by Jeff Peterson
In all plastic decorating processes, the keys to a successful job are a smooth, clean print with fast production speeds and a small number of rejects. For hot stamping applications, the crucial aspect to making all of this happen is choosing the correct hot stamping foil. Many decorators are unaware of the vast number of different foil products that are available on the market for different plastics applications. And although you might struggle and get a hot stamping foil to finally work, analyzing the specific application can save hours of set-up and makeready time, and help considerably decrease your spoilage, which can be extremely important for larger, expensive molded parts.
The first decision you must make is what type and color of hot stamping foil is required. The type of foil will be either a pigment or metallic foil. Holographic patterns could be a third type; however, holographic foils do have a metallic layer and can be categorized as metallic. Pigments are available in a number of different colors in both a gloss and matte finish. Matte pigments are most generally used for marking or identifying while gloss pigments are chosen for decorative as well as marking applications. Metallic foils are also available in a variety of colors, although gold and silver are the most common choices. Because gold and silver are the most common, there is usually a much broader range of foil choices for challenging applications. Special color matches can be done for both pigment and metallic foil products. However, this can be quite costly and requires a minimum order. Unless the usage is going to be very large, it is best to try to find a standard color your foil manufacturer carries.
Once the type and color of the foil has been chosen, a good next step is to determine the foil coverage required. Does the image being hot stamped have fine detail, medium, or heavy coverage? Foils are available with different adhesives (sizing) for the type of coverage required. If the adhesive is too thick, lettering and small images will “fill-in” and cause a flooding effect. If it is a light adhesive made for fine detail, the foil will leave unstamped areas on a medium or heavy coverage application. For example, the coatings used in a wood grain foil are up to two to three times thicker than those of a metallic hot stamping foil used for a cosmetic mascara container. The thicker layers do not allow a sharp crisp edge. Therefore, wood grain designs are applied more as surface decoration in a roll-on type application verses fine detail images and type found on many cosmetic containers.
Specific Resistant Characteristics – There are many plastic hot stamping applications where the environment the product will ultimately be exposed to can have a dramatic impact on the hot stamping foil you choose. These influences are generally categorized into three surface challenges: chemical, physical, and weather. Exposure to these factors should be a strong consideration when choosing the type of foil for your application.
Chemical Resistance – This is very important for cosmetic and other consumer product applications where the foil must have resistance against solvents, acids, oils, grease, plasticizers and perspiration. The stamping foil surface should not be affected over a limited period of time when exposed to these substances. It is always recommended to conduct tests with the specific chemicals or substances that the foil stamped area may be exposed to.
Physical Resistance – Abrasion, mar, and scratch resistance, as well as temperature cycles are all factors that influence the quality of the protective lacquer layer of the hot stamping foil. Advertising specialties that display important company information are a good example of where an abrasion/scratch resistant foil is very important to assure that the stamped image will last over a reasonable period of time. One thing to keep in mind is that high abrasion resistance normally means a reduction of the clean cutting of the foil, because of the required thickness of the protective lacquer layer.
Weather Resistance – Automobile parts, outdoor signage, and outdoor containers are examples of plastic products that are commonly decorated with hot stamping foil and are exposed to the elements. There are specific foils that are formulated to withstand humidity, water immersion, salt, sunlight, as well as other weather conditions. The foil you choose should withstand weathering influences for a reasonable amount of time without blister, loss of brightness, or adhesion.
With certainty, the most important decision when choosing a hot stamping foil for a plastics application is to match the type of plastic with the foil formulation. Conventional plastics are categorized into two basic families. They are referred to in the plastics industry as rigid plastics (styrene, acrylic, PVC, acetate, etc.) and polyolifins (polypropylene and polyethylene). Although this covers the majority of hot stamped plastics, new advanced-engineered plastics are now being developed, such as delrin and surlyn, which create new challenges for hot stamping.
With matte or gloss pigment foils, the type of plastic is usually not as much of a concern as with metallic foils. Many pigment foils formulated for plastics will work equally well on both types. On the other hand, metallic foils are much more sensitive to the type of plastic they are being stamped on. A metallic foil designed for use on a styrene bottle will not stamp well on a polypropylene (PP) bottle. And with more advanced resins, the foil developed for polyolifins may work, but not always. Many times, more heat is required to achieve a proper foil stamped image, which, in turn, can create another challengethe foil will begin to dull or burn.
Another big difference between the rigid plastics and polyolifins is that, in most cases, rigid plastics do not require any pre-treatment methods before hot stamping where as Polyethylene (PE) and Polypropylene (PP) usually do. Common pre-treatment methods include chemical priming, corona treatment, electrical surface treatment, and flame treatment (see Challenges with Screen Printing Polyethylene and Polypropylene in the February/March issue of Plastics Decorating for more information on pre-treatment methods).
If you are unsure of what type plastic you are decorating, there are ways to test the plastic to determine what type it is. One common method is to use a flame and burn the plastic material.
In addition to hot stamping foils that are formulated for different types of plastics, different coverages, and for certain resistances, foils are also formulated to work under different stamping temperatures. This is influenced by the type of die used (metal or silicone rubber) and the type of plastic. For instance, it requires a higher temperature foil to stamp PP or PE than a rigid styrene or acrylic. A metal die, in many cases, is advantageous because it holds its heat much better than a silicone rubber die and it actually debosses the image into the plastic. Of course, silicone rubber dies are necessary if you are stamping an uneven or contoured part. It is best to try to use a low temperature foil for these applications to help preserve the life of the rubber die.
Another important aspect of the stamping temperature to remember is that set-up results may not be the same as when you start production. You may need to increase the temperature setting once you begin production because the speed of the hot stamping press will lower the stamping temperature. If you are using a low temperature metallic foil that worked well during set-up, the increase in temperature during the actual run could cause the foil to begin to dull or burn.
As you can see, there are many factors that must be analyzed before selecting a hot stamping foil for your plastics application. Selecting the right foil can be the difference between a successful job and a potential disaster. The best advice is to gather as much information about the plastic and its final use and work closely with your hot stamping foil supplier to determine the best foil to achieve the best results.
Plastics Decorating would like to thank Tatiana Espinal of Foilmark, Inc. (978-462-7300), Jennifer Hoyt of API Foils (800-255-4605), Sam McElree of Kurz Transfer Products (800-333-2306), and Art Maynard of Webtech, Inc. (609-259-2800) for their assistance with this article.