Patent Publication Number: US-2015064296-A1

Title: Annular co-extrusion die

Description:
TECHNICAL FIELD 
     The present disclosure relates to annular co-extrusion dies for extruding a multi-layer tubular plastic film. 
     BACKGROUND 
     Annular co-extrusion dies have been known for many years. Conventional dies of this kind usually have a series of concentric radially spaced annular die members of different diameters which define concentric radially spaced annular passages for separately feeding different plastic materials to a common annular passage such that a multi-layer tubular plastic film is extruded therefrom. In such annular co-extrusion dies, the radially spaced annular passages are fed either from a central feed block or through side passages, each of which increases the cost and complexity of the die. 
     In recent years, multi-layer co-extrusion dies have been made with die members stacked one above the other to form annular feed passages for the different plastic materials. Such dies have advantages over the conventional concentric radially spaced die member design in that they can be more easily modified to vary the number of layers. Also, the temperature of the different plastic materials can be better controlled. One example of such a multi-layer co-extrusion die is U.S. Pat. No. 5,690,972 to Planeta et al. While such multi-layer co-extrusion dies are well suited to certain applications, they are less well suited to applications requiring a large number of layers. As the height increases with the number of layers, the plastic material which flows through the annular passages formed by lower die members in the stack has to travel longer distances than the plastic material which flows through annular passages formed by higher die members in the stack. This can cause problems in obtaining a satisfactory multi-layer tubular plastic film with a large number of layers. 
     The conventional concentric radially spaced die member design described above has each of the die members providing plastic material to a shared annular passage at different positions. This results in some of the layers flowing separately before combining with other layers and can result in instability of the extruded multilayer material. 
     U.S. Pat. No. 7,097,441 to Sagar et al., the entirety of which is hereby incorporated by reference, illustrates annular co-extrusion dies having frusto-conical die members that are stacked one upon another in a nested configuration. This design is versatile because the dies are easy to take apart and clean and also have a relatively low wetted surface area compared to other dies of a similar size. This design has a shared annular extrusion passage that is formed between the outer surfaces of the side walls of the frusto-conical mandrels and the inner surface of the outer die body member (or members). The Sagar et al. patent also teaches an innovative feed mechanism in which opposed grooves in the upper and lower surfaces of adjacent mandrels form some of the feed channels. 
     Although the annular co-extrusion die taught by Sagar et al. is well-suited to applications in which the die diameter is greater than about 400 mm, adapting the annular co-extrusion die taught by Sagar et al. to smaller die diameters is difficult because the small diameter of the lowermost mandrel leaves little room for feed channels and the air passage(s). Although this difficulty can be surmounted when the number of layers (and hence the number of mandrels) is small, as the number of layers increases, more mandrels are required which in turn require a larger number of feed passages to pass through the lowermost mandrel. 
     SUMMARY 
     The present disclosure improves upon the annular co-extrusion die taught by Sagar et al. by replacing the nested frusto-conical mandrels, whose walls extend radially outwardly as well as upwardly, with nested mandrels whose side walls, and the helical passages formed thereby, extend upwardly substantially parallel to the longitudinal axis of the die. The feed passage structure of the annular co-extrusion die taught by Sagar et al. is retained. The structure described in the present disclosure provides more room in the lowermost mandrel for feed channels and air passages than the annular co-extrusion die taught by Sagar et al. for a die of equal die diameter. 
     Accordingly, there is provided an annular co-extrusion die for extruding multi-layer tubular plastic film, the annular co-extrusion die having a longitudinal axis, the die comprising a lower outer die member having an annular outer die body member. A first inner die mandrel is attached to the lower outer die member. The first inner die mandrel has a base portion with a side wall extending upwardly therefrom substantially parallel with the longitudinal axis of the die, and the outer surface of the side wall has a series of helical grooves that form upwardly extending helical passages coupled to a first longitudinally extending annular extrusion passage formed between an inner surface of the annular outer die body member and the outer surface of the side wall of the first inner die mandrel. The helical passages are coupled to a first feed passage in the lower outer die member to convey plastic material to the helical passages. A second inner die mandrel is nested within the first inner die mandrel. The second inner die mandrel has a base portion with a side wall extending upwardly therefrom substantially parallel with the longitudinal axis of the die. The outer surface of the side wall of the second inner die mandrel having a series of helical grooves that form upwardly extending helical passages coupled to a second longitudinally extending annular extrusion passage formed between an inner surface of the first inner die mandrel and the outer surface of the side wall of the second inner die mandrel. The helical passages of the second inner die mandrel are coupled to a second feed passage in the lower outer die member to convey plastic material to the helical passages of the second inner die mandrel. The first and second inner die mandrels can have a U-shaped cross-section that facilitates nesting. 
     The annular co-extrusion die can also have an inner annular die lip member and an outer annular die lip member that cooperate to form a common annular extrusion passage that joins the first and second annular extrusion passages. The inner annular die lip member and outer annular die lip member can also define an annular extrusion orifice for extruding the material from the common annular extrusion passage. The outer annular die lip member can be attached to the annular outer die body member and the inner annular die lip member can be attached to an innermost nested inner die mandrel. 
     The first feed passage can have a vertical feed passage portion that communicates with a vertical feed passage in the base portion of the first inner die mandrel. The vertical feed passage can be coupled to a series of transverse feed passages extending radially outward within the base portion of the first inner die mandrel, with the transverse feed passages coupled to the helical grooves of the first inner die mandrel. The vertical feed passage portion can be coupled to the vertical feed passage by a horizontal feed passage. The horizontal feed passage can be formed as a groove in any one of a lower surface of the first inner die mandrel, an upper surface of the lower outer die member, and both the lower surface of the first inner die mandrel and the upper surface of the lower outer die member. 
     The second feed passage can have a vertical feed passage portion that communicates with a vertical feed passage in the base portion of the second inner die mandrel, the vertical feed passage coupled to a series of transverse feed passages extending radially outward within the base portion of the second inner die mandrel, the transverse feed passage coupled to the series of helical passages of the second inner die mandrel. 
     The annular co-extrusion die can include a third inner die mandrel nested within the second inner die mandrel. The third inner die mandrel can have a base portion with a side wall extending upwardly therefrom substantially parallel with the longitudinal axis of the die. An outer surface of the side wall of the third inner die mandrel has a series of helical grooves that form upwardly extending helical passages coupled to a third longitudinally extending annular extrusion passage formed between an inner surface of the second inner die mandrel and the outer surface of the side wall of the third inner die mandrel. The helical passages of the third inner die mandrel can be coupled to a third feed passage in the lower outer die member to convey plastic material to the helical passages of the second inner die mandrel. 
     The common annular extrusion passage can combine the first, second and third annular extrusion passages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the various embodiments described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings which show at least one exemplary embodiment, and in which: 
         FIG. 1  is a perspective cut-away view of a cross-section of an embodiment of an annular co-extrusion die having stacked die mandrels; 
         FIG. 2  is a top plan view of an embodiment of an annular co-extrusion die having stacked die mandrels; 
         FIG. 3  is a sectional view taken along line A-A of  FIG. 2 ; 
         FIG. 4  is a partial sectional view showing a first variation of inner and outer annular die lip members; and 
         FIG. 5  is a partial sectional view showing a second variation of inner and outer annular die lip members. 
     
    
    
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without some of these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered as limiting the scope of the embodiments described herein, or of the appended claims, in any way, but rather to serve as a guide to a person skilled in the art for achieving at least some of the advantages of various embodiments described herein. 
     Referring first to  FIGS. 1 to 3 , an annular co-extrusion die  10  is shown having a lower outer die member  12  with a first inner die mandrel  24  secured thereto via circumferentially spaced bolts (not shown). A second inner die mandrel  54  having a similar shape to the first inner die mandrel  24  is mounted on top of and nested within the first inner die mandrel  24  in a stacked relationship. A third inner die mandrel  84 , also similarly shaped to the first and second die mandrels  24 ,  54 , is similarly mounted on top of and nested within second die mandrel  54  in a stacked relationship. Annular extrusion passages  111 ,  112  and  113  ( FIG. 3 ) are formed between the outer die member  12  and the first inner mandrel  24  as well as between adjacent inner die mandrels. Other embodiments can vary the number of inner die mandrels in order to vary the number of annular extrusion passages, and thus the number of layers of the extruded multilayer film. 
     The lower outer die member  12  can comprise an annular outer die body member  14  that can be integral with a base portion  20  of the lower outer die member  12  (see  FIG. 5 , for example) or attached to the base portion  20  of the lower outer die member  12  using bolts. The first inner die mandrel  24  has a base portion  28  and an upwardly extending side wall  26 . The first inner die mandrel  24  has a generally U-shaped cross-section, with the upwardly extending side wall  26  extending substantially perpendicularly from the base portion  28  and substantially parallel to the longitudinal axis A ( FIG. 3 ) of the annular co-extrusion die  10 . An outer surface  30  of the side wall  26  has a series of helical grooves  32  which co-operate with an inner surface  18  of an annular outer die body member  14  to form upwardly extending helical passages therebetween. The helical passages are in communication with the annular extrusion passage  111  between the outer die member  12  and first inner mandrel  24 . The inner surface  18  of the annular outer die body member  14  is also substantially parallel to the longitudinal axis A so that the upwardly extending helical passages also extend in a direction parallel to the longitudinal axis A. The helical grooves  32  have multiple starts (not shown), in the same manner as shown in FIG. 2 of U.S. Pat. No. 5,690,792, which is hereby incorporated by reference. 
     The base portion  20  of the lower outer die member  12  has a circumferentially extending series of feed passages  40  extending radially inwardly from the exterior of the base portion  20 . At its inner end, each feed passage  40  has a 90 degree bend to a vertical feed passage portion  42  which opens onto the upper surface  44  of the base portion  20 . One of the vertical feed passage portions  42  can be located at the radial centre of the base portion  20 , i.e. located on the longitudinal axis A of the die  10 . 
     Referring in particular to  FIG. 3 , the first inner die mandrel  24  has a short vertical feed passage  46  extending upwardly for a short distance from its lower surface and aligned with the vertical feed passage portion  42  of the corresponding feed passage  40 . The base portion  28  of the first inner die mandrel  24  has a circumferentially extending series of transverse feed passages  48  extending in a radially outward direction from the upper end of the vertical feed passage  46 . The transverse feed passages  48  extend to the lower end of the helical grooves  32 . The base portion  28  of the mandrel  24  is bolted to the base portion  20  of the lower outer die member  12  by circumferentially spaced bolts (not shown). 
     In some embodiments, the vertical feed passage portion  42  can be coupled to the vertical feed passage  46  by a horizontal feed passage. In such an embodiment, at its radially outer end, the horizontal feed passage communicates with the top of the vertical feed passage portion  42 , and at the longitudinal axis A the horizontal feed passage communicates with the vertical feed passage  46 . The horizontal feed passage can be formed between complementary grooves in the upper surface  44  of base portion  20  and in the lower surface of first inner die mandrel  24 . Alternatively, the horizontal feed passage can be formed by a groove in either the upper surface of base portion  20  or the lower surface of first inner die mandrel  24 . 
     The base portion  28  of first inner die mandrel  24  also has two circumferentially spaced vertical feed passages  47  extending from the bottom to the top thereof and radially spaced from the longitudinal axis A of the die  10 . As will be explained further below, the two circumferentially spaced vertical feed passages  47  connect with respective vertical feed passages  70 ,  104  in the nested second and third inner die mandrels  54 ,  84 . 
     The second inner die mandrel  54  is similarly shaped to the first inner die mandrel  24  to allow the second inner die mandrel  54  to be mounted on top of the lower mandrel  24  and nested therein, i.e. in stacking relationship therewith. The second inner die mandrel  54  has a base portion  58  and a side wall  56  extending upwardly therefrom. The second inner die mandrel  54 , similarly to the first inner die mandrel, has a generally U-shaped cross-section with the side wall  56  extending substantially perpendicularly from the base portion  58  and substantially parallel to the longitudinal axis A of annular co-extrusion die  10 . An outer surface  60  of the side wall  56  has a series of helical grooves  62  which co-operate with an inner surface  27  of the first inner die mandrel  24  to form upwardly extending helical passages therebetween. The inner surface  27  of the first inner die mandrel  24  is also substantially parallel to the longitudinal axis A so that the upwardly extending helical passages between the inner surface  27  and the outer surface  60  of the side wall  56  also extend in a direction parallel to the longitudinal axis A. As before, the helical grooves  62  have multiple starts (not shown) at their lower ends. 
     Continuing to refer to  FIG. 3 , the vertical feed passage portion  42  of one of the feed passages  40  is in communication with one of the vertical feed passages  47  in the base portion  28  of the first mandrel  24 . This allows material to be fed from feed passage  40  in the lower outer die member  12  through the base portion  28  of the first inner mandrel  24  to feed material to the helical grooves  62  of the second inner die mandrel  54 . In the illustrated embodiment, the second inner die mandrel  54  has a short vertical feed passage  70  that extends upwardly for a short distance from a lower surface of the base portion  58  and communicates with the corresponding vertical feed passage  47  that passes through first inner die mandrel  54 . The base portion  58  of the second inner die mandrel  54  has a circumferentially extending series of transverse feed passages  72  extending radially outwardly from the upper end of the vertical feed passage  70  to the lower end of the helical grooves  62 . The second inner die mandrel  54  is bolted to the first inner die mandrel  24  by circumferentially spaced bolts (not shown). 
     The vertical feed passage  47  in the base portion  28  of the inner die mandrel  24  that communicates with the vertical feed passage  70  in the nested second mandrel  54  does so via a horizontal feed passage  68 . At its radially outer end, the horizontal feed passage  68  communicates with the top of the vertical feed passage  47 , and at the longitudinal axis A the horizontal feed passage  68  communicates with the vertical feed passage  70 . The horizontal feed passage  68  can be formed between complementary grooves in the upper surface of the first inner die mandrel  24  and in the lower surface of the second inner die mandrel  54 . Alternatively, the horizontal feed passage  68  can be formed by a groove in either the upper surface of the first inner die mandrel  24  or the lower surface of the second inner die mandrel  54 . 
     Still referring to  FIG. 3 , the second vertical feed passage  47  in the base portion  28  of the inner die mandrel  24  communicates with a vertical feed passage  57  in the base portion  58  of the second inner die mandrel  54 . This vertical feed passage  57  extends from the bottom to the top of the second inner die mandrel  54 , radially spaced from the longitudinal axis A of the die  10 . The vertical feed passage  57  communicates with a short vertical feed passage  104  in nested third inner die mandrel  84  as explained further below. 
     The third inner die mandrel  84  is similarly shaped to the first and second inner die mandrels  24 ,  54  to allow the third inner die mandrel  84  to be mounted on top of the lower mandrel  54  and nested therein, i.e. in stacking relationship therewith. The third inner die mandrel  84  has a base portion  88  and a side wall  86  extending upwardly therefrom. The third inner die mandrel  84 , similar to the first and second inner die mandrels  24 ,  54 , has a generally U-shaped cross-section with the side wall  86  extending substantially perpendicularly from the base portion  88  and substantially parallel to the longitudinal axis A of the annular co-extrusion die  10 . An outer surface  90  of the side wall  86  has a series of helical grooves  92  which co-operate with an inner surface  55  of second inner die mandrel  54  to form upwardly extending helical passages therebetween. The inner surface  55  of the second inner die mandrel  54  is also substantially parallel to the longitudinal axis A so that the upwardly extending helical passages between the inner surface  55  and the outer surface  90  of the side wall  86  also extend in a direction parallel to longitudinal axis A. As before, the helical grooves  92  have multiple starts (not shown) at their lower ends. 
     As shown in  FIG. 3 , the vertical feed passage portion  42  of one of the feed passages  40  is radially spaced from the longitudinal axis A of the die  10 , and communicates with a vertical feed passage  47  in the base portion  28  of the first inner die mandrel  24  and a vertical feed passage  57  in the base portion  58  of the second inner die mandrel  54 . This allows material to be fed from the feed passage  40  in the lower outer die  12  through the first inner die mandrel  24  and the second inner die mandrel  54  to feed material to the helical grooves  92  of third inner die mandrel  84 . In the illustrated embodiment, the third inner die mandrel  84  has a short vertical feed passage  104  that extends upwardly for a short distance from a lower surface of the base portion  88  and communicates with the corresponding feed passages  47  and  57  through the first and second inner die mandrels  54 ,  84 , respectively. The base portion  88  of the third inner die mandrel  84  has a circumferentially extending series of transverse feed passages  106  extending radially outwardly from the upper end of the vertical feed passage  104 . The transverse feed passages  106  extend to the lower end of the helical grooves  92 . The third inner die mandrel  84  is bolted to the second inner die mandrel  54  by circumferentially spaced bolts (not shown). 
     The vertical feed passage  57  is coupled to the vertical feed passage  104  by a horizontal feed passage  102  as shown in  FIG. 3 . At its radially outer end, the horizontal feed passage  102  communicates with the top of the vertical feed passage  57 , and at the longitudinal axis A the horizontal feed passage  102  communicates with the vertical feed passage  104 . The horizontal feed passage  102  can be formed between complementary grooves in the upper surface of the second inner die mandrel  54  and in the lower surface of the third inner die mandrel  84 . Alternatively, the horizontal feed passage  102  can be formed by a groove in either the upper surface of the second inner die mandrel  54  or the lower surface of the third inner die mandrel  84 . 
     The first, second and third inner die mandrels  24 ,  54 ,  84  are secured to each other and to the lower outer die member  12  to provide annular extrusion passages  111 ,  112 ,  113 . The outermost annular extrusion passage  111  is formed between the inner surface  18  of the annular outer die body member  14  and the outer surface  30  of the side wall  26  of the first inner die mandrel  24 . The next or intermediate annular extrusion passage  112  is formed between the inner surface  27  of the first inner die mandrel  24  and the outer surface  60  of the side wall  56  of the second inner die mandrel  54 . The innermost annular extrusion passage  113  is similarly formed between the inner surface  55  of the second inner die mandrel  54  and the outer surface  90  of the side wall  86  of the third inner die mandrel  84 . Each of the annular extrusion passages  111 ,  112 ,  113  extends substantially parallel to one another and substantially parallel to the longitudinal axis A. 
     Referring now to  FIGS. 4 and 5 , in an embodiment the annular extrusion passages  111 ,  112 ,  113  extend into a common annular extrusion passage  118  that is formed between an inner annular die lip member  110  and an outer annular die lip member  114 . The inner annular die lip member  110  is mounted on top of the third, innermost mandrel  84  and is secured thereto by circumferentially spaced bolts  115  (only one of which is shown in  FIG. 4 ). In some embodiments, the inner annular die lip member  110  can be integral with the third, innermost mandrel  84 . The shape of the upper portion of the side walls  26 ,  56  and the inner and outer annular die lip members  110 ,  114  define how each of the layers flowing in the annular extrusion passages  111 ,  112 ,  113  combine to form multilayer material in the common annular extrusion passage  118  that is then extruded through an annular extrusion orifice  120 ; different variations are shown in  FIGS. 4 and 5 . Design of these surfaces provides control of how each layer of the multilayer film is combined. 
     In operation, different plastic materials are supplied from different extruders through their respective feed passages  40 . As shown, one of the feed passages  40  supplies plastic material through the vertical feed passage  46  and the transverse feed passages  48  to the helical grooves  32  so as to extrude a first film layer into the annular extrusion passage  111 . Another feed passage  40  feeds another plastic material through one of the vertical feed passages  47 , the horizontal passage  68  and the vertical passage  70  to the transverse feed passages  72  and helical grooves  62  to extrude a second film layer into the annular passage  112 . A third feed passage  40  feeds yet another plastic material through the vertical feed passages  47 ,  57 , the horizontal feed passage  102  and the vertical feed passage  104  to the transverse passages  106  and helical grooves  92  to extrude a third film layer into the annular extrusion passage  113 . Plastic material flowing through annular extrusion passages  111 ,  112 ,  113  are joined together in common annular extrusion passage  118  and extruded from annular extrusion orifice  120 . 
     Although the exemplary embodiment is shown with first, second and third inner die mandrels, other embodiments may include only first and second inner die mandrels, or more than three concentrically nested inner die mandrels. 
     The term “substantially” as used with respect to longitudinal axis A allows for a slight variation of a few degrees off center but is preferably aligned with longitudinal axis A. 
     The term “integral”, and its derivations, are used herein to indicate a part that is fabricated from a single piece of material. The term “integral” as used herein specifically excludes other means for maintaining parts fixed together as a single unit. More particularly, the term “integral” is used to specifically exclude known variations where a component can be machined as two separate parts that are later attached together using known mechanical or chemical means. 
     It is to be understood that the disclosed embodiments can be varied by a person skilled in the art while still providing benefits and advantages of the embodiments described herein. Other embodiments and the advantages thereof will be readily apparent to a person skilled in the art, the scope of the claims to be given a full, fair and purposive construction in view of the specification.