Friday, February 23, 2018
Waste matrix stripping or removal in production of self adhesive labels is a very important part of label conversion and is an imperative that leads to a web of labels which can be dispensed on automatic label dispensers in high speed packaging lines. Even though it sounds to be a simple process of stripping the ladder like extra waste after die cutting of labels, yet it remains to be one of the most complex and problematic area of label converting process. A problem with waste removal, like matrix breaking or labels lifting with the waste ladder 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 offline 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. It could be due to the shape of label, size of label, release liner, face stock, adhesive, die cutting process, speed of conversion, die blades or the design of the waste removal section that may affect the correct and efficient removal at the optimum machine speed. Any of these may impact the final result and slow down the machine and the printing process. No one solution can apply to all problems. The traditional waste rewinding system is gradually becoming unpopular due the fact that tension is the key to efficient waste rewind. The rewound waste matrix ladder roll has empty spaces from where labels have been die-cut and as the roll becomes bigger there is lot of irregular tensions leading to breaks. As the market becomes extremely competitive with rising prices of labelstocks printers tend to reduce the gap between the labels to 2mm making the process even more difficult. This article will dwell on most of the variables mentioned here above.
Release Liners: 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 releasing properties to become a proper backing for self adhesive papers. In recent years due to possibility of recycling and reducing the tonnage of waste generated, filmic liners also are being used as backing in labels. Release liners play a major role in die cutting 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 so has to achieve a perfect half cut or kiss cut. The uniform thickness or caliper of the liner is an imperative. If the liner has variations, it will create problems at die cutting and eventually at waste stripping. 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 easy, 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 taking up label conversion.
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%; sharp increase in moisture will affect the strength adversely. The tensile strength of paper at Relative Humidity (RH) up to 50% is maximum after which it moderately decreases with RH up to 65% and on further increase in RH, it drops sharply. The uncoated papers are hygroscopic, so they tend to absorb moisture faster than coated papers. Evidently weather and storage condition of paper does have an impact on waste removal. Even when using emulsion based adhesive 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 case of filmic face stocks, weather may not impact but the condition of die and quality of die cutting does play a major role. If the die is damaged or blunt it may not cut properly resulting in label lifting or film tear.
Adhesive: Commonly available labelstocks are coated with either emulsion based or hot melt pressure sensitive adhesives (HMPSA). In both cases for a perfect waste removal it is necessary that the die cuts through the adhesive as well, as otherwise if the coated film of adhesive is not cut, labels will lift with the matrix. Emulsion adhesives have good die cut ability however hot melt adhesives for better die cutting properties have to be specially selected. In case of HMPSA if the waste is not lifted immediately after die cutting the adhesive may rejoin and lift the labels with the matrix.
Size and shape of a label: These are parameters that are customer driven based on their specific needs, so the converting company cannot request changes from customer. 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 speed of the web. Waste ladder removal of irregular and complex shaped labels with sharp corners like in a star shape, is even more complex to handle. This becomes even more difficult in substrates like BOPP where a small nick may lead to 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 as explained later in this article.
Die design: The die has a definite role to play in waste matrix removal. The subject is extensive and can take a full article to dwell on the nuances. The blade angle, blade height and coating on the die are factors that lead to ease or difficulty of label conversion. 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 case of filmic face material a sharper acute angle is needed to pierce the film as in case of a wider angle the film will stretch and not be cut. An acute angle blade appears to cut better but wears off faster than the wider angle blade dies. Depending on the materials used the die angle varies between 45degrees and 110 degrees. The blade height needs to be adjusted to cut through the face, which maybe paper or film or a laminate, and adhesive without piercing the release liner. If any of the parameters is 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. In case of coated materials like direct thermal and thermal transfer the coatings on the paper are abrasive in nature and tend to make the die wear off soon. In such case laser hardened dies are recommended. Adhesive sticking and building up on the dies also results in uneven cutting and also resulting in early die wear off. 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 as in case of die wear off the gap increases resulting in spaces where labels are not cut and would lift off with the matrix and 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 die should be done regarding cleaner blades, blunt edges or nicks. The dies need to be stored in an environment avoiding excess humidity which may result in rusting.
Machine 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 die-cutting. 2. Taking the die to a larger diameter stripping roller that would support the waste ladder on separation rather than a thin diameter roll that 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 die cutting as this would reduce the tension required to peel off. These measures did help to some extent but complex shapes and a host of issues and factors that impact this process have had machine manufacturers continuously researching this area to keep implementing changes. One such solution that came around some years back was suction of the waste matrix into a suction and shredding system. This does take care of the tension and also manages waste by cutting it to small pieces and compacting it, but such systems have other problems. They are expensive, large in size so difficult to be fitted on presses due to lack of space, costly to operate as they use extra motors, compressor or vacuum and very noisy to run. 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 environment.
The larger established press manufacturers seem to have reached a viable solution. Some years ago they have introduced a big innovation in 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, we pull the entire web up near the rewinder. Here the matrix is peeled off against an idle roll and immediately pasted on the rewinder. Basically this reduces the 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 invented to simplify the label converting process however It does not hold true for all jobs, when a problem comes it can be challenging and creating a solution can be another game changer”!
Written by Harveer Sahni Chairman Weldon Celloplast Limited New Delhi January 2018
NOTE: This article is exclusively written for magazine Label and Narrow Web USA. Publications desirous of reproducing the article may write for permission to Steve Katz editor LNW : firstname.lastname@example.org