Method of manufacturing magnetic, plastic-bonded molded bodies

In the manufacture of magnetic, plastic bonded molded bodies having different magnetic properties but the same physical dimensions, it has been found that variations in the amount of the magnetic mass constituent result in a change in the shrinkage behavior of the mass during and after shaping. Therefore, the shaping tools must be adapted to compensate for this varied shrinkage behavior. In order to prevent this varied shrinkage behavior, according to the invention, magnetic material in the mass is replaced by nonmagnetic inorganic material having the same or similar shrinkage behavior, the constituent of plastic remaining the same in both cases. Since the ratio of binder to material to be bonded remains constant as regards volume, in the final product, molded bodies having strongly different magnetic properties can be manufactured without having to change the shaping tool.

BACKGROUND OF THE DISCLOSURE 
The invention relates to a method of manufacturing a magnetic, 
plastic-bonded molded body which during its manufacture can be adjusted as 
regards its magnetic properties. A powdered magnetic material is provided 
which is mixed with a plastic binder to form a mass which is then 
subjected to a shaping process. This shaping process may be in particular 
extrusion or injection molding. 
During the manufacture of molded bodies from mixtures of plastic and 
magnetic materials, for example, plastic and ceramic powders of permanent 
magnetic material, two criteria are of particular importance for the user 
of such molded bodies: 
(a) the magnetic properties, and 
(b) the physical (mechanical) dimensions and the size tolerances. 
In a manufacturing process, the values of these criteria are selected in 
(a) by controlling the quantity of the component of magnetic material in 
the starting mass, and in (b) by accurately following the process 
specification during the shaping of the plastic and by accurately forming 
the shaping tools. 
When molded bodies having different magnetic properties but having the same 
physical dimensions are to be manufactured, the problem arises that the 
tools have to be changed since, when the same plastic components are used, 
a change in the quantity of magnetic material in the mixture of the 
components causes a different shrinkage behavior of the mass during and 
after shaping. 
Masses having different components of magnetic material thus result in 
products having different dimensions, when they are to be shaped by means 
of the same tool. Variations in the shaping conditions, such as 
plasticizing the mass, temperature of the mass, temperature of the tool, 
pressure during shaping, and pressure during the compaction step after 
shaping, cannot compensate for these differences to such an extent that 
the size of all products will lie within a uniform narrow tolerance range. 
It is known that desired decreased magnetic strength of molded bodies with 
a given filling of magnetic material can be obtained by using fields with 
decreased field strengths during the magnetization of the molded bodies. 
However, this method has the disadvantage that the magnetization takes 
place in the range of the steep slope of the magnetization curve (1st 
quadrant of the hysteresis curve) where small fluctuations in the 
magnetization energy result in large tolerances in the desired magnet 
strength. 
It is an object of the invention to solve this problem and to provide a 
method by means of which molded bodies having the same physical dimensions 
but different magnetic properties can be manufactured by means of the same 
tool. 
SUMMARY OF THE INVENTION 
In a method according to the invention, this object is achieved by adding 
so much of an inorganic non-magnetic filler material to the mass as is 
necessary to obtain a molded body of the desired magnetic properties with 
the volume ratio of binder to material to be bonded in the final product 
remaining the same. 
The magnetic material is preferably a ceramic powder of permanent magnetic 
material, for example, barium hexaferrite powder or strontium hexaferrite 
powder having a grain size of from 1 to 500 .mu.m with 65% of the number 
of grains less than 32 um. 
Iron oxide (Fe.sub.2 O.sub.3) or calcium carbonate (CaCO.sub.3) may 
advantageously be used as a non-magnetic filler material. When choosing 
the magnetic material and the non-magnetic filler it should be ensured 
that both components have approximately the same mechanical characteristic 
properties, mechanical characteristic properties being understood to mean 
in the first instance the grain size distribution and in connection 
therewith the packing density and the shrinkage behavior. At any rate, 
equal constituents by volume of the magnetic material should be replaced 
in the final product by equal constituents by volume of the nonmagnetic 
filler. 
It is also possible to process a nonmagnetic filler with a grain size 
distribution differing from that of the magnetic material and hence with a 
packing density differing from that of the magnetic material. Because the 
magnetic material has to be replaced by components of non-magnetic filler 
which correspond as regards volume in the final product, this means that 
with the same grain size distribution of magnetic material and 
non-magnetic filler, magnetic material may be replaced by non-magnetic 
filler in the ratio 1:1. However, in the case of unequal grain size 
distribution of the magnetic material and the non-magnetic filler the 
replacement ratio has to be calculated separately. The empirical 
determination of the componets of magnetic material and non-magnetic 
filler to be added to the mass, taking into account the material-specific 
parameters influencing the packing density will present no problems to 
those skilled in the art. 
The advantages resulting from the invention are in particular that as a 
result of the constancy in the ratio of plastics to the other constituents 
of the mass (in this case: magnetic material and non-magnetic filler) as 
regards the volume of the final product, molded bodies with different 
magnetic properties can be manufactured without having to change the 
shaping tool. As a result of this it is possible to manufacture a magnetic 
molded body, having a plastic binder, with different magnetic strengths 
while using the same shaping tool--that is to say at minimum costs--since 
the part by volume of the above-mentioned constituents of the mass, hence 
magnetic material and non-magnetic filler, is selected at will and in 
accordance with the requirement of varying quantities of magnetic material 
and non-magnetic filler. 
A further advantage is that magnetization of the molded body can always be 
carried out in the saturation range of the magnetization curve, which 
results in considerably smaller tolerance differences of the magnetic 
properties. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As an example of the invention, the manufacture of rings, having an outside 
diameter of 93 mm and an inside diameter of 70 mm by injection molding 
shows how desired magnetic properties of the molded bodies can be adjusted 
by varying the constituents of permanent magnetic material and filler in 
the masses with constant shrinkage. In these examples a filler was used 
whose mechanical characteristic properties were equal to those of the 
permanent magnetic powder used. A mass according to the examples of the 
invention is prepared so that first the plastic, for example a polyolefin, 
is plasticized via a thermal and compression treatment in agreement with 
the directions of the manufacturer of the plastic. To this plastic mass 
are then added the further constituents of the mass, such as ceramic 
permanent magnetic powder, for example barium hexaferrite powder, filler, 
for example Fe.sub.2 O.sub.3 powder with the same grain size distribution 
as the ceramic permanent magnetic powder, heat stabilizers; for example 
.beta.,.beta.-thio-di-(propionic acid lauryl ester), lubricant, for 
example dioctylphthalate, and flame-retarding additives, for example 
inorganic oxides such as Sb.sub.2 O.sub.3 or organic halogen compounds 
such as perchloropentacylodecane. Dependent on the mixing aggregate the 
mass is mixed to at most 30 minutes until a homogeneous distribution of 
all constituents is obtained, and then supplied to an extrusion press on 
which the compression mass is densified and compressed to thin rods of 
.about.4 mm diameter. The extruded material is granulated and is the 
starting material for a subsequent injection molding process in which the 
final molded bodies, in this case the above-mentioned rings, are 
manufactured at a temperature of approximately 230.degree. C. dependent on 
the plastic used. The magnetization of the molded bodies is carried out 
after shaping the molded bodies. 
During processing of the masses according to the invention, not only the 
extrusion or injection molding process may advantageously be used, but 
also deformation and shaping methods in which powdered masses are 
processed and in which the molded bodies are heated after shaping may be 
used.