U.S. patent application number 10/972620 was filed with the patent office on 2005-03-17 for semi-solid molding method and apparatus.
This patent application is currently assigned to THT Presses Inc.. Invention is credited to Jorstad, John L., Kamm, Richard J..
Application Number | 20050056394 10/972620 |
Document ID | / |
Family ID | 34277909 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050056394 |
Kind Code |
A1 |
Kamm, Richard J. ; et
al. |
March 17, 2005 |
Semi-solid molding method and apparatus
Abstract
A metal alloy is heated to a molten state, and a grain refiner
may be added. The molten alloy is poured into a shallow chamber of
a shot sleeve of a vertical die cast press and on top of a shot
piston. The shot sleeve is transferred to an injection station
while the molten alloy cools to a semi-solid slurry with a
globular, generally non-dendritic micro structure. An inner portion
of the shallow slurry is injected upwardly by the piston through a
gate opening into a die cavity while an outer more solid portion of
the slurry is entrapped in an annular recess. After the slurry
solidifies, the shot piston retracts, and the shot sleeve is
transferred to a position where the residual biscuit is removed.
The shot piston may have internal grooves to provide a large heat
transfer area for cooling water to absorb heat from the molten
alloy.
Inventors: |
Kamm, Richard J.; (Vandalia,
OH) ; Jorstad, John L.; (Richmond, VA) |
Correspondence
Address: |
Alan F. Meckstroth
JACOX, MECKSTROTH & JENKINS
Suite 2
2310 Far Hills Building
Dayton
OH
45419-1575
US
|
Assignee: |
THT Presses Inc.
|
Family ID: |
34277909 |
Appl. No.: |
10/972620 |
Filed: |
October 25, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10972620 |
Oct 25, 2004 |
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10700004 |
Nov 3, 2003 |
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6808004 |
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10700004 |
Nov 3, 2003 |
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10066527 |
Jan 31, 2002 |
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Current U.S.
Class: |
164/113 ;
164/312; 164/900 |
Current CPC
Class: |
B22D 17/007 20130101;
B22D 17/12 20130101; Y10S 164/90 20130101 |
Class at
Publication: |
164/113 ;
164/312; 164/900 |
International
Class: |
B22D 017/12; B22D
025/00 |
Claims
What is claimed is:
1. A method of semi-solid molding a high strength metal part within
a die cavity defined by a die set mounted on a vertical die cast
press, the press including a shot sleeve having a generally
vertical axis and enclosing a shot piston movable axially within
the sleeve with the sleeve and piston defining a shot chamber above
the piston, the method comprising the steps of: melting a solid
metal to form a molten metal, confining the molten metal within the
shot chamber, cooling the molten metal within the shot chamber to
within a predetermined temperature range while the molten metal
within the shot chamber has a horizontal width substantially
greater than its vertical depth and without stirring the molten
metal to form a substantially quiescent and shallow semi-solid
slurry having a globular micro structure, moving the shot piston
upwardly within the shot chamber to inject an inner portion of the
semi-solid slurry from the shot chamber into the die cavity through
at least one gate opening within an inner portion of the shot
chamber, and allowing the semi-solid slurry to solidify within the
die cavity to form the metal part.
2. A method as defined in claim 1 wherein the molten metal is
cooled within the shot chamber into the semi-solid slurry while the
molten metal has a horizontal width at least twice the vertical
depth of the molten metal.
3. A method as defined in claim 1 including the steps of: forming a
downwardly facing annular entrapment recess above the shot chamber
and generally in axial alignment with an inner surface of the shot
sleeve, and trapping a more solidified outer portion of the
semi-solid slurry adjacent the shot sleeve within the entrapment
recess in response to upward movement of the shot piston.
4. A method as defined in claim 1 wherein the molten metal is
cooled within the shot chamber to a temperature range which
produces a range of 40% to 60% solid to form the semi-solid
slurry.
5. A method as defined in claim 1 wherein the molten metal is A356
aluminum alloy and is cooled within the shot chamber to a
temperature within the range of 570.degree. C. to 590.degree. C. to
form the semi-solid slurry.
6. A method as defined in claim 1 and including the steps of:
confining the molten metal within a second shot chamber receiving a
second shot piston, interchanging the second shot chamber and
piston with the first shot chamber and piston after the inner
portion of the semi-solid slurry is injected from the first shot
chamber into the die cavity, and cooling the molten metal within
the second shot chamber to within the temperature range while the
molten metal within the second shot chamber has a horizontal width
substantially greater than its vertical depth and without stirring
the molten metal to form a second charge of the semi-solid
slurry.
7. Apparatus for molding a metal part within a die cavity defined
by a die set mounted on a vertical die cast press, said press
comprising a shot sleeve having a generally vertical axis, a shot
piston movable axially within said sleeve with said sleeve and said
piston defining a shot chamber above said piston, said shot chamber
having a horizontal width substantially greater than its vertical
depth, said die set defining a downwardly facing annular entrapment
recess above said shot chamber and generally in axial alignment
with an inner surface of said shot sleeve, a power operated member
connected to move said shot piston upwardly within said shot
chamber to inject molten metal within said shot chamber into the
die cavity through a gate opening within said die set, and said
entrapment recess being effective to trap a more solidified outer
portion of the molten metal adjacent said shot sleeve in response
to the upward movement of said shot piston.
8. Apparatus as defined in claim 7 wherein said shot chamber has a
horizontal width at least twice the vertical depth of said
chamber.
9. Apparatus as defined in claim 7 wherein said piston defines
passages for circulating cooling fluid within said piston, and said
passages include spaced interconnected grooves within an upper
portion of said piston.
10. Apparatus as defined in claim 9 wherein said interconnected
grooves comprise concentrically spaced grooves interconnected by
ports providing a maze path for circulating cooling fluid.
11. Apparatus as defined in claim 9 wherein said interconnected
grooves are defined within a bottom surface of a removable said
shot piston.
12. Apparatus for molding a metal part within a die cavity defined
by a die set mounted on a vertical die cast press, said press
comprising a shot sleeve having a generally vertical axis, a shot
piston movable axially within said sleeve, said sleeve and said
piston defining a shot chamber above said piston, a power operated
member connected to move said shot piston upwardly within said shot
chamber to inject molten metal within said shot chamber into the
die cavity through a gate opening within said die set, said piston
defining passages for circulating cooling fluid within said piston,
and said passages include interconnected horizontally spaced
grooves within said piston to provide a large heat transfer area
for the cooling fluid to absorb heat from the molten metal.
13. Apparatus as defined in claim 12 wherein said grooves comprise
concentrically spaced grooves interconnected by ports providing a
maze path for circulating the cooling fluid.
14. Apparatus as defined in claim 12 wherein said interconnected
grooves are defined within a bottom surface of a removable said
shot piston.
15. Apparatus as defined in claim 12 wherein said shot chamber has
a horizontal width substantially greater than its vertical depth.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 10/700,004, filed Nov. 3, 2003, U.S. Pat. No. 6,808,004,
which is a continuation of application Ser. No. 10/066,527, filed
Jan. 31, 2002, abandoned.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to semi-solid molding (SSM) of
metal alloys and the equipment and methods used for SSM, and which
are disclosed in many U.S. and foreign patents, for example, in
U.S. Pat. No. 3,954,455, No. 4,434,837, No. 5,161,601 and No.
6,165,411. SSM is also discussed in technical publications, for
example, in a book entitled Science and Technology of Semi-Solid
Metal Processing, published by North American Die Casting
Association in October, 2001. Chapter 4 of this publication was
authored by a co-inventor of the present invention. In conventional
SSM processes, it is necessary to use either a specially treated,
pre-cast billet of appropriate microstructure or a slurry
especially prepared from molten alloy in equipment external to a
die casting press. The cost premiums associated with either the pre
cast specially treated billet that must be sawed to length before
using, or the slurry especially prepared in equipment external to
the die casting press, have severely limited the commercial
applications of the SSM processes. Also, the pre-cast billet is
available from a relatively few sources, is currently made only
from primary alloys, and process offal cannot be reused unless
reprocessed back into a billet.
[0003] Still, SSM provides some important and highly desirable
characteristics. Unlike conventional die castings, die cast parts
which are produced using SSM processes can be produced
substantially free of porosity, they are able to undergo high
temperature thermal processing without blistering, they can be made
from premium alloys, and they provide reliable high levels of
strength and ductility when made using appropriate alloys and heat
treatments. Because of the thixotropic nature of semi-solid slurry
and the non-turbulent way that relatively viscous thixotropic
slurries flow in die casting dies, the SSM process is capable of
producing cast parts having thin sections, great detail and
complexity and close dimensional tolerances, without the entrapped
porosity and oxides which are commonplace in conventional die
casting processes.
SUMMARY OF THE INVENTION
[0004] The present invention is directed to a new SSM process or
method which significantly reduces the costs of producing parts by
the SSM process. The method of the invention is ideally suited for
producing parts having thin sections, fine detail and complexity
and close dimensional tolerances, and which are substantially free
of porosity and oxides, can be processed at elevated temperatures
without blistering and which can provide high and reliable levels
of strength and ductility. The method of the invention avoids any
need to produce a specially treated, pre-cast billet that must be
sawed to length before using or a slurry especially prepared from
molten alloy in equipment external to the die casting press. The
method of the invention is also applicable to a wide variety of
alloys, for example, standard A356 alloy and alloys of the Al--Si,
Al--Cu, Al--Mg and Al--Zn families, all of which can be acquired in
the form of and at prices normal to conventional foundry ingot,
including both primary and secondary origin.
[0005] In accordance with one embodiment of the present invention,
an ingot of commercially available solid metal or metal alloy, such
as aluminum foundry alloy ingot, is heated to the molten state. If
not permanently grain refined, such as by employing a foundry alloy
called SiBloy produced by Elkem Aluminum, AS, an .alpha. aluminum
grain refining material such as 5:1::Ti:B master alloy produced by
numerous suppliers, or a product called TiBloy produced by
Metallurg, is added to the molten alloy in appropriate quantities
to accomplish fine grains in the solidified alloy product. The
grain refined molten alloy is poured directly into a large diameter
shot sleeve or chamber of a vertical die casting machine or press.
The shot chamber receives a vertically movable shot piston which
forms the bottom of the shot chamber, and the diameter of the shot
chamber is greater than its depth or axial length. In a preferred
embodiment of the present invention, the shallow shot chamber is
greater than its depth by a ratio of 2:1 or more. The shot chamber
is then indexed from the initial filling position to a slurry
injection position under a die. The molten alloy is permitted to
cool within the shot chamber to a predetermined temperature range
in which it forms a semi-solid slurry having 40 to 60 percent
solid, the solid fraction having a globular, generally
non-dendritic microstructure. The portion of the slurry immediately
adjacent to the wall of the shot chamber or shot sleeve and the
shot piston become significantly colder and more solid.
[0006] When the semi-solid slurry within the central portion of a
first shot chamber, now in the slurry injection position under the
die, has cooled to the predetermined temperature range in which it
has 40 to 60 percent solid, the shot piston is moved upwardly by a
mechanical actuator or a hydraulic shot cylinder to transfer or
inject the semi-solid slurry within the central portion of the shot
chamber through one or more gate or sprue openings and into one or
more cavities in the die above the shot chamber. The more solid
portion of the slurry adjacent the shot sleeve is prevented from
entering the die cavity or cavities, either by appropriately
distancing the gate or sprue openings from the shot sleeve walls or
by entrapping the more solid portion within an annular recess in
the gate plate through which the gates or sprue openings
communicate with the die cavity or cavities. As a result, the more
solid portion of the slurry remains in the residual solidified
biscuit. After the semi-solid slurry solidifies in the die cavity
or cavities, the shot piston retracts to retract the biscuit intact
with gates or sprues. The shot chamber is then transferred or
indexed back to its initial filling position where the biscuit with
the gates is removed laterally from the shot chamber and piston,
and the shot chamber is then ready to repeat the cycle. After the
die is opened, the part(s) is ejected and then indexed to a
position where it is removed, and the die is ready to repeat the
cycle.
[0007] During the slurry forming, slurry injection and slurry
solidification steps described above relative to the first shot
chamber while in its shot position, a second shot chamber in the
original filling position has similarly been filled with grain
refined molten alloy. When the first shot chamber and its piston
are transferred or indexed back to the initial filling position for
biscuit removal, the second shot chamber and molten alloy are
indexed to the metal transfer or slurry injection position under
the die, and the process of slurry formation, slurry injection and
slurry solidification is accomplished just as with the first shot
chamber. The process is repeated over and over again. In a
modification, the shot piston is provided with internal
interconnected spaced grooves through which cooling fluid or water
is circulated to provide a large heat transfer area for absorbing
heat from the molten metal or alloy within the shot chamber.
[0008] Other features and advantages of the invention will be
apparent from the following description, the accompanying drawings
and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a vertical section through a vertical die casting
press which is used to perform the method of the invention and with
the die set shown in its open position;
[0010] FIG. 2 is an enlarged fragmentary section of the semi-solid
slurry transfer or injection position or station shown in FIG. 1
and with the die set shown in its closed position;
[0011] FIG. 3 is a diagrammatic illustration of the metal
temperature profile of the semi-solid slurry before a central
portion of the slurry is transferred or injected into the die
cavities shown in FIG. 2;
[0012] FIG. 4 is an axial section of a shot piston similar to the
shot piston shown in FIG. 1 and showing a modification of the
invention;
[0013] FIG. 5 is a top view of the shot piston, taken generally on
the line 5-5 of FIG. 4; and
[0014] FIG. 6 is an enlarged bottom view of the shot piston, taken
generally on the line 6-6 of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Referring to FIG. 1, a vertical die cast machine or press 10
is constructed similar to the press disclosed in U.S. Pat. No.
5,660,223 which issued to the assignee of the present invention and
the disclosure of which is incorporated by reference. The press 10
includes a frame 12 formed by a pair of parallel spaced vertical
side walls or plates 14 rigidly connected by top plate 16 a base or
bottom plate 18 and a set of intermediate cross plates or bars 22
and 24 all rigidly secured to the side panels 14. The top cross
plate 16 supports an upper double acting hydraulic clamping
cylinder 30 having a piston rod 32 projecting downwardly on a
vertical center axis of the press. The piston rod 32 carries an
adapter plate 34 which supports a hydraulic ejector cylinder 36
having a piston 37 projecting downwardly to support a plate 38
which carries a set of ejector pins 39.
[0016] An upper die or mold section 40 (FIG. 2) is secured to the
bottom of the plate 38 by an annular retaining plate 41 and has a
pair of recesses 42 which receive corresponding core members 43. A
lower die or mold section 45 is recessed within a circular indexing
or transfer table 48 and defines a pair of cavities 50 which
cooperate with the core members 43 to define the corresponding
metal parts P produced in accordance with the method of the
invention. The transfer or indexing table 48 is mounted on a shaft
52 (FIG. 1) supported by a set of bearings 53 retained within the
frame member 54. The table 48 carries a plurality of at least two
lower mold sections 45 and is rotated or indexed by a pinion (not
shown) engaging periphery teeth 56 on the table 48 and driven by a
stepping motor (not shown). A gate plate 60 is positioned under the
bottom mold section 45 and defines a pair of slightly tapered gates
or sprue openings 62, one for each of the cavities 50. The gate
plate 60 also defines an annular metal entrapment recess or groove
63. It is to be understood that the parts P to be die cast within
the corresponding mold sections 40 and 45 are shown for
illustration only and that the configuration or size of the parts
form no part of the present invention. The parts P may be any size
or shape, corresponding to the desired die cast article.
[0017] A cylindrical vertical column or post 66 is secured to a
plate 67 mounted on the base plate 18 and projects upwardly to
support a rotatable circular table 68 by a set of anti-friction
bearings 69 mounted on a top hub of the post 66. The table 68
supports a plurality or a pair of diametrically opposite
cylindrical shot sleeves 70 which have parallel vertical axes. The
table 68 is also supported by a set of thrust bearings 72 mounted
on the cross bars or plates 22 and 24. The table 68 also has
peripheral gear teeth 74 which engage a pinion (not shown) mounted
on a vertical shaft of an electric stepping motor (not shown).
Actuation of the stepping motor is effective to index the table 68
in steps or increments of 180.degree. for alternately presenting
the pair of shot sleeves 70 between a molten metal receiving or
pour station 80 and a metal injecting or transfer station 82
located under the die sections 40 and 45 and in axial alignment
with the clamping cylinder 30.
[0018] Each of the shot sleeves 70 defines a cylindrical shot
chamber 86 which receives a corresponding shot piston 88. The upper
end portion of each shot piston 88 has a pair of laterally
extending and tapered dovetail slots 92, and a shot piston rod 94
projects downwardly from each piston 88. Each of the shot sleeves
70 and each of the piston rods 94 is provided with internal
passages 87 (FIG. 2) by which cooling fluid or water is circulated
through the sleeves and pistons 88 for cooling the molten metal and
to form a metal residue biscuit B having integrally connected and
upwardly projecting gate pins formed by the gate openings 62.
[0019] A double acting hydraulic shot cylinder 95 is mounted on a
spacer plate 96 secured to the base plate 18 under the metal
transfer station 82 and in vertical alignment from the axis of the
hydraulic clamping cylinder 30. The shot cylinder 95 includes a
piston and piston rod 98 which projects upwardly, and a guide plate
99 is secured to the upper end of the piston rod 98. Another double
acting hydraulic ejection cylinder 110 is substantially smaller
than the cylinder 95 and is mounted on the plate 67 by a spacer
block 112. The cylinder 110 includes a piston and piston rod 114
and a guide plate 116 is secured to the upper end of the piston rod
114. A guide rod 118 projects downwardly from the plate 116 and
through a guide block 121 mounted on the cylinder 110 to prevent
rotation of the plate 116 and piston rod 114. The cylinder 110 is
located in vertical axial alignment with each shot sleeve 70 when
the sleeve is located at the metal receiving or pouring station
80.
[0020] A pair of opposing retaining or coupling plates 126 are
secured to the upper surface of each of the guide plates 99 and
116. Each set of coupling plates defines inner and outer opposing
undercut slots for slidably receiving an outwardly projecting
circular flange 128 formed on the bottom of each shot piston rod
94. Thus when the table 68 and shot sleeves 70 are indexed in steps
of 180.degree., the shot piston rods 94 are alternately connected
or coupled to the piston rods 98 and 114.
[0021] In operation of the vertical die cast machine or press 10 to
perform a semi-solid molding method, a commercially available
permanently grain refined alloy such as SiBloy foundry ingot
produced by Elkem Aluminum AS, or a non-permanently grain refined
alloy such as standard A356 aluminum foundry ingot or foundry alloy
ingot of the Al--Si, Al--Cu, Al--Mg or Al--Zn families, is heated
to a molten state. Preferably, when a melt of non-permanently grain
refined alloy is at a predetermined temperature, for example
650.degree. C. or higher, an .alpha. aluminum grain refining
material, for example, a titanium boron master alloy sold under the
trademark TiBloy and produced by Metallurg, is added at a preferred
melt-to-master alloy ratio according to the manufacturer's
recommendations. The grain refinement step is not necessary when
utilizing a permanently grain refined alloy such as SiBloy. After
the molten grain refined alloy is lowered to a temperature of about
626.degree. C., or within the range of 621.degree. C. to
632.degree. C., the molten alloy is poured into the vertical shot
chamber 86 located at the pour or fill station 80 above the
ejection cylinder 110. Preferably, the shot chamber 86 has a
diameter substantially larger than its depth or axial length, for
example, a diameter over 6 inches, such as 71/2 inches and a depth
of less than 6 inches.
[0022] The shot sleeve 70 confining the molten alloy is then
indexed to the transfer or injection station 82 while a cooling
period occurs. The molten alloy is allowed to cool in the shot
chamber 86 to a temperature range that produces a semi-solid slurry
having a range of 40% to 60% solid, such as approximately 50% solid
and a globular generally non-dendritic micro structure. For
example, the A356 aluminum alloy is allowed to cool to a
temperature range between 570.degree. C. and 590.degree. C. for a
period of fifteen seconds or more from the time it entered that
temperature range to the shot or injection time. When the alloy has
cooled to this temperature within the shot chamber 86 at the
transfer station 82, the temperature profile of the alloy is close
to that shown in FIG. 3 wherein a central portion A of the alloy
has a substantially uniform temperature, and the peripheral portion
of the alloy adjacent the shot sleeve 70 is significantly cooler
due to the cooling effect of the shot sleeve.
[0023] With the mold sections 40 and 45 in their closed position
(FIG. 2) by actuation of the cylinder 30, the injection or shot
cylinder 95 is actuated to move the shot piston 88 upwardly. This
transfers the semi-solid slurry S1 within the central portion A
(FIG. 3) of the alloy upwardly through the gate or sprue openings
62 and into the corresponding die cavities 50 to form the parts P
which have the desired globular, generally non-dendritic micro
structure. The more solidified outer portion of the slurry S2
within the shot chamber adjacent the sleeve 70 is captured or
trapped in the annular recess 63 and prevented from entering the
sprue openings 62.
[0024] While the parts P are solidifying within the cavities 50,
another charge of molten alloy is poured into the second shot
chamber 86 located at the pour station 80. When the parts in the
cavities 50 are solidified, the shot cylinder 95 is actuated to
retract the piston 88 and the residual solidified alloy material or
biscuit B within the shot chamber 86 and to shear the metal within
the gate or sprue openings 62 from the parts P at the interface of
the lower mold section 45 and the gate plate 60. The residual
solidified metal or biscuit B, including the sprees, within the
shot chamber 86 is then transferred by indexing the table 68 to
either a biscuit removal station or to the metal pour station 80.
At this station, the piston 88 is elevated to a level where the
biscuit B is ejected laterally by a fluid cylinder (not shown).
After the parts P are fully solidified, the upper mold section 40
is retracted upwardly by actuation of the cylinder 30 while the
cylinder 36 is actuated to eject or release the parts with the pins
39. The table 48 is then indexed to transfer the parts P to a part
removal station where the parts are lifted and removed, for
example, by a robot (not shown). The above method steps for
semi-solid molding are then repeated for successively molding
another set of parts.
[0025] FIGS. 4-6 illustrate a modification of a shot piston and
shot piston rod constructed and assembled in accordance with
another embodiment of the invention and in which components
corresponding to the components described above in connection with
FIGS. 1 and 2 are identified with the same reference numbers but
with the addition of prime marks. Thus as shown in FIGS. 4-6, a
cylindrical shot piston 88' has an upper surface with a set of
tapered dovetail slots or recesses 92' and is secured to a mating
upper head portion 134 of a shot piston rod 94' by a locating band
135 and a series of axially extending and circumferentially spaced
bolts 136 (FIGS. 4 and 6). The cylindrical shot piston 88' has a
bottom surface with a set of concentric passages or grooves 141,
142 and 143 defined between concentrically spaced walls
part-cylindrical 146, 147 and 148. The grooves are successively
interconnected by passages or ports 151, 152 and 153 formed within
the walls 146, 147 and 148, respectively. As shown in FIG. 6, the
ports 151 and 153 are diametrically opposite the port 152 to
provide a maze path for cooling fluid or water, as shown by the
arrows in FIG. 6.
[0026] Referring to FIG. 4, a pair or set of cooling fluid or water
inlet passages 156 are formed within the shot piston rod 94' and
are connected by a pair of corresponding axially extending passages
158 to the outer circular passage or groove 141 within the shot
piston 88'. The shot piston rod 84' also has a cooling fluid or
water outlet passage 87' which extends within the center of the
shot piston rod 94' to a center chamber 162 (FIG. 6) within the
shot piston 88' and defined within the part-cylindrical wall 148.
As shown in FIGS. 4 and 6, the cooling fluid or water flows
upwardly through the passages 156 and 158 within the shot piston
rod 94' and successively inwardly through the grooves 141, 142 and
143 along the maze path and into the center chamber 162 where the
hotter cooling fluid or water is removed from the shot piston 88'
and shot piston rod 94' through the passage 87'.
[0027] From the drawings and the above description, it is apparent
that a method of semi-solid molding of parts with a vertical die
casting press in accordance with the present invention, provides
desirable features and advantages. For example, the method of the
invention provides for producing die cast parts free of porosity
and which may be heat treated to provide a reliable high level of
strength and ductility. As a result, the parts may have thin wall
sections and be lighter in weight and/or may be complex die cast
parts having close tolerances. The method also extends the service
life of the die sections since the die sections receive less
sensible heat because the injected slurry is at a lower temperature
than fully molten metal and with less heat of fusion since the
slurry is already approximately 50 percent solid when injected.
Also, since the die is required to absorb much less heat in the
process, the overall cycle time may be decreased to obtain more
efficient production of parts.
[0028] The semi-solid molding method of the invention also
eliminates the preparation of special billets or special slurries
and the substantial cost of the preparation equipment, and enables
the reuse of process offal and scrap. That is, by using
conventional foundry ingots or ingots of pure metal, which may be
grain refined, the method of the invention significantly lowers the
cost of input material for semi-solid molding. As another feature,
the large diameter to depth ratio of the shot chamber and the
controlled cooling of the shot sleeves and shot piston provide for
obtaining the desired cooling and temperature profile of the alloy
within the semi-solid slurry S1 in the center portion of the shot
chamber. The annular entrapment recess 63 is also effective to
prevent the more solidified alloy S2 adjacent the shot chamber wall
or sleeve from entering the sprue openings 62 and flowing into the
cavities 50. The short stroke of the shot piston 88, which is
greater than its diameter, also provides for a broad range of
cavity fill rates, for example, when a rapid fill rate is desired
for parts having thin wall sections or a slow fill rate is desired
for parts having heavy wall sections. The diameter of the shot
sleeve and piston are preferably over 6" and may be substantially
more, for example, 24" in order to die cast a large diameter SSM
part such as a motor vehicle wheel or frame member.
[0029] The construction of the shot piston 88' and shot piston rod
94' also provides desirable features. That is, the large area for
heat transfer as provided by the walls 146, 147 and 148 and the
maze path for the cooling fluid or water within the grooves between
the walls provide for rapidly transferring heat from the molten
metal or alloy within the shot chamber 86 to the cooling fluid and
thereby provide for reducing the molding cycle time and/or
temperature uniformity in the molten metal or slurry. The ribs or
walls 146-148 within the shot piston also strengthen the piston
against deflection due to injection pressure, which is especially
desirable for large diameter or area pistons.
[0030] While the method and forms of apparatus herein described
constitutes preferred embodiments of the invention, it is to be
understood that the invention is not limited to the precise method
and forms of apparatus described, and that changes may be made
therein without departing from the scope and spirit of the
invention as defined in the appended claims. For example, while the
vertical die cast press 10 incorporates rotary indexing tables 48
and 68, vertical die cast presses with other forms of transfer
means may be used, for example, a reciprocating shuttle table for
the bottom die section or a tilting mechanism for a single shot
sleeve.
* * * * *