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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
method of manufacturing an electromagnetic device, for example, a yoke used for an
electromagnetic speaker.
First, as a technical background, an entire structure of an electromagnetic sounding body which
is an example of a product to which the molding method of the present invention is applied will
be described. FIG. 2 is a cross-sectional view including the central axis of the electromagnetic
buzzer. The main part of each component is a concentric circular structure except for the lead
frame for terminals and the notch that accommodates it, so it will be easy to understand the
structure even if the overall plan view is omitted. A yoke 1 is made of a high magnetic
permeability soft magnetic material such as permalloy, and a convex cylindrical center pole 1a is
integrated with the central portion of the flat body. A buzzer case 2 is formed by molding a resin
by inserting the yoke 1 and the resin portion holds the peripheral portion of the yoke 1 and
forms a cylindrical wall surrounding the center pole 1a.
Further, two lead frames 3 are insert-molded together with the yoke 1 in a part of the buzzer
case 2. Both ends of the ridge of the drive coil 4 which is ridged in the shape of a flat cylindrical
ring and fitted to the center pole 1a are connected to the exposed portion 3a inside. A permanent
magnet 5 includes a rare earth element and has a ring shape (partly cut away from the mold
portion of the lead frame 1), is disposed around the drive coil 4, and is magnetized in the
thickness direction. A circular diaphragm 6 is a thin plate made of a soft magnetic material. The
diaphragm 6 is mounted on the cylindrical top 2a of the buzzer case 2 and is attracted and
positioned by the permanent magnet 5.
A predetermined gap G is provided between the lower surface of the diaphragm 6 and the top of
the center pole 1a. The sound current flowing through the drive coil 4 changes the amount of
magnetic flux connecting the center pole 1a and the central portion of the diaphragm 6, that is,
the attraction force of the diaphragm 6 downward, and generates sound. Therefore, the size of
the gap G determines the basic amount of magnetic flux, the sensitivity of the diaphragm
amplitude to the drive current, the attraction force and its nonlinearity, or the maximum
amplitude of the diaphragm is limited, so the volume and sound quality of the buzzer etc. It is an
extremely important quantity for product quality control because it sensitively affects acoustic
FIG. 3 shows the shape of the yoke 1. (A) is a plan view, (b) is a cross-sectional view passing
through the central axis. The error of the height H of the center pole 1a, the thickness T of the
flat plate portion 1b, and the warp amount B of the flat plate portion 1b are component
dimensions that directly affect the value of the gap G in the completed product of FIG. Therefore,
in the production of the yoke as a part, although there is a demand for cost reduction as a matter
of course, it is necessary to adopt a manufacturing method in which the shape accuracy of the
parts mainly composed of them is sufficiently satisfied.
2. Description of the Related Art A major method of manufacturing a conventionally used yoke
will be discussed. (1) Header processing method: The tip of a rod-like or cylindrical material at
normal temperature or high temperature is tapped with a mold to form a shape including a
center pole portion. Although the manufacturing cost is low, it is difficult to obtain the
dimensional accuracy of the finished part, and there is a problem that the gap G tends to vary.
(2) Press-in assembly method: The center pole portion machined by an automatic lathe is pressfitted into the central hole of the flat plate portion punched and assembled. The cost is high
because one part is divided into two parts, processed, and then assembled, and the assembling
accuracy also affects the dimensional accuracy of the finished product, which is not particularly
advantageous in terms of accuracy. Further, permalloy, which is a high permeability material, is
desirable as a yoke material, but this material is soft and rich in ductility, and is not very suitable
for the processing methods of the above (1) and (2).
(3) Drawing method: A strip-shaped plate material is processed by several steps little by little
with a remittance mold to gradually form a projection serving as a center pole in the center part
of the plate material by drawing, and finally removing it. Do. Although the processing method
itself is suitable for the permalloy material, the magnetic characteristics deteriorate unless the
processing residual strain is removed by annealing, which makes it difficult to reduce the size
and weight, and also has the disadvantage of increasing the cost of the progressive type.
SUMMARY OF THE INVENTION Electromagnetic devices such as electromagnetic sound
generators are in great demand particularly in portable electronic devices, their applications are
expanding, and new processing methods are desired which improve the disadvantages of the
prior art. An object of the present invention is to provide a new method of manufacturing a yoke
which is low in processing cost, has sufficient product accuracy, and is free from the problem of
residual strain.
SUMMARY OF THE INVENTION In accordance with the above object, the method of
manufacturing a yoke of the present invention has the following features. (1) A process of
preparing a powder material in which magnetic material powder of high magnetic permeability
and synthetic resin material powder are mixed at a predetermined ratio, and melting the powder
material and injecting it into a cavity of a powder molding mold; Cooling the mold and curing the
material in a product shape; removing the cured product in the product shape from the mold;
heating the cured product to vaporize and remove the synthetic resin material component; And
sintering the cured body from which the synthetic resin material component has been removed
at a temperature near the melting point of the magnetic material in this order.
The method of manufacturing a yoke of the present invention may further include the following
features. (2) The high magnetic permeability magnetic material is permalloy, the synthetic resin
material is polystyrene, the predetermined ratio is about permalloy 8 and polystyrene 2 in weight
ratio, and the electromagnetic device is an electromagnetic speaker To be
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic flow chart of a powder
injection molding method, which is an example of the embodiment of the present invention. First,
permalloy powder (weight ratio about 8) and polystyrene powder (weight ratio about 2) are
prepared as raw materials, and these are mixed in step 1 and thoroughly kneaded. In step 2, an
injection molding machine is prepared, a mixture of raw materials is melted and supplied to an
injection mold, filled in a cavity having the shape of a product (finished part), and then cooled in
the mold to Cure.
The product (semi-finished product) cured in step 3 is taken out of the mold and heated in a
drying furnace in step 4 to evaporate and remove the resin component. Then, the semi-finished
product in which the metal fine particles are roughly bonded in step 5 is sintered at a
temperature close to the melting point of the metal material. Thus, a finished product having a
predetermined dense composition is obtained. In the process of sintering, the semi-finished
product shrinks slightly, but the ratio is determined by the processing conditions. The
deformation by this processing method is also slight. The conditions can be sufficiently filled
before mass production.
FIG. 2 shows the structure and operation of the injection mold used in step 2 of FIG. (A) is
principal part sectional drawing just before material injection which shows metal mold ¦ die
structure, (b) is principal part sectional drawing which shows the taking-out operation ¦
movement of the semi-finished product hardened by cooling. A stationary cavity 10 has a runner
10a which is a passage for the molten material. A movable side cavity 11 has a cavity 11a in the
shape of a product (semi-finished product) connected to the runner 10a, and further has a
movable side core pin 11b for determining the height H of the center pole and an ejector pin 11c
for taking out the product (semi-finished product) . The runner stripper plate 12 covers the
runner 10a during molding and moves when the runner is taken out. Although the main part of
the mold is not shown, it also has a heating device or a water passage for forced cooling.
After the material is cooled, the process proceeds to step 3. As shown in FIG. 2 (b), the movable
side cavity 11 and the runner stripper plate 12 are separated from the stationary side cavity 10,
and the semifinished product 13 and the runner 14 (in this case, a cured product inside the
runner 10a which is a passage of molten material) Are separated, and the ejector pin 11 c ejects
the semifinished product 13. The runners 14 are also removed and recovered. The semifinished
product is sent to step 4.
Although the embodiments of the present invention have been described above, it goes without
saying that the present invention is not limited to the above-described embodiments. For
example, the target product is not limited to an electromagnetic sound generator, and may be a
yoke member of an electromagnetic device widely, the material of magnetic metal powder to be
used, material of synthetic resin material, mixing ratio, mold configuration, temperature and time
in each process The management conditions such as can be variously selected so that the effects
of the present invention can be exhibited.
According to the present invention, since powder injection molding is used to manufacture the
yoke, there is no need for molding a single product for a plurality of molds, and since there is no
need for assembling, the manufacturing cost is also compared. As the reproducibility of the
product is small, dimensional accuracy and deformation can be well within the allowable range
by setting conditions beforehand, and there is no residual strain due to processing, so there is no
adverse effect on the magnetic properties, and post-treatments therefor Various effects such as
becoming unnecessary were obtained.
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