WO1995003929A1

WO1995003929A1 - Plant for the production of a multi-layer heat shrinkable polymeric film

Info

Publication number: WO1995003929A1
Application number: PCT/EP1994/001236
Authority: WO
Inventors: Guido Landoni
Original assignee: Soten S.R.L.
Priority date: 1993-08-02
Filing date: 1994-04-21
Publication date: 1995-02-09
Also published as: TW255856B; ITMI931758A1; IT1264550B1; ITMI931758A0; AU6646494A

Abstract

A description is given of a plant for the production of a multi-layer heat shrinkable polymeric film, based on the twin-bubble co-extrusion technique, said plant differing from those already known as to a new multi-layer extrusion die and as to the modifications to and improvements in the steps of: tubular film thickness adjustment and cooling (3, 4); cooled tubular film after-heating (62, 63); film final cooling (10) which, together with a new extrusion die, improve the plant output and the quality of the film produced thereby.

Description

PLANT FOR THE PRODUCTION OF A MULTI-LAYER HEAT SHRINKABLE POLYMERIC FILM Field of the nvention

Object of the invention is a plant for the production of a multi- layer heat shrinkable polymeric film, based on a twin-bubble co- extrusion technique, said plant differing from those already known as to a new multi-layer extrusion die and as to the modifications to and improvements in the steps of

- tubular film thickness adjustment and cooling; - cooled tubular film after-heating;

- film final cooling which, together with a new extrusion die, improve the plant output and the quality of the film produced thereby.

The term "multi-layer" is used herein to mean also a film (and relevant tubular film) consisting of two or more layers made of the same polymeric material.

Prior art

Plants for the production of single- or multi-layer heat shrinkable films, based on the twin-bubble technique, are already known. The relevant procedure is based on the following steps:

- the polymeric material (polypropylene or an equivalent polymeric material, added with resins and/or other additives securing the desired mechanical, chemical and physical properties) is hot extruded and leaves the die as a tubular film consisting of one or more superimposed layers, made of the same polymeric material or of two or more different polymeric materials;

- the extruded tubular film is hot adjusted as to thickness and immediately cooled;

- the cold tubular film is fed to an after-heating system (e.g. consisting at least of an IR-ray oven) , where it is heated to a temperature close to softening;

- the hot tubular film is stretched in the longitudinal direction and, at the same time, inflated e.g. with compressed air: therefore, the tubular film is simultaneously stretched in two orthogonal directions to obtain an adequate orientation of molecules;

- once the film has undergone double hot stretching, it is rapidly cooled by cold air jets which stabilize the inner orientation of molecules;

- the stabilized tubular film is cut in the longitudinal direction and the film obtained is wound round reels.

The production plants based on the "twin-bubble" technique are well known: consequently, no detailed description thereof is given herein. As known, the extent of the crystallization phenomena that take place in the polymeric material (in particular, of the polyolefin type) during the production process, is a critical factor of said technique (not described here being already known) , which can remarkably affect plant productivity and/or the quality of the final product. In fact, a too high crystallinity of the polymeric material negatively affects the mechanical properties (e.g. tensile strength and heat shrinking) of the intermediate products (film adjusted as to thickness, heated film, stretched film) and of the final product (the film). With a higher polymeric material crystallinity, the tubular film and/or film tearing tendency also increases (which causes a decrease in the production process yield) and/or the film heat shrinking capacity decreases (which results in a lower quality and, consequently, in a lower commercial value of the final product) . It follows that the more amorphous the polymeric material during the production process, the better plant productivity and final product quality: in fact, the more amorphous the polymeric material, the more uniform its softening point and, therefore, the simpler the control of its temperature during the whole production process, which allows the obtainment of a better quality film.

A markedly amorphous material can be obtained only through an as accurate as possible control of the cooling system enabling a rapid and uniform decrease in the temperature of the extruded tubular film and of the stretched film (and of the single layers thereof) and/or of the after-heating system enabling a uniform rise in the temperature of the tubular film (and of the single layers thereof) to the value required.

Said control is simpler and more effective if the tubular film and the layers thereof have an extremely precise and uniform thickness throughout the circumference and length of same. The die being an object of the present invention allows the production of a multi-layer tubular film whose layers have an extremely precise and uniform thickness. Objects of the present invention are, therefore, a new extrusion die as well as the modifications to and improvements on an already known production plant based on the twin-bubble co-extrusion technique. Said findings allow a drastic reduction in or, at least, a more accurate control of the degree of crystallization of the polymeric material during the whole production process, by maintaining the polymeric material as amorphous as possible. The efficiency of the production process is thus improved. Furthermore the film obtained is of higher quality and/or has more uniform mechanical properties both within each lot and in all the lots produced. Summary An object of the present invention is a plant for the production of a multi-layer heat shrinkable polymeric film, based on the twin- bubble co-extrusion technique, said plant differing from those already known as to the following operating characteristics:

- the extrusion die, multi-layer type, can produce a multi-layer tubular film consisting of a number of particularly homogeneous and uniform layers;

- the thickness adjusting unit is located very close to the extrusion die outlet and the distance between the die and the thickness adjusting unit can be adjusted; - the thickness adjusting unit outer ring is integral with the extruded tubular film cooling system and its rotary motion is uniform;

- the first and the second ovens of the system after-heating the tubular film from the thickness adjusting unit house several probes for the measurement of the tubular film surface temperature and the control of the intensity of the heating elements inside the_. first and second ovens - in accordance with the surface temperature measured;

- the stretched tubular film cooling system atomizes water into the aforesaid cold air jets.

In a preferred embodiment of the present invention, the second oven consists of two superimposed and communicating chambers of different diameter, wherein the distance between the oven wall and the tubular film wall contained therein is substantially the same. Description of the drawings

The present invention will now be described in greater detail, with reference to an embodiment illustrated in the drawings, to which the present invention is not intended to be confined. In the drawings, Fig. 1 is a simplified block diagram of a "twin-bubble" production plant;

Fig. 2 is a schematic view of a " twin-bubble" production plant including an after-heating system as modified according to the present invention; Fig. 3 is a schematic view of thickness adjusting unit and cooling system (3 , 4) of Fig. 2; Fig. 4 is a longitudinal sectional view of extrusion die 1 of Fig. l ;

Fig. 5 is a cross-sectional view - taken along plane AA of Fig. - of extrusion die of Fig. 4. In the figures enclosed herewith, only the elements having a meaning for the present invention are marked with numbers. Detailed description of the invention

Fig. 1 illustrates a simplified block diagram of an already known plant for the production of polymeric film, based on the twin-bubble co-extrusion technique.

The polymeric material is fed to die 1. The melted polymer leaves the die as a substantially cylindrical tubular film 2, consisting of one or more superimposed layers of the same polymeric material or of two or more polymeric materials. The thickness of tubular film 2 is set by unit 3 and the film is rapidly cooled by the first cooling system 4 (only a sectional view thereof is given in the figure to illustrate unit 3) consisting of an annular chamber containing water (or an equivalent liquid coolant; the liquid coolant referred to hereinafter will exclusively be water) . A uniform water film is caused to adhere to the outer surface of tubular film 2 to increase its cooling rate and facilitate its flowing to unit 3. Tubular film 2 is flattened by wringing apparatus 5, where the water film is removed, fed to after-heating system 6 (consisting of one or more ovens) where it is heated to a temperature close to softening, stretched in the longitudinal direction and, at the same time. inflated, e.g. with compressed air. In the figure, number 8 represents tubular film 2 being stretched in the longitudinal direction and being inflated. After double hot stretching, stretched tubular film 9 is rapidly cooled by a second cooling system 10 (preferably by cold air jets) and cut in the longitudinal direction by slitter 13, not shown in detail in the figure. The film obtained is wound round reels 12 or equivalent means. Figure 2 is a schematic view of a "twin-bubble" production plant including after-heating system 6 as modified according to the present invention. Further modifications and improvements, not shown in Fig. 2, will be illustrated hereinafter with the aid of the following figures. Figure 2 shows extrusion die 1 and tubular film 2 that, after leaving thickness adjusting unit and cooling system (3, 4), is fed to wringing apparatus 5 where the water film is removed and tubular film 2 is flattened. Tubular film 2 is fed through pull rolls or equivalent equipment to after-heating system 6 which, in a plant of known type, consists of two ovens 60, 61. Flattened tubular film 2 is heated in oven 60 to be made "softer", then inflated with compressed air (or other fluid at equivalent pressure) blown, as already known, at the basis of swollen area 8 of tubular film 2; the second oven 61 - not shown in detail in the figure - further heats the tubular film leaving oven 60 up to a temperature close to softening, which is the most favourable for the tubular film transversal stretching by compressed air blowing. At the same time, the different rotation speed of at least one pull- rolls couple (not shown in the figure) downstream of oven 60 and of pull-rolls couple 14 of slitter 13 provides longitudinal stretching of swollen tubular film 8.

Double hot stretching turns out well subject to as uniform and homogeneous as possible thickness of the tubular film (and of the layers forming same), crystalline content, if any, of the tubular film and/or single layers material, heat emission from the ovens heating elements, and temperature distribution along the tubular film wall section. Should one or more of the aforesaid factors fail to be entirely homogeneous, the response of the heated tubular film to the mechanical stresses caused by double stretching would not be uniform, with consequent risks of tubular film tearing and/or worsening of the film characteristics caused by a wrong or incomplete orientation of the film molecules.

The techniques adopted under the present invention to meet the aforementioned uniformity and homogeneity requirements are illustrated below. The die already known was redesigned: the new die 1 - illustrated in figs. 4 and 5 " can produce multi-layer tubular film 2, whose layers have precise and uniform thickness. Furthermore, it allows the type of polymeric material forming each layer of tubular film 2 to be easily modified. Unit 3 and cooling system 4 (illustrated in Fig. 3) are located as close as possible to the outlet of die 1: extruded tubular film 2, being hotter hence more ductile, can be adjusted as to thickness with greater precision and lower power absorption and can be more rapidly cooled, thus preventing or at least hindering the crystallization process of the polymeric material (or materials). On a preferred embodiment, the distance between the outlet of the die 1 and the cooling system 4 is from 10 to 30 cm. For a more uniform cooling of tubular film 2, cooling system 4 (consisting of a water-filled chamber integral with the outer ring of unit 3) is caused to rotate at constant speed around the tubular film itself. The inner surface of the outer ring of unit 3 houses at least two superimposed circular slots (35 of Fig. 3) . allowing water to flow from the chamber with formation of a uniform film along the outer surface of tubular film 2. Constant levels in cooling system 4 are maintained by addition of make-up water; this secures a constant pressure of the water flowing from slots 35- "

Furthermore, the distance between die 1 outlet and thickness adjusting unit and cooling systems 3. 4 can be modified (by known mechanical means) depending on the characteristics of the polymeric material and/or the plant capacity to optimize the operation of same.

Tubular film 2 is made "softer" in oven 60 and further heated in oven 61 (not shown in Figure 2) to a temperature close to softening, which is most favourable to double hot stretching. As shown in Fig. 2, the tubular film made "softer" in oven 60 has a narrow diameter, which rapidly increases to the final value as soon as the polymeric material of the tubular film reaches the optimal temperature. Therefore, oven 61 was implemented in two contiguous and communicating cylindrical sections 62, 63 having a different diameter, wherein the distance between the oven heating elements and the wall of tubular film 2 is always the same. This favours maximum (or at least uniform) energy absorption by tubular film 2 in every area of oven 61; at the same time, air convective movements inside oven 61 - which might negatively affect the uniform heating of tubular film 2 - are avoided.

Furthermore, to secure a homogeneous heating of the material of tubular film 2 and, therefore, a uniform deformation during double hot stretching, the external temperature of tubular film 2 is measured in significant points by several probes (already known) located inside the ovens (in particular in oven 61) controlling, according to a substantially linear law (individually or groupwise) the heating elements inside ovens 60 and 61 and, in particular, the elements inside oven 61. According to a particularly advantageous embodiment for multi-layer tubular films, the heating elements are always kept live independently of the oven operating conditions, which secures continuous heating of the tubular film wall by radiation (as well as by conduction). As known, in a tubular film wall section, heating by radiation and heating by conduction are distributed according to complementary curves (one being concave and one being convex) . By suitably adjusting the amount of heat received by the tubular film wall by radiation, the resulting curve can be "flattened" and the temperature of the whole tubular film section becomes uniform. It follows that the layers forming the tubular film wall are deformed uniformly by the mechanical stresses caused by double hot stretching.

The heating elements preferably consist of IR radiation panels, whose emission spectrum shows a peak substantially coinciding with the peak of IR radiations absorption by the polymeric materials forming the wall (or the various wall layers) of tubular film 2. The highest efficiency is thus secured, the IR radiations being almost completely absorbed by the tubular film without any overheating of the surrounding air. Finally, water atomization into air jets from cooling system 10 provides a more rapid cooling of stretched tubular film 8, which increases the plant production rate. Furthermore, the mechanical properties of the film thus obtained would be better by 20% in respect of those of a film being cooled exclusively by air jets. Fig. 3 is a schematic representation of cooling system 4 (consisting of a water-filled annular chamber integral with the outer ring 30 of unit 3) which is caused to rotate at constant speed (as already known) around the tubular film itself (not shown in Fig. 3)- In the embodiment illustrated in the figure, outer ring 30 bearing chamber l is driven by motor 33 through pinion 32 engaged in crown gear 3 _* The inner surface of outer ring 30 of unit 3 houses two superimposed circular slots (35 of Fig. 3) connected to chamber 4 through passages 36 to allow water regularly to flow from the chamber for the formation of a uniform film along the outer surface of tubular film 2. The presence of at least two slots 35 secures the formation of a uniform water film even if one or more passages 36 is/are at least partially clogged with foreign matter and/or impurities of the chamber make-up water. For the sake of simplicity. Fig. 3 does not show body 4l (represented in Fig. 4), substantially cylindrical and preferably water cooled, located inside central hole 3 of outer ring 30. Said body and said outer ring 30 constitute thickness adjusting unit 3- A particularly accurate surface finishing of 4l improves tubular film 2 sliding during cooling and reduces body wear. Figs. 4 and 5 are, respectively, a longitudinal sectional view of extrusion die 1 of Fig. 1 and a cross-sectional view, taken along plane AA, of said extrusion die of Fig. 4.

Die 1, multiple screw type, consists of body 42 housing channels 43, 44, and 45, which connect the feed points of each polymeric material meant to form each layer of tubular film 2 to the corresponding extrusion screws 46, 47, 48; of body 49 including said extrusion screws 46, 47, and 48 and unit 50 forming tubular film 2 by compression of the layers leaving extrusion screws 46, 47, and 48. Extrusion die 1 differs from the dies already known in that i) body 42 consists of three superimposed elements 51, 52, and 53 fastened together and to body 49 by several bolts or equivalent removable fasteners. ii) as shown by Fig. 5 - which represents a cross-sectional view, taken along plane AA, of element " channels 44 and 45 (which are connected with intermediate extrusion screw 47 and external extrusion screw 48, respectively) are symmetrical to central channel 43, which is connected with internal extrusion screw 46. Said superimposed elements 1, 52, and 53 can be rotated at will in respect of each other and/or in respect of second body 49 by predetermined degrees (multiple of 30° in the example shown in Fig. 5, where each of said screws 46 to 48 includes 12 extrusion channels) , thus allowing the feed points of each material meant to form each layer of tubular film 2 to be every time in a preferred and/or more advantageous position. In particular, a film with external layers consisting of the same polymeric material can be easily obtained by lining up the feed points of internal extrusion screw 46 and of external extrusion screw 48, and feeding them simultaneously (e.g. through element 5 of Fig. 4) with the same polymeric material. iii) each of extrusion screws 46, 47, and 48 has a section and a gap in respect of the corresponding external spindle which are reduced by 10% to 30% (preferably by 12% to 18%) in respect of those of the extrusion dies already known. It follows that - the volume of extruded polymeric material in the unit of time being the same and the screws inclination being unchanged - the length of said screws increases by 10-15% . A slower and more uniform passage of the polymeric material through the gap between each screw and the corresponding spindle is obtained : this secures a more uniform distribution of the polymeric material along the extrusion screw peripheral area, which provides a better uniformity and homogeneity of the layer of tubular film 2 extruded by each screw and , consequently, of tubular film 2.

TABLE 1

PRODUCER SPINDLE DIAMETER SCREW LENGTH GAP nun nun nun

S0TEN 220 220 0.6

ALPINE 200 190 0.7

GHI0LDI 200 190 0.8

On Table 1, some characteristics of the external extrusion screw 48 of an extrusion die according to the invention (labelled "S0TEN") is compared with those of two extrusion dies of known type.

Numerous modifications and improvements suggested by normal practice and by the natural evolution of the technique may be effected by those skilled in the art without departing from the scope of this invention, which contemplates a plant for the production of a multi- layer heat shrinkable film with low crystalline content.

Claims

CLAIMS 1. Plant for the production of a multi-layer heat shrinkable polymeric film, based on the twin-bubble co-extrusion technique, including in the order - extrusion die (1) ; - thickness adjusting unit and cooling system (3, 4) of tubular film (2) leaving said die (1), said cooling system (4) being integral with outer ring (30) of said thickness adjusting unit (3) and including an annular chamber containing a liquid coolant; - after-heating system (6) of said tubular film (2) adjusted as to thickness, consisting of at least a first (60) and a second oven (61), each of said ovens (60, 61) including at least a number of heating elements; - unit for the simultaneous stretching, in the longitudinal and transversal directions, of said tubular film (2) heated by said after-heating system (6); - cooling system (10) of said stretched tubular film (9) for the supply of a number of cold air jets; - slitting unit (13) for cutting said stretched and cooled tubular film (9) in the logitudinal direction for the obtainment of said polymeric film (11); and - reels (12) for the winding of said polymeric film (11), wherein said extrusion die, multi-layer type, can produce a multi-layer tubular film (2) consisting of a number of particularly homogeneous and uniform layers; - said thickness adjusting unit (3) is located very close to the outlet of extrusion die (1) and the distance between die (1) and said thickness adjusting unit (3) can be adjusted; - said outer ring (30) of thickness adjusting unit (30) is integral with said cooling system (4) of said extruded tubular film_. (2) and its rotary motion is uniform; - said first and second ovens (60, 61) of after-heating system (6) of said tubular film (2) adjusted as to thickness house several probes for the measurement of the surface temperature of tubular film (2) and the control of the intensity of the heating elements inside the first and second ovens (60, 61) - in accordance with the surface temperature measured; - cooling system (10) of said stretched tubular film (9) atomizes water into the aforesaid cold air jets. 2. The plant according to claim 1, wherein said distance between said die (1) and said thickness adjusting unit (3) is from 0 to 30 cm. 3> The plant according to claim 1, wherein the inner surface of said outer ring (30) of unit 3 houses at least two parallel circular slots (35) connected, through passages (36), to said annular chamber of said cooling system (4) of said extruded tubular film (2). 4. The plant according to claim 1, wherein said heating elements of said after-heating system (6) of said tubular film adjusted as to thickness (2) are IR radiation sources, the intensity of said IR radiation emitted by said radiation sources being controlled by several probes according to a linear law. 5- The plant according to claim 4, wherein said IR radiation sources are always kept switch on. 6. The plant according to claim 4, wherein said IR radiation sources show a peak of emission of said IR radiation and said emission peak coincides with the peak of said IR radiation absorption by the polymeric material forming said film (11). 7- The plant according to claim 1, wherein said second oven (6l) of after-heating system (6) of said tubular film (2) adjusted as to thickness consists of two superimposed and communicating cylindrical sections (62, 63) having a different diameter, and the distance between the inner wall of each of said sections (62, 63) of said second oven (61) and the external wall of said tubular film (2) contained therein is substantially constant in both sections (62, 63). 8. The plant according to claim 1, wherein multi-layer die (1) consists of a first body (42) housing channels 43, 44, and 45, which connect the feed points of each polymeric material meant to form each layer of tubular film 2 to the corresponding extrusion screws (46, 47, 48); of a second body (49) including said extrusion screws (46, 47, and 48) and unit 50 forming tubular film (2), wherein, in said extrusion die (1) : said channels (44, ) connecting said feed points to said intermediate extrusion screw (47) and said external extrusion screw (48) , respectively, are symmetrical to said central channel (43) connecting said internal extrusion screw (46) to the corresponding feed point; - said first body (42) consists of three superimposed elements (51, 52, 53) fastened together and to said second body (49) by several equivalent removable fasteners; - said feed points of each of said polymeric materials forming each of said layers of said tubular film (2) are positioned by rotating said superimposed elements (51, 52, 53) of said first body (42) in respect of each other and/or in respect of body (49) by predetermined degrees; - the distance between each of said extrusion screws (46, 47, 48) and the corresponding external spindle is reduced by 10% to 30% in respect of that of the extrusion dies already known; - the section of each of said extrusion screws (46, 47, 48) is reduced by 10% to 30% in respect of that of the dies already known; - the length of said extrusion screws is greater by 10% to 15% than that of the extrusion dies already known. 9- The plant according to claim 8, wherein - the distance between each of said extrusion screws (46, 47, 48) and the corresponding external spindle is lower by 12% to 18% than that of the extrusion dies already known; - the section of each of said extrusion screws (46, 47, 48) is reduced by 12% to 18% in respect of that of the dies already known. 10. The plant according to claim 8, wherein the distance between each of said extrusion screws (46, 47, 48) and the corresponding external spindle is 0.6 mm and the length of said extrusion screws is 220 mm. 11. The plant according to claim 8, wherein said superimposed elements (51. 52, 53) of said first body (42) are rotated in respect of each other and in respect of said second body (49) by multiples of 30° . 12. The plant according to claim 8 for the production of heat shrinkable polymeric film, with external layers consisting of the same polymeric material , wherein said feed points of said internal extrusion screw (46) and of said external extrusion screw (48) are lined up and are simultaneously fed with the same polymeric material .