As with most processes, success depends on doing your homework and making the right choices with regard to materials and suppliers, and I’m grateful to Vetaphone, Paragon Inks and Plasfilms for their input and advice, and to Berkshire Labels for their first-hand experience of producing shrink sleeves on narrow web presses.
I began by speaking with John Walker of Plasfilms, who outlined the basic requirements of heat-shrink substrates.
NC: What are the key properties required for good shrink film products?
JW: The main characteristics are controlled and consistent shrinkage, mechanical properties, clarity, and stability at a wide range of temperatures. This is full body labeling, so 360-degree, high-quality graphics are essential with ease of processing (printing and converting), with tamper evidence and, of course, recyclability. The shrinkage performance is determined by both the base material and formulation, plus the stenter/orientation process.
NC: In terms of film products, which stack up best against these criteria?
JW: The main substrates are PVC, OPS and PETG. PVC has been established and is the lowest cost shrink film but has limited shrinkage, around 60%, and is less appropriate for difficult or complex shapes. OPS has good shrink capability, around 70% with minimal MD shrinkage. But the best is probably PETG, which has the highest shrink capability of 78-80% and is capable producing complex shapes. It also has the highest level of clarity and has strong mechanical properties that make it suitable for downgauging. On the downside, however, it is the most expensive.
NC: Does the suitability of the film depend on which print process is used?
JW: In simple terms, all of these materials are suitable for printing by flexographic, rotogravure and using UV curing. PVC is less resistant to printing solvents and likewise OPS is more challenging to convert, being sensitive to solvent choice and UV. PETG is more resistant to solvents and therefore has replaced PVC and OPS as the most popular substrate.
NC: Inevitably, I have to ask about the environmental aspect of growth in this market sector – how do you view its impact?
JW: PVC is perceived to be environmentally unfriendly although it is relatively easy to recycle. The problem is that the recycling chain is not sufficiently developed. OPS is regarded as recycling friendly, but PETG comes out on top as it’s environmentally friendly and readily recycled because the recycling chain is well developed for PET bottles. I’d like to add that energy recovery is also available, and that chemical recycling is also growing in importance. The problem is not at the front end of the process with film production but at the rear end with disposal and re-usage.
Of course, the key factor with all shrink sleeve production is that it uses plastic film as its substrate. Being non-absorbent, films can be notoriously difficult to achieve consistently high print quality owing to the variance of surface energy between the liquid ink and the film surface. Who better to explain the role of surface treatment than Danish manufacturer Vetaphone, which invented the process back in the 1950s and has subsequently pioneered what the world knows now as corona treatment. I spoke with the company’s VP of technical sales, Kevin McKell, about the potential issues of printing shrink film.
NC: What are the main issues with using heat-shrink film?
KM: Shrink film is not really different from other plastic materials. You have to control the surface energy between the liquid ink and the plastic substrate. If the surface tension of the film is too low, the inks bead up on the surface and don’t adhere – we call this "wetting," and you need the ink to "wet out" to form a strong bond. We measure the surface energy and the substrate’s adhesive ability in dyne/cm. The higher the dyne reading, the better the adhesion.
NC: How do you persuade ink to adhere to a non-absorbent substrate like film?
KM: You modify the surface of the substrate with a high frequency electrical discharge at close range. This is known as corona treating, and it breaks up the long molecular chains in the surface of the film and oxidizes it. This allows the ink to bond securely. The process is harmless to the substrate as it affects only a microscopic layer at the surface and changes neither the strength nor appearance of the film.
NC: Does the print process affect the need for or type for corona treatment?
KM: Fundamentally, no it doesn’t. The issue, or to be more precise the variable in the equation, is the substrate. While all synthetic materials are surface treated at the extrusion stage, it’s important to remember that two pre-treated rolls of the same material from the same supplier may not be consistent throughout their length or width, which is why you need to surface treat immediately prior to printing.
NC: What other factors affect a film’s ability to perform consistently?
KM: There is one intrinsic issue and that is with the additives that are sometimes used in the manufacturing process. These can migrate throughout the film and not necessarily in an even way, and then affect the chemical composition of the surface being treated. This, along with transport and storage issues, can cause the dyne level of the film to have dropped by the time it’s delivered to the printer/converter. Excessive heat, cold and humidity will adversely affect the film during storage, so it’s vital to ensure that it gets used within the date recommended by the supplier. One piece of advice we give to everyone is "always dyne test the material before you use it."
NC: Looking ahead, how will surface treatment cope with the new and more complex substrates being developed?
KM: Since we invented corona process and have constantly developed surface treatment, we have always been one step ahead of the market. Is corona the answer to all applications? Most, but not all. Some of the latest substrates have been developed to produce unique products, typically using special inks and lacquers, lamination, multi-web applications and other techniques to produce sophisticated packaging. The constituents of these substrates require a chemical treatment process, as well as a physical one – and in these instances plasma treatment is necessary to ensure bonding. But that’s not the case for shrink sleeve production as its stands today.
Choosing the right substrate and preparing it correctly for its intended use is only part of the process of producing high-quality shrink sleeves. The choice of ink is critical, as Amanda Jones, business development and marketing director at Paragon Inks, explains.
NC: What are the key rules and points that need to be addressed for the successful production of shrink sleeves using UV flexo inks?
AJ: The fundamental point to make is that UV inks adhere by co-adhesive electrostatic bonds and do not interreact directly with the substrate being used. For UV flexo inks, the rule is "less is more." The more ink applied, the thicker and heavier the layer, and this brings a greater risk of poor adhesion because the bonds sticking the ink to the substrate are weaker. The lighter and thinner the layer, the better the adhesion.
NC: What part does curing play in the process?
AJ: It depends which process you are using. Curing an ink properly is a balancing act between the wavelengths emitted by the lamps needed to activate the photo initiators and absorbed by the different colored pigments, also known as a pigment window. For example, the darker the ink the more difficult it is to cure using conventional lamps – this is the opposite with LED/UV. Curing the surface of the ink is easy, but the thicker the layer the more difficult it is to effect a complete cure.
NC: How do you test for a full cure?
AJ: There are various methods you can use on-press. Touching or rubbing the ink surface with your finger to see if its dry is the easiest and most common, as any smudging indicates insufficient curing. You can also conduct a tape test, which will also check adhesion. It’s very simple but effective. Apply the tape to the printed and cured surface securely and then peel it off. Any residual ink staining on the substrate indicates that it is not fully cured. If it’s clean, it is fully cured.
NC: What are the main issues that you have to troubleshoot?
AJ: The main one is poor adherence. Incomplete curing normally gets the blame, but it’s often a combination of substrates giving false dyne readings that lead to incorrect surface treatment. This normally occurs when there are surfactant additives involved, typically used in PET and PVC manufacturing, which disperse unevenly and create adhesion problems by forming a thin invisible surface layer. The age, storage and suitability of the substrate for UV inks also need to be taken into account. If none of these is causing the problem, you need to look at the volume of ink applied, and also its formulation. Not all inks behave the same with all substrates, so seek advice from your supplier.
NC: Are there issues after printing that converters need to be aware of?
AJ: There are a number of potential problems, including ink flaking off after the shrink process, an embossed effect appearing, a dulling of certain colors and an inconsistent gloss effect to name a few – and each one needs to be analyzed in detail to eradicate the fault – but they are all known issues and can be remedied if the converter, ink, and substrate supplier use their individual expertise and work together. Like all processes, there is a learning curve, but there is plenty of experience available to help make it work.
The last word on the subject needs to go to someone who is involved in day-to-day commercial production of shrink sleeves, and I spoke with Paul Roscoe at Berkshire Labels, one of the UK’s top producers of high-quality printed packaging using narrow web technology.
NC: What attracted you to shrink sleeves in the first place?
PR: We’re constantly looking to innovate and grow the business year on year, and we saw an opportunity to bring something new into the shrink sleeve marketplace in terms of quality, lead times, customer service and product offerings.
NC: Did you need to install new equipment, or could you accommodate it on your existing technology?
PR: We invested around £1.8m on new equipment to ensure we could deliver our commitment to customers. Our investments included full in-house studio software for pre-distortion and 3D mock-ups, a new 17” Mark Andy flexo press, a new HP Indigo digital press, Karville finishing equipment and an Accraply sleeve applicator.
NC: So, you print the sleeves flexo and digital?
PB: We wanted to offer the full package on both short run digital and long run flexo. So, it was essential that we profiled digital to flexo so we could offer everything to all customers both big and small. We selected the Mark Andy Performance Series, with the full filmic kit for its superior handling of thin films and perfect registration. Following a successful Mark Andy P7 installation, two years earlier, we installed a P9 that gives us greater speeds and more sleeve and filmic capacity.
NC: What has been your experience of shrink sleeve manufacturing to date?
PR: We are six years in now and following a steep initial learning curve we have built up a good reputation in the shrink sleeve market, which even saw us winning an award at last year’s AWA International Shrink Sleeve ceremony. Our aim was to say "yes" to most things while also creating some new opportunities along the way. Our ability to offer very quick turnarounds on some exciting new products has won us new business against what was typically seen as a more traditional industry approach. Full in-house control from artwork distortion all the way through to finishing and application has put us in a good position. We set our standards very high in terms of quality and finishing at the beginning and have never deviated from these.
NC: How do you see the market developing and what future plans do you have for shrink sleeves?
PR: I have mixed thoughts on the market development because on the one hand we are aware that a plastic sleeve on a plastic bottle can be frowned upon, whereas being an ISO14001 approved business, and always looking to promote environmentally friendly options, we have helped many customers improve their recycling credentials along with a reduction in waste, cost and inventory. Our future plans are to continue to push the boundaries in both label and sleeve technologies. This will see further embellishments added to our portfolio and rolled out onto sleeves.
This has been by no means a fully comprehensive view of the shrink sleeve market, but in a sector that is attracting a great deal of interest and producing some highly attractive packaging options, narrow web converters that are considering a move away from "sticky" labels can be reassured by the depth of knowledge and expertise available to tap into, and the security of knowing that there is plenty of room for growth.