Is Prestretch ?
is the process that allows stretch film to be stretched before
(pre) being applied to a load.
- The film
threads past two rollers (Primary and Secondary) in which the
second rollers' rotational speed is faster than the first, thus
stretching the stretch film between the two.
As you stretch the film , the tensile strength of the film increases
so it is significantly (2X) stronger once you pass the point of
- FILM MEMORY
The memory of the film wants to return it to its initial form
. However the more you prestretch the lower the memory . (which
is why Bi-stretch did not work)
If the film is applied to a load with no film force, minutes later
you can observe that the film has tightened due to the film elastic
memory as it tries to return .
is this film memory (like an elastic band on your hand) that differentiates
stretch film from other means of unitizing . Settling that occurs
during shipment can loosen other methods of unitizing where stretch
film memory takes up the slack and continues to secure the load
Film Memory is the single most important reason for prestretching
- FILM ECONOMIES
The cost of consumables (film) is reduced drastically compared
to wrapping a load with film that has not been prestretched ie.
Payback of capital expenditure of 6-12 months is easily obtainable.
- STRETCH CONSISTENCIES
Stretching using tension or a brake (post stretching) does not
provide uniform stretch and thus is not able to repeat its performance
day after day .
This can lead to intermittent load failures
As prestretch is more controllable than post stretch it is typically
more desirable to maximize prestretch and minimize post stretch
How Is Film
- Film is
prestretched when it is elongated mechanically between the two
film carriage rollers due to the gearing ratio and diameter of
- As the surface
of the secondary roller is rotating faster than the Primary the
film gets pulled between them and stretches.