A METHOD OF PRODUCING PLASTIC BAGS tions of layers of film may cross each other during the
.,.ir. ~- TM„„ ... time the resin is extruded. A resin-feeding opening of
BACKGROUND OF THE INVENTION one of ^ djes Js communicated with an extruding ma
It is well known that when heated plastic resin is ex- chine for supplying with low-heat-plastic resin, and the
panded, the material is apt to be oriented in the direc- 5 rotation of this die is stopped when the resin is ex
tion of being stretched to increase its tensile strength truded. The other two resin-feeding openings of the
toward the orientation, so that in case the material is other dies are respectively communicated with one or
stretched in both directions of length and breadth at two extruding machines which is or are supplied with
the same co-efficient of stretch, the strength of the ma- similar or different kinds of heat-plastic-resin. The
terial tends to be increased in any directions. 10 three annular dies are assembled with the annular inner
The packing bags made of polyethylene of low den- die inserted in a space of a middle annular die which in
sity of molecules, for packing heavy fertilizer, are man- turn is inserted in a space of the other annular die.
ufactured by the "method of inflation" which makes a Before the film is pressed and flattened, two kinds of
cylindrical film. For example, to make a bag for fertil- resin are extruded so as to form two kinds of bubbles
izer of 25 Kg., a cylindrical film of 450 to 500 mm in 15 through annular gaps of outer and inner annular dies,
diameter and 0.2 mm in thickness is formed by an an- and both bubbles are formed in cylindrical bubbles,
nular die having a slit whose diameter is 250 mm and both of which are contacted by the method of inflation,
its width is 1.00 mm. The co-efficient of the lengthwise The bubble at the outer side is cooled by a cooling me
stretch of the cylindrical film is 1.0 mm/0.2 mm = 5, dium to a certain degree while the inner one may not
while that of transverse stretch of the film is 2 X (450 20 become solidified. The two kinds of resin are extruded
to 500)mm/3 X 250 -M .3. by the rotation of the annular die for outer layer of film.
Accordingly, it is clear that the former is larger than In a space of the rotatable annular die for the outer the latter, namely, the lengthwise stretch is about four layer of film, another annular die for the inner layer of times as great as the transverse one. For the purpose of film is inserted, of which resin-feeding opening is cornincreasing the transverse stretch of the flattened cylin- 25 municated with an extruding machine in which lowdrical film, it is necessary to form such a film of 0.2 mm heat-plastic resin is supplied, while the annular die for in thickness, by welding two cylindrical films cross- the outer layer of film is also provided with a resinwisely. Thus formed stratified film is well known to be feeding opening which is communicated with an other strong enough for packing any kind of heavy materials. extruding machine in which another kind of heat
30 plastic-resin is supplied.
To manufacture above mentioned strong bags several troublesome steps are necessary such as gluing two flattened cylindrical films crossing in lengthwise directions For a more particular description of the invention, to each other with paste; cutting the films in desired reference is to be had to the accompanying drawings, length; folding back the cut film along its center line; 35 forming a part thereof, in which: sticking the edges opposite the center line so as to form FIG. 1 is a perspective view illustrating stratified film a tube; and forming a bottom to make a bag from the formed by an embodiment of the invention, tube. For the purpose of utilization of the method, our FIG. 2 is a vertical cross sectional view of the apparainvention is to provide a novel method and apparatus. tus for manufacturing the stratified film illustrated in
FIG. 3A and FIG. 3B are respectively illustrating dynamically the stretching directions of inner and outer
The object of the invention is to provide a method for layers forming the stratified film shown in FIG. 1.
manufacturing flatten cylindrical film of heat-plastic FIG. 4 is a vertical cross sectional view of another
resin in layers stretched and stuck one over the other. 45 embodiment of the invention for manufacturing the
Before the cylindrical films are flattened and stuck to- stratified film.
gether a bubble of melted plastic resin is expanded by In an example (1) of the first embodiment, polyethylthe "method of inflation" so that the layers of film may ene of high density of molecules is supplied to an exbe contacted or welded with each other. The bubbles truding machine (not shown in drawings) having two are extruded concentrically through annular gaps discharging openings for melted resin with which, the formed between two annular dies when one of the dies apparatus of the invention is communicated, rotates or both dies rotate in reverse directions. As shown in FIG. 2, inside of an annular outer die II One die is inserted in a space of another die and each for forming film of an outer layer is provided a space die is provided with a feeding opening for melted resin 5J into which an annular inner die I for forming film of an and communicated with an extruding machine having inner layer is inserted so as to rotate in reverse directwo extruding openings for the same kind of material, tions with each other.
or communicated with two extruding machines for the To the top of an inner shaft 1 of the inner die I, an different kinds of materials. inner member 2 is fixed, around of which is provided Another object of the invention is to provide for 6Q an inner cylinder 3, to the lower portion of which a gear manufacturing flattened cylindrical film of heat-plastic wheel 4 is fixed which is driven through a chain 5 by a resin in layers stretched and connected one over the driving means 6 so as to rotate the inner die I for formother. Before the cylindrical films are flattened bubbles ing film of an inner layer, which inner disc comprises of melted plastic resin are expanded by the "method of the inner member 2 and the inner cylinder 3. To the inflation" so that the layers of film may be contacted 65 bottom of the cylinder 3 is fitted, through a packing 7, or welded with each other. The bubbles are extruded a feeding means 8 for introducing resin. Between the through each annular gap formed in three annular dies, feeding opening 8 and the inner cylinder 3 is arranged two of which are rotated so that the stretched direc- a bearing 9 for rotation. Melted resin supplied from an