Web Exclusive: Slitting Film

Despite the fact that crude oil prices have risen sharply, in turn elevating the price of oil-based raw materials used in plastic film manufacture, this group of materials still has largely untapped packaging possibilities. Plastic films and newer combinations of existing synthetic and natural materials in the form of co-polymerization products continue to be developed.

The possibility for the development of radically new plastic film is slight; rather developments are expected to take the form of new combinations, particularly through the process of co-extrusion. Combinations with aluminum foil will be increasingly substituted by vacuum-metallized plastic films with, for example, retortable pouches and cans made out of plastic and foil as an alternative to metal cans. Stretch films will be favored in place of shrink films, particularly for pallet overwraps, due to their lower energy costs. Stand-up pouches—particularly suited to changing market demographics, i.e., smaller households, single portion—will find new applications in the food sector.

Since there are so many types of plastic films available for use in applications such as packaging, packaging technologists, converters, etc., must make themselves aware of the various characteristics in order to make an informed selection. Characteristics to consider include tensile strength (the force necessary to break the material in relation to a given area), tear strength (an important property governing the use of many films), and the ability of certain types of processing machines to handle them. Other properties that have to be borne in mind are stiffness, impact resistance, temperature resistance, moisture resistance, gas barrier properties, elongation, hardness, haze and gloss, film slip coefficient of friction (COF), elasticity, and dimensional stability.

Tips on Film Handling
Plastic films need to be handled with care if a supplier is to obtain maximum yield from the material and keep waste to the minimum. Film rolls should be kept wrapped and only unwrapped when necessary. Even when unwrapped, a roll of filmic material, which may have been kept in cool conditions, should be allowed to rest and reach the same temperature as the working surround.

Film must be stored carefully, off the floor and in a moisture-free environment. Excessive moisture can wick into wrap layers and later cause problems such as blocking.

If this wasn’t enough, care must be taken on machine, when slitting and rewinding, etc. Film must be unwound and fed into the processing zone, then rewound with tension appropriate to the zone, taking into account the extensible nature of many of the materials in question.

To run plastic (flexible) materials such as polyethylene and polypropylene at commercially acceptable speeds and to high quality, all aspects of a machine must be controlled in unison. If one element of control fails, a flawed product and/or high levels of costly waste will occur.

Effective web control is essential. For instance, too low a web tension may cause scratch marks or web slip leading to overlapping when slitting. On the other hand, too high tension threatens the integrity of the plastic web, leading to web stretching or web breaks.

Poor tension control may prevent a slitter from handling different widths and thicknesses effectively. It also will mean that the slitter cannot run at optimum speed. On all machines, constant unwind tension is imperative to ensure good tracking and slitting.

To facilitate the rewinding of narrow slit rolls and to avoid core crushing or telescoping of the finished rolls, accurate and stable tension control is once again necessary. Control of the material in feed is also vital; developments such as the vacuum “Hug Drum,” facilitates the passing of material through this zone without the creasing that can arise from web slackness.

Today’s converter is fortunate in that machines such as slitter/rewinders have evolved into powerhouses of production, offering greater accuracy through developments such as automated knife positioning systems, drives, PC/touch-screen control, and better overall machine design. With the increasing sophistication of the market, there is now a wider choice of machines than ever before, it’s just a case of making an informed choice.

Keith Fordham is chief engineer for Ashe Converting Equipment, Ipswich, Suffolk, U.K. Contact him at sales@ashe.co.uk.

Supplier Information
Ashe Converting Equipment—www.ashe.co.uk


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