Pneumatic gauge adjustment of edge-pinned cast web

A process for promoting uniform thickness in cast webs of thermoplastic material comprising use of variable air forces selectively directed toward a cast web with pinned edges along the line of initial proximity with a quenching surface. Especially useful to mitigate thinning adjacent to the pinned edges.

TECHNICAL FIELD 
This invention relates to gauge adjustment of cast webs of thermoplastic 
material as the webs are cooled by proximity to a moving quenching surface 
and the web edges are pinned against the quenching surface by 
electrostatic or other forces. The process of the invention utilizes 
variable air forces directed toward the cast web before application of the 
edge pinning, along the line of initial proximity between the cast web and 
the quenching surface, and exerted at predetermined locations and 
velocities adequate to alter local web shrinkage, during the quench, to 
result in a more uniform quenched web thickness. The invention is 
particularly suited for gauge adjustment of web edges adjacent to the 
pinning. 
PRIOR ART 
U.S. Pat. No. 3,597,515 issued Aug. 3, 1971 on the application of Widiger 
discloses the use of an oscillating air knife to direct a nonuniform air 
stream against a cast, molten, web of polymeric material applied to a 
quenching drum. The web of the patent is not pinned at the edges and there 
are, therefore, no substantial transverse tension forces generated during 
the quenching. The process of that patent is directed toward causing 
thickened areas of the unpinned web to migrate from edge to edge across 
the web and create random gauge nonuniformities in a wound roll, rather 
than permitting such nonuniformities to remain at the same place, thus 
causing a thickened longitudinal lane which would result in wound rolls of 
poor quality. 
U.S. Pat. No. 3,904,725, issued Sept. 9, 1975 on the application of Huskey 
et al. discloses a method for quenching a molten web of polymeric material 
by means of directing air forces diagonally from an edge of the web across 
the web and in the direction of web travel after the web has been laid 
over a quenching drum and its edges have been pinned by electrostatic 
forces. Gas forces are provided by means of individual, closely-spaced, 
air jets with a possibility for individual adjustment. Because the air 
forces are applied after the web has met the quenching drum and been 
pinned, excessive air can be locally entrained and sealed between the web 
and the quenching drum. Such locally entrained air acts as an insulator 
between the web and the quenching drum preventing efficient heat transfer 
and resulting in some gauge nonuniformities. 
Canadian Pat. No. 848,852, issued Aug. 11, 1970 on the application of Roth 
et al., discloses an arrangement for pinning and quenching a molten web of 
viscous polymeric material by means of electrostatic forces applied across 
the web in a line where the web comes into actual contact with a quenching 
drum followed by application of gas forces uniformly across the web and 
around the quenching drum extending from the line of web contact to a 
point of web submersion in a cooling liquid. There is no provision for 
adjustment of the air forces transversely across the web. 
SUMMARY OF THE INVENTION 
Thermoplastic material, molten and cast as a web onto a moving quenching 
surface, has long been pinned to the quenching surface at the lateral 
edges of the web. Such edge pinning eliminates loss of web width due to 
shrinkage during the quench and prevents the web edges from wandering on 
the quenching surface. Because the web edges are pinned during quenching, 
thus preventing any change in web width, transverse forces are generated 
across the web and the web is stretched to the extent that it shrinks as 
it cools. Such stretching occurs at the weakest points on the web 
including: hot edges directly adjacent to the pinning; web areas of 
highest temperature; and areas wherein excessive air has been entrained 
and sealed between the web and the quenching surface thereby reducing the 
heat transfer between the web and the quenching surface. This invention 
relates to a process for controlling the temperature of a web by 
controlling its proximity to the quenching surface which, in turn, 
controls the areas of the web which will be stretched during the quench 
and, thus, accomplishes gauge adjustment. This invention relates to a 
process for affecting a variable force transversely across a molten web to 
control the local proximity of the web to a quenching surface. 
It has been well established that, in processes wherein a molten web is 
cooled by proximity to a moving quenching surface, the web does not come 
into actual contact with the quenching surface. In the absence of some 
intimate pinning force, such as electrostatic pinning force, there is a 
thin layer of air entrained between the web and the quenching surface. For 
the purpose of the description of this invention, the word "contact", as 
it refers to a relationship between a web and a moving quenching surface, 
means that the web is in proximity with the quenching surface and is 
separated therefrom by only that thin layer of air. To the extent that the 
entrained layer of air has a uniform thickness transversely across the 
web, thermal conductivity between the quenching surface and the web will 
be substantially uniform. Any air entrained between the web and the 
quenching surface causing the layer to have a nonuniform thickness, causes 
nonuniform heat transfer. 
For purposes of the description of this invention, the term "pinning" 
refers to holding a web in actual contact with a quenching surface with 
forces adequate to prevent lateral movement of the edges of the web 
despite transverse tension forces generated across the web due to 
shrinkage when the web is cooled. 
According to this invention, there is provided a process for quenching and 
adjusting the gauge of a molten thermoplastic web which includes extruding 
the web onto a quenching surface, pinning each lateral edge of the web to 
the quenching surface, and directing air against the web along the line of 
initial contact between the web and the quenching surface. At the lateral 
edges, the air is directed against the web before application of the 
pinning forces. The air is directed through a plurality of individually 
adjustable jets and, in one embodiment, the jets are adjusted, at the 
pinned edges, to have air forces generally decreasing inwardly from the 
pinning. In central portions of the web, the jets are adjusted to have air 
forces which are greatest against the relatively thin areas of the web.

In FIG. 1, molten thermoplastic web 11 is extruded from extrusion die 12 to 
quenching surface 13 moving in the direction of arrow 14. Air jet pack 15 
is mounted above quenching surface 13 (the mounting is conventional and 
not shown). Air is introduced into air jet pack 15 through individual 
ports 16.sup.1, 16.sup.2, 16.sup.3, . . . , 16.sup.n from separate air 
sources capable of individual air pressure adjustment. Air is directed 
from air jet pack 15 against web 11 along the line 17 of initial contact 
between web 11 and quenching surface 13. After web 11 has been subjected 
to the air forces of air jet pack 15 and before the web has built 
substantial shrinkage forces, lateral edges of the web are pinned by 
electrostatic probes 18. For purposes of this description, ports 16.sup.1 
and 16.sup.2 and 16.sup.n and 16.sup.n-1, representing lateral edge widths 
of about 2 centimeters, are located directly over the portion of web 11 
which would be pinned by probes 18. 
In operation of the process of this invention, the air jet pack provides 
air forces against the molten web to accomplish web gauge adjustment in at 
least two ways. First, in the vicinity of the electrostatically pinned 
edges, the air ports are located to direct air against the web before 
application of the pinning forces and they are adjusted such that air 
forces against the web are greatest where the web will be pinned and are 
generally decreased inwardly from that site. Such adjustment of air forces 
provides that molten thermoplastic material adjacent to the electrostatic 
pinning is forced closer to the quenching surface in a way which will 
cause more rapid web cooling nearest to the area where the edges will be 
pinned. More rapid web cooling nearest to the area where the edges will be 
pinned mitigates excessive stretching and consequent thinning at the edges 
which would be caused by shrinkage in cooling. Air forces applied in the 
vicinity of where the edges will be pinned generally extend inward to 
about the points in the web adjacent the pinned edges whereat minimum web 
thicknesses would occur in the absence of the air forces--a distance of, 
generally, 2 to 15 centimeters and usually of about 5 centimeters. 
Second, in the vicinity of the center of the web between the lateral edges 
in cases wherein the web may have been cast with local relatively thick or 
thin areas, the air ports are adjusted to provide air forces which are 
greatest in areas of the web which are relatively thin. That adjustment 
creates nonuniformity in the entrained air layer, causes more rapid 
cooling of relatively thin web areas and leaves relatively thick web areas 
at a higher temperature and more likely to be stretched during the 
shrinkage in cooling. The vicinity of the center of the web is generally 
taken to be the remainder of the web inward from the vicinity of where the 
edges will be pinned--a distance of, generally, 2 to 15 centimeters, and 
usually about 5 centimeters. 
When the edges of the web are pinned to the quenched surface, some air from 
beneath the pinned edges is displaced to a position adjacent the pinned 
edges. This results in a relatively thicker air layer adjacent the pinning 
and, by the mechanism described above, results in excessive thinning. The 
air jet pack can be located such that one or more of the air jets are 
positioned over that portion of the web which will be pinned to the 
quenching surface. When air forces are directed against the web areas to 
be pinned, some of the air between the web and the quenching surface is 
eliminated and, as a result, when the web edge is pinned, less air is 
displaced under the web adjacent the pinning. In this way, the air layer 
is more uniform and heat transfer is more uniform. 
The width of web which is affected by application of electrostatic pinning 
forces is usually about 2 to 3 centimeters. Air jets directed toward that 
affected width of pinned web are said to be directly over the pinning. The 
outermost air jets adjusted to provide air forces furthest from the 
central portion of the web are said to be directed at the lateral edges. 
With reference to FIG. 1, the gauge adjustment air forces are usually 
greatest for 16.sup.2 and 16.sup.n-1 and usually gradually decrease 
inwardly from each edge through 16.sup.3, 16.sup.4, and so forth and 
16.sup.n-2, 16.sup.n-3, and so forth, to a position immediately above the 
area of the web which would exhibit a minimum thickness in the absence of 
the air forces. Air forces from 16.sup.1 and 16.sup.n are often adjusted 
to be less than forces inward and adjacent to the pinning. Adjustment of 
other air forces is made such that relatively greater forces are directed 
toward relatively thinner web areas. The relatively thinner web areas 
occur as a matter of normal thermoplastic web manufacture, often as a 
result of some local deviation in the opening of the extrusion die. 
Thickness variations in web manufacture can be detected by any of several 
well-known methods, such as by noncontacting radiation gauges or 
mechanical thickness measuring devices. 
The web thickness can be continuously monitored by noncontacting means and 
indications of thickness deviations can be used to automatically control 
and adjust air forces directed toward the web to cause correction in the 
deviations. Of course, the deviations can also be determined manually and 
the air forces can be manually adjusted accordingly. 
In FIG. 2 there is shown a four-part air jet pack 20 in partial cut-away to 
illustrate the interior of the device. Air is introduced into the jet pack 
through ports 21 from individually adjustable air pressure sources which 
are conventional and, therefore, not shown. The air passes through ports 
21 into individual chambers 22 and is then directed through slit openings 
23 in the side of the pack which, in operation, faces the thermoplastic 
web. Chambers 22 are individually separated by walls 24 and, at the slit 
end, walls 24 have beveled edges 25 to afford a continuum of air forces 
between neighboring slit openings 23. Of course, air jet pack 20 can have 
as few or as many individually adjustable air jets as are required or 
desired for a particular use. As few as one jet at each lateral edge 
provides some of the benefit of the present invention but at least four at 
each edge are preferred, and six to eight are usually used. The air jet 
packs can extend across the complete width of a web; and, for adjustment 
of gauge, it has been found that the slits should not be more than about 5 
centimeters long and usually not more than 2 centimeters long. The slits 
can be as short as desired but, because air is supplied to each slit from 
an individually controlled air supply, the shortness of the slits becomes 
a matter of economics and convenience of operation. For that reason, slits 
are generally not less than about 0.5 centimeters long. 
The air jet pack should be located at a distance from the web to avoid 
contact with the web during operation and yet to maintain coherent and 
individual air forces from each port against the web. The jet pack is 
usually located about 3-6 millimeters from the cast web. 
In FIG. 3 there is shown, in cross section, a preferred device for practice 
of the present invention. The device 30 comprises a combination of 
particular web manufacturing elements as previously disclosed in U.S. Pat. 
No. 4,017,575 issued Apr. 12, 1977 on the application of Heyer, the same 
inventor as herein. Air bearing 31 is used with pressure chamber 32 and 
the air forces of the present invention are applied therebetween and 
before application of pinning forces. In operation, web 33 is cast from 
extrusion die 34 and is then forced out of a catenary path by air bearing 
31 and, thereafter, pressed into proximity with quenching surface 35 
moving as indicated. 
The molten web is held taut by and stretched around air bearing 31 but does 
not come into close proximity with quenching surface 35 across the web 
until it reaches point 36. At point 36, air is directed toward the web 
from air jet pack 37 as has been disclosed. Subsequent to the air forces 
of air jet pack 37, the pressure chamber 32 provides a constant air 
pressure against the web on the quenching surface from air bearing 31 to 
point 38 and across the entire web. To assure that lateral edges of web 33 
are fixed to quenching surface 35, electrostatic pinning forces are 
applied thereto immediately after the air jet pack 37. 
In the practice of this invention, it is necessary that the air forces of 
the air jet pack must be directed toward the web at or near to the point 
where the web makes initial contact with the quenching surface to avoid 
entrainment of excess air and to provide an air layer of desired thickness 
between the web and the quenching surface. 
In FIG. 4 there is shown a graphical comparison of cast web thickness 
profiles with and without the process of this invention. The abscissa 
represents distance, in centimeters, from the lateral edges of a cast film 
web and the ordinate represents thickness of the cast web. Line 41 
represents the thickness profile of a quenched web cast using the device 
of FIG. 3 without air pressure supplied to the air jet pack. To make the 
webs characterized by the lines of this FIG. 4, polyethylene terephthalate 
was cast at a temperature of about 285.degree. C. onto a quenching surface 
moving at about 35 meters per minute and having a surface temperature of 
about 25.degree. C. The pressure to the air bearing was about 30 kPa and 
there was a uniform plenum pressure of about 25 Pa. The nominal thickness 
of the web was 178 micrometers. The web was subjected to electrostatic 
pinning at about 10 millimeters inward from each lateral edge (indicated 
at 42) and it is noted that, from the edges to about 2-3 centimeters 
inward, the web thickness is considerably greater than the nominal 
thickness. The thickened edges 43 are intended and are necessary to 
provide material for gripping by web handling devices subsequently in film 
manufacturing procedures. In line 41 the extreme thinness 44 which occurs 
immediately inward from the thickened edges and which is followed by 
another thickened area 45 are characteristic of webs made without use of 
the present invention, and are especially noted. 
Line 46 represents the thickness profile of a quenched web cast using the 
same device as above under the same conditions with the exception that, 
near each lateral edge air jet packs were positioned such that the first 
of eight slits 9.5 millimeters long was at the edge of the web. The slits 
were directly adjacent one another and were 0.75 millimeters wide. Air 
pressures at the slits in each air jet pack were adjusted such that the 
velocity of air from the individual jets was as is indicated in FIG. 4 and 
the slits were positioned to be 3.8 millimeters from the cast web. Air 
velocities from jets over the sites of electrostatic pinning (jets L1, L2, 
L3, R1, R2, and R3) were adjusted to be generally greater than jets inward 
from the pinning. Exact adjustment of the jets cannot be predetermined for 
any given situation. The web thickness profile is inspected and the jets 
are adjusted in accordance with the teaching herein. 
Line 46 evidences a cast web of regularly thinning profile without 
thickness reversals. It is noted that line 46 does not indicate either an 
area of extreme thinness or a thickened area inward therefrom as were 
represented by 44 and 45, respectively, in line 41. 
A cast web of the improved thickness profile of line 46 is the result of 
this invention and the result is even more pronounced in FIG. 5 where is 
shown a graphical comparison of the cast webs of FIG. 4 after being 
subjected to biaxial orientation by being stretched 3.4.times. in the 
machine direction and 4.3.times. in the transverse direction to yield a 
film with a nominal thickness of 12 micrometers. In FIG. 5, line 51 
represents the oriented film product of cast web 41 in FIG. 4 and line 52 
represents the oriented film product of cast web 46 in FIG. 4. It is noted 
that the thickness of the film of line 51 deviates substantially from the 
nominal 12 micrometers thickness for a distance 10-13 centimeters inward 
from one edge and a distance of 15-18 centimeters inward from the other 
edge for a total of about 25-31 centimeters of scrap. Such deviations 
represent considerable waste in film which must be scrapped for being 
outside of acceptable thickness limits. On the other hand, the thickness 
of the film of line 52 directly approaches the nominal thickness and stays 
there with a total of less than 13 centimeters of scrap. 
In FIG. 6 there is shown a graphical comparison of thickness profiles of 
another cast web with and without the process of this invention. Line 61 
represents the thickness profile of the quenched web cast using the device 
of FIG. 3 without air pressure supplied to the air jet pack. To make the 
webs characterized by the lines of this FIG. 6, polyethylene terephthalate 
was cast at a temperature of about 285.degree. C. onto a quenching surface 
moving at about 85 meters per minute and having a surface temperature of 
about 16.degree. C. The pressure to the air bearing was about 69 kPa and 
there was a uniform plenum pressure of about 22 pa. The nominal thickness 
of the cast web was 64 micrometers. The web was subjected to electrostatic 
pinning at about 10 millimeters inward from each lateral edge (indicated 
at 62) and it is noted that, from the edges to about 4-5 centimeters 
inward, the web thickness is considerably greater than the nominal 
thickness. 
In line 61 the thickness minima 63 and the inward maxima 64 are noted as 
gauge deviations to be adjusted by the process of this invention. 
Line 65 represents the thickness profile of a quenched web cast using the 
same device as above under the same conditions with the exception that, 
near each lateral edge, air jet packs were positioned such that the first 
of eight slits 6.5 millimeters long was at the edge of the web. The slits 
were directly adjacent one another and were 0.75 millimeters wide. Air 
pressures at the slits in each air jet pack were adjusted such that air 
from the individual jets was as is indicated in FIG. 6 and the slits were 
positioned to be 4.3 millimeters from the cast web. Air velocities from 
jets over the sites of pinning were adjusted to be greater than jets 
inward from the pinning and air velocities from the jets at the lateral 
edges, L1 and R1, were adjusted to be less than air velocities from jets 
directly over the pinning, L2 and R2. Air velocities from jets adjacent to 
the pinning were adjusted to decrease inwardly from the pinning. 
Line 65 evidences a cast web of regularly thinning profile without the 
maxima and minima of line 61. 
The optimum pressures to the air jet packs useful for practicing this 
invention vary greatly with changes in kind and character of the 
thermoplastic material and changes in conditions of casting and 
stretching. The above teaching is of the best mode presently contemplated 
for practicing the invention and for changed materials or conditions, air 
pressures and other process conditions can be altered to provide gauge 
adjustment by simple inspection of web thickness profiles followed by 
adjustment of air pressure in accordance with the teaching disclosed 
herein. 
While this invention can be used in the manufacture of any thermoplastic 
cast web in accordance with the teaching herein, the invention is best 
suited for use in manufacture of cast webs of: polyolefins such as 
polyethylene and polypropylene; polyamides such as polyhexamethylene 
adipamide and polycaproamide; vinylidene chloride; and polyesters such as 
polyethylene-2,6-naphthalate and polytetramethylene-1,2-dioxybenzoate and 
is especially useful with polyethylene terephthalate.