Selecting the right vinyl : cast or calendered
You may ask the question why do pressure sensitive film manufacturers have so many different types of films? Why is there such a huge range of different vinyls, let alone polyesters, reflective and other specialty films?
Well the simple reason is that,
no one type of film will do it all!
David Newman, technical marketing representative, Avery Dennison – Graphics Division Australia and
New Zealand – elaborates on
‘why to select the right material.’
Whether you are new to the graphics industry or have been around for many years, you have most likely heard terms such
as cast, high performance, durable, long-term, polymeric calendered, intermediate, monomeric calendered and promotional. It is safe to say
that most people may not know the actual ingredients or manufacturing processes for these films. They do, however, know that a promotional film should not be used for a vehicle wrap and that a cast or high-performance film is overkill for a point-of-purchase (PoP) display that will only last for six months to one year. Let us explain the differences between cast and calendered films as well as give examples of where each type of film would be used.
Most vinyl films are made from the same basic raw materials. We begin with polyvinylchloride (PVC) polymer, which is simply basic plastic, and is, by nature, relatively rigid. Other ingredients are then added to the PVC. These ingredients include plasticizer to make the film flexible; pigment to make the desired colour; and additives to help achieve specific properties such as UV absorbers to improve resistance to UV radiation, heat stabilizers, fillers and processing aids. These raw materials can be chosen from a wide range of quality levels. Of course, for a film with limited durability, often the least expensive raw materials are chosen.
Apart from the type of raw materials that are used, the manufacturing process and the type of plasticizer used create the main differences of vinyl films. Vinyl films can either be made by calendering or by casting. Each of these processes renders different qualities of films. The grade of plasticiser that is used to make the film flexible also greatly affects the grade of the film. Generally for pressure-sensitive adhesive films, a choice is made between polymeric and monomeric plasticisers. We won’t go into detail on the plasticizers in this article, but for simplicity’s sake consider polymeric to be the higher grade and monomeric to be the economy grade plasticizer. The combination of these factors greatly determines the durability of vinyl films.
Cast films, also known in the industry as durable films, are considered to be a premium product with excellent durability and conformability characteristics. The term “cast” refers to the manufacturing process of this type of vinyl. The vinyl begins as a range of ingredients known as the formulation, which includes solvent. These materials are added to a mixing churn in a predetermined order while mixing at specific speed and for a set amount of time to ensure a complete and consistent mixture. This liquid mixture, known as organosol, is then “poured” or cast onto a moving webbed carrier known as the casting sheet and is then processed through a series of ovens, which allows for the evaporation of solvents. When the solvents are evaporated, a solid “film” is left behind. The film is then wound up in large-diameter rolls for subsequent adhesive coating. The casting sheet determines the texture of the film.
Because the vinyl is cast on the casting sheet in a relaxed state, this material offers very good dimensional stability. This process also allows the film to be very thin (most cast films are 50 microns), which helps with the conformability of the product. Material manufacturers recommend the use of cast films on substrates such as fleets, vehicles, recreational vehicles or boats where the customer wants a “paint-like” finish that will last a long time, usually five to ten years, depending on how the film is processed.
Advantages of cast films are numerous like shrinkage is the lowest of all vinyl films because the “casting sheet,” not the film itself, is pulled through the machine. Since the film has not had any stress applied during the manufacturing process, it does not try to resume or shrink back to its original form. Durability of cast films is generally higher than that of other vinyl films due to the manufacturing method and the raw materials used and cast films can be made very thin, which produces a conformable product that allows application over substrates with rivets, corrugations, and complex curves. Also, once applied, this low caliper makes the graphic less vulnerable to abrasive forces.
These films also maintain their colour and other properties better than other vinyl films. This results in better performance of pigments and UV absorbers and the manufacturing process of cast films makes it easy to run small productions of custom colours for colour matching. It is relatively easy to change colour during production making colour matching in small batches possible.
And like cast, calendered film also gets its name from the manufacturing process. These films may also be referred to as intermediate or promotional. Calendered vinyl is formulated with similar raw materials as cast, except that no solvents are used. The batch is mixed and heated to a molten state that resembles pizza dough. Once the film reaches this molten state, it is extruded through a die and is then fed through a series of calendering rolls. These polished steel rolls progressively squeeze and stretch the vinyl into a flat sheet. Because the film is stretched into shape, it has some degree of memory and, therefore, is less dimensionally stable than cast vinyl films. This means that when a calendered film is exposed to heat, the film will have a tendency to shrink or pull back towards its original form. Calendered films also tend to be thicker (usually 80 to 100 microns) than cast films because of the limitations of the calendering process. Unlike casting where a textured or smooth casting sheet is used to produce the film finish, calendering implements a special finish cylinder at the end of the process while the film is still warm. This process is extremely fast and is ideal for bulk production runs. Therefore, colour matching is not very easy on these machines. However, due to its bulk production with high yields, calendered films are relatively inexpensive.
The quality of calendered films can range from economy to intermediate with durability of one to five years. These films generally are not recommended for vehicle applications because they are thicker, less conformable and less durable than cast films. Whereas advantages of calendered films include greater production yields equals less cost; stiffer/thicker film equals easier handling and application characteristics; and thickness of film increases resistance to abrasion.
As with anything else, the finished product is only as good as what you put into it. This begins with choosing the right vinyl for the job. If you are doing a full vehicle wrap where you want the graphic to conform so that it looks and performs similar to paint, you should choose a cast material. Calendered films are ideal for applications that do not require the film to stretch or conform around contours. Examples of calendered film uses would be floor graphics, wall murals, and POP displays.
