Welding machine for thermoplastic web

A displacement device in a thermoplastic web welding apparatus withdraws the leading edge of the web from the welding apparatus during pauses in feeding and/or production to prevent melting and sticking of the leading edge.

BACKGOUND OF THE INVENTION 
The invention relates to a machine for producing welds in a thermoplastic 
web. Machines of this kind are generally used for producing bags from the 
separated ends of a tubular web of thermoplastic synthetic which is 
intermittently fed to the welding station, said ends of said tubular web 
being advanced past the welding station. A difference is hereby made 
between the so-called separation seam welding, in which the end marked off 
by the weld is at the same time separated from the web by said weld, and 
bottom welding, in which, simultaneously with the production of the weld, 
a cut is made which lies next to the weld. The web ends thus separated 
during welding then form the bags which are stacked after they emerge from 
the welding station and are then carried away, either manually or by 
machine. 
In separation seam welding, a welding bar, movable upward and downward, 
acts in conjunction with a counter roll which forms a base for the web in 
the welding station. After each welding operation, after the front end of 
the web is separated, the leading edge of the web remaining in the welding 
station remains in contact with the counter roll until it is advanced 
during the succeeding feeding step. The leading edge of the web is warmed 
and softened during the welding. During stoppage, the softened leading 
edge may stick to the counter roll, thereby causing interruption of 
production until the adhesion is corrected. 
In bottom welding, the weld is usually produced by an upper and lower 
welding jaw. Here, during the short interruption of the intermittent feed 
or during an interruption of the welding process for a longer pause, the 
leading edge of the web can become softened by proximity to the heated 
lower welding jaw and stick onto it. This interrupts production until 
cleared. 
Through DT-AS 1942410, U.S. patent application filed Sep. 16, 1968, Ser. 
No. 760,048, a machine for separation seam welding is known in which, 
during the stoppage of the intermittently operating feed mechanism, the 
welding bar is adjustable in such a way that even when its upward and 
downward movement continues, it attains a position in which it can no 
longer touch the web. The web, now resting due to the standstill of the 
feed mechanism, is to be thus prevented from coming into contact with the 
still-operating welding bar, so that the welding bar is temporarily 
rendered inoperative without the whole machine having to be turned off. 
However, when the interruption of the intermittent feed extends over 
several work cycles of the machine, the leading edge of the web is capable 
of adhering to the base, as described above, when operation of the feed 
mechanism is resumed. This is especially disadvantageous in the welding 
station which includes a lower welding jaw, for example, in a bottom weld 
machine. 
It is an object of the invention to provide a machine for producing welds 
on a thermoplastic web, in which adhesion of the last welded leading edge 
of the web in the welding station during an interruption of the 
intermittent feed can be certainly avoided. 
SUMMARY OF THE INVENTION 
These and other objects are achieved in the apparatus in accordance with 
the invention. According to the present invention, the feed mechanism is 
mounted displaceable against the feeding direction from a first position 
into a second position. During normal pauses in intermittent feeding 
and/or longer interruption of the feed, the feed mechanism is shifted 
against its feeding direction into its second position which pulls the 
just welded leading edge of the web out of the welding station. This 
displacement of the feed mechanism can be done manually such as by a 
hand-operated lever which may also be coupled to the means for switching 
off the intermittently operating feed mechanism. 
An automatic displacement drive can also be provided for shifting the feed 
mechanism. 
In a preferred embodiment, the displacement drive is regulated by a control 
mechanism such that the feed mechanism is shifted into its second position 
immediately after the end of each welding and separation operation. The 
displacement drive shifts the feed mechanism into its second position at 
the end of each welding operation and shifts it back into its first 
position before the next welding operation begins. Thus, during each 
stoppage of the web in the regularly repeated interruptions of the 
intermittent feed, the leading edge of the web is pulled out of the area 
of the welding station after the welding operation and adhesion of the 
softened leading edge is thus avoided. 
The control mechanism may optionally be arranged to pull the leading edge 
of the web back from the welding station only or also during longer 
interruptions of the web feed. The welding operaion can thus be 
interrupted for as long as desired without the remainder of the operation 
of the machine having to be interrupted and having to render the welding 
apparatus inoperative. By permitting the welding apparatus to continue 
during the interruption, for example, the welding bar touches the counter 
roll in each work cycle. The counter roll is always kept at the same 
normal operating temperature as during welding. This permits resumption of 
welding without the faulty welds normally resulting while waiting for the 
counter roll to heat up to operating temperature. Known counter rolls 
consist of heat-resistant material, such as a silicone rubber layer coated 
with teflon-reinforced glass fiber, and are not damaged by regular contact 
with the welding bar.

DETAILED DESCRIPTION OF THE INVENTION 
The embodiment, represented in FIGS. 1 and 2, of a machine for producing 
welds on a thermoplastic web 10, has an intermittently operating feed 
mechanism comprising two feed rolls 11 and 12 which rotate in the 
directions of the arrow, as shown, and thus advance the web 10 in the 
feeding direction 13, indicated by an arrow, and guide it into the welding 
station shown generally at 14. The web is guided into the gap between the 
feed rolls 11 and 12. 
Feed rolls 11 and 12 are rotatably mounted with their ends in two levers 15 
and 16, and are rotatably connected to each other through gears 17. Thus, 
when the lower feed roll 11 is rotated in the rotating direction, the 
upper feed roll 12 rotates simultaneously in the opposite direction. 
Levers 15 and 16 are rigidly joined to each other at their lower ends by a 
tube 18. A shaft 19, rotatably mounted in side walls 21 and 22 of the 
machine frame, runs through the tube 18. The levers 15 and 16 thus are 
permitted to swivel on the shaft 19 as a swivel axle, between the first 
position shown in FIGS. 1 and 2 in continuous lines, and into the second 
position shown in FIG. 2 in dotted lines. The first and second positions 
of the levers 15 and 16 are defined by abutment with stops 23 and 24, 
respectively, (FIG 2). Stop 24 consists of an adjustable screw which can 
be fastened with a lock nut to adjust the second position of the levers 15 
and 16. 
A sprocket pulley 25 is fastened to the shaft 19 inside the side wall 21, 
and another sprocket pulley 26 is fastened to the shaft 19 outside the 
side wall 21. The sprocket pulley 25 is connected by a sprocket belt 27 to 
a shaft 31, said shaft 31 being driven alternatively in opposite rotating 
directions, in the known way, through a brake-clutch combination 28, 29. 
The sprocket pulley 25 is therefore similarly intermittently rotated 
through appropriate regulation of the brake 28 and the clutch 29. The 
sprocket pulley 26 is connected by a sprocket belt 32 to a sprocket pulley 
33 which is mounted on a shaft 34 on which also the lower feed roll 11 is 
mounted. A pneumatic stroke cylinder 35 with a piston 36 is provided as a 
displacement drive for swivelling both levers 15 and 16 and thus for 
adjusting the feed mechansim comprising the feed rolls 11 and 12 from the 
first into the second position, and back. A second stroke cylinder (not 
shown) may be disposed outside the side wall 22. The stroke cylinder 35 is 
mounted on the side wall 21, swivelling on an axle 37. Its piston rod 38 
is swivelably connected to the lever 15 through a joint 39. 
The welding station 14 has a welding bar 41 extending horizontally and 
transverse to the feeding direction 13. The welding bar, during stoppage 
of the web, is movable by a drive, not shown, from the raised resting 
position shown into a welding position pressing against a counter roll 42. 
When touching the web 10 introduced into the welding station 14, the 
welding bar 41 separates the end 43 of the web 10 projecting over the said 
welding bar 41 and said end of the web is carried away by a conveyor 
mechanism 44. 
The upward and downward movement of the welding bar 41 is produced in the 
known way such as by continuously rotating cam discs or a crank mechanism 
or intermittently operating electrical or fluid actuating means. The 
intermittent drive for the feed rolls 11 and 12, adapted to this upward 
and downward movement of the welding bar, is caused by a corresponding 
regulation of the brake 28 and the clutch 29, as indicated above. For this 
a control device 45 of any type known in the art is actuated by rotating 
cams or other means. The rotating cams may be rotated by the common drive 
of the machine, which also drives the welding bar upward and downward. 
This kind of control is generally known so that a detailed description is 
superfluous. 
The supply and discharge of compressed air on both sides of the double 
acting piston 36 of the stroke cylinder 35 takes place through pipes 46. 
The pipes 46 are connected through a valve 47 to an opening, not shown, 
for supply or exhausting compressed air. The valve is also controlled by 
the control device 45. The welding operation may be stopped while the 
welding device continues to run without melting the leading edge of the 
web. This is accomplished in the embodiment represented in FIGS. 1 and 2 
by the control device 45 actuating the stroke cylinder as a welding 
operation is completed and the welding bar 41 starts to raise from the web 
10. The feed rolls 11 and 12 are stopped at this time and the stroke 
cylinder 35 moves the feed rolls 11, 12 from the first position into a 
second position, shown in FIG. 2. In the second position, the leading edge 
of the web 10 is withdrawn from the welding station 14 and lies on a base 
48 which is not heated during welding. The welding bar 41, continuing to 
move up and down, now directly touches the counter roll 42 without 
touching the leading edge of the web 10. The continued periodic contact 
between the welding bar and the counter roll at the normal cyclic rate 
maintains the counter roll near its usual production temperature. Welding 
can therefore be resumed at will wlthout waiting for the counter roll to 
heat up. 
When welding is to be resumed, the drive of the feed rolls 11 and 12 is 
switched on and stroke cylinder 35 is actuated at the proper time by 
control device 45 so that the normal welding operation can begin again. 
In the embodiment represented in FIG. 3, all the parts which essentially 
correspond to the embodiment in accordance with FIGS. 1 and 2, have the 
same reference numbers so that through this indication, the preceding 
description can be referred to. 
The embodiment in accordance with FIG. 3 differs from the preceding 
embodiment principally in that this embodiment involves bottom weld 
machines in which the weld is produced by an upper welding jaw 141 and a 
lower welding jaw 142, without at the same time cutting the web at the 
weld. A cutting apparatus is provided for separating the one end 43 of the 
web 10 which forms a bag. The cutting apparatus has upper and lower blades 
51 and 52, respectively. The upper blade 51 is moved up and down together 
with the upper welding jaw 141. In other respects, the set up and the 
function of the machine shown in FIG. 3 is the same as in the machine 
shown in FIGS. 1 and 2. 
The embodiment shown in FIG. 4 differs from the preceding embodiment in 
that here, instead of using a stroke cylinder 35 for displacement of the 
feed mechanism 11, 12, a cam disc 53 is provided which engages one or both 
levers 15 or 16. This cam disc 53 moves the feed mechanism 11, 12 against 
the strength of a spring 54 at regular intervals from the first position 
into the second position and then controls its return movement through the 
strength of the spring 54. The shaft 55 carrying the cam disc 53 is 
rotated synchronously with the drive shaft for the welding mechanism, so 
that through appropriate shaping of the cam disc 53, the control realized 
by the said cam disc at the same time forms the displacement drive which 
shifts the feed mechanism 11, 12 into its second position immediately 
after the end of a welding and separating operation and causes the spring 
54 to shift the feed mechanism 11, 12 back to its first position promptly 
before the next welding operation. In this embodiment, therefore, the 
control of the displacement drive is released by each welding operation. 
A bolt 56 interferes with the return of the feed mechanism 11, 12 during a 
longer interruption of the welding process while the welding mechanism 
continues to run. This pulls back the leading edge of the web from the 
welding station and keeps it in this position for the duration of the 
stoppage of the welding operation. The bolt 56 may be automatically moved 
by the control mechanism 45 into the path of the lever 15 when the lever 
is in the second position and the intermittent drive of the feed rolls 11, 
12 has been stopped. When the welding process is resumed, the bolt 56 is 
moved by the control mechanism 45 out of the path of the lever 15 so that 
the above-described continuous operation can take place again. Manual 
operation of the bolt 56 may also be employed. 
For simplification of the description, only one cam disc 53 and one stroke 
cylinder 35 for interaction with the lever 15 are mentioned above. It is 
within the contemplation of the present invention that each of levers 15 
and 16 be provided with one cam disc 53, stroke cylinder 35 or bolt 56 
each. 
In the foregoing, the invention was described on the basis of embodiments 
in which feed rolls 11 and 12 are provided for the intermittent feed of 
the web 10. Instead of the feed rolls 11 and 12, any other kind of feed 
mechanism suitable for an intermittent feed of a web can be provided. 
In the embodiments in accordance with FIGS. 1, 2 and 4, in order to prevent 
the counter roll 42 from being unequally warmed by the contact with the 
welding bar 41 and at the same time to hasten the separation of the end 43 
from the web 10 in separation seam welding, in both the cases mentioned, 
the counter roll 42 is driven at a slightly greater peripheral speed than 
the feed rolls 11 and 12. 
It will be understood that the claims are intended to cover all changes and 
modifications of the preferred embodiments of the invention, herein chosen 
for the purpose of illustration which do not constitute departures from 
the spirit and scope of the invention.