Foiling or foil blocking is a dry printing process which uses very thin aluminum foil in a variety of metallic colors – such as gold, silver, red or blue – rather than inks, from which to embellish parts of the label design.
Hot foil stamping is a printing process that uses heat and pressure to transfer the image from a metal printing plate to a label substrate, using a metallic or colored foil to produce the desired result.
A more recent development of foil blocking is the cold foil process, in which a print unit is used to print a special adhesive on the label web where the metallic effect is required. When the metallic foil is brought into contact with the adhesive it adheres to it to produce the printed foil design on the label.
A comprehensive insight into the cold foiling process will also be provided later.
HISTORICAL EVOLUTION OF FOILING
Hot foil stamping, a method of transferring a metallic or pigment finish from a carrier strip of paper or film – known as a foil – onto a label substrate using heat, pressure and dwell time, was first patented by Dr Ernest Oeser in 1892. This first hot stamping process used both gold and colored stamping foils made from 23 carat gold or bronze powder with a dye to obtain the necessary color. Both these types of foils were supported on a glassine carrier strip.
Then, in 1931, the world’s first vapor deposit stamping foil using real gold was invented and patented by Konrad Kurz. This was mainly used in industrial applications for the foil printing of pencils, in bookbinding, and in the hat industry.
The early stamping foils had a number of limitations. The use of real gold made the foils very expensive, while the bronze powder foils were easily tarnished. In addition, the coloring dyes used with the metallic powders were not light stable and tended to fade. This meant that other alternatives for metallic image printing continued to be considered.
However, it was not until the 1950s that vacuum or vapor metallized foils using aluminum were developed and introduced by Leonhard Kurz GmbH & Co.
This company, founded in the 1890s by Leonhard Kurz as an enterprise for the production and marketing of gold leaf, now had a product that was to revolutionize metallic image printing throughout the graphic arts industry – and is still used today.
In this hot foil stamping process, a pigmented or metalized coated foil was transferred from a carrier, more usually today a polyester film, and fused to a substrate by heat in a hot stamping machine.
This means that the pigmented or metalized coating has to be compatible with the material to be stamped. For this reason, hot stamping foils are manufactured in various formulations designed to give quality prints on a specific material. Also incorporated in the foils are qualities such as abrasion resistance, oil and grease resistance, and chemical resistance.
Over the years an enormous range of foil types has become available. Where metallics are concerned, there is every conceivable shade of gold (and silver), in bright, satin or matt finishes. These are supplemented by metallic colors, brushed finishes and a wide range of gloss and matt colored pigments. In addition, there are numerous patterned colored and metallic foils which are particularly suitable for background effects. To meet more specialized requirements today, there are also fluorescent, magnetic, pearlized and holographic printing foils.
Roll-on stamping machines for applying the foils were introduced by Kurz in the 1960s and vertical stamping machines in the 1970s.
Many other companies have also contributed to the development and growth of metallic image decoration on labels, from the likes of Gallus, Nilpeter, Edale and other early 20th century roll-label press manufacturers, to dedicated hot and cold foil materials companies such as George M Whiley and API Foils.
A more in-depth narrative on the historical evolution of print and embellishing processes can be found in The History of Labels: The evolution of the label industry in Europe by Michael Fairley and Tony White.
HOT FOIL STAMPING
Hot foil stamping is a decorative embellishment which uses heat and pressure to transfer an image from a metal printing plate to a label substrate using metallic foil to give a highly reflective surface. Foil is often gold or silver, but it is also manufactured in various colors, patterns and special finishes (see Figure 2.1). A very hard thermoformed plastic plate can be used instead of metal for very short runs.
Figure 2.1 - Example of hot foiling
The use of metallic finishes can considerably enhance both the quality and the added value of the label giving excellent solid images, opaque finishes and producing a very high density of color.
THE FOILING PROCESS
A lacquered aluminium foil is positioned with the adhesive layer face down on the substrate. Pressure is applied with a heated “imaged” die to activate the adhesive layer. The foiling film is then separated from the substrate leaving the metallized image of the die reproduced on the substrate surface.
The stamping foil is made up of a number of layers - film carrier - release coating – reflective lacquer and adhesive.
Figure 2.2 illustrates the structure and composition of the ‘foil’ when positioned over the substrate to be foil printed and prior to contact with the heated foiling die.
The foil is positioned with the adhesive face down on to the substrate and pressure is applied with a heated engraved die which activates the adhesive. The activation of the adhesive is for a very short period, known as the ‘dwell time’.
Figure 2.2 - Structure and composition of foil
The control of die temperature and pressure between the substrate, foil and die, is very important in ensuring that the adhesive is activated sufficiently for the adhesive and lacquer film to transfer to the substrate.
For effective transfer to take place consideration must also be given to the type of foil, the release factor of the adhesive and the surface of the substrate being foil blocked.
METHODS OF FOILING
Hot foil stamping (also known as foil blocking) can be used as a single off-line process or can be incorporated into a multi-process operation, running in-line with other printing and embellishing processes.
Units designed for the off-line hot-foil printing or decoration of labels, come in a variety of configurations and widths. Stand-alone hot foil blocking machines are generally narrow web, but the standard self-adhesive production widths are used where combination print processes include foil blocking. Larger sheet-fed presses are used for foil blocking of large sheets of glue-applied or in-mold labels.
There are three different types of foiling methods each having its own distinct advantages:-
Flatbed foiling - offers an easier set-up (make-ready) and tooling costs are significantly less.
Round flatbed foiling - is better suited for the shorter runs lengths and uses the same flat die tooling as the flatbed system.
Semi Rotary and Full Rotary foiling - is suitable for longer run lengths, gives higher running speeds and is excellent for fine details, but the cost of the tooling is much higher than flatbed.
Flatbed foiling machines work on the same principle as the ‘platen’ printing presses and die-cutting machines.
The imaged dies are usually located onto the electrically heated bottom platen using a honeycomb base and held with tensioning clips. These clips allow the operator to make positional adjustments to the die to achieve the correct print registration (see Figure 2.3).
Figure 2.3 - Honeycomb base with embossing plate mounted
The substrate and foil lays between the two platens and the upper and lower platens are brought together under pressure over the full area being foiled (see Figure 2.4).
Figure 2.4 - Platen foiling
The platens then separate and the foil moves forward, leaving the foiled image on the surface of the substrate. A typical flatbed foiling embossing unit is shown in Figure 2.5.
Figure 2.5 - Typical flatbed foiling/embossing unit
ROUND FLATBED FOILING
In round-flatbed foiling, the ‘top’ platen section is replaced by a rotating cylinder. The steel ‘chase’ onto which the die is attached moves forward and backwards in synchronization with the cylinder and pressure is applied at the narrow point of pressure when the cylinder, imaged die, the foiling material and the substrate meet (see Figure 2.6).
Figure 2.6 - Round flatbed foiling
This type of foiling system is used widely in wet glue label manufacturing when the foiling of large sheet sizes is required.
The semi-rotary foiling process is mainly used in the sheet-fed label market, where the foiling process is typically carried out on a converted letterpress machine, which is configured with a flatbed base and a large impression cylinder.
The ink duct and roller train is removed, giving excellent access to the flatbed section of the machine whilst the sheet feeder and delivery remain in position.
The foiling die mounting operation is usually carried out off-line to reduce the down time during the make ready operation. The flat foiling die is mounted on to a flatbed base and involves the use of a honeycomb base with the foiling dies secured via mounting toggles.
Rotary foil stamping is one of the most popular systems of foil printing in the label industry. As this method is a full rotary system, it allows easy fitment onto rotary label presses, giving the advantage of faster running speeds and excellent foiling quality.
The biggest drawback with full rotary foiling is the cost of the ‘tooling’ i.e. the manufacturing and imaging of the brass foiling dies and the waste deriving from the gap which can occur between each image on the die.
There are systems however that can overcome some of the ‘gapping’ waste issues. These are called foil saver systems and are detailed later.
In Figure 2.7 a typical layout of a rotary foiling unit is illustrated. The brass foiling die is positioned over a rubber impression roller and the substrate to be ‘foiled’ passes between these two cylinders. The foil is sandwiched between the surface of the foiling die and the substrate. A major advantage offered by the rotary system is that it does not require the same amount of impression strength as the flatbed method. The point of contact between the foiling image, the foiling material and the substrate is narrow and therefore allows a very clean point of contact and a quick separation of the foiling material from the substrate surface, allowing very fine detail to be achieved. Figure 2.8 shows a typical rotary foiling unit mounted on a reel fed labeling press.
Figure 2.7 - Principle of rotary foiling
Figure 2.8 - Shows a modern hot foil unit located on a combination press
TYPES OF DIE AND MOUNTING SYSTEMS
The manufacturing costs of foiling dies can be very high, in particular metal dies.
Metal dies (brass in particular) give a very good heat transfer and the heat can be accurately controlled, which is an important factor when producing a quality foiling result.
There are a number of different types of foiling dies used:-
Flatbed – in magnesium, copper and brass
Brass segmented rotary die – sections are used instead of a full cylinder
Flexible metal sleeve attached to a magnetic cylinder.
The metal dies used in the flatbed process are not as sophisticated as those used in the full rotary system. The flatbed die does not have any curvature issues and can be etched or engraved in the flat. Once the image has been engraved or etched the die is ready for mounting in the press.
FLATBED MAGNESIUM DIES
Magnesium is the softest of the materials used for metal dies and is the least expensive. This type of die is best suited for flatbed use, particularly when ‘single’ image short runs are required. (Figure 2.9)
Figure 2.9 - Flatbed foiling die
The imaging of a magnesium plate is done using a chemical etching process. A photo sensitive coating is applied to the magnesium plate to be imaged and a film negative of the image to be produced is then placed in contact with the plate surface and exposed to a light source before being photographically developed to produce the image. The plate is then chemically etched to remove the ‘non-image’ area leaving the ‘image’ area in relief.
FLATBED BRASS DIES
Whereas magnesium dies are chemically etched, the flatbed brass dies used in the label industry are imaged using a CNC (computer numerical control) digitally driven engraving system. This method of imaging applies to both flatbed and rotary dies.
The flatbed die is engraved in the flat. The engraving head travels over the die moving through the X and Y axis and rising and falling as the digital data instructs, to produce a very fine detailed image.
Brass is an excellent conductor of heat and is the preferred material for the hot stamping processes used in label manufacturing. The heat level of a brass die is more controllable than the magnesium and because of its durability is more suitable for longer production runs.
FLATBED COPPER DIES
Copper dies are imaged using an etching process similar to magnesium die etching.
The copper die is harder than magnesium and therefore more suitable for longer production runs and multiple image work. Good image etching characteristics will give excellent foiling results.
FULL ROTARY BRASS DIE
The manufacturing of the rotary foiling die requires more engineering than the flatbed die.
The manufacturing process starts with a length of metal that is machined to the correct outside diameter of the required print length for the job to be printed and foiled (Figure 2.10).
Figure 2.10 - Machining the die cylinder to correct diameter
The inner part of the die is machined out to create a tube and special end sections are fitted to the die. Oil feed connectors are fitted to these to allow heated oil to flow in and out of the center of the die.
Alternatively the die can be heated using electric elements which are inserted into it and secured to the end sections of the die.
The rotary die is imaged using exactly the same principle as flatbed engraving, but instead of the engraving head traversing on the X and Y axis the engraving head moves only on the X axis. The rotary die rotates back and forth on the Y axis with the engraving head rising and falling as required.
This complex system of engraving is driven by a digital file which contains the image to be engraved. Figure 2.11 shows finished rotary die with engraved images.
Figure 2.11 - Images engraved onto rotary die
SLEEVED ROTARY DIES
Another option for creating a rotary foiling die involves a sleeve system. This type of die reduces the metal content of the rotary die by imaging a sleeve that is manufactured to the width of the image area required.
In Figure 2.12 the imaged brass sleeve is mounted onto a heated base cylinder before being secured. The sleeve is slid onto the cylinder and can be positioned both laterally and circumferentially to give the correct registration position
The image also shows the utilization of a narrow web of foiling material even though the web width of the job being processed may be much wider.
Figure 2.12 - Sleeved segmented brass die (curved)
The die shown in Figure 2.13 is called a segmented die. This is a novel method of mounting individual brass dies onto a full rotary cylinder without having to use an expensive solid brass die.
Figure 2.13 - Segmented die - curved brass foiling dies mounted onto a rotary honeycomb base
The cylinder is manufactured in a honeycomb configuration which facilitates the insertion of securing ‘clips’ which are located onto each edge of the plate.
The curved segmented die is secured to the honeycomb base using a tensioning key to tighten the clips.
Magnetic foiling dies work on the same principle as the steel flexible dies used for the profile cutting of the label shape.
The die is made up of a flexible ‘shim’ with a steel backing and a copper surface layer which carries the foiling image. Copper is used because of its good heat conductivity.
The imaged shim is positioned onto a heated magnetic cylinder (see Figure 2.14) and the cylinder and die are mounted in the foiling unit in the same way as a solid rotary die.
Figure 2.14 - Magnetic base cylinder
Imaging of this type of foiling shim is done using the same type of etching process as that used for imaging the copper flatbed die.
Fitting and removing foiling shims is a simple operation particularly as some magnetic cylinders are fitted with pins that correspond with locating holes in the shim itself, making registration an easy operation.
One of the main advantages of this type of die is the easy changeover between production jobs.
TYPES OF HEATING SYSTEMS
The use of oil or electric systems to heat the foiling die is generally one of preferential choice by the printer, as there is very little difference in the overall performance between the two systems (see Figure 2.15).
Figure 2.15 - Diagram showing layout for oil and electric heating of the die
The critical factor is control of the surface temperature of the die and the evenness of the die temperature across the full width of the web.
The surface temperature of the die must be within a tolerance that is governed by the release factor of the foiling material and the optimum running speed of the job being foiled. This temperature must be maintained throughout the production run.
To establish the optimum die temperature, the printer must ensure that the pressure (impression) between the die, substrate, foil and the bottom platen (flatbed foiling) or the rubber covered impression roller (rotary foiling), is at the correct setting.
To ensure a good foiled image is achieved the foiling material must leave the surface of the substrate with a clean break and some tension needs to be maintained so that the foiling material does not become loose or ‘baggy’.
At this point the printer can then adjust the impression, running speed, heat control and foil material tension to get the optimum printed result.
FOIL SAVER SYSTEMS
One of the downsides of hot foil stamping is the amount of wasted foiling material which occurs in the gap between each foiled image. In Figure 2.16 the foiled image area (the horse) and the gap between each image is clearly visible.
Figure 2.16 - The impact of using a foil saver A) Hot foil stamping without use of foil saver system B) Hot foil stamping with a foil saver
Once this reel of foiling material has passed through the press it cannot be reused.
The problem of waste foil material has been overcome by the use of ‘foil saver’ units. These devices use a controlled variable ‘pull through’ of the foiling material to minimize wastage and to give optimum use of the foil.
An incremental amount of foil is fed into a rotary foiling unit which is synchronized with press speed (die speed) through an optical encoder attached to the press drive.
The foil feed mechanism is controlled by a stepper motor which accelerates and decelerates the foil web and maintains the proper speed and tension.
A) Foil substrate
B) Foiled image
C) Foiling die
A) Hot foil stamping without use of foil saver system
B) Hot foil stamping with a foil saver
This movement is achieved using a method of reciprocating rollers. The roller system which carries the foil travels forward on the printing cycle and then reciprocates back, pulling the foil between the gap in each image on the die and then repeating the motion ready for each image (see Figure 2.17).
Figure 2.17 - Foil Saver System used for the transfer of holographic foiled images
Another big advantage with this system is the facility to use individual ribbons of foil material (see Figure 2.18).
This allows the printer to make optimum use of the foiling materials and make additional reductions in foil wastage. For instance the facility to utilize any narrow width ‘off-cut’ reels that may be held in stock.
Figure 2.18 - A foil saver system showing multiple webs of foiling material
The latest foil saver units control the movement of the foil material using a vacuum system which eliminates the need for dancer rollers. One of the big advantages with this type of foil saver system is the high speeds (20 movements per second) which can be achieved, coupled with a high degree of foiling accuracy (essential for holographic applications), when small label gaps are being printed.
Cold foiling is an alternative method of foil stamping in which a print unit is used to apply a special adhesive on the web in the area where the metallic foil is required.
When the metallic foil is brought into contact with the adhesive it adheres to it to produce the printed foil design. Figure 2.19 shows an example of cold foiling.
Figure 2.19 - Example of cold foiling
The use of cold foiling or ‘die-less’ foiling as it is sometimes known, has increased over the last decade and is now used widely in the label industry.
The simplicity and low cost of adapting a label press for the cold foiling process makes this technique easily accessible to a printer who does not want to invest in conventional hot foiling equipment. Cold foiling is a cost effective alternative to hot foiling, which requires an expensive engraved die and needs a controlled heat source which can also incur expensive running costs.
PRINCIPLES OF PROCESS
The principle of the cold foiling process is very simple.
A standard flexo or screen print unit is used to print a UV adhesive image of the area to be foiled. This printing can be done using a standard printing plate or screen.
After the substrate has been printed with the adhesive image, it is then part-cured using UV curing, which changes the adhesive to a tacky consistency.
The foiling material is positioned over the face of the substrate and the two materials are nipped together with both the substrate and the foil material running at the same web speed (see Figure 2.20).
Figure 2.20 - The cold foiling system
The web then travels through a ‘take off’ roller and the foil is stripped away from the substrate leaving the foiled material adhered to the adhesive image leaving a perfect metallic image adhered to the substrate.
Cold foil quality depends on there being enough glue for the foil to adhere to, and absorbent substrates can be a problem.
Filmic and gloss substrates are the most suitable for the cold foiling process and results comparable with hot foiling can be achieved.
The adhesive is an important part of the process and there are two different types of adhesives: free radical and cationic. The free radical adhesive reacts only when exposed to UV light whilst cationic adhesive has an initial cure to make it ‘tacky’ and a post cure to complete the cross linking process.
The cationic adhesive system was used extensively in the early days of cold foiling, but the system proved unpredictable and required a good press operator to get the best results. The adhesive system now favored is the free radical process because it is more user friendly and press stable.
One of the major benefits that cold foiling offers the self-adhesive label printer is the facility to run it as an ‘in-line’ process with the other printing processes.
The cold foiling process will give faster running speeds and achieve excellent rub resistant (most important for quality specifications).
Cold foiling also allows converters to add embellishments in sectors of the label market that have previously not been viable.
One of the areas still to be fully exploited by the designer is the opportunity to foil half-tone images. This is an exciting feature of the cold foil process which uses a printing plate in the process to enable true halftone metallic effects to be created.
DIGITAL DIE-LESS COLD FOILING
The digital die-less cold foiling process uses an inkjet system to generate the image. An adhesive pattern (image) is created on the surface of the substrate to be foiled by the inkjet head, driven by a digital file which holds the image profile. No plates or dies are required using this process.
The foil material is then placed in contact with the adhesive image and the substrate and foil materials are nipped together and then exposed to a UV light source which cures the adhesive.
The foil material is then stripped away leaving the foiled image (see Figure 2.21).
Figure 2.21 - Digital die-less foiling
One of the big advantages of digital die-less foiling is the facility to produce variable data with a foiled surface finish.
SUMMARY OF ADVANTAGES AND DISADVANTAGES OF HOT FOILING VERSUS COLD FOILING
Hot foil stamping gives a brighter more reflective color
Hot foiling gives a wide range of quality finishes including holographic
It is suitable for foiling and embossing combinations
Hot foiling is a more expensive process than cold foiling, plus the cost of dies and heating
The prep time for hot foiling is longer than cold foiling, dies are usually outsourced
More steps in the manufacturing chain
Close registration issues can sometimes occur
Cold foiling units can be easily retrofitted to an existing press
Cold foil does not require an imaged magnesium, copper, or brass die
It only requires a simple printed image to provide the foiling area
Cold foiling can achieve faster running speeds than hot foil stamping
When printed as an in-line process the registration between print and the foiled area is very accurate
The cold foiling process does not have the potential high set up costs incurred with hot foil stamping
Halftone foiling possible
Cold foiling does not give the very fine detailed foiled image achieved with hot foiling
The cold foil result is not as bright and reflective as hot foiling
Dark colored foils tend not to be recommended for cold foiling
FOIL SPECIFICATION AND PERFORMANCE
With all types of foiling the selection of the correct foil type and the adhesive release factor is very important (Figure 2.22).
Figure 2.22 - Foil specification
The adhesive has to be compatible with the surface of the substrate being foiled and the optimum temperature of the die surface combined with the dwell time (which is governed by the press speed) must be established and maintained throughout the job run.
The lacquer coating has to provide the correct degree of gloss finish after foiling and must meet the required quality control hardness and scuff resistant testing procedures on both rigid and flexible materials.
The optimum press speed is governed by the release factor of the foil being used. This release factor can be varied by the foil manufacturer, giving the printer the opportunity to specify the foil/adhesive release factor which will give the best results for quality and press speed.
FOILING - DESIGN CONSIDERATIONS
The use of metallic foils will significantly improve the added value content of the label, particularly when advantage is taken of the very wide range of face materials it can be applied to, including paper, metal, plastics and filmic.
As with all label designs which involve a printing or embellishing process, the more understanding and appreciation the designer has of the processes involved, the greater the design scope and benefits for the finished label.
It is important the label designer understands the limitations of the foiling process for both hot and cold foiled images.
Any design concept which involves a foiled image needs to consider the type size of the text and the content of any illustrations required. Fine lines and text should not be too close as this could create filling in and bridging and small type and fine serifs can cause problems. Tonal work must not be too fine and by using a coarser screen the value of the dots will be spaced sufficiently apart to allow a good foiling result.
It is possible that the foiled area may require overprinting. If this is the case consideration of the print process to be used for the overprinting needs to be carefully considered at the design stage.
Overprinting ink onto foils and foil substrates is a trend worthy of note.