Mold for direct press molding of optical glass element

A mold used for manufacturing optical glass parts by the direct press molding of lumps of raw optical glass. The pressing surface of the mold is made of a material comprising .alpha. type SiC or amorphous SiC or a mixture of both. The pressing surface may be a coated film on a base body of hard alloy or high density carbon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a mold used for the manufacturing of optical 
glass parts such as a lens and prism by directly pressing lumps of optical 
glass materials to form such optical glass parts. 
2. Description of the Prior Art 
In recent years, there is a tendency for the optical glass parts, 
especially the optical glass lenses, to have aspherical surfaces so as to 
simultaneously achieve the simplified construction as well as the 
lightweight lens of a lens system in an optical instrument. With the 
existing grinding method employed as a manufacturing method of the optical 
lens, difficulties are involved in the workability and quantity production 
while the direct press molding method is regarded as a promising method 
for manufacturing an aspherical lens. 
In this direct press molding method, a pair of molds 1 and 2, e.g., as 
shown in FIG. 1, which is finished beforehand to the desired surface 
quality and surface accuracy and has aspherical pressing surfaces 3 and 4 
as shown in FIG. 1 is used for molding by heating the lump of the optical 
glass on the mold or by pressing the lump of the preheated optical glass, 
without requiring any further post processes such as a grinding process 
after the press molding of the optical lens. 
However, said manufacturing method of the optical glass lens is required to 
provide the produced lens with good quality to an extent that the quality 
of the image formed by the lens is not damaged after the press molding. 
Therefore, the mold is required to have the least chemical action to the 
glass at high temperature, hard glass pressing surface which is not 
susceptible to damage such as scratches, and high breaking resistance to 
the thermal shock caused by repeated rapid heating and rapid cooling, 
among other properties. 
To meet such requirements, materials such as silicon carbide and silicon 
nitride are said so far to be suitable for the mold, and various studies 
are being made. 
However, many polymorphisms are present in the crystal structure of silicon 
carbide which is regarded as the most prospective material for the mold 
used for the above purposes and such polymorphisms extremely differ in 
their chemical and physical properties. There is a problem among others 
that some of such polymorphisms have violent reactions against the 
components of glass composition, and it cannot be simply said that silicon 
carbide is a suitable material for the mold by merely verifying that it is 
a compound of Si and C through analysis. 
SUMMARY OF THE INVENTION 
Because the optical glass parts, in particular the optical glass lenses of 
good image forming quality, are manufactured by the continuous direct 
press molding, the material of the direct press mold of this invention is 
either of the .alpha. type SiC or the amorphous SiC or otherwise a mixture 
of both, which is the characteristic of this material. 
More particularly, the following three types of the mold of this invention 
are available. One type of the mold for the press molding of this 
invention consists of the sintered substance of the .alpha. type SiC. 
The other type of the mold of this invention is the one of which improves 
the defect wherein it is difficult to work the simple substance of the 
.alpha. type SiC of the above into the shape of a mold. This improved mold 
is made from the base material of sintered hard alloy mainly consisting of 
tungsten carbide (WC) and cobalt (Co) and having better workability than 
the simple substance of SiC. The base material is worked into a mold 
having a shape of a lens to be molded. 
This mold is further coated over its press molding surface with the film of 
uniform thickness of the .alpha. type SiC or the amorphous SiC or 
otherwise a mixed phase of both. 
Another type of the mold of this invention is made from the base material 
of a block of high density carbon instead of said sintered hard alloy. In 
this case, however, the carbon used as the base material has extremely 
good workability and can be worked into any shape but is a little porous. 
If, however, the .beta. type SiC film is formed on this base material by 
the CVD method or PVD method, the working generally becomes more difficult 
than that of the simple substance of carbon whereas the .beta. type SiC is 
relatively easily worked in the silicon carbide (SiC). Therefore, if a 
block of high density carbon is worked into a mold of aspherical surface 
beforehand, a thick film of the .beta. type SiC is formed on the pressing 
surface, each pressing surface is worked further into the one with higher 
accuracy again, and then coated consecutively thereon with the film of 
uniform thickness of the .alpha. type SiC or the amorphous SiC or 
otherwise the mixed phase of both, then such mold is excellent for the 
direct press molding. 
The reason that the film surface of the .beta. type SiC is not directly 
used for the pressing surface is because the .beta. type SiC reacts easily 
with the glass containing much alkali metal such as Na, K or Ba. Forming 
the film of the .alpha. type or amorphous SiC or the mixture of both on 
the pressing surface makes a mold which does not react with the glass of 
above. 
The three types of the mold described above are characterized by the fact 
that the material constituting their pressing surfaces has high hardness, 
can stand the rapid heating and rapid cooling, and, is of the .alpha. type 
SiC or the amorphous SiC or the mixed phase of both which does not easily 
react with glasses of various composition. 
By the direct press molding of the lump of the optical glass material using 
the above-mentioned types of the molds, it has become possible to 
manufacture the optical glass parts of highly accurate shape and good 
image formation quality.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described hereinafter in connection with the 
preferred embodiments. 
Embodiment 1 
FIG. 1 shows an embodiment of this invention for the mold. 
Two each of the sintered .alpha. type silicon carbide cylinders of 30 mm in 
diameter and 50 mm long having the compositions shown in the Table 1 are 
prepared, one of which is worked into the top mold 1 having the concave 3 
of the radius of curvature of 46 mm provided with cuts on its periphery as 
shown in FIG. 1 while the other is worked into the bottom mold 2 having 
the concave 4 of the radius of curvature of 200 mm so as to make a pair of 
molds for press molding. 
Press molding surfaces of each pair of these blocks are mirror polished 
with superfine abrasive grains of diamond and finished to the maximum 
surface roughness of less than 0.02 .mu.m. The pair of molds 1 and 2 made 
in such manner is set in the press machine shown in FIG. 2 and the 
spherical optical glass lump 11 of 20 mm radius constituted by one of the 
two kinds of compositions stated below is pressed to mold into a biconvex 
lens in the atmosphere of argon gas containing 2% of hydrogen. The press 
machine is constructed to have the piston cylinders 7 and 8 arranged with 
the heaters 5 and 6 so that the pair of molds 1 and 2 can be installed and 
heated in a vessel which can be closed tightly to control the atmosphere. 
The raw glass lump 11 is clamped and held by the raw glass feeding fixture 
10, passed through the glass preheating tunnel kiln 9 and inserted inside 
the press machine. 
The raw glass lump 11 is put on the bottom mold 2 to be further heated and 
pressed by moving the pair of molds 1 and 2 by means of the piston 
cylinders 7 and 8. The molded glass after pressing is taken out from the 
discharging opening 12. One of the two kinds of the optical glass which 
are press molded is an optical glass of borosilicate alkali system 
consisting of 68 wt % of SiO.sub.2, 11 wt % of B.sub.2 O.sub.3, 10 wt % of 
Na.sub.2 O, 8 wt % of K.sub.2 O, and the very small quantity of 
composition in the balance, while the other is an optical glass of 
borosilicate barium system consisting of 31 wt % of SiO.sub.2, 17 wt % of 
B.sub.2 O.sub.3, 50 wt% of BaO and the very small quantity of composition 
in the balance. 
For the press molding of the glass lump, the molds are heated to 
800.degree. C., the glass lump is pressed under a press pressure of 40 
kg.cm.sup.-2, allowed to cool down to 400.degree. C. in the pressed 
condition, and the molded glass and molds are taken out together. Table 2 
shows the results of the press molding of the glass of borosilicate alkali 
system of the above and Table 3 the results of the press molding of the 
glass of borosilicate barium system respectively. 
For comparison, a mold of the .beta. type SiC which is another type of the 
silicon carbide is manufactured, set in the press machine in FIG. 2 in the 
place of the mold of this invention, and the similar optical glass lump to 
the above-mentioned was press molded under the similar conditions. The 
results of this press molding are also shown in Table 2 and Table 3. 
The mold of this .beta. type silicon carbide was finished to the shape of 
the press mold of the .alpha. type silicon carbide (specimen No. 1 of 
Table 1), however, the crystal structure of 3 C of 200 .mu.m thick, i.e., 
the film of the .beta. type silicon carbide is formed by the CVD method on 
the surface contacting the glass lump during the press molding, and this 
surface was mirror polished with abrasive grains of diamond up to the 
maximum surface roughness of 0.02 .mu.m. 
The mold of the .beta. type silicon carbide reacted with the glass during 
the press molding and was found unsuitable as the mold material. 
As can be seen from Table 2 and Table 3, the press mold of the preferred 
embodiment of this invention excels extremely in the press moldability of 
the optical glass as compared with the mold of the .beta. type silicon 
carbide which has the same chemical composition but differ considerably in 
the crystal structure. 
TABLE 1 
______________________________________ 
Ratio of composition 
No. Types 2H 4H 6H 15R 33R 
______________________________________ 
1 Specimen 1 
-- Very 98 Very -- 
small small 
quantity quantity 
2 Specimen 2 
-- 98 Very Very Very 
small small small 
quantity 
quantity 
quantity 
3 Specimen 3 
98 Very -- -- -- 
small 
quantity 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Press Moldability of Borosilicate Glass 
Condition of the 
Deteriorating 
glass after pressing 
condition of 
Smoothness 
the pressing 
Separa- 
of the 
surface of 
tion of 
pressing 
mold (after 
the mold 
Trans- 
surface 
20 continous 
and 
No. 
Types of mold 
parency 
of mold 
pressing) 
glass 
__________________________________________________________________________ 
1 .alpha. type SiC 
Specimen 1 
o o o o 
2 2 o o o o 
3 3 o o o o 
4 .beta. type SiC 
x x x x 
__________________________________________________________________________ 
Circles (o) denote "good". 
TABLE 3 
__________________________________________________________________________ 
Press Moldability of Borosilicate Barium Glass 
Condition of the 
Deteriorating 
glass after pressing 
condition of 
Smoothness 
the pressing 
Separa- 
of the 
surface of 
tion of 
pressing 
mold (after 
the mold 
Trans- 
surface 
20 continuous 
and 
No. 
Types of mold 
parency 
of mold 
pressing) 
glass 
__________________________________________________________________________ 
1 .alpha. type SiC 
Specimen 1 
o o o o 
2 2 o o o o 
3 3 o o o o 
4 .beta. type SiC 
x x x x 
__________________________________________________________________________ 
Circles (o) denote "good. 
As described above, because the mold for the direct press molding of the 
optical glass parts stated in the present invention employs the silicon 
carbide material which is indicated as the .alpha. type SiC, in case the 
glass lump for the optical glass part is press molded, no chemical 
reaction occurs on the contacting surfaces of the mold and the glass nor 
is the transparency of the glass surface lost after molding, and further 
the excellent effect of good separation of the glass and mold during the 
molding is obtained. Using this material for the mold therefore makes the 
direct press molding method of considerable productivity feasible as 
compared with the past method in connection with the manufacturing of the 
optical glass parts. 
Embodiment 2 
Two each of the hard alloy cylinders of 30 mm in diameter and 50 mm long 
containing 2 wt% of Co and the balance having WC of average grain diameter 
of 0.5 .mu.m are prepared. One of the two cylinders was machined by 
electric discharging to make the top mold of similar shape shown in FIG. 1 
of the embodiment 1 having a concave of radius of curvature of 46 mm and 
cuts on the periphery thereof, while the other cylinder was machined in 
similar manner to make the bottom mold having a concave of radius of 
curvature of 200 mm so as to provide a pair of molds for press molding. As 
the result of the mirror polishing of the press molding surfaces of each 
pair of these blocks with superfine abrasive grains of diamond, the such 
surfaces were mirror polished to the maximum surface roughness of 0.02 
.mu.m inside of two hours. Then, the 2 .mu.m thick film of the amorphous 
silicon carbide (SiC) was formed on such mirror polished surface by 
sputtering so as to make the mold for press molding of the glass. 
The molds made in such manner are set in the press machine of similar 
construction as shown in FIG. 2 of the embodiment 1 and the two kinds of 
spherical optical glass lumps of 20 mm radius are pressed to mold into 
biconvex lenses. 
One of such spherical optical glass lumps which are press molded is an 
optical glass of borosilicate alkali system consisting of 68 wt% of 
SiO.sub.2, 11 wt% of B.sub.2 O.sub.3, 10 wt% of Na.sub.2 O, 8 wt% of 
K.sub.2 O, and the very small quantity of composition in the balance, 
while the other is an optical glass of borosilicate barium system 
consisting of 31 wt% of SiO.sub.2, 17 wt% of B.sub.2 O.sub.3, 50 wt% of 
BaO, and the very small quantity of composition in the balance. 
For the press molding of such glass lumps, the molds are heated to 
800.degree. C., the glass lumps pressed under a press pressure of 40 
kg/cm.sup.2, allowed to cool down to 400.degree. C. in the pressed 
conditions, and the molded glass and molds are taken out together. The 
specimen No. 1 in Table 4 shows the results of the press molding of the 
glass of borosilicate alkali system of the above and the specimen No. 2 of 
Table 5 the results of the press molding of the glass of borosilicate 
barium system respectively. 
For the purpose of comparison, molds of the simple substance of the silicon 
carbide (SiC) and the simple substance of the silicon nitride (Si.sub.3 
N.sub.4) which were used in the past are manufactured, set in the similar 
press machine as stated above in the place of the molds of the preferred 
embodiments of this invention, and the glass lump of similar composition 
as stated above was pressed under the similar conditions. 
These molds of the silicon carbide and the silicon nitride are finish 
ground to the similar shape to the sintered hard alloy mold after electric 
discharging and mirror polished of its surface using the similar abrasive 
grains of diamond as stated above. For this mirror polishing process 
alone, it took 40 to 50 hours to finish both molds to the maximum surface 
roughness of up to 0.02 .mu.m. The time required is 20 to 25 times as 
compared with the grinding of the sintered hard alloy molds. Results of 
press molding by these two molds are shown in Table 4 (specimen No. 11 and 
No. 12) and Table 5 (specimen No. 11 and No. 12) as examples for 
comparison. 
As is apparent from Table 4 and Table 5, the press molds of the specimen 
No. 1 through 6 of the preferred embodiments of this invention are better 
than the silicon carbide molds used conventionally and can be manufactured 
easily. It is also apparent from the Tables that these molds demonstrate 
excellent performance over the conventional molds especially when the 
quantity of Co is in a range of 2-20 wt% and the grain diameter of WC in 
the range of 0.05 .mu.m-0.5 .mu.m. 
TABLE 4 
__________________________________________________________________________ 
Results of Press Molding 
of Borosilicate Alkali Glass 
Grain 
Max. Glass 
dia- 
surface surface 
Mold surface 
Spec- 
Q'ty 
meter 
roughness Type of 
condition 
condition 
imen 
of Co 
of WC 
of mold: 
Coating 
coated 
after 
after 100 
No. (wt %) 
(.mu.m) 
Rmax. (.mu.m) 
method 
film pressing 
pressing 
__________________________________________________________________________ 
1 2 0.05 
0.013 Sput- 
Amor- 
Good Good 
tering 
phous 
2 5 0.10 
0.015 Sput- 
Amor- 
Good Good 
tering 
phous 
3 15 0.25 
0.017 Sput- 
Amor- 
Good Good 
tering 
phous 
4 20 0.50 
0.02 Sput- 
Amor- 
Good Good 
tering 
phous 
5 5 0.10 
0.019 CVD** 
.alpha. 
Good Good 
6 5 0.10 
0.015 CVD Amor- 
Good Good 
phous 
7* 5 0.10 
0.015 CVD .beta. 
Dis- Reacted with 
colored 
glass. 
8* 1 0.10 
0.05 Sput- 
Amor- 
No good 
Glass attach- 
tering 
phous ed to mold. 
9* 25 0.10 
0.016 Sput- 
Amor- 
Good Filmed peeled 
tering 
phous off. 
10* 5 0.6 0.08 Sput- 
Amor- 
No good 
Glass attach- 
tering 
phous ed to mold. 
11* Sintered 
0.02 N.A. .alpha. + .beta. 
No good 
Reacted with 
SiC mold glass. 
12* Sintered 
0.02 N.A. .alpha. + .beta. 
No good 
Reacted with 
Si.sub.3 N.sub.4 mold glass. 
__________________________________________________________________________ 
*Specimen No. 7.about.No. 12 are examples of comparison. 
**Chemical Vapor Depostion 
TABLE 5 
__________________________________________________________________________ 
Results of Press Molding 
of Borosilicate Barium Glass 
Grain 
Max. Glass 
dia- 
surface surface 
Mold surface 
Spec- 
Q'ty 
meter 
roughness Type of 
condition 
condition 
imen 
of Co 
of WC 
of mold: 
Coating 
coated 
after 
after 100 
No. (wt %) 
(.mu.m) 
Rmax. (.mu.m) 
method 
film pressing 
pressing 
__________________________________________________________________________ 
1 2 0.05 
0.013 Sput- 
Amor- 
Good Good 
tering 
phous 
2 5 0.10 
0.015 Sput- 
Amor- 
Good Good 
tering 
phous 
3 15 0.25 
0.017 Sput- 
Amor- 
Good Good 
tering 
phous 
4 20 0.50 
0.02 Sput- 
Amor- 
Good Good 
tering 
phous 
5 5 0.10 
0.019 CVD** 
.alpha. 
Good Good 
6 5 0.10 
0.019 CVD Amor- 
Good Good 
phous 
7* 5 0.10 
0.019 CVD .beta. 
No good 
Glass attach- 
(Glass 
ed to mold. 
became 
slightly 
opaque.) 
8* 1 0.10 
0.05 Sput- 
Amor- 
No good 
Glass attach- 
tering 
phous 
(Glass 
ed to mold. 
became 
slightly 
opaque.) 
9* 25 0.10 
0.016 Sput- 
Amor- 
Good Sputtered 
tering 
phous film peeled. 
10* 5 0.6 0.08 Sput- 
Amor- 
No good 
Glass attach- 
tering 
phous 
(Glass 
ed to mold. 
became 
slightly 
opaque.) 
11* Sintered 
0.02 N.A. .alpha. + .beta. 
No good 
Glass attach- 
SiC mold (Glass 
ed to mold. 
became 
slightly 
opaque.) 
12* Sintered 
0.02 N.A. .alpha. + .beta. 
No good 
Glass attach- 
Si.sub.3 N.sub.4 mold (Glass 
ed to mold. 
became 
slightly 
opaque.) 
__________________________________________________________________________ 
*Specimen No. 7.about.No. 12 are examples of comparison. 
**Chemical Vapor Deposition 
Embodiment 3 
Two each of the cylinders of high carbon density of 30 mm in diameter and 
50 mm long are prepared, one of which is worked into the top mold of 
similar shape shown in FIG. 1 of the embodiment 1 having a concave of 
radius of curvature of 46.1 mm and cuts on the periphery thereof, while 
the other cylinder is worked into the bottom mold having a concave of 
radius of curvature of 200.1 mm so as to provide a pair of molds for press 
molding. 
Then, these molds are put into a furnace heated to 1400.degree. C. flowing 
a gas mixture of silicon tetrachloride (SiCl.sub.4), ethylene (C.sub.2 
H.sub.2) and hydrogen (H.sub.2), and the .beta.-SiC films of 0.2 mm thick 
are formed on the concave surfaces of respective molds. The concave of the 
top mold is then machined to a radius of curvature of 46.0 mm and that of 
the bottom mold to a radius of curvature of 200.0 mm. The top and bottom 
molds are polished for four hours using superfine abrasive grains of 
diamond until the maximum surface roughness of 0.02 .mu.m is obtained. 
Furthermore, the amorphous SiC films of 2 .mu.m thick are formed by 
sputtering on these mirror polished surfaces to make molds for press 
molding of glass. 
The molds made in such manner are set in the press machine of similar 
construction as shown in FIG. 2 of the embodiment 1 and the two kinds of 
spherical optical glass lumps of 20 mm radius are pressed to mold into 
biconvex lenses. 
One of such spherical optical glass lumps which are press molded is an 
optical glass of borosilicate alkali system consisting of 68 wt% of 
SiO.sub.2, 11 wt% of B.sub.2 O.sub.3, 10 wt% of Na.sub.2 O, 8 wt% of 
K.sub.2 O, and the very small quantity of composition in the balance, 
while the other is an optical glass of borosilicate barium system 
consisting of 31 wt% of SiO.sub.2, 17 wt% of B.sub.2 O.sub.3, 50 wt% of 
BaO, and the very small quantity of composition in the balance. 
The molds are then heated to 800.degree. C., the glass lumps pressed under 
a press pressure of 40 kg/cm.sup.2, allowed to cool down to 400.degree. C. 
in the pressed conditions, and the molded glass and molds are taken out 
together. The specimen No. 1 in Table 6 shows the results of the press 
molding of the borosilicate alkali glass and the specimen No. 1 in Table 7 
the results of the press molding of the borosilicate barium glass 
respectively. 
For the purpose of comparison, molds of the simple substance of the silicon 
carbide (SiC) and the simple substance of the silicon nitride (Si.sub.3 
N.sub.4) which were used in the past are manufactured, set in the similar 
press machine as stated above in the place of the molds of this invention, 
and the glass lump of similar composition as stated above was pressed 
under the similar conditions. 
These molds of the silicon carbide and the silicon nitride are rough ground 
and then mirror polished of its surfaces with the similar abrasive grains 
of diamond as stated above. 
For this mirror polishing process alone, it took 40 to 50 hours, which is 
10 to 15 times of the time required to finish the .beta.-SiC surface on 
the cylinder of high density carbon up to 0.02 .mu.m. Results of press 
molding by the silicon carbide mold are shown in Table 6 (specimen No. 11 
and No. 12) and in Table 7 (specimen No. 11 and No. 12) as examples of 
comparison. 
As is apparent from Table 6 and Table 7, respective press molds of the 
specimen No. 1 through No. 8 of the preferred embodiments of this 
invention tested under various conditions are better than the molds of 
silicon carbide and silicon nitride used conventionally and can be 
manufactured easily. Specimen No. 9 through 12 in Table 6 and Table 7 are 
mentioned for comparison and outside of the scope of the present 
invention. 
TABLE 6 
__________________________________________________________________________ 
Press Moldability of Borosilicate Alkali Glass 
Method Type 
Time needed 
Film coated 
Glass 
of of to finish 
on mirror pol- 
surface 
coating film 
mold to max. 
ished .beta.-SiC 
condi- 
Mold surface 
Spec- 
.beta.-SiC 
coated 
surface 
Coat- 
Type 
tion condition 
imen 
on on roughness 
ing of after 
after 100 
No. carbon 
carbon 
of 0.02 .mu.m 
method 
film 
pressing 
pressing 
__________________________________________________________________________ 
1 CVD .beta.-SiC 
4 hours 
Sput- 
Amor- 
Good Good 
tering 
phous 
2 CVD .beta.-SiC 
4 hours 
CVD Amor- 
Good Good 
phous 
3 CVD .beta.-SiC 
4 hours 
CVD .alpha.-SiC 
Good Good 
4 CVD .beta.-SiC 
4 hours 
Sput- 
.alpha.-SiC 
Good Good 
tering 
5 Sput- 
.beta.-SiC 
4 hours 
Sput- 
Amor- 
Good Good 
tering tering 
phous 
6 Sput- 
.beta.-SiC 
4 hours 
Sput- 
.alpha.-SiC 
Good Good 
tering tering 
7 Sput- 
.beta.-SiC 
4 hours 
CVD Amor- 
Good Good 
tering phous 
8 Sput- 
.beta.-SiC 
4 hours 
CVD .alpha.-SiC 
Good Good 
tering 
9* CVD .alpha.-SiC 
45 hours 
CVD .alpha.-SiC 
Good Good 
10* CVD .beta.-SiC 
4 hours 
CVD .beta.-SiC 
Glass 
Glass attach- 
became 
ed to mold. 
slightly 
opaque. 
11* Sintered 50 hours 
N.A. 
N.A. 
Glass 
Glass reacted 
SiC mold became 
with mold. 
slightly 
opaque. 
12* Sintered 45 hours 
N.A. 
N.A. 
Glass 
Glass reacted 
Si.sub.3 N.sub.4 mold became 
with mold. 
slightly 
opaque. 
__________________________________________________________________________ 
*Specimen No. 9.about.No. 12 are examples of comparison. 
TABLE 7 
__________________________________________________________________________ 
Press Moldability of Borosilicate Barium Glass 
Method Type 
Time needed 
Film coated 
Glass 
of of to finish 
on mirror pol- 
surface 
coating film 
mold to max. 
ished .beta.-SiC 
condi- 
Mold surface 
Spec- 
.beta.-SiC 
coated 
surface 
Coat- 
Type 
tion condition 
imen 
on on roughness 
ing of after 
after 100 
No. carbon 
carbon 
of 0.02 .mu.m 
method 
film 
pressing 
pressing 
__________________________________________________________________________ 
1 CVD .beta.-SiC 
4 hours 
Sput- 
Amor- 
Good Good 
tering 
phous 
2 CVD .beta.-SiC 
4 hours 
CVD Amor- 
Good Good 
phous 
3 CVD .beta.-SiC 
4 hours 
CVD .alpha.-SiC 
Good Good 
4 CVD .alpha.-SiC 
4 hours 
Sput- 
.alpha.-SiC 
Good Good 
tering 
5 Sput- 
.beta.-SiC 
4 hours 
Sput- 
Amor- 
Good Good 
tering tering 
phous 
6 Sput- 
.beta.-SiC 
4 hours 
Sput- 
.alpha.-SiC 
Good Good 
tering tering 
7 Sput- 
.beta.-SiC 
4 hours 
CVD Amor- 
Good Good 
tering phous 
8 Sput- 
.beta.-SiC 
4 hours 
CVD .alpha.-SiC 
Good Good 
tering 
9* CVD .alpha.-SiC 
45 hours 
CVD .alpha.-SiC 
Good Good 
10* CVD .beta.-SiC 
4 hours 
CVD .beta.-SiC 
Glass 
Glass reacted 
became 
with mold. 
slightly 
opaque. 
11* Sintered 50 hours 
N.A. 
N.A. 
Glass 
Glass reacted 
SiC mold became 
with mold. 
slightly 
opaque. 
12* Sintered 45 hours 
N.A. 
N.A. 
Glass 
Glass reacted 
Si.sub.3 N.sub.4 mold became 
with mold. 
slightly 
opaque. 
__________________________________________________________________________ 
*Specimen No. 9.about.No. 12 are examples of comparison. 
Various changes and modifications can be made in the described structure 
without departing from the scope of the invention defined in the appended 
claims.