Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"SEALING AND GUIDING DEVICE FOR THE INJECTION PISTON OF A
HOT CHAMBER PUMP FOR CORROSIVE ALLOYS"
The present invention relates to the sealing devices used in pumps
for the injection die forming of metallic pieces, and in particular for the
hot
chamber die casting of corrosive light alloys.
It is known that even if the use of hot chamber pumps, in which the
injection pump is totally or partially immersed in the molten alloy, solves
most of the problems of cold chamber pumps, yet it presents the great
drawback that when said alloy at melting temperature is corrosive for the
ferrous materials, the members of the pumps are rapidly etched by it.
An example of a traditional hot chamber pump dating back to 1940 is
disclosed in DE-C-745.583 relating to an improved arrangement for the'
alignment of the injection piston with the cylinder abutting against the flat
top of the gooseneck. This traditional type of pump does not allow high
injection pressure and is not suitable for corrosive alloys.
The continuous research of new corrosion-resistant materials,
capable of assuring a sufficient life and reliability to the parts exposed to
the contact with the corrosive alloys, has led to the development of alloys
of various elements such as titanium, boron, silicon, carbon, chromium
and aluminum and rarer elements such as yttrium, lanthanum, scandium,
cesium, samarium, zirconium, etc. The aim of the research of alloys more
and more corrosion-resistant is that of extending the operating life of the
pump, mainly as far as the most critical members such as the piston and
the cylinder are concerned, which are not only subject to the corrosion by
the molten alloy, but they also have to withstand the abrasion caused by
the motion of the piston sealably sliding in the cylinder.
In conventional pumps, the play which occurs between piston and
cylinder owing both to the thermal expansion and the surface corrosion is
extremely damaging for the correct working of the pump. In fact, the
introduction of the molten alloy into the cylinder usually takes place
through an opening in the side wall of the cylinder which is closed by the
piston in its downward stroke with the consequent impossibility of using
low rigidity piston rings which would be damaged by the passage on the
side opening. FR-A-1.178.540 discloses an example of such a pump,
wherein the replacement of the members undergoing corrosion and wear
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is easy, fast and economical. However this pump is designed for the
casting of magnesium alloys, which are not corrosive for the types of
metallic materials used nowadays and allow the use of elastic piston rings.
In other cases, such as in patents CH-625.439, US-A-3.467.171 and
US-A-3.469.621, the piston has its lower end cut at 45 or somehow
machined to obtain therein a loading mouth so as to allow the inflow of the
molten alloy into the cylinder without extracting completely the piston and
without forming openings in the side wall of the cylinder. Nonetheless, the
piston must sealably slide in the cylinder, and therefore the problem of the
coupling tolerances between piston and cylinder remains. Even if metallic
piston rings can be applied in this case in order to improve the sealing,
said rings wear down rather rapidly thus requiring the replacement thereof
after few thousands of cycles. Moreover, their presence implies a limitation
of the maximum operating pressure, so as to prevent excessive friction
and wear, which in some cases is insufficient to obtain casts of the
required compactness.
The maximum pressure may be considerably limited also by sealing
problems between the container cylinder wherein the injection piston
slides and the seat of the gooseneck siphon wherein said cylinder is
housed. This occurs especially if said members are made of different
materials, such as in the typical case of a cylinder made of corrosion-
resistant ceramic material and a siphon made of coated steel. A further
problem stems from the fragility of said ceramic materials which are
sensible to bending stresses.
From the above it is apparent that in prior art pumps special
surfacings are needed for the critical coupling between piston and
cylinder, in which account must be taken of the problems of thermal
expansion, friction between the parts, corrosion of the contacting surfaces
and possible oxide scales on said surfaces. Similar problems arise in the
coupling area between cylinder and siphon, and the whole of these
problems implies a shortening of the life of the above-mentioned critical
members of the pump with consequent costs, both in terms of pieces
replacement and machine stop times for the inspection and/or
maintenance thereof.
The applicant has already been granted the US patent n.5.385.456
which discloses a hot chamber pump with a plunger piston. In this way, the
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cylinder is integral with the siphon, and the sealing is not performed
between piston and cylinder but through seals of compressed yielding
material located at the mouth of the siphon. Though it substantially solves
several of the above-mentioned problems, said pump has limited
achievable pressure and injection speed due to the presence of said
yielding materials. In fact, it is necessary to limit the pressure in order to
prevent an excessive expansion of said materials in the direction
transverse to the lateral surface of the piston, in addition to limiting the
maximum piston speed in order to prevent an excessive heat production
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due to the friction.
Therefore it is desirable to provide a sealing and guiding device suitable to
overcome the above-mentioned operating limitations.
According to an aspect of the present invention, there is provided a hot
chamber injection apparatus for corrosive alloys including a body, immersed in
a
molten alloy contained in a crucible, in which a chamber and an injection
cavity
therebelow are formed wherein an injection piston coaxially slides with a
vertical
reciprocating motion, an upper portion of said chamber being in communication
with the crucible through suitable ducts and said cavity being in
communication
with the crucible and with a mold through suitable ducts, and sealing/guiding
means for said piston which extend between said upper portion and the cavity,
wherein said sealing/guiding means include a bush, having an outer diameter
smaller than the inner diameter of the chamber, interposed between an upper
and
a lower centering member housed in the chamber, said bush being made coaxial
with the chamber by a pair of opposite surfaces of revolution formed on said
centering members, the upper surface and the upper centering member
performing a pressure-tight sealing against the upper side of the bush and the
sidewall of the chamber, the outer lateral surface of the bush being in
communication with the injection cavity through a channel in the lower
centering
member, said upper side of the bush being immersed in the crucible at a depth,
measured from the lowest level reachable by the molten alloy, which is greater
than the maximum travel of the piston, and the inner surface of the bush being
a
surface of revolution with diameters larger than the piston.
A first advantage of an embodiment of the present invention is that it is
made up of high-rigidity members which allow high injection pressures.
A second considerable advantage consists in achieving a reliable
hydrodynamic guide with no direct contact between the members, with take up of
the radial and axial plays and without problems of speed limit.
These and other advantages and characteristics of the device according to
the present invention will be clear to those skilled in the art from the
following
detailed description of an embodiment thereof, with reference to the only
drawing,
annexed herewith as fig.1, which schematically illustrates a vertical
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cross-section thereof.
Referring to said figure, there is seen that a hot chamber die casting
pump consists of a body 1, immersed in the molten alloy contained in a
crucible
(not shown), in which an injection cavity 2 is formed at the bottom, wherein a
cylindrical plunger piston 3 slides with a vertical reciprocating motion V.
The
feeding of the molten alloy into cavity 2 takes place through a channel 4
provided with suitable means for the opening and closing thereof, while a
sprue
5 takes the alloy under pressure to the mold (not shown) as indicated by arrow
S.
A cylindrical chamber 6, wherein the sealing and guiding device
according to the present invention is housed, is formed in the upper portion
of
body 1 with a diameter larger than the injection cavity 2 and coaxial
therewith.
Starting from the bottom, said device includes a lower centering ring 7, a
bush
8, an upper centering ring 9, a compression sleeve 10 and a threaded locknut
11. The lower ring 7 rests on the abutment at the bottom of chamber 6 and is
centered therein, since its outer diameter is equal to that of chamber 6, same
as the upper ring 9. On the contrary, bush 8 interposed between rings 7 and 9
has an outer diameter smaller than chamber 6 but larger than the inner
diameter of the centering rings, and it is made coaxial with chamber 6 and
piston 3 by a pair of opposite, preferably conical, surfaces of revolution 12
and
13
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respectively formed on the upper side of the lower ring 7 and on the lower
side of the upper ring 9.
The annular space 14 included between the outer surface 15 of bush
8 and the wall of chamber 6 is in communication with the injection cavity 2
through a channel 16 formed in the lower ring 7, or possibly through
leakages at the lower seat 12. On the contrary, the upper seat 13 is
pressure-tight and the sealing between the upper ring 9 and chamber 6
may be further assured by a known device such as an 0-ring 17.
The space 18 of chamber 6, above bush 8, is in communication with
the crucible through channels 19 formed in the wall of body 1, of sleeve 10
and of ring 9, or in other suitable ways. The feeding of the molten alloy
into cavity 2 can thus take place also by partially or totally extracting
piston 3 from bush 8, depending on whether the former is shaped at its
end to form a loading mouth or not. Anyway, a scraping ring 20 can be
placed along the edge of the upper ring 9 so as to prevent the bath floss
from being taken by piston 3 inside bush 8. The diameter of piston 3 is just
smaller than the inner diameter of bush 8, whereby a thin chamber or
channel 22, which has been considerably enlarged in the drawing for the
sake of clarity, remains between the inner surface 21 of bush 8 and the
lateral cylindrical surface of piston 3. In order to reduce possible
hydrodynamic unbalances, the inner surface 21 may be interrupted by
grooves orthogonal to the axis.
After having schematically described the members of the pump, now
the operation thereof will be described, while defining P' the pressure on
the free surface of the bath of molten alloy, and P" the maximum pressure
which can be generated by the motor of piston 3 inside cavity 2.
It is clear that while piston 3 enters cavity 2 the pressure increases
from P' (neglecting the different heights of the various members) to a value
P lower than or equal to P". Since the pump is of the volumetric type, value
P can equal P" if the flow rate of the losses due to leakages is lower than
the effective flow rate generated by piston 3, this being so much easier as
the losses are small. With reference to what said above about the sealing
between space 14 and space 18, these losses can only occur through
channel 22 and proportionally to the characteristics thereof. In particular,
the losses increase with the increase in pressure P and in the width of
channel 22, and they decrease when the length of the latter, measured
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along the generatrix, increases. In order to achieve a good working of the
pump it is essential to find a good compromise for the width of channel 22.
In fact, it has to be sufficiently large to allow a proper play at high
injection
speed without causing excessive friction, yet sufficiently small as to limit
the losses and provide an effective guide to piston 3.
As mentioned in the introductory part, the ceramic materials which
resist corrosion and friction have a good behaviour in case of compression
stress but do not withstand high bending stresses. Due to this, piston 3
which is subjected almost to Pascal's pressure can be made of ceramic
materials, whereas body I has to be made of properly coated metal.
Moreover, one has to take into account the considerable differences in the
coefficient of thermal expansion between ceramic and metallic materials,
with the consequent coupling problems which can give rise to excessive
plays or interferences. Therefore it is clear that bush 8 has to be made of a
material similar to that of piston 3, with similar or equal coefficients which
leave unchanged the width of channel 22 upon varying of the temperature.
This implies that bush 8 be not subjected to tensile stress, and that its
housing in chamber 6 be made so as to prevent the onset of plays which
jeopardize the sealing or of interferences which generate dangerous
stresses thereon.
The device according to the present invention allows to overcome the
above-mentioned drawbacks by making the other sealing and guiding
members, apart from piston 3 and bush 8, of suitable metallic alloys
having thermal expansion coefficients compatible with one another, and
therefore with couplings defined on the base of the operating temperature.
The scraping ring 20, if present, can be made of ceramic material so as to
maintain the correct play with piston 3.
The system for centering bush 8, consisting of the surfaces of
revolution 12 and 13, allows the coupling between materials with different
thermal expansion by simultaneously adjusting the radial and axial play of
bush 8 with respect to body 1, even pre-loading the former if necessary.
This is achieved by pressing downwards the upper ring 9 through sleeve
10 by acting on locknut 11, which also allows, upon stopping of the pump,
the unlocking of the device prior to the beginning of the cooling so as to
prevent possible damages caused by the thermal shrinkage.
The feeding of the molten alloy into the mold substantially takes
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place in three steps. During the starting step of ejection of the air from the
mold, piston 3 is lowered slowly and generates into the injection cavity a
pressure P close to P. During the intermediate step of mold filling, piston 3
is lowered very rapidly and generates a high pressure P for a very short
time. During the final step of feeding of the shrinkages of the solidifying
cast, the pressure becomes and remains very high, but piston 3 is lowered
slowly according to the speed allowed by the little flow rates of the
shrinkages and of the leakages.
When the pressure in cavity 2, and thus also in the annular space
14, reaches a certain value P, the outer surface 15 of bush 8 is subjected
to said constant pressure P along its generatrix, as schematized by
diagram K. On the contrary, the inner surface 21 is subjected to a
decreasing pressure while going up along a generatrix, namely from value
P in cavity 2 to value P' in space 18, as exemplified by diagram D. The
exact law of variation of the pressure along surface 21 depends on the
conformation of channel 22. Therefore the pressures acting on the lateral
surfaces of bush 8 have resultants directed towards the longitudinal axis,
whose values can be obtained from the difference between diagram K and
diagram D.
Furthermore, it should be noted that bush 8 is also subjected to axial
compression due to the pressure P>P' acting on the lower side, and to the
corresponding reaction of seat 13 acting on the upper side. This push of
pressure P causes an expansion of ring 9 and the consequent pressure-
tight sealing thereof against the wall of chamber 6.
Since piston 3 and bush 8 are made of materials with similar
characteristics, the effect of the centripetal pressure increasing along the
generatrix is that bush 8 contracts more than piston 3, also due to the
decreasing pressure acting on the latter, thus leading to a decrease in the
width of channel 22. Through a proper sizing of bush 8, it is possible to
define the axial development of the width of channel 22 according to the
characteristics of the alloy to be cast, thus allowing high injection speeds
and low losses due to leakages. In particular, bush 8 preferably has
increasing inner diameters towards space 18, in the absence of stresses,
so as to obtain an inner cylindrical surface 21 during the final feeding step,
35 when the bush is in the stressed condition. In fact, the greatest leakage
flow rates occur in said final step due to the combination of high pressure
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and long duration of the step, whereas in the two preceding steps the flow
rate is negligible since pressure (in the first step) or time (in the second
step) are very small.
The above is valid supposing that piston 3 remains substantially
cylindrical; therefore it is necessary to prevent that during its vertical
reciprocating motion the temperature changes along the generatrix are
such as to cause significant differences of diameter in its active portion,
i.e. the portion which performs the sealing within bush 8. To this purpose,
the scraping ring 20, if present, or the upper edge of bush 8 anyway, are
immersed in the molten alloy at a depth L greater than the maximum travel
C of the piston, said depth L being measured from the lowest free surface
23 which can be reached by the molten alloy bath. In this way, the active
portion of piston 3 is constantly at the bath temperature since it is still
immersed therein even at the maximum travel, thus remaining cylindrical.
It is clear that the above-described and illustrated embodiment of the
device according to the invention is just an example susceptible of various
modifications. In particular, the law of variation of the inner surface 21 of
bush 8 may be designed according to the specific requirements of the
case, and the same is valid for the angles of the surfaces 12 and 13.
Moreover, bush 8 can also extend beyond the latter.
