Apparatus for making bags from a two-ply web of thermoplastic material

Parallel, spaced apart backing rods are carried by a rotating cylinder and serve the continuously supplied web. Back-pressure bars are extensible to engage and cooperate with welding bars. The backing rods and back-pressure bars are movable from and toward each other and provided with means for guiding a web with low friction. Those welding bars which are disposed near that sector of the cylinder in which the backing rods are disposed when they are retracted are moved concentrically to and at the same angular velocity as the back-pressure bars outside that surface of the cylinder over which the back-pressure bars are disposed. The back-pressure bars consist of rotatably mounted rolls, which are provided with a covering of elastic material which resists the welding heat. The welding bars have knife edge-like welding edges facing the rolls.

This invention relates to apparatus for making bags or tube sections from a 
two-ply web of heat-sealable or weldable material, comprising parallel, 
spaced apart backing rods carried by a rotating cylinder and serving to 
support the continuously supplied web, and a transverse welding and 
severing device having back-pressure bars which are extensible to engage 
and cooperate with welding bars, the backing rods and the back-pressure 
bars being movable from and toward each other to cause the web to extend 
between them in a zig-zag line and being provided with means for guiding 
the web with low friction, and those welding bars which are disposed near 
that sector of the cylinder in which the backing rods are disposed when 
they are retracted and in which the welding seams to be separated are made 
being moved concentrically to and at the same angular velocity as the 
back-pressure bars outside that surface of the cylinder over which the 
back-pressure bars are disposed, in accordance with U.S. Pat. No. 
4,115,183 (hereinafter referred to as "the abovementioned patent). 
In the apparatus according to the abovementioned patent, each back-pressure 
bar consists of a retaining bar having a knifelike blade, which is adapted 
to be forced against the end face of the heated welding bar in that 
portion of the sector of the cylinder in which the seam welds to be 
severed are to be made, rods which are parallel to the knifelike blade are 
disposed on opposite sides thereof, and rollers for guiding the web are 
rotatably mounted on said rods. These rollers reduce the friction and 
permit the web to be formed into a zig-zag shape as the backing rods are 
retracted. 
In the apparatus according to the abovementioned patent, it has been found 
that the welding bars covered by a polytetrafluorethane woven fabric are 
subjected to increased wear because the knife edges of the knifelike 
back-pressure bars always impinge on the same areas. 
For this reason it is an object to provide the apparatus according to the 
above-mentioned patent with a more durable transverse welding and severing 
device. 
This object is accomplished according to the invention in that the 
back-pressure bars consist of rotatably mounted rolls, which are provided 
with a covering of eleastic material which resists the welding heat, and 
the welding bars have knife edge-like welding edges facing the rolls. In 
the apparatus according to the invention the back-pressure bars consisting 
of rotatable rolls serve not only to guide the web with low friction as 
the same is pulled to assume a zig-zag shape, so that there is no need for 
additional rods disposed on opposite sides of the back-pressure bars are 
provided with rollers, but each roll is abutted by the associated welding 
bar different generatrices of the roll during different welding operations 
so that the fact that random positions are assumed by the roll after every 
formation of the zig-zag shape ensures that the entire periphery of the 
roll will be used as an abutment and will thus be uniformly stressed and 
suffer virtually no wear. 
The cover of the roller consists suitably of a silicone rubber cover and an 
overlying polytetrafluoroethane woven fabric. 
According to a preferred further feature, the backing rods disposed 
adjacent to the sector in which the concentrically moved welding bars are 
disposed are moved toward the back-pressure bars by the cams which control 
the retracting movement of the backing rods. As the backing rods retracted 
to their inner end position are thus raised, the sheeting web held in a 
zig-zag shape is slackened so that the fresh seam weld will not be 
subjected to tension and will not be prematurely severed. Only when the 
seam welds have sufficiently cooled, the backing rods are moved back to 
their inner end position so that the web is subjected to tension adjacent 
to the knife edge-like welding edges and the sections provided at their 
ends with transverse seam welds are severed from the web. 
According to another preferred embodiment of the invention, the bearings 
for the rolls are slidably mounted in radial guide grooves formed in the 
end discs of the cylinder and are provided with brackets, in which rollers 
are mounted, which run on cams arranged around the cylinder, prestressed 
compression springs are held between the bearings or carriers connected to 
said bearings, on the one hand, and the discs or brackets connected to 
said discs, which springs urge the rollers against the cams, and the cams 
are interrupted adjacent to the sector in which the concentrically moved 
welding bars are disposed, so that the rolls bear on the welding bars. 
This arrangement ensures that during the welding operation the rolls which 
constitute back-pressure bars will engage the knife edges of the welding 
bars under the pressure which is determined by the springs. 
The combination of a knife edge-like welding bar with a welding roll which 
constitutes a back-pressure bar is known per se from German Pat. No. 
2,309,719.

The end discs 4 of the welding cylinder 1 are formed with radial grooves 3, 
in which backing rods 2 are mounted to be radially slidable parallel to 
themselves. The backing rods 2 are axially extensible and retractable and 
are rotatably mounted in a radially slidable sliders. Guide members 
provided with cam follower rollers are secured to the sliders and guided 
in cam slots 5 which are fixed to the frame of the apparatus. By means of 
the cam slots 5, the backing rods are controlled to be radially retracted 
between the rolls 6, which constitute the back-pressure bars, when the 
rolls 6 have been pushed over the sheeting web 9 above the flight cylinder 
described by the rolls. The sheeting web 9 is supplied over the guide 
roller 8. In the above-mentioned patent it has been described more in 
detail how the sheeting web 9 is confined between the backing rods and the 
back-pressure bars before the backing rods are moved beyond the flight 
cylinder of the back-pressure bars to form the web into the zig-zag shape. 
Cam rails 10 are disposed outside the welding cylinder 1 adjacent to the 
sector thereof which is indicated by the angle .alpha. and have curved cam 
faces, which are concentric with the cylinder. Welding bands 12 secured to 
endless chains 11 revolve around the curved cam face. The guide chains are 
provided with brackets 13, in which cam follower rollers 14 are mounted. 
The latter roll on the cam rails 10 and guide the welding bands 12 
adjacent to said rails. 
It is apparent from FIG. 2 that the bearings 15 for the rollers 6, which 
constitute the back-pressure bars, are screw-connected to guide members 
16, which by means of sliders 18 are radially slidably guided in radial 
grooves 17 of the end discs 4. The forked upper end portion of the guide 
member 16 carries a pin 19, which carries the slider 18 and on which a cam 
follower roller 20 consisting of a ball bearing is mounted. The roller 20 
runs on a cam face formed by an annular cam rail 21, which is fixed to the 
frame of the apparatus and surrounds the cylinder. 
A bracket 22 is connected to the guide member 16 and has bores 23, which 
are aligned with bores 24 in a carrier 26 that is secured to the discs 4. 
The bores 23, 24 receive the ends of compression springs 27, which urge 
the cam follower rollers 20 against the cam faces of the cam rails 21. 
Adjacent to that sector of the cylinder which is bridged by the 
concentrically curved cam rails 10, the cam rails 21 are interrupted so 
that the springs 27 urge the rolls 6 against the welding bands 12, which 
are disposed radially outwardly of the cam rails 21 and revolve at the 
same velocity. 
Stops 28 for limiting the radially outwardly directed movement of the rolls 
6 are secured to the end discs 4. 
The rolls 6 consist of tubes 30, which are mounted by means of needle 
roller bearings on journals 29 and carry a covering 31 consisting of 
silicone rubber and an overlying polytetrafluorethane woven fabric. 
The welding bars 12 consist of welding bands, which are provided with knife 
edge members 32. These welding bands are supplied with welding current in 
known manner by way of sliding contacts. 
In the intermediate portion of that sector of the welding cylinder 1 which 
is bridged by the concentrically curved cam rails 10, the backing rods 2 
are moved toward the rolls 6 by the portion 33 of the cam slot 5 to 
slacken the web which is held in zig-zag shape between the backing rods 2 
and the rolls 6. As a result of that slackening, the several sections 
which have been provided with seam welds and severed from the web cannot 
be separated from the web before the seam welds have been sufficiently 
cooled. Adjacent to the end of the concentrically curved cam rails 10, the 
cam slot 5 causes the backing rods 2 to be retracted so that the sections 
34 provided at their ends with transverse seam welds are separated from 
the web 9.