Device for transverse cutting and welding of webs

In a device for transversely welding and cutting tubular film into bags, feed rolls advance the film to a gripping rail which grips the leading end of the film during operation of cutting and welding devices located behind the gripping rail in the direction of feed. To ensure proper tensioning of the film, the feed rollers are reversed after the film is gripped and prior to operation of the cutting and welding devices.

BACKGROUND OF THE INVENTION 
This invention relates to a device for transverse cutting and welding of 
webs of thermoplastic plastic, more particularly webs in the form of a 
tubular film used for the manufacture of bags. The device comprises a pair 
of feed rolls for feeding the web in steps equal to the length to which 
the web is to be cut, a transverse cutting knife, preferably parallel to a 
transverse welding device and preferably heated, and two vertically 
movable rails placed parallel to each other in the direction of web feed. 
The rail which is downstream in the direction of feed acts as a gripping 
rail for pressing the end of the web, fed between the jaws of the 
transverse cutting device, against a support. The second rail, which is 
arranged between the cutting knife and the gripping rail, acts as a 
holding rail for tensioning the web which is gripped between the pair of 
feed rolls and the gripping rail before the cutting knife cuts the web. 
In a device of the above kind, as described in German patent publication 
3,204,492 A, it is possible for a section of the web which has been 
advanced between the jaws of the transverse welding device and under the 
cutting knife, and which has been held between the pair of feed rolls and 
the gripping rail, to be so slack that it may not be clamped in the 
desired manner by lowering the gripping rail. 
If insufficient tension is produced by lowering the gripping rail, it is 
not possible for a severing cut to be produced. Therefore jamming between 
the cutting edges--as may be caused the heated severing knives--may not be 
reliably overcome. 
SUMMARY OF THE INVENTION 
One object of the invention is to design a device of the initially 
mentioned type, in which the gripping rail produces a tensioning effect in 
the gripped section of web so as to ensure a good severing cut and 
overcoming any potential jamming problem. 
In a device in accordance with the invention, the feed roller part is 
subject to a drive movement drawing back the web during an interval of 
time which comes before the formation of the transverse weld seam. The 
pair of feed rolls perform a small reverse motion after feeding of the web 
so that any folds or slack in the web are taken up by drawing the web 
taut. 
A further feature of the invention is that the pair of feed rolls may 
advance the web in steps through a distance which is greater than the 
length to which the web is cut. The pulling back motion causes the excess 
length to be pulled back again by the pair of feed rolls, so that the web 
is pulled to smooth it out. In this respect the amount of the backward 
pulling motion is chosen to match the excess amount by which the web is 
advanced. 
It is convenient if the pulling back of the web takes place after the web 
has been gripped by the holding rail. When the advanced leading end of the 
web is gripped between the holding rail and a support, the web may be 
drawn into a properly smooth state by pulling it backwards. 
It is convenient if the web is pulled back along a length which is equal to 
the difference between the length of advance and the length to which the 
web is cut. The length along which the web is pulled back may be adapted 
to suit the desired state of tensioning. The holding rail may draw the 
advanced section of web taut even while it is moving back, or later, and 
only after the web has been pulled back. 
Furthermore it is possible for the knife to cut the web even while it is 
being pulled back.

DESCRIPTION OF PREFERRED EMBODIMENT 
In a device for the production of plastic bags shown in the drawing, a pair 
of feed rolls 1,2 advance a plastic web 3 of tubular film in steps equal 
to the length of a bag. During the dwells of the feed rolls 1 and 2, the 
web 3 is clamped between the rolls. It is possible to provide air nozzles, 
not shown, to cause the leading end of the web of tubular plastic film to 
be fed in a taut condition. When the feed rolls 1 and 2 have fed a 
sufficiently long piece of tubular web, a depressing device, consisting of 
a gripping rail 4, moves down onto a support 5 so that the web is gripped 
on the one hand between the feed rolls 1 and 2 and on the other hand 
between the gripping rail 4 and the support 5 or, respectively, the stack 
of bags (shown in chain dotted lines) accumulating on the support. Then a 
holding rail 6 moves downwards and presses the web 3 sufficiently firmly 
in the downward direction that the latter is drawn taut over a groove 7 in 
a mating gripping rail 8. The web 3 is then severed by moving down an 
upper cutting knife holder 9 with a transverse cutting knife 10 thereon. 
Generally simultaneously with this, or directly thereafter, a permanently 
heated pair of welding jaws 11 is used to form a bottom transverse weld in 
the web. 
Directly after the out has been made, the holding rail 6 moves even further 
downwards and while so doing presses the bag severed from the web 3 onto a 
loading rail 13 which is connected with the mating rail 8. During this 
working step, it is possible for the welding jaws 11 to be still shut and 
to form the transverse weld seam. The loading rail 14 has holes through 
which needles 12 extend upwards so that the newly formed bag is spiked on 
such needles. The needles 12 are firmly mounted on an angle section of 
support 5. 
The gripping rail 6 also has holes so that the individual bags may be 
slipped onto the needles 12. A recess 14 under the groove 7 in the mating 
rail 8 serves to receive the edges of the bags without their being 
crumpled. 
The device described with reference to the drawing is essentially similar 
to the device described in the German patent publication 3,204,492 A so 
that for further details reference may be had to same. 
However there is a departure in the described device from the earlier known 
device, since the fed rolls and 2 forming a pair of advancing rolls may be 
turned back against the direction of feed, (indicated by the arrow,) of 
the web 3 prior to operation of the cutting knife and the welding jaws. 
This backward rotation of the feed rolls 1 and 2 means that the section of 
web 3, which has previously been freely drawn forward, is tensioned so 
that it does not have any slack and does not lie in folds. Reverse 
rotation of the feed rolls may, for example, be effected by any suitable 
form of reversing drive.