Waste matrix stripping or removal in the production of self-adhesive labels is a very important part of the label converting process. It is a function that leads to a web of labels that can be dispensed on automatic label dispensers in high-speed packaging lines. Even though it sounds like a simple process of stripping the ladder, the leftover waste after diecutting labels, it remains one of the more complex and problematic areas of label converting.

Matrix breaking or label lifting with the waste ladder represents a challenge, as it may slow down the machine or in some cases make it extremely difficult to remove it online. Converters may have to resort to removing the waste manually off-line, making the process unproductive and costly.

A host of parameters affect the process, and it is difficult to address the issue in a singular way. With so many variables that impact the waste removal process, it is difficult to predict a simple solution. The shape of a label, size of the label, release liner, facestock, adhesive, diecutting process, speed of converting, die blades or the design of the waste removal section could affect the correct and efficient removal at the optimum machine speed. Any of these elements can impact the final result and slow down the machine and the printing process. No one solution can apply to every potential problem.

The traditional waste rewinding system is gradually becoming unpopular due to the fact that tension is the key to efficient waste rewinding. The rewound waste matrix ladder roll has empty spaces from where labels have been diecut, and as the roll becomes bigger there is lots of irregular tension leading to breaks. As the market becomes extremely competitive with the rising prices of labelstocks, printers tend to reduce the gap between the labels to 2mm, making the process even more difficult.

Release Liner
The most widely used base papers as release liners in self-adhesive label materials are glassine, super calendared Kraft and clay coated Kraft. These are uniform caliper, densified and non-porous papers that have adequate strength and accept a uniform coating of silicone, giving excellent release properties to become a proper backing for self-adhesive papers.

In recent years, the possibility of recycling and reducing the tonnage of waste generated has led to filmic liners being used as backing in labels. Release liners play a major role in diecutting, and in turn impact the waste removal process. The die blade has to cut through the laminate and stop at the face of the liner to achieve a perfect half cut or kiss cut. The uniform thickness or caliper of the liner is imperative. If the liner has variations, it will create diecutting problems, and eventually  waste stripping issues. If the release gets thicker, the die will pierce the liner, making a through cut and exposing paper fibers to the adhesive.  This also may result in web breaks. If the liner gets thinner, the die will not cut resulting in labels lifting with the matrix.

Release level of the liner is also very important. If the release level is tight, the matrix will tend to break due to tension and if it is too loose, labels will tend to lift with the ladder. Uneven silicone coating or pinholes in coating may also create problems. If the labelstock, prior to waste matrix removal goes through a nip roll that has excessive pressure between them, the edges may develop micronic nicks that may render the face paper susceptible to web breaks. The paper rolls may also develop these rough edges in transportation and mishandling. The web needs to be inspected thoroughly before the converting process begins.

Face Paper
Paper and films are generally used as face materials. A fairly high strength paper will perform well if all other parameters are addressed. If the gap in labels is too small, 2mm or less, the matrix will tend to break repeatedly. Moisture content in paper should ideally be between 3.5% and 5.5%; a sharp increase in moisture will affect the strength adversely. The tensile strength of paper at a relative humidity (RH) up to 50% is the maximum, after which it moderately decreases with an RH up to 65%. Further increases in RH lead to sharp drops. The uncoated papers are hygroscopic, so they tend to absorb moisture faster than coated papers.

Atmosphere and storage conditions of paper also have an impact on waste removal. Even when using emulsion-based adhesives, if the adhesive is not dried properly, the face paper will tend to absorb the residual moisture from the adhesive and result in deterioration of paper and affect waste stripping. In the case of filmic facestocks, climate may not cause an impact, but the condition of the die and quality of diecutting do play a major role. If the die is damaged or blunt, it may not cut properly, resulting in label lifting or film tear.

Adhesives
Commonly available labelstocks are coated with either emulsion-based or hot-melt pressure sensitive adhesives (HMPSA). In both cases, for perfect waste removal it is necessary that the die cuts through the adhesive. Otherwise, if the coated film of adhesive is not cut, labels will lift with the matrix. Emulsion adhesives have good diecut ability, however hot-melt adhesives for better diecutting properties have to be specially selected. In the case of HMPSA, if the waste is not lifted immediately after diecutting, the adhesive may rejoin and lift the labels with the matrix.

Size and Shape
These are parameters that are customer-driven based on their specific needs, so the converting company cannot request changes from customers. Small labels have a very limited area of contact and reduced tack holding it to the release liner, and with little force the label may fly off or lift off with the matrix. In such a case, die makers suggest packing self-adhesive foam in the die shapes, so as to push the label back on the release liner. Other times printers have found limited success in addressing this problem by increasing the speed of the web.

Waste ladder removal of irregular and intricately-shaped labels with sharp corners, such as a star shape, is even more complex to handle. This becomes increasingly difficult with substrates such as BOPP, where a small nick may lead to a web break. Converters need to slow down the machine to a great extent to finish the labels online. Machine manufacturers have addressed this issue of handling complex shapes.

Die Design
Dies play a key role in proper waste matrix removal. The subject is so extensive a full article on the topic would be required to cover all of the nuances. The blade angle, blade height and coating on the die are all factors that effect the ease or difficulty of the process. Thickness of the face materials, type of adhesive and thickness of release liners are all imperative inputs that are needed before a die is put into production.

A die that is designed for paper material is not recommended for filmic materials. Blade angle for paper is kept wider so that after penetration of around 80% into the paper, the rest of the cut happens by crush or bursting of the material before stopping at the surface of the liner. In the case of films, a sharper acute angle is needed to pierce the film, as is the case with a wider angle where the film will stretch and not be cut. An acute angle blade appears to cut better but wears off faster than dies with wider angle blades. Depending on the materials used, the die angle varies between 45 degrees and 110 degrees. The blade height needs to be adjusted to cut through the face, which may be paper or film or a laminate, as well as adhesives, without piercing the release liner. If any of the parameters are not right, the waste matrix removal will become a challenge. If the blade pierces the liner even slightly, it may expose the release paper fibers to the adhesive and get stuck to them, causing waste ladder breakage. If the blade does not cut through the adhesive, labels will lift with the matrix.

With regard to coated materials, such as direct thermal and thermal transfer, the coatings on the paper are abrasive in nature and tend to make the die wear off sooner. In such cases, laser-hardened dies are recommended. Adhesive sticking and building up on the dies also results in uneven cutting and also leads to early die wear. This is more evident where aggressive high tack hot-melt adhesives are used. For this reason, special non-stick, coated dies are available so that the adhesive will not stick to them. The standard gap between the magnetic cylinder and the anvil is also very important. When die wears, the gap increases, this results in spaces where labels are not cut and would lift off with the matrix. So to get a perfect cut the die pressure is increased. This results in faster wearing off of the bearers, leading to a smaller gap and over-cutting.

Care has to be taken in die storage and handling. Before commencing any job, proper inspection of the die should be done regarding cleaner blades, blunt edges or nicks. The dies need to be stored in an environment avoiding excess humidity, which can cause rusting.

Manufacturers have been consistently making efforts to address the issue of waste matrix removal to aid faster converting.

Some of the steps taken include:

1. Lifting the waste matrix immediately after diecutting.
2. Taking the die to a larger diameter stripping roller that would support the waste ladder on separation rather than a thin diameter roll, which would provide a sharp angle to waste being stripped off.
3. By rethreading the paper in such a manner that the label web is peeled off the matrix instead of the matrix being pulled off.
4. De-laminating the web and re-laminating it before diecutting, as this would reduce the tension required to peel off. These measures help to some extent, but complex shapes and a host of issues and factors that impact this process have had manufacturers continuously conducting research in this area to keep implementing changes. One such solution that came around some years back was using suction system for the label waste matrix to reach a shredding system. This does take care of the tension and also manages waste by cutting it into small pieces and compacting it, but such systems have other problems. They are expensive and large in size, making them difficult to be fitted on presses due to lack of space. Some are also noisy and costly to operate as they use extra motors, compressors or vacuums. Yet there is a brighter side to it. There is development going on to separate the waste and recycle it inline so as to reduce the impact on the environment.

The larger established press manufacturers seem to have reached a viable solution. Some years ago, they introduced a big innovation to the industry by designing a simpler contact system rewinder for waste matrix. The idea was very simple; instead of pulling only the matrix up to the rewinder, it pulls the entire web up near the rewinder. Here the matrix is peeled off against an idler roll and immediately pasted on the rewinder. Basically, this reduces travel of the matrix from 1 meter to hardly 5 cm, and the journey is even supported by a roll. This system has now become the standard with many label press manufacturers.

Simple solutions are continuously invented to simplify label converting, however, it does not hold true for all jobs. Certain problems that arise during the converting process can be quite challenging to overcome, and creating a solution can be another game changer.


About the author: Harveer Sahni is Chairman of Weldon Celloplast Limited in New Delhi, India. In 2017, he was recognized by the Label Manufacturers Association of India with a Lifetime Achievement Award.