A few words about Shrink Packaging

  1. Introduction
    Shrink packaging has been well established for over three decades. Due to it's practical and cost related advantages, it has found its use in many industries at various stages of packaging process.
    Shrink Packaging can be divided into following groups:
    a)Primary- where the product is totally enclosed for protection from abrasion, dust, moisture and for improved appeal.

    b)Secondary- where a group of items is wrapped together to form a Sales unit, (Sixpacks of Beer, Juice etc)

    c)Distribution packaging
    -
    where product is wrapped into a distribution unit which may stacked directly on the pallet ready for transportation.
    When walking through supermarket one only has to look on top of the shelves to see the extent of use of this packaging method.
    Most grocery and high volume beverage products will use cardboard trays for added

    rigidity of the pack. While the use tray is not always necessary, it does provide improved handling in automated warehouses.

  2. Shrink Packaging Process
    Shrink wrapping, bundle wrapping, etc, is a process during which a product item or a group of items in wrapped in a loose sleeve or envelope of plastic film, which upon application of heat shrinks and tightly conforms to the shape of enclosed contents.
    The key element of this process is Shrink Film.

    Shrink Film can be made from a variety of materials each having different strength, shrink characteristics transparency and luster.
    PolyVinylChloride (PVC), PolyPropylene (PP) and PolyEthylene (PE) are the three most commonly available materials.
    PVC, being one of the first materials used, is now being phased out due to its toxic properties, although still used abundantly in underdeveloped countries.
    Low Density Polyethylene (LDPE) is best suited for general packaging applications due to it's relatively high strength and low cost.

    Polyethylene Shrink film is manufactured in an vertical extrusion process where ethylene granules are heated under pressure to produce Polyethylene polymer.
    The polymer is forced upwards through a circular extrusion die to produce a very thin walled continuous tube of material, still in a semi-molten state as it emerges from the die.

    A controlled supply of air is fed up through the center of the die to stretch the tube in the radial direction, thus forming a bubble. at the same time the tube is stretched in length by winding the film at a faster than it is extruded.

    As the film cools, the induced stretch is "memorized" in the film. Upon reheating the film, if unrestrained, will shrink a certain percentage in the width, referred to as % Shrink in TD (transverse direction), and in the length, % shrink in MD(machine direction)

    At the top end of the bubble the film tube is cool enough to be flattened and wound off as Lay-flat tubing, alternatively, it can be slit at one edge to produce Center-folded material or slit at both edges to produce Flat sheet.

    a)- Wrapping Process
    The essential aim of the wrapping process is to place shrink film around the product prior to shrinking. There are two principle forms of wrapping:
    i)Sleevewrapping.
    ii)Overwrapping

    Sleevewrapping -
    best suited for secondary and tertiary packaging. The process is easily automated, uses low cost material and has low labour input. Sleevewrapping is readily applied to regular shaped products that need to be collated into larger, easier to handle packs for palletising, warehousing or distribution.

    Sleevewrapping is performed using two rolls of flat sheet shrink film. The ends of the sheets are heat sealed together forming a vertical curtain. Product is pushed through the film curtain to a position behind the Welding bar.
    The welding bar descends in front of the pack, completing the sleeve of film, parting the sleeve from the rest of the film and reforming the film curtain for the next pack.

    The sleeved pack then passes to the shrink tunnel where film shrinking takes place.
    The resultant pack will is Not totally enclosed. Each end of the pack has an opening due to the original wrap being only a sleeve.

    Sleevewrapping Machines (Bundle wrappers)
    A wide range of machinery is available and the choice depends on application and budget. Simple manually operated machines are the lowest level. In use they require the operator to position the product, operate the jaw, by means of a pull bar or a foot pedal initiate the seal cycle and release the Jaw.
    At the next level are operator controlled semi- automatic machines with pneumatic assistance. These require the operator to position the pack and initiate the automatic welding cycle. Medium speed automatic systems are the next option, all machinery cycles being initiated by the arrival of product onto the machine.
    This type of machinery features intermittent motion operation and is capable of speeds up to 30 packs/min. High speed continuous motion machinery is available with speeds up to 100 cycles/min.

    Overwrapping
    Overwrapping Achieves a total enclosure of the product by a perimeter sealing process performed on L-bar type machines that utilize Centrefolded film. Total enclosure can also be achieved using flat sheet film on a Flow wrapping machines.

    Overwrapping is best suited for low height, rectangular shaped products such as stationery, magazines, video tape, CD's etc. Although the writer has in the past designed L-Bar type machines 2.4m long x 1.2m wide with a suitable 1.35m wide Shrink Tunnel.

    When performed on an L-bar sealing machine the operation is as follows The end of the film is sealed across the width and forms one of the sealed sides of the pack. Product is inserted between the layers of film, then transferred together to the sealing station. Here the remaining two sides are sealed and product is now completely enclosed by the film.

    From there the package is moved onto the shrink tunnel conveyor to complete the operation. Simple L-Bar machines are manually operated although automatic models are also available.

    There are also "All in one" L=Bar type units where the Sealing and shrinking are performed
    in one operation. The Sealing station consist of a hood, where the product is enclosed in film. Once the seal is complete, Hot air is blown around the pack to shrink the package. These machines are best suited successful in low speed, office or dispatch applications.

    b)Shrinking Process
    Once the product is wrapped, the plastic film around it has to be shrunk to complete the process.
    Shrinking of the film can be achieved through a number of methods which depending on application may range from hand held gas or electric hot air blowers, to fully automated room size ovens.However, the best results are obtained through the use of a Shrink Tunnel.

    Shrink Tunnel,
    is simply a recirculated hot air chamber sitting on top of a conveyor which carries the product through the chamber.
    The essential requirements for a good shrink tunnel are a constant yet adjustable air temperature and conveyor speed, symetrical air circulation pattern.

    As pre-wrapped product is transported through the hot chamber the film temperature raises and the film shrinks to conforms to the shape of enclosed product. At that stage the film is not strong enough to compress the contents.

    When the pack emerges from the hot chamber the film cools and shrinks a little more, tightening the pack into a stable, easily handled unit.
    The degree of final compression is determined mainly by the thickness and type of film used.

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