THE EMBOSSING PROCESS
Embossing is one of the most commonly used embellishments in the label industry. This is the process of creating a raised relief 3D image (see Fig 4.1) or alternatively a sunken image (de-bossing) on the label substrate.
Figure 4.1 - Examples of embossed images
The embossed image is created using a male and female die. The female die has an engraved or etched recessed image whilst the male die has a raised image.
The substrate being embossed is sandwiched between the male and female dies and pressure is applied which forces the male relief image into the female recessed image. This action pushes the label substrate into the recessed female image creating a raised profile on the label surface (see Fig 4.2).
Heat may also be applied to assist in producing the optimum embossed image.
Figure 4.2 - The embossing process
No ink is used for the actual embossing process but very often the embossed image will lay over the printed area of the label. This will involve close register between the printed image and the embossed image and this is called a ‘registered emboss’.
If the embossed image lies in a non-printed area this is called 'blind embossing' .
Another type of embossing process is ‘de-bossing’ which uses exactly the same method as theembossing process, but the function of the male and female dies are reversed.
Whereas an embossed image is created with a ‘raised’ image in the label substrate, de-bossing is a ‘recessed image’ in the surface of the substrate.
The process involves the application of pressure to the face side of the label substrate, forcing the material downwards into the female thereby creating the recessed profile (see Figure 4.3).
Figure 4.3 - Example of de-bossed image
METHODS OF EMBOSSING
The embossing process can be undertaken using a flatbed, semi-rotary or a fully rotary system (see Figures 4.4a, b and c). The choice of method is generally governed by the type of label required, the run length and the cost of the dies.
The flatbed embossing unit works on the same principle as the flatbed die-cutting unit and the flatbed foiling unit.
The illustration above (Fig 4.4a) shows the flatbed type of press configuration.
The substrate, which can be in sheet-fed or web fed format is positioned between the two plates which hold the embossing dies. The top plate moves up and down sandwiching the substrate to be embossed (as indicated by the arrow).
Figure 4.4 - Embossing systems
Because of the heavy construction and high impression strength of this type of ‘platen’ configuration, it is used extensively for both embossing and die-cutting in both sheet-fed and reel fed label manufacture.
Figure 4.5 shows a typical flatbed embossing unit with the top and bottom plates holding the embossing dies visible.
Figure 4.5 - Flatbed embossing unit
The semi-rotary embossing process is mainly used in the sheet-fed label market.
The embossing process is usually 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.
impression cylinder are clearly visible with the sheet feeder positioned at the top. The female flat die is mounted onto the flatbed base.
This usually involves the use of a honeycomb base and the dies are secured with mounting toggles (Fig 4.7).
Figure 4.6 - Semi-rotary embossing unit
Figure 4.7 - Honeycomb base with embossing plate mounted
The mounting operation is usually carried out off-line to reduce the down time during the make-ready operation.
The interesting thing about this type of embossing is that the male element of the process is created by the press operator, involving the following process sequence;
The dies are mounted onto the honeycomb base and positioned in the press
The dies are manually inked
The cylinder is dressed with card or blotting paper
The operator feeds a sheet through the press with the cylinder on impression allowing a print to be taken of the die showing if the dies are in register.
If any of the dies are not in register then the operator re-positions them, using the toggles to make any adjustment.
The process is repeated until all the dies are in correct register
The card and blotting paper are removed and a special embossing make ready material is fixed to the cylinder. This material is dampened and the press is then turned over under impression
The material is forced, under pressure, into the female die and the press is run for several minutes until the male image is formed and the material has dried and hardened. This process has created the male embossing element of the process
If any individual areas require additional pressure the operator can paste tissue onto the specific area, thereby ensuring that the substrate being embossed is fully contacting the base of the female die
EMBOSSING ON INTERMITTENT FED PRESSES
With intermittent feed presses a flatbed embossing unit is used. The embossing head is positioned to allow the web to travel between the two embossing dies. The web travels with a stop-start motion and the embossing takes place when the web is stationary.
As the web stops, the embossing dies come together to create the embossed image. The plates then open and the web moves forward. This action is called the ‘pull’ and the distance of the pull is set by the printing width of the plate cylinder (see Figure 4.8).
Figure 4.8 - Embossing on intermittent fed press
FULL ROTARY EMBOSSING
Rotary embossing is one of the most popular systems in use in the label industry, because it allows easy fitment onto rotary label presses, thereby giving the advantage of faster running speeds and excellent embossing quality (see Fig 4.8a).
Figure 4.8a - Rotary embossing unit on a modern label press
The biggest drawback with full rotary embossing is the cost of the ‘tooling’ i.e. the manufacturing and imaging of the embossing dies. The configuration of rotary embossing on a rotary label press (Figure 4.9)
Figure 4.9 - Full rotary embossing
There are four factors that control the quality and consistency of good embossing.
Each of these factors is explained in more detail in the following section.
The depth of the female die. In the ‘embossing’ process the depth of the female die will govern the height of the embossed image - the deeper the die the greater the profile of the image.
Consideration must be given to the type of substrate to be embossed and what depth can be achieved before the substrate fibers break. The required depth can be assessed using the original artwork allowing the engraver to determine the correct die depth.
Applied pressure. Quality embossing requires considerable pressure to be applied to the dies during the embossing process. It is important that the substrate contacts the bottom of the female die and this can only be done effectively by pressure on the male die.If there is insufficient pressure and the dies do not fully contact then some of the embossed image, particularly in the fine detail may be lost.
Too much pressure will damage the surface of the substrate and can in some cases puncture the substrate, destroying the embossed area.
The two methods of embossing, flatbed and rotary, can vary in the amount of direct pressure required. Flatbed embossing needs far greater ‘overall’ pressure (tonnage) than rotary.
The amount of pressure needed can be reduced by using a ‘rocking’ technique. The top plate, which holds the male or female die (dependent on whether it is a de-bossed or an embossed image) is set to contact the bottom plate using a rocking motion.
This allows the two embossing dies to make contact in stages, thereby reducing the amount of pressure required to achieve the correct embossed depth.
Figure 4.10 - Principle of applying pressure on an embossing unit
Rotary embossing automatically benefits from this principle and therefore requires less pressure than the flatbed process, which requires heavy pressure.
Substrate and heat
The application of heat can be an advantage during the embossing process. The use of a heated die can aid the molding of the substrate fibers during the embossing process, but care must be taken not to apply too much heat, as this will distort the substrate.
The type of substrate to be embossed is also an important consideration for the label designer and converter. Substrates vary in the amount of pressure they can absorb before the material fibers break and the substrate surface is damaged.
The function of the embossing die is to mould the paper fibers into the profile of the embossed image without damaging the substrate surface, but allowing the embossed image to maintain its profile after the process is completed.
Synthetic materials are unsuitable for embossing as they are cannot be used to create a raised image and these substrates will not accept any direct pressure from an imaged die and are easily punctured.
TOOLING AND ENGRAVING
There are five different types of embossing dies used in the label industry.
The flatbed and semi-rotary processes use steel, magnesium, copper and brass dies, whereas full rotary embossing generally use only brass or steel dies.
The manufacturing costs of embossing dies can be high, particular when tooling and engraving full rotary male and female brass dies.
FLATBED EMBOSSING DIES
The metal dies used in flatbed embossing 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.
The characteristics of each of the die materials are reviewed below.
Flatbed magnesium embossing 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.
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.
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 or steel dies
Unlike magnesium embossing dies that are chemically etched, flatbed brass and steel embossing dies are imaged using a CNC digitally driven engraving system. This method applies also to the imaging of rotary dies made from brass or steel.
The flatbed embossing die is engraved in the flat and the engraving head travels over the die moving through the X and Y axis and rising and falling as the digital data instructs. This method of engraving will produce a very fine and detailed image.
Copper embossing dies are imaged using the etching process similar to magnesium die etching. Due to the fact that the copper embossing die is harder than magnesium it is more suitable for longer production runs.
The copper die is also better suited for multiple image work and with good image etching characteristics it will give an excellent embossed result.
ROTARY EMBOSSING DIES
Rotary brass or steel dies
The manufacturing of the rotary embossing die requires more engineering than the flatbed embossing die. The manufacturing process starts with a length of brass that is machined to the outside diameter of the required print length for the job to be printed and embossed (see Fig 4.11 ).
Figure 4.11 - Pair of imaged embossing cylinders – positive and negative
The ends of the cylinder are then machined to create the end assemblies, the dimensions of which are dependent on the type of press and the specification of the embossing unit being used.
The embossed image (both male and female) 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.
Figures 4.12 and 4.13 show examples of final imaged embossing cylinders with close-up of positive and negative images.
Figure 4.12 - Examples of combination foil/embossing
Figure 4.13 - Close-up of imaged embossing cylinders
Sleeved rotary embossing dies
Another option for rotary embossing involves what is known as a sleeve system.
This system creates a sleeve which slides over a reusable base cylinder, thereby reducing the volume of metal required to produce the embossing die. The sleeve is manufactured to the width of the image area required.
Once the sleeve has been slid onto the base cylinder it can be positioned both laterally and circumferentially to give the correct registration position.
The embossing process can also be combined with hot foil stamping and this is called combination foil/embossing. The process involves embossing and foiling in one operation using a single combination brass die. The die has a sharp cutting area around its edge to ensure that the foil substrate strips evenly on each embossing cycle.
Combinations of foiling and embossing are still carried out in label decoration, but they were used extensively for the manufacture of ‘seals’ for both self-adhesive and non self-adhesive applications.
Seals provided a superb and unique method of product decoration, particularly on high weight metalized substrates.
Fig 4.14 shows an example of a substrate which has been blind embossed (right) and also combination foil embossed (left).
COMBINATION PRINTING AND EMBOSSING
Die stamping is a method of printing and embossing the image using the same engraved die. The die is manufactured in steel or copper. The flatbed male and female dies are mounted in the same way as normal embossing dies and placed in a die-stamping press. This press usually has a letterpress roller inking system which deposits a film of ink onto the surface of the female die. The substrate to be die stamped is positioned between the two dies which are then pressed together under extreme pressure leaving a printed and embossed raised image.
Holographic embossing is used extensively for the mass production of hologram images.
The process reproduces very fine holographic patterns onto a polymer substrate carrier which provides the reflective metallic effect and brightness of the hologram.
The process for producing the embossed hologram involves passing the reflective polymer substrate between the die pressure roller onto which is fixed the holographic nickel shim. A pressure roll makes contact with the shim and with a high temperature and pressure the image is transferred onto the surface of the polymer and thus producing the holographic image.
More information on holograms and their production can be found in this article.