Abstract:
A method of manufacturing a bonded metal assembly having a form and a powdered metal-filled plastic member with a cylindrical surface generally surrounding the form includes providing a mold having a cylindrical interior surface corresponding to the cylindrical surface of the resulting powdered metal-filled plastic member and providing a gate to the mold. The gate extends at least partially along a radius of the cylindrical interior surface of the mold. With the form positioned in the mold, a powdered metal-filled plastic material is injected through the gate to the mold and flowed around the form. A resulting bonded metal assembly includes a form and a powdered metal-filled plastic member having a cylindrical surface generally surrounding the form. The powdered metal-filled plastic member includes an injection-molding gate line extending at least partially along a radius of the cylindrical surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to an injection-molded article and method of manufacturing thereof and, in particular, to an injection-molded powdered metal-filled plastic member and method of manufacture thereof. While the invention is illustrated with manufacture of a permanent magnet molded over a form, such as a rotor of a brushless DC motor, it may apply to other types of bonded metal assemblies, such as electrical generators, electro-magnetic sensors, and the like. 
     It has long been known to manufacture powdered metal-filled plastic members, such as permanent magnets using injection-molding techniques. This is accomplished by injecting or shooting a molten or liquid powdered metal-filled plastic material into a hollow cavity, known as a mold, and allowing the material to cool to a point of solidification or otherwise curing the material. 
     SUMMARY OF THE INVENTION 
     While this technique has worked well when manufacturing relatively small powdered metal-filled plastic members, such as on the order of magnitude of 4 to 5 grams, the high specific gravity of powdered metal-filled plastic material has made molding of larger parts impractical. The present invention provides the ability to be able to manufacture larger components, for example, components of up to 28 grams or more. As brushless DC motors and other bonded metal assemblies find additional applications, there is a need for bonded metal assemblies of a larger size. However, the invention is not limited to any particular size. 
     A method of manufacturing a bonded metal assembly having a form and a powdered metal-filled plastic member having a cylindrical surface generally surrounding the form, according to an aspect of the invention, includes providing a mold having a cylindrical interior surface corresponding to the cylindrical surface of the resulting powdered metal-filled plastic member and providing a gate to the mold. The gate extends at least partially along a radius of the cylindrical interior surface of the mold. With the form positioned in the mold, a powdered metal-filled plastic material is injected through the gate to the mold and flowed around the form. 
     At least a portion of the material may flow generally tangentially along the cylindrical interior surface of the mold. The gate may open in a direction forming an angle to the radius of the cylindrical interior surface at the gate. In this manner, the gate has a component that is tangential to the cylindrical interior surface of the mold. The gate may be at an angle of between approximately 30 degrees and approximately 60 degrees to the radius of the cylindrical interior surface. 
     The bonded metal assembly may be ejected radially from the mold. The method may be used to manufacture a bonded metal assembly having a powdered metal-filled plastic member with a length to diameter ratio that is greater than 1 to 1. The ratio may be greater than 2 to 1. The gate may extend at least 50% of the length of the cylindrical inner surface of the mold and may extend at least 80% of the length of the cylindrical inner surface of the mold. The method may be used to manufacture a bonded metal assembly weighing at least approximately 28 grams and may be used to manufacture a permanent magnet assembly having a cylindrical shaft, such as a rotor of a brushless DC motor producing at least approximately 50 watts of mechanical power or greater. 
     A method of manufacturing a powdered metal-filled plastic member having a curved outer surface, according to another aspect of the invention, includes providing a mold having a curved interior surface corresponding to the curved outer surface of the member. A gate is provided to the mold. The gate opens at an angle to the radius of the curved outer surface of the member. A powdered metal-filled plastic material is injected through the gate to the mold and at least a portion of the material flows generally tangentially along the curved interior surface of the mold. 
     The curved interior surface may be a generally cylindrical surface. The gate may extend at least 50% of the length of the cylindrical surface and may extend at least 80% of the length of the cylindrical surface. At least a portion of the material may flow more than half of the circumference of the cylindrical surface and may flow substantially the full circumference of said cylindrical surface. The gate may open at an angle of between approximately 30 and 60 degrees to the radius of the curved outer surface of the magnet. The gate may open at an angle of between approximately 30 and 60 degrees to the radius of the curved outer surface of the member. 
     A form may be positioned in the mold prior to the injecting. At least a portion of the material may flow generally unidirectionally along the curved interior surface around the form. The method may be used to manufacture a permanent magnet. The form may be a cylinder having a central portion and outer portions, with the central portion having a greater radius than the outer portions. The powdered metal-filled plastic member may extend outwardly past the central portion. 
     A bonded metal assembly, according to an aspect of the invention, includes a form and a powdered metal-filled plastic member having a cylindrical surface generally surrounding the form. The powdered metal-filled plastic member includes an injection-molding gate line extending at least partially along a radius of the cylindrical surface. 
     The powdered metal-filled plastic member may have a length to diameter ratio that is greater than 1 to 1 and may be greater than 2 to 1. The gate line may extend at least 50% of the length of the cylindrical surface and may extend at least 80% of the length of the cylindrical surface. The form may be a cylindrical shaft having a central portion and outer portions. The central portion may have a greater radius than the outer portions. The powdered metal-filled plastic member may be a permanent magnet and may extend outwardly past the central portion. The central portion may have a knurled outer surface. 
     The powdered metal-filled plastic member may exclude any substantial knit line generally circumferentially opposite the gate line. The member may weigh at least approximately 28 grams and may have a substantially uniform diameter along the length thereof. The bonded metal assembly may be used in a brushless direct current motor. Such brushless direct current motor may produce at least approximately 50 watts of mechanical power. 
     These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a bonded metal assembly, according to an embodiment of the invention; 
         FIG. 2  is a side elevation of the assembly in  FIG. 1 ; 
         FIG. 3  is an end elevation of the assembly in  FIG. 1 ; 
         FIG. 4  is a sectional view taken along the lines III-III in  FIG. 3 ; 
         FIG. 5  is a perspective view of a runner attached to the bonded metal assembly in  FIG. 1 ; 
         FIG. 6  is a schematic view of a molding process illustrating material flow through the gate and mold; 
         FIG. 7  is the same view as  FIG. 6  of an alternative embodiment thereof; 
         FIG. 8  is a sectional view of a mold assembly used to mold a bonded metal assembly; 
         FIG. 9  is a right side elevation of a left mold half; 
         FIG. 10  is a left side elevation of a right mold half; 
         FIG. 11  is a sectional view taken along the lines XI-XI in  FIG. 9 ; 
         FIG. 12  is a sectional view taken along the lines XII-XII in  FIG. 10 ; 
         FIG. 13  is the same view as  FIG. 7  of an alternative embodiment thereof; and 
         FIG. 14  is the same view as  FIG. 13  during a subsequent manufacturing step. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and the illustrative embodiments depicted therein, a bonded metal assembly  10  is shown including an insert, such as a cylindrical shaft  16  and a powdered metal-filled plastic member  14 , surrounding the shaft ( FIG. 1 ). Member  14  has a cylindrical surface  15  that generally surrounds shaft  16 . Bonded metal assembly  10  and, in particular, powdered metal-filled plastic member  14  are manufactured utilizing injection molding techniques, as will be discussed in more detail below. In particular, member  14  is molded over shaft  16  from a powdered metal-filled plastic material. A powder can be spherical, flake or other shape of particle. Various blends of material may be used, including an iron powder, such as magnetic NdFeB or a magnetic ferrite, in a plastic, such as polyamide, polyphenylene sulfide, or the like. However, other metals, such as tungsten, and the like, may be used. In the illustrated embodiment, bonded metal assembly  10  is a permanent magnet assembly  10  in the form of a rotor of the type used in a brushless direct current (DC) motor. It may be used in various sizes, such as up to 28 grams or more. It is capable of production of a greater output than conventional bonded magnet brushless DC motors, such as greater than 50 watts and even 350 watts to 500 watts or greater. However, bonded metal assembly  10  may find different applications, such as electrical generators, electro-magnetic sensors, and the like. Also, certain aspects of the invention may find application to metallic members other than permanent magnets and to powdered metal-filled plastic members that are not formed around an insert. 
     In the illustrated embodiment, powdered metal-filled plastic member  14  has a length L and a diameter D. The ratio of LID is at least 1 and may be greater than 1, such as 2/1, 4/1, and the like. For example, length L may be three (3) inches and diameter D one (1) inch. The ratio of L/D is relevant to injection molding of assembly  10 . In order to provide for adequate uniformity of metal powder in the plastic material, the powdered metal-filled plastic member includes an injection-molding gate line  18  that extends at least partially along the radius R of the cylindrical surface. This may also be referred to as a tangent line because the line that extends along the radius of a cylinder is a line where a plane tangent to the surface intersects the surface. In the illustrated embodiment, gate line  18  extends at least 50% of the length L of cylindrical surface  15  and extends at least 80% of length L, although it may extend upward of 90% and even 100% of length L. Gate line  18  is formed by the flow of powdered metal-filled plastic material through a gate  25  which produces a runner  26  which extends from gate line  18  during the injection-molding process. In the illustrated embodiment, the cross-sectional area of the gate can be larger than entry to the mold. This reduces the distance the material must flow from gate point entry until the location where the material stops flowing. This reduces cycle time of the process. 
     In order to form gate line  18  in this manner, runner  26  is in the shape of an angular fan that extends outwardly from a sprue  28 . As is understood by the skilled artisan, runner  26  and sprue  28  are left over material in the channels of the mold that supply material to form powdered metal-filled plastic member  14 . A cold slug  30  of scrap material extends from sprue  28  distal of runner  26  and is formed from the first portion of the material shot injected toward the mold, which is at too low of a temperature to form a portion of the final core, as is known in the art. Runner  26  is removed in a conventional secondary operation thereby leaving leave gate line  18  remaining. As is also conventional, if powdered metal-filled plastic member  14  is to be a permanent magnet, it is magnetically treated after it is molded using an electrical current applied to coils in order to apply magnetic fields to core  14  in order to define two or more sets of magnetic poles (indicated by the symbols N, S, N, S, N, S in  FIG. 3 ), thereby forming a magnetized permanent magnet. All references herein to “permanent magnet” may refer to member  14  either before or after it is magnetically treated. 
     In the illustrated embodiment, powdered metal-filled plastic member  14  is formed in a mold  50  defining a gate  25  that opens at an angle to radius R of powdered metal-filled plastic member  14 . This causes the powdered metal-filled plastic material to travel generally tangentially along the curved surface  52  of mold  50  in which the powdered metal-filled plastic member is molded onto shaft  16 . As illustrated in  FIG. 7 , the material flows generally unidirectionally around the mold until it reaches the material entering through gate  25  at a knit line  54 . Thus, at least a portion of the material flows more than half of the circumference of the cylindrical surface of the mold forming core  14  and may flow substantially the full circumference of the cylindrical surface of the mold. The highest density of powdered metal is generally nearer the gate and the lower density is generally furthest from the gate. Therefore, any significant knit line illustrated schematically at  54  is formed adjacent gate line  18 . The absence of a knit line opposite gate  25  provides more uniform distribution of the powdered metal in the plastic. This provides better magnetic characteristics when used to produce a permanent magnet as would be understood by the skilled artisan. In the illustrated embodiment, gate  25  is at an angle α of between proximately 30 and 60 degrees to the radius of the curved outer surface of the permanent magnet. However, a greater or lesser angle could be used. 
     In another embodiment illustrated in  FIG. 6 , a bonded metal assembly  10 ′ is formed in a mold (not shown) having a gate  25 ′ that opens generally in line with radius R of powdered metal-filled plastic member  14 ′. In this embodiment, the powdered metal-filled plastic material travels generally tangentially, but in opposite directions around the mold. This results in a knit line  54 ′ that is opposite gate  25 ′. 
     In the illustrated embodiment, mold  50  used to manufacture the bonded metal assembly is defined by a left mold half  56  and a right mold half  58  that may open along a part line  60  defined along a plane generally at runner  26 . This allows bonded metal assembly  10  to be ejected radially from the mold using ejection pins (not shown) operating on outer portions  22  of cylindrical shaft  16 . This results in a shorter ejection motion that if permanent magnet assembly  10  were to be ejected axially as would be with conventional end gate injection-molding technology. Also, it allows cylindrical surface  15  to have a substantially constant radius throughout its length. This is because it does not need to be tapered to accommodate axial ejection from the mold. Cylindrical shaft  16  may be positioned in the mold during the injection-molding process using magnets (not shown) engaging outer portions  22 . However, other techniques may be used. While only one cavity is illustrated in  FIGS. 8 through 12 , another cavity may be provided that is the mirror image of that shown. This would allow a common sprue  28  to be used for two bonded metal assemblies. 
     In the illustrated embodiment, cylindrical shaft  16  has a central portion  20  and outer portions  22  on opposite axial sides of central portion  20 . Central portion  20  has a radius that is greater than the radii of outer portions  22 , as can be seen in  FIG. 4 . An intermediate step  32  may be provided between central portion  20  and each outer portion  22 . Powdered metal-filled plastic member  14  extends axially outwardly past central portion  20 . This causes the powdered metal-filled plastic member to envelope central portion  20  in order to provide a more mechanically stable assembly. Central portion  20  may have a knurled outer surface  34  in order to provide better rotational engagement between powdered metal-filled plastic member  14  and shaft  16 . In the illustrative embodiment, shaft  16  is made from stainless steel, although other materials may be used according to the application of the magnet assembly, such as solid iron, laminated iron, partial plastic, or the like. Also, other shapes for the form besides cylindrical may be provided. 
     A bonded metal assembly  110 , according to another embodiment, includes a recessed tangent gate line  118  that does not extend beyond the cylindrical surface  115  of a powdered metal-filled plastic member  114 . Gate line  118  is made using a gate  125  ( FIGS. 13 and 14 ). A matching recess  170  is formed in core  114  to provide rotational balance to the assembly. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.