by Neil Meador
Like silk screening, hot stamping suffers with a historic name, which refers to the origins of the process. There was a time when silk really was used to make silk screens, and there was a time when high pressure stamping really was a vital part of hot stamping. There was a time when gold foil was called gold leaf because it really was beaten gold. The point here is that modern hot stamping, contrary to its name, is an evolved printing process that requires good process controls, mechanical stability, and very fine adjustments to obtain the lightest consistently achievable impression.
Stamping often brings to mind large tonnage presses, burned in images, metallic dies, and ragged edged printing. High tonnage and ragged images just arent part of the modern hot stamp process. Current application methods include traditional press work along with very high quality roll-on peripheral decorating of rounds, ovals and squares in one continuous operation. In these applications, print pressure is no longer measured in tons but rather in ounces and grams. Some machines are able to monitor the print pressure with force gauges in the tooling or in closed loop servo driven heads.
Regardless of the application machine, the benefits of hot stamping remain. It is a dry process, a very permanent and durable process. The image is thermally fused to the substrate. Even after years of outdoor exposure or complete failure and wash out of the pigment over time, a readable image often remains on the part. Hot stamping upsets the molecular structure of the material with heat and surface melting. Thus combining the heat activated adhesive coat of the foil directly with the substrate being decorated.
Hot stamping is better described as a die selective heat transfer process. The transfer only occurs where the die contacts the part. In its simplest form, hot stamping utilizes a heated die generally mounted to the upper platen of a press. The multi-layer foil is compressed between the die and a part, which is fixtured on the press work table. The foil is held sandwiched between the die and the fixtured part for a period of time known commonly as dwell time. At the completion of the dwell time, the press opens and the pigment layers of the foil have been bound to the part by heat and pressure only where the die made contact. The foil carrier is then stripped upward from the part. During the stripping operation, the pigmented layers of the foil will separate from the carrier only in the areas where heat and pressure have been applied. The foil web is not actually punched out in the die contact areas, rather, specific layers are activated by the heat and pressure to adhere to the part and release from the web material that the foil is made on which is usually polyester film. After printing, the spent foil will show areas of clear carrier where the pigment has been transferred off.
Foils used in hot stamping consist of three primary layers. In practical application most manufacturers use other layers in between the three primaries, but these are often for improved performance, weathering, and opacity. In its simple form foil is made with the layers in the opposite order that they will be on the finished part. That is, the first layer applied to the foil during manufacturing is the top layer or finish of the applied decoration. The first layer of coating, which is put on the mylar during manufacture, is what is called the release layer. It is a material which when heated will become fluid enough to allow the successive layers to be pulled off the carrier. Again, the release layer actually is the top or finish layer when the hot stamping is applied. The next layer in the foil is the color layer, this can be a vacuum metalized layer as in bright silvers and golds or a plain color in the case of pigment foils. The last layer that is applied is called the sizing, sticker, or adhesive coat. It is the layer which will actually bond to the product being printed. In particular, this layer must be compatible with the material that is being decorated.
When the foil is in contact with the part and die, heat travels from the die through the foil to the part. As the foil heats, the release layer becomes fluid, the adhesive layer is activated and sticks to the part and the color layer is carried between the two. At the correct time in the application process, the die is separated from the part, removing the heat source, then the foil is lifted off the part while the release layer is still fluid but the adhesive layer has grabbed. The color layer is stripped from the foil, trapped between the adhesive and the release, permanently bonded to the part.
The direct hit process is the most rudimentary form of hot stamping. It has its place in the line up of different applications but it is limited to flat surfaces, cylindrical sections less than 90 degrees, and very limited applications on convex compound curved surfaces. Very large areas of solid copy tend to trap air. There are die making techniques, which can help with this but they increase the required pressure dramatically. Contoured dies made for cylindrical sections or small sections of compound surfaces require absolute precision alignment of the die to the part surface being printed. Part variations are not well tolerated under these circumstances. Direct hit hot stamping is the most economical method to start with and large flat platen presses are the mainstay in the greeting card, book binding, and desktop promotional markets. Contoured die direct hit applications are prevalent in cosmetic compacts, housewares, appliances, and industrial product identification.
Hot stamping dies are available in two basic types: hard or soft. Common hard die materials are photo-engraved or machine engraved magnesium, brass, copper, and hardened steel. Hard dies do not compensate for any variation in parts or fixtures. Soft die materials are silicone, which has excellent heat resistant properties and recently some photo polymers which have a short life at the temperatures required for good results. The vulcanized silicone used for hot stamping dies ranges from 40 to 95 durometer. The big benefit of silicone dies is the ability to conform to the surface of the part without extraordinary pressure. This gives some leeway in part variation and fixture fit. The down side of silicone is that its a terrific insulator. Keeping the surface hot on a high speed line requires pumping a lot of calories into the die, and the more heat you push through a silicone die, the faster it dries out and hardens to the point of being useless.
Direct hit tipping, or foil tipping as it is sometimes referred to, is a variant of hot stamping that doesnt require a die because the part being printed acts as a die. A good example of tipping is novelty and promotional automotive license plate frames. Commonly the letters on a license plate frame are raised from the surface by .040 to .100 inches. They are usually hot stamped with bright exterior metallic foils. To accomplish this type of stamping all that is needed for a die is a flat piece of silicone slightly bigger than the outer boundaries of the area to be tipped. The flat die or pad is mounted to the heater block. Since the flat pad will only contact the tops of the raised letters (as long as the pressure is set right), the foil will only transfer on the tops of the letters, hence the name tipping.
The next application method utilizes an externally heated silicone roller. This application technique is best suited when large relatively flat areas have to be covered edge to edge. It is a method used often in the appliance industry for face panels hot stamped with wood-grain and brushed metallics. In a further variation, roller heads are used to apply wood-grain and other patterns completely around TV cabinets and flower pots. There are two different approaches when using a heated roller applicator. In one approach the flat part is held stationary, the foil is draped over the part and the heated roller rolls across the part while pressing the foil onto the surface. The foil is stripped off after the application roller passes over the part. The other approach uses a stationary roller and moves the part and foil against the roller. Neither method is intrinsically better than the other, but the best choice is made by considering up stream and down stream operations that may have to interface to the decorating machine.
Similar to the way flat surfaces are decorated by moving a cylindrical roller across them, cylindrical items like caps, jars, mascara bottles, syringes, and pipettes are decorated by rolling the round part against a flat die. In these applications, the parts are loaded on a roller set, on a mandrill or between centers. The flat die is brought into contact with the part, and it moves across the die. In some machines the die moves across the part. Rotational drive is usually provided to keep the part rolling across the die without skidding or slipping. Modern multi-axis servo driven peripheral hot stamping machines use one axis of servo to bring the die into contact with the part, one axis to provide rotational drive, and one axis to move the rotating part against the die. Other machines employ various methods of linear and rotational drives including air cylinders, hydraulic checked air cylinders, stepper motors, cables, pulleys, and gears.
The final caveat in peripheral hot stamping is applying a continuous hot stamp around the periphery of a non-round part. Typical applications for this method are banding oval and square cosmetic caps, gilding the edges of compacts, applying full wrap on mascara bottles, and large applications like TV cabinets, automotive, and appliance parts. This process can be accomplished using special shaped gears or cams although most machine builders now utilize multi-axis servos operating in a simultaneous contouring mode. The machines create the correct motion profile to make an oval or square part roll across a die or against a heated roller as if it were round. These machines are equipped with some type of learning or teaching device, which allows the machine to feel its way around a part, establishing a mathematical model of the part. This data is then used to calculate the correct velocities and distances for multiple segments in a contoured path. Like force sensing in direct hit machines, some manufacturers are building sensing systems into the machine, which actually sense each part individually while printing and therefore eliminate the learning or teaching requirements.
There are five ingredients that you need to get started in Hot Stamping:
1. A product to stamp. Products that hot stamp very well are usually designed that way right from concept. Decorating should always be considered in any initial part design.
2. Artwork and die. Consult with a die maker or hot stamp machinery manufacturer about your artwork unless you have a lot of experience. There are minimum element sizes, some especially difficult patterns and some guidelines for type spacing and blocking that if modified before the first print can save you a lot of trouble later.
3. Foil which is the color you desire and which is compatible with the product you want to stamp. Foil is available in virtually any color and sheen from matte to bright metallic silvers and gold along with special effects like holograms, opalescents, and continuous patterns. Regardless of the color of foil you select, it must be compatible with the substrate you intend to stamp. Foil manufacturers make different formulations for PET, PP, PE, Wood, Paper, and many others.
4. A fixture or nest to hold the part during the stamping cycle. Theres been some good articles written here about hot stamp fixturing. (Oct/Nov 2003 Nesting Fixtures Key to Hot Stamping Jobs.) The most important aspect of a nesting fixture is that it provides full intimate contact and rigid support underneath the print area and prevents part shifting as the die contacts, dwells on, and leaves the part. This holds true right from the basic direct stamping to multi-axis contouring machines.
5. A machine to bring it all together. Hot stamping machines today range from hand lever operated machines, pneumatic direct acting machines, pneumatic and hydraulic hybrids, a variety of pneumatic and mechanical toggle and wedge approaches, and most recently all electronic servo driven presses and multi-axis decorating centers. Selecting the correct machine for the job may be influenced by the quantity of the first job you have to run and other products that you may consider using the machine for in the future. It is suggested that you consider all the products you might want to print with one machine. Then contact a decorating machine manufacturer and let it propose which machine in its line will be the best choice for you.
Hot stamping is a lot more than just a press and a die. Many applications today do not use a press at all and dont do anything that remotely resembles high pressure stamping. Modern hot stamping is a mature process whose parameters have been well defined for more than 50 years as a mainstream process. Once considered an art, hot stamping is now an exact science where good engineering and sound principles yield consistent and predictable results.
Neil Meador is an engineer who has been involved in decorating throughout his entire career. After years of in-house development, Meador’s first machine was patented and made public in 1985. One of his ground-breaking designs was the first machine to employ multi-axis servo controls to continuous peripheral decoration of non-round wares. He is now manager of Permadur Industries and still has involvement in the full spectrum of decorating technologies.