Converting heat shrink sleeve labels

It is tangential rotary shear slitting that produces the cleanest cut and the least impact and stress on the slit edges of the film. This in turn maximizes our opportunity to produce a quality seam with minimal to no visibility on the finished product. Figure 5.2 represents an example of a roll with raised edges on each side of the film roll. 

These raised edges may be the result of one of two unsuitable slitting methods.

The first unsuitable slitting method for shrink sleeve film is crush slitting (or score slitting, Figure 5.3), which does not produce the clean, crisp cut to the film that the shrink sleeve process requires. The second unsuitable slitting method is razor-in-air slitting (Figure 5.4), which also produces an inferior cut to the film. 

In either case, as illustrated in Figures 5.6 and 5.7, the edges of the roll have been deformed and rendered unsuitable for subsequent solvent seaming (or welding). The requirement for the finished seam on a shrink sleeve label to be as minimally visible as possible calls for flat edges and a clean cut. The only slitting method that will provide crisp, clean edges is shear slitting (Figure 5.5).

As a further illustration of the appropriateness of shear slitting, Figures 5.8 and 5.9 represent a 50x magnification of two slit edges. Figure 5.8 represents film cut using the shear slitting method, which has a demonstrably cleaner cut. Figure 5.9 represents film cut using the razor slitting method, which results in a much less crisp cut that makes the subsequent seaming process infinitely more difficult. Shear slitting is always the recommended method for slitting shrink film.

WHAT IS SHEAR SLITTING, AND HOW DO YOU IMPLEMENT IT?

Shear slitting is the process by which two rotating circular blades cut a moving web of film at the point where the two blades contact each other (Figure 5.10). This process is identical to the process of how cutting shears, or scissors, slice one piece of paper into two.

There are two types of shear slitting: wrap shear (Figure 5.11) and tangential shear (Figure 5.12). Traditionally, wrap shear is deployed for thinner materials, as the wrap curvature on the bottom blade opposes vertical deflection, preventing the web from dipping under the blade. Tangential shear slitting employs the use of entry and exit rollers, and its top blade is offset to its bottom blade. As a result, this type of shear slitting produces the same cut as wrap shear but with less risk of deformation to the edges of the film. Thus, tangential shear is preferred over wrap shear for shrink film.

Now that we have established shear slitting as most appropriate for shrink sleeve film, we now turn our attention to the five keys to shear slitting success, which are:

• Depth (Overlap): The depth is set by how far the tangent point of the top blade is engaged beyond the tangent point of the bottom blade (see Figure 5.13).

• Cant Angle: The cant angle forces the shear knife to contact at the overlap entrance point (see Figures 5.14 and 5.15). The cant angle used will vary depending on the type of material being shear slit, as is explained in detail in Figure 5.16. 

• Knife/Blade Profile: The type of material being slit again determines the knife/blade profile. The primary top blade profiles used are 25, 45 and 60 degrees; 25 degrees is used for stiff, high-density webs; 60 degrees is used for thicker, lower density webs; and 45 degrees is used as the default tangential shear top blade. Satisfactory results with shrink film have been achieved using both 45 degree and 25 degree blade profiles.

• Side pressure (force): Side pressure, or force, is the number of pounds per square inch of pressure that the top knife places on the bottom knife. The objective here is to use the right amount of side pressure; too much can negatively impact knife sharpness and blade wear, while too little will not fully enable the bottom band to drive the top blade.

• Knife Sharpness: Knife sharpness is critical to create the clean slit edge that the shrink sleeve process requires. Improper cant angle positioning and the use of too much side pressure will negatively affect knife sharpness.

THE SEAMING PROCESS

Having achieved crisp, clean slits without deformation of the roll edges, we are now ready to move to the seaming (or welding) step. While our objective is to transform the flat, printed film into a seamed tube (see Figure 5.17), the goal should always be to form the perfect seam that is minimally visible and tactile. Our further goal is to do this with maximum throughput and minimal waste. Throughput is a function of speed and uptime on the equipment, while waste tends to be a function of using the right ingredients (i.e., film and solvents), equipment features and knowledgeable operators.

At this point in the process, we need to introduce some industry terminology and important concepts, which are also depicted in Figures 5.18 and 5.19:

• Layflat: A finished shrink sleeve that is ready for application to a container will be in the form of a flat tube. Measuring from edge to edge of this flat tube, the term used to describe this distance is called the layflat, or the layflat width.
   
• Slit width: If the layflat tube is unfolded, it provides the slit width, which describes the width of the material as it came off the slitter and entered into the seaming machine.

• Overlap: The overlap is the portion of the shrink sleeve tube where the seam is welded.

• Seam location: The layflat tube is bonded together using a solvent, which creates a seam. The position of the seam on the tube is called the seam location, and the type of container being decorated and/or the preferences of the brand owner will typically determine the seam location. Container shape is a significant determinant; the seam location on a round container may be of no importance, but with square containers, oval containers or trigger bottles, seam location is very important. For example, it is generally preferable to avoid having a seam located on the front center panel of a product such as a household cleaning agent that is packaged in a trigger spray container.

• Solvent: As previously mentioned, the layflat tube is formed using a solvent weld and typically not a glue or adhesive. It is critical that the solvent and film chemistry be right to chemically bond the film material.

• U-folds: It is desirable to avoid crushing the edges of the material during or after the seaming process. The rationale for this is to avoid introducing fold lines that may be visible on the finished label, or worse still introduce a risk of cracking the ink, which will be visible on the finished label (see Figure 5.20). 

• Solvent placement and consistency: To achieve the perfect seam every time, converters must be particularly mindful of the amount and placement of the solvent used to form the seam. Figure 5.21 visually demonstrates good solvent placement and consistency. More specifically, the solvent reaches to the edge of the seam without going over the edge, and a consistent amount of solvent is applied across an even width of the film. 

Figure 5.22 provides four commonly observed instances of poor solvent placement and consistency:

• Not to the edge of the overlap: When the solvent placement falls short of the edge of the overlap, the result is a coarse edge that the brand owner may reject.

• Past the edge of the overlap: When the solvent placement falls outside of the edge of the overlap, the result is that the solvent will bond with another adjacent layer of the film roll, which will cause ’blocking’ as the roll is unwound.

• Skips/voids: When the solvent placement is inconsistent on the film, the seam will not fully form and will likely open or come apart on the container if not before even being placed on the container.

• Inconsistent flow line: Two factors can contribute to having an inconsistent flow line. The first may be due to a problem with the solvent flow control, or solvent delivery system. In the example shown, the narrowing of the solvent line may be due to an inconsistent flow of solvent due to the use of, for example, a peristaltic pump.

THE PROCESS OF SEAMING – HOW A TYPICAL SEAMER OPERATES

The process of seaming requires a stand-alone machine that has several key characteristics, all of which are illustrated in Figure 5.23. The unwind portion of the seaming machine is where the flat material is loaded and webbed through the machine. 

The web guide makes corrections and adjustments to the web when necessary.

The linear perforation section, which is located just before the forming station, perforates the web in the machine direction, just before the forming process takes place. The purpose of this perforation is to give the consumer the ability to remove the sleeve from the container for recycling purposes.

The forming section is where the flat web is converted into a sleeve, or tube. This is also where the solvent is delivered to the web. The nip section is used to isolate tension, and it is also where all air is removed from the newly-formed tube before moving to the rewind section. The rewind section with oscillation is where the completed roll of seamed material is offloaded. During this process, the rewind function must have the ability to oscillate the roll. The need for this is illustrated later in Figures 5.28 and 5.29.

SOLVENT CONTROL

To form a shrink sleeve label, a seaming machine forms the material into a tube using a solvent, which produces a chemical reaction that welds the material together. There are three ways to control the flow of solvent to the web (Figure 5.24) when forming the material into a sleeve (or tube).

They are gravity-fed, pressure system, and servo pump systems. It is important to achieve consistent results throughout the roll, and acceleration and deceleration, or running the web at different speeds, can influence the result. The amount of solvent and the rate at which it is applied, proportionate to web speed, is critical to achieve a properly formed sleeve with minimal waste. 

SOLVENT DELIVERY METHODS

Solvent wheel, top wick, bottom wick, and needle are the types of delivery methods most frequently used to apply solvent to the web (see Figure 5.25). Solvent wheels are the least popular of the four. Wicks, both top and bottom, and needles are more commonly used, though both have their advantages and disadvantages. Using a wick gives the operator the ability to easily apply solvent to the edge of the seam. The problem is that wicks tend to pick up contamination and can leave striations in the seam. Some converters that print shrink sleeves using the offset printing process may actually require a wick. A fountain solution that has excessive ink contamination can cause problems for seaming solvents if ink residue is deposited in the clear seaming area. This ink residue impedes the chemical reaction between the solvent and the film, and a weak seam can result. In this case, using a wick to wipe the solvent onto the surface may help as it abrades the surface of the film and aids penetration of the solvent. High-speed seamers bring greater levels of sophistication to this process, but with the increased speed comes a need for greater solvent placement accuracy. It is for this reason that high-speed seamers typically use a needle to deliver solvent to the web. 

SOLVENT DELIVERY LOCATIONS

A seaming machine can deliver solvent to the web in two locations: before the forming table while the material is still flat and stable as it passes over a roller, and on the table while the material is already formed in a tube, and just prior to it being nipped. The two solvent delivery locations are pictured in Figure 5.26. Either location is acceptable, but each has a different requirement of the seaming solvent used. 

Suppliers offer a slower reacting solvent for slower machines that utilize solvent location 2. Suppliers will provide a quicker reacting solvent when using faster, higher speed machines that are delivering solvent to location 1.

Regardless of the solvent delivery location, the speed of the equipment and its ability to monitor and maintain tolerances is what will affect production quality and efficiency.

FOLDING AND FORMING
The folding and forming tooling that makes the web into a sleeve has evolved greatly over the years. Advanced folding and forming tooling design has taken much of the guesswork out of the process, as this aspect has historically been complicated and time-consuming for machine operators. Several variations of folding tooling are used in the industry (Figure 5.27). The earliest tooling used was a fixed-size shoe. Operators would insert a single, custom-sized forming shoe into the table area that corresponded to the layflat size they were running. It is rare to see this tooling used today except for high volume, dedicated production runs.

The greatest limitation to using a fixed-size shoe is that it can only be used for one specific size. Because of this limitation and the implausibility of purchasing fixed-sized tooling for every layflat, tooling evolved to accommodate the needs of converters. Manual tooling consists of forming shoes, pins and cranks, all of which are placed into position using hand levers (shown in red) that need size measuring. Semi-automatic tables utilize a hand crank, which moves the tooling into place while also notifying the operator of the exact measurements on screen. The most advanced tooling on the market is fully automatic tables. This type of sophisticated equipment enables the operator to type in the desired layflat on the screen, and the tooling will automatically move into position.

REWIND AND OSCILLATION
An oscillated rewind is an important component of the shrink sleeve process. When examining a shrink sleeve, one will observe that the thickness is tripled at the seam reaction area. If this finished roll of shrink sleeve labels is rewound in the traditional sense, all of the tension would build up in that seam area (Figure 5.28). 

The reel is then likely to telescope and topple over, wasting the production time and material used in this process. The seam therefore needs to be spread back and forth, which is referred to as oscillation (Figure 5.29). 

Some co-packers and end users require a specific amount of oscillation, but a good rule of thumb for operators and customers to follow is to oscillate twice the overlap.

MONITORING AND CONTROL

Historically, operators would start and stop their machine on a regular basis to collect, assess and measure product samples. The industry then moved into optical monitoring with indicators, which still required the operator to assess and monitor performance while the machine was running at production speed. Today, the industry relies on ultrasonic sensing technology; an ultrasonic sensor, with proper software, can identify layflat webs running through the equipment, monitoring and qualifying the layflat size (Figure 5.30). A more recent advance gives the operator the ability to download the production data to a USB drive as well as print said data for later reference. Furthermore, many decorators require a certificate of analysis (Figure 5.31) to show the decorator that the product was monitored and qualified.

THE FINISHING STEPS

The finishing steps of the seaming process consist of unwinding and rewinding the seamed web using a Doctor Machine® with oscillation, changing the core size (when necessary), changing the application rewind direction (when necessary), and building finished sized rolls. It is also at this step that splices in the seamed rolls are ‘repaired’ and made suitable for passing through the automatic label application equipment, before being detected and ejected from the line prior to the shrink tunnel.

SHEETING

Following the finishing steps of the seaming process, the labels will either be applied to the container automatically or by hand. If the labels will be applied automatically with label application equipment, then the finished product will remain in roll form. If the labels will be applied by hand, then the labels will need to be cut into individual sleeves. A sheeting machine is required to produce individual cut labels. Should manually applied sleeves require perforation, then that too must be inserted using this equipment (Figure 5.32).

PERFORATIONS IN SHRINK SLEEVES

The primary purpose of shrink sleeve perforation is for tamper evident sealing, and this perforation is typically performed on a seaming machine with linear perforation as previously outlined in Figure 5.23. Examples of shrink sleeve perforations for tamper-evidence can be seen in Figure 5.34. 

The secondary purpose of shrink sleeve perforation is for recyclability, or more specifically, to provide the consumer with an easy means of removing the shrink sleeve label from the container so that the container can be recycled. 

Tamper evident labeling (Figure 5.35) is also used for tax strips and anti-counterfeit purposes. These labels typically incorporate linear perforation produced on the seam machine, and they are often combined with a tear-strip or holographic material, which is generally an easy add-on to seaming equipment (Figure 5.36).

There are also other types of tamper evident sleeves with tear tabs that come down the container wall and then perforate around the container, and are typically used for a full body sleeve that also requires a tamper evident seal. Sometimes a brand owner will require a partial body tamper evident seal where the T-perforation is used around the container lid while still maintaining the container’s brand identity on the neck and body of the container.

Most of this is done on the automatic application equipment unless it is going to be sheeted for hand application, in which case, the perforation is completed on the sheeting equipment using a perforation device.

CONCLUSION

The secondary converting steps materially affect the quality of the finished label on the shelf, and each step requires great attention to detail. It is therefore of critical importance to use the best ingredients – film, ink, solvent, equipment, and trained people – to ensure the production of the highest quality, most consistent shrink sleeve label in the market.