GB2137550A

GB2137550A - Web cutting

Info

Publication number: GB2137550A
Application number: GB08408316A
Authority: GB
Inventors: Arthur Hall
Original assignee: ALPINE PLASTIC MACHINERY LIMIT
Current assignee: ALPINE PLASTIC MACHINERY LIMIT
Priority date: 1983-03-31
Filing date: 1984-03-30
Publication date: 1984-10-10
Also published as: GB2137550B; GB8408316D0

Abstract

A plastics web 13, for example in bag manufacture, is cut by an endless band 10 extending round rollers 11 and generally in an inclined plane to the web. Guide rollers 13 make a length 10a of the band at right angles to the web. The band may provide multiple teeth with cutting edges and is moved at a high speed, e.g. 200 to 700 m/min, so that the web is cut by movement of the band through only one or a few teeth pitches, for example in 0.5 millisecond. The web is moved into and out of engagement with the band. The web may be preformed bags but the cutter may be associated with welding heads. The web is stationary during welding and cutting. <IMAGE>

Description

SPECIFICATION Web cutting This invention relates to web cutting apparatus and methods. The invention is primarily but not exclusively concerned with the cutting off of thin (less than 0.250mm) polyethylene web in the course of bag manufacture.

In bag manufacture there is a demand for high cutting speeds in order to increase productivity.

Current practice uses a flying knife which moves across the web and in which the time to make a cut is proportional to the width of the web. There is a limited speed at which a flying knife can operate and hence there is a minimum time at which a web of given width can be cut. A chopping blade which moves up and down can be used in lieu of a flying knife but such blades can typically only have a cutting speed of 0.8 m/sec.

In accordance with one aspect of the present invention a method of cutting a flexible web comprises applying on a face of the web a continuously moving cutting band. The band may have multiple teeth providing multiple cutting edges moving at a velocity such that a cut is effected by the band moving one, or only a few, teeth pitches. The band may move less than eleven tooth pitches.

According to another aspect of the invention apparatus for cutting a web comprises an endless cutting band, means for continuously moving the band across the web to cut the web, and means for effecting relative movement between the band and the web for bringing the web to cutting position.

The means for effecting relative movement may comprise means for moving the web towards the band.

The band can typically be moving at 200 to 700 M/min. with a tooth pitch of 5 mm. If a cut is performed in three teeth pitches then the time of cut (regardless of width of web) is about 0.5 millise conks. The cut introduces very little heat.

Preferably the endless band moves generally in a plane which is inclined to the web and the band is given a twist by guide rollers to present the band at right angles to the web.

The invention may be performed in various ways and two specific embodiments will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a plan view of one arrangement with parts omitted; Figure 2 is an elevation; Figures 3A, B,C,D and Eshow possible forms of tooth or band profile; Figure 4 is a side elevation of another arrangement; Figure 5 shows part of Figure 4 on a larger scale; Figure 6 is a timing diagram; and Figure 7shows part of a band and driving roller.

In Figure 1 an endless toothed band 10 having teeth 1 Ob is driven over a pair of pulleys 11, one of the pulleys being driven by electric motor 1 The pulleys rotate about parallel axes on shafts 11 b in frame structure 1 1c and in a plane which is inclined to the horizontal (see angle A in Figure 2). The pulleys 11 are spaced apart to allow a web 12 to pass under the band 10 where the band performs a straight run 10a parallel to the upper face of the web.

On each side of the band 10 there are guide roller pairs 13 which rotate about axes generally at right angles to the web and which apply a twist to the band and which guide the band 10 so that the length 10a passes over the web 12 at right angles to the web 12. Rollers 14 are mounted for rotation in structure 14b and propel the web 12 in the direction shown by arrow 15. For example one roll 14 is driven by electric stepping motor 14a and drives the other roll through timing belts 14b.

In Figure 2 there is additionally shown a clamp 16 (not shown in Figure 1) having an upper spring loaded jaw 17 urged by spring 17a towards the web 12 and a lower movable powered jaw 18. The jaw 18 is moved up and down by suitable means. The lower jaw 18 also carries a cutting slot 19 transverse to the web 12.

In use, the rolls 14 move the web 12 (which may be a series of formed but uncut bags) through the open clamp 16 at upto 3 metres/sec. Periodically feed of web 12 is stopped and the lower clamp jaw 18 is rapidly raised. This closes the clamp 16 to hold the web 12 and lift it on to the continuously moving band 10 so that the band is applied to the web face on a line across the web face and the web is cut on a line completely across, the band 10 entering slot 19.

With the cut completed the jaw 18 is lowered and the feed rolls 14 restarted. The now cut or separated bags form a stack. The period between successive cutting movements of the jaw 18 can be varied by timing devices in order to adjust to bags of different lengths and moving at different speeds.

Figure 3A shows a serrated form of toothed band with cutting edges 30 on the low inclination edge of the teeth.

Figure 3B shows a band with a wavy tooth form with cutting edges 31 at least on the leading lower edges of the teeth.

Figure 3C shows a symmetrical tooth form with cutting edges 32 ground on the leading lower edges.

The band can typically be moving at 200 to 700 M/min. with a tooth pitch of 5 mm. If a cut is performed in three teeth pitches then the time of cut (regardless of width of web) is about 0.5 miliseconds. The cut introduces very little heat.

The tooth pitch and the speed of the band 10 can be varied. In one arrangement the cut is achieved with the band 10 moving only one tooth pitch. In general the cut is completed with the band moving only a few tooth pitches, for example ten or less (less than eleven).

Figures 4 and 5 show use of the invention in bag making. The web 12, which is a flattened tube and thus in effect is two layers of flexible thin material of total thickness for example of 0.50 mm, is fed by rolls 14 between upper and lower air jets 30,31 which produce air flows in the direction of web travel, the upper jet 30 producing the faster air flow to produce uplift on the web. The jets are supplied by pump 32 via valve 33. The web passes between upper and lower transverse welding heads 34,35, over band 10a and a series of transversely spaced spikes 36.

The head 34 is carried on springs 37 from structure 38. Movable with head 34 is structure defining downward facing slot 19 and apertures 39 for receiving spikes 36. Structure 38 includes two uprights 40 on either side of the web connected by top transverse member 41, to which springs 37 are connected, and bottom transverse member 42. Uprights 40 are reciprocated in guides 43 by electric stepping motor 44 driving connecting rod 45 through eccentric 46.

Figure 6 shows a typical timing diagram where X is time, Y is welding head 34 displacement, Z is web velocity. At point 50, web velocity is at its maximum and the head 34 is at its top position. The speed of motor 14a now begins to reduce and the motor 44 begins to move head 34 down. At point 51 the web is stationary and the band cutter 10a is beginning to cut. By point 52 the cut is completed and the welding heads 34,35 close to effect a weld during time J. The head 34 now begins to rise and at point 52 the rolls 14 restart, the cutter 10a having moved clear of the web, for the next cycle. N indicates that jets 30, 31 are on. The periods G, H, J, K, L may for example be 75,40,80,40, 100 milliseconds; the distances B, C 25 and 10 mm. The cut-off bags form a stack 60 on spikes 36.When a selected number of bags are in stack 60 the spikes 36 are lowered by a pneumatic ram and the stack is removed on a conventional band support shown diagrammatically at 62.

The motors are controlled by a micro processor 61 which is programmed to enable the timing cycle to be adjusted for differing web speeds and bag lengths.

The width of the web may be for example 1.6 metres and the thickness of band 10 may be 0.046 cm. The width of the band 10 may be 1.6 cm. In some cases the centre of length 1 0a may be supported on a wear-resistant roller. A band sharpener may be fitted to engage as desired the non-operative length of the band. The band may have no teeth but an endless sharp edge 70 Figure 3D Figure 3E. Preferably the band has only one cutting edge slope 71 to provide vertical face 72 leading to edge 70 so that the cut bags move only downwards when forming the stack 60.

The band may be slightly inclined upwards from the horizontal (Figure 8 - much exaggerated) in the direction of travel, for example at an incline 80 which corresponds to a nominal diagonal of the flat web, although in practice the web may bow downwards slightly. The height of the incline may be 10 mm across the web, for example an angle O. & to the horizontal, in which case the point of cut would transverse across the web.

As shown in Figure 7, pulleys 11 have a crowned or convex surface 76 about twice the width of the band 10.

It will be understood that during cutting and welding the web is gripped on both side of the cut and weld, by roll 14 and spikes 36 respectively. In some cases an additional clamp can be included between the cutter and the drive rolls.

Claims

1. A method of cutting a flexible web comprising applying on a face of the web a continuously moving cutting band.

2. A method as claimed in claim 1, in which the band has multiple teeth providing multiple cutting edges moving at a velocity such that a cut is effected by the band moving less than eleven teeth pitches.

3. A method as claimed in claim 2, in which the band moves only one tooth pitch.

4. A method as claimed in any preceding claim in which the band is applied on a line on the web face.

5. A method as claimed in any preceding claim, in which the web comprises a series of formed but uncut bags.

6. Apparatus for cutting a flexible web comprising an endless cutting band, means for continuously moving the band across the web to cut the web, and means for effecting relative movement between the band and the web for bringing the web to cutting position.

7. Apparatus as claimed in claim 6, in which the band has multiple teeth providing multiple cutting edges.

8. Apparatus as claimed in claim 7, in which the endless band moves generally in a plane which is inclined to the web and the band is given a twist by guide rollers to present the band at right angles to the web.

9. Apparatus as claimed in claim 6 or claim 7, in which the means for effecting relative movement comprises means for moving the web towards the band.

10. A method of cutting a flexible web as claimed in claim 1 and substantially as herein before described.

11. Apparatus for cutting a flexible web substantially as herein before described with reference to and as illustrated-in Figures 1 and 2, or Figure 3A, or Figure 3B, or Figure 3C, or Figure 3E or Figure 3D, or Figures 4 and 5, or Figure 7, or Figure 8, of the accompanying drawings.