Understanding the basics of label application

Accurate die-cutting is an essential element in successful label application. The cutting die – flat, rotary or wraparound – should only cut through the face material and adhesive of the pressure-sensitive laminate (see Figure 2.2.), but not cut into or through the backing paper. The matrix is then removed to leave the desired label shapes on the web (Figure 2.3.). 

The printed, die-cut and waste-stripped labels that come off the press are usually in large master reels, often with a number of label repeats across the web.

These master reels are slit and rewound into smaller, single-width, applicator-sized, reels before being despatched to the customer. This process is called rewinding and is the operation of unwinding printed die-cut labels from the press roll through a machine that slits them to the required widths, and maybe inspects them to enable faulty or missing labels to be replaced. The number of labels on each roll is counted and the labels rewound into smaller rolls of the correct size ready for end-use application, with the correct leading edge for the specific application line.

Poor or faulty die-cutting may lead to problems at both the waste stripping and application stages. Too loose or too tight rewinding of the labels and the wrong leading-edge direction can cause application line problems. If there is no gap between each label on the liner, the leading edge of the label cannot be released and peel away on the beak or peel plate.

IMPORTANCE OF REWIND AND UNWIND DIRECTION

Most label application machines will only work with one label unwind direction, which needs to be known before slitting and rewinding of press rolls takes place. Pressure-sensitive labels wound in the wrong feed direction or orientation will almost certainly cause production delays and extended down time. Figure 2.4 indicates the range of rewind options used with pressure-sensitive applicators.

Depending on the end-use application requirement, labels can be rewound on the inside or outside of a roll, and with either the narrow edge leading or the wide edge leading. Fig 2.5 shows labels wound on the outside, with the right edge leading. The diagram also shows how the terms label gap, label length, label width and label roll core are defined.

If both front and back labels are being applied then the rewinding specifications and requirements may be different for each.

DETERMINE CORRECT APPLICATOR REEL SIZE

As dispensing and application machines come in many different shapes and sizes it is also important to know the minimum and maximum outside diameter of the label reels that can be held in the unwind holder (magazine). The reel diameter measurement is also the larger the outside diameter of the unwind holder, the larger the reel size that can be accommodated.

Larger reel sizes on the application equipment will mean less changeover time and more production efficiency. However, the reel size must not be so large that it cannot be handled safely by the operator. Knowing the maximum outside diameter of the reels will allow the number of labels per roll and the size of roll to be calculated.

ESSENTIAL COMPONENTS OF LABEL APPLICATION

There are many different types of machines and devices for dispensing or applying labels, but they all need to incorporate a number of key features:

1. The Labeling Head, which incorporates an unwind reel holder or magazine that will carry the reel of labels, and a means of controlling the web path at the required speed through the applicator. The liner pulls the web through the labeling head and there are elaborate tension controls to ensure that the pull is always greater than the push.
2. The Applicator, with a means of peeling away the backing material (release liner) by pulling it back (see Figure 2.6.), normally over an acute metal beak, so leaving the label free to be applied to the product, either by hand or automatically, together with a means of feeding the semi-detached label forward and pressing/applying the label to the surface to be labeled. 

Extensive testing has shown that with a 2mm radius on the peel tip or beak, the optimum angle to pull the liner is 7 degrees.

The labeling head and the applicator are regarded as one single unit by most equipment manufacturers.

3. The product handling system, which is designed to consistently place the product or item being labeled in the right position and speed for the applicator to do its job.

The labeling process involves unwinding a reel of die-cut labels and then pulling the web under tension around the beak (Figure 2.7).

As the backing material is taken around the sharp angle of the plate the front edge of the label peels away from the backing and the label is pressed to the surface, while the backing liner runs around the beak to be removed and rewound on the rewind take-up spindle. The aim throughout is to apply differential pressure and/or degree of wrap to achieve a consistent tension at the beak.

Many applicator systems include a separate rewind motor to maintain accurate and consistent pull tension at the beak.
                 
UNWINDING AND CONTROLLING THE LABEL WEB

The labeling head incorporates a control module which manages the applicator functions and tells it when to stop and start. 

The roll of labels to be applied is placed in the unwind reel holder or magazine, and threaded through the applicator. In many cases the label web will go around a dancer roller that helps control the unwind rate and enables a free flow of labels from the unwind and through the applicator system.

A light source and a phototransistor read the gap between labels to tell the labeler when to stop. The web path may also include an idler roller to help with a smooth passage of the web.

An adjustable tension device or shoe (Figure 2.8) is commonly located above the stripper plate or beak. This is adjusted to apply slight pressure to the label being dispensed to keep it laying flat until it reaches the plate or beak tip, ensuring an ideal pressure through a gap sensor and aiding the peeling of the label from the backing release liner.

To pull the label web through the applicator system a drive shaft roller and a pinch roller are usually located towards the end of the web path. These rollers also feed the backing liner to the liner rewind spindle.

A product sensing device is used to read the presence of products to be labeled.

REMOVING THE LABEL FROM THE BACKING

One of the commonest methods of dispensing or removing die-cut labels from the web at the point of application is by using a metal beak.

Depending on the equipment manufacturer this may alternatively be called a stripper plate, a tongue, a dispensing blade, a peeler or peel plate, a bar, or a peeling lip. In all cases the label web is bent around the beak or peel tip at a sharp angle, with the front edge of the label peeling away from the backing liner as a result of the liner pulling action. The liner continues on to be rewound.

Stripper plates or beaks are found in a variety of configurations, including fixed plates and floating plates. Both variations can be seen in Figure 2.10.

Rigid stripper plates are usually used for very uniform products, while spring-based plates may be recommended for certain side labeling applications. Floating stripper plates, either solenoid or pneumatically operated, may be used to place labels accurately into recessed areas on a pack. Powered stripper plates need to be accurately controlled by electronic timers, so that the label is pre-dispensed or maintained in contact with the pack or product for the required period.

Most systems will actually float the label onto the container surface at shallow angles. The leading edge of the label is often ‘pre-dispensed’ by a small length to help break the bond of the adhesive to the release coating. The 90 degree stripper plate design has a minor impact on release, as the liner is traveling marginally faster than the actual label around the guide pulley.

APPLYING THE DISPENSED LABEL

Once the labels start to be detached from the backing there are various ways of feeding them forward and pressing (applying) them to the pack, container or product. The effectiveness of the labeling operation is determined by the uniform application of pressure to ensure a positive contact of the adhesive with the surface of the product being labeled.

The three most commonly used methods of uniformly applying pressure to the applied label are:

1. Direct transfer
2.  Tamp
3. Air-jet

Direct transfer application. Direct transfer, also referred to as roll-on or wipe-on, makes use of a rubber (see Figure 2.11) or foam roller and sometimes a drum applicator device to press and fix the label to the product or pack as it passes below the applicator.

May also be supplemented by a brush or brushes.

Roller or drum application devices are one of the most common methods of affixing pressure-sensitive labels to products or containers.

Ideal for flat, regular surfaces or cylindrical containers or packs, they may also be used to label concave and convex surfaces.

By employing two application heads it is possible to apply labels to the bottom and top of objects (Figure 2.12) simultaneously with only one pass through the machine.

By setting the applicator heads horizontally and facing each other it then becomes possible to also apply labels to both sides of objects (Figure 2.13). 

It is important that the label being dispensed is traveling in the same direction and speed as the item being labeled during the label application process.

Direct transfer label application is widely used for primary and secondary labeling and for wrap-around applications.

Tamp application. With tamp application the label is dispensed from the release backing and sucked into place and held in position on a vacuum tamp pad or plunger before being extended to the product. When the product is in the correct position, the suction is released. The label is driven to the application position by an air cylinder and applied to the product by direct pressure. See Figure 2.14.

The result is a highly accurate labeling system. With such systems – which may also be referred to as tamp-on, pressure-pad or pneumatic plunger – it is possible to select any surface, direction or angle, up or down, simultaneously to both sides, for label application – for example to place labels at an inclination of 45 degrees.

By employing a longer plunger arm it is possible to apply labels into deep recesses or even into the bottom of something like a cup or glass. Depths up to 150mm (5.1 inches) or so may be reached. Some of these variations can also be seen in Figure 2.14.

Air jet/air blow application. This is another frequently used method of label application. The labels are stripped from the backing as usual and then, after release, are retained by a vacuum on a honeycomb grid, from where they are subsequently blown onto the product or container by applying air pressure (see Figure 2.15).

Sensors at the peel tip ensure there is no double labeling or missing labels, allowing for fast, accurate placement. Tampers or brushes may additionally be used to press the label firmly down for wrinkle-free label attachment.

Using this method it is possible to apply labels in positions that would be virtually impossible by other methods. Absolutely no damage will be done to the surface or product to be labeled.

A variation of air jet application is where the application head is built into a mechanical arm which can then be positioned to label products such as television tubes. This type of application is illustrated in Figure 2.16.

Other application options. Other methods of uniformly applying pressure to the label to ensure that it sticks with a positive contact include hybrid applicators (using two or more of the methods already described), vacuum wrap, dual-web and multi-label applicators.

It is possible to achieve great flexibility with one labeler handling a range of different products. Examples of some of the different types of dispensing and application devices are given on the next page.

With some systems it is feasible to apply labels on and around the corners of boxes, such as for medicines or confectionery box sealing, or even three panel labeling. These can be seen in Figure 2.17.

A variation of conventional roller application used on some machines is the use of rollers made of knurled aluminium rings, so allowing packs with uneven surfaces to be handled, as in Figure 2.18.        

To ensure that labels are applied accurately and consistently to products, pressure-sensitive label applicators may be fitted with various label or product tracing/sensing/control devices such as the one shown in Fig 2.19.                              

Label sensing or tracing devices monitor the gap between the individual die-cut labels and interrupt the web feed at the end of each label. This resets the system to receive the next start signal from the product tracing devices, which include micro-switches, photocells or spot color readers.

Other controls on the web may be used to detect missing labels, the end of the reel or web breaks (see Figure 2.20 and 2.21).                                            

With many applicator systems the actual product or container triggers the application of the label through the use of product tracing/control devices.

This means that even with erratic or irregular product flow the system will automatically compensate and only dispense a label as required. 

Container or conveyor speed encoders may also feed back to the applicator so that the next label is applied at the correct speed. A fully loaded in-line slat conveyor will run faster if containers arrive with big gaps.

Many different scanning/sensing technologies are available on pressure-sensitive label applicators, from photocells with reflector mirrors, transmitter/receivers, proximity switches, spot or color readers or, for certain applications, micro-switches.

For semi-automatic applications foot switches may be used.

It is also common for label applicators to be part of a larger packaging or assembly line in which they will receive the pack or product to be labeled from a previous automation stage, apply the label, and then feed the labeled item onto the next operation or stage in the process.

Sophisticated handling, product orienting, conveyor belt and other control systems or devices may then be required, as well as a means of product detection and ejection (see Figure 2.23.).

Where containers are labeled empty - before filling - there are usually feedback loops with queue sensors which will slow the labeling machine if the filler stops or the bottle feed is interrupted. Accuracy is better where systems slow rather than completely stop.