Flexible packaging: pre-press, printing and converting

Studio Designer enables a 3D pack preview, with all text and artwork graphics, to be spun around and looked at from any angle as if being held in the hand. Make a change to the artwork and it can immediately be seen on the 3D pack.

The operator can also see instantly whether seals or gussets will obstruct design elements. If the designs are opened in Studio Visualizer it is possible to check the opaque white backing and see any metallic inks and sealing reliefs. Visualizer can even show how the backside of the pack will appear. In addition, brand owners can see upfront how the brand will look, both in-store and when compared with the competition.

Realistic real-time images can also simulate the various printing and finishing operations, in the correct order, and on the right substrate – whether gloss, matte or coated paper, clear or white plastic films, Pantone colors and spot colors, reverse printing, and much more.

So, let’s take some examples of the flexible packaging types described in the previous chapter and show how images in the Shapes Store then appear as structural images in Studio, starting with a fairly simple Pillow Pouch or Pillow Bag construction (see Figures 4.2 and 4.3).

The exact physical dimensions, bottom, top and back sealing areas, and the area available for pack graphics can all be displayed and changed as required. With this construction the front face of the pack, and the two areas either side of the center of off-center vertical back seal are available for graphic design images, brand identity, contents, ingredients, barcode, etc., as necessary to meet legislative and brand owner requirements.

Now let’s compare the Pillow Pouch construction with that of a Gusseted Bag or Pouch construction, where the side gussets will normally be unprinted. This can be seen and explained in Figures 4.4 and 4.5.

This time, the pouch needs to contain side gussets, enabling the pack to expand and contain more content than the pillow pouch. These gusset areas on each side of the pack are clearly indicated in the structural diagram. Again, all the necessary physical dimensions, sealing areas, and the space required for pack graphics are clearly indicated. And can be changed as required.

During pouch manufacture on the form, fill and seal machine, the gussets will be pushed inwards to create the gusseted structure, the pack filled and then sealed at the top. As with the Pillow Pouch, there will be a seal area down the center or off center on the back of the pack, leaving one full face and two half faces for the graphic and text requirements.

On pillow and gusseted packs the back seal can be folded (a Fin Seal) or glued (a Lap seal) in one of two ways. Left over right or right over left. These seal types will be further explained and illustrated later. Depending on how the seal is selected the image will automatically adjust.

If we now look at the design and construction of a Stand-up Pouch or flexible bag which has top, left, right and optional bottom seals and a gusset in the bottom, the structural layout is again quite different. Different types of stand-up capability can be achieved depending on the materials used. Paper constructions and plastic construction offer different options. However, let’s take just one example of a Stand-up Pouch. See Figures 4.6 and 4.7.

With this particular stand-up pouch construction, the structural layout includes a bottom double fold so that the pack can stand upright, as well as showing the side seams. There is also a larger top sealing area. The stand-up pouch has a large double front and back face area for graphics. Again all the physical dimensions and sealing areas can be created to meet the specific design parameters and form, filling and sealing machine requirements.

A further example of a pack structure that is perhaps particularly interesting to narrow- and mid-web label converters because of its simplicity and relatively small size (multiples of which can be stepped and repeated across the width of a narrow-web press) is that of the three- or four-side seal sachet, the latter as shown in Figure 4.8 and structurally in Figure 4.9.

Both the face and the reverse of this type of sealed sachet are available for graphics. They can be sealed on all four sides, or on three sides with a fold at the bottom and seals along the left, right and top sides.

It should be noted that Studio Toolkit supports both three-seal sachets with a fold and three seals, and four seal sachets.

Converters looking to produce other types of flexible packaging can find structural templates in Esko’s Toolkit for Flexibles. Dimensions and content can be easily changed and there are various tools that interact with the various shapes. Esko’s solutions also allow companies to reuse text and content from existing artwork.

Esko’s WebCenter includes a controlled and automated process for updating existing artwork and for creating multiple variants for a single product. All stakeholders use a common database for content, automatically pulling text statements and other regulatory content into the artwork to ensure compliance.

Artwork and color management considerations. While the flexographic process used for the majority of flexible packaging is able to produce excellent results, this is only possible if the designer takes into account the limitations of the flexo process. Designs are typically printed with a combination of the CMYK process and spot colors.

Small areas of text and print edges printed in CMYK can then sometimes look slightly out of register on the printed pack due to small dot registration shifts as the web runs through the press. If large areas of the artwork are to be solid color then the use of spot colors, rather than CMYK, is likely to produce better results.

Where transparent film is to be printed it needs to be backed with a solid white color in order to make the subsequent colors stand out. Printing direct on to a transparent film, without white, will generally be harder to read, and the printed product will not look very good. If a clear window area is not necessary, then a white sealant web, such as white PE or CPP, can be used to save on ink cost.

Brand color management across different substrates can therefore be critical to successful flexible package printing. A brand color will almost certainly appear different depending on the particular substrate being used, whether paper or clear, white or metalized films. Materials will need initial fingerprinting before origination can commence. Color pallets will need to be carefully managed at the design and artwork stages and color retouching will generally be required for optimum results.

Brand color management solutions today also include intuitive software that facilitates fast, accurate press-side correction of ink formulations, with the software automating the whole process of delivering absolute consistency from press-to-press, shift-to-shift, and plant-to-plant. Accurate in-line spectral measurement will additionally enable packaging converters to achieve absolute color consistency, with automated L*a*b* measurement on film, paper, or board, ensuring that all printed products are within the customers’ color specifications.
When flexible packaging artwork requires vignettes or gradient screens, it should be remembered that flexo presses cannot print a zero percent screen. Moving from a very fine screen to a zero percent screen will generally create a defined hard line edge . best avoided in the artwork design. With some substrates there may also be a tendency towards dot gain with very fine screens.

Plate gaps. An important consideration when producing plates for flexible packaging is that of the printing plate ‘gap.’ When flexo plates are mounted on print cylinders there is always a small gap where the top and the bottom of the plate come together. This is the plate gap, which results from mounting a flat plate on a round cylinder. This gap is typically between 1.6mm (1/8.) and 3.2mm (1/16.) wide. In most cases it can be worked in to the artwork design, enabling the gap to be barely noticeable . although still there. If the design calls for a continuous solid color with random gaps . such as a reverse-out or a negative image . then a defined plate gap is noticeable. Depending on the color being printed it may be possible to use a secondary plate cylinder to print over the gap, but this will generally only work on dark colors.

A more successful solution to the problem of plate gaps is to use in-the-round (ITR) sleeve imaging, such as DuPont Cyrel FAST ITR system. Although more expensive than conventional plate mounting, ITR systems deliver a continuous print flexo sleeve optimized for precision printing, eliminate the use of adhesive mounting tapes, offer a shorter turn-around time and improved productivity . typically running at higher speeds than flat plates, as well as working well for long runs or for short repeat runs because they eliminate the mounting process.

Imaging in position on a sleeve ensures perfect registration accuracy, eliminates butt joins, provides simplified sleeve mounting on a mandrel containing air channels (see Figure 4.10), delivers longer plate life on the press, and opens opportunities for seamless designs. The imaged sleeve simply slides over a print cylinder mandrel containing compressed air channels. When the air is withdrawn, the sleeve fits tight on the mandrel with minimal risk of plate lift.

PRINTING

Label converters looking to extend their production into flexible packaging will already be skilled in label printing technologies . whether flexo, offset, combination process, digital or even hybrid. It is not proposed to go into these printing technologies in this book. The Label Academy books on Conventional Label Printing processes and Digital Label and Package Printing amply cover these topics.

The aim with this title is to look more at areas where differences in press design, printing, inks, web handling, etc., become increasingly important when producing flexible packaging . particularly films, foils and laminates.

Certainly, flexo is the dominant printing process for both self-adhesive labels and flexible package printing process worldwide, with more than 80 percent of printers using this process. Offset is used for some flexible packaging, while digital printing, from a small base, is now growing faster than conventional press investment.

Gravure is still an important process for wide-web flexible packaging of very long runs. While some label converters may have a gravure unit on their presses, this is more likely used for coating and lamination requirements.

Flexo undoubtedly excels at printing on a range of flexible packaging materials: plastic, foil, acetate film, brown and white paper, and other materials typically used in flexible packaging. That means that to print packaging like bags, sachets, pouches, wrappers, flexible tube films and laminates, as well as self-adhesive labels and lidding, flexo is likely to be the first choice for the many applications of medium and shorter run jobs that are of interest to the label converter. The process offers fast turn-around times for plates and finished products, and can be used with a wide variety of inks, including water-based, solvent-based and UV curable.

Digital printing, or digital hybrid presses, will increasingly come into play for ultra-short runs, multi-versions and variations, variable text or graphics and sequential coding or numbering. Certainly, emerging capabilities to print directly onto thin plastic films using digital inkjet technology are fast improving print productivity, lowering costs, and enabling not only short runs and personalization that meet changing market needs, but also offer brand owners product packaging that exudes a clean look and professional feel that has previously been difficult to replicate.

In addition, the development of HD Flexo printing now also enables converters to successfully compete with offset for quality labels and packs, and with the gravure printing process used for wide-web flexible packaging. In addition, digital plate exposure today ensures greater consistency in plate production and in flexible package printing.

As a general consideration, whatever printing process is being used, it is essential that both raw materials and finished roll handling is undertaken with care. Dirt or dust on rolls can affect print and rewind results and create possible print defects, web splits or tears. See reference under web cleaning later in this chapter. How and where incoming and finished rolls are stored can also be important. That includes temperature and or humidity levels during storage and handling.

The same comments should be applied to the press. Materials running at high speed can attract dirt and dust from the atmosphere and in and around the press. It is therefore important that presses are kept spotlessly clean to minimize such problems. This includes proper cleaning and inspection of inking rollers, impression cylinders, transfer rollers and systems to ensure that particles are not transferred to plates, cylinders, sleeves or substrates during printing.

Let’s therefore concentrate on those areas of flexible package printing and converting where more particular attention and investment may be required by the label converter as the product portfolio expands and changes.

Press web width. When printing flexible packaging to be used with vertical form, fill and seal machines it is important to remember that the film, paper or foil material width must be at least twice the bag or pack width, plus the width of the longitudinal seam. Conversely, this means that the bag or pouch is half the film width, minus the width of the longitudinal seam. This can be explained in Figure 4.12.

In the example shown, the face of the pack is 146mm wide; the back of the pack is in two halves, each 73mm wide, on either side of the longitudinal seam. The seam allowance in this case is 18mm in total, with 9mm on either side of the pack overall width. This gives a total pack and substrate width for this example of 310mm, plus any additional edge trim required.

This enables one pack across, say, a 330mm web width press.

Even with a press width of 330mm (13”) the market opportunities for a label converter to move into flexible packaging can be somewhat limited. Successful narrower web converters producing both labels and flexible packaging are more likely to be obtaining orders with flexo presses with 430mm (17”) or 450mm (18”), or even wider wide web widths.

Seams. It should be noted that the longitudinal seam may be formed in one of two different ways, which affects the seam width, and press width measurements. In a straightforward overlap seam, one edge is placed over the other edge (Figure 4.13 left) and the two edges are sealed together. 

In all of the laminating processes described the resulting laminated web is then rewound into a finished roll.

Label converters moving into laminating will probably already be most familiar with adhesive laminating.

It should be noted that adding an overlaminate may require the packaging equipment to run at higher temperatures and therefore a destructible bond is required between the overlaminate film and the base film structure. It is highly recommended that the overlaminate is one that has been specifically designed for flexible packaging applications.

The two main methods of applying a flexible packaging overlaminate to the web on a narrower or mid-web label press are:

• Either, a self-wound thin PET laminate can be applied on the press. This is similar to using an overlaminate on a label press. Special handling of the overlaminate on the press may be required due to aggressive adhesive and thin film.
• Or, a wet adhesive laminate using UV lamps for the curing. This will require UV lamps to be in premier state, with a curing time for the structure between 24 and 72 hours.

BARRIER COATINGS

A barrier coating or sealing coat can be applied to flexible packaging webs to prevent migration of ink, adhesive, or other substances through the face material. Barrier and other functional coatings encompass materials that are coated onto substrates to provide a barrier to protect selected packaged goods.

Barrier coatings, providing barriers for food packaging requirements, may include protection against oxygen and aromas, liquid water and water vapor, oils, and grease. An effective barrier can prevent both losses from the packaged product, and penetration into the package, both of which can affect quality, and shorten product shelf life. Packaged food products are being maintained fresh longer as a result of new materials, and food processing developments. For example, O. scavengers are now being used that work within a sealed package to limit O. reaction with a food product. Combined with effective O. barrier packaging, food packagers have the ability to improve shelf life, preserve product appearance, and flavor, while minimizing preservative use.

Additionally, antimicrobials, while under siege, have been proven effective as additives to coatings, and packaging films, in combating food sourced illnesses. Nanotechnology is being applied to improve the gas barrier properties of coatings. In doing so, nanoclay is dispersed in barrier coatings, resulting in a platelet orientation that creates a 'torturous path' for gas molecules to traverse, yielding a very thin film, effective gas barrier.

Knowledge around migration and/or barrier protection is crucial for servicing the flexible packaging food markets. Suppliers must be able to provide the right barrier or seal for a given application and be able to support clients with ‘fitness for use’ testing.

PACKAGE SEALING CONSIDERATIONS

Cold Seal Packaging. Many temperature sensitive confectionery items (e.g. chocolate) are packaged and sealed using cold seal adhesives. These surface printed constructions consist of a surface printed ink and a cold seal release lacquer (CSRL), which prevents the printed ink from offsetting against the cold seal glue when in the printed roll.

Inks for cold seal packaging should not be formulated with any kind of fatty amide (erucimide) or PFE waxes as these are said to ‘poison’ the cold seal adhesive if left in contact with the adhesive for any length of time in the printed roll.

Heat Seal Packaging. Both surface printed inks and lamination inks can also be heat sealed. In this instance a heat sealable film is used or a heat sealable coating applied to the packaging material is used to combine two films by applying heat to achieve the seal. Sealing temperatures and pressures can vary so when formulating these types of inks, it is important to know and test the inks under the specific sealing conditions. A typical sealing specification may be 350 degrees Fahrenheit for half a second at 40p.s.i.

COEFFICIENT OF FRICTION/SLIP

Before leaving this chapter it was considered important for label converters moving in to flexible packaging to have an understanding of the role that coefficient of friction/slip can have in successful printing and subsequent packaging line performance.

Coefficient of friction (COF) is a term that is significant in the world of packaging and package printing and is critical to the optimum processing and handling of packaging from filling operations through to the consumer. By definition, COF is a measure of slip, or how one surface moves across another. The importance of this is in how a substrate moves through and across print stations, conveyor belts, on the form, fill and seal machine, or released from a mandrel or loading after processing.

It should be noted that there are two kinds of coefficient of friction: static, which is the force needed to begin movement, and kinetic, the force required to maintain movement. Generally, static COF is of greater concern for stacked or palletized items, while kinetic COF is important when using roll films. It provides a relative indication of frictional characteristics and is routinely specified in substrates such as plastic films used by flexible packaging converters.

Films with COF values greater than 0.5 are considered non slip. COF is usually specified for a given process and adjusted by the printer with inks or varnishes as needed. Depending on the packaging application, a high or low COF may be desired. But why does it matter?

In flexible packaging applications where a film web is pulled over a forming collar, such as with vertical form, fill and seal equipment (explained in the next chapter), a low COF is considered to be favorable. If too high, the flexible film may bind when sliding over the filling collars. However, if the static COF is too low, problems in maintaining stability in stacks could result, as well as difficulty pulling materials through automatic processing machinery. The importance of COF requirements in different filling and packaging applications can be summarized as follows:

• In VFFS (vertical form fill and seal) systems, too much friction of the sealant side of the film can cause poor film feeding over metal forming collars and inconsistent package sizes
• In HFFS (horizontal form fill and seal) systems, too much friction of the sealant side of the film can lead to film dragging or jamming as it passes over metal plates
• In either system, too much friction can result in lateral slipping that leads to poor seals (leakers)
• Too little friction on the outside can cause packs to slip or fall-off-of inclined conveyor belts
• Too much friction on the outside can slow packages’ progress down delivery chutes
• Too little friction on the outside can result in packages sliding off of stacks or pallets.

Controlling COF gives the form, fill and seal processors the ability to optimize performance and avoid problems in forming, filling, transporting, and storing of the finished packages. 

Getting the COF right is mostly a process of qualifying a packaging product and then supplying this product at a consistent COF specification based on a given instrument and testing procedure. It is important to remember that many converting variables cause COF to vary. COF can be affected by a number of factors, including antiblock additives, corona treatment, inks, varnishes, adhesives, application viscosity, coating weight, drying conditions, and environmental humidity, while in energy-cure systems, COF is affected by the degree of cure. Running to proven standards will ensure low waste and fewer field problems and returns.

In packaging films, the traditional technology to lower COF is to incorporate fatty amide, waxes, and silicones, and add silica particles to increase COF. Newer technology uses more stable proprietary non-migratory slip packages.

Customized COF’s are achieved by adding a ‘slip agent’ to a film resin during production. The traditional approach to reducing COF involves adding a compound that is incompatible with the film resin, and will migrate to the surface of the film over time. Non-migratory slip agents offer benefits in the area of thermal stability and consistency, but can affect film clarity.

Label converters moving into flexible packaging production should consider investing in COF testing equipment, of which there are two common basic types — sliding plane and incline.

CONFIGURING A LABEL AND PACKAGE PRINTING PRESS

What can be seen from the information in the preceding pages is that a narrow/mid-web press designed for label and package printing will need to meet certain web handling, control, print and finishing requirements. In particular, the press will need to be able to print and cure/dry all the necessary types of inks, varnishes and adhesives (see more information in the next chapter), whether on supported or unsupported films, and in web widths increasingly offered by key press manufacturers of 420mm, 430mm, 450mm, 480mm, 500mm, 520mm, 620mm and even going up to 750mm wide.

Stages in the flexible packaging printing and converting process are:

• Printing
• Coating (varnishes, primers)
• Laminating
• Inspection/slitting/rewinding
• Forming, filling and seaming

Factors to be considered include register control, web tension, rewind tension, methods of cure/drying, heat control, smell, and special applications such as heat-seal and cold-seal. In terms of register and web tension control the converter will likely need to consider a short web path, a free running counter pressure roller, web transport by chill drum, low tension and paper tension rewind.

In terms of curing, the flexible packaging printing and converting press has to be able to cure/dry using all types of flexible packaging inks, varnishes and adhesives:

• UV
• Mercury
• LED (hybrid or not)
• Nitrogen
• Hot air
• Water-based
• Solvent-based (explosion proof)
• Infra-red
• EB

New generations of label/flexible packaging press may be hybrid machines, have application modules on rails and incorporate intelligent web transport systems.

Where lamination of flexible packaging webs is to be carries out this may be undertaken all in one pass (print/UV laminate/slit/rewind). Alternatively, the web may be printed and laminated in one pass, stored for 8 to 24 hours and then slit and rewound. Another option is to print, to laminate, store for 24 hours and then again slit and rewind.

Laminated flexible packaging webs may be two layer laminated products (Duplex webs) or three layer laminated products (Triplex). This can be seen in Figure 4.16.