This invention relates generally to the making of permanent magnets by molding magnetizable material under the influence of a magnetic field. More particularly, it relates to the injection of a matrix material comprising a binder and magnetizable material into a mold which provides not only saturation magnetization, but also means for releasing the newly formed magnet from the mold by demagnetization of the molding apparatus. Description of the Prior Art

Injection molding of magnetizable materials to form permanent magnets is known in the prior art. Typically, the injected material is a matrix of approximately 90% barium ferrite and 10% binder. The more binder that is used, the less magnetic material is available in the particular part, and the less magnetization there will be. The binder may be nylon or polyethylene or other non-magnetic material, the viscosity of which at certain temperatures permits injection into a molding apparatus. The temperature of the molding is set by the type of plastic binder. For example, a typical temperature for nylon is approximately 260 degrees to 280 degrees C. The material is maintained under pressure after it is injected into the mold to compact the material while it hardens into the desired magnet.

In the prior art, a coil is placed around the molding tool and energized to produce a small electromagnetic field. This aligns the magnetic grain structure of the matrix, producing a weak magnet. However, this procedure does not saturation magnetize the matrix. Most molds are not designed to maximize magnetic flux density in the part being molded. Accordingly, it is very difficult to create enough flux in the molding tool

to saturation magnetize the part. The unit of magnetizing force is termed the oersted, while the unit of flux density is measured in gauss. An oersted is measured in ampere turns per inch, or per meter, in the magnetizing coil, while a gauss is one maxwell per square centimeter. The strength of the magnetic field produced by the coil is dependent upon the number of turns in the coil and upon the current flowing through the coil. Approximately 3,000 oresteds is required in an efficient magnetic circuit to totally saturation magnetize the part.

Therefore, after the part is physically ejected from the mold, if a weak magnet is not acceptable for a given application, the part must be further magnetized to make it a stronger magnet. A commonly known method for later saturation magnetizing such a part is to discharge a capacitor into a coil in which the ejected magnet has been placed to produce a very high current for a very short time. The necessity in the prior art of a multiplicity of magnetization operations decreases efficiency of manufacture by increasing the labor, time, and cost requirements of producing a new magnet.

A typical prior art apparatus for making a permanent magnet is disclosed in Steingroever, U.S. Patent No. 3,564,654. In this patent, a magnetizable powder is compressed into a press hole and subjected to a "radial" magnetic field from a field coil. A separate demagnetizing coil is then energized to demagnetize adjacent parts of the press. This reference differs from the present invention in that a magnetizable powder is only partially subjected to a "toroidal" magnetic field. In addition, the strength of this field is dissipated by a ram which must be introduced into the press hole in order to compress the magnetizable material. The magnetizing coil is placed around the outside of the press and creates only a small magnetic field in the

pressing hole. While this may result in a serviceable magnet, the electromagnetic field typically produced is never enough in a practical tool to totally saturation magnetize the part. In order to achieve the approximately 3,000 oersteds of coercive magnetizing force necessary for saturation magnetization, the present invention differs from the prior art in that it creates a closed loop magnetic circuit in which the only gap in the loop is filled by the magnetizable material that is to be molded. Thus, the magnetizable material in the present invention effectively comprises a flux path segment surrounded by non-magnetic material such that the coercive magnetic force is maximized in the magnetizable material. Another prior art method for pressing and aligning magnetic powders to produce a so-called radially oriented torodial magnet is disclosed in Leupold, et al., U.S. Patent No. 4,592,889. This patent differs from the present invention in that two coils, disposed at a distance apart from the magnetizable material, are energized to create magnetic flux which is conducted to the magnetizable material along relatively small diameter metal rods. The magnetizable material is not placed within the strong magnetic field itself. Also, magnetic flux is introduced into the cavity containing the magnetic material from two directions in order to create the required saturation magnetic flux density. This is necessary due to the limited flux carrying capability of the small diameter rods. The present invention provides an advantage over the prior art in that the magnetizable material is disposed within the actual magnetic field of the coil in such a way that a previously open magnetic circuit is closed by the magnetizable material. Magnetically impermeable material surrounds the magnetizable material such that the lines of magnetic flux are maximized or

concentrated in the magnetizable material to thereby produce a much more powerful magnet than was previously possible.

The present invention also solves the problem of non-destructive separation of the newly fabricated magnet from the mold apparatus. Magnets made by the prior art methods and apparatus tend to stick to adjacent metal parts of the mold, including ejection mechanisms, which also become magnetized when the matrix is subjected to the electromagnetic field. This creates obvious handling difficulties. The present invention overcomes this problem by providing a relatively small AC current pulse for temporarily demagnetizing the mold when ejecting a new magnet from the apparatus. Summary of the Invention

Accordingly, it is an object of the present invention to provide an improved magnet molding apparatus and a method for making permanent magnets.

Another object of the present invention is to provide an apparatus and method for molding and magnetizing material such that the labor, time and cost requirements for making magnets are reduced and efficiency of the molding and magnetizing operation is increased. A more specific object of the present invention is to provide an apparatus and method for molding and magnetizing material such that, upon completion of the molding and magnetizing operations, the magnet so formed is released from magnetic attraction to adjacent metal surfaces of the molding apparatus or other equipment. To accomplish these objects and others, the present invention provides a magnet mold apparatus for forming magnets from magnetizable material under the influence of a magnetic field. The apparatus includes a frame having at least one chamber into which the magnetizable material is injected by injection means via

an aperture connecting each chamber to the exterior of the apparatus. The magnetizable material may be a matrix of ferromagnetic material in a plastic carrier. A removable cap seals the magnetizable metal in the chamber, while an electromagnetic field is established by at least one coil conducting direct current from a direct current source.

The chamber for containing the magnetizable material is disposed with respect to a coil such that the circuit of the magnetic fields produced by the coil passes directly through the magnetizable material contained in the chamber without any air gaps whatsoever. That is, the magnetizable material is positioned to become a pole piece of an electromagnet. Preferrably, the magnetizable material is positioned in the core of the coil. Thus, the magnetizable material itself comprises a portion of the magnetic circuit such that the lines of magnetic flux are concentrated and maximized within the material. The maximization of magnetic flux is aided by a shell of non-magnetic material which surrounds each die cavity such that the lines of magnetic flux are concentrated and focused within the cavity containing the material to be magnetized. This intensifies the magnetizing force within the chamber containing the magnetizable material. The electromagnetic field thus established aligns the magnetic particles in the magnetizable material and saturation magnetizes the molded part to create a permanent magnet. After this saturation magnetization step and the tool is opened, the molded part remains in the tool in part due to the magnetization of the tool. The part must be ejected out of the mold cavity, typically by pushing an ejecting pin forward. To overcome the magnetization of the tool and also the ejector if it is of magnetic material, at least one.coil is momentarily

energized by alternating current from an alternating current source. This alternating current pulse must be of sufficient power to demagnetize the tool just enough to enable the molded part to be ejected from the tool but not strong enough to destroy the permanent magnetism of the molded part. In other words, the AC demagnetizing current pulse enables the molded magnet to be ejected by the ejector pin without being magnetically attracted to either the pin or the mold apparatus. Brief Description of the Drawings

Figure 1 is a partially exploded perspective view of a preferred embodiment of a molding apparatus according to the present invention.

Figure 2 is a vertical cross-sectional view of the molding apparatus shown in Figure 1.

Figure 3 is a schematic drawing of control circuitry in a preferred embodiment of a magnet molding apparatus according to the present invention.

Detailed Description of the Preferred Embodiment Referring to Figures 1 and 2, an apparatus according to the present embodiment provides a die tool or mold 10 for forming two injection molded magnets 25. The mold apparatus 10 is comprised generally of a container 12, a cap 14, and a non-magnetic section 20 which define the die cavities 28 which hold the magnetizable material. The container 12 and cap 14 are separable along a parting line 16.

The container 12 is made of a magnetic material, and includes a coil 18. Coil 18 is energized to apply a magnetic field 34 to the parts 25 to be magnetized, as shown in Figure 2.

In a preferred embodiment, the coil 18 is located within the body of the container 12 and disposed circumferentially around two die cavities 28. In the preferred embodiment, the two cylindrical die cavities or chambers 28, each receive injected magnetizable material

25. The chambers 28 are closed at their uppermost end by cap 14 such that magnetically conductive material in the cap 14 seals the ends of the chambers without any air gaps between the cap 14 and the injected magnetizable material. This is important in order to create a completely closed magnetic circuit through the chambers. The non-magnetic section 20 which defines the sides of the die cavities or chambers 28 is comprised preferably of stainless steel. It is important that the material defining the chambers 28 exhibit structural strength. At the same time, the material must not significantly weaken the coil's magnetic field in the area of cavities 28 except along the path of the magnetic circuit formed by the lines of magnetic flux 34. Any material having the characteristic of non-magnetic conductivity and also having good structural strength may be used to define the side walls of chambers 28 of the mold 10.

It will be appreciated that the non-magnetic material in section 20 aids in maximizing the magnetic flux density in the cavities 28 and consequently in the magnetizable material 25. The cavities 28 are disposed in parallel alignment with the magnetic poles of the coil 18. The position of the cavities 28 is such that they preferrably form the only gap in the magnetic circuit formed by the lines of magnetic flux 34. Once the cavities 28 are filled with the magnetizable material 25, this completes the magnetic circuit and enables an especially strong magnetizing force to be applied which saturation magnetizes the parts 25. Magnetizable material is injected into the chambers 28 through a conventional trough 17 along the parting line 16. The chambers 28 are sealed at one end by a removable cap 14 and at the other end by ejector pins 30 and 32, respectively. The ejector pins 30, 32 are preferably metal since they are normally positioned at one end of the chambers 28 and as much metal as

possible should be there in order to create a goo _d magnetic field at the pole. As shown in Figure 1, the pins can be actuated to provide the ejection function using mechanical or other conventional driving means. The present invention takes advantage of the fact that the most efficient magnetic circuit is a closed loop with a coil around it. Normally, magnetic material has an intrinsic coercivity in the order of 2,000-3,000 oersteds, and generally more is required due to circuit inefficiency. In the present invention, there are preferrably 80-100 turns in the coil 18, requiring that a minimum of 30-40 amps be coupled through the coil in order to produce the magnetic flux required to saturation magnetize the part being molded. As will be explained, a bank of capacitors in the control circuit of the present invention can be used to increase this magnetization current by four to five times.

It is an object of the present invention to minimize the gap in the magnetic circuit and to create as closed a magnetic circuit as possible. As can be seen from Figure 2, preferrably the only gap in the magnetic circuit shown by lines of flux 34 is provided by the chambers 28 for receiving the magnetizable material. The chambers 28 are aligned in parallel orientation with the magnetic lines of flux and the magnetic pole of the coil 18. It is important that the material to be magnetized completes the magnetic circuit thereby enabling the maximization of lines of magnetic flux in the chamber. In other words, the magnetizable material is placed directly in the magnetic circuit where.the lines of magnetic flux are most intense. The flux density of the magnetic field in the present invention is not dissipated, as in prior art methods, by being carried along magnetic conductors or being required to cross air gaps. The present invention prevents loss of coercive magnetic force by minimizing any gaps in the magnetic

circuit path shown by the magnetic lines of flux ₃₄ . The existence of one or more gaps in prior art mold devices necesitates the generation of a larger magnetic field in order to generate the same magnetic flux in the molded part. Moreover, excess heat would be generated in the mold as a result of such gaps and the resultant need for a stronger magnetic field.

The gap in the magnetic circuit path 34 is eliminated when magnetizable material is injected into the chambers 28. When coil 18 is energized, the injected magnetic material within chamber 28 provides a completed circuit path for the magnetic lines of flux 34. This configuration, which includes the non-magnetic section 20 for positioning the magnetizable material directly in the magnetic field produced by the coil, maximizes the amount of coercive force applied to the magnetizable material.

The magnetizing force of the present invention is also maximized through the use of the non-magnetic material in section 20 outside of the magnetic circuit path. This non-magnetic material prevents the deflection of magnetic flux along any path outside of the molded part. That is, the magnetic field is not shorted out or deflected away from the magnetizable material in chamber 28 by any magnetic conductor formed in the walls of chambers 28.

Furthermore, as seen in Figure 2, stainless steel or some other non-magnetic material is preferably used for risers 100 and/or the back plates 110. Such non- metallic materials prevent losses or4 short-circuiting of the magnetic lines of flux 34 through the framework of the molding apparatus.

Accordingly, the present invention provides a significant advantage over prior art devices by enabling a magnet to be formed and saturation magnetized in a single process through the application of a strong magnetizing field from within the mold ^" apparatus such

that a closed loop magnetic circuit is provided directly through the material to be magnetized. In the prior art, the location of a coil around the outside of the mold apparatus may have been sufficient to align the magnetic grain structure of the magnetizable material, but was never enough to totally saturation magnetize the part. According to the present invention, during the injection process, when the magnetizable material is still in the liquid state, coil 18 is energized to apply a magnetic field to the apparatus. The magnetic grain structure in the thermoplastic part being magnetized is thus aligned in a desired orientation by energizing the coil, and the part is magnetized. That is, a magnetic field is created between two poles which is of sufficient strength to orient the grains of magnetic material in the material to the axis of the desired magnet. When the magnetizable material has solidified within the chamber 28 and before the mold is opened, a large magnetic field is created, as will be described, by discharging a bank of capacitors into coil 18 to provide a very high current for a brief period of time. This is the commonly known method of saturation magnetization. After the part 25 is saturation magnetized, it remains in the mold apparatus when the mold is opened along parting line 16. To overcome the difficulty of ejecting the strongly magnetized part from a mold according to the present invention, a control circuit causes an alternating current to be passed through the coil 18 to create an alternating magnetic field around the opened area of the mold. This prevents the magnetized part from sticking to the end of an ejector 30, 32 or any other part of the mold apparatus, and the magnet normally is thereby removed by gravity from the mold. The mold can then be closed again and the process repeated. This provides the advantage of a greatly accelerated magnetizing process and obviates the need to manually remove each magnet from

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the mold when magnetization is completed.

The control circuitry of the present invention will now be described with reference to Figure 3. The control circuitry is coupled by input/output leads 22a and 22b to coil 18 of the injection molding apparatus. The control circuit for controlling magnetization of the part is divided into three sections. The first section 42 provides a DC output voltage in the range of approximately 10-12 volts to generate a low current in coil 18 that orients the magnetic particles during the injection molding process. The second section 52 comprises the saturation magnetization circuit for saturation magnetizing the part. The third section 62 provides the alternating current pulse or jolt which demagnetizes the mold and enables the ejectors 30 and 32 to separate the magnetized parts from the mold apparatus without affecting the saturation magnetization of the parts.

Individual selection of a given circuit section 42, 52 and 62 for output to the coil 18 is controlled by relays Rl, R2 and R3. Conventional circuitry (not shown) ensures that if any one relay is engaged, the remaining relays, and consequently the other circuit sections, are disabled. Note also that solid state devices are preferrably used to do the switching performed by Rl, R2, and R3 described hereinbelow.

When the mold is closed, and while magnetizable material is being injected into the mold apparatus, circuit section 42 is activated to couple a low current to the coil 18. The first circuit section 42 includes a VARIAC isolating transformer 44, a first rectifier circuit (full wave bridge) 48, a capacitor 50 and a relay or solenoid activated switch Rl. This circuit section provides a DC output voltage in a range of 10-12 volts from the rectifier 48' to the coil 18 in the mold apparatus. In operation, relay Rl is switched on and

kept on during the entire process of injecting magnetizable material into the die cavity 20. Relay R _l may also remain energized during the cooling period for the magnetizable material. Consequently, circuit section 42 causes the grains of magnetic material injected into ^' cavities 28 to be oriented in a desired direction by coil 18. Typically, the injection time is approximately 3-5 seconds, and the cooling time is approximately 7-10 seconds. The second control circuit section 52 provides the output current to coil 18 for saturation magnetizing the parts 25. Circuit section 52 is controlled by relay R2. Normally, R2 is activated either at the end of the injection period or at the end of the cooling period of the magnetizable material. Circuit section 52 includes a VARIAC isolating transformer 54 having its output applied to full wave rectifier 56. This produces a full wave rectified AC voltage output, which approximates a DC voltage source of 300 volts, at the output node of rectifier 56. This voltage source is then applied to charge up a bank of capacitors 58a, 58b, 58c and 58d connected in parallel. ^" This capacitive system is used to generate the very high current needed to saturation magnetize the parts 25. The capacitors 58a, 58b, 58c and 58d are discharged through the coil 18 in a very short time, on the order of 1-2 milliseconds, by means of relay R2. This discharge time is a function of the capacitance of the capacitors 58 and the voltage at which they have been charged. The current created by this discharge of capacitors 58a, 58b, 58c and 58d is fed to coil 18 and provides the high density magnetic flux which saturation magnetizes the parts 25 in the mold 10. The magnitude of this current may be 100 amps or more. The duration of this current pulse is kept short to prevent overheating of the coil 18. As mentioned above, coil 18 has between 80 and 100 turns and thus a resistance of about 2-3 ohms.

Note also that while capacitors 58a, 58b, 58c and 58d are discharging through the coil 18, relay R2 disconnects rectifier 56, and thus the voltage source output therefrom, from these capacitors. When the mold apparatus is opened, relay R3 is activated. Relay R3 controls the third section 62 of the control circuitry. Circuit section 62 produces an AC voltage to demagnetize the mold apparatus. Normally, the voltage applied by this circuit to coil 18 is about 7-8 volts but may vary depending upon the type of setup.

Only a very small amount of current is created by this voltage, just sufficient enough to cause the parts 25 to separate from the mold apparatus. Relay R3 is activated during the time that the mold apparatus is open. It can be seen that the apparatus according to the present invention enables magnets to be made and saturation magnetized in the same process. The apparatus according to the present invention also enables a magnet to be quickly and easily separated from a mold device by providing a small AC current to demagnetize the mold apparatus at the completion of the manufacturing process, thereby enabling the magnetic part to separate from the mold.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.