Abstract:
A die plate is shown suitable for extruding a viscous material into a convenient form for the preparation of small pellets or particles. The die plate includes solid support portions and perforated portions. Each perforated portion of the die plate includes a major portion of holes through the die plate spaced from each other by a first distance and a transition portion separating the major portion from the solid support portions, with the transition portions having holes spaced from each other by a distance that is greater than the first distances.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a die plate for use in extruding a viscous material. More particularly, a die plate is shown that permits rapid extrusion of material and that has an extended service life. 
     2. Background of the Invention 
     Existing die plates used in the extrusion of viscous materials are formed from flat metal plates with a multiplicity of holes drilled through the plates. The holes through the plates are of substantially equal diameter and of substantially uniform diameter throughout their lengths. The holes through existing extrusion die plates are also spaced parallel to each other and substantially equal distance from each other. 
     A common problem with existing extrusion die plates is that the area around the perimeter of a group of holes through the plate tends to rip or tear away from the solid portion of the plate under the pressure of the extruder. The extruder generally forces a viscous or pasty material through the holes in the extruder die plate to form a plurality of cylinders of the viscous material that may be broken up into small particles for the preparation of solid granules. 
     The thickness of the extruding die plate controls the length of the holes through the die plate, which constitutes one of the factors affecting the power consumed by the extruder. The thickness of the die plate also affects the strength of the die plate, and hence resistance to failure by tearing along the boundary between the perforated section of the die plate and the solid support portion of the die plate. A change in the thickness of the die plate, and therefore in the length of the holes through the die plate, presents problems because it affects not only the pressure required to extrude the viscous materials through the die plate but also the rheology or flow characteristics of the material extruding through the die plate. The feed rate at which the viscous material is extruded through the die plate can also be changed in order to affect the pressure exerted on the die plate. A problem with reducing the feed rate in order to lower the back pressure on the die is a resultant lower production rate and reduced economy. The characteristics of the material being extruded, such as viscosity, can also be changed in an attempt to reduce the stress on the die plate during extrusion. Changes to the material characteristics of the extrusion materials is generally either impossible or undesirable as a result of the required characteristics of the end product. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems with conventional die plates, the die plate according to one aspect of the present invention was developed to permit rapid extrusion of a solid viscous material under pressure while maintaining an extended service life of the die plate as compared with conventional die plates. 
     A die plate according to an aspect of the present invention includes a solid support area and a perforated area that borders on the solid area. The perforated area includes a main portion having holes through the die plate that are spaced from each other by first distances, and a transitional area separating the main portion from the solid support area and having holes through the die plate that are spaced from each other by second distances greater than the first distances. 
     The solid supporting section of the die plate separates the perforated portion of the die plate into a plurality of smaller perforated extruding sections. Each of the smaller perforated extruding sections has a major portion of holes through the die plate that are spaced from each other by first distances and a transition portion of holes through the die plate that are spaced from each other by second distances greater than the first distances. The transition portions surround the major portions in each smaller perforated extruding section and separate the major portions from the solid supporting section. 
     The solid supporting section of the die plate is shaped in accordance with the structure of the extruder used to force viscous material through the die plate. A typical extruder screw used for forcing the viscous material through the die plate is a screw-type extruder such as the six inch extruder sold under the trademark “EXTRUD-O-MIX” made by Hosokawa Bepex Corporation of Minneapolis, Minn. The extruder includes a beveled blade that wipes viscous material over the face of the die plate and pushes the material through the holes in the die plate. 
     The solid support section on the die plate borders perforated areas of the die plate through which the viscous material is extruded. The spacing, and therefore the amount of die plate material between the holes through the die plate in the transitional areas adjacent the solid support section is greater than the amount of material between the holes in a major portion of each perforated area. The increase in the amount of material between holes through the die plate closer to the solid support sections improves the strength of the die plate at the perimeters the perforated areas, thus increasing the service life of the die plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional die plate design with uniformly spaced holes. 
     FIG. 2 shows a die plate according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one embodiment of the present invention, as shown in FIG. 2, a die plate  10  approximately six inches in diameter and approximately 0.04 inch thick includes a solid support section  20  and perforated sections  30 . The solid support section  20  includes a center hub  22 , radially extending spokes  24  and an outer rim  26 . The areas of the die plate  10  defined between the center hub  22 , the radially extending spokes  24  and the outer rim  26  of the solid support section constitute the perforated areas  30 . Each of the perforated areas  30  includes a major portion  32  of holes through the die plate and a transition portion  34  of holes through the die plate. The transition portions  34  completely separate the major portions  32  from the solid support portions  20 . 
     In the embodiment shown in FIG. 2, the holes through die plate  10  in major portions  32  of perforated areas  30  are 0.7 mm (0.027 inch) in diameter. The holes are shown to be substantially equal distance from each other within each of the major portions  32 . The holes within the major portions  32  are spaced from each other approximately 1.0 mm (0.04 inch) center-to-center. 
     The holes through the die plate  10  in the transition portions  34  of perforated areas  30  are also 0.7 mm (0.027 inch) in diameter. The holes within the transition portions are spaced from each other approximately 1.5 mm (0.06 inch). The transition portions  34  include at least 3 rows of the 0.7 mm (0.027 inch) diameter holes spaced 1.5 mm (0.06 inch) from each other center-to-center extending around the outer periphery of each major portion  32 . 
     Although the embodiment shown in FIG. 2 has holes through the plate in both the major portions and the transition portions of the perforated sections that are substantially equal diameter round holes, a skilled artisan will recognize that the holes could have other shapes. The shape of the holes through the die plate determines the shape of the tubes of extruded material being forced through the die plate. Additionally, the spacing between the holes in the major portions and the transition portions could vary as long as the holes in the transition portions are spaced by greater distances than the holes in the major portions, such that the transition areas have an increased strength and resistance to tearing away from the adjacent solid sections. 
     In the following examples a conventional extruder die plate was compared to die plates made in accordance with aspects of the invention. 
     EXAMPLES 
     In the following examples, intimate mixtures of sodium 4-sulfophenyl-[(1-oxyalkanoyl)amino]hexanoatee (alkanoyl=C 8 -C 10 ), citric acid or sodium citrate dihydrate, linear alkanesulfonate (Ufaryl 85™), and sufficient water to moisten the mix were extruded, using a six inch extruder sold under the trademark “EXTRUD-O-MIX™” (manufactured by Hosokawa Bepex Corporation of Minneapolis, Minn.), to form small pellets. 
     Examples 1 and 2 use conventional die plates, and example 3 uses a die plate made in accordance with aspects of this invention. 
     Example 1 
     A mixture of 123.8 kg of sodium 4-sulfophnyl-[(oxyalkanoyl)amino]hexanoate, 18.9 kg of LAS, and 16.2 kg of citric acid was extruded, using a die plate 0.7 mm (0.027 inch) thick with 50% open area which consisted of 0.7 mm (0.027 inch) holes (FIG.  1 ). Power drawn by the extruder was measured as the extrusion rate was increased from 226 to 376 kg/hr. The power consumed by the extruder rate was increased gradually from 1.28 to 3.63 kw. When an attempt was made to increase the extrusion rate further, the die failed by ripping of the perforated portion of the die away from the solid portion. It was observed during the run that the water level in the feed mix was not critical. 
     Example 2 
     In this example, a slightly thicker die plate 1 mm (0.04 inch) thick was used. The die failed after only 5 min running time, at an extrusion rate of 393 kg/hr. The feed material was the same as in Example 1, except that sodium citrate dihydrate was substituted for citric acid. The power drawn by the extruder at the time the die failed was 4.4 kw. The die failure was the same way as in Example 1. 
     Example 3 
     In this example, a die plate as described in this specification and illustrated in FIG. 2 was used. The die plate was 1 mm (0.04 inch) thick and contained 50% open area in which the 0.7 mm (0.027 inch) diameter holes spaced 1.0 mm (0.040 inch) apart from center-to-center were surrounded by holes spaced 1.5 mm (0.06 inch) center-to-center apart. The feed material was the same as that of Example 1. The extruder operated without interruption and with no die failure, at extrusion rates of 210-250 kg/hr, and at extruder power consumption as high as 7.6 HP (5.7 kw). 
     It is apparent from comparison of Examples 1 and 2 with Example 3 that die plate “ripping” that occurs with the conventional plate which contains only 0.7 mm holes spaced equidistant is avoided when the die plate of this invention is used. In Example 1, the plate failed when the extruder power consumption (a measure of the back pressure on the die plate) was only a little over 3.6 kw. In Example 2, in which a thicker plate was used, failure occurred at power consumption of 4.4 kw. In Example 3, in which the plate thickness was the same as Example 2, but the improved die plate was used, failure did not occur at power consumption as high as 5.7 kw. The power consumption varied during the run because of changes in the temperature and viscosity of the feed. 
     It will be understood that various modifications and changes can be made in the configuration of the extended life die plate according to the present invention. The thickness of the die plate can be varied to affect the pressure drop through the die plate. A thicker die plate could be used with a less viscous material while keeping total pressure drop through the die plate the same. The shape of the holes through the die plate could be round, square, trapezoidal, polygon, oval, or any other desired configuration, with a resultant change in the shape of the strands of material extruded through the die plate. The spacing between the holes through the die plate can be varied as long as the holes in a transition area adjacent solid support sections of the die plate are spaced further apart than holes in a major portion of the perforated area separated from the solid support sections by the transition area. The solid support section of the die plate can be varied to conform to different extruder devices. While a center hub, radially extending spokes and outer rim configuration for the support section is shown, other configurations for the solid support section could include various grid patterns, or even just a simple square or circle without a center solid support area.