Note: Descriptions are shown in the official language in which they were submitted.
CA 0223~903 l998-04-27
~ethod and Device for Plastic Lamination of Metal Strip by
me~ns of Direct Extrusion.
The invention relates to a method for plastic lamination
5 of a metal strip by me<~ns of direct extrusion, in which
the metal strip is moved in its longitudinal direction,
and is heated; by means of a sheet die, a film of molten,
thermoplastic plastic material is deposlted directly onto
one side of the moving strip, this plastic film is pressed
o onto the metal strip, while being led through a gap
between two rollers, by which the roller adjacent to the
plastic film (laminato:r roller) is maintained below the
melting temperature of the plastic material; if necessary
the other side of the metal strip may be coated with a
15 ]plastic film in simila:r fashion and, in a subsequent
treatment, the laminated metal strip is heated to a
temperature in the region of the melting point and finally
cooled rapidly to a temperature below 40_C.
20 Furthermore, the invention also relates to a device for
plastic lamination of a metal strip by means of direct
extrusion.
In a known method of the type mentioned above (US 5 407
25 702, Fig.3), an aluminium strip is heated, before
laminating with PET (polyethyleneterephthalate), to a
temperature in the range of 204-260_C, preferably 215-
246_C and led through the gap between two rollers. Before
entering the gap, the liquid plastic film is deposited on
CA 0223~903 l998-04-27
one side of the aluminium strip. The roller pressing on
the plastic film is a chrome steel roller and is kept at a
temperature of 150-200 C. The roller adjacent to the
unlaminated side of the aluminium strip is a rubber
5 encased roller and has a surface temperature of 205_C, in
order to keep the aluminium strip up to temperature. The
layer thickness of the deposited plastic film should be
approx. 8-20 ~m, preferably lO~m. After the aluminium
strip, which was laminated on one side, has left the first
o pair of rollers, lamination on the other side is
achieved by means of a second sheet die and a second
identical pair of rollers. In this procedure, the
adhesion of both plastic films to the aluminium strip is
slight at first, but just large enough, that the plastic
15 films do not separate from the aluminium strip in the
course of further processing, which is described as "green
peel strengthn. After the aluminium strip has been
laminated in this way on the second side too, it is led
through an induction heater, where it is heated to approx.
20 215_C. As a result of this heating, the bond between the
plastic fi~lms and the aluminium strip should be completed.
Next, the bonding system is cooled using spray jets to a
temperature firstly, which allows the half-cooled,
laminated aluminium strip to be fed into a water bath via
25 a deflection roller, where it is cooled down to a
temperature below 40_C. In this known method, line
contact or contact between the roller adjacent to the
liquid plastic film and the plastic material is only
achieved via a relatively small contact surface.
CA 0223~903 1998-04-27
rhe cooled roller is therefore only in contact with the
plastic film for a very short time. Separating the roller
surface from the plastic film may only result, however,
when at least the surface layer of the plastic film is
s solid since the plastic film adheres otherwise to the
roller and contaminates the latter. So that the plastic
material in the gap region can cool sufficiently, a very
low speed must be used for the strip, making economic
production impossible. Laminating a steel strip in place
o of an aluminium strip and using greater layer thicknesses
of plastic material with for example 200~m, this method
could definitely not be used since heat could not be
dissipated quickly enough through the adjacent roller
with line contact only because of the higher heat capacity
15 and lower heat conductivity of the steel strip and because
of the greater layer thickness of the plastic film. In
addition, the adhesion between the plastic film and a
steel strip would not be adequate after leaving the
rollers to prevent a separation of the same from the steel
20 strip using greater film thicknesses, in which there is a
high occurrence of shrinkage while cooling.
Therefore, the object underlying the invention is to
indicate a method of the type mentioned at the beginning
25 which can be accomplished in economic conditions, i.e.
with sufficiently high strip speeds and which results, at
the same time, in excellent adhesion between the steel
strip and the plastic film, which remains in place even
during deep-drawing, but particularly also during
CA 0223~903 l998-04-27
sterilisation. In add:Ltion, the object underlying the
invention is to create a device for plastic lamination of
a metal strip by means of direct extrusion, which makes it
possible to laminate metal strips at high strip speeds and
s with excellent adhesion between the metal strip and the
plastic film.
The method according to the invention is characterised in
hat, during lamination of a steel strip, the latter is
o heated to such a temperature that the temperature lies
above the melting temperature of the respective plastic
material in the depositing region of the liquid plastic
:Eilm, in that, between the roller pressing on the plastic
film (laminator roller) or a continuous belt pressing on
15 the plastic film (laminator belt), there is surface
contact and that this surface contact is maintained by
synchronous conveyance of the surfaces of the plastic film
,~nd the laminator roller or laminator belt, which are in
contact, via a contact time or contact length which
20 ,suffices for cooling at least the surface layer of the
plastic film at a belt speed of at least 50m/min and with
a cooling rate of at most 400 W/m2_C to a temperature lying
~t least around 30_C below the melting point of the
respective plastic before contact is lost between the
25 plastic film and the laminator roller or laminator belt.
.~dvantageous processing measures are given in the Sub-
~laims 2-13.
30 Devices according to the invention for plastic lamination
CA 0223~903 1998-04-27
of a metal strip by means of direct extrusion are
characterlsed in the Claims 14-24.
During lamination, the liquid plastic film is extruded
5 directly onto the steel strip. Then, the plastic film is
pressed onto the steel strip by the laminator roller or
laminator belt. The l~minator roller or laminator belt
then takes over the job of cooling the plastic film in
order to convey it from the liquid phase into the solid
o phase. The laminator roller or laminator belt, which for
the sake of simplicity are known only as laminator in the
following, can serve, because of their surface structure,
for shaping the surface of the plastic film as well.
While the plastic film is cooling, the latter experiences
transverse shrinkage which, if certain measures were not
taken, would lead to the plastic film peeling off the
steel strip. This is particularly the case when the
plastic film has a thickness greater than up to 200~m. It
must be established, that the adhesion capacity in the
20 bond between the steel strip and the plastic film
increases more quickly while cooling, than the shrinkage
capacity in the plastic film. For this purpose, certain
measures are mandatory, namely heating the steel strip so
that, in the depositing area of the liquid plastic film,
25 temperatures are kept above the melting point of the
plastic (e.g. for PP [polypropylene] : 190~C, for PET
[polyethyleneterephthalate] : 290~C and for PE
[polyethylene] : 130~C, also pressing the liquid plastic
film onto the steel strip with sufficient contact time or
CA 0223~903 1998-04-27
contact length and sufficient pressure and also with a
cooling rate which is effected by the laminator and which
is not more than 400 W/m2_C.
5 Separation of the laminator from the plastic film can only
result when at least the surface layer of the plastic film
is converted by cooling into a solid condition. The
adhesion of the plastic film to the steel strip must be
greater than that of the adhesion to the laminator.
o~therwise, particles of plastic material adhere to the
laminator, which leads not only to contamination of the
laminator but also to a momentary separation of the
plastic film from the steel strip and to an irreversible
loss of adhesion as well as eventual transverse shrinkage
rn order to achieve adequate cooling of the plastic film
at belt speeds of over 50m/min and preferably more, which
permit rational production, the invention envisages that
while the steel strip is moving, the plastic film is held
20 :in position on the laminator for an adequate period of
contact by surface contact between the plastic film and
lhe laminator for the time required for cooling and that
:it is pressed onto the steel strip at the same time. A
:Longer contact time and greater contact length (in the
2s direction of movement of the belt) is particularly
required for converting at least the surface layer of the
plastic film, which is adjacent to the laminator, into a
solid state by cooling when there are greater film
t:hicknesses of for example 200~m and an essentially
CA 0223~903 1998-04-27
smaller heat conductivity relative to aluminium, and also
a higher heat capacity in the steel strip.
'~hen the plastic film is being cooled by the laminator,
5 the cooling rate should not be higher than 400 W/m2_C since
otherwise, when using the size of film thicknesses
mentioned, and because of too speedy transverse shrinkage
of the plastic film, the latter separates in part from the
~steel strip and a loss of adhesion occurs.
So that the liquid plastic film is pressed onto the steel
strip to a sufficient degree by the laminator, this should
be done with a force of at least 60 N/mm of steel strip
width.
So that the adhesion between the plastic film and the
steel strip is improved, an adequate reaction time must
exist between the liquid plastic material and the surface
of the heated steel strip. In order to achieve this, the
20 temperature of the steel strip in the depositing area of
the plastic film should be at least around 10~C preferably
however around 20~C or more above the melting point of the
respective plastic material.
25 .~ sufficiently long contact time between the plastic film
and the laminator can be achieved by leading the steel
strip with the plastic film which is adjacent to a
laminator roller under tension over a part of the
circumference of the laminator roller.
CA 0223~903 1998-04-27
continuous laminator belt is likewise led with tension
together with the laminated steel strip round part of the
~ircumference of the roller, the plastic film, which is
s -ooling down and adjacent to the laminator belt, being
held on the laminator belt till at least its surface layer
is converted into a solid condition. Cooling results in
this case by means of the laminator belt, which can
usefully consist of steel, on the one hand, and by means
o of the partly encircled roller on the other.
This method is particularly appropriate for laminating
~steel strips on both sides, the encircled roller being
esigned as a cooling laminator roller which presses the
:Eirst plastic film to one side of the steel strip while
lhe second plastic film is pressed by the laminator belt
onto the other side of the steel strip and then cooled.
l?or the abovementioned reasons, during cooling of the
20 plastic film by means of laminators, the cooling rate
~heat transmission coefficient) should not be greater than
400 W/m2_C, to ensure the desired level of adhesion.
This cooling rate is however not sufficient for keeping
25 t:he growth of crystallites or spherulites small,
particularly using PP. Exceeding a critical spherulite
cliameter leads with PP to a clouding of the plastic film
and to so-called white breakage during reshaping of the
laminated steel strip into packaging material. This is
30 a~lso true to a lesser extent for PET, the spherulite
CA 0223~903 l998-04-27
growth rate of which is however considerably smaller than
t:hat of PP. However, even with PET a plastic film in an
amorphous form with as few crystallites as possible is
aspired to, in order to guarantee a high degree of
5 ductility. In order to reduce the amount of spherulites
or to make an amorphous structure, the completed laminated
<,teel strip is heated after the laminator to a temperature
above the melting point of the respective plastic
rnaterial, e.g. with PP above 200~C, with PET above 300~Cr
o ',ubsecIuently, rapid cooling must occur by plunging in
water at room temperature, so that, when remaining below
t:he crystallisation temperature which lies immediately
below the deformation point, renewed growth in crystals
does not occur (spherulite growth). For homo-PP the
15 cooling rate should be at least 200~C/s, and for a random-
~?P at least 100~C/s. In order to achieve this, heat
t:ransmission coefficients for the laminated steel strip
nnust be produced in water of at least 3000 W/m2~C or at
least 1800 W/m2~C. This implies the necessity for high
20 relative speeds between the laminated metal strip and the
water (30-100 m/min), to ensure turbulent transportation
of material or heat.
When both sides are to be laminated with plastic materials
25 which have varying melting points, care must be taken,
t:hat the plastic material which is deposited first, has
t:he higher melting point, since, at thelbeginning, the
steel strip has the highest temperature. When the plastic
f.ilm which is deposited first, e.g. PET, is pressed onto
CA 0223~903 1998-04-27
the steel strip by a first laminator and is thereby
cooled, the steel strip is then also cooled simultaneously
to a lower temperature. As long as the temperature of the
steel strip remains above the melting point of the second
5 plastic film however, e.g. PP, the PP plastic film is
extruded on the now cooler steel strip. The strip
temperature difference for the two laminations of PET and
PP is about 100 ~C.
o The deposition of the liquid plastic material on the steel
3trip is achieved by means of a sheet die, at a width for
this purpose which is greater than the width of the steel
3trip. The thickness of the emergent plastic film is
determined by the adjustment of the gap in the die. Since
he steel strip possesses a higher strip speed relative to
the die exit speed, the plastic film is pulled
Longitudinally and becomes thinner. Thereby, a reduction
:in breadth also takes place, which leads to an even
distribution in thickness of the film over its breadth.
20 The edge areas are thicker than the central region. For
t:his reason, a plastic film is produced which has a
greater width than that of the steel strip, with the
result that the thicker edge areas on the steel strip
e~xceed approx. 20-30mm. In order to protect a pressing-on
25 roller which is located opposite the laminator roller from
contamination from pro~ecting plastic film, continuous
Teflon strips are led on bolth longitudinal edges of the
steel strip in the depositing area and ad~acent to said
strip synchronously with the steel strip, until the
CA 0223~903 1998-04-27
11
sections of plastic film projecting sideways over the
steel strip are cooled adequately to below melting point.
After the plastic film has set, the protruding plastic
film on the steel strip can be trimmed. If the plastic
5 films on both sides of the steel strip are different, they
are suctioned off separately, so that they can be sent for
recycling.
Devices according to the invention for plastic laminating
o on both sides of a metal strip by means of direct
extrusion are described in greater detail in the following
with the aid of embodiments which are represented
schematically in Fig. 1-3 of the drawing.
15 In all three embodiments, the heating device, the device
for subsequent heating of the laminated steel strip and
the cooling device are all the same, for which reason they
are described in greater detail only in the embodiment
example represented in Fig. 1. The metal strip is
20 preferably a steel strip, which can also be surface
treated with tin plating, chrome plating or conversion
laminating. With the devices according to the invention
however, other metal strips, for example aluminium strips
can also be laminated. The metal strip can have a
25 thickness of 0.05 -0.5mm. Thermoplastic plastic
materials, such as PET, homo-PP, block PP, random PP and
PE can be used for laminating. The film thicknesses can
thereby be 5 - 200~m on one side and 3 - lO~m on the other
side. Both sides of the metal strip can be laminated with
CA 0223 j903 l998 - 04 - 27
12
the same or different plastic materials depending on
application requirements. The operation can be carried
out with belt ~peeds of 50-400 m/min.
5 According to Figure l the metal strip M is directed
firstly through a heating device l. Next to this is a
first laminating station. The latter has a first sheet
die for directly depositing the molten thermoplastic
plastic material in the form of a first liquid plastic
lO film 3 onto the first side of the heated metal strip M.
The plastic film can consist of two layers in a known
fashion. The layer orientated towards the metal strip can
ensure, especially with a steel strip, the adhesion of the
plastic film to the metal strip. The outer layer should
15 be selected for the integrity of the packaging, with
:respect to the contents, which is produced from the
:Laminated metal strip or for its resistance with respect
to outer stresses. In order to produce a plastic film
with a two-layer design, both layers can be extruded
20 simultaneously from the same sheet die, which is known
per se and therefore is not described in more detail.
13ehind the sheet die l, there are two rollers 4,5 which
are pressed against one another. The roller 4, which is
described as the pressing-on roller in the following, has
25 .~ sleeve 4a made from rubber elastic material. The other
:roller 5, which is described as the laminator roller in
_he following, is cooled with cooled water, which flows
through the interior of the laminator roller 5. The metal
.strip M with the ~till liquid plastic film 3 is fed
30 ~hrough a gap 6 between the two rollers 4,5 and thereby
CA 0223~903 l998-04-27
13
pressed by the laminator roller 5 onto the metal strip M~
The liquid plastic film 3 should be pressed onto the
metal strip M with a force of at least 60 N/mm applied to
the width of the steel strip. The cooling rate of the
s laminator roller 5 should be set in such a way that a
cooling rate of at most 400W/m2_C results. While the
plastic film 3 is adjacent to the laminator roller 5, lts
surface layer at least must be converted by cooling into a
firm condition, before the surface of the laminator roller
o 5 is separated from the plastic film. For that reason, a
second cooling laminator roller 7 and a second pressing-on
roller 8 are arranged in such a way that the metal strip M
is looped round the preceding first laminator roller 5 on
a part of its circumference and the metal strip with the
still liquid plastic film adjacent to the laminator roller
5 is held in contact at the first gap 6 over a part of the
circumference of the roller 5 in an arrangement with the
latter. In front of the second laminator roller 7, a
second sheet die 9 is arranged, with which a second liquid
20 plastic film 10 can be extruded on the second side of the
metal strip. Said second liquid plastic film is then
pressed by means of the cooling laminator roller 7 onto
the second side of the metal strip in the previously
mentioned manner. The sheet die 9, the lamina~or roller 7
25 .~nd the pressing-on roller 8 together form the second
:Lamination station. In the direction of the belt behind
the second-laminator roller 7 a deflection roller 19,
which may likewise be designed as a cooling roller, is
once again arranged such that the metal strip M loops
30 round the second preceding laminator roll 7 on a part of
CA 0223~903 1998-04-27
14
its circumference and the metal strip with the still
liquid plastic film 10 which is adjacent to the laminator
roller 7 is held in contact at the second gap 11 over a
part of th'e circumference of the second laminator roller
5 in an arrangement with the latter, till at least the
surface of the second film 10, which is adjacent to the
laminator roller 7 was cooled down into a firm condition~
The length of the encircling of each laminator roller 5,7
or the length of contact depends upon the speed of the
o belt, the thickness and type of metal strip, the thickness
of the plastic films and the temperature of the laminator
rollers. Experiments with a steel strip of 0.26 mm
thickness and a PP plastic film of 200~m thickness gave
the result that, with a laminator roller temperature of
15 40~C, the contact time on the laminator roller must be 60
ms and the contact length, with which the plastic film
must be kept in position on the contact roller, must be
200 mm at a belt speed of 200 m/min. If the temperature
of the laminator roller is at 60~C, then the contact time
20 must be 80 ms and the corresponding contact length 270 mm.
In order to make sure of a flexible gap compensation in
the gap 11, in case variations in film thickness occur the
pressing-on roller 8 can usefully have a sleeve 8a, which
25 iS made of rubber elastic material and which is surrounded
concentrically by a thin, outer steel sleeve 8b which is
flexible in a radial direction. Using a steel sleeve 8b,
prevents the pressing-on roller 8 from leaving behind
unwanted patterns on the first plastic film 3, which may
CA 0223~903 1998-04-27
occur if the sleeve made of rubber elastic material were
to sit directly on the plastic film.
For reasons which have already been described in greater
5 detail in the description of the method, the width of the
sheet die 2 is greater than the width of the metal strip
M. This leads to the fact that the plastic film 3 juts
out on each side of the metal strip M by 20 mm to 30 mm.
The liquid plastic film would adhere to the pressing-on
o roller 4. In order to prevent this, continuous Teflon
strips are provided on both sides of the steel strip M,
said strips being directed via the pressing-on roller 4
and two deflection rollers 13,14. The deflection roller
14 is thereby likewise arranged such that the protruding
plastic film is pressed onto the laminator roller 5 until
it is converted into a solid condition by cooling. The
protruding plastic film is therefore only separated from
the Teflon strips 12 after cooling and setting of the
plastic material also. The protruding part of the plastic
20 films is later cut off using trimming rollers 15, which
are arranged on both side~ to the laminated metal strip,
and sent for recycling.
In the description of the method, it was explained
25 extensively, that subsequent treatment of the laminated
metal strip by heating and then fast cooling down in a
water bath are mandatory. This is achieved by means of
the heating device 16, which is only represented in Figure
1, and the connected cooling device 17 which consists of a
30 water bath.
CA 0223~903 l998-04-27
The embodiment example which is represented in Figure 2
corresponds essentially to the previously described
embodiment example. Devices and parts with the same
5 function are thus designated with the same reference
numbers. So that repetitions are avoided, reference is
made to the embodiments shown in Figure 1.
In the embodiment example represented in Figure 2, the
o second laminator roller 17 is arranged such that it faces
the first laminator roller 5 and can be pressed onto the
latter. In order to ensure a certain flexibility here in
the gap 11 as well, the second laminator roller 7 can have
a sleeve 7a made from rubber elastic material, and be
15 surrounded concentrically by a thin outer steel sleeve 7b
which is flexible in a radial direction. In this
arrangement, the second pressing-on roller drops out of
use, since the first laminator roller takes over the
function of the second roller 7.
In the solution which is represented in Figure 3, a first
sheet die 21 is arranged on the first side of the metal
strip M and a second sheet die 22 on the opposite side of
the same. Underneath both sheet dies 21,22 a first
25 laminator roller 24 and a second laminator roller 25 are
situated beside one another and can be cooled
appropriately by means of water so that they can be set at
a temperature of 20-80~C. A deflection roller 23, which is
connected to the first laminator roller 24, makes certain
CA 0223~903 1998-04-27
17
that the metal strip, which surrounds the laminator roller
1 by around 180~ in this case, is in contact with the gap
26 which is formed between the two laminator rollers 24,25
which are pressed against one another. It is ensured in
5 this way, that the film is pressed onto the metal strip M
by means of the first sheet die 21 firstly in the gap 26
through the laminator rollers 24 and 25 which face one
another and then the metal strip with the first plastic
film 27 which is at first still liquid is held in contact
10 with the gap 26 in an arrangement with the first laminator
roller 24 until the first plastic film 27 is cooled down
by the laminator roller 24 and converted into a solid
condition. Only then does the first plastic film 27,
which is adhering securely to the metal strip M, separate
15 from the surface of the laminator roller 24.
A continuous laminator belt 28, which consists
appropriately of steel, is directed over the second
laminator roller 25, which appropriately has a rubber
20 elastic sleeve 25a. Furthermore, two deflection rollers
29, 30 are provided for directing the laminator belt 28,
said rollers being designed appropriately as cooling
rollers. The deflection roller 29 is arranged such that
the laminator belt surrounds the first laminator roller 24
25 in that region, in which the laminator roller 24 is also
surrounded by the metal strip M. In this way, the second
plastic film 31 which has been extruded by the second
sheet die 22 can be pressed onto the metal strip by the
second laminator roller 25 by inserting the laminator belt
CA 0223~903 1998-04-27
18
in the gap 26. The second plastic film 31 is then, in
contact with the gap 26, pressed onto the metal strip M,
furthermore, during the partial surrounding of the first
laminator roller 24 until the second plastic film is
5 cooled down into a solid condition. Cooling is achieved
thereby using the laminator belt 28, which has been cooled
down by the cooling rollers 29,30 and the second laminator
roller 25. Additional cooling is achieved using the first
laminator roller 24. This modus operandi is adopted when
o similar plastic materials, with similar melting points are
to be deposited on both side~ of the metal strip e.g. PET
on both sides or PP on both sides.
As can be seen from Figure 3, a pressing-on roller 32 is
also assigned to the first laminator roller 24, said
pressing-on roller having a sleeve 32a of rubber elastic
material appropriately. The laminator roller 32 is
arranged in the direction of the circumference of the
first laminator roller at a greater spacing from the gap
20 26, which is formed between the two laminator rollers
24,25. The first sheet die 21 can be adjusted from its
first pouring position, which is fully opened-out in
Figure 3, into a second pouring position which is shown in
Figure 3 with dotted lines. If plastic materials with
25 different melting points are to be deposited on both sides
of the metal ~trip M e.g. on the first side PET and PP on
the second side, the first sheet die is then brought into
its second pouring position, which is shown with dotted
lines. The steel strip is then directed round the
30 pressing-on roller 32 and through the gap 33 which is
CA 0223~903 l998-04-27
19
formed between the pressing-on roller 32 and the first
laminator roller 24, as is shown likewise with dotted
lines in Figure 3. By means of the first die 21, the
plastic material with the higher melting point, e.g. PET
(melting point 280~C) is extruded on the steel strip M in
the region of the pressing-on roller 32. In the gap 33,
the still liquid plastic film 27a is pressed through the
laminator roller 24 onto the first side of the metal
strip. Similarly, as in the embodiments which are
o described and represented in Figures 1 and 2, the first
plastic film 27a is still held after the gap by the metal
strip in an arrangement with the laminator roller 24 and
cooled down. At the same time, the metal strip is also
cooled down. The spacing between the pressing-on roller
15 32 and the gap 26 is decided upon in such a way that the
metal strip in the gap 26 always has a temperature above
the melting point of the second plastic film, e.g. PP
(melting point 140 -160~C). By means of the second sheet
die 22, the second side of the metal strip M can be
20 laminated with the second liquid plastic film 31,
consisting of PP, in the manner already described.
In order to avoid contamination of the pressing-on roller
32, continuous Teflon strips 12 are also provided here in
25 the region of the pressing-on roller 32, said Teflon
strips 12 corresponding in design and function to those
shown in Figure 1. The description given for this purpose
in connection with Figure 1 can be applied logically also
to Figure 3. The same is also true with respect to the
CA 02235903 1998-04-27
2~
device for subsequent heating and the cooling device which
are not shown in Figure 3.
