The shrink wrap process involves two stages - the enveloping
of the pack in shrink wrap either totally or partially and - the application of
heat to the shrink wrap film which activates the material's memory of its non-stretched
molecular chains.
The application of shrink wrap can be divided into two
principal types - transit and display.
The purpose of transit packaging is to offer the lowest cost
option for packaging. The material used is almost exclusively polyethylene,
which will provide the appropriate strength at the lowest available cost. A
typical application is the collation of individual packs with or without tray
support. The most frequent use of the forgoing involves the packaging of a
quantity of cans or bottles or cartons for distribution.
The equipment used involves a sleeve or bundle sealer
working in conjunction with a shrink tunnel. Two rolls of material produce a
sleeve of film around the pack, which is then processed through the tunnel.
Generally, these open sleeves do not cause any disadvantages to the pack
integrity although a modification can be made to the tunnel whereby an attempt
is made to have the hole closed through excess, annealed film.
A considerable amount of automation is offered for this
process so that one may find a full range of manual, semi and fully automatic
sealers and at maximum through-put, one can find lines with speeds of up to 200
packs per minute.
The processing of polyethylene is distinguished by the need
for the shrink wrap film to reach an almost melt state, after which cooling
will provide significant additional shrink. As a consequence, tunnels require
specific cooling devices if elevated output speeds are needed.
All of this equipment may be used to wrap individual packs
where collation is not required but a degree of protection is needed.
Applications may vary from radiators through to office doors and bolts of
cloth.
The identical processes are also applied but with display
shrink film where products have a natural requirement to use a sleeve. Gift
wrap and wallpaper rolls fall into this category. The principles remain the
same although both the sealer and tunnel will be modified to accommodate
individual rolls with appropriately small diameters.
At the most basic level, this is achieved using an L-Sealer
and folded film. The product is manually placed between the two layers of film
and again manually placed into the sealer. The seal is completed and the pack
placed on the tunnel conveyor. This is a continuous process that also produces
an element of scrap film.
Automation is applied to the seal head and also to the pack
support. Then, the seal process and the removal of the pack to the tunnel
become automated. Generally, this is regarded as semi-automation. The clamping
of the sealer head will be carried out using either a solenoid or a pneumatic
ram. Full automation of an L-sealer will allow packs to be automatically fed
into the film and through to the sealer.
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