Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
polymer are important factors. In the early days of such equipment, the filling speed
was the rate determining factor but with increasing filling speeds, the limit is now set
by the speed of sealing.
For materials sealing at the same temperature, the maximum speed of sealing is
likely to be determined by the speed of crystallization and this, in turn, is affected by
density. The higher density grades crystallize at a higher temperature and so set up
more rapidly. Of course, they also require higher seal temperatures but in practice
sealing jaw temperatures are higher than the minimum required for a good seal and it
is unlikely that a significant increase would be necessary for the high density grades.
Heat sealers have also been developed which work by applying a blast of hot air to
the film surfaces. They are particularly useful for heavy duty sacks used for fertilizers
or other products liable to cause contamination by powder of the two mating
surfaces. Any dust particles are completely encapsulated by molten polyethylene and
do not affect the heat seal strength.
SEALING OF ORIENTED FILM:
The sealing of oriented films presents particular difficulties. When heated, the film
naturally tries to return to its original, outstretched state in the vicinity of the heat seal
and this leads to cockling of the seal. Highly oriented films are also liable to
crystallize readily and if the cooling rate is slow then large spherulites are formed,
giving brittle seals. Rapid cooling gives small spherulites and good strength at the
seal. Secure clamping of the film and cooling of the areas adjacent to the heat seal
can ameliorate these troubles but does not always cure them.
Another method which was designed especially for oriented polypropylene film is
known as ‘multi-point’ sealing. As its name implies the heating jaws consist of a large
number of very small points and the heating effect is, therefore, extremely localized
thus preventing the tendency to shrinking of the whole heat seal area. Its
disadvantage is that it is not a complete heat seal and air and moisture vapor can
eventually diffuse through the seal.
Another more satisfactory method is to coat the oriented film with a polymer having a
lower softening point and use the coating to affect the heat seal. Since the oriented
film is not heated to its softening point the tendency to shrink is avoided. This method
has the additional advantage that the oriented film gives support during the heat
sealing operation provided that the heat seal temperature of the coating is far enough
below the softening point of the oriented film substrate.
27