Abstract:
Mixing means in the form of a torpedo for an injection molding machine having a plurality of alternately isolated and intersecting flow paths for repeatedly dividing and recombining material flow streams to achieve uniform properties throughout the material. The paths are formed as right and left-hand helical channels on the circumference of the torpedo body, with the helixes of one hand having a pitch different from that of the other such that the flow paths intersect in a pattern which is bilaterally asymmetrical with respect to an axial net flow direction. The nonsymmetrical arrangement is formed to provide substantially greater mixing action than that obtainable with similar symmetrical arrangements.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to material mixing means and, more specifically, to mixing means of the type in which a plurality of material flow streams are continuously divided and recombined. 
     PRIOR ART 
     Material mixing devices of the type to which the present invention is directed have among their various uses application in injection molding machines. In injection molding machines, particularly of the ram-feed type where the inherent agitating action of a screw feed is not present, an undersized plug, called a &#34;torpedo&#34; in the trade, has been utilized in the ram cylinder or other flow passage leading to the mold cavity. This plug or torpedo has been employed to increase the working of the material being molded by restricting the area of the flow passage through which it must pass. It is also known to provide various external channels and contours on such plugs to obtain flow and mixing charactertistics for improved homogenization of the material. Examples of the prior art are illustrated in U.S. Pat. Nos. 2,669,750; 3,130,448; and 3,593,375. The teaching of the former two of these patents is that exterior channels following left and right-hand helixes are to be substantially symmetrical in both directions such that the pitch and other characteristics of the channels of opposite hands are the same. 
     SUMMARY OF THE INVENTION 
     The invention provides means for homogenizing flowable material, such as thermoplastic or thermosetting, moldable material, wherein material flow is directed through a plurality of intersecting paths which are bilaterally asymmetrical to a net flow direction. It has been discovered that an asymmetrical pattern of intersecting flow paths produces greatly improved material mixing action when compared to similar symmetrical arrangements. 
     In the preferred embodiment, the intersecting flow paths are provided as open channels helically oriented on the exterior of a circular plug. The plug is adapted to be disposed in the path of material to the mold of an injection molding machine. The axis of the plug defines the net flow direction of material over the plug. The channels cooperate with a surface of a bore in which the plug is received to form a plurality of paths in both right and left-hand helixes, which are alternately isolated and intersecting. The intersecting paths are arranged in asymmetrical relationship to a line on the surface of the plug parallel to its axis and the net flow direction by virtue of the left and right-hand helixes having different pitches. As illustrated, the paths of each helix direction are formed as equally spaced, multiple helixes, with the pitches of both directions being selected such that pairs of opposite hand paths intersect at common points at an entrance stage and at common points at an exit stage. Preferably, the helix pattern of both directions are also arranged such that the intersecting points at the inlet and outlet stages are in axial alignment with one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an elevational view of a ram cylinder of an injection molding machine with portions thereof in cross section to reveal a material mixing plug constructed in accordance with the principles of the invention; 
     FIG. 2 is an enlarged, elevational view of the mixing plug of the invention; 
     FIG. 3 is a planar development of the circumference of the plug of FIG. 2; 
     FIG. 4 is a fragmentary, cross sectional view of a lead end of the plug, taken along the line 4--4 of FIG. 2; and 
     FIG. 5 is a fragmentary, cross sectional view of a portion of the plug illustrating the profile of an external groove. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A conventional ram cylinder, indicated generally at 10, of an injection molding machine forms in its inner bore a charging chamber 11 through which flowable material is forcibly driven into a mold cavity 12 by operation of a cylindrical ram 13. Thermoplastic or thermosetting, moldable material is delivered to the zone of the charging chamber 11 through an inlet or supply 14 by conventional means (not shown). The illustrated ram cylinder is conventional in construction, and is of the general type illustrated in U.S. Pat. No. 3,194,868 to Shaw. During the molding cycle, the ram 13 reciprocates axially to forcibly induce flow of the molding material through the chamber 11 and through a nozzle assembly 17. 
     The nozzle assembly 17 includes an end cap 18 which is removably threaded onto a lower end, as viewed in FIG. 1, of a cylindrical, tubular body 19, forming the charging chamber 11. The end cap 18 retains a nozzle body 20 on the lower end of the cylinder 19. A jacket 21 encircles the cylinder 19 to form a closed chamber 22 in which water or other fluid may be circulated to control the temperature of the cylinder 19, and therefore the temperature of the molding material in the charging chamber 11. In a usual manner, a lower end of the nozzle body 20 is provided with a spherical surface to facilitate sealing engagement with an aligned recess 24 of the gate of the mold. The nozzle body 20 includes an axially extending, central, through passage or bore 26 having at its upper end a flared throat or entrance 27 which, in the illustrated case, is conical and forms a 90° angle. 
     Means for intensely mixing and homogenizing material flowing through the charging chamber 11 is provided in the form of a circularly shaped plug 31. The plug 31 includes a tapered or conical forward portion 29, a generally cylindrical, intermediate main body portion 28, and a trailing, truncated, conical portion 30. With reference to FIG. 2, the plug 31 preferably is a solid body having its cylindrical exterior surface formed with a plurality of grooves 32, leaving intervening land portions 33. The upper or lead end 29 of the plug is tapered from the full diameter of the plug 31 to a somewhat blunted or rounded point 36. A series of flutes 38 are angularly disposed about the plug end 29 flaring radially and circumferentially outwardly in a forward-flow direction, i.e., downwardly in FIG. 2. The flutes 38 are centered on and in communication with associated inlet grooves 39 on the cylindrical portion 28 of the plug. 
     The grooves are disposed in multiple right and left-hand helix patterns, designated 32R and 32L, respectively. In the illustrated embodiment, there are an equal number of leads or grooves, namely eight, in both the right and left-hand directions. As illustrated in FIG. 5, the grooves 32 are milled or otherwise formed on the circumference of the cylindrical plug portion 28 and are rounded at their interiors to facilitate material flow therethrough and cleaning of the plug between periods of use. Preferably, the plug 31 is machined from tool steel and is heat-treated subsequent to machining operations to a hardness of 50-55 on the Rockwell C scale. The external surfaces of the plug ideally are polished to improve material flow characteristics and fatigue resistance. 
     The grooves of each hand 32R and 32L are parallel and equally spaced from each other on the periphery of the cylindrical portion 28, and are disposed thereon such that right and left-hand grooves 32R and 32L intersect at common points 43 and 44 on imaginary stage lines 46 and 47 lying in planes transverse to the axis 35 of the plug. The relatively short inlet grooves or passages 39 extend axially from the intersecting points 43 of the grooves 32 at the upper stage line 46. Similarly, a set of axially extending grooves 48 on the trailing end portion 30 of the plug 31 communicate with the grooves 32 at points 44 of their intersection on the stage line 47 to provide an exit for material. 
     As seen most clearly in FIG. 3, where the grooves 32 shown in a planar development are straight, the helix angle or pitch of the right-hand grooves is somewhat steeper than that of the left-hand grooves 32L. The grooves 32R and 32L intersect at a plurality of points 51 on the area of the cylindrical plug portion 28. Owing to the unequal pitches of the right and left-hand grooves 32R and 32L at each of these intersections 51, the grooves are bilaterally asymmetrical with a corresponding reference line 56 drawn in a direction parallel to the axis 35 of the plug 31. Preferably, the grooves 32 are arranged such that their intersections 43, 44 at the stage lines 46, 47 and the corresponding inlet and outlet grooves 39 and 48 are angularly aligned, i.e., lie on common lines parallel to the axis 35 of the plug. Further, an inspection of FIG. 3 reveals that the difference in pitch of the grooves is such that the steeper right-hand pitch results in an angular course of the right-hand grooves 32R about the circumference of the cylindrical portion 28 of three leads or intersecting points 43 from one end to the other, while the less steep left-hand pitch results in an angular course of the left-hand grooves 32L of four leads or intersecting points. 
     By way of illustration, a plug constructed in accordance with the invention has the following physical characteristics: 
     
         ______________________________________Diameter          2.5 in.Length betweenstage lines       5-3/8 in.Width of grooves  5/16 in.Depth of grooves  5/16 in.Right-hand pitch  14.333 in. (8 leads)Left-hand pitch   10.75 in. (8 leads).______________________________________ 
    
     The outside diameter of the plug 31 is selected with respect to the diameter of the charging chamber 11 to provide a relatively close fit so that the bore of the chamber effectively closes the grooves 32 and prevents communication between the grooves across the lands 33. The plug 31 is axially supported by abutment with the conical nozzle surface 27. Material within the charging chamber 11 is forcibly driven forwardly or downwardly as viewed in FIG. 1, and is directed over the fluted end 20 of the plug. The material is thereby separated into a plurality of flow streams, at the inlet grooves 39, in the illustrated embodiment, eight in number. At the forward stage line 46, the eight flow streams are divided into the left and right-hand grooves 32 to double the number of flow streams. Thereafter, as material flows through the paths defined by the grooves 32 and bore of the chamber 11, it is successively recombined and redivided at each intersection of the right and left-hand grooves. The reasons for the greatly improved mixing action of the nonsymmetric arrangement of the grooves are not fully understood. The result may be produced because the grooves of the lower pitch are proportionately longer than the leads of the higher pitch, so that the net resistance along these grooves is slightly higher than that of the other, and at each intersection the forces and velocities to the left and right are unequal. Either or both of these factors may produce a degree of turbulence or mixing action in the immediate vicinity of the intersections 51 not obtainable with more symmetrical groove arrangements. The improved mixing or blending action is of great advantage in injection molding machines, particularly of the ram-feed type disclosed, for working, unifying, and homogenizing typical molding materials. It is especially useful for mixing naturally colored material and color additives into a product of consistently uniform color. It is contemplated, for example, that the grooves may be suitably arranged in a planar path with a pattern corresponding to that of FIG. 3. 
     Although preferred embodiments of this invention are illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention disclosed and claimed herein.