Abstract:
A method and apparatus for extruding plastic products having complex cross sections, which require a complex die cross section, is described in which the cross section of the die is divided into regions of critical flow and separate flow streams are supplied to each region in proportion to the relative size of the region to the overall die cross section.

Description:
BACKGROUND OF THE INVENTION 
     In the field of extruding complex shapes one of the more challenging items to produce is the flexible gaskets used for sealing refrigerators, automobile doors, hatches, and the like. These products utilize a complex cross section which requires considerable dexterity for the die to accurately reproduce the product. In order to accurately extrude these shapes, the die must completely fill while maintaining a continuous flow of plastic. The dies used in this type of process are generally referred to as profile dies. In manufacturing such dies it is often necessary to construct the die, use the die to see how it works and then, through a series of corrections, gradually bring the die into tolerance. This trial and error method is time consuming and expensive. 
     It is a purpose of this invention to construct a profile die which provides a balanced flow to the extremities of the die passage and to provide a more accurate and simpler way to construct a profile die within closer tolerances. A die with close tolerances will allow the product to be extruded with less material with a resulting savings in cost. 
     SUMMARY OF THE INVENTION 
     A profile die system is constructed for extruding a flexible product having a complex cross section. The die system of this invention consists of an assembly of axially aligned generally cylindrical modules. A series of passages are provided to deliver molten plastic from the input to the die in a balanced flow. To accomplish this a distribution module is constructed with an inlet to receive plastic from an extruder. The distribution module has a plurality of distribution channels extending downstream in the die system. The distribution channels are sized and positioned to supply selected regions of the extrusion passage. The distribution channels exit at the downstream face of the distribution module. A transition module is positioned downstream of the distribution module and is constructed with a series of transition passages extending from its upstream to its downstream face. In the assembled position, the inlets of the transition passages are aligned with the exits of the distribution channels to receive molten plastic therefrom. The cross section of the transition passages gradually converts a generally cylindrical flow to a cross sectional shape representative of the region of the die selected for the particular passage. 
     In constructing the distribution channels, it is first necessary to analyze the shape of the extrusion passage and divide its cross sectional area into regions for concentration of plastic according to the nuances of its shape. The number of distribution channels is determined by the number of regions selected. The cross sectional area of each of the distribution channels is designed to be less than the cross section of the profile die system inlet and consistent with the flow area of the region serviced by the channel. In this manner the overall flow volume from the extruder is metered proportionally into the selected regions. Each of the transition passages are constructed to have a reduced cross sectional area from that of the distribution channel with which it communicates. Therefore, the flow volumes in each of the regional flow streams is gradually reduced from the inlet of the distribution module to the extrusion die, thereby creating an overall funnel effect in each of the regional streams. In the transition module the passage is constructed to convert the generally cylindrical flow to a shaped flow more compatible with the entrance to the die passage in each of the selected regions. 
    
    
     DESCRIPTION OF THE DRAWING 
     The preferred embodiment of the invention is described in more detail below with reference to the drawing in which: 
     FIG. 1 a  is a cross sectional view of the profile die system of this invention; 
     FIG. 1 b  is an end view of the upstream face of the transition module of this invention; 
     FIG. 1 c  is an end view of the downstream face of the transition module of this invention; 
     FIG. 1 d  is an end view of the upstream face of the die module of this invention; and 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1 a , a profile extrusion die system  1  is constructed having a distribution module  2 , a transition module  3  and a die module  4 . An die plate  5  retains the modules in the assembled condition and provides a straight exit  21  for the flowing plastic from the die module  4 . Inlet flange  6  is shown integral with the distribution module  2  and encloses inlet  11  to the profile die system  1 . The inlet flange  6  is constructed to connect to the extruder which supplies the plasticized material to the profile die system  1  for extrusion into a complex shape as illustrated in FIG. 1 d.    
     A series of distribution channels  7 ,  8 ,  9 , and  10  (not shown) are constructed by electric discharge machining or other means within the distribution module  2 . The number and size of the distribution channels are selected relative to the complexity of the die. The cross sectional area of each of the distribution channels  7 - 10  is less than the cross sectional area of the inlet. In the example shown, four distribution channels are selected to meter the flowing plastic into four regional flow streams having volumes relative to regional portions of the extrusion passage  19 . Each of the distribution channels exit at the downstream face  20  of the distribution module  2 . 
     A transition module  3  is assembled adjacent to the downstream face  20  of the distribution module  2 . Transition passages  11  through  14 , as shown in FIG. 1 b , are constructed by electric discharge machining or other means. The passages  11 - 14  extend from entrances in upstream face  15  (FIG. 1 b ) of the transition module  3  to exits in the downstream face  16  (FIG. 1 c ) in module  3 . In the assembled position, the entrances of the transition passages  11 - 14  align with the exits of the distribution channels  7 - 10  respectively to receive flowing material. 
     Consistent with the overall flow strategy of the profile die system of this invention, the flow area of each of the transition passages is reduced relative to the flow area of each of the distribution channels to which it is connected. In addition the cross section of the transition passage changes over its length to convert the flow from generally cylindrical to a shaped flow consistent with the shape of the selected region of the die cross section. The exits of the passages  11 - 14  are shown in FIG. 1 c.    
     To accomplish the balanced flow, the shape of the cross sectional shape of die passage  19  is analyzed and accordingly divided into a plurality of regions, for example; A, B, C, and D shown in FIG. 1 d . Each region is selected to coincide with areas of possible distribution difficulties where complete filling of the die is critical. The filling of the die throughout its volume must be accomplished with constant velocity and flow. After the regions are selected, the percentage of flow volume for each region is calculated and related back to the size of the distribution channel which supplies the particular region. 
     For illustration purposes, the cross section of extrusion passage  19 , may be divided into regions A, B, C, D as shown by dotted lines in FIG. 1 d . In order to determine the proportional flow area of the channels  7 - 10 , the ratio of the area of the region supplied by a channel, i.e. channel  7  supplies A, channel  8  supplies C, etc., to the overall area of the die profile is determined. The channel is sized to accommodate a flow stream consistent with the requirements of the related region. 
     To insure an overall uniform flow at a consistent velocity, the regional flow paths constructed by the assembly of distribution channels  7 - 10 , transition passages  11 - 14 , and extrusion passage  19  are designed for a funnel effect. This is accomplished by reducing the cross sectional areas of adjoining portions of the flow path from the upstream to the downstream ends of the regional flow paths. This assists in maintaining a constant velocity of the flow. The gradual increase in pressure which arises within the funnel shaped flow paths forces the molten material into the most remote section of the die. 
     The embodiment shown is for illustration purposes only, as the possible die shapes are infinitely variable. In each instance, depending on the cross section of the product, a specific flow path must be designed. In very complex dies it may be necessary to use multiple transition modules. Where necessary, bushings may be used to throttle the flow from the distribution module to the transition module as a means of adjusting flow to balance or correct for manufacturing inaccuracies. 
     In addition for special applications involving the extrusion of multiple materials, it may necessary to provide separate inlets and distribution channels to individual regions. A particular region would be supplied separately and maintained as an independent channel from the inlet to its outlet in order to extrude a product having components of different materials. This could be accomplished in a common or independent distribution module. In some instances a transition module for one component could be used as the distribution module for another component. The extruded components would be joined and welded together in the final product. In this manner extruded assemblies having two or more components of different materials can be processed in the same profile die assembly.