Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
when the jaws are opened and the seal is left unsupported so that there is a danger
of the seal being ruptured.
One early way round the problem was used to make bags from lay flat polyethylene
tubing. The end of the tubing was held by two pieces of cold metal so that about a
centimeter of tubing protruded. A Bunsen flame was played on the protruding film
which quickly fused and gave a heavy bead seal to the tubing. The metal jaws had
the function here of conducting the heat away from the rest of the film. The method
was of course, slow as well as a potential fire hazard. Another method of overcoming
these difficulties is the use of an ‘impulse sealer’. In an impulse sealer the jaws are of
light construction and have attached to them a resistance wire or ribbon covered with
PTFE, which is heated electrically. Because of their small heat capacity the jaws heat
up quickly and cool quickly when the current is switched off. Alternatively, models are
available with water cooled carrier bars. However, these have the drawback that
under humid conditions, condensation may occur on the bar and this can affect the
contact between the bar and the heating unit or lead to deposition of water onto the
film.
The sequence in impulse heat sealing is as follows. The bag is placed between the
jaws, the jaws are heated for a certain time (the current is usually switched on by the
action of the jaws on a micro switch, the current is switched off (by a timer) but the
bag is retained by the jaws until cool. The jaws are then opened and the bag is
removed. The cooling cycle is usually pre-set with a sequence timer and a red light is
on during the heating and cooling cycle. Since the bag is held by the jaws while the
seal cools there is no danger of seal rupture during withdrawal. There is a limit to the
width of seal which can be made by impulse sealing because the wider the heating
strip the longer it takes to cool and eventually the operation becomes uneconomic.
However, even narrow impulse seals can be quite strong and are satisfactory for
most uses.
Impulse heat sealers are intrinsically slower than heated jaw sealers because of the
cooling sequence in part of the cycle. It is possible to incorporate various cooling
methods in tandem with heated jaw sealers so giving heating and cooling without
holding up the passage of the film during cooling. More specialized developments of
the heated bar principle are rotary bar sealers and heated rotary band sealers.
When heat sealing laminated materials, it is generally preferable to use heated jaw
sealers since laminates do not suffer from the disadvantages of single, fully fusible
films. It is possible, therefore, to make use of the high speeds of the continuous
heating method of sealing and embossed jaws can be used to give extra strength. In
general, a good heat seal, using heated jaw or impulse sealers depends on the
temperature at the interface, contact or dwell time, pressure between the jaws and
the nature of the film.
In the particular case of low density polyethylene film used on form, fill and seal
sachet making equipment, the speed of cooling and the strength of the molten
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