Note: Descriptions are shown in the official language in which they were submitted.
~0~0~ 7
.
The present invention relates in general to a
precision casting method and, more partlcularly, to a method
of manufacturing a profiled three dimensional structure,
such as a propellor, using investment or precision casting
molds.
; Investment or precision casting is a process which
has been used for many hundreds of years for casting parts of
complex shape, and it has recently been employed for casting
parts which, even if they have a comparatively simple shape,
have to be cast to precise dimensions. This avoids the need
for subsequent cleaning or machining processes, which cause
a loss of expensive material and may also result in an alter-
ation of the properties of at least the surface portions of
the casting.
In the conventional process a pattern made of wax
or polystyrene (or a substance having similar properties)
is formed, either directly or by using a solid master-profile,
and then the pattern is dipped into a slurry of a refractory
material. The pattern is normally sprinkled with sand after
it has been dipped, so that a refractory coating is produced
on the pattern. Back up material, such as dry sand, is then
poured around the pattern, whereby an investment mold is
defined by the back up material. The investment mold is
allowed to dry, the wax or other substance defining the
pattern is melted and caused to flow out of the mold, the
mold is preheated to a temperature suitable for receiving a
particular molten metal or alloy, and then the molten metal
is poured into the mold and allowed to solidify, and then
the molten article is removed.
Since the shape of a casting is defined by an
integral mold or pattern, it is possible to achieve much
-- 2 --
- ~.6~ t~
greater precision than is possible in casting processes in
which different portions of the casting are defined by separatP
patterns or mold parts, and for this reason investment casting
is commonly used in the aircraft industry, for example. Con-
ventionally, however, it has been found extremely difficult or
impossible to produce molds of sufficient strength to permit
casting of large articles, and in the aircraft and other
industries the investment casting process is almost entirely
limited to the production of small articles.
For the precision casting of large articles such as
generator turbine blades having a length of from 40 cm to 130
cm and a weight of from 1 kg to 70 kg, or marine screw pro-
pellors having a diameter of the order to 200 cm and a weight
of the order of 250 kg, it has been usual to employ a process `
in which a pair of molds, such as green molds, jell molds or
; ceramic shell molds, is employed over a pattern, then stripped
from the pattern, fitted together again, and solidified, i.e.,
the shaw process or a modification thereof. However, in this
process the mold comprises at least two portions which must
be fitted in an exactly matching relationship and held clamped
together, and apart from the fact that fitting together of
mold halves constitutes an extra work step, it is found
difficult to ensure that the mold halves are exactly fitted
together. Cast parts are often produced with overhang pro-
jections in the region where the molds fit together, there-
fore usually require subsequent machining, which results in
wastage of material. Furthermore, with some materials such
as stainless steel, is difficult to obtain a required degree
of precision during machining.
From hydrodynamic considerations, the thickness of
a blade of a ship's screw propellor varies in the direction
, , .
of the longitudinal axis thereof. If such a propellor is
cast by a conventional process, since the green mold employed
has a comparatively low heat-resistance, which imposes limits
on the temperature to which the mold may be heated during the
pouring process, there is likely to be inefficient filling or
short-run of the mold by the poured metal, particularly when,
in order to achieve improved strength and resistance to wear
and corrosion, stainless steel is used instead of a copper
alloy. In addition, blowholes or burning stainless steel are
sometimes present on the surface of the cast part. Because
of these faults, combined with the fact that good dimensional
precision of a cast part cannot be guaranteedl as noted above,
such castings always have unnecessary metal or other material
attached thereto, which must be removed by machining which
increases the cost of manufacture of a propellor.
These disadvantages are not limited to the manufac-
ture of ships' propellors, and they also apply to the manu-
facture of similar large articles of complex shape, for example
the blades of large supercharger turbines, compressors or
condenser.
In the conventional investment casting processes,
because of the properties of the pattern material employed,
it is necessary to employ comparatively large and complex
injection molding equipment for injection of the thermally
fusible material. The pattern made of a thermally fusible
material may be formed manually by an artwork engraving,
but for production on an industrial scale, split molds made
of a metallic alloy may be used for casting a master pattern.
In this case, the mold halves must be given a correct finish
by machining or a similar process, and then clamped together
to define a single mold into which thermal]y fusible material
13'7
.
can be injected by an injection molding machine. However,
use of an injection molding machine results in increased manu-
facturing costs, since the split mold defining the required
shape must be extremely strong and able to resist the high-
pressure flow of injected thermally fusible material. Also,
`~if the mold cavity has to contain a core, special measures
must be taken to ensure that the core is not moved during the
forcible injection of the thermally fusible material, and the
complicated procedures necessary to ensure that the core
remains correctly positioned are often the cause of delay in
the manufacturing processes.
-It is accordingly a principal object of the present
invention to provide an improved method for precision casting
profiled members of three dimensional structure, such as
propellors, impellers, diffusers, condensers, turbine blades
and the like which reduces some of the problems inherent in
conventional casting processes.
According to one aspect of the invention there is
provided an investment casting method which comprises (a) pre-
20 paring a thermally fusible pattern from a material selected -
;from the group consisting of naphthalene and para-dichloro-
benzene, with or without the addition of one or more polymers
each having a vinyl radical, which thermally fusible pattern
is substantially a replica of the article to be cast; (b)
forming a refractory investment around the thermally fusible
pattern by coating the pattern with a refractory slurry; (c)
preliminary melting the outer parts of the thermally fusible
pattexn so as to produce a small gap between said thermally
fusible pattern and said refractory investment by dissolving
the surface portions of said pattern by means of exposing the
-pattern to the vapour of an organic solvent for the pattern
-- 5 --
9~
material; (d) completely melting and removing the residue
of the thermally fusible pattern from the refractory invest-
ment by heating so as to leave the refractory investment with
a cavity previously occupied by the thermally fusible pattern,
said cavity having all the details of said thermally fusible
pattern, whereby there is provided a rigid ceramic mold of
one-piece construction; (e) preheating said refractory invest-
ment constituting said mold in an oven to a temperature close
to the temperature of the molten metal to be cast; (f) pouring
said molten metal into the mold while the latter is heated to
minimize the temperature difference between the mold and the
molten metal; (g) solidifying the molten metal within the mold;
and (h) removing the solidified metal from the mold in the
form of the desired casting.
According to another aspect of the invention there
is provided a refractory mold of one-piece construction which
is manufactured by preparing a thermally fusible pattern from
a material of the group consisting of naphthalene and para-
dichloro~benzene, with or without the addition of at least one
polymer having a vinyl radical, which thermally fusible pattern
is a substantial replica of a desired casting to be made by
the use of said mold, subsequently forming a refractory invest-
ment enveloping the thermally fusible pattern, preliminarily
melting the thermally fusible pattern so as to produce a small
gap between saicl pattern and said refractory investment by
dissolving a portion of said pattern by means of contacting the
pattern and investment with the vapour of an organic solvent,
and then completely melting and removing all the residue of
the thermally fusible pattern from said refractory investment
by the heating thereof so as to leave the refractory investment
having a cavity that was formerly occupied by said thermally
fusible pattern, said cavity having all the details of said
.
,
,
-` ~.C~S~ 37
`
thermally fusihle pattern.
It is an advantage of the invention, at least in
preferred forms, that it can provide a method for casting
large profile members which does not require the execution
of complex casting procedures.
It is a further advantage of the invention, at least
in preferred forms that it can provide a method for precision
casting of profiled members which is not limited to casting
of propellors or the like of particular size or of particular
material, and which permits production of profiled members of
three dimensional structure with highly precise as-cast
dimensions and smooth casting surfaces.
It is yet another advantage of the invention, at
least in the preferred forms, that it can provide an investment
casting method in which production of investment casting
patterns of profiled members of three dimensional structure
is fairly easily achieved and costs of mold-making are com-
paratively low.
Preferably the thermally fusible material is poured
into a mold during the production of a pattern at a pouring
speed in the range of from 0.1 kg/sec to 5 kg/sec.
Because of the gap produced in the preliminary
melting stage" stress on, and cracking of, portions of the
shell due to expansion of the pattern are avoided during the
complete melting stage and a shell mold is thereby produced
which enables the production of a propellor or other casting
with a good surface finish and precise dimensions.
These and other advantages and features of the
invention will become apparent from the following detailed
description of preferred embodiments of the invention when
read with reference to the attached drawings, in which like
3 7
., .
numbers refer to like parts, and:
Fig. 1 is a cross-sectional view of a thermally
fusible pattern defining a ship's screw propellor coated with
a ceramic shell according to one form of investment casting
method of the present invention;
Fig. 2 is a cross-sectional view of a heating means
for removal of the pattern-defining thermally fusible material
and illustrates the preliminary removal of the thermally
fusible material employed in Fig. l;
Fig. 3 is a cross-sectional view similar to Fig. 2
which illustrates the removal of the remainder of the thermally
fusible material;
Fig. 4 is a cross-sectional view of a mold positioned
in readiness for investment after removal of the thermally
fusible material therefrom; and
Fig. 5 is a cross-sectional view similar to Fig. 4,
showing a mold defining the propellor positioned in a flask
immediately prior to pouring of the metal.
The description below refers to the casting of a
ship's screw propellor, but it should be understood that the
method of the invention is equally appliable to the casting
of other types of three dimensional structure, such as
impellers, diffusers, condensers, turbine blades and the like.
In Fig. 1, a hot-melt or thermally fusible pattern
1 defines a screw propellor or similar element to be cast
and has a coating of a ceramic forming a shell 2 of suitable
thickness.
The thermally fusible pattern 1 may be made by any
known pattern-making method, such as employed for example in
investment casting or similar processes. Preferably, however,
a split mold is employed which is made of gypsum and comprises
- 8 -
~ .. . . . .
'.: ~ ' , . `: .: ' ' '
two halves, and in which portions corresponding to the thin
portions of a screw propeller master-mold, not shown, define
cavities. The dimensions of this gypsum mold are selected
bearing in mind the shrinkage of the poured metal and the
material which must be removed to produce the finished part.
As a propellor must define a hole to permit mounting
thereof on a drive shaft, such a hole is provided in the
propellor-defining pattern, and for reasons which will be
apparent later, a recessed portion corresponding to the shaft
hole may be provided in the master mold.
Before the thermally fusible material of the pattern
1 is poured into the central area defined by the two halves
of the split mold which are clamped together, a core 3 is
positioned in the split mold which defines a hole for the
propellor drive shaft and is provided with à flange 4 at the
bottom.
Next, the thermally fusible material, which has a
composition described later, and has been rendered fluid by
being heated to 85C, is poured into the split mold. The
properties of the thermally fusible material poured are such
that use of an injection molding machine is unnecessary.
This does not necessarily mean that an injection molding
machine is never used in this invention. However, even for
the casting of comparatively large parts, an injection molding
machine, if employed, need have only a simple construction.
A particularly noteworthy fea-ture of the method is
the speed at which the thermally fusible material can be
poured into the mold. A preferred thermally fusible material
is material of the naphthalene system. Although such material
has many advantages, it has been found that use thereof in
conventional methods leads to surface holes and local porosity
...... .... , . _ . .. _ . _ _ .. _ .... _ . _ _ _ .. _ . _ ... .. . _ .. . .. .... _ ...
in a pattern produced. Research undertaken by the inventors
showed that the principal cause of such faults was the
adherence of steam produced during the pouring process to
the wal]s of the mold. From the results of further research and
; tests made it became clear that this could be avoided by
keeping the pouring speed in the range 0.1 kg/sec to 5 kg/sec.
When fluid material is poured into a mold at a speed greater
than 5 kg/sec, swirling occurs and air is entrapped against
the mold surface, leading to surface roughness in the finished
pattern. In addition to this, there is a tendency for an
applied release agent to be stripped off, with the result
that separation of the pattern from the mold becomes difficult.
On the other hand, if the pouring speed is slower than 0.1
kg/sec there may be local porosity and, since poured material
tends to cool excessively before new material is poured in,
step-lines or zones are produced in the surface of the pattern,
and the required dimensional precision fails to be achieved.
When the pouring speed is kept in the range 0.1 kg/sec to 5 kg/
sec, however, a pattern with precise dimensions and a smooth
s-~rface finish can be obtained.
After being poured into the split mold, the thermally
fusible material defining the pattern 1 is allowed to solidify
and is then removed from the mold. The pattern 1 thus removed
still supports the core 3.
The thermally fusible material employed for the
pattern may be para-dichloro benzene or naphthalene,
and, in addition, polystyrene resin or vinyl acetate
may be present employed singly or as a mixture.
Preferably, however, the total weight of the
-- 10 --
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naphthalene is kept within the range of 0.5~ - 10~ if a
mixture of naphthalene and polystyrene resin is employed,
in the range of 1 - 5% if a mixture of naphthalene and a
copolymer of ethylene acetate is employed, and in the range
of 3 - 10~ if a mixture of naphthalene and polyethylene resin
is employed.
The properties of naphthalene and styronaphthalene,
i.e., a mixture of naphthalene and polystyrene resin, are
shown in Table 1. For comparison, the properties of represen-
tative conventional waxe~ are noted in Table 2. Comparingthe values in these two tables, it is seen that the addition
of polystyrene increases bending strength.
-- 11 --
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When the thennally fusible pattern 1 is for~ed a refracto-
ry ceramic shell 2 is formed therearound by repeating a coat-
ing and sand-sprinkling processes a set number of times
determined with reference to the required strength of the
shell 2. For example, coating and sand-sprinkling are
alternately repeated 6-7 times if :it is required to subse-
quently cast a screw propellor hav:ing a diameter of 400 mm,
and 10-12 times if it is required to cast a propellor hav-
ing a diameter of 1,200 mm. The pattern 1 is completely
enclosed in the shell 1 except for an opening 10, which
for a propellor pattern is located on the opposite side of
the pattern to the core 3.
Each sand-sprinkling step is designed to strength-
en the ceramic chell 2, and the sand emplos~ed is suitably -
a dry sand such as alumina sand or fusible silica, which -
may be applied in a flow bed tray, or be blown or poured
onto the shell 2.
After completion of the requisite number of cycles
of coating and sand-sprinkling, the core 3 supported by
the pattern 1 is held mechanically by the flange 4 to the
shell 2, in order to ensure that the core 3 remains in place
after the pattern 1 is subsequently melted out of the shell
2. . :
After completion of the final coating and sand-
sprinkling cycle, preliminary and completion melt-out steps
are performed in order to remove the pattern 1 and leave
a central hollow space in the shell 2 defining the shape
of the propellor it is desired to cast. The first of these
steps is perfor~ed in a preliminary melt-out oven 5 shown
- 14 - .
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.
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in Fig. 2, to which reference is now made.
A horizontally disposed partition board 6 is provided
across a lower portion of the melt-out oven 5 so that a heating
compartment 7 is defined in the lowermost portion of the oven S.
The heating compartment 7 is filled with a fluid medium, such
as oil. An electric heater tube 8, to which power is supplied
by power line 9, is mounted in a lower side-wall portion of the
oven 5 and projects into the compartment 7. When power is
. supplied to the heater tube 8, the fluid medium in the compart-
ment 7 is heated and the partition board 6 is also heated.
The pattern 1 coated with the shell 2 is held by
supports 11 in the oven 5 in such a manner that the uncoated
opening 10 thereof faces downwards, so that melted material of
the pattern 1 may fall onto the board 6, on which it forms a
layer 12. A suitable quantity of an organic solvent is supplied
into the oven 5 in the form of an alkene or chloro-hydro-carbon,
for example, such as 1-1-1 trichloro-ethane tCH3:CCQ3), 1-1-2
trichloro-ethane, (CHCQ:CCQ2), or 1-1-2-2 tetrachloro-ethane
. (CQ2C:CCQ2), in order to dissolve the material of the pattern 1.
; 20 The properties of various different solvents which
may be used in the invention are shown in Table 3.
- 15 -
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To avoid the necessity of using of an unduly large
amount of solvent, and also to prevent atmospheric pollution,
cooler pipes 13 are provided around the upper portion of the
inner walls of the oven 5. Cooling air or other fluid is
constantly circulated through the pipes 13 by external con-
ventionally known means (not shown). With this arrangement,
as solvent vapour rises to the upper portion of the oven 5, it
is cooled by the cooler pipes 13 and forms droplets 14, which,
since the solvent employed is heavier than air, as indicated
in Table 2, run down the inner wall of the oven 5, so that the
solvent is recovered.
The thermally fusible material of the pattern 1 melts
due to the effect of the latent heat`of vaporization of the
solvent and is also dissolved by the vapour of the solvent.
At the same time, the solvent vapour passes through micro-pores
in the ceramic shell 2, and the shell 2 is heated due to the
effect of latent heat of li~uefaction of this vapour. It is
undesirable to leave the pattern shell assembly in the oven
for a long time once the varporization of the solvent has
commenced, and after such an amount of the thermally fusible
material has been melted that a gap 3a of several millimetres
is provided between the outer surface of the pattern 1 and the
inner surface of the shell 2, the assembly of the pattern 1
and shell 2 is removed from the preliminary melt-out oven 5
and transferred to a complete melt-out oven lS.
Referring now to Fig. 3, the pattern and shell
assembly is supported in the complete melt-out oven 15 with
the opening 10 facing downwards by supports 20 of the same
construction as the supports 11 in the melt-out oven 5. The
30 assembly is heated by air at a temperature of 350 - 450C,
which is supplied into the interior of the oven 15 via one or
.. _ .. . .
'3~{?B~
more pipes 17 by a high-pressure burner unit 16. This hot
air is preferably directed into the oven 15 in a direction
such that it is not blown directly from the pipe or pipes 17
onto the pattern and shell assembly. The top of the oven 15
is closed, and air may leave the oven 15 via suitable ducts
(not shown).
This two-stage melt-out process is very advantageous
in terms of maintenance of dimensions in the cast parts. Al-
though cracking of the ceramic shell can be avoided in the
conventional processes for the melt-out of pattern defining
material, in which a pattern and shell assembly is immersed
in boiling water or in which a pattern and shell assembly is
heated by an air blast at a temperature in the range 350 - 450C
without any preliminary treatment, in the first process immer-
sion of the assembly for too long results in a weakening of
the binder material and consequent distortion of the mold, while
in the latter process it is difficult to control the path of
heat transfer from the boss portion of the propellor to the
blade tips or edges, and if the pattern-defining material
contains a large amount of polystyrene, i.e. 3% or more, there
may be breakage of large portions of the mold intended to define
the blade edges since the material expands before it melts.
With the two stage method described above, however,
these problems are avoided, since the first stage of the melt-
out is accompanied by practically no expansion of the pattern-
defining material, as it takes place at relatively low tem-
perature and usually for a short time, since it is only
necessary to effect production of a gap 3a of the order of 0.5 -
1.0 mm between the pattern defining material and the ceramic
-'0 shell interior in order to ensure that thermal expansion has
no adverse effects in the subsequent complete melt-out stage.
- 18 -
Melt out of the thermally fusible material in the
oven 15 results in the production of a hollow ceramic shell
or mold 2a. To remove any water or residual pattern material
which may still adhere to the inner surface of the mold 2a,
and also to render the mold 2a strong and stable, the mold is
-held for a set time at a temperature in the range 500 - l,100C
in a heating furnace 22 such as that shown in Fig. ~, to which
reference is now made. The furnace 22 has steel walls which are
lined with a refractory lining material 23, and a mold support
stand 24 is provided in the centre of the lower wall. The mold
2a is supported on the stand 24 with the boss portion thereof
underneath and the open portion thereof facing upwards, the upper
surface of the stand 24 being flat in order to ensure stable
support for the mold 2a. Nozzles 27 are defined in the support
stand 24 to provide communication between the interior of the
furnace 22 and a high-pressure burner 25 provided below the
stand 24 outside the main body of the furnace 22, and to which
gas is supplied by a line 26. Upon actuation of the burner 25,
therefore, the mold 2a is subjected to a hot blast effecting
complete removal therefrom of water and other residual material.
In addition to drying the mold 2a, the furnace 22 also serves
to heat the mold to a temperature as close as possible to the
temperature of molten metal subsequently poured thereinto.
For example, the mold 2a may be held in the furance 22 for
about three hours if it is required to heat the mold to a
temperature of the order of 400C to 700C.
'The mold 2a is then placed in a flask 28, such as
- that shown in Fig. 5, with the opening of the mold 2a facing
upwards and now constituting a sprue lOa. Steel shot, chromite
sand, zircon sand, or similar dry sand material 29 is packed
around the mold 2a, only the sprue lOa being left projecting
19
PC1~7
~ .
. .
above the level of the sand material 29. This projecting
portion is suitably wrapped with insulating material 30 made
of ceramic fibre, for example.
; In the condition shown in Fig. 5, the mold 2a is
ready to receive the molten metal. One of the main advantages
of this method is that the metal to be cast may be stainless
steel. Although recently some materials such as aluminium
bronze have sometimes been used in place of high tension brass
(HBsC-l), which was previously the main material used for the
manufacture of ships' propellors, there has previously been no
equipment or methods available permitting the use of stainless
steel in an economical manner. This is because, conventionally,
use is made of the so-called sweeping mold methods employing
C2 or green molds, and problems associated with run and other
factors make it necessary to leave 2 - 3 mm of metal to be
machined off after casting. This is not an excessive amount
for conventional copper alloys, which are comparatively cheap
and easily machinable, but is excessive for stainless steel
which is very difficult to machine, as well as being expensive.
The result has been that in conventional methods, processing
costs when propellors are cast in stainless steel are 3 - 5
times higher than when copper alloys are employed. With the
method of the invention, however, as noted in greater detail
below, an extremely good finish and close dimensional tolerances
can be achieved in the casting of propellors even when stainless
steel is employed, and the amount of material to be removed
after the casting is only in the order of 0.3 mm. In other
words, the invention offers the advantage that in terms of
overall processing costs, there is very little difference
between using stainless steel and copper alloys for manufacture
of propellors, i.e., propellors may be easily and economically
- 20 -
''.' ' , , ", ' ' ~' ': ' '' ' .
~ (3~ S ~ 7
.; '
made of stainless steel, which for this purpose is far superior
to copper alloy materials.
Examples of suitable types and compositions of
stainless steels which may be employed for the manufacture of
propellors are given in Table 4.
Table 4
.._
Material C Si Mn Cr Ni ~lo Cu
(Code)
__ _ .. _ . _ __
No. 1 KSP-l 0.0~ 1.4 1.2 18.8 8.5 1.0
..__
No. 2 KSP-2 0.05 0.8 0:8 13.0 4.0 0.7 _
.
Using the method of the invention, there was found
to be no difference in the casting surface regardless of
whether the casting was effected by bottom pouring or by top
pouring, but the latter type of pouring is preferred since it
presents advantages with respect to the preparation of molds.
When a pouring well is employed, there is only a minor amount
of inclusion of slag or dross in the finished casting.
From the point of view of ease of the casting process, the
best process is to use a tea-spout ladle to effect prelimi-
nary pouring and then to effect a top pour.
Subsequent to pouring, the castings are allowed tocool, and then the risers are cut off and machining or other
finishing processes are effected in a known manner, to
produce ~inished propellors.
; In contrast with the average value of surface
roughness of 50 - 140 ~ of propellors cast in conventional
sand molds, the surface roughness of 18-8 stainless steel
propellors cast by the method of the invention is very
small, and is in the range 5 ~ 15 ~. Thanks to this, only
a small amount of finishing machining is necessary, and the
required time and expense for producing a finished propel-
- 21 -
S~ '7
lor are accordingly much less.
SpeciEic examples of the good results obtained
by the method of the invention are given in Table 5, which
notes as-cast dimensions of propellors cast by the above-
described method.
Table 5
. ____ . . ........ _ . .
Size Dimensional precision
__ . .~
<25 mm < ~0.2 mm
.. _ . ...
25 - 75 mm ~0.15 ~ 0.5 mm
75 - 200 mm ~0.4 - 1.0 mm
_ . .
200 - 400 mm ~0.8 - ].. 5 mm
_ ... _
~ 400 - 600 mm ~1.2 - 2.0 mm
_
600 - 800 mm ~1.8 - 2.4 mm
~800 mm _0.2 - 0.4 mm
'
In further illustration of the advantages of
the invention, Table 6 provides a comparison of the propellor
blade pitch achieved by conventional green mold casting
methods and by the me-thod of the invention~
- 22 -
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Ideally, dimensions for the first to the third blades
should be the same, but in practice there is inevitably
some difference in the dimensions or pitch between the blades
As seen from the above -table, whereas this difference is
large in conventionally cast propellors, it is small in
propellors cast by the method of the invention. In other
words, the invention makes it possible to manufacture
propellors which may be rotated at high speed but are sub-
ject to little vibration.
A specific Example of manufacture of a stainless
steel propellor according to one form of the method of the
invention is as follows.
Example
1. Propellor dimensions
Diameter 810 mm
Expanded area ratio 45 ~
Weight 15 kg
Number of blades 3
2. Manufacturing stages
a) A gypsum mold was prepared in a size dimensioned to
allow for shrinkage and the material to be removed to give a
finished product of the required dimensions.
b) Napthalene was melted at a temperature of 85~C,
a 1% addition of polystyrene was made thereto and melted
therein, and the mixture was poured into the gypsum mold
to form a pattern.
c) After the pattern had hardened, it was released
from the mold and then allowed to cool to room temperature.
d~ The propellor pattern was coated by being
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3~
dipped in a slurry consisting of silica flour throughly
mixed in colloidal silica, and while still wet had grains
~ of silica sprinkled thereon, and was then dried. This
coating and sand-sprinkling process was repeated 8 times,
resulting in the formation on the pattern of a ceramic shell
having an average thickness of 6 mm.
e) The pattern coated in this manner was dried
for approximately 12 hours and then immersed in a trichloro-
ethylene vapour bath fQr approximately 15 minutes to effect
preliminary melt-off of approximately 1 mm of the outer
surface of the pattern, after which the pattern and shell
assembly was transferred to a hot air furnace in which
it was exposed for approximately 30 minutes to a hot air
blast at a temperature of 350C, to completely melt out the
pattern material and produce a shell mold.
f) The shell mold thus produced was dried and
hardened for approximateIy 15 minutes in a heating furnace
employing high-pressure burners, and was then heated to
red heat (approximately 650C).
g) The heated mold was packed in dry sand, and
then 18-~ stainless steel,which had been melted in an
ultrasonic electric furnace,was poured thereinto.
h) After pouring, the cast metal was allowed to
cool to room temperature and was then removed from the
mold.
3. As-cast dimensional precision
The thickness at two places on each of 20 propellors
produced by the abovedescrlbed process was measured, and
it was found that the variation ln thickness for the enti.re
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sample of 20 propellors was no more than +0.41 mm and the
standard deviation was 0.12. This is as opposed to conven-
tional sand-mold casts for which the variation in thickness is
+1.5 mm or more.
4. Cast surface roughness
The surface roughness was measured at three locations
on each of the 20 propellors, and the as-cast surface roughness
was found to be in the range of 8 - 12 ~, which is much
less than the range of 50 - 140 ~ achievable by conventional
methods.
Although the present invention has fully been
described in conjunction with the preferred embodiments
thereof with reference to the accompanying drawings, it is
to be noted that various changes and modifications are ap-
parent to those skilled in the art. Accordingly, such
changes and modifications are to be understood as included
within the true scope of the present invention as defined by
the appendant claims.
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