COEXTRUSION DIE

Abstract: A coextrusion die for the extrusion of plastics materials in the shape of a tubular film comprising supply means (2) for separately supplying at least two plastics materials to said die, a distribution channel system connected to each supply means, each distribution channel system comprising at least two arched channels (24, 25, 26) provided in different axial planes, the ends of each arched channel (24, 25) being connected to the middle portion of the following arched channel (25, 26) and the distribution channel systems being located at different distances from the centre axis of the die, collecting chambers (18-20, 33-35) communicating with said distribution channel systems and an annular extrusion slot (5, 36) communicating with the collecting chambers (18-20, 33-35).

Description: COEXTRUSION DIE Technical Field This invention relates to an extrusion die for the coextru- sion of plastics materials in the form of a tubular film, said apparatus comprising means for separately supplying at least two extrudable materials in fluid state to said apparatus, a system of distribution channels connected to each supply means to form partial streams of said extrudable materials, each system of distribution channels opening into an annular collecting chamber, and an annular extrusion slot communicating with said annular collecting chamber(s) . Background Art In a prior art coextrusion die of the above mentioned type each extrudable material is supplied in liquid state through a central¬ ly located supply conduit which is connected to a system of radially extending distribution channels, each system being provided in a distribution plate, and is then passed into an annular collecting chamber from which it is passed into a common annular extrusion slot. Due to the presence of the radially extending distribution channels in the prior art coextrusion there is insufficient space for the strong clamping bolts which are required to maintain the distribu¬ tion plates sufficiently tensϊoned to avoid leakage of extrudable mate¬ rials between said plates at high extrusion pressures. The prior art supply system for extrudable materials com¬ prising centrally located supply conduits also suffer from the draw- back that it allows no space at the centre of the die for the provision of driving means when it is desired to rotate the inner and outer die parts relative to one another or for the provision of supply and discharge conduits for gaseous coolant, such as air to cool and expand the extruded tubular film . In the absence of such cooling , the produc¬ tion rate is considerably reduced . The object of the invention is to provide a coextrusion die providing sufficient space for the provision of clamping bolts, means for rotating one of the die parts relative to one another and supply and discharge conduits for gaseous coolant. Disclosure of I nvention According to the invention there is provided an extrusion die wherein the. distribution channel system for each extrudable ma¬ terial comprises at least two arched channels provided in different axial planes , the ends of each arched channel being connected to th middle portion of the following arched channel and wherein the distri bution channel systems are located at different distances from th centre axis of the extrusion die. I n the following the invention will be described with re ference to an extrusion die for the coextrusion of three extrudabl materials, wherein the arched distribution channels are circular arche and wherein the distribution channel systems are located concentricall because the use of such distribution channels and the concentri mounting of these systems are particularly advantageous _as far as th manufacture of the extrusion die is concerned . However, it should be understood that the die may compris two or more than three distribution channel systems and that th arched distribution channels should not necessarily have the shape o circular arches and even may be composed of linear channel section and that the distribution channel systems do not have to be concentri cally mounted . The invention is based . on the discovery that by effectin the distribution of each extrudable material in a labyrϊnthϊc manner i different distances from the centre axis of the die and in differen axial planes thereof, it is possible to avoid conduits for supplyin extrudable materials to the die and radially extending distributio channels in the central zone of the die. Thus, the supply conduit can be connected to the end surface or exterior surface of the di and can be connected to the first arched channel of each distributio channel system through connection channels provided in the die Thus , the necessary clamping bolts can be inserted betwee the distribution channel systems and and driving means , if any, fo the rotation of the inner die part as well as conduits for gaseou coolant can be mounted in the central zone of the die. By providing the distribution channel systems at differen distances from the centre axis of the die, it is also possible to pro vide within the die drainage conduits for draining extruded materia leaking from the distribution channels . US patent specification No . 3.698.988 discloses an extrusio die for the extrusion of a tubular film on a preformed tubular body. The prior art extrusion die which is no coextrusion die comprises supply conduit for supplying a plastics material to a first arche distribution channel which is provided in a piate member. Each end of said first distribution channel is connected to a second arched distri¬ bution channel provided in an adjacent plate. The ends of said latter distribution channel are connected to an annular inner distribution channel provided within the same plate and having a diameter which is less than that of the first and second distribution channels . Thus, the flow of material from said arched distribution channels to said annular distribution channel which opens into an annular extrusion die is a radial flow and not an axial flow. The part of the extrusion die of the invention in which the distribution channels are provided preferably consists of elements which are clamped together and the arched channels are preferably formed from grooves provided in the surfaces of said elements. An extrusion die composed of such elements presents seve¬ ral advantages as far as its manufacture is concerned . Thus, it is much simpler to form grooves in the surface of such elements than to form inner channels therein . Furthermore, it is easy to maintain and clean such a die because the elements can easily be replaced, and when the die has been disassembled the grooves forming the distribu¬ tion channels are easily cleaned. The arched distribution channels and the connecting chan¬ nels connecting the ends of one arched channel with the middle por¬ tions of the following channels may have any cross-sectional shape. In practice, however, it has been found preferable to use channels hav¬ ing an essentially rectangular cross-sectional shape with rounded corners in order to obtain a uniform flow through said channels and to prevent a thermal conversion of slowly flowing material . An embodiment of the extrusion die of the invention com- prises arched distribution channels each formed from two grooves, one groove being formed in the surface of one element and the other in the surface of the adjacent element. This embodiment is particularly advantageous because it allows distribution channels having circular cross-section or having rounded corners to be formed in a simple manner. In another preferred embodiment of the die of the invention the elements are disc-shaped and the arched channels are intercon¬ nected through connecting channels extending through the inter- O. PI mediate discs . Such disc-shaped elements are particularly preferre because they can be clamped together to form distribution channel without dead zones , even if the elements are warped e . g . as a resul of a high temperature cleaning of the distribution channels . The elements are preferably discs which extend perpendi cular to the axis of the extrusion die and in that case the connectin channels preferably are parallel to said axis . The portion of the extrusion die comprising the distributio channel system may also be composed of shell -shaped elements inserte in one another and the channels connecting the arched channels ar preferably formed from grooves formed in the surfaces of the shell shaped elements . Such a die is easily manufactured because all distri bution channels of the distribution channel system can be formed b cutting . A particularly uniform distribution of the extrudable mate rial in the annular collecting chambers can be achieved by providin guide grooves at the points where the distribution channels open int the collecting chambers , said guide grooves being inclined relative t the centre axis of the die and the depth of said grooves gradual l decreasing in the longitudinal direction of the die. Due to the presence of said guide grooves the extrudabl materials are subjected to a transverse movement after passing int the collecting chambers and said transverse movement contributes t the elimination or reduction of irregularities in the tubular bodie formed in the collecting chambers . A similar effect is achieved by using an extrusion die i which at least one chamber wail in each collecting chamber is rotatabl relative to the openings through which the distribution channels ope into the collecting chamber considered . The number and the length of the arched channels ar preferably selected such that the channels through which a give distribution channel system is connected to a collecting chamber i uniformly distributed over the periphery of the collecting chamber . I this manner the extrudable material supplied to said collecting chambe is uniformly distributed over the whole periphery of the tubular fil formed in the extrusion slot. Brief Description of Drawings Fig. 1 is a diagrammatic perspective view of an embodi¬ ment of an extrusion die according to the inven¬ tion fed by three extruders , Fig . 2 is a longitudinal sectional view of the extrusion die of Fig . 1 taken along the line l l -l l , Fig . 3 is a part of the distribution channel system of the extrusion die of Fig . 2, shown in unfolded view, and Fig . 4 is a longitudinal sectional view of part of another embodiment of the extrusion die according to the invention. Detailed Description of the Preferred Embodiments Fig . 1 illustrates a coextrusion die 1 which through supply conduits is fed by three extruders 3. Each extruder 3 comprises a supply funnel 4 for extrudable material . The extrusion die 1 has an annular extrusion slot 5 for the extrusion of a tubular film 6. Figs . 2 and 3 shows the extrusion die of Fig . 1 in enlarged scale. The supply conduit 2 which has one end inserted in a hole 10 in the end of the extrusion die 1 communicates with the middle of the three distribution channel systems through a channel 11 . The rear end of the extrusion die to which the extrudable materials is fed is composed of plate elements 12-15 which are clamped together and which are attached to the front end 16 of the extrusion die by clamp¬ ing bolts 17 which are screwed into holes in said front end 16. Three collecting chambers 18-20 are provided in said front end , one end of each collecting chamber communicating with a distribution channel system and the opposite ends of said collecting chambers 18-20 being connected to the annular extrusion slot 5. The location of the extrusion slot 5 and consequently the thickness of the extruded tubular film is adjustable by means of ad- Justing screws 21 and 22. I n the embodiment illustrated in Fig . 2 the extrusion die is provided with electric heating Jackets 23 on both the exterior and interior surfaces thereof. The three distribution channel systems are in principle identical and only the middle system will be described with reference to Fig . 3. The channel 11 connecting the supply conduit 2 with th distribution channel system opens into the middle portion of a firs arched distribution channel 24. Each end of said channel is connecte to the middle portion of a group of arched distribution channels 25 The ends of said channels 25 are in turn connected to the middl portion of another group of arched distribution channels 26. The end of the distribution channels are connected to the collecting chambe 19 through connecting channels 27. In order to obtain a good distribution, of the partial stream leaving the channels 27, guide grooves are provided in the wall o the collecting chamber 19, said guide grooves 27 being inclined rela tive to the longitudinal axis of the die and having a depth whic gradually decreases . The extrusion die illustrated in Fig . 4 consists of a sta tionary part shown generally as 30, two rotatabie parts 31 and 32 an located between parts 31 and 32 three collecting chambers 33, 34 an 35, each being connected to an annular extrusion slot 36. The rotatabie part 32 is mounted on a central shaft 3 cooperating with driving means (not shown) mounted in a central bor in the stationary part ' 30 of the die. The exterior surface of th rotatabie part 31 has the shape of a toothed wheel which is In engage ment with a pinion (not shown) . A guide ring 39 with a washer 40 attached to the stationar part 30 Is inserted in an annular guide groove 41 in the rotatabi part 32. Similarly a guide ring 42 with washer 43 which are als attached to the stationary part 30 are inserted in an annular guid groove 44 in the rotatabie part 31 . The distribution channel system of the die Illustrated i Fig . 4 corresponds to that of Figs . 2 and 3 but the arched dϊstrϊ bution channels 24, 25 and 26 are composed of grooves which ar provided in the conical surfaces of an inner element 45 and two sur rounding conical shells 46 and 47 as well as an outer shell 48 whic has a conical Inner surface, and a channel 49 connecting a suppl conduit 2 with the first arched channel 24. The elements 45-48 containing the distribution channe system are clamped together by a plate 51 comprising a set of hole 50 and bolts 52 which are inserted in said holes 50 and attached t the elements 45-48. Due to the conical shape of the contact surface of the elements 45-48 only a small force and consequently relativel C thin and short bolts are required to clamp together the portions forming the stationary part of the die. The space between the outer rotatabie part 31 and the in¬ ner rotatabie part 32 comprises two elements 53, 54 of which one 53 by means of fastening means (not shown) having a small cross-sectional area is connected to the rotatabie part 31 and the second one 54 by means of similar fastening means (not shown) is connected to the other rotatabie part 32. When using the embodiment of the die illustrated in Figs . 2 and 3, the streams flowing out of the channels 27 are smeared out during the passage through the zone In which the guide grooves 28 are provided . I n the embodiment Illustrated in Fig . 4 the streams are smeared out as a result of the movement of the rotatabie parts 31 , 32. These parts can be rotated in opposite directions (contrarotated) or in the same direction at different or equal speeds of rotation . Alternatively, one part can be rotated while the other part is station¬ ary. It should be understood that the embodiment illustrated in Figs . 2 and 3 also might comprise rotatabie die parts and that guide grooves also might be provided in walls of the collecting chambers illustrated In Fig . 4. The embodiments of the extrusion die shown in the draw¬ ings are designed for the manufacture of laminates consisting of three layers located parallel to the surfaces of the laminates . However, if the extrusion die shown is modified such that one or two distribution channels systems open into the same collecting chamber, products could be manufactured wherein segments of the extrudable materials are located side by side in one or more layers or extending from one surface to the opposite. Such a modified embodiment of the extrusion die according to the Invention would be suitable for the manufacture of products of the type disclosed In US patent specifications Nos . 3.565.744, 3.505.162, 3.632.282 and 3.690.982 and part of the extru¬ sion die of the invention which comprises the distribution channel system couid be combined with the apparatus described in said US patent specifications .

Claims: C l a i m s 1. An extrusion die for the coextrusion of plastics materials in the form os a tubular film, said apparatus comprising means (2) for separately supplying at least two extrudable materials in fluid state to said apparatus, a system of distribution channels connected to each supply means to form partial streams of said extrudable materials, each system of distribution channels opening into an annul¬ ar collecting chamber (18-20, 33-35) and an annular extrusion slot (5,36) communicating with said collecting chamber(s), c h a r a c - t e r i z e d in that the distribution channel system for each extrud¬ able material comprises at least two arched channels (24,25,26) pro¬ vided in different axial planes, the ends of each arched channel (24,25) being connected to the middle portion of the following arched channel (25,26), and wherein the distribution channel systems are located at different distances from the centre axis of the extrusion die. 2. An extrusion die as in claim 1, c h a r a c t e r i z e d In that the arched distribution channels (24,25,26) are of a circular shape and that the distribution channel systems are located concen- trϊcally relative to one another. 3. An extrusion die as in claim 1, c h a r a c t e r i z e d in that the part of the die comprising the distribution channel system is composed of elements which are clamped together (12-15, 45-48) and that the arched channels (24,25,26) are formed from grooves provided in the surfaces of said elements. 4. An extrusion die as in claim 3, c h a r a c t e r i z e d in that each arched channel (24,25,26) is formed from two grooves, one being provided in the surface of one element and the other being provided in the adjacent surface and another element. 5. An extrusion die as in claim 3, c h a r a c t e r i z e d in that the elements (12-15) are disc-shaped and that the arched channels (24-26) are interconnecting channels (27) extending through the intervening disc-shaped elements (12-15). 6. An extrusion die as in claim 3, c h a r a c t e r i z e d In that said part of the die comprises shell-shaped elements (45-48) inserted In one another and that the arched channels (24-26) are interconnected through connecting channels formed from grooves provided in the surfaces of said shell-shaped elements. OMPI 7. An extrusion die as in claim 1, c h a r a c t e r i z e d in that the collecting chambers (18-20, 33-35) comprise guide grooves (28) which are provided in the zones in which the distribution chan¬ nels open into said collecting chambers, which are of gradually de- creasing depth and which are inclined relative to the longitudinal axis of the die. 8. An extrusion die as in claim 1, c h a r a c t e r i z e d in that at least one wall of each collecting chamber (33-35) is rota¬ tabie relative to the openings through which the distribution channels (24-26) open into the collecting chamber considered.

Inventors: JENSEN B, RASMUSSEN O
Applicants: JENSEN B, RASMUSSEN O
Application year: 1981
Main classification: B29D7
IPC codes: B29C47, B29L23