- February 01, 2008, By Keith Fordham, Ashe Converting Equipment
Materials today often are more than they may seem at first glance; many are manufactured with non-conventional properties. Label and semi-glossed enamel stocks are engineered to provide adhesive hold out and to provide good folding properties, making them suitable for high-speed labeling applications.
Co-polyester films and coatings are made that are permeable to moisture and gases such as CO2 and O2, but at the same time are non-porous and thus impermeable to liquids such as water, blood, and bacteria. When used as a mono-layer film, co-polyester thermoplastic elastomer (TPE) resins have a desirable non-stick tendency making them useful for packaging many food items; when used as a multi-layer structure, they have an affinity with ethylene vinyl acetate (EVA) and ethylene-methyl acrylate (EMA) co-polymers, and enhance performance properties of other polyesters and nylon.
Films such as high-density polyethylene (HDPE) are available in multi-layer, cross-laminated construction, as well as in single ply, and can be clay coated as a specialty product offering increased printability. If a film needs to be reinforced for strength and yet needs to remain flexible, and the film selected for the application is polypropylene (PP), calcium carbonate can be added.
Many materials now available are complex in manufacture, ultra-thin, and can be a challenge when slitting or undertaking another converting process. To obtain the desired results, a combination of care on the part of the converter and sensitive and responsive processing equipment is needed. If one element in the production/processing equation fails when handling or slitting these materials, many of which are expensive, lost productivity, high levels of waste, and much more will occur.
Unwinding a web at one end of a machine, then drawing that web through a machine so that a product can be slit into more manageable and marketable products (or for some other value-added process to occur, i.e., printing), then winding the roll or rolls up at the other end ready for customer shipment or additional processes can be an area fraught with difficulty.
Regardless of ease of convertibility, materials are designed to meet aesthetic and functional requirements. So no matter how challenging the material, rolls must be wound with the right degree of hardness, rolls must look good, and they must be of the right shape and consistency. Slit width, winding tension, or web path variations can affect nominal roll shape, producing visual roll edge changes. Width changes can be caused through variations in web tension through the slitter; width changes also can result if a slitter blade is allowed to move. A bottom slitter blade, for example, if allowed to wobble, will cause the edge to be cut into a scallop. This scalloped slit edge then will wind up at speed to produce an unsightly "bulls eye" interference pattern on the edge of the finished roll.
Variation in material caliper also can produce difficulties. Areas of relatively high caliper can take a disproportionate share of nip pressure, which may damage delicate materials; diametrical variations can result in an uneven stretching of the web around the roll so that the appearance of bagginess becomes a cause for concern. Diametrical variation also may force a web to steer towards the larger area.
Rolls may become out-of-round during the winding process, sometime due to materials with a high co-efficient of friction, sometimes due a vibrational bounce induced by bulky material. More often though, out-of-round or deformed rolls occur during handling and storage, hence employees need to exercise care when moving material around. An approach is to optimally wind rolls up in the first place and to keep interlayer pressure or friction in equilibrium.
Rolls must be slit and wound accurately using the appropriate winding technology and web tension. Thin materials can deform easily and require lower levels of tension. Thicker materials, on the other hand, may not bend as easily as thinner materials; therefore, higher diameter rolls are needed. Thicker materials may require more power and higher tension levels.
If the problems outlined were the only difficulties that may arise when processing many materials, that would be bad enough, but in reality there are many more.
For instance, the non-porous nature of films allows for easy entrapment of air between wrap layers and between web and rollers. The seemingly smooth nature of a film's material surface can complicate matters further in that it increases the incidence of surface scratches and abrasions.
In designing slitting and rewinding equipment able to process these increasingly challenging materials at commercially acceptable speeds, innovative ways have been developed to reduce the impact that processing transients have on production quality.
Taking into account the extensible nature of many of these materials and their thin gauge handling requirements, equipment is available that utilizes automatic closed loop tension control for consistent and repeatable control on slitting and rewinding machines. In addition, a vacuum drum can be used as the main pull roller in place of nip, preventing creasing due to web slackness.
Selecting the optimum winding mode for an application can be a challenge. Developments that incorporate all the favored winding modes in a single compact machine address this issue. These machines provide optimum wind up and roll quality using center winding with differential slip and lock bar, or center winding with lay on in differential and lock bar, or they can surface wind with differential and lock bar.
Alternatively they can provide constant gap winding with differential and lock bar. No equipment change is needed to move between the various formats therefore making them highly productive and versatile.
Web run is kept as short as possible between knives and rewind so that the web is maintained at a constant distance throughout diameter build up. The differential system and cantilevered rewind shafts also facilitate ease of loading/unloading.
Waste saving and monitoring strategies help make the modern slitter a more efficient processing tool. Innovations such as digital drives that allow smoother soft start material transitions on unwind and rewind positions together with features such as automatic knife positioning systems contribute to enhanced productivity and workflow as does increased throughput possibilities.
Keith Fordham is chief engineer for Ashe Converting Equipment, Ipswich, Suffolk, U.K. Contact him at email@example.com.
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