Integral film tear strip

Disclosed is plastics film containing a tear strip and a method for producing it in which the film is co-extruded with a bead which adheres to the film surface. Because of the heat retained by the bead, blown expansion of the film following extrusion causes the film to undergo a gauge reduction along and adjacent to the bead which provides a preferential tear line for the film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a film containing an integral tear strip 
and to a method for producing a tear strip for a plastic film during the 
manufacture of the film itself. 
2. Discussion of the Prior Art 
Prior attempts to provide preferential tear lines for plastic materials, 
particularly plastic films, have involved the steps of mechanically 
processing the material to provide perforations or score lines therein 
along which the material may easily separate. Providing weakened areas 
such as these requires additional processing of the film and additional 
equipment costs which naturally adds to the overall cost of film 
production. In addition, providing perforations in a material is an 
undesirable manner of introducing a weakness line since the material 
itself penetrates into the perforations and does not entirely provide an 
acceptable product. 
Another method for providing a line of weakness in a plastics film is to 
laminate a material having substantially different properties to an 
underlying substrate material which has preferential tear properties in a 
predetermined direction. The material which is then laminated thereon is 
provided with a high relative strength in its longitudinal direction so 
that it may remain substantially intact. By pulling on the laminated 
material, it tears the substrate material. The difficulty with providing a 
tear line using this technique is that additional process steps are 
required to laminate additional materials on a substrate material so that 
the processing cannot be conveniently done in line during the manufacture 
of the underlying plastic substrate. 
SUMMARY OF THE INVENTION 
One object of the invention is the provision of a line of weakness in an 
extruded plastics material along which a tear can preferentially 
propagate, the line of material weakness being introduced during the 
manufacture of the plastics film and not requiring any additional post 
manufacturing processing steps. 
An additional object of the invention is the provision of a method for 
forming a preferential tear line in an extruded plastics film which does 
not require extensive or complex equipment or processing steps, and which 
can be done in line with the manufacture of the film itself. 
These and other objects, features and advantages of the invention will be 
more readily discerned from the following detailed description of the 
invention which is presented in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the invention, a plastics bead is coextruded with and on 
a surface of an extruded plastics film. The bead adheres to the plastics 
film and the heat retained by the bead causes a gauge reduction in the 
thickness of the plastics film during blown expansion of the film 
subsequent to extrusion. This gauge reduction forms a line of weakness 
along which the film will preferentially tear. 
Turning to FIG. 1, a portion 11 of the wall of an extruded film 23 of the 
invention is shown containing thereon an integrally extruded bead 13. On 
opposite sides of bead 13 and extending along bead 13 are formed areas 15 
of reduced film thickness. These are the areas along which the film 23 
will preferentially tear. 
FIG. 2 illustrates the extrusion and blowing operations associated with 
producing the plastics film 23 shown in FIG. 1. In FIG. 2, a tubular 
extrusion die 17 is used to extrude the tubular film 23. Co-extruded with, 
and on the tubular film 23 is a bead 13. The bead 13 may be in contact 
with the film 23 as both exit the die 17, or the bead 13 may contact with 
the film 23 at a contact point spaced slightly above the exit orifice of 
die 17 and below the frost line 21. 
After the bead 13 and film 23 have been coextruded, the film 23 is expanded 
below the frost line 21 of the film using known blown expansion techniques 
in which a pressurized gas exits orifice 41 which causes an expansion of 
the film in the transverse direction (TD) causing the areas 15 of the film 
adjacent bead 13 to undergo the thickness reduction shown in FIG. 1. 
During blown expansion the nominal thickness "a" of the film is reduced 
somewhat, but a greater thickness reduction occurs at the areas 15 to 
produce a film thickness "b" which is less than the nominal thickness "a". 
After blown expansion, the film 23 is collapsed by a pair of nip rollers 
25 in conventional fashion. 
FIG. 3 illustrates a tubular die 17 which may be used in the method 
illustrated in FIG. 2. The tubular die includes an outer die wall 27 and 
an inner die wall 29 defining between them an annular flow passage 35 
through which the plastics film 23 is extruded. The terminating end of 
annular flow passage 35 is the die orifice 31 through which the extruded 
plastics film exits the die. 
The bead flow passage is illustrated as 37 in FIG. 3. Whereas the flow 
passage 35 for the film is annular the flow passage 37 is not, having a 
generally circular cross section which extends in an generally axial 
direction of the die. Flow passage 37 is inclined toward the die axis so 
that a bead exiting passage 37 at a die orifice 33 is directed towards the 
external surface of the film being extruded through annualar die orifice 
31. The angling of the bead flow passage is such that contact of the bead 
with the extruded film occurs approximately 1" above the annular die 
orifice 31 and approximately 4" below the frost line of the film. (FIG. 
2). The exit orifice 33 of the bead flow path 37 could also terminate 
directly adjacent annular die orifice 31 so that the bead 13 and film 23 
are co-extruded and adhered at the time they both exit the die. 
A common melt feed flow path 39 is shown in FIG. 3 for the bead flow 
passage 37 and annular film flow passage 35. Thus, with the die of FIG. 3 
the same plastics melt is used to extrude both the film 23 and bead 13. Of 
course, if desired, a separate flow path could be provided for each of the 
annular flow passage 35 and the bead flow passage 37 to allow for 
co-extrusion of different melts. 
A low density polyethylene has been extruded from the die illustrated in 
FIG. 3 having a 2 1/2" diameter annular die orifice 31. The extrusion gap 
was machined to produce a 2 mm film thickness. The angling of the orifice 
toward the film path was made so that contact between the bead and film 
was made approximately 1" above the die orifice and approximately 4" below 
the frost line 21. The bead profile easily adhered to the film surface, 
but the increased mass of the bead 13 retained heat longer and thus took 
longer to cool, making areas 15 adjacent the bead hotter than other areas 
around the circumference of the blown film. As the film bubble was 
subsequently expanded, as shown in FIG. 2, to its final dimension, the 
plastics film 23 stretched preferentially in the local hot spots to reduce 
the gauge of the film from approximately 2 mm to approximately 0.5 mm in 
the areas 15 adjacent the bead profile. When the subsequently cooled film 
was torn, the tear easily propagated along the lines 15 of thin gauge. 
Because the method as shown in FIG. 2 is integrated with the extrusion of 
the plastics film, no additional steps are required to provide a line of 
weakness in the film making the method of the invention easy to implement 
without requiring additional processing steps or machinery. 
While one example of an extruded material has been described above, it 
should be apparent that the invention can be used with many different 
types of thermo-plastics materials, for example, high and low density 
polyethylene, polypropylene, polystyrene and others. 
While an embodiment of the invention in both its method and product aspects 
has been shown and described above, it should be apparent that many 
modifications can be made without departing from the spirit and scope of 
the invention. Accordingly, the invention is not limited by the foregoing 
description, but is only limited by the claims which are appended hereto.