Apparatus for stretching film

An apparatus for stretching polymeric film is disclosed wherein a heating unit is located slightly above and between the parallel tracks of a tenter frame and wherein the heating unit includes an air duct with an air entrance opening and a series of apertures directed between the tenter frame tracks wherein the air duct is separated, by internal intermediate walls, into adjacent isolated zones extending parallel with the tracks of the tenter frame and wherein each isolated zone includes a heater. Air introduced into the duct is conducted through the zones where it is heated and directed through the apertures onto a web of polymeric film moving in the tenter frame prior to stretching.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is a method of and apparatus for stretching polymeric film 
in which a web of film is heated in a tenter frame by an air duct/multiple 
heater combination positioned closely adjacent the web whereby improved 
preferential heating in a selected lane is brought about to obtain 
improved film properties. 
2. Description of the Prior Art 
Methods of selectively, and preferentially, heating polymeric film prior to 
stretching are known to the art. 
U.S. Pat. No. 3,161,711 to Tassler is exemplary of the known art wherein a 
series of radiant heaters is suitably mounted over the film immediately 
ahead of the stretching device, such as a tenter frame. Such heaters are 
disposed, preferably, for the full width of the film web and are spaced 
apart, parallel to each other in the direction of web travel. The heaters 
may be individually controlled preferentially to heat the web while it is 
substantially free from tension along at least one selected longitudinal 
lane. 
U.S. Pat. No. 3,649,726 to Knowles shows a method of controlling the 
thickness of the polymer film structures in which heaters of the type 
shown in U.S. Pat. No. 3,161,711 are used preferentially to heat a web of 
material prior to stretching. Briefly, such method includes measuring the 
thickness of the film at a location across the width before stretching; 
measuring the thickness of the film at any location across the width 
thereof after stretching; computing the cumulative mass values of the film 
structure across the width thereof before and after stretching; and 
adjusting the thickness of the film at any point across the width thereof 
before stretching responsive to deviation from a preselected value in the 
thickness of the film after stretching corresponding to the cumulative 
mass value of the film before and after stretching. Again, as with the 
Tassler patent preferential heating method, the effectiveness of the 
heating depends upon the effectiveness of the heater used. 
Other patents showing other than radiant means for heating a web of film 
prior to stretching include U.S. Pat. No. 2,339,451 to Bailey et al which 
shows heating during transverse direction stretching by a plurality of gas 
burners. The burners are positioned in a zigzag configuration to prevent 
overheating of individual lanes. 
Further, U.S. Pat. No. 2,412,187 to Wiley et al shows the controlling of 
heat input from internally steam heated radiators to separate portions of 
the web during machine direction, and prior to cross-machine direction 
stretching and U.S. Pat. No. 3,304,205 to Ballard et al shows heating 
alternate edges of the web in a tenter preheat zone to control the 
transverse direction stretching pattern. The heating means disclosed are 
radiant or air. 
A problem with the teachings of this art, and the other known art, is that 
it does not show a way to effectively control within very narrow limits 
the heating of a web in a tenter frame prior to stretching primarily 
because of the distance of the web from the radiant heater or by failure 
effectively to isolate the heated air that impinges on the web. 
There have been numerous other attempts to improve thickness properties in 
extruded films; apparatus and methods, for example, have evolved to sense 
thickness deviations and effect adjustment of film thickness at specific 
locations usually at or near the point of extrusion of the polymeric 
material in the film forming operation. 
For instance, U.S. Pat. No. 3,122,782 describes a mechanical arrangement 
for adjusting the thickness of selected hypothetical lanes across a film 
structure corresponding to thicker or thinner portions of the film. In 
such patent, two interconnected multiple-junction terminal boards are 
disposed intermediate a film thickness sensing guage and an adjustment 
means for changing the width of the extrusion orifice from whence the film 
structure issues. The film thickness gauge measures the thickness of the 
film structure and transmits a bias signal through suitable relay elements 
for energizing motors which are so constructed and arranged to cause 
opening or closing of the orifice opening in the extruder in accordance 
with the deviation in the thickness of the film that is sensed by the film 
thickness measuring gauge. 
In U.S. Pat. No. 3,455,175, an extrusion apparatus is disclosed with 
adjustable lip members containing a plurality of adjustment screws along 
the orifice slot. The screws are arranged closely together on about 20 mm 
(0.787 inch) spacing and are threaded into the lip members which have been 
modified near the extremities by locally weakened cross sections. Manual 
rotation of the small diameter screws results in displacement of the lip 
members and by that means the slot opening defining the thickness of the 
web is varied. Confining the deflection of a lip member to a narrow area 
approximating the width of a single adjustment screw is most difficult due 
to the inherent interaction of adjacent screws. Each time the setting of a 
screw is changed, there is a corresponding unavoidable disturbance of the 
setting of neighboring screws resulting in an unpredictable deflection of 
the lip members. To counteract the undesirable deflection, the settings of 
the neighboring screws must also be corrected. As a consequence, narrow 
band or streak non-uniformities cannot easily be eliminated without 
upsetting the basic gauge setting of the lips. As is apparent, adjustment 
means of this sort require constant personal attention and trial and error 
manipulation by skilled operating personnel. 
Conversely, the difficulties of reliance on skilled operators and trail and 
error control are to some extent obviated by an automatic gauge control 
system, such as is disclosed in U.S. Pat. No. 3,122,784, which features a 
plurality of speed controlled adjustor motors mechanically coupled to 
displaceable lip segments on a web extrusion apparatus. The motors respond 
to signals generated by a traversing thickness sensing means located at 
some distance below the extrusion apparatus which measures the final 
thickness of the web from standard reference deviations. Electrical 
signals are thus generated, translated into thickness corrections and 
distributed by way of switching means sequentially to the input of the 
motors. This system cannot, however, eliminate narrow streak and gauge 
band non-uniformities because the individual adjustment means control 
relatively wide segments of the orifice slot. Moreover, the nature of the 
nonuniformities is often too subtle and complex for corrective adjustment 
by mechanical displacement of die lip members. 
Another method of improving the thickness uniformity of film is shown in 
U.S. Pat. No. 3,819,775 wherein means are provided for selectively 
controlling (by heating or cooling) the temperature of isolated parts of 
die lips of an extrusion die whereby to improve thickness uniformity. 
There is no web heating immediately prior to stretching. 
And, lastly, U.S. Pat. No. 3,341,388 discloses a method of and apparatus 
for extruding thermoplastic film wherein heaters are positioned in the die 
body whereby to heat those areas of the die to help maintain the thickness 
of the sheet substantially constant. 
While these patents and others show polymeric film thickness-controlling or 
related devices, they do not show controlling such thickness of a web of 
film, in an improved fashion, by positively controlling the heating in a 
tenter frame of a selected web lane using air plus a heater positioned in 
a thermally isolated zone to assure that only a preselected area of the 
film is preferentially heated as in the instant invention. 
The accidental, or incidental, heating of lanes adjacent to the lane 
selected to be heated is a real problem here. This invention resolves that 
problem by substantially preventing the escape of heat into adjacent lanes 
using a duct/heater apparatus of the invention. By so controlling heating, 
it is possible to induce a substantial temperature rise in a given web 
lane while raising the temperature of adjoining lanes a minimal amount. In 
so doing, improved film properties result. 
SUMMARY OF THE INVENTION 
Briefly summarized, this invention is a method of stretching film including 
the steps of: 
preheating the film preferentially prior to stretching in a tenter frame by 
heated air by means of an air duct positioned between and closely adjacent 
the inner edges of the tenter frame clips carrying the film, the bottom 
wall of the duct having at least one aperture therein extending across the 
film and such aperture being closely spaced, preferably no more than 1 
inch, from the top surface of the film and thereafter stretching the film 
in the tenter frame. 
In greater detail, such a method includes the steps of: 
moving a web of unstretched film into operative relationship with a heating 
unit, such heating unit including a duct having a top wall, a bottom wall, 
side walls, and a plurality of intermediate walls between the side walls 
extending from top to bottom of such duct and throughout its length in the 
direction of film travel, such duct further having means defining at least 
an aperture in a wall of said duct, said aperture being closely adjacent a 
surface of the web of film; 
selectively heating air moving through the duct by heaters positioned 
between adjacent side and intermediate walls whereby preferentially to 
heat air passing through a selected zone defined thereby without changing 
the velocity of the air passing therethrough and through the duct and 
aperture; and 
thereafter stretching the heated film. 
In such method the edges of a web of the film is grasped by tenter frame 
clips mounted on an endless chain carried by a sprocket wheel; and 
such film is selectively heated while it is substantially free of tension 
and before it is stretched transversely in a tenter frame along at least 
one selected longitudinal lane by directing heated air toward a surface of 
the web, which air is heated by varying the temperature of an electric 
heater positioned above the web, between air guide walls, at such lane. 
An apparatus for performing this method of stretching a web of film 
includes: 
a tenter frame including a plurality of tenter frame clips carried by two 
endless chains and guided by tracks having parallel and diverging 
sections; 
a heating unit for preferentially heating a web of film moving therepast to 
a stretching temperature positioned between the tenter frame clips and 
closely adjacent a surface of the web of film, said heating device 
including: 
a duct having a top wall, a bottom wall and side walls, such duct further 
having a plurality of intermediate walls extending parallel to the side 
walls with adjacent side and intermediate walls each defining, with the 
top and bottom walls, a thermally isolated zone in the duct; 
said duct further having means defining a plurality of apertures extending 
across the bottom wall of the duct and normal to said side and 
intermediate walls; 
a web heater positioned between each of the adjacent walls defining a 
thermally isolated zone; and 
means for varying the temperature of each heater whereby air moving through 
a given thermally isolated zone may be heated thereby, the thus heated air 
passing through the apertures in the bottom wall whereby preferentially to 
heat the web. 
Preferably, the heaters extend from the top wall of the duct perpendicular 
to the upper surface of the web at the entrance of said duct. In another 
embodiment the heaters are electric and extend the length of the duct. 
The bottom wall of the duct is spaced less than 2 inches from the upper 
surface of the web of film and the side walls of the duct are positioned 
less than 1/4 of an inch from the inner surfaces of the tenter frame 
clips, in preferred embodiments. 
The heated air, as substantially thermally isolated in the zones of the 
duct, emerges from the apertures and impinges on the web of film and 
preferentially heats it, in a novel and useful manner, as will be 
explained. 
A key to this invention is the isolating of the heated air flow in the duct 
whereby it is brought to bear only on a selected area of the web. Such air 
flowing through the duct, as selectively heated, leaves the apertures at 
constant velocity. There is a tendency, however, and this is one of the 
problems the method and apparatus of this invention solves, for the air 
next adjacent to the side walls of the duct to be somewhat cooler than 
that in the center of the duct, because such walls serve as a heat sink. 
By positioning the air heating means or electric heaters between 
intermediate walls of the duct, it is possible independently to control 
them separately and thereby to increase the temperature of the heated air 
between those intermediate walls and, of course, to control the 
temperature of the air next to the side walls using the heater in the zone 
defined, in fact, by such side walls. This, in turn, means that the air 
passing out of the part of the aperture (in the selected zone) will be 
heated hotter than the air in adjacent zones and by isolating the heated 
air passing through a particular zone, then through the aperture so that 
it impinges on a particular area of the web, it is possible to bring about 
controls heretofore unknown to the art. The isolation, again, of this 
heated air is the key. Without the walls and without the selective heating 
means, the heat would be dissipated into adjacent zones and would produce 
diminished lane resolution and prohibitively long response times, for 
example. By thermally isolating the air into preselected zones, more 
effective response times and better controlled heat are available and are 
realized in shorter time. Thus, the art has available to it a means and 
method of controlling film properties heretofore not within its group.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention is a method of stretching polymeric film including steps and 
means of selectively heating lanes of that film prior to stretching in a 
tenter frame. 
In its broadest sense the invention involves, in a method of heating a web 
of polymeric film prior to transverse stretching, the improvement 
comprising: 
heating the film selectively in lanes by heated air directed toward a 
surface of such web, such air being heated by multiple heaters and the 
temperature of the air in a selected lane being controlled by the selected 
heater in that lane and such heater being located between air guide walls 
of a duct wherein such air is directed onto the web at a preselected 
location thereof. 
A better understanding of the specifics of such method, in greater detail, 
may be had by considering the steps of the method as hereinafter explained 
in detail in conjunction with the drawing. 
Referring to such drawing it will be seen that a method of stretching a web 
of film of this invention includes the steps of: 
grasping, in an apparatus generally designated 10 also of this invention, 
the edges of a web of polymeric film F in a tenter frame 11 by tenter 
frame clips 12 mounted on an endless chain 13 carried by a sprocket wheel 
14; 
selectively heating such film F while it is substantially free of tension 
and before it is stretched transversely in the tenter frame along at least 
one selected longitudinal lane by directing heated air toward a surface S 
of the web, which air is heated by varying the temperature of an electric 
heater positioned immediately above the web, between air guide walls, at 
such lane. 
The apparatus 10 for stretching the web of film F includes: 
such tenter frame 11 including the plurality of tenter frame clips carried 
by the two endless chains and guided by tracks, as shown, having parallel 
and diverging sections designated 15 and 16; 
a heating unit 20 for preferentially heating the web of film F moving 
therepast to a stretching temperature positioned between the tenter frame 
clips 12 and closely adjacent the surface S of the web of film, such 
heating device including: 
a duct 21 having a top wall 22, a bottom wall 23 and side walls 24, 25, and 
an entrance opening 26 and a closed end 27, such duct further having a 
plurality of intermediate walls 28 extending parallel to the side walls 
24, 25 with adjacent side and intermediate walls each defining with the 
top and bottom walls a thermally isolated zone, designated Z-1, Z-2, Z-3 
and Z-4, in the duct 21; 
such duct 21 further having means defining a plurality of apertures 29 
extending across the bottom wall 23 of the duct and normal to the side and 
intermediate walls 24, 25 and 28; 
an electric heater 30 positioned between each of the adjacent walls 
defining a thermally isolated zone; and 
means 31 for varying the temperature of each heater 30 whereby air moving 
through a given thermally isolated zone may be heated thereby, the thus 
heated air passing through the apertures 29 in the bottom wall 23 whereby 
preferentially to heat the web of film F. 
Preferably the electric heaters 30 extend from the top wall 22 of the duct 
perpendicular to the upper surface of the web at the entrance opening 26 
of the duct. In another preferred embodiment, the electric heaters 30 are 
tubular and extend the length of the duct, as best seen in FIGS. 2 and 3. 
In an important aspect of this invention, the bottom wall 24 of the duct is 
spaced less than 2 inches from the upper surface S of the web of film F 
and in a preferred embodiment, the bottom wall 23 of such duct is 
positioned less than 1/2 of an inch from the upper surface of the web. 
Further, the side walls 24 and 25 of the duct are positioned less than 1/4 
of an inch from the inner surfaces 12' of the tenter frame clips 12, in a 
preferred embodiment. 
This positioning of the duct with respect to the surface of the web being 
heated and with respect to the inner surfaces of the tenter frame clips -- 
in other words, the duct is positioned between the tenter frame clips and 
closely adjacent the upper surface of the web -- is a key factor in 
bringing about improved film properties, in accordance with this 
invention, as now to be explained in greater detail. 
It is well known, of course, to use a tenter frame in order to stretch webs 
of plastic films, fabrics, foils, etc., in the direction of their width. A 
typical tenter frame generally includes a plurality of tenter frame clips 
mounted on two endless chains and guided by a pair of diverging tracks. 
The clips are adapted to grip the edges of a web of material, such as 
polyethylene terephthalate film, as it emerges from a machine-direction 
stretching device. Each track is composed of two rails, an inner guide 
rail and an outer guide rail. The chains are adapted to be moved about two 
or more motor driven members (usually sprockets) while being disposed in 
guiding contact with the rails of the diverging tracks during their 
movement. In this way, the clips on the chains serve to move the web 
longitudinally, while simultaneously stretching the web transversely. 
Before the film can be stretched, however, it must be at or above a certain 
temperature known as the stretching temperature, which varies with the 
composition, structure, type and the stretching history of the film (i.e., 
whether it has previously been stretched). For this heating or preheating 
of the film, the tenter frame is usually provided with heaters, such as 
radiant or hot-air. During preheating, the film is maintained under low 
transverse tension to prevent tearing before it reaches the thermoplastic 
state, so for this step the rails are substantially parallel. 
Often it is desirable or necessary to heat the film to a greater or lesser 
extent in different longitudinal lanes of the film, especially to apply 
greater heat to the film close to the tenter clips, which may act as heat 
sinks, resulting in a difference in temperature between the adjacent part 
of the film and the major portion of the film, causing thickness and 
property differences. 
Among the causes of the inability to selectively control radiant heat are 
poor absorption characteristics of film, caused by low absorption 
coefficients (such as in polyolefins), high reflectively, such as in film 
filled with white or reflective particles, and the high absorption by 
metal tenter clips causing them to become too hot. With air heating, in 
which air is heated at a remote location, with heat adjustments by 
changing the flow of air through separate ducts by dampers, precise 
control is difficult because variations in flow cause pressure 
differences, resulting in spill-over from lane-to-lane. 
The method and apparatus of this invention provides for selectively 
adjusting the heat input to individual lanes of a film as it enters a 
tenter frame before transverse stretching. The primary means to heat is to 
impinge heated air on the film. This invention further provides for 
separation of the heated air into a plurality of zones parallel to the 
direction of motion of the film, and auxiliary adjustable electric heaters 
in each zone so as to enable changes in the temperature of the air and 
thus vary the temperature of the web in parallel longitudinal lanes so 
that gauge variation may be minimized. 
As previously stated, it is known in the art to preheat a web prior to 
stretching in a tenter frame by means of heated air passing through 
apertures in a duct positioned over the tenter frame clips. This, again, 
as previously stated, differs from the method and apparatus of the instant 
invention in that the aperture in the bottom wall of the duct is a 
considerably greater distance (more than 2 inches, and around 8 to 12 
inches) above the surface of the web that is heated. This creates 
additional problems, or, at least, lessens the ability to control the 
heating of the web, within closely defined limits. 
In these known devices, using a duct for moving heated air onto the upper 
surface of the web as it is moved between the parallel tracks of the 
tenter frame, most problems occur because of the distance between the 
bottom of the duct and the upper surface of the web, but also because the 
tenter frame clips tend to adsorb considerable heat adjacent the side 
walls of the duct and inadequate or undesirable heating often occur, 
particularly adjacent the edges of the film. This has resulted in film 
losses, primarily of these edges. 
A first key feature of this invention is that for the first time, the air 
heating duct is positioned between the tenter clips and closely adjacent 
(no more than 2 inches) from the surface of the film being carried by the 
tenter clips. This enables air from apertures in the bottom wall of the 
duct more effectively to heat a web moving under the duct and this brings 
about dramatic improvements in film properties. 
Problems still existed, however, in that the air adjacent to and due to 
their proximity to, the side walls of the duct, tended to be somewhat 
cooler and, hence, uneven heating often occurred at the edges of the web. 
And other problems, this positioning of the duct between the tenter clips 
left unsolved, again well known to the art, is that in many instances it 
is highly desirable to provide selective heating of some lanes whereby to 
improve thickness properties in the stretched web. For example, it is 
known to change the properties of the stretched web by varying the heat 
along a lane of that web in keeping with the thickness variations. The 
positioning of the duct between the tenter frame clips left this problem 
unsolved. 
The second key step, then, of this invention is the provision of means, in 
the form of the heaters positioned in the isolated zones in the duct, and 
between intermediate walls of the duct, whereby to provide a selective web 
heating capability while keeping the velocity of flow onto the web and 
close to the web, the same. With these principles in mind, the invention 
will now be described in further detail. 
The heating unit 20 includes a plurality of separately adjustable, 
elongated tubular heaters 30, positioned in the zones of the duct close to 
the web of film at its upper surface. Each heater 30 or 30' is placed in 
the zones in close, suitably spaced apart, relationship across the width 
of the web of film, and, importantly, in the zones Z-1 and Z-4 adjacent 
the tenter frame clips so that the edges of the web held by these clips 
may be preferentially heated during the preheating step. A suitable heater 
of the elongated tubular type is shown in U.S. Pat. No. 3,161,711 to 
Tassler, which patent is incorporated herein by reference. 
The heaters 30 or 30' (such as Calrod heaters) are normally operated at 50% 
of the rated capacity with adjustments to raise or lower the temperature 
made by raising or lowering the power above or below the 50% level. 
In the preferred apparatus the duct is made up of a group of identical 
parallel longitudinal modules so that by combination of an appropriate 
number of the modules the width of the heating unit can be adjusted to 
heat the film as close to the tenter clips as possible. Each of the 
modules has an individual internal heater, and the entire group is coupled 
to a control panel having power (voltage) adjustment means (Variac 
adjustable autotransformers), ammeters and thermocouple indicators for 
respective modules. These heaters 30 or 30' may extend parallel to the 
direction of web travel thoughout the length of the duct but, preferably, 
are in the form of tubular heaters positioned perpendicular to the upper 
surface of the web and extending from the top wall of the duct to a 
position closely spaced from the bottom wall in a given duct zone. (See 
FIG. 4). 
Means 31 are provided for adjusting power to these heaters whereby their 
temperature may be varied in accordance to desire. By selectively 
increasing the power to an electric heater in a given duct zone, the 
temperature of the air flowing in the duct past that heater in that zone 
is heated hotter than the air in the zone next to it. The intermediate 
walls 28 isolate this hotter air from the air flowing in adjacent zones 
and such selectively heated air passes through the apertures 29 and onto 
the upper surface of the web positioned closely adjacent to the duct, 
preferentially to heat the web in that location. While there tends to be a 
slight dissipation into adjacent zones it is minimal and, thus there is 
provided a proven and effective way to control, in a tenter frame, 
preferential heating of a web passing therethrough. 
The film after being heated is moved to a stretching means either to a 
machine direction stretching means, such as spaced apart nip rolls or, as 
shown, to a tenter frame, as is known in the art, where the film is 
stretched. The film is then generally heat-set in known manners to various 
heatsetting temperatures and thereafter is wound on to a windup roll and 
stored for use. 
In practicing this invention, typical heating temperatures prior to 
stretching are shown in U.S. Pat. No. 2,823,421; typical tenter frame 
apparatus are shown in U.S. Pat. Nos. 3,118,212 and 3,727,273; typical 
stretching techniques are shown in U.S. Pat. Nos. 2,823,421 and 2,995,779, 
which patents are incorporated herein by reference. 
By following the method and by using the apparatus as described, 
preferential, carefully controlled heating of a selected lane or group of 
lanes of a polymeric film immediately prior to subjecting it to stretching 
forces is made possible. In the short period of time between heating and 
stretching the thicker portions cool more slowly than the thinner portions 
of the film, thus sharpening the temperature differential between them and 
allowing the thicker portions to stretch more upon application of force 
and tending to improve the thickness properties of the film. 
In a typical method of making a film such as polyethylene terephthalate, 
the molten polymer is extruded from a die onto a cooled drum where it is 
quenched, leaving the film in a substantially amorphous condition. This 
film then passes to a first stretching means. This stretching can be in 
the longitudinal direction by stretching the film between two sets of 
rolls by operating one set at a higher speed than the other or the 
stretching can be in the transverse direction by means of a tenter frame 
wherein the film is stretched by the use of moving, diverging tenter frame 
clips. Generally, it is typical to stretch the film in both directions 
with one of the above stretching operations following the other, the 
stretching operation which precedes being a matter of preference in the 
particular case. 
By monitoring the web thickness, it is possible to actuate the appropriate 
lane heater(s) which heats the lane or group of lanes of the amorphous 
film web after it leaves the quench drum and before it passes into the 
transverse direction tenter stretcher. 
To accomplish the specific lane heating, this invention employs the zone 
heaters, as described, comprising a plurality of tubular element 
electrical heaters disposed parallel to each other in the longitudinal 
direction. Such heaters are positioned over the film immediately ahead of 
the slow nip rolls or immediately ahead of the film's entry to the tenter 
frame stretcher, according to the stretching sequence to be followed. The 
distance between the heaters and slow nip rolls or tenter frame is such 
that the unintentionally heated thin portions of the film will become cool 
prior to stretching. There are many variables that determine this 
distance, e.g., the rate of film travel, the thickness deviation of the 
film and the temperature of the heaters, and can best be determined from 
the conditions of a particular run. 
Preferably, the heaters are disposed for the full width of the film web and 
are spaced with a center-to-center distance of one inch. 
The temperature to which the film is heated is regulated by power attached 
to each heater. This voltage is set in advance according to the thickness 
deviation of the film and the rate of film travel. The rate of stretching 
of thicker lanes is effectively altered if the temperature difference 
between the thicker and thinner portions of the film is as little as 
0.1.degree. C. Of course, the heater for the thickest lane will be turned 
to a higher voltage than the remaining heaters. The voltage of the other 
heaters are adjusted according to the extent to which the corresponding 
lane was thicker than the desired film. 
This invention is effective in use immediately before stretching either in 
the longitudinal or transverse directions. Any crystalline or 
crystallizable thermoplastic polymer can be used in the manufacture of 
films according to the process of this invention. Such polymers would 
include: polyolefins and blends thereof; polyethylene-2,6-naphthalate; 
polytetramethylene-1,2-dioxybenzoate; polyethylene-1,5-naphthalate; 
polyhexamethylene adipamide; polyhexamethylene sebacamide; 
polycaprolactam; and copolymers of ethylene terephthalate and ethylene 
isophthalate. Films of substantially amorphous non-crystallizable polymers 
such as polystyrene can also be employed. A preferred method relates to 
the treatment of substantially amorphous polyethylene terephthalate film. 
This invention is particularly suited for the production of filled, 
stretched polyethylene synthetic paper by a flat-sheet method. It is 
adaptable to the transverse stretching of other thermoplastic films, such 
as those of polyethylene terephthalate. The essence of the invention is 
not that it changes the overall heat input to the film, but that it 
provides a means to selectively adjust the heat input to individual 
longitudinal lanes of the film. 
This use of this modular duct heating unit in the preheat area of the 
tenter frame positioned between the tenter rails and very close to the 
upper surface of the film assures lane temperature control and even 
stretching over the entire sheet right up to the tenter clips. This 
invention is peculiarly adapted to preheat filled films. 
In brief again, this invention is a method of stretching film including the 
steps of: 
preheating the film preferentially prior to stretching in a tenter frame by 
heated air by means of an air duct positioned between and closely adjacent 
the inner edges of the tenter frame clips carrying the film, the bottom 
wall of the duct having at least one aperture therein extending across the 
film and 
thereafter stretching the film in the tenter frame. 
In the duct, intermediate walls define duct heating zones and a heater is 
positioned in each zone, and such method further including the steps of: 
supplying power to a selected heater whereby further to heat the air moving 
therepast in this selected zone, such heated air being substantially 
isolated from the air in the other zones and being moved through the duct 
and out the aperture in the bottom wall whereby preferentially to heat the 
film moving under the aperture in the selected zone. The heated air 
emerges from the apertures and impinges on the web of film and 
preferentially heats it.