An Introduction to Shrink Wrapping Covering Applications as well
as Materials and Machinery
Shrink wrapping is a particularly well-established form of
packaging that has developed over the past four to five decades.
Essentially, material is produced such that molecules are
stretched as part of the manufacturing process. When this
material is subjected to heat, the elastic memory of the plastic
is activated, causing the material to shrink around the product.
This is a traditional division between packaging for transit and
packaging for display, whilst the latter has a further division
into food and non-food applications.
The key to the application of shrink wrap for transit is cost so
that the material and its application provide the lowest cost
solution in delivering the product from the manufacturer to the
end user.
The conventional definition of a transit wrap involves the use
of a sleeve sealer and polyethylene film. An individual or
bundled wrap provides the necessary solution of achieving the
shipment at the lowest cost.
A typical wrap offers two open sides as a consequence of the use
of a sleeve although in certain applications the sleeve may be
effectively eliminated by the use of appropriate machinery.
Generally, the latter is not a particular requirement.
Polyethylene has the attributes of strength at a certain
thickness together with shrink ratios and slip. Accordingly, a
user will be able to specify a thickness to provide the
necessary pack integrity to the end user and shrink ratios to
ensure that the wrap is tight. Slip can be altered to ensure
that packs stack appropriately.
There are further benefits of shrink wrap since waste disposal
after delivery is relatively insignificant whilst the material
has clarity to allow product to be identified. In addition,
there are tamper-evidence and moisture-prevention benefits
depending on the exact application.
Normal applications involve the collation of products although
individual packs are readily benefited from transit wrapping.
The market also offers developments in this area such as
coloured and printed films. Polyethylene is not a particularly
good medium for product enhancement through printing or
pigmentation as it has a naturally soft surface which allows
scuffing and dust retention whilst it is also relatively cloudy.
However, polyethylene may frequently offer sufficient optics so
that it is acceptable in these applications.
As technology has advanced in this market so have applications
crossed over traditional definitions in certain areas.
Consequently, certain transit packs are found using normal
display films where the pack and its marketing benefit from the
increased investment in a material that offers better optics and
machineability.
There are highly specialised display films that are designed to
offer the same strength as polyethylene but with far better
optics and the marketing benefit is such, in these very specific
areas, that the greater film cost can be justified.
Display shrink wrap is traditionally oriented towards non-food
point of sale packaging where the principal goal is product
enhancement with some limited pack protection through an overall
wrap.
The latter is the main distinguishing feature of display
packaging, the partial wrap with a sleeve generally found is
replaced with a total wrap involving the creation of a two
dimensional bag around the product.
Some years ago, the conventional film used for this purpose was
PVC - polyvinylchloride with plasticiser; an amorphous
crystalline structure.
PVC has largely been replaced by polyolefin shrink film although
PVC continues to be offered.
PVC has particular benefits in that it is the easiest of any
films to seal and shrink but it suffers from problems on
sealing, whilst it also has strength and storage issues.
PVC's sealing temperature is very close to its degradation
temperature at which a number of by-products are created. These
include very small quantities of hydrogen chloride gas and
carbon deposits on the sealer.
The HCL may be dealt with through ventilation. It is to be
stressed that all materials should be used with regard to good
manufacturing practice and as a consequence, polyolefin films
will also require ventilation in the same way. The by-products
of combustion should always be considered in relation to the use
of any specific machine operator.
Carbon deposits require regular cleaning whilst the presence of
HCL will give rise to a need for regular maintenance of the
sealer. As polyolefin's have no chlorine, this does not arise
with their use.
The plasticiser in PVC will harden in cold conditions and soften
in hot conditions and this may well cause strength problems in
cold weather and equally machineability problems in hot. Again,
as polyolefin material has no plasticiser, these are not issues
where this film is concerned.
PVC requires storage at normal room temperature failing which it
is likely to start to shrink. Polyolefin will tolerate higher
storage temperatures.
The overall machineability of PVC still gives this material some
market share but, increasingly, polyolefin has dealt with its
lower overall machineability whilst offering the benefits noted
above so that it is now by some considerable margin, the
principal display material.
Display wrapping involves the creation of a two dimensional bag
around the product.
This is conventionally performed using an L Sealer with centre
folded film. The sealer allows continuous production of wrapped
packs that are then processed through a shrink tunnel.
L sealers are found in a variety of forms - manual, semi and
fully automatic. A large part of the manual and semi automatic
market has been taken by combined seal and shrink machinery.
These machines are commonly referred to as chamber machines.
Fully automatic L sealers have found increasing use as their
cost and versatility have improved.
The highest output speeds are offered by flow wrappers using
side seal or overlap mechanisms. Nevertheless the common feature
of a two dimensional bag remains the principal similarity
between all these sealers.
Polyolefin films offer a variety of attributes so that a very
large range of applications can be made.
In general, these are not found in food applications although
once more, definitions continue to be stretched. Polyolefin is
quite well used in certain applications notably pizza and egg
wrapping as well as produce packaging. However, a more precise
definition might be that polyolefin is not used to extend the
shelf life of a food product. This applies generally except
where shrink barrier film is used although this latter product
is particularly specialised.
Normally, other materials than shrink are used to extend shelf
life and as a consequence, shrink is found only in certain food
applications although volumes can be significant.
Display applications for polyolefin have almost no limits as
materials have been engineered to deal with any given
requirement.
Soft shrink as well as perforation and printing are standard
offers. At a more specialised level, slip and anti-fog
treatments are also available.
There are a few notable ranges of polyolefin shrink film that
offer better machineability and performance than the majority of
the market and these are particularly distinguished by technical
differentiation in manufacturing. These lead to irradiated
grades which offer enhanced strength and multi-layer complexes
that may be tailored to a particular requirement.
Polyolefin manufacture typically uses three or five layer
extrusion and this confers performance benefits as a consequence.
It will be noted that the key to market movement is the
development of material to address specific requirements, since
its application is relatively uncomplicated.
There is no doubt that polyolefin shrink film will continue to
develop so that it addresses even more uses than at present.