by Jeff Peterson
The use of heat transfers for plastics decorating has continued to become a viable choice for a variety of applications. In simple terms, a heat transfer is a printed image on a release carrier in roll form. They can be used on all types of plastic materials and are also applied to vinyl, carpet, specialty fabrics, and even painted metal. Heat transfers allow the decorator to apply multi-colored images in one single pass using standard hot stamping equipment that is equipped with an electronic indexer to read the position of each transfer. The operator places the part to be decorated in a specially designed holding fixture. The indexer positions the decal over the part, and a die transfers the entire image to the product in a single stamp, which permanently fuses the inks to the product material. Although heat transfers are most commonly applied with a vertical hot stamping press, transfers can be applied with a roll-on press as well for cylindrical or curved plastic parts.
Probably the biggest advantage of heat transfers is that a molder/decorator can decorate multi-color images without becoming a printer. It is a completely dry process with a minimal amount of training needed to run the operation.
From a decorator’s standpoint, it may not seem important how heat transfers are printed on the roll or web. However, there are advantages of each process that could help you evaluate the best type of heat transfer for your specific application. Heat transfers are printed in one of three printing methods – screen printing, rotogravure, and flexography. Plastics Decorating will evaluate each of these different printing methods and discuss the advantages of each.
Screen Printing
The majority of heat transfers are screen printed, mainly because of relatively low set-up costs and the ability to achieve bright opaque colors. A screen printed transfer utilizes a different screen for every color required. The screen is composed of a mesh material made of either a nylon fabric or fine metal threads. Art is provided to the heat transfer company, who in turn manipulates it by computer to create the positives to produce the different screens. A positive is created for each color that is then burned into the screen material, which has been coated with an emulsion. After exposure, the screen is washed and the area where the image was exposed washes away, allowing the ink to pass through the mesh. The screens are then locked in position on the press. There is a separate station for each color and a drying process between each lay down of ink.
Screen printing can provide either spot (a specific color) or process color (black, yellow, magenta, and cyan). Again, the main advantages of screen printed transfers are low set-up costs for short to medium runs and bright opaque colors that can be achieved. Even white inks can be used and applied to a black surface without showing through. And because of the heavy ink deposit inherent with the screen printing process, it provides an actual dimension to the printed image. Quantities for screen printed heat transfers usually run from small up to 500,000. However, depending on the specific application, quantities can be even higher, especially if quantities are ran at different times as the customer needs them. Another advantage of screen printed transfers is their ability to pass extreme testing conditions due to the thickness of the label (i.e. automotive specifications).
The disadvantages of screen printed heat transfers is that the actual printing speed is much slower than flexo or rotogravure, and therefore, not as feasible for very large runs. It also does not provide 4-color process quality achievable with rotogravure, although this has continued to improve and is more than acceptable for many applications.
Rotogravure
The popularity of clear printed labels has increased the use of heat transfers in recent years and has opened up opportunities for extremely large consumer product applications. With this, rotogravure printed heat transfers have become a feasible option for certain situations.
The rotogravure process is a type of intaglio printing in which the image is actually etched into the surface of a metal cylinder. Tiny wells are engraved into the cylinder to create the printed image. When the cylinder passes over the fountain of ink, the excess ink is removed by a doctor blade, leaving the ink in the engraved wells to transfer to the web of heat transfers. The size and depth of the ink wells determines how much ink will be deposited on the substrate.
The number one advantage of rotogravure printed transfers is the extremely high running speeds that can be achieved once the press is set-up. If the application is for 500,000 or more, than you should definitely consider rotogravure as an option. Another advantage to rotogravure is the high quality print inherent with the process. Rotogravure printing produces some of the most beautiful printing available on the market.
As you can imagine, the downside of rotogravure is the high cost of set-up. This is why small to medium sized runs would simply become unfeasible because of cost. In addition, rotogravure printing applies a light ink deposit compared to screen printed transfers, meaning opaque colors cannot be applied to the plastic surface.
Flexography
Flexographic printing is achieved through the use of a flexible plate, usually made of rubber or photopolymer. The plates are photo-etched, leaving the printed image raised above the surface of the plate. The plate is then mounted on a cylinder that runs next to another cylinder called the anilox roll. The anilox roll delivers the ink to the flexo plate, which, in turn, prints the web of plastic or paper material. Flexographic inks are fast drying, making it an ideal process for printing materials such as plastics and foils.
Like screen printing, a flexo press is relatively fast to set-up, meaning it can be used for fairly short runs. In addition, it runs at high speeds (although not as fast as rotogravure), so it is feasible for long runs as well.
The drawback to flexo printed heat transfers is the overall quality of print you can achieve. Currently, the applications for flexo are for low-end type of products. However, the quality of flexo printing has continued to improve in recent years. And like rotogravure, flexography applies a light ink deposit. The opaque coverage found with screen printed inks is not achievable.
As mentioned above, heat transfers provide decorators an excellent alternative for applying multi-colored images without having to utilize a printing process themselves. You just didn’t know there was so much involved in how they are produced! Hopefully, the information above will help you select the best product for your specific application.
Plastics Decorating would like to thank Lee Johnson and David Klaiman of Si-Cal, Inc. (508-898-1800) and Gina Reilly of Electrocal (860-290-3900) for their assistance with this article.
