General Considerations
Sealing
Time As the power is turned on, the material
heats up and its temperature rises. Naturally, as the
temperature rises, heat is conducted off through the dies
and the air until a state of heat balance is reached. At
this point, the amount of heat generated within the
plastic material remains constant. This temperature,
indicating a sort of equilibrium condition between heat
generated and heat loss in order to seal must be above
the melting plastic. It is time required ( measured in
seconds or fraction thereof ) to reach this melting point
that is defined as the “sealing time”. The heat loss is
without a doubt greater with thinner material and less
with thicker material. Indeed, very thin materials ( less
than .004”) loose heat so rapidly that it becomes very
difficult to seal them. (see paragraph on Buffers). The
usual sealing period ranges from one to four seconds. To
minimize failures, it is suggested that the timer
determining the sealing cycle should be set slightly
above the minimum time found necessary for a good
seal.
Pressure The electrodes provide the sealing
current to melt the materials and the pressure to fuse
it. Generally the lower the pressure the poorer the seal.
Conversely, a higher pressure will usually produce a
better seal. However, too much pressure will result in an
undue thinning out of the plastic material and in an
objectionable extrusion along the sides of the seal. as a
result of the two electrodes moving closer to each other,
arcing may be caused, damaging the plastic, the buffer,
and possibly the die.
To obtain high pressure and yet avoid the
above disadvantages, the moving die is restrained in its
motion by a “stop” on the press which is set to prevent the
dies from closing completely when there is no material
between them. This prevents the die from cutting completely
through the material, and at the same time gives a seal of
predetermined thickness. When a tear seal type of die is
used, the stops are not set on the press, since a thinning
of the tear seal area is desired.
To insure a uniform seal, the proper
pressure must be obtained at all points of the seal. To
effect this, the dies are made or ground perfectly flat ,
and held parallel to each other in the press. The dies must
also be rigidly constructed to prevent warping under
pressure.
Power
The amount of power required for a good
seal is directly proportional to the area of the seal.
Moreover, thicker materials require less power than thinner
materials. Our
Sealing Area
Chart shows the
maximum area of seal obtainable with each unit. However, it
must be kept in mind that these figures are for long thin seal,
and for certain materials that are hard to
seal.
Adjusting Power, Time, and
Pressure
When setting up a new sealing job the
first test should be with minimum power, moderate time and
around 80 psi pressure. If the seal is weak, power should be
increased gradually. For greatest freedom from burning or
arcing, the power should be kept as high as possible,
consistent with good sealing.
The dies must be held parallel to produce
even pressure at all sections. If there is too much
extrusion or if the seal is too thin, the press sealing stop
should be used. To set the stop, place half the total
thickness of the material to be sealed on the lower plate.
Close the press and adjust the stop-nut finger tight. Then
insert the full thickness of material in the press and make
a seal. Check the result and lower or raise the stop as
required.
If the seal is weak at certain spots, the
dies are not level. The leveling screws should be checked
and adjusted. If these adjustments are still unsatisfactory,
the die can be shimmed up at the problem areas or the die
may have to be surface ground.
After making a number of seals, the dies
then warm up somewhat and the time and power may require
re-adjustment after several hours of operation . To
eliminate re-adjustment many machines are equipped with
heated upper platens to pre-warm dies to operating
temperatures. Use of heated platens is desirable when doing
tear seal applications.
Pre Seal time and a heated platen can
also change these factors Heated Platens also maintain
consistency.
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