CONTACT WEB CLEANING FOR PRODUCT VIABILITY, COMMERCIAL VIABILITY
14 February 2008
CONTACT WEB CLEANING FOR PRODUCT VIABILITY, COMMERCIAL VIABILITY
By Hunter Paterson
New, better, improved, different, updated, revised - doesn't really matter which word you decide upon, for the producer of film, for the printer of film and for the converter tasked with adding value, it's fundamentally about adaptability, being up to the challenge, being adept at coping with constant change. Producing and processing film is arguably more challenging than many other material segments, for example, unlike life or business where one is constantly striving to overcome barriers, for films for flexible packaging its often about adding barriers. By adding layers and by combining various structures a film, or film in combination with another material gains added functionality - providing better resistance against gases, moisture, light and much more.
New filmic structures can include layers that not only provide better barrier properties but also add stiffness and/or strength or improve sealant properties, or enhance final product appearance, etc. The composition and thickness of layers, which may number as many as nine may include an exterior layer, providing dimensional stability, heat resistance, low co-efficient of friction (CoF), clarity and printability. An exterior layer option may be of BOPP (bi-axially oriented polypropylene), OPET (oriented polyester) and BON (bi-axially oriented nylon).
The barrier layer added may be for protection against moisture, oxygen, odour, but also to provide chemical resistance and bond strength. Typical barrier materials employed include: SiOx (silicone dioxide), PvdC (polyvinyldene dichloride) and EVOH (ethylene vinyl alcohol). The latter, a co-polymer barrier resin continues to find new uses in the high barrier flexible packaging market because of its outstanding gas barrier properties, excellent resistance to odour and flavour permeance, and ease processability on a wide range of co-extrusion equipment.
A sealant layer prevents gases from penetrating through the seals into the package. Typically applied to the inside layer of a structure, the most common materials used are LDPE (low density polyethylene), ULLDPE (ultra-linear low density polyethylene), LLDPE (linear low density polyethylene) or EVA (vinyl acetate).
Flexible packaging and flexible film development in general is driven by the needs of its customers. Exploration, innovation and general dynamism are hallmarks of these sectors. Innovative concepts gaining acceptance include the use of ethylene gas, polymerised to make polyethylene resin, which is then extruded into PE. Although structurally it lacks the stiffness it is adequate for many applications.
While films such as PE and PP have distinct individual uses, marrying them together in one way or another provides greater breadth of application. The ability to down-gauge for reasons of cost makes film such as polypropylene worthy of consideration when clarity and good stability at room temperatures is needed for lamination and co-extruded purposes.
Each stage of the evolutionary development of filmic materials has not occurred in isolation. Collaboration between formulators and producers is essential, determining printability and convertibility on a wide range of machines, from presses to coaters is equally critical. For instance, colour sells, it attracts consumers and influences decision-making.
To begin with, in order to achieve good ink adhesion it is essential for ink and material to have compatible surface tensions. Surface tension refers to the energy level with which molecules cling to each other. Non-porous in nature, films need to be treated, a process which often requires a degree of trial and error using corona or flame treatment processes.
Printing with certain colours on film can present difficulties, such as when the colour standard require that opaque white ink is used as a layer under the colour. In situations such as flexo printing on clear films for bread wrappers, the light scattering characteristics of the opaque inks can really affect the appearance and colour of the finished product.
But its not just colour and the quality of graphics that can mar the appearance of film; the quest to produce a quality product is as likely to be thwarted by airborne and other contaminants that settle on or become engrained in a high speed web. In print processes such as flexo printing airborne contaminates may be transferred from the web and onto the impression roller, resulting in a distorted print image. Dust and other particles can also be transferred directly onto the printing plate, producing the ubiquitous hickey.
There are many sources of contamination, furthermore, and unlike other processing inconsistencies that usually arise at a single zone on a converting machine, the cause of particulate contamination may reside in the material being put on the machine.
For instance, filmic material is manufactured on a continuous facility, in itself a possible cause for contamination. The process may deposit lubricating oil and grease, metal from bearings, dirt from the air drawn onto the web, fibres from an operator's clothing, hair, skin, flecks of paint, and much more besides. Inadequate mixing of polymers, carry over of impurities from tasks previously carried out on the machine prior to production such as cleaning residuals exacerbates the problem of contamination. Value added and preparation processes such as slitting can generate dust. The way in which a roll of material is transported, stored and handled can make for difficulties during a converting process. In a coating room ambient conditions should have a relatively high humidity so that static is not generated and dust in the air is attracted to the surface. A web cleaner is essential.
More than novelty value a web cleaner maintains material integrity, improves product yield and is an important contributor to the efficiency of the overall business. A contact cleaner such as the Teknek NWP for narrow web printing or a model from the XCHP system line up removes contaminants as small as 1 micron from the web, enabling a converter to run his/her processes at optimum machine speed with confidence. Material waste is kept to the minimum, machine downtime is minimised, machine uptime maximised and quality is maintained at the highest level.
Particulate contamination on base film is without doubt a major source of product waste and disrupted production. While many types of cleaning methods are available, including non-contact ultrasonic, vacuum or air impingement systems, they are not very effective because they cannot penetrate and overcome the boundary layer of air on the web surface, which holds debris fast. Contact cleaners, those that rely on the mechanical action of static, rotary or traversing brushes also pose problems in that they can easily damage a sensitive web surface, they too, are unable to remove small particles of debris.
The thin film of air attached to all moving webs, the so-called boundary layer of air is now better understood and is associated with the ‘Bernoulli Effect.' Fluid dynamics has a great influence on the removal of particles from a flat surface. The physicist Bernoulli explained that there is a reduction in pressure exerted by a fluid due to its flow, in general, the faster the flow the greater the effect. However, the principle of the boundary air layer also explains why it is difficult to remove small particles from a surface using air alone. Immediately against any surface the speed of fluid flow is zero - ‘the boundary air' layer is the distance from the surface to any point where fluid flow reaches full stream speed. What does this mean? It means that many small particles held within the boundary layer ‘will never' be lifted using moving air systems as the air stream will physically not be able to touch them. The result, loose particles are held on the web surface by static charge, surface energy and gravity. Increasingly though, customers demand ever more precise removal of contaminants, especially with the introduction of sensitive RFID and other electronic devices.
Teknek has devised a solution with its contact-cleaning concept based on the use of an elastomer roller with a high-energy surface grab. This roller runs in contact with the web, penetrating the boundary air layer to remove the smallest of particles, lifting the contaminants, and transferring them to a special reverse wound adhesive roll where they become permanently trapped. When the adhesive roll becomes saturated, the outer-sheeted layer is simply removed exposing the next underlying layer ready for use. An anti-static system component then ensures dust is not re-attracted to the web.
The simple description outlined above belies the complexity of the engineering and adhesive technology involved. Systems may be easily retrofitted to existing label, printing presses, coaters, laminators, holographic embossers and other equipment. OEM's in industries other than converting also specify the technology, for example, PCB surface mount technology and other ‘clean' critical applications within the electronic, medical and pharmaceutical sectors, etc.