HK1066759A - Methods and apparatus for extruding a tubular film - Google Patents

Description

Method and apparatus for extruding a tubular film

The invention relates to a method and a device for extruding a tubular thin layer of polymeric material, wherein a material is used which is contained in circumferentially equal spiral grooves extending in a plane or cone, which are formed in one or more flat or conical die part surfaces and which guide the flow of the material outwards. The aim of the invention is to be able to fully exploit the particular possibilities offered by the particular arrangement of the helical grooves.

Patent documents relating to methods and devices for extrusion forming (particularly for co-extrusion forming) include: british patent GB-A-1.384.979(Farrell), European patent EP-A-0.626.247(Smith), WO-A-00/07801(Neubauer) and WO-A-98/002834(PlanetｃA et al).

Figure 1 of the drawings is based on the above reference. This figure shows that annular extrusion (single extrusion or co-extrusion) can use in-plane or conically extending grooves that equalize peripheral material flow, which has a number of advantages over more conventional systems where the uniformity of the periphery is established by cylindrically extending grooves (i.e., grooves formed in the surface of one or more cylindrical die parts).

Thus, when the polymeric material is extruded outwardly, the periphery is equalized by the grooves at the same time, and the space in the die can be fully utilized. This means that the mould can be very compact, the importance of which is not only the saving of steel material but also the easy assembly and disassembly, while a uniform temperature can be reached quickly and safely. Furthermore, there is the great advantage for the cleaning operation that most of the channels are formed between the clamped-together mould parts and are easily accessible after simple disassembly.

The peripheral distribution (the original grooves formed in the cylindrical surface) is accomplished by the spiral grooves and the space for the overflow between the grooves, which was first described about 30 years ago. The cross-section of each spiral groove and the cross-section of the spaces between adjacent grooves in the distribution system allow for overflow, so that progressively less material flows through each groove and progressively passes over adjacent grooves, while the depth of the grooves progressively reaches zero.

A single spiral groove (extending several turns around the annular die) has been proposed which allows perfect circumferential distribution, providing a groove design and intermediate space for the overflow which is precisely adapted to the rheological characteristics of the molten polymeric material in the prevailing conditions. However, this is the principle, and in practice the polymer flow must first be divided in one way or another into several partial flows, each partial flow entering a spiral groove before and being provided with spaces for overflow between the different grooves. The greater the number of branches and grooves, the shorter the spiral portion of each groove, but in any case the design of the grooves and the space for the overflow depends essentially on the rheological characteristics of the polymeric material in the molten state.

As with most of the techniques described in the aforementioned documents, the present invention is primarily concerned with co-extrusion, although both aspects of the invention are applicable to single extrusion simultaneously. The first aspect of the invention relates to covering an intermediate foil with a surface layer which has a relatively high melt flow index (and thus a relatively low melt viscosity) compared to the intermediate foil. This feature is very important for co-extrusion but as will be explained below, prior art type dies are not suitable for this application.

The second aspect of the invention consists in a completely new concept for the inventors, namely the extrusion of a thin layer of thermoplastic polymer through orifices located at the periphery of the die, a system offering new possibilities of production of this thin layer. Extrusion from the periphery of an annular die is used to make the food construct, whereas in the above-mentioned WO 00/07801(Neubauer) a tube is made by using a die plate inside the cross-section of the mould cavity (e.g. between moving corrugated bands). However, it is not used to make blown tubular films.

A third aspect of the invention relates to the actual adjustment of the overflow between the spiral grooves. With the techniques known today, large and expensive die parts must be replaced to adapt one and the same die to different polymers with quite different rheological properties, or expensive feedback systems can be used alternately to compensate for the insufficient function of the helical groove equalization. The feedback system applies different amounts of cooling air on the periphery of the laminae (which are blown), or sets different temperatures at different circumferential positions of the discharge fitting of the mould, all automatically controlled by the automatic thickness readings in the array.

In contrast to this expensive prior art system, the third aspect of the invention aims to propose a relatively inexpensive solution, using the geometrical arrangement of helical grooves formed in a flat or conical surface, for embedding means to allow relatively simple adjustment of the overflow, as will be explained below.

Returning now to the object of the first aspect of the invention, i.e. to produce a laminate having a surface layer with a relatively high melt flow index, a relatively important example is the coating of High Molecular Weight High Density Polyethylene (HMWHDPE) with Linear Low Density Polyethylene (LLDPE) or another ethylene copolymer (melt flow index of 0.5-1 or higher) on both sides of the polyethylene (HMWHDPE) having a melt flow index (m.f.i.) of about 0.1 or lower according to ASTM D1238 condition E. HMWHDPE provides, in particular, sheet strength when oriented, while the surface layer provides improved adhesion characteristics and/or improved gloss and/or increased coefficient of friction. The surface layer is actually made of a copolymer (having a higher melt flow index) because copolymers having a higher melt flow index are more readily available on the market, achieve higher gloss and make welding easier.

HMWHDPE is tubular coextruded with a surface layer of a copolymer having a higher melt flow index, generally made by using an annular coextrusion die, wherein circumferential equality is established by a system of spiral grooves (in overflow) extending in a geometric arrangement along a cylindrical surface. However, the prior art dies use a flat or conical arrangement of helical grooves (with several advantages as described earlier), which is for example very unsuitable for co-extrusion with HMWHDPE (melt flow index of 0.1 or lower) of ethylene copolymer (melt flow index of 0.5 or higher) (cf. ASTM D1238 condition E). The same is true for the co-extrusion of polypropylene with a high melt viscosity similar to HMWHDPE with copolymer, which in practice applies to the surface layer on the polypropylene film.

These known co-extrusion dies consist of disc-shaped or shell-shaped ("bowl-shaped") elements which are nested within one another in a "bowl" or shell (consisting of several parts screwed together), between the cylindrical or conical inner surface of this "bowl" and the outward facing surface of the nested elements (see fig. 1). The bonding of the materials occurs continuously (sequentially). A surface component first combines with the component that will become its adjacent part, and then the two components advance together a relatively long distance along the outward facing surfaces of an element nested within one another before they combine with the co-extruded third component. If more than 3 components are required in the final laminate, these steps are repeated, always with a relatively long distance between the locations where bonding occurs. This is required for constructional reasons. If 3 or more components are extruded and at least 2 of these components have very different melt viscosities, in the case of HMWHDPE, this means that a channel of more than 5-10 cm or a longer length extends through the die, the viscosity of the component in contact with one surface of the channel being substantially different from the viscosity of the component in contact with the opposite surface of the channel. This combination produces a disturbed layer distribution, which may be shown as lateral stripes, for example.

The present invention is described in the prior art by a method and apparatus for extruding a tubular polymer film using circumferentially equal helical grooves formed in the surface of one or more flat or conical die members and extending in a plane or cone. More specifically, the invention relates to a method and an extrusion die for forming a tubular film by extrusion of at least one thermoplastic polymer material A, the annular extrusion die used having at least one inlet for the material A and a discharge channel, ending in an annular discharge orifice, each inlet being closer to the axis of the annular die than the discharge orifice, while the material a in the molten state flows outwardly towards the discharge orifice, and in the process the shaping of the flow of material a is established by the arrangement of mould parts having flat or conical surfaces, the mould parts are clamped together so that the surfaces are provided with grooves to form channels, to equalize the flow over the periphery of the discharge orifice, the flow between each inlet and the discharge opening is divided into a plurality of substantially helical partial flows, passing at least a part of each channel and having a space for overflow between said parts.

With respect to the present process, the first aspect of the invention is defined as the co-extrusion of at least one thermoplastic polymeric material a with at least two thermoplastic polymeric materials B and C (having a melt flow index at least twice that of material a) (the test conditions of which will be described below), material B being applied to one side of material a and material C being applied to the other side. Thus at least the co-extrusion of material a follows the above-described method and is characterized in that the materials a and B are joined at the same location as or in close proximity to the location where the materials a and C are joined, the material a flowing at least immediately outwardly before joining with the materials B and C, while the materials B and C flow immediately towards each other before joining.

The coextrusion dies used to accomplish this process have similar characteristics, but their use is certainly not limited to coextrusion of ingredients having a specific relationship between rheological properties.

Circumferential equalization of polymeric materials B and C is typically (but not necessarily) accomplished in the same manner as the circumferential equalization of material a. Good uniformity of the surface composition is not always necessary, however, as each composition occupies less than 15% or less than 10% of the volume of the structure and thus is simplified and less efficient, peripheral uniformity of well known devices can also be applied.

The melt flow index can be referenced to ASTM standard D1238-90 b. If the complete melting range of each polymeric material is below 140 ℃, condition E (i.e., temperature of 190 ℃ and load of 2.16 kg) should be used. Provided that the highest limit of the melting range of any one polymeric material is from 140 ℃ to less than 180 ℃, condition L (i.e., temperature of 230 ℃ and load of 2.16 kg) should be used. If the highest limit of the melting range of any one polymeric material is from 180 ℃ to 235 ℃, condition W (i.e., temperature of 285 ℃ and load of 2.16 kg) should be used. It is not actually believed that the upper limit of any polymeric material would exceed 235 c.

The first aspect of the invention is particularly useful for the coextrusion of at least one intermediate layer consisting of a polyethylene-based material having a melt flow index of 1 or less according to the above condition E, said intermediate layer consisting of at least 50% of coextruded lamellae and a surface layer having a higher melt flow index, as described above.

The first aspect of the invention is also particularly useful for the coextrusion of at least one intermediate layer consisting of a polypropylene-based material having a melt flow index of 0.6 or less according to the above condition L, said intermediate layer consisting of at least 50% of coextruded lamellae and a surface layer having a higher melt flow index, as described above.

The condition that the flow split or channel must be substantially helical does not limit the invention to a regular helical form, for example in the form of a two or three dimensional curve defined by a point moving at a fixed angular velocity around another point lying in a plane or around an axis lying in space while moving at a fixed linear velocity and (if in a 3 dimensional condition) its projection on the axis moving continuously at the same time. Although this particular regular pattern is generally well suited to channel configurations, it is not necessary for a preferred equality. Thus, as an example, if there are many partial flows (e.g., 16 or more), the "generally helical" portion of each partial flow may be very short at a small angle and thus may be a linear shape, the angle being the angle made by the tangent to the circle intersecting the short linear portion and being made by a point rotating around the mold axis. Another example of an irregular but generally helical form that may be suitable for the configuration of the channels is an offset form in which a first portion of a generally helical flow split is followed by a channel that is annular around the axis of the die, and then the channel curves to cause the first mentioned flow split to project into a "track" before the flow split meets the adjacent flow split and further away from the axis of the die. The second portion of the channel continues to be circular, and then the channel curves outward to a third "track" and so on, again before the two streams meet each other. As will be explained later, this staggered form is advantageous, for example, in combination with special means for adjusting the overflow.

The first aspect of the invention is not limited to co-extrusion of three polymeric materials. It may further have a composition as described in claims 17 and 18, so that the co-extruded die may have more than 3 sets of channels, as described in claims 52 and 53.

The partial flows can extend substantially in a planar manner (which can be used in all three aspects of the invention), or they can extend in a geometric arrangement along an annular conical surface. Due to the construction, a right circular conical surface is preferred, i.e. the generatrix (generatrix) of which is a straight line but may also be curved, for example like a parabola (whose axis is parallel to the axis of the mould but spaced apart from this axis). In either case, the plane of tangency of the conical surface preferably makes an angle with the axis of the die of at least 20 degrees, more preferably 45 degrees, covering at least most of the downstream portion of the surface. In the case of a right circular conical surface, the angle is the angle between the straight generatrix and the axis.

As mentioned above, the flow of material a is divided into several partial flows before being equally circumferentially equalized. It should be noted that in the case of co-extrusion according to the first aspect of the invention the symbol a refers to a polymeric material having a lower melt flow index, whereas in the case of co-extrusion according to the second and third aspects of the invention the claims refer to one component only (although it is not limited to a single extrusion but comprises co-extrusion at the same time) and this component is referred to as a. The following description relates to all three aspects of the invention.

The split is preferably split by the system of U.S. patent No. 4,403,934 (Rasmussen et al), which is considered a labyrinth split, but some splitting may be accomplished by other systems before the labyrinth split. The labyrinth separation is readily apparent from fig. 3 and 9, fig. 9 representing a ring segment developed through three flat, disc-shaped die parts. By labyrinth separation is meant that a main flow is branched into two substantially annular, arcuate, equal-length and mutually symmetrical first branch flows, which substantially occupy 50% of the circumference of the corresponding circle, after which each first branch flow is branched in a similar manner into two substantially annular arcuate second branch flows, of which 4 total second branch flows also substantially occupy 50% of the circumference of the corresponding circle. In the same way, 8 or 16 or 32 or even 64 partial flows are formed by successive division. Some minor modifications to the annular arrangement are possible, for example four second substreams may constitute four sides of an octagon, eight third substreams may constitute eight sides of a 16-sided regular polygon, etc.

Labyrinth separation is first described in us patent No. 2,820,249 (Colombo), which relates to extrusion coating of cylindrical objects. A first description of an extruded labyrinth separation for blown sheets can be found in the above-mentioned us patent No. 4,403,934 (Rasmussen et al) and relates to the subsequent equalization of overflow by means of a helical channel.

At least a portion of the passage for labyrinth separation may be integrally formed with the passage for substantially helical flow between the flat or conical surfaces of the first die component by a groove in at least one of the pair of contacting surfaces.

This is illustrated in figure 3. Alternatively or additionally, at least in the initial phase of the labyrinth separation is established by using a second mould part with a flat or conical surface, which is clamped together with the first mould part, the arrangement of the passages for the initial phase of the labyrinth separation being established partly by grooves in the contact surface between the second mould part or a second part and a first part and partly by interconnecting passages through the second and/or first part. See fig. 7, 8 and 9.

In either case, it is preferred to form a relatively wide continuous cavity around the axis of the mold. This is useful for efficient application of internal cooling air for electrical connection devices.

The choice of either or both types of labyrinth separation is largely dependent on the die diameter and the preferred size of the continuous cavity around the die axis.

When any of the three aspects of the invention is used in co-extrusion, one of the co-extruded polymeric materials is susceptible to heat degradation at the temperatures required to extrude the other co-extruded material, preferably or necessarily providing thermal insulation between the die fittings that form the channel system for the two polymeric materials. One example is the coating on both sides of HMWHDPE with ethylene/vinyl acetate copolymer, having a melt flow index of less than 1 according to the ASTM test described above. This is conveniently accomplished using a co-extrusion die as shown in fig. 2a and 3, but because the convenient rapid extrusion of HMWHDPE requires an extrusion temperature of about 200 ℃ or higher, and given the tendency of the copolymer to degrade as it passes through the die above about 180 ℃, a suitable thermal insulation needs to be made in the die between the two polymeric materials. Thus, with reference to fig. 2a, the disk-shaped die detail 7a should be divided into two half disk-shaped details and thermally insulated from each other, while likewise the disk-shaped die detail 7b should be divided into two half disk-shaped details and thermally insulated from each other. The thermal insulation is preferably established by means of an air volume, i.e. one or both of the fitting halves forming 7a or 7b are provided with ribs, recesses, handles or similar means, which are precisely machined so that the fittings can be clamped firmly and positively together. There must be an effective seal at the boundary adjacent to a polymer flow to avoid material leaking between the two fitting halves and damaging the heat insulation. For example, the sealing portion may be a teflon or bronze ring. The flow of intermediate component a from its inlet to the point where it joins with the other components actually maintains its temperature while heat transfer between the fitting halves is minimized.

When the die components 7a and 7b are conical as shown in fig. 5, similar heat insulating portions can be arranged. When the first aspect of the invention is completed, the discharge passage may direct the combined common flow of B, A and C further outward and then axially, or the common passage may direct the common flow in a substantially axial direction immediately without further outward passage, so that the combined material flows substantially axially as it meets at the discharge orifice. The first possibility is shown in fig. 2a, 2b and 6, while the last possibility is shown in fig. 12.

A third possibility is that the vent channels direct the common flow of B, A and C perpendicular to the mold peripheral surface, as shown in fig. 4a, 4b, 6 and 7, but this possibility will be explained in more detail in the third aspect of the invention.

The embodiment shown in fig. 12, which belongs to the first aspect of the invention, is further characterized in that spiral grooves for circumferential equalization of a surface composition are formed in a cylindrical die member surface. It may also be located in two cylindrical surfaces facing each other, or these surfaces may be conical but closer to cylindrical, for example with generatrices making an angle of no more than 30 degrees with the axis. In this way it is in fact possible to make the common discharge channel cylindrical in the starting position, thus minimizing its length and the pressure drop to the discharge orifice when the material is joined. This pressure drop is important for the peripheral equality of the surface component, a low pressure drop being preferred when the melt viscosity is significantly lower than that of the intermediate component.

A second aspect of the invention is shown in figures 4a, 4b and 5 and is characterised in that the discharge channel directs the molten material perpendicularly to the peripheral surface of the mould where the discharge orifice is located and the tubular sheet is spaced from the discharge orifice and at an angle of at least 20 degrees to the axis of the mould, and a regulated overpressure is applied to the interior of the tubular sheet to form the desired diameter of the pipe while it is drawn down and solidified. Thus, it is not possible to produce a tube with a similar mould fitting, and to transport it immediately on leaving the fitting to the interior of a transport mould (for example WO 00/07801, Neubauer). According to a third aspect of the invention, when the tubular film is under normal extrusion, the tubular film leaving the die from its periphery can be blown directly by internal air maintained under an overpressure, feedback controlled by automatic registration of the diameter, while the thickness of the film is moved downwards and axially away by conventional means (driven rollers, crushable frames, etc.). However, the tubular film in the molten state is preferably spaced from the peripheral surface of the mould and is held in contact with a ring concentric with the mould, thereby reducing the angle between the axis of the mould and the direction of movement of the film and creating a friction between the ring and the film to assist in the molecular orientation of the film and to pull the film over the ring. This feature enables higher machine direction orientation than can be achieved by extrusion of a conventional blown film, which is particularly useful when the polymeric material comprises a high content of high molecular weight material, for example, at least 25% HMWHDPE (with a melt flow index of 0.1 or less (ASTM test, condition E above) or at least 25% polypropylene (with a melt flow index of 0.6 or less (ASTM test, condition L above).

For example, when thin layers are used to make cross-laminates, a higher degree of machine direction orientation ("melt orientation") associated with extrusion is important. For this purpose, the tubular lamina may be cut in a spiral manner before being laminated in a widely known manner and may be further oriented in different stages of processing, as is widely known, see european patent EP 0624126 (Rasmussen).

The second aspect of the present invention is applicable to both single extrusion and co-extrusion. In addition to the advantage of improved melt orientation due to the arrangement of the ring, the second aspect of the invention has the advantage that the passage from the circumferentially uniform termination point to the discharge orifice can be minimized in the case of co-extrusion from the point where the different polymeric materials are joined to the discharge orifice.

The ring is preferably rounded at least over a part of the surface in contact with the foil and is preferably mounted in the immediate vicinity of the discharge opening. Thermal insulation is preferably achieved from the hot mold part, whether by installing a thermal insulating material or by a support means passing through a hollow space around the center of the mold.

It is preferred to cool the ring to avoid the tubular film sticking too strongly, but this is not always necessary if the film is particularly thick. The cooling can be accomplished by circulating water or oil at a suitable temperature. If the temperature of the surface of the ring is below the lower limit of the melting temperature range of the polymeric material with which it is in contact, the thin portions of the thin layer will solidify and thus the tendency to stick can be avoided or reduced. This solidification is usually temporary, so the thin part of the thin layer melts again when it has left the ring. The person skilled in the art can decide how to adjust the optimal cooling conditions (or if cooling is always needed) for obtaining an optional constant amount without risking a production stop due to the adhesion of the thin layer to the ring. The circulation of the cooling medium is preferably achieved by leading the medium in and out through a suitable number of ducts through hollow chambers surrounding the mould axis.

By bringing this ring close to the die, the co-extrusion can be conveniently done without bonding the polymeric material inside the die, but with the polymeric material welded together when it is joined on the ring.

When making very thin layers or layers with a surface with a very high coefficient of friction at room temperature, the cooling of the ring is not sufficient to avoid too much adhesion or too high friction, a phenomenon related to the strength of the layer, while the layer passes outside the ring. In this case, the ring may be designed to be suitable for supporting the sheet on an "air pillow", i.e. pressurized air is blown onto the sheet from the inner space in the ring through one or more closely spaced fine holes in an annular array around a portion of the ring, which fine holes are directly adjacent to the sheet. The details of the construction of the ring are used to support the sheet on the air, which is not a problem for those familiar with the "air pillow" art. This air is preferably cooling air, and therefore it also acts as an effective medium for internal cooling.

The ring must be suitable for effectively achieving a peripheral equalization of the flow of compressed air before this air comes into contact with the fine holes of the annular array. The air is preferably directed from the compressor and freezer, through a (preferably multiple) conduit passing through a hollow chamber around the axis of the mold, and out of the mold through at least one other conduit connected to the interior of the thin film bubble. (the cavity around the axis of the mould must be isolated from the external environment, so that an overpressure can be maintained inside the bubble). A valve is provided at the air outlet to control the pressure in the air bubbles.

The inventor's idea is to choose the die periphery for the location of the annular discharge orifice and to turn the lamella onto the die in combination with said ring concentric to the die, which is inventive in itself and independent of the peripheral equality achieved by using a spiral groove with overflow and a special arrangement for the above-mentioned groove. Independently of the nature of the generally tubular lamina passing through the ring, a particular embodiment of the second aspect of the invention is characterised in that the discharge orifice is bounded on at least one side by a lip member which is sufficiently flexible to permit adjustment of the clearance of the orifice and to provide means for such adjustment.

It can be immediately appreciated that this adjustment is feasible and practical when the vent passage is flat near the vent hole up to the vent hole, since then the annular die is comparable to a flat die, where the overflow from the vent hole is almost always adjusted in the same way. However, some conicity may be allowed in the discharge passage even at a location before the passage meets the discharge orifice. The magnitude of the allowable taper depends on the detailed construction, but can be determined by one skilled in the art. In any event, however, a conical passage may be leveled before it meets the discharge orifice.

A third aspect of the invention is characterized in that said overflow between the partial flows can be adjusted by means of exchangeable inserts between the mould parts or by means of adjustably positioned device parts opposite the groove. These features are applicable to both single extrusion and co-extrusion, for example, which can serve the additional function of a co-extrusion die of the known type shown in fig. 1. As shown in fig. 2a, 2b, 4a, 4b, 5 and 7, and further explained in the description of fig. 2a, the replaceable insert may be an insert shim (8a) by which the distance between two channel-forming die parts can be adjusted, the shape of which prevents overflow between the channel parts, which must be prevented and which permits overflow if desired. When the flow pattern is as shown in fig. 3 (corresponding to fig. 2a), the upstream limits of the areas where overflow is desired are preferably jagged or staggered, as shown by the dashed line (16) and the associated dashed circle (16a), except for an overflow area where the flow is stationary. Thus, with this type of groove, the boundaries of the embedded shim (8a) preferably have the zigzag or staggered form.

As mentioned above, the form of the channel between the parts having the overflow may be staggered in that a first portion of the generally helical flow split follows an annular channel around the axis of the mould, whereby the channel bends to cause the first flow split to project into a "track" and further separate from the axis of the mould just before this flow split meets the adjacent flow split. This is a suitable pattern of generally helical flow to avoid "dead" areas while maximizing the utilization of the mold parts as much as possible. In this case, the downstream boundary of the embedded shim may be annular.

However, in the preferred form of the staggered spiral groove, it progressively changes from "track" to "track", from a circular form with generally radial connections therebetween to a continuous spiral form, i.e. the form is circular in one or some "tracks", then it changes to a regular spiral, with increasing inclination with respect to a circle from "track" to "track", and with decreasing length of generally radial connections.

Alternatively, the replaceable insert may be a cavity-filling insert. In the embodiment without an insert, a space for the overflow is provided, which is partially filled with the replaceable insert. The insert (8b) is shown in fig. 2a, 2b, 4a, 4b and 5.

Instead of using a replaceable insert, the overflow between the partial flows can be controlled as described by means of an adjustably positioned device fitting opposite the groove. Continuous adjustment is preferred. The device may comprise a flexible sheet in the form of a flexible, flat, generally ring, which is fixed at its inward and outward borders to a rigid mould part forming part of the channel system, or it may comprise a plate in the form of a rigid, flat, generally ring, which is hinged at its inward and outward borders through a flexible sheet in the form of a flexible, generally ring to the rigid mould part, in each case with an annular array of adjustment members on the side of the sheet or plate in the form of a flat, generally ring opposite to the flow. The flexible sheet is preferably a thin metal sheet which may be integrally formed with the rigid mold part.

This is further explained with respect to fig. 10 and 11. The device shown in this figure utilizes a rotatable plug for adjustment, although other devices such as screws or wedges could be used instead.

The present invention will now be described in further detail with reference to the accompanying drawings.

Fig. 1 is prior art. There is shown an axial view of a co-extrusion die for 5 components and based on WO 98/00283.

Fig. 2a, which is to be studied in conjunction with fig. 3, shows the axial parts marked c-d in fig. 3. It represents a specific embodiment of the invention in which each helical distribution channel system is for 3 components (which are combined in a mould) and is formed integrally with the aforementioned labyrinth partition system, and in which the channels of the system are constituted by grooves in discs clamped together. The figure further shows the discharge channel diverting the co-flow, so that the direction of extrusion becomes axial at the outlet, and shows two different inserts for adjusting the overflow between the spiral grooves.

Figure 2b (similar to figure 2a) shows a small modification to the mould shown in figure 2 a.

Fig. 3 shows three sections (denoted a-b) perpendicular to the axis (1) in fig. 2a, 2b, 4a, 4b and 6. Figure 3 shows grooves for labyrinth separation which are formed integrally with spiral grooves for equalisation. The portion shown in fig. 3 does not extend beyond the outer limit (16c) of the spiral distribution system.

Fig. 4a (which is similar to fig. 2a) represents an embodiment of the invention which, unlike the terminal portion of the through-mold passageway shown in fig. 2a, begins generally along a plane perpendicular to axis (1) and ends at the periphery of the mold. The figure also shows the inverted extruded sheet, a cooled ring immediately after it exits the die and shows a lip member of a flexible and adjustable vent.

Fig. 4b is similar in nature to fig. 4a, but shows a variation of the arrangement of the co-flow of the 3 components.

Fig. 5 is generally similar to fig. 4a, except that in fig. 5 the channel is configured as a conical surface rather than a planar surface.

FIG. 6 is similar to FIG. 2a but shows the co-extrusion of 5 components.

Fig. 7, which must be studied in conjunction with fig. 8 and 9, is the axial portion designated by e-f in fig. 8. It is generally similar to figure 4a, except for the construction of the labyrinth partition system. In fig. 7, the partitioning system starts in grooves formed in the additional disc surface, which is clamped on the disc with grooves for the labyrinth partitioning and the final step of the spiral grooves.

Fig. 8 represents the axial portion denoted by e-f in fig. 7, and represents portions g-h and i-j simultaneously, except for the inlet portion. Which shows the final step for labyrinth separation and is formed integrally with the spiral portion of the groove.

Fig. 9 is an expanded view of the ring portion formed by rotating each line k-l of fig. 7 around the mold axis (1). Showing the first two steps of the labyrinth separation.

Figures 10 and 11 are detailed sectional views (which are similar to figure 2b but enlarged) showing means for adjustment of the positioning of the overflow between the helical grooves, instead of the replaceable insert for component a shown in figure 2 b.

Fig. 11 is a development view of a ring portion formed by rotating the line m-n in fig. 10 about the mold axis (1).

Fig. 12 is also an axial view, limited for simplicity to the final part of the channel, showing a modification of the die of fig. 2a, showing the formation of helical grooves for one surface component in a cylindrical surface, for the other surface component in a flat surface, for the intermediate component in a conical surface, with a common outlet channel axially from the bore to the outlet orifice.

The prior art mould shown in figure 1 has an axis (1) and comprises a disc and a shell-like or bowl-like fitting clamped together. Thus (2a) and (2b) together constitute a housing or "bowl-shaped fitting", in which (3a) to (3i) are discs fitted. The 5 components are fed into a die for co-extrusion, shown with two inlets. All channels for the co-flow of the 5 components and 2 or more of the components, except for the inlet channels, are constituted by the spaces between the disc or shell-like ("bowl-like") fittings, so that each component is equal in circumference, which equalisation is established by helical grooves (4a) to (4e) extending substantially along a plane perpendicular to the axis (1) and here in cross-sectional view. The groove is formed in the surface of one of a pair of adjacent discs or between the "bowl fitting" and the adjacent disc. (alternatively, grooves may be provided in the two surfaces facing each other, which is also applicable to the present invention).

The figure shows that for each component fed directly from the inlet there is only one spiral groove for the component, but there are typically several substantially parallel grooves for each component, with one or the other distribution channel system between the groove for the component and the inlet. This is prior art.

As shown in the figures, there is an overflow between the different fittings of each channel (which fittings are adjacent when viewed in axial view), or, when there are several channels per component (also prior art), an overflow is arranged between different adjacent channels. Each trench is relatively deep initially and becomes progressively shallower until the depth becomes zero at the end. In such a spiral distribution system, the ratio between the different dimensions is important for the equality of the flow over the periphery, which depends on the rheological parameters of the melt extruded at known temperature conditions and output.

As mentioned above, this configuration of the extrusion die has the advantage of allowing co-extrusion of multiple components, but has the disadvantage that the components must have relatively similar rheological properties or the thickness of the layers can become non-uniform. This is because the different components are combined successively one after the other with a relatively long distance between the positions of the combination. It will thus be appreciated that the high extrusion pressures required result in a greater thickness of each disc. However, as previously described, if one component in contact with one channel surface has a high viscosity and a second component in contact with the opposite channel surface has a lower viscosity, the co-flow will quickly become irregular.

In the embodiment of the invention shown in fig. 2a and 3, with some minor modifications in fig. 2b, an annular mold with an axis (1) is made of: two housing (bowl) shaped fittings (5) and (6), two disk-shaped fittings (7a) and (7b), and further disk-shaped fittings (7c) in fig. 2b, three inserts (8a) and (8b) for adjusting the overflow between the spiral channels, and a ring (9) for adjusting the discharge orifice.

The molten thermoplastic polymeric material (A) having a higher melt viscosity and the two molten thermoplastic polymeric materials (B) and (C) having a lower melt viscosity are fed through separate inlets (10). It is divided by a labyrinth system of channels, first into two partial flows in the channel (11), then further into 4 partial flows in the channel (12) and 8 partial flows in the channel (13). (depending on the die size, it is of course possible to construct a greater or lesser number of partial flows, but in any case an integer power of 2).

In the direct continuation of the "labyrinth" separation, the partial flows in the channels (13) continue in a spiral distribution system via the grooves (14) so that a suitable balance is established between the flow via the spiral grooves (14) and the overflow between the grooves, which overflow takes place in the narrow gap of the space 15, the starting position of which is shown by the broken line (16) in fig. 3.

The insert for regulating the overflow will be described below. The circle (16a) shown by the dashed line in fig. 3 relates to the device for continuously adjusting the overflow shown in fig. 10 and 11, and not to the mould part shown in fig. 2a and 2 b.

The dashed line (17) in fig. 2a and 2b shows a cross-sectional view of the channel, which connects to the outside of the part shown in the figure.

After passing through the spiral equalization system of channels, A, B and C advance toward a common annular discharge channel (18) so that B and C are joined to A through bores (19) and (20), respectively. The two bores are immediately opposite each other (or have a slight axial distance between them) at the same axial position. The common passage terminates in a discharge orifice (21).

In fig. 2a, B and C join a at a sharp acute angle, which in some cases has rheological advantages, while in fig. 2B and C extend perpendicularly to a. This solution may be chosen, for example, if it is desired to shorten the diameter of the discharge orifice. The tubular co-extruded flow B/a/C passes through an annular discharge orifice (21) and leaves the die, pulling it down and blowing it in a conventional manner. The arrangement and performance of the adjustable lip-shaped ring (9) will be explained later.

The shell-like and disk-like die parts (5), (6), (7a), (7b) and (7c) in fig. 2b are screwed together by two annularly arranged bolts (22a) and (22 b). (only one bolt is shown in figures 2a and 2 b). These components may be precisely fitted together and secured by means of grooves (not shown).

In fig. 2a (but not fig. 2b), the overflow between the spiral grooves for component a is adjusted by the embedded shim (8a) (as described earlier). A plurality of different thicknesses of the embedded shim may be adapted for adjustment. For example, the thinnest thickness is 0.5 mm and the thickest thickness is 3 mm, while the depth of the spiral groove (14) at the starting position may be 5-20 mm. The embedded shim (8a) has an annular inner limit and an indented outer limit as defined by the dotted line (16) and the circled portion (16b) drawn with the dotted line in fig. 3. The inserted shim (8a) is supported in place by bolts (22a) and (22b), preferably also by notches. Thus, each groove is used for "labyrinth" separation, and the beginning of each spiral groove is a closed channel, while each of the remaining spiral grooves becomes open for overflow. It will be appreciated from figure 2a that the thickness of the embedded shim also has an effect on the thickness of the flow of a where this component meets B and C, in other words on the gap of the "bore" for a. However, when the object of the invention is to use a mold and to combine a with a higher melt viscosity with B and C with a lower melt viscosity, in particular when the throughput of a should be higher than that of B and C, then the clearance for the inner bore of a should in any case be much larger than the clearance for the inner bores of B and C (as is known in the art), so that a smaller variation of the clearance for the inner bore of a is generally not important. Typically, the clearance of the bores for B and C is between 0.5-1 mm, while the clearance of the bores for A is typically between 2-4 mm.

Since variations in the thickness of the embedded shim (8a) cause different axial positions of the shell-like fitting (5) relative to the shell-like fitting (6), the embedded shim (8a) disturbs the outward flow from the discharge orifice (21) unless the difference is compensated for. This can be done by replaceable lip rings (9) of different axial length corresponding to embedded shims (8a) of different thickness. The lip-shaped ring (9) is radially adjustable relative to the shell-shaped fitting (5). Which is secured to the shell fitting (5) by means of an annular array of bolts, while the threaded holes in the lip ring (9) are large enough to allow this adjustment.

In fig. 2b, the overflow for component C is adjusted by means of similar embedded shims (8 a). This is possible because, as shown in the figure, the two walls of the internal bore (20) for the component C are cylindrical, so that small variations in the axial position of the shell-like fitting (6) relative to the disc (7b) do not have a significant effect on the combination of C and a. Conversely, the embedded shim (8a) is not typically used when the bore wall is significantly conical (as in the walls of bores (19) and (20) in fig. 2 a).

For adjusting the overflow, i.e. using the gap (15) for components B and C in fig. 2a, another type of replaceable insert, i.e. a cavity-filling insert (8B), may be used. This does not affect the clearance between the inner bores (19) and (20). Similar inserts are shown in fig. 2B for components a and B, but embedded shims (8a) may be used for all three components.

Whereas the inserted shims (8a) regulate overflow by adjusting the distance between adjacent shell-like or disc-like die parts, the cavity-filling inserts (8b) regulate overflow by more or less filling the hollowed-out space in a disc or housing that is located face-to-face in a spiral-fluted portion in an adjacent disc or housing.

The cavity-filling insert (8b) can start at the inlet as an inserted shim (8a) up to a "labyrinth" separation system for the respective component, but can start at a later stage as shown in the figure. In fig. 2a and 2b, the cavity-filling insert (8b) is screwed onto the fitting (5), (6) or (7 c).

A variation of the cavity-filling insert (which is used to allow for adjustment of the overflow, typically continuously without disassembly of the mold), is as described above in fig. 10 and 11 and will be described later.

As shown in the figures, it is preferable to provide a large hollow continuous space extending from the axis (1) of the mold to the cylindrical surface of the innermost layer of the clamped-together mold parts (e.g., the surface may be conical rather than cylindrical). This space is very useful, for example, for creating an efficient internal cooling of the extruded tubular thin layer.

In order not to render the study of the drawings too difficult, it can be simplified in several places. Thus, the dimensions of the grooves for components A, B and C in the "labyrinthine" partition and spiral overflow system are the same, although the die is primarily used to co-extrude thinner surface layers B and C onto the thicker intermediate layer a. In order to avoid unnecessarily long residence times for B and C, the channel system for each component should therefore preferably have a lower volume than the channel system for component A.

Furthermore, it is of course impractical for the inlets (10) for each of the three components to pass along the same axial plane, to be angularly spaced from each other in the axial direction, and preferably not to extend through a conduit which extends into the central cavity of the mould as shown in figures 2a and b, but which is formed as a bore through the disc or housing. The heating elements are not shown. The helical portion of the groove shown is particularly short.

Finally, the figures do not show any venting system that is necessary when a channel for extrusion is formed between the die parts clamped together. In the absence of proper venting, inevitable leakage can result in too high a pressure between the mold parts. Since such emissions are known in the art, they will not be further described herein.

In fig. 4a, the part of the die shown that is configured below the discharge channel (18) is identical to fig. 2a, but in fig. 2a the channel is bent 90 degrees in order to be extruded axially into a partial flow B/a/C, which in fig. 4a advances radially out, while the discharge holes (21) are located at the die periphery. After leaving the discharge orifice, the molten tubular B/A/C film is inverted on a cooled ring (22) and drawn, blown and cooled by conventional means (not shown). The ring (22) is fixed directly to the shell fitting (6) of the mould via a thermally insulating material (23). The ring (22) is hollow and the cooling effect is achieved by circulation of water or oil, the temperature of which can be controlled. The cooling medium is pumped into the ring (22) or out of the ring (22) via lines, one of which (24) is shown. The conduit preferably passes through a cavity located in the region around the axis of the mould.

One of the annular lip members (25) of the discharge orifice (21) is preferably made flexible and adjustable by means of a row of screws (26) as shown. This method of adjustment is known for the construction of common flat moulds and in fact the mould of figure 4a can be regarded as a flat mould, although the discharge orifice (21) is not flat but annular. The screw (26) is shown pressing on the lip of the die fitting (25) but at the same time pulling the die lip, but the pressure in the melt will bring about a sufficient opening force to avoid any pulling action of the screw. Alternatively, a means of controlling the spacing by thermal expansion of the components may be used. This device is known for other die configurations and is used in particular to automatically avoid variations in thickness by feedback of an automatically measured thickness over the width of the extruded thin layer.

It is clear that the flexibility required for the adjustment of the discharge orifice (21) does not cause any problems when the flow at the outlet is a direct radial flow, however it should be noted that the flow may be conical to some extent without impairing the adjustment capability. In this connection, it depends on the magnitude of the cone degree allowed, but this can be easily determined by those skilled in the art.

The purpose of fig. 4B is to show a variant of the design of the invention in which, instead of component a, one of the surface components is used for co-extrusion, where component B flows in a planar, radial manner upstream of the bores (19) and (20), while a and C flow angularly towards the bores. The arrangement is still as described in claim 1, with a flowing outwardly (although not in a planar, radial manner) relative to the mold axis (1) immediately before meeting with B and C, while B and C flow toward each other immediately before joining.

The conical shape of the mould parts is shown in fig. 5, which is advantageous as described above, in particular when the discharge orifice (21) has a large diameter, because the conical form is mechanically stable against higher melt pressures, so that the mould parts clamped together can be made thinner.

Similar parts to those of fig. 3 are omitted as the conical shape will make it considerably more complicated and the channel shape of the mold of fig. 5 is sufficiently understood from fig. 3.

The mould of figure 5 is similar to that of figure 4a except that it is of conical form, with the discharge orifices (21) arranged at peripheral positions and a cooling ring (22) fixed to the mould for rotating the molten tubular B/a/C film. The replaceable embedded shim (8a) shown in the figures is similar to the embedded shim (8a) of figures 2a, 2b, 4a and 4b, except that it has a conical shape with downstream front surfaces (16) and (16a) that are parallel to the axis (1) (not shown here, but shown in figure 3).

Replacing the flexible lip (25) in fig. 4a with screws (26) for adjustment, a replaceable drain ring (27) can compensate for the different thicknesses of the replaceable embedded shim (8a) and at the same time provide proper mutual centering of the two surfaces of the extruded tubular material by small up and down movements. The cavity-filling inserts (8b) as shown in fig. 2a, 2b, 4a and 4b are not shown for simplicity, but may be present. In fig. 6, in addition to the 5 shell-like or disk-like fittings (5), (6), (7a) and (7b) in fig. 2a, 2 shell-like ("bowl-like") mold fittings (28) and (29) are provided. Passages are established in these fittings for labyrinth separation and the equality of the spiral grooves of the other two molten polymeric materials D, E, namely the passage for D between die fittings (28) and (7a) and the passage for E between die fittings (7b) and (29), which terminate in bores (30) and (31) immediately adjacent to bores (19) and (20) for B, C. Fig. 3 also helps to understand the figure. Any insert for adjusting the overflow between the spiral grooves is not shown, but it is of course possible to arrange the above-mentioned inserts (8a) or (8b) if desired. If the melt viscosity of B is close to that of D, then the two material streams may be fully combined with each other before co-extrusion with A, if desired, or B may be combined with D after D is combined with A. The same applies to the combination of C and E.

In contrast to fig. 4a, the mold shown in fig. 7, 8 and 9 comprises additional discs (32), (33) and (34). Starting from the inlet (10), here an aperture in the disc (32), each molten polymeric material A, B and C is distributed over two channel branches (35a) and (35b) (see fig. 9), here shown as grooves in (32) and (33), but may be a groove in only a portion. Starting from each end of the branch, each component passes through a hole in the disc (33), each of the two partial flows being distributed into two partial flows (36a) and (36b) at the other surface of the disc (33), for a total of 4 branches, so that each component A, B and C has now become 4 partial flows. At the end of each of the 4 streams, each component passes through holes (37) in the disc (34) which lead into the die fittings (5), (7a) and/or (7 b).

Each hole (37) continues as a hole (38) and extends through the shell-like fitting (5), see fig. 7. For component B, the holes (38) directly form the 4 inlets and reach the channel system between (5) and (7 a). For components A and C, the hole (38) may continue as a hole (39) and extend through (7 a). For component a, the holes (39) form directly 4 inlets to the channel system between (7a) and (7 b). For component C, the hole (39) may continue as a hole (40) and run through (7b), and this hole directly forms 4 inlets to the channel system between (7b) and (6). Since the sections e-f, g-h and i-j are identical except for the inlet, FIG. 8 shows in fact a continuous flow system for each component B, A and C. The mould parts (5), (7a), (7b), (6) and the insert shim (8a) are clamped together by two annularly arranged bolts (41) and (42).

As shown in fig. 8, each of the 4 partial flows is divided into 2 partial flows, so that each component constitutes a total of 8 partial flows, see fig. 8, and the 8 partial flows proceed through the spiral groove by means of overflow. Alternatively, not only 4 partial flows per component (but also all 8 partial flows) can be formed by labyrinth separation upstream of the die fittings (5), (7a) and (6), or, in particular for dies with large discharge orifice diameters, it is preferable to distribute them in excess of 8 partial flows (for example, in 16 or 32 partial flows). The outlet holes (21) of the discs of fig. 7-9 are in the peripheral surface.

In fig. 10 and 11, the cavity-filling insert (8a) has a flexible annular-shaped region extending between an annular inner limit (16a) and an annular outer limit (16 c). In this figure (16a) corresponds to (16a) in fig. 3, and (16c) corresponds approximately to the end of the helical groove. The insert (8b) is rigid upstream (inward with respect to the axis of the mould) and downstream of the flexible annular region, so that the flexible region can be considered as an annular membrane. The hard portion on the downstream side, i.e. towards the outside of the limit (8c), is fixed to the adjacent mould disc (7c) by means of an annular array of bolts (one of which (43) is shown in the figure) welded to the insert (8 b).

The pressure in the composition A pushes the membrane part (8b) against an annular array of helically curved plugs (44), each located on a rotating shaft (45) which nests in a hole in the mould disc (7 c). There are a plurality of such shafts with plugs, which extend in a star-like fashion through the disc (7 c). By rotating the shaft, the position of the membrane and the overflow between the spiral grooves can be continuously adjusted. Means for rotating and adjusting the plurality of shafts (45) and securing them in place (e.g., by using a spindle and a wheel of the spindle) are not shown.

In fig. 12, equalization of component B occurs between the inner cylindrical surface of (5) and the outer conical surface of (7a), with spiral grooves (14) disposed on the inner cylindrical surface of (5). Equalization of component a occurs between the inner conical surface of (7a) and the outer conical surface of (7b), on which also spiral grooves (14) are arranged. The equalization of component C takes place between the opposing surfaces of (7b), which are substantially flat, and the flat surfaces in (6), which are provided with helical grooves. In the figures (5) and (7a) the formation of the "bowl" is not seen, except for its annular form, since the mould preferably should have a continuous cavity around its centre. Similarly, (6) is an annular disk and (7b) is an annular truncated cone. The 4 die parts are bolted together in a manner similar to that shown in most of the figures, while in the upstream portion of the spiral channel, components A, B and C are distributed into multiple partial streams by a labyrinth separation similar to that shown in the other figures. The inner bores, which introduce the flows of materials (B) and (C) into (a), face each other almost directly, and for rheological reasons it is preferred that the length of the common channel (18) from the inner bore to the discharge orifice is as short as practically possible.

Claims (69)

1. A method of forming a tubular laminate by co-extruding at least one thermoplastic polymeric material A with at least two thermoplastic polymeric materials B, C having a melt flow index at least twice that of material A, material B being applied to one side of material A and material C being applied to the other side, the extrusion being carried out through an annular extrusion die having at least one inlet for each component and a common discharge passage terminating in an annular discharge orifice, each inlet being closer to the axis of the annular die than said orifice and the material in the molten state being extrudable discharge orifice flowing outwardly, during which each flow of each component is established by the arrangement of a first die part having a flat or conical surface, the mould parts are clamped together by said surfaces provided with grooves for forming a channel for each flow of polymeric material in order to equalize the flow over the periphery of the discharge orifice, the flow between each inlet and the discharge orifice being divided into a plurality of substantially helical partial flows at least a part of each of which passes and is provided with a space for overflow between said helical parts, said partial flows with overflow being progressively joined into a common annular flow, characterized in that the position where the materials a and B are joined is the same as or immediately adjacent to the position where they are joined with the material C, the material a flows outwardly with respect to the axis of the mould immediately before joining with the material B, C, and the material B, C flows towards each other immediately before joining.

2. A method according to claim 1, wherein the substantially helical form of the partial flow extends in a substantially planar manner.

3. A method according to claim 1, characterised in that said substantially helical form of the partial flows can extend in a geometric shape along an annular conical surface, the tangential plane of which forms an angle of at least 20 ° with the axis of the mould, at least over a substantial part of the downstream portion of said surface.

4. The method of claim 3, wherein the angle is at least 45 degrees.

5. A method according to claim 3, wherein the surface extending in a spiral is a right circular conical surface.

6. The method of claim 1, wherein each partial flow is formed by a labyrinth distribution in one or more flow stream dies.

7. The method of claim 6, wherein at least a portion of the passageway for labyrinth distribution is formed integrally with the passageway for substantially helical flow between the flat or conical surfaces of the first die component by a groove in at least one of the pair of contacting surfaces.

8. A method according to claim 6, wherein at least the initial stage of the labyrinth dispensing is established by using a second mould part having a flat or conical surface, which is clamped together with the first mould part, the arrangement of the passages for said initial stage of the labyrinth dispensing being established partly by grooves in the contact surface between the second mould part or between a second part and a first part and partly by interconnected passages through the second and/or first part.

9. The method of claim 1, wherein said overflow between the split streams is adjustable by a replaceable insert between the first mold parts or by an adjustably positionable device part opposite the channel.

10. A method according to claim 1, wherein the flows of different polymeric materials after joining, the common flow in the common discharge channel is diverted axially or i.e. proceeds in this direction and flows substantially axially when it reaches the discharge orifice.

11. A method according to claim 1, wherein the flows of different polymeric materials are combined such that the common flow proceeds perpendicular to the peripheral surface of the mould at the location of the discharge orifice and leaves the discharge orifice at an angle of at least 20 degrees to the axis of the mould, and a regulated overpressure is applied to the interior of the tubular film to form the desired diameter of the tube, while it is drawn down and solidified.

12. A method according to claim 11, wherein after the tubular film in the molten state leaves the discharge orifice, a ring concentric with the mould engages and is fixed to it, the film is turned over onto the outside of the ring, thereby reducing the angle between the axis of the mould and the direction of movement of the film and creating friction between the ring and the film to assist in the orientation of the molecules of the film and to pull the film onto the ring.

13. A method according to claim 12, characterized in that the cross-section of the ring is round at least over a part of the surface in contact with the layer.

14. The method of claim 12, wherein the ring is cooled by internal circulation through a cooling system.

15. The method of claim 12, wherein the ring is mounted in a region immediately adjacent the discharge orifice.

16. A method according to claim 11, wherein at least one side of the discharge orifice is defined by a lip member which is sufficiently flexible to permit adjustment of the clearance of the orifice, means being provided for said adjustment.

17. The method of claim 1 wherein at least one additional thermoplastic polymeric material D, other than B and C, is combined with B or C at any stage after flow equalization of B and C, said material D having a melt flow index of at least twice a.

18. The method of claim 1 wherein another ingredient, E, having the same or lower melt flow index as a, is co-extruded and a and E are directly bonded to each other before they are combined with the flows of B and C, or at substantially the same location when they are combined with the flows of B and C.

19. A method of forming a tubular laminate by extrusion of at least one thermoplastic polymeric material A through an annular extrusion die having at least one inlet for A and a discharge passage terminating in an annular discharge orifice, the inlet being closer to the axis of the annular die than the discharge orifice and A in a molten state flowing outwardly towards the discharge orifice, in which process the formation of the flow of A is established by an arrangement of die parts having flat or conical surfaces, the die parts being clamped together such that said surfaces are provided with channels for forming passages to equalize the flow over the periphery of the discharge orifice, the flow between the inlet and the discharge orifice being divided into a plurality of substantially helical partial flows passing through at least a portion of each passage and being provided with a space for overflow between said helical parts, characterised in that the discharge channel directs the molten material in a direction perpendicular to the peripheral surface of the mould, the discharge orifice being located at the peripheral surface, the tubular sheet leaving the discharge orifice at an angle of at least 20 degrees to the axis of the mould, and a regulated overpressure is applied to the interior of the tubular sheet to establish the desired diameter of the pipe while it is being drawn down and solidified.

20. A method according to claim 19 wherein at least one more thermoplastic polymeric material is co-extruded with a, the polymeric material being bonded to a in the molten state.

21. A method according to claim 19, wherein the tubular film in the molten state, after leaving the discharge orifice, is brought to engage and be fixed to a ring concentric with the mould and is turned over outside the ring, thereby reducing the angle between the axis of the mould and the direction of movement of the film and creating friction between the ring and the film to assist in the orientation of the molecules of the film and to pull said film onto the ring.

22. The method of claim 21, wherein the cross-section of the ring is rounded at least over a portion of the surface in contact with the lamina.

23. The method of claim 21, wherein the ring is cooled by internal circulation of a cooling medium.

24. A method according to claim 21, wherein the ring is mounted in the region immediately adjacent the discharge aperture.

25. A method according to claim 19, wherein said substantially helical form of the partial flow extends in a substantially planar manner.

26. A method according to claim 19, wherein said substantially helical form of the partial flows can extend in a geometric arrangement along an annular conical surface, the tangent plane of which forms an angle of at least 20 ° with the axis of the die, at least in the downstream portion of the surface.

27. The method of claim 26, wherein the angle is at least 45 degrees.

28. The method of claim 28, wherein the surface extending in a spiral pattern is a right circular conical surface.

29. The method of claim 19, wherein each of said partial flows is formed by a labyrinth distribution in one or more flow stream dies.

30. A method according to claim 19, wherein at least one side of the discharge orifice is defined by a lip member which is sufficiently flexible to allow adjustment of the clearance of the orifice and which is provided with means for said adjustment.

31. The method of claim 19, wherein the overflow between the partial flows is adjustable by a replaceable insert between the mold parts or by an adjustably positionable device part opposite the groove.

32. A method of forming a tubular laminate by extrusion of at least one thermoplastic polymeric material A through an annular extrusion die having at least one inlet for A and a discharge passage terminating in an annular discharge orifice, the inlet being closer to the axis of the annular die than the discharge orifice and A in a molten state flowing outwardly towards the discharge orifice, in which process the formation of flow of A is established by the arrangement of a die assembly having a flat or conical surface which is clamped together, the surface being provided with grooves to form passages to equalise flow over the periphery of the discharge orifice, flow between the inlet and the discharge orifice being divided into a plurality of substantially helical flow splits which pass through at least part of each passage, and a space is provided for the overflow between the screw parts, characterized in that said overflow between the partial flows can be adjusted by means of exchangeable inserts between the mould parts or by means of adjustably positionable device parts opposite the groove.

33. A method as claimed in claim 32, wherein the adjustable position device fitting comprises a flexible, flat, generally ring-shaped sheet secured at its inward and outward boundaries to a rigid mould fitting forming part of the channel system, or a rigid, flat, substantially ring-shaped plate hinged at its inward and outward boundaries through a flexible, substantially ring-shaped sheet to the rigid mould fitting, in each case with an adjustment device in the form of a ring arrangement on the side of the flat, substantially ring-shaped sheet or plate opposite the flow.

34. A method as claimed in claim 31, wherein the adjustable position device fitting comprises a flexible, flat, substantially ring-shaped sheet secured at its inward and outward borders to a rigid mould fitting forming part of the channel system, or a rigid flat, substantially ring-shaped plate hinged at its inward and outward borders through a flexible, substantially ring-shaped sheet to the rigid mould fitting, in each case with an adjustment device in the form of a ring on the side of the flat, substantially ring-shaped sheet or plate opposite the flow.

35. A method according to claim 19, wherein the adjustable position device fitting comprises a flexible, flat, substantially ring-shaped sheet fixed at its inward and outward borders to a rigid mould fitting forming part of the channel system, or a rigid, flat, substantially ring-shaped plate hinged at its inward and outward borders through a flexible, substantially ring-shaped sheet to the rigid mould fitting, in each case with an adjustment device in the form of a ring arrangement on the side of the flat, substantially ring-shaped sheet or plate opposite the flow.

36. An annular co-extrusion die for co-extrusion of at least one thermoplastic polymeric material A with at least two thermoplastic polymeric materials B and D, material B being applied to one side of material A and material C being applied to the other side to form a tubular film, the annular extrusion die having at least one inlet (10) for each component and a common discharge passage (18) ending in an annular discharge orifice, each inlet (10) being closer to the axis (1) of the annular die than the discharge orifice and the extrudable material flowing outwardly towards the discharge orifice (21), and each flowing formation of each component being established by the arrangement of a first die part (5, 6, 7, 28, 29) having a flat or conical surface, which die parts are clamped together, the surface of the fitting is provided with grooves (14) to form channels (11, 12, 13) for each flow of polymeric material to equalize the flow over the periphery of the discharge orifice (21), between each inlet (10) and discharge orifice (21) the flow of at least A is divided into a plurality of substantially helical partial flows (13), and a space (15) is provided for overflow between said partial flows and adapted to said partial flows, said partial flows with overflow being gradually combined into a common annular flow, it is characterized in that the position of the combination of the materials A and B is the same as or close to the position of the combination of the materials A and B and the material C, the channels serve to cause material a to flow outwardly relative to the axis of the mold at least immediately prior to combining with materials B and C, while channels (19, 209) are used to flow materials B and C towards each other immediately before they are combined with a.

37. A co-extrusion die as claimed in claim 36, characterised in that the substantially helical channels (11, 12) extend in a substantially planar manner.

38. A co-extrusion die as claimed in claim 36, characterised in that the substantially helical channels (11, 12) are formed in a conical surface, the tangential plane of which forms an angle of at least 20 degrees with the die axis over a substantial part of the downstream portion of the surface.

39. The co-extrusion die of claim 38, wherein the angle is at least 45 degrees.

40. The co-extrusion die of claim 38, wherein the conical surface has a positive taper.

41. The co-extrusion die of claim 36, wherein each of said substantially helical channels is formed as a continuation of the labyrinth distribution system of said channels.

42. The co-extrusion die of claim 41, wherein at least a portion of the passageway for labyrinth distribution is integrally formed with the substantially helical form of the passageway between the clamped-together first die parts by a groove in at least one of the pair of contact surfaces.

43. Co-extrusion die according to claim 41, characterised in that at least the first part of the labyrinth distribution system comprises second die parts (32, 33, 34) having flat or conical surfaces, with which the second die parts are clamped together, the arrangement of the passages for the parts of the labyrinth distribution being established partly by grooves (35, 36) in the contact surface between the second die parts or between a second part (34) and a first part (5), and partly by interconnected passages (37, 38, 39, 40) through the second and/or first part.

44. The co-extrusion die of claim 36, wherein the overflow between the partial flows is adjustable by replaceable inserts (8a) in the die or by an adjustably positionable fitting opposite the channel.

45. A co-extrusion die as claimed in claim 36, characterised in that in the downstream part of the location for combining the flows of different polymeric materials, the passage for the co-flow (18) is turned axially or extends substantially axially from this location, said flow being directed substantially axially when it reaches the discharge orifice (21).

46. A co-extrusion die as claimed in claim 36, characterised in that, in a downstream part of the location for combining the flows of different polymeric materials, the passage for co-flow (18) advances towards the peripheral surface of the die where the discharge orifice (21) is located, whereas at the discharge orifice said passage for co-flow (18) is at an angle of at least 20 degrees to the die axis and means are provided for pulling down the extruded tubular film while applying a controlled internal overpressure to establish the desired diameter.

47. A co-extrusion die as claimed in claim 46, comprising a ring (22), the ring (22) being concentric with the die and fixed to the die at a height such that the tubular sheet can be inverted over the ring surface by means for pulling the sheet substantially axially.

48. Co-extrusion die according to claim 47, characterised in that the cross-section of the ring (22) is rounded over at least a part of the surface in contact with the lamella.

49. Co-extrusion die according to claim 47, characterised by means (24) for cooling said rings by internal circulation of a cooling medium.

50. A co-extrusion die as claimed in claim 47, wherein the ring is mounted in the region immediately adjacent the discharge orifice (21).

51. A co-extrusion die as claimed in claim 46, wherein at least one side of the vent aperture is formed by a lip member (25) which is sufficiently flexible to allow adjustment of said gap, and the die includes means for such adjustment.

52. A co-extrusion die as claimed in claim 36, characterised in that in addition to the overall system of passages for B and C, a system of passages (10, 11, 30) is provided for co-extrusion of at least one further thermoplastic polymer material D, the passages terminating in an internal bore (30) for joining D to B or C in the downstream portion of the passage for equalising the flow of B or C.

53. The co-extrusion die of claim 52, wherein the location for combining D with B or C is substantially the same as the location for combining A with B and C.

54. An annular extrusion die for forming a tubular laminate comprising at least one thermoplastic polymeric material A, the annular extrusion die having at least one inlet (10) for A and a discharge passage (18) terminating in an annular discharge orifice (21), the inlet being closer to the axis of the annular die than the discharge orifice (21), and A being directed to flow outwardly towards the discharge orifice (21), and in which die formation of the flow of A is established by the arrangement of a die part (7a, b) having a flat or conical surface, the die parts being clamped together, the surfaces of said parts being provided with grooves (14) to form channels (11, 12, 13) for equalising the flow over the periphery of the discharge orifice, the flow between the inlet and the discharge passage being divided into a plurality of substantially helical flow splits (13), between which there is a space (15) for overflow, characterized in that the discharge channel for a is arranged to guide the material towards the peripheral surface of the die, the discharge orifice being located at said peripheral surface, and that the discharge channel (18) meets this orifice at an angle of at least 20 degrees to the die axis, and that means are provided for pulling down the extruded tubular film while applying a controlled internal overpressure to form the desired diameter.

55. An extrusion die as in claim 54 wherein means are provided for coextruding A with at least one more thermoplastic polymeric material.

56. An extrusion die as claimed in claim 54, comprising a ring (22) concentric with the die and fixed to the die at a height such that the tubular film can be inverted on the ring by means for pulling the film substantially axially.

57. Extrusion die according to claim 54, characterised in that the cross-section of the ring (22) is round over at least a part of the surface in contact with the lamella.

58. An extrusion die as in claim 56 wherein said means for cooling said ring is by internal circulation of a cooling medium.

59. An extrusion die as in claim 56 wherein the ring is mounted in the region immediately adjacent the vent opening.

60. Extrusion die as in claim 54, wherein the substantially helical channels (13) extend in a substantially planar manner.

61. Extrusion die according to claim 54, characterised in that the substantially helical channel (13) is formed in a conical surface, the tangent plane of which forms an angle of at least 20 degrees with the die axis, at least over a substantial part of the downstream portion of the surface.

62. The extrusion die of claim 61, wherein the angle is at least 45 degrees.

63. The extrusion die of claim 61, wherein the conical surface has a positive taper.

64. Extrusion die according to claim 54, characterised in that each channel (13) of substantially helical form is formed as a continuation of the labyrinth distribution system (11, 12) of channels.

65. An extrusion die as claimed in claim 54, wherein at least one side of the vent aperture is defined by a lip member (25) which is sufficiently flexible to allow adjustment of the clearance, the die including means for such adjustment.

66. An annular extrusion die for forming a tubular laminate of at least one thermoplastic polymeric material A, the annular extrusion die having at least one inlet for A and a discharge passage (19) terminating in an annular discharge orifice (21), the inlet (10) being closer to the axis (1) of the annular die than the discharge orifice (21), A being directed to flow outwardly towards the discharge orifice (21), in which die the formation of the flow of A is established by the arrangement of die parts (7a, b) having flat or conical surfaces, which are clamped together, the surfaces of said parts being provided with grooves (14) to form channels (11, 12, 13) for equalising the flow over the periphery of the discharge orifice, the flow between the inlet and the discharge passage being divided into a plurality of substantially helical flow splits (13), between the partial flows there is a space (15) for overflow, characterized in that the overflow between the partial flows can be adjusted by means of exchangeable inserts (8a) in the mould or by means of adjustably positioned device fittings (8b) opposite the groove.

67. Annular extrusion die according to claim 66, characterised in that the adjustable position means fitting comprises a flexible, flat, essentially ring-shaped sheet (8b) which is fixed at the inward (16a) and outward (16c) borders on a rigid die fitting forming part of the channel system, or a rigid, flat essentially ring-shaped plate which is hinged at the inward and outward borders through a flexible, essentially ring-shaped sheet on the rigid die fitting, which in each case has an annular arrangement of adjusting means (45, 46) on the side of the flat essentially ring-shaped sheet (8b) or plate opposite the flow.

68. Annular extrusion die according to claim 36, characterised in that the adjustable position means fitting comprises a flexible, flat, essentially ring-shaped sheet (8b) which is fixed at the inward (16a) and outward (16c) borders on a rigid die fitting forming part of the channel system, or a rigid, flat essentially ring-shaped plate which is hinged at the inward and outward borders through a flexible, essentially ring-shaped sheet and to the rigid die fitting, which in each case has annularly arranged adjusting means (45, 46) on the side of the flat essentially ring-shaped sheet or plate opposite the flow.

69. Annular extrusion die according to claim 54, characterised in that the adjustable position means fitting comprises a flexible, flat, essentially ring-shaped sheet (8b) which is fixed to a rigid die fitting at the inward (16a) and outward (16c) borders forming part of the channel system, or a rigid, flat essentially ring-shaped plate which is hinged through a flexible, essentially ring-shaped sheet at the inward and outward borders to the rigid die fitting, which in each case has an annular arrangement of adjusting means (45, 46) on the side of the flat essentially ring-shaped sheet or plate opposite the flow.