Apparatus for uniting concentrically extruded tubular films of thermoplastic synthetic resin

Apparatus for uniting at least two concentrically extruded tubular films of thermoplastic synthetic resin includes a mandrel located in the innermost of the films and having a leading conical portion and a trailing cylindrical portion to expand the innermost film in transverse direction substantially to the diameter of the outermost film. The mandrel is heated by electromagnetic waves produced by a high-frequency transmitter connected to a high-frequency coil surrounding the tubular films in the region of the mandrel. A temperature sensor connected to the mandrel senses the temperature of the latter and the sensed temperature is transmitted by an ultra shortwave transmitter connected to the sensor and ultra shortwave receiver connected to the high-frequency transmitter to control the latter and to maintain the temperature of the mandrel at a desired constant temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for uniting at least two 
concentrically extruded tubular films of thermoplastic synthetic resin 
wherein at least one of the films is heated to a temperature slightly 
below the melting point of the resin from which it is formed. 
In order to obtain an end product of high tear strength, the extruded 
tubular films are stretched in such an apparatus in different directions. 
The outermost tubular film is stretched longitudinally without changing 
its diameter and each of the inner extruded tubular films, which have a 
smaller diameter than the outermost film, are stretched in a direction 
transverse to their longitudinal direction by a heated mandrel located 
within the innermost film and having a maximum diameter substantially 
equal to the diameter of the outermost film. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus for 
uniting at least two concentrically extruded tubular films of 
thermoplastic synthetic resin by heating at least one of the films to a 
temperature slightly below its melting point, while simultaneously 
stretching the inner film in a direction transverse to their longitudinal 
direction to a diameter substantially equal to the diameter of the 
outermost film. 
It is a further object of the present invention to provide an apparatus of 
the above-mentioned type in which the stretching of the inner films in 
transverse direction and heating of at least one of the films is carried 
out by a heated mandrel located within the innermost film. 
It is an additional object of the present invention to heat the mandrel 
located in the innermost film by means of a high-frequency transmitter 
connected to high-frequency coil located outside of the films in the 
region of the mandrel. 
It is also an object of the present invention to provide means for 
maintaining the heated mandrel at a desired predetermined temperature. 
With these and other objects in view, which will become apparent as the 
description proceeds, the apparatus according to the present invention for 
uniting at least two concentrically extruded tubular films of 
thermoplastic resin to each other mainly comprises mandrel means located 
within the innermost of the concentrically extruded films and constructed 
to expand the latter in a direction transverse to its longitudinal 
direction so that the peripheral surfaces of the concentrically extruded 
films will engage each other. The apparatus includes further heating means 
for heating the mandrel means and comprising a high-frequency coil 
surrounding the outermost of the concentrically extruded films in the 
region of said mandrel means and a high-frequency transmitter connected to 
this coil. 
The apparatus preferably includes further control means connected to the 
high-frequency transmitter for switching the latter on and off to thereby 
maintain the temperature of the mandrel means at a predetermined 
temperature. 
Such control means preferably comprise temperature sensing means within the 
mandrel means, an ultra shortwave transmitter connected to the temperature 
sensing means, and an ultra shortwave receiver connected to the 
high-frequency transmitter. 
The mandrel means has preferably a leading conical portion and a trailing 
cylindrical portion. The leading conical portion is preferably hollow and 
the heating means may include a container for a liquid downstream of the 
cylindrical portion of the mandrel means and passage means extending 
through the cylindrical portion and providing communication between the 
container and the hollow conical portion so that steam developing from the 
liquid by heating the same by the above-mentioned heating means can pass 
through the passage means into the hollow conical portion to heat the 
latter to the desired temperature which may be controlled by the 
temperature sensing means extending into the liquid in the container and 
by transmitting the sensed temperature by an ultra shortwave transmitter 
connected to the temperature sensing means to an ultra shortwave receiver 
connected to the high-frequency transmitter to switch the latter on and 
off in dependency on the sensed temperature to maintain the mandrel means 
at a desired predetermined temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically shows a plurality of tubular concentrically extruded 
films extruded from an apparatus well known in the art, not forming part 
of the present invention and not illustrated in the drawing. These tubular 
films of synthetic thermoplastic resin include an outermost film 1 having 
a diameter L and an innermost film 2 having a smaller diameter 1. 
Laminated to the inner surface of the outermost film 1 is a tubular film 
1' which has a lower melting point than the films 1 and 2 and which, when 
heated, closely adjacent to its melting point has excellent adhesive 
qualities. Instead of laminating the film 1', to the inner surface of the 
outermost film 1, it is also possible to have such a film 1', with the 
characteristics mentioned above, between the films 1 and 2 and having a 
diameter between the diameters 1 and L. 
The outermost tubular film 1 consists of a synthetic resin of polyolefin, 
such as a high density polyethylene or polypropylene, having a high 
tensile strength and a considerably higher melting point than the film 1', 
which preferably consists also of a thermoplastic synthetic resin, such as 
ethylenevinyl-acetate polymer or ethylene-ethyl acrylate polymer and 
which, as mentioned above, has a lower melting point than the tubular film 
1. The inner tubular film 2 is preferably made of the same resin as the 
outermost film 1. 
Located within the innermost film 2 is a metallic mandrel 3 having a 
leading conical portion 4 and a trailing cylindrical portion 4' of a 
diameter equal to the maximum diameter of the leading conical portion. The 
mandrel 3 is suspended within the innermost film 2 on a suspension rod 18 
which is anchored at its upper end, not shown in the drawing, on a pair of 
upper squeeze rollers, likewise not shown in the drawing, in a manner as 
clearly described in the above-mentioned copending application Ser. No. 
330,043 to which reference may be had with regard to the anchoring means 
of the suspension rod 18. Located below or downstream of the cylindrical 
portion 4' of the mandrel 3 is a container 5 adapted to contain a liquid, 
for instance water, and the interior of the container 5 communicates 
through a passage 7 extending through the cylindrical portion of the 
mandrel 3 with a cavity 8, preferably also of conical configuration, 
provided in the conical portion 4 of the mandrel. The container 5 and the 
passage 7 are preferably surrounded by heat insulating material 6. A 
high-frequency coil 9 surrounds the outermost film 1 in the region of the 
container 5. The high-frequency coil 9 is connected to a high-frequency 
transmitter 10 which, in turn, is connected to a source of electric 
energy, not shown in the drawing and arranged to transmit electromagnetic 
waves through the coil 9 to the mandrel so that by actuating the 
high-frequency transmitter the liquid in the container 5 may be heated. 
Sensing means 11 extend into the liquid in the container to sense the 
temperature of the latter, and the temperature sensing means 11 is 
connected to an ultra shortwave transmitter 12, which, in turn, cooperates 
in a known manner with an ultra shortwave receiver 13, connected to the 
high-frequency transmitter 10, to switch the latter on and off in 
dependency on the temperature sensed by the temperature sensing means 11 
so as to keep the temperature of the mandrel 3 at a constant predetermined 
temperature. The mandrel 3 includes a further cylindrical portion 14 
located below the container 5 and connected to the upper cylindrical 
mandrel portion 4' by a plurality of bars 15. A tank 16, into which a 
cooling fluid, such as cold water, is continuously fed and discharged 
therefrom, as indicated by the arrows at the lower inlet and upper outlet 
for such cooling fluid, is located downstream or below the mandrel 3 and a 
pair of squeeze rolls 17, driven by means not shown in the drawing, is 
located adjacent the bottom of the tank 16. The tubular films 1, 1' and 2 
are passed between the nip of these rolls. The stretched and united 
tubular film F is cooled rapidly in the water of the tank 16 and fed out 
of the same by guide rolls shown in FIG. 1. 
The above-described apparatus will operate as follows: 
At the start of the operation the high-frequency transmitter 10 is actuated 
to thereby transmit high-frequency waves through the coils 9 connected 
thereto to the mandrel 3 and the container 5 connected thereto. The 
electromagnetic waves of high-frequency transmitted to the mandrel will 
create eddy currents therein and heat the mandrel without heating the 
tubular films and the heat insulating material around the container 5, to 
thereby heat the liquid in the container 5. 
Due to the heating of the liquid the latter is vaporized and the steam 
passes through the passage 7 into the cavity 8 at the upper conical 
portion of the mandrel 3 to thereby keep the outer surface of the conical 
portion 4 at a temperature at which the tubular film 1', having a lower 
melting point than the films 1 and 2, is softened so that as the inner 
tubular film 2 passes over the cylindrical portion 4' of the mandrel, the 
three films are united with each other. As the inner tubular film 2 passes 
over the conical portion 4 of the mandrel onto the cylindrical portion 4' 
it is simultaneously stretched in a direction transverse to its 
elongation. 
The temperature sensed by temperature sensing means 11 is transmitted by an 
ultra shortwave transmitter 12 connected thereto to the ultra shortwave 
receiver 13 connected to the high-frequency transmitter 10 to switch the 
latter on and off, depending on the temperature sensed to thereby assure 
that the mandrel portions 4 and 4' and especially the conical mandrel 
portion 4 is always automatically kept at the necessary desired 
temperature. 
The cone angle .alpha. of the conical portion 4 of the mandrel may be 
changed within the wide limits. However, as shown in FIG. 2, for a given 
height h of the conical portion of the mandrel the final radius of the 
stretched inner tubular film will be either r or r', dependent on the cone 
angle. The maximum cone angle has therefore to be chosen in such a manner 
so as not to exceed the stretchability of the inner tubular film in 
slightly softened condition since otherwise the apparatus would not be 
operable properly. 
As mentioned in the copending application Ser. No. 330,043 the outer 
tubular film 1 is stretched in longitudinal direction due to the fact that 
the squeeze roller 17 at the bottom of the tank 16 are rotated at a higher 
angular speed than upper squeeze roller, not shown in FIG. 1, whereas the 
inner tubular film 2 is stretched in a direction transverse to its 
elongation so that the united film F emanating from the tank 16 will have 
a very high tear resistance. The united film F is cooled rapidly by the 
cold liquid in the tank 16 so that crystallization of the material of the 
film is prevented. As a result, a tubular film F having a high 
transparency can be produced. 
With the apparatus according to the present invention the mandrel 3 located 
within the films is heated and maintained at a predetermined necessary 
temperature by the above-described arrangement without the necessity of a 
direct connection of the heating means within the mandrel with an outside 
source of energy, but heating of the mandrel in the construction of the 
present invention is produced by the high-frequency transmitter 10, the 
operation of which is controlled by the temperature sensing means 11 and 
the ultra shortwave transmitter 12 connected thereto as well as the ultra 
shortwave receiver 13 connected to the high-frequency transmitter. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
apparatus for uniting at least two concentrically extruded tubular films 
of thermoplastic synthetic resins differing from the types described 
above. 
While the invention has been illustrated and described as embodied in an 
apparatus for the aforementioned kind in which the inner film is stretched 
over a mandrel to the diameter of the outermost film and in which the 
mandrel is heated by electromagnetic waves produced by a high-frequency 
transmitter, the operation of which is controlled without a direct 
connection between the high-frequency transmitter and the mandrel, it is 
not intended to be limited to the details shown, since various 
modifications and structural changes may be made without departing in any 
way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.