Figure 5.1 Wet-glue labeling is a cost effective way of applying highly decorative labels to glass bottles
The key uses for glue-applied labels are in the high speed, high-volume, low changeover labeling of drinks bottles and for canned foods – both human and pet food – where application speeds up to 60,000 – 80,000 or more containers per hour are available.
Although used mainly for primary decoration some wet glue labels are also applied as top straps over the closure as a tamper evident or security feature.
WET-GLUE LABELING SYSTEMS
The wet-glue labeling system comes in three main formats:
Punched labels where the labels are ram-punched into specific profile shapes, which are compatible with the container profile. These are also referred to as 'patch labels'.
Cut labels which are rectangular or square in shape. This profile is suitable for 360 degree wrap- round decoration or as patch labels.
Roll-fed labels, supplied in reels for 360 degree decoration. These are unconverted labels and are not cut to shape, but cut singly from a reel at the point of application.
The majority of wet-glue applied labels are manufactured by a conventional multi-stage operation and are printed and embellished in sheet form (Figure 5.2). In the case of high-volume gravure printed wet-glue labels the printing would be carried out on a web fed gravure press and the printed reels would be transferred onto a sheeting unit which can be an in-line or off-line.
There is however an increasing use of medium size (350-650mm web width) multi-process web-fed presses which print and embellish in one pass.
Figure 5.2 Heidelberg Speedmaster multi-stage sheet-fed printing press
The first stage of this operation is the production of sheets of printed labels. If foil blocking, embossing, bronzing, or over-varnishing is required then the printed sheets are further processed through each separate stage. The finished sheets of labels are then guillotined, and square cut or profile punched (Figure 5.3), batched, packaged and palletized.
Figure 5.3 Blumer Strip cutting and ram punching unit profile cutting a wet-glue label
MULTI-PASS SHEET FED WET-GLUE LABEL MANUFACTURING VERSUS SINGLE PASS MANUFACTURING
Manufacturing using multi-pass processing can have a production cycle of several days, or in some instances several weeks dependant on the quantities required, with perhaps 10 people involved in the processes as sheets pass through the printing, embellishing and converting processes.
The length of this extended manufacturing window is a problem, with brand owners demanding more frequent short run orders and quicker delivery schedules. The problem of this lengthy manufacturing window has led to the use of web-fed combination/platform presses.
These presses have evolved via the self-adhesive industry and can replicate the printing, embellishment and finishing techniques used for conventional wet-glue label manufacturing.
Multiple processes can be combined using all the major printing and embellishing processes i.e. offset litho, rotary screen, UV flexo, gravure and digital printing plus foiling, embossing, laminating and converting into single labels, all in one pass.
Figures 5.4 and 5.5 illustrate the effect that one-pass printing and converting has on the manufacturing window. The significant difference is the removal of multiple passes when labels are printed in sheet-fed format.
Figure 5.4 Standard sheet fed wet-glue label manufacturing with multiple passes
Figure 5.5 The in-line reel fed one pass wet-glue label manufacturing process
SUBSTRATES USED FOR WET-GLUE APPLICATION
Paper face-stocks such as Chromolux 60, 80, 90gsm materials are commonly used but increasingly popular forms of wet-glue paper labels are foil-laminates and more recently metalized papers. These offer a reflective metallic finish, which can be printed on, offering excellent design opportunities.
Metallized paper, as opposed to foil-laminates, uses less aluminium and could be said to be more environmentally friendly, using less non-renewable natural resources.
The label papers are available in standard, wet strength, embossed and high gloss versions to cover all the technical and marketing requirements and are suited to a wide variety of other applications, such as box covering, lamination, food wrapping and interleaving.
TYPICAL WET GLUE PAPER SUBSTRATES
The main types of wet-glue paper materials and their characteristics are summarized here;
Less suitable for quality image reproduction
More suitable for quality image reproduction
When selecting a substrate for use in wet-glue label applications the following factors must be considered;
Direction of the grain and the resulting effect on label curl in uncontrolled humidity
Substrate moisture content during storage and transportation
Humidity levels in the printing environment
Humidity levels in the filling and label application areas
High gloss lacquers are used on the face of wet-glue applied labels for scuff resistance and increased shelf impact. For lithographic printing, the norm is the use of ultra violet (UV) lacquers, whilst for photogravure printed labels, a range of solvent and water-based lacquers are used. A combination of matt and gloss lacquers can give increased shelf impact.
Embossing is used where a specific part of the design is raised to enhance the appearance of the label.
With photogravure printing, in-line all over or partial embossing is possible, producing a tactile feel to the label in addition to helping improve line efficiencies, especially in relation to metalized paper labels.
Foil-blocking is used where parts of the design can appear in a high gloss reflective foil. This is available in a wide range of colors.
WET-GLUE LABEL APPLICATION
In general terms this system of label application places a single cut label from a stack of labels, direct into the container using either wet-glue adhesive or a hot melt adhesive system.
The adhesive can be applied to either the full area of the reverse side of the label or by gluing the trailing and leading edges of the label. The type of adhesive application can also be selected from options such as skip, pattern or stripe, depending on adhesion, application speed or drying speed.
The key methods used for applying a wet-glue label are simply explained here.
In can labeling adhesive is first applied as a stripe to the container and a second stripe of adhesive is applied to the trailing edge of the label as it rotates over the label magazine.
The stages involved in the labeling of a can are illustrated in Figure 5.6.
The application of a typical patch label is illustrated in Figure 5.7. In simple terms the cut label is picked from a stack, held in place by a vacuum, whilst wet-glue is applied to its reverse by a glue application head and before it is transferred to the bottle or container.
Figure 5.6 Stages involved in labeling a can
Figure 5.7 A typical patch label application sequence
Figure 5.8 shows a system operating two hot melt gluing units. Gluing unit 1 applies a vertical strip of glue to the container. The container revolves as it passes the label magazine and the label is then picked from the magazine by the glued strip.
Gluing unit 2 applies glue to the trailing label edge and the label is wrapped around the container. The label is glued together at the overlap.
Figure 5.8 Single label leading and trailing-edge gluing using a hot melt double head. Key- 1 Hot melt gluing head, 2 Pre-cut single labels in magazine, 3 Hot melt gluing head
Figure 5.9 shows the positioning of strips of adhesive on the leading and trailing-edges of the label. This method uses much less adhesive than systems that apply adhesive all over the label.
Figure 5.9 Wrap round label featuring hot melt glue strips on leading and trailing edges
WET-GLUE WRAP AROUND LABELING
The format of the wrap around label is exactly as the wording describes i.e. the label wraps fully around the container giving 360 degree graphics.
Labels are usually rectangular in shape and are very popular for decorating bottles, jars and cans.
Application can be from a stack or a reel and usually features an overlap which allows the leading edge and the trailing edge to be glued together.
A key advantage of the wrap around label is they are cost effective as the container can be decorated with a single label and not multiples as some applications are i.e. body, neck and back labels.
Figure 5.10 illustrates a typical reel-fed wrap round label application.
Figure 5.10 Typical reel fed wrap round applicator
WET-GLUE – ADVANTAGES AND DISADVANTAGES
Wet-glue paper labeling offer the following benefits:
The lowest label cost of all methods decoration
High-speed application rates, exceeding 1,000 bottles per minute
Excellent ability to reproduce photographic designs, tone-work, metallic colors, etc.
Relatively low design origination costs for lithographic plates
No waste matrix or liner to dispose of
Easy removal of the labels from returnable bottles in a caustic wash bath.
The constraints of wet-glue labels are as follows:
Multi-stage manufacturing with longer lead times
High cost of application equipment – including change parts and adhesive
Cannot be applied to highly-shaped areas of containers
Paper labels have limited moisture resistance compared to plastics labels
Poor product and damage resistance (paper).
Wet-glue labeling continues to dominate in the high volume sectors for bottle labeling, notably beer and spirits, mineral water and canned foods.
More self-adhesive suppliers are however, competing in the traditional wet-glue markets, particularly in the wine and spirit sectors and in those markets that use filmic substrates.
Many end users that have invested in wet-glue application systems are reluctant to change to self-adhesive for high volume run lengths.
Wet-glue labeling in comparison to the self–adhesive labeling system has much higher application speeds facilitated by label substrates specifically developed for high speed application.