HOW THEY GET A BALE WRAP JUST RIGHT…
Black, shiny and stretchy.
That about sums-up most
peoples description of bale
wrap film. But this high-tech
plastic has a number of
characteristics that enable
it to perform its important
role. Andy Moore reports
IT LOOKS a simple enough material. But beneath that shiny exterior lie some sophisticated ingredients that make silage stretch film rather more complex than may at first be considered.
"At just 25 microns thick, bale wrap is extremely thin," says John Hammond, distributor director at Clansman Films. "Yet it has to be exceptionally strong, with good perforation resistance, consistent stretch and elasticity, and be capable of maintaining an air-tight seal for a long time."
Incorporating these characteristics in a product like Clansmans Silaflex involves a careful blend of raw polythene and special ingredients, as well as a manufacturing process that can turn out huge tonnages of finished film to a consistent specification.
"Effective quality control is essential to ensure stretch wrap has the required characteristics," says Mr Hammond.
With 25 miles of film per hour reeling off one production line at Clansmans Potters Bar factory in Herts, film thickness, stretch and elasticity are measured regularly.
In addition to these laboratory-style tests, however, Clansman also puts samples of Silaflex through a simulated bale wrapping exercise using a steel "bale" on a farm wrapper to ensure it meets the required standards of tensile strength, adhesion and ability to unwind cleanly from the roll through the pre-stretch units.
The manufacturing process itself starts by pouring the raw polymer – in granular form – into an extruder where the material is heated, compressed and squeezed from a slowly rotating circular die.
A chemical which gives the stretch film its characteristic stickiness is added during this process in such a way to ensure it is incorporated on one side of the film only. Otherwise, wrapped bales would attract any amount of debris, would stick together in storage and would be difficult to remove intact from handling machinery.
The warm, soft plastic is kept in a circular column that stretches high into the roof of the factory by blowing air through the centre of the die. This also helps cool and solidify the material, which is then carefully collapsed over rollers, cut and separated to form two separate ribbons of plastic.
These are trimmed to the familiar 500mm or 750mm widths – or the increasingly popular 250mm width used for wrapping mini round bales of silage and horse forage – and wound on to rolls, boxed and stacked on pallets ready for dispatch to distributors.
With 750,000 miles of film used in the UK in 1998, UK farmers clearly have an insatiable appetite for wrap.
"The past two years have seen the strongest real demand for bale wrap in Britain, and with the supply chain fairly slow, it is advisable to secure supplies in good time to ensure film is available when it is needed," says Mr Hammond. "The 750mm film is still gaining popularity, especially amongst contractors wanting to wrap bales quicker with fewer rotations of the bale." *
• Perforation resistance To resist punctures from stemmy material such as thistles, stones and stubble when the wrapped bale is dropped from the wrapper on to the ground. Also needed to withstand mechanical handling.
• Strength To cope with high tension loads as the film is peeled from the roll and stretched through the pre-stretch rollers.
• Stretch For more economical use of film and to help produce a tight binding around bales, bale wrap is pre-stretched on the wrapper by 50% or, more commonly, 70%.
• Elasticity The films ability to try to return to its original length. Vital for ensuring a tight wrap, an air-tight seal and to help maintain bale shape.
• Tack Adhesion balance has to be just right for the film to peel off the roll easily but then to stick again to achieve an air-tight seal. This can be affected by ambient temperature. Manufacturers recommend removing bale wrap from cool storage some time before it is used.
• "Necking" resistance Ability to maintain the correct film width under tension between the pre-stretch rollers and the bale. Pre-stretch roller tension often needs to be adjusted because day time temperature changes affect these characteristics.
• UV inhibition Exposure to ultra violet light causes stretch film to go brittle. An inhibitor is incorporated as part of the production process to preserve the original characteristics of the film for as long as possible.