Ins and Outs of Hot Stamping v. Heat Transfer
by Keith Hillestad, United Silicone
The hot stamping industry as we know it today began in the 1940s with the introduction of roll leaf, which was hot stamping foils manufactured in roll form. The early hot stamping foils used acetate or cellophane as the carrier. Hot stamping applications greatly expanded through the late 1940s as the first vacuum metalizing of a carrier film was introduced in Europe. Simulated gold foils were used primarily in the graphics industry where stamping on silk, paper, and leather became increasingly popular. The first mechanical and pneumatic hot stamping machines also were introduced at this time. The 1950s brought the introduction of pigment foils, as well as the introduction of foils to the plastics industry. Appliance, automotive, and consumer electronics were the first industries to utilize hot stamping to enhance their products. The mid-1960s found the introduction of printed pattern foils like marble, wood-grain, granites, leathers, and other texture-look patterned foils that became increasingly popular as the plastics industry boomed.
Heat transfers were first introduced in the early 1960s. The popularity of heat transfers grew rapidly with the capability of doing multi-color decorating in a single application. Some early applications included checkbook covers and eyeglass cases followed by logos on golf balls. Heat transfers were introduced to the plastics industry, namely consumer electronics, in the early 1970s. The process gained in popularity as manufacturers realized they could not only eliminate multiple set-ups for multi-color printing but also, the decoration could be done at the molding press rather than in a separate area. This process provided labor savings, reduced overhead costs, and improved the quality of parts produced.
The overall capabilities in production of hot stamping foils and heat transfers, along with the improvements to the equipment, have allowed the processes to evolve from very temperamental, unknowing, trial-and-error-type processes to highly sophisticated, consistent, quality processes. Today, hot stamp decorating is one of the most widely used processes due to the convenience, versatility, and results produced.
Hot Stamping v. Heat Transfer
Hot stamping is a dry process. There are no EPA concerns, storage problems of flammable materials, offensive odors, ink mixing, and messy clean-up. The environment including heat and humidity do not affect the process. The decorated part is ready to handle or package immediately from the press. Hot stamping will produce a variety of finishes on a wide selection of materials and is the only dry process where bright gold and silver metallic finishes can be produced. In addition to matte and gloss pigment colors, Day-Glo colors, wood-grains, brushed effects, and multi-color graphics can be applied. Besides plastic materials where the emphasis is on thermoplastics, additional items including thermosets, wood, book cloth, leather, textiles, paper, cardboard, and pre-painted metals can be decorated by utilizing the hot stamping process with a large degree of success.
Shapes including flats, cylinders, slightly conical parts, and simple compound curves can be decorated with hot stamping. Smooth surfaces, light textures, multi-level configurations, raised graphics, and beads also are hot stamped. Fine detail graphics to bold/solid coverage also can be hot stamped. New colors or designs involve changing a dry roll of foil on the hot stamp press and artwork changes involve a die change. Hot stamping is a permanent decoration with excellent adhesion and abrasion resistance. Advancements in foils have allowed the decoration of almost all plastic materials in production today.
Like hot stamping, heat transfer is a dry process where the application of single- or multi-color artwork is applied to the part. The largest advantage of heat transfers is that the printing and registration of artwork is done on a high-tech press where multiple colors, or layers of the transfer, are printed in registration to each other. Image quality, along with the art registration, can be inspected prior to application of the heat transfer to the part, thus reducing scrap. The capability to print fine detail graphics along with bold/solid images allows the heat transfer process the ability to apply the transfer to the part during the same application. New colors or designs involve changing a dry roll of heat transfers on the heat transfer press.
Heat transfers are an engineered product where custom colors and formulations are incorporated in the design. Colors are easily matched from PMS numbers or submitted samples. This eliminates the variable color shift that is possible when working with a direct printing process. The formulations are consistent because inks are formulated in a single batch, at a location by engineers/chemists, and are specific to meet the customer requirements and specifications for the substrates being decorated. Formulas specific to the individual product include chemical and abrasion resistance, UV stability, adhesion specifications, and other exact requirements depending on the specifications for the product being decorated. As with hot stamping, shapes including flats, cylinders, slightly conical parts, and simple compound curves can be decorated with heat transfers. Smooth surfaces, light textures, and a variety of substrates can be decorated as well. Heat transfers produce a permanent decoration with excellent adhesion and abrasion resistance. Advancements in ink chemistry have allowed the decoration of almost all plastic materials in production today, in addition to many other substrates including fabrics, metals, and glass.
Both processes, hot stamp and heat transfer, are applied in much the same way both processes require the correct combination of heat, time, and pressure. Heat is determined by the substrate or part being decorated in combination with the foil or transfer being applied. Time is determined by the size and type of image or graphic fine, medium, or bold. Pressure is required to ensure the correct contact of the die to the part. It is this contact that then allows the heat and time to transfer the image to the part.
The difference between hot stamping and heat transfers is where or how the art/image is produced. With the hot stamping process, the image is in the stamping die and generally a solid color foil is used. With the heat transfer process, the image is pre-printed on a carrier, and the entire image is transferred to the part being decorated.
Selecting the Right Machinery
When selecting a press for the application of hot stamp foils and heat transfers, many variables must be considered. Some of the issues that need to be addressed include the following:
Graphic. What is the size, or area, and type of graphic that is going to be applied to the part? To determine the required pressure to apply the hot stamp foil or heat transfer, the following general rule of thumb can be used:
- It takes 350 PSI of contact area of the silicone rubber die face to hot stamp graphics.
- It takes 500 PSI of silicone rubber die contact area to apply a solid coverage hot stamp area or heat transfer.
- It takes 1,000 PSI for contact area of metal dies (magnesium, brass, or steel).
2″ x 4″ silicone rubber die with 60% artwork area.
2″ x 4″ = 8″sq. x 0.60 = 4.8″sq. x 350 (PSI) = 1,680 PSI required.
1,680 divided by 2,000 (pounds per ton) = 0.84 tons of stamping force. This application would require a minimum 1-ton stamping press.
Consider the same artwork area of 2″ x 4″, only this time it is solid coverage for a heat transfer application.
2″ x 4″ = 8″sq. x 500 (PSI) = 4,000 PSI required. 4,000 ÷ 2,000 = 2 tons of stamping force required. This application would require a minimum 2½-ton heat transfer machine.
Part Size. What is the physical size of the part being decorated?
- How high (tall) is the part, and will the hot stamp press have enough daylight or space under the stamping head to allow the part to fit? Will there be room for the required tooling, part nesting fixture, and stamping die, in addition to the stroke of the machine?
- Where is the graphic being placed on the part? Will the press have enough throat depth or space from the centerline of the graphic (where the stamping die should be mounted) to the uprights supporting the head?
Solutions to these issues include slide tables that can be mounted to the press to accommodate parts that must be supported on a tall and/or hard-to-load fixture that offers support to the decorating area. Custom machine frames can be made to accommodate most parts. Overhead mounting of the stamping head can be used to accommodate the parts that would not fit into a standard machine frame. Note: multiple parts being decorated at the same time also would require a larger press.
Part Shape. The part shape, along with the graphic requirements, will determine if a vertical, peripheral, or roll-on machine is required.
- The general rule for applying graphics with a vertical machine is that up to 90 degrees of the circumference can be applied. Tooling to hold the part and dies to apply the graphic must be manufactured to exact tolerances to fit the part.
- Application of graduations 90 degrees or more around a circular-shaped part would require a peripheral decorating machine to apply the artwork. More than that amount of circumference area requires either rolling the part under a flat die or using a roller to apply the graphics.
- For applying heat transfers with a vertical heat transfer machine, up to 15″ sq. of decorating area can be applied. Above that area, it is recommended that a roll-on-type machine be used. Roll-on equipment is used for large area applications that would lend themselves to air entrapment if applied in a vertical motion.
- Parts that are manufactured by the extrusion process require the use of equipment designed for continuous application of foil or heat transfers in-line with the process equipment.
Heated Head Size. Sometimes the graphic area on a part is small, but the area may be split between two or three different locations on the part. This type of application may require a low stamping pressure, but a large head to accommodate the die size and location. Custom head sizes can be made to fit special applications. An example would be if the decoration needed to be applied down into a recessed area on the part.
Foil Feed. A larger press or head size may be required to feed more than one foil at a time. Hot stamping on parts with different color foils applied to them at the same time using a musical chair fixture would be an example.
Other Parts. Is the machine dedicated to one part, or are other parts going to be decorated on the same press? When additional parts are going to be decorated on the same press, considerations should be made for tooling to be built with common stamping heights and standard mounting holes to facilitate quick tooling/part changes.
Production Rate. Will the parts be decorated one at a time, offline, or in cycle with a molding press? Will multiple parts be decorated at the same time? For multiple-up decorating, the time required for loading and unloading of the press becomes a concern v. cycle time of the press. Larger presses usually require longer cycle times due to the mass of the head assembly moving. Most often, a smaller press with a small head can operate at a faster rate of speed and loading/unloading of a single part is faster than loading/unloading multiple parts.
Work Environment. Where is the press going to operate? Will the press operate in the same room as a molding press? Is the area climate-controlled? Are there drafts from open doors or windows to consider? Clean rooms have a different requirement for a press than one used in a general molding area. Special considerations need to be addressed for contamination. Depending on the class of clean room, a pneumatic oiler should not be used, exhaust air should be plumbed out of the room, and special coatings and plating of certain machine components may be required.
Part and Heat Transfer Orientation. Not only must the part fit into the hot stamp/heat transfer press to facilitate loading and unloading of the part but also, the heat transfer orientation on the web must match the graphic requirements of the part. Coordination of the image on the web and fixture holding the part must allow the product to be decorated in the correct orientation.
First, it is necessary to understand the environment of the finished product. Not only does the location of where the process will be performed need to be considered but also, the environment to which the decorated part will be subjected. Will the part be exposed to or have requirements to meet a certain level of UV exposure, temperature, humidity, abrasion/scuff resistance, or chemical resistance? What are the customers requirements regarding the presence of the decoration? Is this a single-use item or will it have a long life expectancy, and must the decoration last the life of the product? Is there additional assembly, machining, or packaging to be done to the part?
Also to be considered is the industry for which the part is being produced. For example, the automotive industry has different requirements for under-hood applications than what is required for interior or exterior applications. Cosmetics have a different chemical requirement due to what is being packaged than that of appliance manufacturer requirements where cleansers and food products determine the chemical resistance requirements.
There are a number of industry standards that have been established to determine the quality of the foil or heat transfer. An industry standard for adhesion is a crosshatch tape test. There are established lab tests to check for abrasion or scuff, and light exposure boxes to check for UV stability and weather effects on the products. Specifications for certain chemical resistance can vary from a simple wet rag wipe to a full emersion for a period of time. Each industry has a set of physical properties to meet and the decorated products must be submitted to these tests. The chemical make-up of the foil or heat transfer determines the capability of meeting these specifications. When introducing a part to production, a full series of parts needs to be submitted to verify that the established specifications are being met. Once released to production, quality checks during the application process will help to ensure that the application is correct, along with periodic checks to verify that the overall quality is consistent with the specifications and requirements of the product.
Keith Hillestad has over 30 years of experience in the plastics decorating industry, including hot stamping, heat transfers, pad printing, spray painting and screenprinting. As national sales manager for United Silicone, an ITW Company, Hillestad has presented a variety of technical papers at both TopCon and ANTEC over the past several years and has conducted numerous regional training seminars on decorating. For more information, email email@example.com or visit www.unitedsilicone.com.