GB2124966A - Process and equipment for winding a flattened extrusion blown tubular plastics foil ribbon into a coil - Google Patents

Abstract

A flattened plastics foil ribbon (3), which is drawn from a blown foil extrusion unit at a prescribed constant drawing speed and is wound into a coil (4), is subjected before winding to a periodic angular displacement of a thickness error (10), which is effected by a displacement tool e.g. the extruder die (1), which rotates at a certain angular displacement velocity (w). To produce a coil 4 of truly cylindrical external form, the angular displacement velocity (w) is reduced as the plastics foil ribbon is wound, in proportion to the increase in the winding times for successive foil layers, thereby maintaining a constant displacement arc of the thickness error (10) for each of a long succession of foil layers. Equipment for carrying out this process incorporates a monitor (15) which emits a control signal based on the number of foil layers and/or the coil thickness, the rest of the control equipment being adapted so that the controller (14) for the displacement tool driving means (13) maintains a constant displacement arc. <IMAGE>

Description

SPECIFICATION Process and equipment for winding a flattened plastics foil ribbon into a coil This invention relates to a process for winding a flattened plastics foil ribbon, produced in a blown foil unit, into a coil comprising a plurality of foil layers, in which the plastics foil ribbon drawn out of the blown foil unit at a prescribed constant drawing speed is wound in conformity with the drawing speed and subjected before winding to a periodic thickness error displacement, specifically with the aid of a thickness error displacement tool which moves at a certain angular displacement velocity on a displacement circle of prescribed radius and for each layer of foil in the coil describes a displacement are on the displacement circle, which displacement arc represents the product of the angular displacement velocity times the radius of the displacement circle and the winding time for the layer of foil concerned. The invention further relates to equipment for carrying out the said process, having a displacement tool, driving means for the displace menttool, a controllerforthesaid driving means, and a winding station. In the context of the invention, the term "foil layer" covers the two layers of a flattened blown foil tube, as it were joined together at two folding edges, and single layers made therefrom by separation. The invention can be applied to any known thickness error displacement tool (cf. Schenkel's "Kunststoff-Extrudertechnik"/ Plastics Extrusion Technology/, Munich, 1963, pp.

390-393). More particularly, the extrusion head used to extrude the plastics tube for expansion into a tubular blown foil can function as the displacement tool. However, it is equally possible to employ separate displacement devices having displacement bars or the like to act as the displacement tool. The periodic thickness error displacement and corres pondingly the periodic movement of the displace ment tool consists of either a periodic reciprocation or a periodic circumferential movement, depending on technical preferences and the facilities provided by the displacement tool.

Within the prior art, the angular displacement velocity of the displacement tool remains constant. It is adjusted so that the foil tube is laid flat and free from folds in spite of the thickness error displace ment, and conseqnently it must not be excessive. In general, operations are carried out at the maximum safe angular displacement velocity. This produces an adequate thickness error displacement and avoids the formation of bulges on the coil which would otherwise develop from the super-imposition of thickness errors and could interfere with subsequent operations. The displacement arc described on the displacement circle by the thickness error displacement tool in this case becomes longer as layer is wound upon layer.Although this is not detrimental as regards the thickness error displace ment, the finished coil often becomes excessively distorted from the true cylindrical form. In fact, one often encounters so-called cylindrical form errors.

For example, instead of remaining truly cylindrical, the coil can become conical or acquire the form of a semi-hyperboloid of rotation. It is a nuisance when a distorted coil is further processed in subsequent machines, for example to make bags, and one must apply tools to the coil periphery and/or take measurement and derive other parameters. It is possible with the known devices to set the angular displacement velocity to a prescribed setting when adjusting the device or equipment, but there is no attempt to adjust the angular displacement velocity during the coil winding process, if indeed such adjustments are at all possible.

The object of the invention is to modifythebasic process so that the cylindrical form errors referred to above are avoided and a coil is built up with a sufficiently true cylindrical form.

According to the present invention, the angular displacement velocity is reduced as the plastic foil ribbon is wound, in proportion to the increase in the winding times for successive foil layers, thereby maintaining a constant displacement arc for a succession of foil layers.

The invention arises from the realisation that the displacement pitches of the thickness errors in the foil ribbon during and after winding must be held constant for successive foil layers if a cylindrical form error is to be avoided, irrespective of whether one considers the first or one of the early foil lyaers or the n-th layer for large values of n. Under these conditions the displacement pitches become additive over the breadth of the coil, with the surprising effect of maintaining a truly cylindrical form within very narrow tolerances. By adopting the teaching of the invention, this addition of displacement pitches is spontaneously maintained.

Adopting the notation wn for the angular displacement velocity for the n-th foil layer, Atn for the winding time for this n-th foil layer, and r for the radius of the displacement circle, the invention teaches that the product WnrAtn is held constant for all values of n up to large numbers, so that wn is reduced in proportion to the increase in Atn.

There are numerous ways of carrying out the process of the invention. One particularly simple embodiment of the invention is characterised in that the displacement arc is held constant over the entire long succession of foil layers from which the coil is formed. However, it is possible when forming thick coils in particular to modify the process of the invention so that the displacement arc is held constant over an initial large number of foil layers, until the angular displacement velocity has fallen to about 1/2 to 1/3 of its initial value, whereupon the angular displacement velocity is restored approximately to its initial value and the displacement arc is again held constant at this setting; the restoration sequence can be repeated over a number of successive stages.The effect of this procedure applied to thick coils is to maintain an adequate distribution of thickness errors out to the outermost layers. It is particularly effective when the angular displacement velocity is restored every time the displacement tool completes one cylce of its movements.

Another object of the invention is to provide equipment for carrying out the process as described, having a displacement tool, driving means for the displacement tool, a controller for the said driving means, and a winding station, which is fitted with a monitor which emits a control signal based on the number of foil layers or the thickness of the coil, while the controller for the displacement tool driving means maintains a constant displacement arc in conformity with the control signal, with optional feedback.

An embodiment of equipment for carrying out the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagram of the equipment for carrying out the process of the invention; Figures 2a, 2b and 2c depict the movements of the displacementtoolforvariousfoil layers, when applying the process of the invention; Figure 3 is a graph showing the variations in the angular displacement velocity when applying the process of the invention; and Figure 4 is a similar graph to Figure 3, but relates to another way of applying the process of the invention.

Figure 1 shows the so-called blowing head 1 of a blown foil unit, disposed in front of an extruder not shown. It also shows the inflated foil tube 2 and beyond the break in the drawing, a plastics foil ribbon 3 made by flattening the tube to form a double layer. The final coil 4 is also shown. Thus a flattened plastics foil ribbon 3 produced in a blown foil unit is wound into a coil 4 containing a plurality of foil layers, and since the coil diameter increases during winding the successive foil layers vary in length. The double arrow 5 in Figure 1 indicates the length of one foil layer. The plastics foil ribbon 3 is drawn from the blown foil unit at a prescribed constant speed. It is wound in conformity with this draw-ing speed, so that the winding speed decreases as the number of foil layers on the coil increases.The plastics foil ribbon is subjected before winding to a periodic thickness error displacement.

In the embodiment shown, this is effected with the aid of the blowing head 1, which functions as a thickness error displacement tool by rotating back and forth as shown by the double arrow 6 in Figure 1.

The thickness error displacement is effected, as clearly shown in Figures 2a, 2b and 2c, at an angular displacement velocity w along a displacement circle 7 of radius r. For each of the n foil layers in the coil there exists a displacement arc 8 related to the displacement circle 7 of the thickness error displacement tool 1,which arc can be expressed by the formula w#rAt#, where tn is the winding time for the n-th foil layer. A corresponding displacement pitch 9 is set up in the plastics foil ribbon 3. Referring to a thickness error formed at the point 10 in Figure 1 by the periodically moving blowing head 1, this error is displaced approximately along the curve 11 in Figure 1 and lies in the plastics foil ribbon 3 along a curve 12 with a peripheral displacement pitch 9 which can be traced along the foil layer.The displacement pitch 9 is proportional to the previously mentioned displacement arc 8 orto w,mt. The displacements are shown on a larger scale in Figure 1, for the sake of clarity.

The outer circle 7 in Figure 2a denotes the displacement circle of the thickness error displacement tool 1,for example the mouth gap in the blowing head 1. Consider now the n-th foil layer in the coil 4. Figure 2a shows the related angular velocity wn and the corresponding time Atn required to wind this n-th foil layer. The radius of the displacement circle is r, so that the semicircular arc 8 in Figure 2a denotes the displacement arc, which according to the invention is to be held constant for all the many foil layers in the coil 4. The situation for the (n+x)-th layer is depicted in Figure 2b.It can be seen that the angular velocity w,+, has become smaller than in Figure 2a, but the time Atn+x has become longer so that the product wn+xrAtn+x is still the same as in Figure 2a and is again denoted by the semicircle 8. The same applies to Figure 2c, in which the angular velocity is smaller still at wn+y and the time Atnwyforwinding the (n+y)-th foil layer is longer.

Summing up therefore, it can be seen that as the winding of a tubular plastics foil ribbon progresses the angular displacement velocity w when winding the n-th foil layer is reduced in proportion to the longer winding time Atn, so that the displacement arc 8 depicted in Figures 2a, 2b and 2c, representing the product wnrAtn, is held constant over a plurality oftubularfoil layers.

Figure 3 shows that the angular displacement velocity w becomes correspondingly smaller. Its values are plotted on the vertical axis against the layer number n on the horizontal axis. It can be seen that the angular displacement velocity w is inversely proportional to the number n of layers or the coil thickness, - including the drawing speed as a parameter n the numerator. In Figure 3 the process has been adjusted so that the displacement arc wnrAtn, denoted by reference number 8, remains constant for the entire series of foil layers forming the coil 4. In Figure 4, on the contrary, the process is so adjusted that the displacement arc wnrAtn is only held constant for an initial series of foil layers, terminating when the angular displacement velocity w has fallen to approximately 1/3 of its initial value.

Thereupon the anguler displacement velocity w is restored to approximately its initial value and winding is continued with a constant displacement arc at this initial value, - and so on over a series of restoration stages. It is possible in this case, as shown in Figure 4, to restore the angular displacement velocity w after each movement cycle of the displacement tool 1.

As shown in Figure 1, the equipment for carrying out the process of the invention, incorporates driving means 13 for the displacement tool, a controller 14 for the displacement tool driving means 13, and a winding station and the coil 4, which winding station is fitted with a monitor 15 which emits a control signal based on the number of foil layers in the coil 4 and/or the coil thickness. The controller 14 for the displacement tool driving means 13 maintains a constant displacement arc of wnrAtn value in conformity with this control signal, with optional feedback.

The entire system can easily be constructed from the components available in modern driving and controlling technology.

Claims (7)

1. A process for winding a flattened plastics foil ribbon produced in a blown foil unit, into a coil comprising a plurality of foil layers, in which the plastics foil ribbon drawn out of the blown foil unit at a prescribed constant drawing speed is wound in conformity with the drawing speed and subjected before winding to a periodic thickness displacement, specifically with the aid of a thickness error displacement tool which moves at a certain angular displacement velocity on a displacement circle of prescribed radius and for each layer of foil in the coil drescribes a displacement arc on the displacement circle, which displacement arc represents the product of the angular displacement velocity times the radius of the displacement circle and the winding time for the layer of foil concerned, the angular displacement velocity being reduced as the plastic foil ribbon is wound, in proportion to the increase in the winding times for successive foil layers, thereby maintaining a constant displacement arc for a succession of foil layers.

2. A process as in Claim 1,wherein the displacement arc is held constant over the entire long succession of foil layers from which the coil is formed.

3. A process as in Claim 2, wherein the displacement arc is held constant over an initial large number of foil layers, until the angular displacement velocity has fallen to about 1/2 - 1/3 of its initial value, whereupon the angular displacement velocity is restored approximately to its initial value and the displacement arc is again held constant, repeating the restoration sequence over a number of successive stages.

4. A process as in Claim 3, wherein the displacement velocity is restored every time the displacement tool completes one cycle of its movements.

5. Equipment for carrying out the process as in any one of Claims 1 to 4, having a displacement tool, driving means for the displacement tool, a controller for the said displacement tool driving means, and a winding station which is fitted with a monitor which emits a control signal based on the number of foil layers and/or the thickness of the coil, while the controller for the displacement tool driving means maintains a constant displacement arc with optional feedback.

6. A process for winding a flattened plastics foil ribbon, substantially as hereinbefore described with reference to the accompanying drawings.

7. Equipment for carrying out the process as in Claim 6, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.