not want to limit their capabilities by having just one type of laminator. For shops that want the capability of laminating hot or cold with just one machine, there are many units available that will mount and laminate both hot and cold (see the Laminator Update in this issue). Machines that offer the most options will thermally encapsulate, or apply hot or cold films or adhesives to both sides of a substrate in one pass, or mount with hot or cold adhesives. And most important of all is your vendor. It is important that your vendor be there with training, support, and service, not just after the sale, but for the life of the machine.
Because of the range of available materials and techniques used in lamination, there may be exceptions to some of the statements presented in this article. If you have questions about your finishing problems, please contact your vendor, or feel free to contact the editors or contact the author.