Abstract:
An improved extrusion die and a method for using the same that minimizes the deflection of the die exit by providing the walls of the extrusion die with areas of increasing thickness that increase from the die ends to a central location. The die wall thickness is varies such that there is minimal deflection created at the die exit.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to an extrusion apparatus and more particularly to extrusion dies for extruding thermoplastic material to produce a sheet of substantially uniform thickness.  
         BACKGROUND OF THE INVENTION  
         [0002]    An extrusion die is used to extrude molten thermoplastic material into a relatively thin film or sheet. Conventional extrusion dies have a die cavity with the general shape of a coat hanger. These extrusion dies have been generally referred to a coat hanger dies. Typical coat hanger dies include an inlet, an inlet manifold, a generally triangular shaped preland portion, a melt well, and a die exit or gap. In these dies, a back edge of the preland portion includes linear edges that form a taper converging towards a die entrance. The preland portion provides a resistance to flow that varies over the width of the die to uniformly spread the thermoplastic material across the entire die.  
           [0003]    The rheological characteristics of the molten thermoplastic materials and the pressure these thermoplastic materials exert on the die bodies result in clamshelling, or non-uniform deflection. Clamshelling results in uneven flow of the thermoplastic material through the die exit. These or other nonuniform patterns are undesirable for the production of flat sheet materials. The nonuniformities can be corrected by adjusting the die lip exit gap. Most extrusion dies are equipped with some form of lip adjustment mechanism to adjust the gap. If a particular die is to used to form a large quantity of a single layer of material, and the extrusion process is not subject to any interruptions, the adjustment of the gap is an acceptable method of removing or at least addressing these nonuniformities. However, since the performance of an extrusion die is influenced by a number of factors including flow rate, temperature, the nature of the thermoplastic materials, and the like, the use of the lip adjustment mechanism to remove or minimize these nonuniformities is not practical where a die is to be used for short runs to produce different products. For each production change the die lips must be the empirically adjusted to the new conditions. This results in a loss of production time and a waste of material as off specification product is produced during start up.  
           [0004]    The nonuniformity that is created in these prior die designs is more serious when two or more layers of a thermoplastic material are to be coextruded. In these cases, the formation of the M or W shaped boundaries between the co-extrusion layers creates an interfacial deformation that cannot be corrected by adjusting the die exit gap using the lip adjustment mechanism.  
           [0005]    In the past, there have been proposals to address these problems mostly through the design of different manifold configurations. In one such design, the back line of the preland portion is structured to be a uniform distance from the die exit. While this particular design does minimize the above problems, there are concerns relative to maintaining the flow through the die without creating areas of substantially higher residence time that over time could lead to the degradation of the thermoplastic material.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, an extrusion die comprises mating die portions having die walls defining a die inlet, a substantially straight-line die exit and a die cavity. The die cavity includes a manifold that is in fluid communication with the die inlet and the die exit, and has a back wall located a varying distance from the die exit, and side walls. The die walls have a wall thickness that varies dependent on a pressure exerted on the die exit such that there is substantially uniform deflection of the die exit.  
           [0007]    In a further embodiment of the present invention, a method of controlling the flow of an extrudate from an extrusion die comprises the steps of extruding the extrudate from a die manifold between a substantially straight line die exit, where the die manifold has a back wall located a varying distance from the die exit; and controlling a deflection of the die exit by providing a die wall having a wall thickness that varies in thickness along a length of the die wall corresponding to a pressure level exerted on the die exit by the extrudate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is an isometric view of one embodiment of the extrusion die of the present invention;  
         [0009]    [0009]FIG. 2 is a plan view of the embodiment of FIG. 1 taken from the opposite side;  
         [0010]    [0010]FIG. 3 is a plan view of the exterior of the upper die body of FIG. 1;  
         [0011]    [0011]FIG. 4 is a plan view of the exterior of the lower die body of FIG. 1;  
         [0012]    [0012]FIG. 5 is a plan view of the interior of the upper die body of FIG. 1;  
         [0013]    [0013]FIG. 6 is a plan view of the interior of the lower die body of FIG. 1;  
         [0014]    [0014]FIG. 7 is a cross sectional view taken along line  7 - 7  of FIG. 2;  
         [0015]    [0015]FIG. 8 is a view similar to FIG. 2 of an alternative embodiment; and  
         [0016]    [0016]FIG. 9 is a view similar to FIG. 2 of a still further embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    As shown in FIGS.  1  to  4 , an extrusion die  20  has an upper die body  22  and a lower die body  24 . The upper die body  22  and the lower die body  24  are joined together by a series of body bolts  26 . Two end plates  28  are affixed to the upper die body  22  and the lower die body  24  as the two die bodies  22  and  24  are joined together. The end plates  28  seal the sides of the die  20  near an upper die lip  30  and a lower die lip  32 . The upper die lip  30  and the lower die lip  32  define a die exit  34 . The upper die body  22  includes a die lip adjustment channel  36  that can accept any conventional die lip adjustment mechanism (not shown). The upper die body  22  has two upper die body ends  38  and  40 . The lower die body  24  also has two similar lower die body ends  42  and  44  that are coextensive with the upper die body ends  38  and  40 . Each of the upper die body  22  and the lower die body  24  has a central area  46  and  48  intermediate of the respective die body ends  38 ,  40 ,  42 , and  44 . Between the die ends  38 ,  40 ,  42  and,  44  and the central areas  46  and  48  are areas of increasing thickness  50 ,  52 ,  54 , and  56 .  
         [0018]    A die inlet  58  is located within the central areas  46  and  48 . The die inlet  58  can be connected to any conventional extrusion device (not shown) by conventional means. A die inlet seal area  60  surrounds the die inlet  58  to enable the extrusion die  20  to form a fluid tight seal when attached to the extrusion device. A series of heater holes  62  extend through the upper die body  22  and the lower die body  24 . Any conventional heating device (not shown) can be inserted into these holes to provide a source of heat to the extrusion die  20 . A series of thermocouple orifices  64  are provided in upper die body  22 . Any conventional thermocouple device (not shown) can be inserted into the thermocouple orifices  64 . The thermocouple devices cooperate with the heating devices to maintain the proper temperature control for the extrusion die  20 . Also shown on the lower die body  24  are a series of deckle tabs  65 . The deckle tabs  65  enable a conventional deckle device (not shown) to be attached to the extrusion die  20  to vary the width of the film produced from the die exit  34 .  
         [0019]    The extrusion die  20  includes the areas of increasing thickness  50 ,  52 ,  54 , and  56 . These areas of increasing thickness  50 ,  52 ,  54 , and  56  provide sufficient rigidity to the extrusion die  20  such that the die exit  34  either does not deform or deforms at a relatively uniform rate across the length of the die exit  34  when the flow rate or the composition of the polymer to be extruded is changed. The areas of increasing thickness  50 ,  52 ,  54 , and  56  increase in a linear fashion from the respective die ends  38 ,  40 ,  42 , and  44  to the central area  46  and  48  surrounding the die inlet  58 . It is believed that the areas of increasing thickness  50 ,  52 ,  54 , and  56  provide increased rigidity to the extrusion die  20  to minimize any deformation of the die exit  34  because of changes to the flow rate, temperature, or composition of the polymer to be extruded through the extrusion die  20 . The exact ratio of thickness between the respective die ends  38 ,  40 ,  42 , and  44  and the central areas  46  and  48  is determined empirically. It has been found that a relative increase of between about 75 to about 150%, and preferably from about 100 to about 135%, provides adequate increase rigidity to the extrusion die  20  to minimize changes to the die exit  34 . The extrusion die  20  as shown in FIGS.  1  to  4  is about 5.5 inches thick at the die ends  38 ,  40 ,  42 , and  44  and about 12 inches thick at the central areas  46  and  48 . This is a relative increase in thickness of about 118%. While the variation in wall thickness has been described relative to FIGS.  1  to  4  as increasing from the die ends  38 ,  40 ,  42 , and  44  to the central areas  46  and  48 , for certain die configurations it is possible that the variation in wall thickness might be a complex relationship that both increases and decreases in thickness across the width of the die.  
         [0020]    As shown in FIGS. 5 and 6, the body bolts  26  pass through body bolt holes  66  to join the upper die body  22  to the lower die body  24 . The body bolt holes  66  are located outside of a manifold  68 . As shown in FIG. 5, a sealing surface  70  is formed around the die inlet  58  and a back edge  72  of the manifold  68 . This sealing surface  70  forms a seal when the upper die body  22  and the lower die body  24  are joined together. The manifold  68  is a standard coat hanger type die manifold. In this regard, the manifold  68  includes a cavity  74  defined by the back edge  72  and by a preland back line  76  of a preland portion  78 . The manifold  68  also includes a secondary manifold  80  that is downstream from the preland portion  78 . In the area immediately downstream from the secondary manifold  80  and upstream from the die exit  34  is a land portion  82 . The exact structure of the manifold  68 , the cavity  74 , the preland portion  78 , the secondary manifold  80  and the land portion  82  can be varied and may conform to any of a number of different conventional designs for a coat hanger type extrusion die.  
         [0021]    As shown in FIG. 7, the upper die lip  30  and the lower die lip  32  of the extrusion die  20  form the die exit  34 . The upper die lip  30  includes the die lip adjustment channel  36  that can accept any of a number conventional die lip adjustment mechanisms. The upper die lip  30  includes a relatively thin flex zone  84  that enables the upper die lip  32  to be adjusted by the die lip adjustment mechanism.  
         [0022]    [0022]FIG. 8 shows an alternative embodiment of the present invention where an extrusion die  120  has an upper die body  122  and a lower die body  124 . The upper die body  122  and the lower die body  124  have areas of increasing thickness  150 ,  152 ,  154 , and  156  that increase in a linear fashion from the die ends  38 ,  40 ,  42 , and  44  to a central line  186 . The extrusion die  120  will perform in a manner similar to that of the extrusion die  20  described above. In the embodiment of FIG. 8, the relative ratio between the thickness of the extrusion die  120  at the die ends  38 ,  40 ,  42  and  44  and the central line  186  is about 125%.  
         [0023]    [0023]FIG. 9 shows a further embodiment of the present invention where the change in the thickness of an extrusion die  220  is non-linear. An upper die body  222  and a lower die body  224  have areas of increasing thickness  250 ,  252 ,  254 , and  256  that increase in an elliptical fashion between die ends  38 ,  40 ,  42  and  44  and a central line  286 .  
       INDUSTRIAL APPLICABILITY  
       [0024]    The preceding embodiments are applicable to extrusion dies for use in extruding sheets of thermoplastic films without deformation of the die exit. This prevents the formation of variations in thickness or other irregularities in the extruded product.  
         [0025]    Numerous modifications to the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as only illustrative or exemplary of the claimed invention and is presented for the purpose of enabling those skilled in the art to make and use the invention and to teach the best mode of carrying out same. The exclusive rights to all modifications which come within the scope of the appended claims are reserved.