Shrink wrap requires a pack to be enclosed in shrink film before the pack and film are introduced to a heat source which activates the film's memory to produce the desired effect.



There are two ways in which pack may be wrapped – the pack may be either wholly enclosed or partially enclosed using a sleeve.

The sleeve is not the concern of this article. Sleeve wrapping generally involves polyethylene material and collations of packs where the medium provides the lowest packaging costs for moving products in bulk.

The total enclosure of a pack allows it to be offered principally for display in a thin material that enhances its appeal.

The issue is then to be able to create a bag around a product.

This is achieved either by an L-sealer or by an automatic machine using one or two rolls of flat film where a variety of alternative sealing methods leads to the same objective – the pack is wholly enclosed.

It is impossible to achieve an appropriate shrink result unless the film is adequately secured around the product.

Hence, flat sheet film will not produce a shrink result.

There has been some work to offer shrink bags for the market.

These can be available in polyethylene but they are not available in either polyolefin or PVC.

It is not possible to open bags to allow them to be used efficiently.

The consequence of these demands is that where production runs through either manual or semi-automatic equipment, an L-sealer is used.

The machine requires, then, that material is used that is centrefolded and presented, on the reel.

Material is available in a variety of thicknesses and a variety of widths but the common theme is that the material is presented to the sealer such that the reel is unwound from the right to the left with the fold of the film at its rear.

The use of the expression L-sealer is marginally inaccurate since sealing takes place using an inverted L.

The L-seal takes place such that the foot of the letter L is rotated through 180 degrees – the vertical axis of the L is unaltered but the base of the L is rotated so that the foot points to the left and not the right.

The film is loaded onto the sealer and split in half using a splitter plate. This allows one half of the film to pass underneath the plate and one half over the top of the plate.

The adjustment of the width of the film together with the adjustment of the splitter plate is made to accommodate the dimensions of the pack.

Before the first pack is introduced, the film is sealed for the first time.

The sealer works so as to cut the film and seal both sides of the cut.

The first cycle creates the left hand seal – and it is noted again that the film is already effectively sealed at the rear by virtue of the film being folded in half.

Having been inserted on the splitter plate, the pack and film are then moved into the sealing area of the sealer.

The base of the sealer will also need to be adjusted for the height of the pack.

The pack is appropriately positioned and the film is also positioned to ensure a minimum of creases and a lack of tension.

The sealing cycle creates the front seal – and scrap - together with the right hand seal and as already mentioned the left hand seal for the next pack.

As a consequence, the sealer may be continuously cycled to produce sealed packs.

The rate of scrap produced at the front of the pack requires to be optimised.

If there is too much, it is just uneconomic.

If there is too little, it is likely to cause slow production speeds together with inadequate seal strength as the film is put under tension to ensure that it meets on the seal beds at the front of the machine.

Although height adjustment is preferable, to allow seals to reside on the sides of any pack, the true purpose is to minimise the film width.

It is essential that half the film covers the bottom of the pack and half its height dimension and equally, half the film covers the top of the pack and half its height dimension.

The fundamental principle remains the need to create two dimensional bags around three dimensional objects.

L-sealing is most certainly also used on fully automatic equipment where the same principles as above hold.

There are some variations with regard to accommodating the third dimension being height.

At the simplest level, this is ignored.

At the more sophisticated level, the position of the seal head and beds is altered to ensure that these components – rather than the pack – move to accommodate the pack height.

It is worth highlighting the need to appreciate that plan sealing dimensions for any L-sealer must be adjusted to account for pack height.

At their maximum, the length and width of any sealer must be reduced by half the height of the pack to be wrapped.

Although an L-sealer can cope with an amount of film pleating, this will inhibit a good quality seal.

As a consequence, film does require to be both flat as well as not under tension on the seal beds.

The key to successful L-sealing is the use of adequate temperature and pressure with the complete absence of tension.

For automatic applications, films require to have excellent machineability, particularly with regard to tear resistance so that the film scrap may be continuously removed from the sealer.

The selection of the correct material with the correct thickness and width is crucial to successful L-sealing.