Inorganic media for dry barrel finishing

Inorganic media for barrel finishing produced by sintering a media green body formed from a mixed material comprising clay fine grains as a binder, abrasive grains and aluminum hydroxide fine grains as a brittleness imparting agent. The inorganic media is excellent in finishing performance, with which rough finishing can be conducted by dry barrel finishing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improved inorganic media for dry barrel 
finishing. The dry barrel finishing herein means barrel finishing using no 
water that is conducted for removing flushes, rounding edges, leveling and 
burnishing of an article to be finished. The inorganic media of the 
invention can be used in various barrel finishing apparatuses such as 
rotary barrel, vibration barrel, flow barrel and centrifugal barrel. 
2. Description of Related Art 
As media used in the dry barrel finishing (particles having abrasive 
power), those having an arbitrary shape, such as a sphere, a triangular 
pyramid and a triangular prism, in a size of from several millimeters to 
several tens millimeters have been used. The media generally comprise an 
abrasive compound assuming the abrasive power and a binder (matrix) 
maintaining the abrasive compound to substantially constitute the media. 
The media are roughly classified into three classes, organic media, 
metallic media and inorganic media, depending on the main component of the 
binder. 
The organic media are generally obtained by mixing and heating an inorganic 
abrasive grains and a binder such as a polyamide resin, followed by 
forming, as described, e.g., in Examined Published Japanese Patent 
Application No. 6-55446. While the barrel finishing using the organic 
media causes less roughening of a finished surface due to their soft 
nature, they have a small specific gravity and small abrasive power. The 
organic media are thus used for intermediate finishing such as leveling. 
The metallic media are composed of metal balls themselves, as described, 
e.g., in Unexamined Published Japanese Patent Application No. 60-180765. 
Since the metallic media do not contain an abrasive compound, 
substantially no abrasive power is expected. They are used for fine 
finishing (mirror finishing) such as polishing utilizing their function of 
crushing and leveling micro-projections on the surface of an article to be 
finished. 
The inorganic media are formed by sintering a mixed material of abrasive 
grains such as alumina and an inorganic binder such as bauxite, as 
described, e.g., in Examined Published Japanese Patent Application No. 
44-23873. Since the inorganic media are inexpensive and have large 
abrasive power, they are used for rough finishing. The rough finishing 
herein is the opposite of the fine finishing and includes flush removal 
and edge rounding. 
In the wet barrel finishing, a mass composed of works (articles to be 
finished), media and abrasive water is filled in a finishing barrel, and 
the mass is fluidized by rotation, revolution or vibration of the barrel 
or rotation of a rotating plate equipped at the bottom of the barrel to 
conduct finishing. As a result of flow of the mass, flush removal, edge 
rounding, leveling or polishing is conducted by friction between the works 
and the media. The wet barrel finishing has a characteristic feature in 
that a large number of articles having a complicated shape can be treated 
at once, and it has been employed in production processes of a wide range 
of products. However, the wet barrel finishing requires disposal ment of 
waste abrasive water, and there is a demand for making the waste water 
treatment easier. 
In order to solve the above problem associated with the wet barrel 
finishing, various dry barrel finishing methods using no abrasive water 
have been proposed. 
A dry barrel finishing is proposed in which a spindle oil or a fat and oil 
is used instead of abrasive water, as described, e.g., in Examined 
Published Japanese Patent Application No. 50-13994 and 57-57105. In this 
method, however, the oily component adhered on the articles to be finished 
must be removed when the following step is coating or plating. Thus, in 
this dry barrel finishing, waste water generates after washing the oily 
component, and as a result, a problem of waste water treatment arises. 
Another dry barrel finishing is proposed in which no liquid material is 
added, as described, e.g., in Unexamined Published Japanese Patent 
Application No. 50-13994. However, the use of this finishing method is 
generally limited to final finishing such as polishing. 
When rough finishing is conducted according to this dry barrel finishing 
using an inorganic media, the abrasive power (sharpness) of the inorganic 
media is soon decreased, and rough finishing cannot be stably conducted. 
The reason of the use of inorganic media is that the inorganic media has 
the largest abrasive power among the three types of media. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an inorganic media having 
excellent finishing performance for dry barrel finishing capable of 
conducting rough finishing. 
The inorganic media of the invention is produced by sintering a media green 
body formed from a mixed material comprising clay fine grains as a binder, 
abrasive grains and aluminum hydroxide fine grains as a proper brittleness 
imparting agent. 
A sintering agent may be added to the media green body. Examples of the 
sintering agent include at least one of iron oxide (II), iron oxide (III), 
manganese oxide (II), manganese oxide (III) and manganese oxide (IV). The 
sintering agent may be added in an amount of from 0 to 5% by weight based 
on the amount of the green body. 
The characteristic feature of the invention resides in the point in that 
aluminum hydroxide is further added to and mixed with the conventional 
mixture of clay grains and abrasive grains, followed by sintering. As a 
result of the use of the composition to which aluminum hydroxide is added, 
the reduction in abrasive power due to the continuous dry barrel finishing 
is suppressed to increase wear of the media, in comparison to the 
conventional inorganic media. This is supported by the Examples described 
later. 
While the function of aluminum hydroxide cannot be clearly concluded, it is 
considered as follows. 
Aluminum hydroxide is converted to alumina as a result of dehydration 
decomposition at a temperature of from 200 to 300.degree. C. to reduce its 
volume as a solid component, and hydroxyl groups escape into the air as 
water vapor. Based on this knowledge, the inventors have found that fine 
pores or cracks are generated in the course of sintering of the media due 
to the volume reduction of aluminum hydroxide and the escape of water 
vapor, and the fine pores or cracks remain after sintering. 
The inorganic media is in a state owing to the presence of the fine pores 
or cracks, in which the media is suitably subjected to brittle fracture 
(cleavage fracture and grain boundary fracture). After repeated collision 
with articles to be finished in suitable times, the surface of the 
inorganic media is suitably peeled off to expose a non-worn abrasive 
material (abrasive grains) present inside the media. As a result, the 
reduction in abrasive power is suppressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The inorganic media of the invention is produced by sintering a mixed 
material comprising clay grains, abrasive grains and aluminum hydroxide 
fine grains in the form of media grains. The percents used as a unit 
showing mixing ratio is percents by weight unless otherwise defined. 
The clay grains used herein include an aggregate of fine water-containing 
silicate minerals generally having a grain diameter of from 0.01 to 0.002 
mm or less, which exhibits plasticity by mixing with water and is sintered 
by baking. Specifically, kaolin, bentonite, woody clay, quartz-containing 
clay(shale clay) and agalmatolite can be used as the clay grains. The clay 
grains function as a binder (matrix), and general-purpose clay used for 
pottery and roof tiles having good miscibility with the abrasive grains 
and aluminum hydroxide can be used. 
The abrasive grains are not particularly limited if they have miscibility 
with the clay grains, and general-purpose abrasive grains can be used. The 
abrasive grains having a grain size of #32 to #250 of Tyler mesh (0.495 to 
0.061 mm opening) can be used. When the grain size is too small, rough 
finishing is difficult to be conducted. When the grain size is too large, 
the abrasive grains are liable to drop from the media on barrel finishing. 
The abrasive grains may be natural materials or artificial materials, and 
materials generally called abrasive materials can be used. Examples of the 
natural materials include sandstone, corundum, emery, garnet, flint and 
diamond sand. Examples of the artificial materials include artificial 
corundum, synthetic diamond, various nitrides (boron nitride, aluminum 
nitride and titanium nitride) and various carbides (silicon carbide and 
boron carbide). Among these, White Alundum #100 (manufactured by Norton 
Co., artificial corundum (white aluminaous abrasive grains)) is 
particularly preferred. 
Aluminum hydroxide is expressed by chemical formula Al(OH).sub.3. Not only 
alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) but also alumina 
monohydrate can be used. 
In the case where there is a possibility that the brittleness of the media 
owing to aluminum hydroxide is too high, i.e., in the case where wear of 
the media proceeds too fast, the brittleness (easiness of cleavage) can be 
reduced by adding a sintering agent. 
The sintering agent is a component to be added to accelerate sintering. The 
sintering agent forms a liquid phase at a sintering temperature, and 
enhances wetting of the grains to accelerate migration of the components. 
Specifically, iron oxides having a relatively low melting point (FeO, 
Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4) and manganese oxide (MnO, Mn.sub.3 
O.sub.4, Mn.sub.2 O.sub.3 and MnO.sub.2) may be used. 
The ratios of the components are decided depending on the desired 
characteristics, for example, whether maintenance of sharpness precedes or 
suppression of wear precedes, and generally can be as follows. 
Clay grains (binder): about from 40 to 70%, preferably about from 45 to 60% 
Abrasive grains: about from 15 to 45%, preferably about from 20 to 40% 
Aluminum hydroxide: about from 5 to 45%, preferably about from 10 to 35% 
Sintering agent: about from 0 to 5%, preferably about from 1 to 3% 
In the above ratios, when the amount of the clay grains is too small, it is 
difficult to exhibit the role as a matrix (binder). When it is too large, 
the ratios of the abrasive agent and aluminum hydroxide become relatively 
small, to make difficult to maintain the power of rough finishing of the 
invention. When the amount of the abrasive grains is too small, it is 
difficult to reach the power of rough finishing of the invention. 
When the amount of the abrasive grains is too large, the ratios of the 
binder and aluminum hydroxide become relatively small, to make difficult 
to attain the effect of the invention, i.e., maintenance of power of rough 
finishing for a long period of time. 
When the amount of aluminum hydroxide is too small, it is difficult to 
supply the media with the suitable brittle fracture property, to make 
difficult to maintain the power of rough finishing of the invention for a 
long period of time. When the amount of aluminum hydroxide is too large, 
the brittleness fracture property becomes too high. 
The sintering agent is appropriately added, and when the amount thereof 
exceeds 5%, it is difficult to prepare a media having a prescribed shape. 
The inorganic media of the invention can be prepared by a process similar 
to that for an inorganic media for the conventional wet barrel finishing, 
i.e., a suitable granulation process corresponding to the shape 
(triangular pyramid, column, polygonal column, sphere and cone) selected 
depending on the object of finishing can be employed. Examples of the 
granulation process include rolling granulation, fluidized bed 
granulation, stirring granulation, press granulation, spray granulation 
and extrusion granulation. In the case of extrusion granulation, for 
example, the components in a powder form are weighed and mixed with each 
other, and mixed and kneaded with from 10 to 20% of water to form a 
slurry. The resulting slurry is put in an extruder and extruded to have 
the prescribed cross sectional form, followed by cut into the prescribed 
length. 
The resulting green media are sintered at a temperature of from 1,000 to 
1,500.degree. C. for from 1 to 3 hours, preferably at a temperature of 
from 1,200 to 1,300.degree. C. for from 1.5 to 2.5 hours. When the 
sintering temperature is too low, it is difficult to obtain a mechanical 
strength sufficient as media. The sintering temperature is too high, it is 
difficult to maintain the shape of media because of generation of thermal 
cracks. 
The range of the suitable sintering temperature should be adjusted 
depending on the composition of the media. Accordingly, in the case where 
media having a new compositional constitution and/or a new shape are 
sintered, the suitable sintering temperature should be previously searched 
by conducting a sintering test of a test sample. 
The inorganic media thus produced are put into a dry barrel finishing 
apparatus along with articles to be finished to conduct barrel finishing. 
On barrel finishing, the surface of the inorganic media is suitably peeled 
off to expose non-worn abrasive grains on the surface of the media, as 
described above. As a result, the inorganic media do not suffer from the 
reduction in the abrasive power with the lapse of time as in the 
conventional case, and the abrasive power of the inorganic media is 
maintained for a long period of time. 
Description will now be made of a number of non-limiting examples and 
comparative examples in which tests were conducted to ascertain the 
advantages of the invention. 
EXAMPLES 1 TO 3 AND COMATIVE EXAMPLES A TO C 
In Examples 1 to 3, inorganic media of the invention were prepared where 
the mixing ratios of the clay fine grains, abrasive grains, aluminum 
hydroxide and a sintering agent were changed. In Comparative Examples A 
and B, inorganic media for the conventional wet barrel finishing and 
inorganic media for comparison were prepared respectively. The 
compositions of Examples and Comparative Examples are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Sintering agent 
Binder Abrasive grains 
Aluminum 
Manganese 
Iron oxide 
(Clay fine grains) 
WA #120 
WA #240 
hydroxide 
oxide (IV) 
(III) 
__________________________________________________________________________ 
Example 1 
50% 24% -- 24% 2% -- 
Example 2 
48% 24% -- 24% 2% 2% 
Example 3 
50% -- 40% 10% -- -- 
Comparative 
75% 25% -- -- -- -- 
Example A 
Comparative 
72% 25% -- -- 3% -- 
Example B 
__________________________________________________________________________ 
As aluminum hydroxide, "Hydilite H031" manufactured by Showa Denko Co., 
Ltd. was used. Its chemical composition and properties obtained from the 
catalogue of the manufacturer are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Chemical Composition 
Al(OH).sub.3 
Fe.sub.2 O.sub.3 (%) 
SiO.sub.2 (%) 
Na.sub.2 O (%) 
w-Na.sub.2 O 
__________________________________________________________________________ 
99.8 0.01 0.01 0.16 0.02 
__________________________________________________________________________ 
Adhered water 
Average grain size 
+75 .mu.(%) 
pH (30% slurry) 
Specific surface 
content (%) 
(.mu.) area (m.sup.2 /g) 
__________________________________________________________________________ 
0.10 18 0.02 9.3 1.4 
__________________________________________________________________________ 
As manganese oxide and ferric oxide for a sintering agent and clay fine 
grains for a binder, the following commercial products were used. 
Manganese oxide (IV): "Manganese Dioxide 65" manufactured by Heiwa 
Chemicals Co., Ltd. 
Iron oxide (III): "Ferric Oxide 85" manufactured by Heiwa Chemicals Co., 
Ltd. 
Clay fine grains: "SB-83" manufactured by Maruo Ceramics Co., Ltd. 
For the inorganic media of Examples and Comparative Examples, the raw 
materials in a powder form were mixed according to the formulation shown 
in Table 1, and about 15% of water was added thereto to form a slurry. The 
resulting slurry was put in an extruder to extrude with a circular cross 
section having a diameter of about 5 mm, and cut to a length of about 15 
mm, followed by drying, to prepare green media. The green media was put in 
a heat resistant container and sintered at a temperature of from 1,200 to 
1,300.degree. C. for about 2 hours, to produce the intended inorganic 
media. 
The resulting inorganic media for dry barrel finishing were actually used 
for dry barrel finishing. The results obtained are shown in FIGS. 1 to 4. 
Comparative Example C in FIGS. 2 and 4 was a result obtained by conducting 
the same dry barrel finishing with the use of resinous media (NVRT-10 
manufactured by SintoBrator Ltd.) for comparison. In these experiments, 2 
kg of stainless steel-made electronic parts in a cap form having a 
diameter of 12 mm and a height of 8 mm as works and 10 litters of the 
media were put in a dry flow barrel finishing apparatus (EVF-04D 
manufactured by SintoBrator Ltd.) to conduct dry barrel finishing. Five 
test pieces made of S45C having a diameter of 22 mm and a length of 15 mm 
were mixed with the articles to be finished on dry barrel finishing, and 
the abrasive power was obtained from the average of weight decrease of the 
test pieces. 
FIG. 1 is a graph showing the abrasive power of the Examples of the 
invention, and FIG. 2 is a graph showing the abrasive power of the 
Comparative Examples, in which the ordinate is the weight decrease of the 
articles to be finished on barrel finishing in 30 minutes, and the 
abscissa is passage of time from the start of finishing. According to the 
results shown in FIG. 2, Comparative Example A, a general-purpose 
inorganic media for wet barrel finishing, maintained high abrasive power 
of a weight decrease of 31 mg in the first 30 minutes immediately after 
the start of finishing, but the abrasive power was decreased to a weight 
decrease of 23 mg in the second 30 minutes (30 to 60 minutes after the 
start of finishing), and was further decreased to a weight decrease of 20 
mg in the third 30 minutes (60 to 90 minutes after the start of 
finishing). The decreasing ratio of abrasive power in this case was about 
35%. 
On the other hand, according to the results shown in FIGS. 1 and 2, Example 
1, in which 24% of aluminum hydroxide and 2% of manganese oxide were 
added, exhibited abrasive power of a weight decrease of 33 mg, which was 
only slightly larger than Comparative Example A. However, the abrasive 
power of Example 1 was stably maintained to a weight decrease of 33 mg 
even in the third 30 minutes (60 to 90 minutes after the start of 
finishing) with no reduction. It is understood that the addition of 
aluminum hydroxide and manganese oxide contributes improvement in 
stability of the abrasive power of the media. 
In Example 2, in which 2% of ferric oxide was further added, the hardness 
of the media is slightly increased, and the abrasive power was slightly 
decreased accordingly. However, Example 2 did not suffer decrease in 
abrasive power with the lapse of time, and the abrasive power was stably 
maintained to a weight decrease of about 30 mg. Example 3, in which 10% of 
aluminum hydroxide and no sintering agent was added, exhibited abrasive 
power of a weight decrease of 24 mg in the first 30 minutes, whereas the 
abrasive power was gradually decreased to a weight decrease of 20 mg in 
the third 30 minutes (60 to 90 minutes after the start of finishing). 
However, the decreasing ratio of abrasive power in this case is only about 
17%, and the addition of only 10% of aluminum hydroxide contributed the 
stability of the abrasive power of the media. 
In comparison to Examples exhibiting stable finishing performance, 
Comparative Example B, in which 3% of manganese oxide was added but no 
aluminum hydroxide was added, exhibited a weight decrease of 26 mg in the 
first 30 minutes and 15 mg in the third 30 minutes (60 to 90 minutes after 
the start of finishing). The decreasing ratio of abrasive power in this 
case was as large as about 42%, and therefore the addition of only 
manganese oxide did not contribute the stability of the abrasive power. 
Comparative Example C, in which a commercially available resinous media 
was used, exhibited low abrasive power in the initial stage as a weight 
decrease of 21 mg in the first 30 minutes, and furthermore, the decreasing 
ratio of abrasive power was as large as about 43%. The resinous media thus 
could not be used for rough finishing. 
FIG. 3 is a graph showing the worn amount of the media of the Examples, and 
FIG. 4 is a graph showing the worn amount of the media of the Comparative 
Examples, in which the ordinate is the worn amount of the media in 30 
minutes, and the abscissa is passage of time from the start of finishing. 
According to the results shown in FIGS. 3 and 4, the worn amounts of the 
media in 30 minutes in Examples were stable as similar to the abrasive 
power, where as those in Comparative Examples were unstable. It is 
understood that the addition of aluminum hydroxide contributes the 
stability of worn amount of the media. According to the results shown in 
FIGS. 3 and 4, the worn amount in Example 1 in the first 30 minutes was 
4.4%, which was slightly larger than but was not abnormally larger than 
3.6% in Comparative Example A. It is generally known that the abrasive 
power is proportional to the worn amount. The slight larger worn amount in 
Example 1 means that such a general relationship between the abrasive 
power and the worn amount can be applied to the case where aluminum 
hydroxide is added. Therefore, a larger amount of aluminum hydroxide is 
added when the abrasive power and its stability are important, whereas a 
smaller amount of aluminum hydroxide is added when the suppression of the 
worn amount is important. Accordingly, it is indicated that media suitable 
for the intended use can be produced by considering the addition amount of 
aluminum hydroxide as a factor of design.