Method and apparatus for pinning the edges of an extruded mass

Abstract

A method for pinning the edges of an extruded polymeric mass and an edge-pinning group are described.

Claims

1. A method for pinning edges of an extruded polymeric mass (10), comprising the following steps: extruding a polymeric mass (10) from an extrusion die (12); pre-cooling in at least two pre-cooling steps, wherein: two areas are defined in the extruded polymeric mass (10) along each of external edges of the polymeric mass, having a width respectively equal to (a) and (b) (15 and 15) and a length equal to the length of the extruded mass (10), wherein the area having the width (a) (15), which is a variable width, is arranged flush with an edge of the extruded mass (10), and wherein the area having the width (b) (15) is adjacent to the area having the width (a) (15), internally with respect to the edge of the extruded mass (10), and a third area is defined having a width equal to (c), (16), and a length equal to the length of the extruded mass (10), adjacent to the area having the width (b) (15), internally with respect to the edge of the extruded mass (10); wherein in a first pre-cooling step of the at least two pre-cooling steps, the area (16) having a width (c) of the polymeric mass (10) is pre-cooled by action of an air flow, before the extruded mass (10) is cast and comes into contact with a surface of a rotating cylinder (11) positioned at a distance (d) from the extrusion die (12); wherein in a second subsequent pre-cooling step of the at least two pre-cooling steps, the area (15) having a width (b) is pre-cooled by action of a second air flow, before the extruded mass (10) is cast and comes into contact with the surface of the rotating cylinder (11) positioned at the distance d from the extrusion die (12); and electrostatically charging, in at least two electrostatic charging steps, the extruded polymeric mass (10), wherein: in a first electrostatic charging step of the at least two electrostatic charging steps, the area having the width (b) (15), after the pre-cooling, is electrostatically charged to favor its adhesion to the surface of the rotating cylinder (11) and ensure a desired heat exchange between the area having the width (b) and the rotating cylinder; and in a second electrostatic charging step of the at least two electrostatic charging steps, the area having a width (a) (15), arranged flush with the edges of the extruded mass (10), externally with respect to the area having a width (b) (15), treated in the first electrostatic charging step, is electrostatically charged.

2. The method according to claim 1, wherein the extruded polymeric mass (10) is a polyethylene film having a thickness ranging from 6 microns to 20 microns, an extrusion rate ranging from 500 m/min to over 800 m/min, a value of the width (a) (15) being variable within a range of 0 to 10 mm, a value of the width (b) (15) being included within a range of 15 to 25 mm, and a value of the width (c) (16) being variable within a range of 20 to 50 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] The structural and functional characteristics of the present invention and its advantages with respect to the known art will appear even more evident from the following description, referring to the attached schematic drawings, which show an embodiment example of the invention itself. In the drawings:

[0084] FIGS. 1-6 are schematic representations relating to cast technology solutions with and without edge-pinning devices, according to the state of the art;

[0085] FIG. 7 is a schematic representation of an edge-pinning group according to the present invention;

[0086] FIG. 8a is a plan view of an embodiment of an edge-pinning group according to the present invention;

[0087] FIG. 8b is a side view of an edge-pinning group according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0088] With reference to the figures, in particular, FIG. 7 schematically shows the edge-pinning group 13 object of the present invention, consisting of a pair of supports 18 and 18, each support 18 and 18 bearing at least one electrode 19, and a pair of nozzles 20 and 21, each of which is supplied with compressed air or with air from an independent blower.

[0089] The precise spatial positioning of the elements constituting the invention is best exemplified in the following FIGS. 8a and 8b, with particular reference to the sequence in which said elements must be present.

[0090] FIG. 8a shows in particular the spatial interaction between the various elements with reference to the center line of the extrusion surface (S), passing through or containing the extrusion direction of the machine Y and with respect to the axis Y which obviously corresponds to the axis of the extrusion line.

[0091] In particular, following the axis Y, it can be seen that the first element present is the nozzle 21, which partially pre-cools the area of extruded melt 10 which, with reference to the center line of the extrusion surface (S), corresponds to the area 16 indicated in FIG. 4, said area having a width (c).

[0092] This area of the melt, in the absence of this pre-cooling, would tend to slide towards the center of the chill-roll, generating the above-mentioned area 16 having a reduced thickness; through the action of the nozzle 21, on the other hand, the formation of this area 16 having a reduced thickness can be completely avoided, decreasing its potential by removing heat, thus prefiguring the formation of a film with a constant thickness (except of course for the area 15 affected by the neck-in).

[0093] The position of this nozzle 21 appears even clearer by looking at FIG. 8b, where it can be seen that the action of the air blow ensured by this nozzle 21 envelops the area 16 of the melt 10 before it even touches the surface (R) of the chill-roll 11 and is therefore subjected to the cooling process which relates to the whole film, and which therefore cannot be controlled sectorally.

[0094] The second element present in spatial order, again following the axis Y, is the nozzle 20, whose action allows the partial pre-cooling of the edge area of the melt 10 (said area having a width (b) 15 of the same) affected by the electrostatic discharge of the electrodes before it comes into contact with the chill-roll, allowing better control even in the case of high velocities and reduced thicknesses and thus avoiding said draw-down-resonance phenomena.

[0095] From FIG. 8a, the perfect alignment between this nozzle 20 and the support 18 in the direction Y, and the positioning of the nozzle 20 in correspondence with the area having a width (b), 15, with respect to the edge (variable) of the melt and at the center line of the extrusion surface (S), can be particularly appreciated.

[0096] FIG. 8b on the other hand shows how the action of this nozzle 20 takes place preferably (but not exclusively) on the melt 10, immediately before it comes into contact with the surface of the chill-roll 11, for the same reasons explained in the previous paragraph.

[0097] Continuing along the axis Y, the next element encountered is the support 18, bearing at least one electrode 19, which, as already indicated, is perfectly aligned with the nozzle 20; its positioning with respect to the center line of the extrusion surface (S) is therefore also identified by the area having a width (b) 15, previously mentioned.

[0098] The function of this support is, as already mentioned, that of classic edge-pinning devices, i.e. it must electrostatically charge a portion of the melt as small as possible in order to favour its adhesion to the surface of the chill-roll and consequently ensure the correct heat exchange between the two elements. This portion corresponds to the area having a width (b), 15, previously defined.

[0099] Subsequently, again following the axis Y, there is the support 18, which is preferably, but not exclusively, positioned further downstream with respect to the support 18 in the extrusion direction Y, but above all it is displaced with respect to the center line of the extrusion surface (S), in the area having a width (a) 15 towards the variable edge of the melt 10.

[0100] As indicated above, the extent of the width of the area (a) 15 is subject to even major and significant variations based on the type of polymer and the process conditions.

[0101] The action of this support 18, also equipped with at least one electrode 19, is essentially that of blocking the variable portion of melt corresponding to the area having a width (a) 15, which is thus caused to adhere to the chill-roll, avoiding the formation of the peduncle shown in FIG. 6, but which, even in its absence, (due as said to the variability of the width of the melt), in no way jeopardizes the stability of the film thanks to the action of the first support 18.

[0102] The various elements forming part of the invention can obviously be supported in the most varied ways, i.e. by means of micrometrically or electrically adjustable supports (to ensure the repeatability of positioning), just as the relative positions between the various elements can, indeed must, be able to be regulated within a range wide enough to take into account the production requirements.

[0103] The constructive forms of the relative supports, besides being able to acquire the most varied physiognomies, have not been schematized for the sake of simplicity of display and specifically because they change in shape and size.

[0104] The essential elements are obviously all the elements forming the edge-pinning system according to the present invention and the interaction between the same, fundamental for ensuring the correct functioning of the invention.

[0105] The protection scope of the present invention is therefore defined by the enclosed claims.