Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
AND TRADE AsGSO RESEARCH ~ 1 - AGENTS REF- 361~
_ _ _
METHOD OF CASTING USING
EXPF,NDABLE PATTERNS
.
The present invention relates to the cas-ting of
shapes of molten metal by a technique involving the use
of a so-called expendable pattern. In this technique a
pattern made of a heat destructible material is surrounded
by a mould material in a mould box; molten metal is
brought into contact with the pattern which is vaporised
or burnt out to form a cavity which is filled with molten
metal which, upon solidification, forms a cast shape.
The use of an expendable pattern was probably
first proposed by Shroyer, see British patent 850331 (1960).
Many proposals have been made to improve the technique: see
e.g. British patents 9~5208; 955021; 999316; 1039086 and
1076198. A significant advance was made in the -technique
with the use of an unbonded sand as the mould material, see
British patent 1127327 (1968) r In the improved technique,
the expendable pat-tern is placed within a binder-free flow-
able sand in a mould box and the sand is then subjected -to
vibration, preferably (sic) at the mould material ultra-
sonic frequencies.
There have been many variations on the techniqueO
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In one set of proposals, a vacuum is applied -to -the sand,
with or without vibration. It is known to fluidise the
sand in order to insert the expendable pattern, then to
collapse the fluldised bed and app~y a vacuum to aid
compaction. Such proposals are exemplified by British
patent 1254592 (1971), 1572860 (1980) and US patent
3842899 (1974). It has also been proposed to seal the
body of sand by placing a plastic sheet on the top of the
box and applying a vacuum to es-tablish a uniform vacuum
level in the body, see for example British pa-tents 1401239
(1975) and 1403240 (1975).
It has also been appreciated that the pattern
itself can be treated with a gas-permeable refractory paint
or coating, see British patents 945208 (1963), 999316 (1965),
1039086 (1966).
There is disclosed in US paten-t 4222429 (September
1980), a method of casting in which a bed of sand is
flu:Ldised, a coated pattern is forced into the sand, the
sancl is defluidised, and -the bed may be subjected to
vibration and a vacuum may be drawn~optionally placing a top
cover on the bed to establish a uniform vacuum. Metal is
then cast and allowed to cool following which the sand is
refluidised and the casting is heat treated.
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Despite the range of proposals available, the use
of an expendable pattern in unbonded particulate material
has problems and risks. None of the proposals is wholly
reliable. Three areas still cause anxlety: the risk of
pollution caused by burnout o~ the expendable pattern, the
risk of explosion caused by inadequate removal of the
products of vaporisation of the pattern, and collapse of the
mould which happens unpredictablyO
The invention is based upon the discovery tha~
many of the drawbacks o~ the prior proposals may be overcome
by creating a controlled pressure gradient in the height of
the particulate material in the box. ~or this invention,
the top of the box must be open to the atmosphere and the
particulate material must be compacted i.e. the bulk denslty
thereo~ must exceed a minimum valu~.
According to one aspect of this invention there
is provided a method o~ casting a metal article in a
mould box having a top open to the atmosphere, comprising
locating an expendable pattern in the box, the pattern
having a gas permeable refractory coating thereon, plac-
ing unbonded particulate material about the pattern and
compacting the material to maximise the bulk density of
the material in contact with ~he coated pattern, ~upplying
molten metal into the box so as to vaporise or burn away
the pattern and form the article of defined shape and
applying a vacuum to the compacted particulate material
during casting such as to create sufficient pressure
gradient in the height of the compacted material to
maintain the integrity of the gas permeable refractory
coating.
The gas permeable refractory coating may be
selected from the many available in the literature and
having regard to the metal being cast. The permeability
of the coating causes a pressure drop through the coating
layer under the vacuum applied during casting thus holding
the coating layer in intimate contact with the compacted
particulate material when the expendable pattern has
vaporised. The degree of permeahility required of a
coating when used in the invention is that the coating must
be sufficiently impermeable to create a pressure drop across
the coating layer to provide adequate support or the
compacted particulate material and to prevent metal pene~
tration, yet permeable enough to allow the gases arising
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~ 5 - AGENTS REF: 36 _
from the vaporisation of the p~t-tern to escape through the
coating. The re~ractoriness required will depend on the
metal being cast and suitable refractory materials are well
known and available. The coating may be applied by a
variety of methods; brush, spray, dipping, overpouring,
etc. More than one layer may be applied sequentially.
Most preferably the coating has a low binder content so that
it does not dry to form a-hard crackable coating. As is
known, the refractory materials will be selected according
to the metal being cast.
Preferably the patterns are made in expanded
polystyrene or like polymers having a density of abou-t 20
kg/cu.m. Low density patterns are prone to flexing
during moulding and damage during handling, whereas high
density patterns produce e~cessive gas.
In a modification of the method, the coated
expendable pattern is removed by heat before casting,
leaving the gas permeable refractory shell within the
compa~ted particulate material. In such case, the pattern
may be coated with a ceramic slurry which is chemicall~
cured or allowed to dry to form a shell. The pattern may
he vaporised or burned out before or after investing the
shell in the particulate material. The method is seen to
good advantage especially when used with relatively thin
~g~6~
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shells since such shells are well suppor-ted.
A ~eature of the invention is the deliberate
compaction of the particulate material to a predetermined
degree. The purpose of compaction in this invention is
twofold, firstly to cause the particulate moulding material
to flow into intimate contact with the surface of the coated
pattern irrespective of its contours so eliminating the need
for cores and secondly to compact the mass of the ma-terial
by bringing the individual particles in close contact,
ideally until they can be brought no closer together. One
way of determining the degree of compaction is by measuring
the bulk density of the material used and subjecting that
material to compaction so as to maximise the bulk density
where it contacts the coated pattern. A preferred method
of compaction to achieve the maximum is vibration since
this is efficient and can be used where the mass of
particulate material is large; high frequency low amplitude
vibration is preferred and the force rating of the vibrator
is preferably of the order of 0.75 of the total load it is
vibrating, giving the moulding box an acceleration of about
1~5g. A frequenc~ of at least 40 Hert~ is preferred to
cause the material to flow about complexly shaped patterns.
Vibration can be performed b~ a vibrator attached to the
side of the moulding box, but preferably the box is mounted
on a vibrating table since vibration is more uniormO Both
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electric and air vibra~ors are suitable~ Maximum
consolidation appears to be achieved in a short time, be-tween
3Q and 60 seconds, depending upon pattern complexity, and
this may be detected visually by -the fall in level of the
material in -the box and then the presence of a shimmer or
rolling of t~e top surface of the sand, which shimmer or
rolling is constant. It must be stressed that the purpose
o~ compaction is to bring the particles together, not to
evacuate the air between the particles, and for this reason
the application of a vacuum does not produce compaction for the
purpose of this invention. Alternative methods o~ compaction
include centrifuging, mass dropping, jolting and the like.
The coated pattern is placed in the unbonded
particulate material below the top surface thereof and the
height of unbonded particulate material above the expendable
pattern is of importance in the method. I-f the height is
less than about 20 cm~ for example in the case of ferrous
metals, the metallostatic pressure arising during casting
may cause deformation or lifting or even collapse of the
mould. The minimurn height ensures that a minimum pressure
reduction is applied to the granular material at the top of
the pattern. In some earlier proposals weights are placed
on the top surface of the material to counteract the lifting
tendency; such weights are not re~uired in the method of
this invention, The maximum height is determined by the
size of the mould box.
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The level of vacuum needed will be related inter
alia to the de~ree of compaction of the particulate
material, the metal being cast and the properties of the
gas permeable refrac-tory coating present on the expendable
pattern. Insufficient vacuum will not create enough
pressure gradient and there will be a risk that the mould
will collapse; too great a vacuum may cause the pattern
to deform and the gas permeable refractory coating to crack;
it may also cause penetration of metal into the refractory
coating giving poor surface finish of the casting. The
vacuum removes the gases and fumes from the mould and this
contributes to reducing the risk of explosion. In addition
however, the vacuum reduces the pressure of air contained in
the voids between the grains and so increases the frictional
force between them. In this way the body of the compac-ted
particulate material iS held together to resist a tendency
to collapse. The level of vacuum applied is preferably o~
the order of about 130 mm to abou-t ~50 mm mercury in the
region of the coated pattern.
It is a much preferred feature of the invention
that the vacuum be drawn from the bottom of the box. Because
the top surface of the compacted unbonded particulate material
is exposed to the atmosphere when the vacuum is applied to the
body of the material there is a pressure gradient -through the
height of the compacted particulate material and the s~stem
is thus dynamic. The vacuum may be drawn using a medium
pressure vacuum pump, preferably a liquid ring pump. The
rate o~ application of vacuum will depend on the permeabilit~
of the particulate material and the power of the vacuum pump
being used. Using a 50 A~ sand, permeability number 180 to
200, a flow rate of about 15 cubic metres/minute/square metre
(about 50 cubic feet/minute/square root) of box area is
preferred.
The vacuum can be established in a matter of
seconds before it is wished to pour mol-ten metal into the
mould. The vacuum pressure can be measured by means of a
probe gauge inserted into the body of the particulate
material. The vacuum should be maintained following casting
until the casting has started to solidify to the point at
which it will not distort or is self supporting. This will
depend on the size of the casting: i.n the case of a small
casting t`he vacuum may be removed two to three minutes
~ollowing casting and for a large body the period may be five
to ten minutes following casting.
Th~ particulate material is preferably a sand.
The sand must be sufficiently fine to support the coating on
the pattern and sufficiently coarse to allow the removal of
the gaseous products of vaporisation or combustion of the
expendable pattern. Commercial sands (e.g. Chelfor ~ 50
- 10 - AGEMTS REF: 36
available in Great Britain) are sui-table. The sand must
offer support to the coating on the expendable pattern bu-t
characteristics of the sand will dictate the level of vacuum
that can be achieved for a given flow rate of air, This is
directly related to the sand permeability which is related
to grain fineness and shape, It is preferred that sand
grains be rounded since such grains can ~low and compact
better under vibration.
In evaluations performed using the method of the
invention it was observed that a number of patterns in one
box may be cast in succession without a fall o~ in qualit~,
l'he invention may be applied to a variety of metals,
both ferrous and non-ferrous,
In order that the invention may be well understood
it will now be described by way of illustration7 with
reference to the following examples.
EXAMPLE _
A mould box about 91 cm long and 91 cm wide and
having a depth of 7~ cm was used in this Example. Below
the box were pipes leading to a liquid ring ~acuum pump~
The unbonded particulate material used was a silica sand,
sub--angular, 50 AFS (Amerlcan Foundryman's Society),
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permeability o~ about 180 to 200. Two polystyrene patterns
about 24 kg/cu.m. were used in each case, one belng shaped to
form a simple block and the o-ther being a complex shape to
~orm a valve. Core pieces were not used. The metal cast
was steel and in each case -the casting weighed about 50 kg.
Where a gas-permeable refractory coating was used this was
a semithixotropic paint comprising zircon in a non-aqueous
carrier having a low binder content.
A. The mould was filled with the sand and the
pattern was placed 20 cm below the top sur-face of the loose
sand. The pattern had a paint coating of 0.5 mm. A
vacuum was applied to the box at the flow rate of 15 cu.m/
minute/sq.m. It was observed that in the case of the
complex shape the mould collapsed and the valve formed had
a poor surface. In the case of the block the mould also
tended to collapse and the casting formed had a poor surface.
This test shows that the use of a vacuum both to
compact the loose sand and during casting does not lead to
a successful result.
B~ The process of test A was repeated but the
sand was first subjected to vibration at the rate o~ 35 Hz,
less than lg acceleration. The vibration was stopped and
a vacuum was applied just before casting to induce a flow
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rate 15 cu.m/min/sqOm. The results obtained were as in the
case of the first evalua-tion which shows that inadequa-te
vibration does not lead to a successful resultO
C~ The process of test B was repeated but this
time the sand was vibrated at 50 Hz and an acceleration of
1 to 1.5g for about 60 seconds~ until the level of the sand
in the box fell by about 10%, to a bulk density of about
1.6 gm/cu.cm and the top surface had a steady appearance.
The vacuum was applied just before cas-ting to induce a flow
rate of 15 cu.m/min/sq.m. until surface solidification of
the casting had taken place. Both the complex shape and
the simple block shape formed good quality castings; the
mould did not collapse and the working environment was
found to be acceptable. At the end of casting the box was
inverted and the loose sand was cooled for immediate re-use.
I`est C was repeated several times and in each case
a totally reliable result was obtained.
D~ The process of test C was repeated bu-t this
time the vacuum flow rate was reduced to 6 cu.m/min/sq.m~
It was observed that the casting tended to break through the
top surface of the sand, the mould tended to collapse and
there was some evidence of inclusions of gas in the casting
formed.
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E. The process of test C was repeated but this
time a higher vacuum flow rate was used. The use of a
higher flow rate increased the risk of metal penetration;
this was offset by increasing the thickness of the painted
coating, but it was observed that when the flow rate reached
2~ cu.m/minJsq.m. 9 the surface of the casting formed was
poor. It was therefore decided not to use higher flow rates.
F. In this test the process of test C was
repeated except that the head of compacted sand above the
pattern was reduced to 5 cm. The casting broke through
the top surface of the sand.
G. The process of test C was repeated but using
two uncoated patterns. Despite the required head of
compacted sand and the required flow rate, the casting formed
had a very poor surface and the mould tended to collapse.
This shows that a refractor~ gas-permeable coating is needed.
The results of the tests of this Example show that
when the sand is compacted by vibration to the specified bulk
densityl a gas permeable refractory coating is presen-t on the
polystyrene pattern and the sand is subjected to vacuum at
the required stage to induce the required pressure gradient,
a reliable casting is achievedO
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EXAMPL~ II
. _ .
Using the mould box of Example 1 the sand was
compacted by vibration at 50 Hz and an acceleration of lg.
The sand was sub angular silica sand 50 AFS. The level of
vacuum and the depth of sand in the box according to flow
rate was measured and the results obtained are shown on the
accompanying graph of Figure 1. This graph shows that
because the top surface of the compacted surface is
uncovered, a pressure gradient is present in the sand.
This gradient is a characteristic of the method of invention
and is a feature leading to its success.
EXAMPLE III
The process of Example I test C was repeated using
a silica sand having a permeabilit~ of 100 units and a vacuum
flow rate of 7.5 cu.m/min/sq.m; good quality cas-tings were
obtained.
EXAMPLE IV
. ~
The process of Example I test C was repeated but the
mould box con-tained a pattern shaped to form five interlinking
chain links each measuring about 140 mm x 180 mm~ The casting
was done sequantiall~ and each was cast perfectly despite the
time interval in casting from the first to the Iast.
As will be clear from the foregoing description and
examples, the success of the invention is due to the controlled
pressure gradient in the height of compacted particulate
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material in the mould box. As indicated hereîn, -the
pressure gradient may be crea-ted by leaving -the -top of -the
box open to the atmosphere and drawing a vacuum frorn below
but the invention includes other ways of creating the
controlled pressure gradient for example applying a positive
pressure to the top of the particulate material and drawing
the vacuum from o-ther locations.
The apparatus used in the me-thod herein comprises
a mould box and associated piping for drawing a vacuum, and
l~ pumps, and the like. Such apparatus is well known in the
art and for this rèason -those skilled in the ar-t will be
able to cons-truct and assemble suitable apparatus without
further elaboration.
Without further elaboration the foregoing will so
fully illustrate our invention that others may, by applying
cur:rent or future knowledge, readily adap-t the same for use
under various conditions of service~
