Differential pressure, countergravity casting with alloyant reaction chamber

A casting mold (e.g., a cope and drag) is disposed on a drag slab having a sprue and a reaction chamber containing an alloyant to be selectively introduced into the melt as it is drawn through the reaction chamber during differential pressure, countergravity casting. A rubber formed between the mold and the drag slab communicates the reaction chamber to a plurality of narrow mold ingates that supply the melt treated (alloyed) in the reaction chamber to the mold cavity during countergravity casting.

FIELD OF THE INVENTION 
This invention relates to an improved apparatus and method for the 
differential pressure, countergravity casting of a melt in such a manner 
as to pass the melt through a reaction chamber to selectively introduce 
alloyant into the melt as it is countergravity cast into the mold. 
BACKGROUND OF THE INVENTION 
A vacuum countergravity casting process using a gas permeable mold 
sealingly received in a vacuum housing is described in such patents as the 
Chandley et al U.S. Pat. Nos. 3,900,064; 4,340,108 and 4,606,396. That 
countergravity process involves providing a mold having a porous, gas 
permeable upper mold member (cope) and a lower mold member (drag) 
sealingly engaged at a parting plane, sealing the mouth of a vacuum 
housing to a surface of the mold such that a vacuum chamber formed in the 
housing confronts the gas permeable upper mold member, submerging the 
underside of the lower mold member in an underlying melt and evacuating 
the vacuum chamber to draw the melt upwardly through one or more narrow 
ingates (pin gates) in the lower mold member and into one or more mold 
cavities formed between the upper and lower mold members. 
In practicing the vacuum countergravity process to produce nodular iron 
castings, the melt is typically prepared in a melting vessel (e.g., a 
cupola) using a charge of pig iron to which additions of alloyants are 
made to provide the desired base melt chemistry. For example, in casting 
nodular iron, ferromanganese (Fe--Mn), ferrosilicon (Fe--Si) and other 
additions are made to the base pig iron charge to provide a desired base 
melt chemistry. 
Once the desired base melt composition is achieved, the melt is transferred 
from the melting vessel to a ladle where a nodularizing agent (e.g., a 
magnesium bearing alloy such as Fe--Si--Mg) is added to spherodize 
(nodularize) the carbon. The treated base melt is then transferred from 
the ladle to a casting vessel to provide a melt pool from which a 
plurality of molds are successively vacuum countergravity cast over time. 
However, prior art workers have experienced great difficulty in maintaining 
an effective concentration (i.e., at least 0.02 percent by weight) of 
magnesium in the melt over the extended time required to cast a plurality 
of molds in succession from the melt. This difficulty is attributable to 
the rapid evaporation of magnesium from the melt after the initial 
treatment with the nodularizing agent in the transfer ladle. Erratic, 
uncontrolled loss (also known as fade) of the fugitive magnesium from the 
melt over time has been experienced and resulted in off-chemistry melts in 
so far as magnesium content is concerned and correspondingly inconsistent 
nodularization. 
As a result of this inability to reliably control and maintain the melt 
chemistry (i.e., to maintain the magnesium content above the desired 
effective level) over the time required for casting a plurality of molds 
in succession, use of the countergravity casting processes described in 
the aforesaid patents in high volume production of nodular iron parts has 
been rendered impractical and/or uneconomical to date. 
Moreover, in order to produce iron castings having different 
compositions/microstructures (e.g., corresponding to the known ferritic 
nodular grade 4010 or pearlitic nodular grade 5203), the practice has been 
to prepare separate base melts of the desired different compositions using 
pig iron charges to which appropriate alloy additives are made in the 
melting vessel and then ladling and countergravity casting the separate 
base melts from the casting vessel as described above. This practice 
amounts to producing castings of one composition/microstructure in one 
batch and castings of another different composition/microstructure in a 
separate batch with preparation as well as subsequent handling, treatment 
and casting of different base melts for each batch. 
It is an object of the present invention to provide an improved apparatus 
and method for the differential pressure, countergravity casting of a melt 
wherein the melt is drawn through a reaction chamber formed in a drag slab 
to introduce alloyant therein above a predetermined effective 
concentration and the treated (alloyed) melt is then supplied to a casting 
mold, such as a cope and drag, disposed atop the drag slab. 
It is another object of the invention to provide an improved apparatus and 
method for the differential pressure, countergravity casting of a melt 
wherein a fugitive alloyant, such as a Mg nodularizing agent used to 
nodularize iron, is introduced into the melt in a reaction chamber of a 
drag slab disposed between the mold and the melt to maintain a 
predetermined effective concentration of the fugitive alloyant in the melt 
supplied to the casting mold, thereby counteracting any previous loss (or 
fade) over time of the fugitive alloyant from the melt. 
It is another object of the invention to provide an improved apparatus and 
method for the differential pressure, countergravity casting of a melt 
wherein the melt is treated (alloyed) in a reaction chamber in a drag slab 
disposed between the mold and the melt and supplied to the mold to produce 
a casting having a composition/micro- structure different from that 
obtainable from the underlying melt and tailored for a particular intended 
use, thereby eliminating the need to prepare, handle, treat and cast 
separate base melts. 
SUMMARY OF THE INVENTION 
The present invention contemplates an improved apparatus and method for the 
differential pressure, countergravity casting of a melt wherein a casting 
mold (e.g., a gas permeable cope and a drag) is disposed on an underlying 
drag slab having a sprue with a lower inlet adapted for engaging an 
underlying source of melt, and an alloyant-containing reaction chamber 
between upper and lower sides of the drag slab for receiving melt from the 
sprue. The casting mold includes a mold cavity and one or more ingates 
(e.g., pin gates) on the mold underside. The mold ingates are communicated 
directly or indirectly (e.g., via runners) to the reaction chamber for 
supplying the treated melt from the reaction chamber to the mold cavity 
during casting. 
A sufficient differential pressure is established between the mold cavity 
and the underlying source of melt when the sprue inlet and the source are 
engaged to draw the melt upwardly through the sprue into the reaction 
chamber where the melt so contacts the alloyant as to have the alloyant 
introduced therein. The treated (alloyed) melt then is drawn through the 
mold ingates to fill the mold cavity with the treated melt. 
In one embodiment of the invention, a lateral runner system is formed in 
the underside of the casting mold (e.g., in the underside of the mold 
drag) and communicates the narrow ingates (e.g., pin gates) formed in the 
mold drag to the reaction chamber in the drag slab. 
In another embodiment of the invention for countergravity casting of 
nodular iron, the reaction chamber in the drag slab contains a fugitive 
magnesium-bearing nodularizing agent contacted by the melt drawn through 
the reaction chamber during countergravity casting so as to introduce the 
nodularizing agent into the melt immediately prior to its entering the 
mold cavity. The concentration of the fugitive nodularizing agent in the 
melt entering the mold cavity is maintained above a predetermined 
effective concentration for nodularizing the carbon in the melt. This 
alleviates problems of loss (or fade) over time of the nodularizing agent 
from the melt. 
In another embodiment of the invention, an alloyant is introduced into the 
melt in the reaction chamber in the drag slab for supply to the mold to 
form a casting having a composition/microstructure tailored to a 
particular end use and different from that obtainable by solidification of 
the underlying melt. For example, copper can be introduced into a ferritic 
nodular iron melt as it is drawn through the reaction chamber prior to 
entering the mold cavity to produce a casting having a microstructure and 
mechanical properties (e.g., tensile and yield strength) corresponding to 
a pearlitic nodular iron grade.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 1 depicts a pool 2 of melt 4 which is to be drawn up into a casting 
mold assembly 6 comprising a gas-permeable casting mold 7 disposed on a 
lower drag slab 11 at a parting line 13. The casting mold 7 includes a gas 
permeable upper mold portion 8 (mold cope) and a lower mold portion 10 
(mold drag) engaged at a parting line 12 and defining a mold cavity 14 
therebetween. The melt 4 is contained in an underlying casting furnace or 
vessel 3 heated by one or more induction coils (not shown) to maintain the 
melt 4 at a desired casting temperature. 
The drag slab 11 is sealed to the mouth 18 of a vacuum box 20 defining a 
vacuum chamber 22 via a seal 24 (e.g., high temperature rubber, ceramic 
rope, etc.). The seal 24 is affixed to the lower edge of the depending 
peripheral side 25 of the vacuum box 20 to this end. The vacuum chamber 22 
encompasses the upper mold portion 8 and communicates with a vacuum source 
23 (e.g., a vacuum pump) via conduit 26. 
The seal 24 on the peripheral wall 25 of the vacuum box 20 may engage the 
upper mold portion 8 or the lower mold portion 10 in lieu of engaging the 
drag slab 11. In situations where the seal 24 engages the upper or lower 
mold portion 8,10, the exterior surfaces of the mold 7 and/or drag slab 11 
exposed outside the vacuum box 20 may be substantially sealed as by 
coating with a core wash material (e.g., a silica slurry) well known in 
the art to reduce the gas permeability of those surfaces and provide 
better control of the negative differential pressure established between 
the inside and the outside of the vacuum box 20. Moreover, if exposed 
outside the vacuum box 20, one or both of the parting lines 12,13 may be 
glued to this same end; i.e., to reduce or prevent drawing of air into the 
mold assembly 6 at the parting lines 12,13. 
The upper mold portion 8 comprises a gas-permeable material (e.g., 
resin-bonded sand) which permits gases to be withdrawn therethrough from 
the mold cavity 14 when a vacuum is drawn in the vacuum chamber 22. The 
lower mold portion 10 and the drag slab 11 may conveniently comprise the 
same material as the upper mold portion 8 or other materials, permeable or 
impermeable, which are compatible with the material of the upper mold 
portion 8. The drag slab 11 includes an upstanding levee 26 surrounding 
the seal 24 and isolating it from the melt 4 for purposes described in 
U.S. Pat. No. 4,745,962 and assigned to the assignee of the present 
invention. The upper and lower mold portions 8,10 and the drag slab 11 are 
each made in accordance with known mold practice where a compliant 
(shapeable) mixture of sand or equivalent particles and a settable binder 
material (e.g., an inorganic or organic thermal or chemical setting 
plastic resin) is formed to shape and then cured or hardened against 
respective contoured pattern plates (not shown). Alternately, the drag 
slab 11 could be made of a different material resistant to degradation in 
the melt to enable repeated use in casting multiple disposable molds in 
succession. 
The drag slab 11 includes a plurality of anchoring sites 28 engaged by 
T-bar keepers 30 of the type described in commonly assigned U.S. patent 
application Ser. No. 147,863, abandoned in favor of patent application 
Ser. No. 286,051, providing means for mounting a mold assembly to the 
vacuum box 20. In particular, the drag slab 11 is provided with a 
plurality of anchoring cavities 32 adapted to receive the T-bar keepers 30 
via slots 34 in the shelves 40 overlying the anchoring cavities 32 and 
attached to the drag slab 11. A 90.degree. rotation of the T-bar carrying 
shafts 36 (e.g., by air motors 38) causes the T-bar keepers 30 to engage 
the underside of the attached shelves 40 overhanging the cavities 32 to 
secure the drag slab 11 to the vacuum box 20. Other known mold 
assembly-to-vacuum box mounting means can also be employed in practicing 
the invention (e.g., see U.S. Pat. No. 4,658,880). 
The upper mold portion 8, the lower mold portion 10 and the drag slab 11 
are pressed into sealing engagement (i.e., at the parting lines 12 and 13) 
by means of a plurality of plungers 42 so as to eliminate, if desired, the 
need to glue the upper and lower mold portions 8,10 at the parting line 12 
and lower mold portion 10 and drag slab 11 at the parting line 13. Feet 44 
on the ends of the plungers 42 distribute the force of the plungers 42 
more widely across the top of the upper mold portion 8 to prevent 
penetration/puncture thereof by the ends of the plungers 42. Pneumatic 
springs 46 bias the plungers 42 downwardly to resiliently press the upper 
mold portion 8 against the lower mold portion 10 and the lower mold 
portion 10 against the drag slab 11 as the mold assembly 6 is being 
positioned in the mouth 18 of the vacuum box 22. Schrader valves 48 on the 
air springs 46 permit varying the pressure in the springs 46 as needed to 
apply sufficient force to press the upper mold portion 8, the lower mold 
portion 10 and the drag slab 11 into sealing engagement, and, as needed, 
to prevent destructive inward flexure of the mold assembly 6 when the 
casting vacuum is drawn. The force applied by the plungers 42, however, 
will not be so great as to overpower and damage the anchoring sites 28, 
dislodge the mold assembly 6 from the mouth 18 of the box 20, or break the 
seal formed thereat. 
Referring to FIGS. 1-3, in accordance with one embodiment of the invention, 
the drag slab 11 includes a single sprue 50 having a lower inlet 50a for 
engaging the pool 2 and supplying the melt 4 to a reaction chamber 52 when 
the lower side 11a of the drag slab 11 is immersed in the pool 2 with the 
mold cavity 14 evacuated. The sprue 50 extends between the lower side 11a 
and the upper side 11b of the drag slab 11 and is in flow communication 
with the reaction chamber 52 via a recessed inlet chamber 54 formed in the 
upper side 11b of the drag slab 11. 
The reaction chamber 52 includes a horizontal bottom wall 52a and 
upstanding (slightly outwardly diverging) side walls 52b. The top of the 
reaction chamber 52 and the recessed inlet chamber 54 are closed off by 
the bottom side 7a of the casting mold 7 (i.e., by the bottom side of the 
mold drag 10) as best shown in FIG. 3. 
As will be explained in more detail hereinbelow, an alloyant 60 in suitable 
form (e.g., alloy particulate) is disposed in the reaction chamber 52 to 
be so contacted by the melt 4 drawn upwardly through the sprue 50 and into 
the reaction chamber 52 during countergravity casting as to introduce a 
selected quantity of the alloyant 60 into the melt 4 immediately prior to 
its entering the mold cavity 14. 
The alloyant 60 is positioned in the reaction chamber 52 of the drag slab 
11 before the drag slab 11 is assembled with the casting mold 7. In 
particular, the appropriate quantity of the alloyant 60 is first placed in 
the reaction chamber 52 of the drag slab 11 and the casting mold 7 is then 
disposed atop the drag slab 11 as shown in FIG. 1. Although the mold 7 is 
described as including a single mold cavity 14, the invention envisions 
supplying the treated melt 4 from the reaction chamber 52 to a plurality 
of mold cavities 14 in one or more molds 7 disposed on the drag slab 11. 
Referring to FIGS. 2-3, the reaction chamber 52 includes an inlet 52c at 
its juncture with the recessed inlet chamber 54 and an outlet 52d at its 
juncture with a primary, lateral (horizontal) runner 62 formed in the 
bottom side 7a of the mold 7. The runner 62 communicates in flow relation 
with a secondary lateral (horizontal) runner 64 also formed in the bottom 
side 7a of the mold 7. The secondary runner 64 in turn communicates in 
flow relation with each of the plurality of narrow upstanding ingates 16 
(i.e., pin gates) formed in the lower mold portion 10 and extending 
between the mold cavity 14 and the bottom side 7a. 
Referring to FIG. 3, the primary runner 62 includes a riser portion 62a 
proximate the outlet 52d of the reaction chamber 52. The riser portion 62a 
has an increased vertical dimension (compared to other portions 62b of the 
runner 62 remote from the reaction chamber outlet 52d) so as to trap dross 
inclusions and other floating debris in the treated melt 4 exiting the 
reaction chamber 52. The outlet 52d of the reaction chamber 52 
communicates directly with the riser portion 62a formed in the bottom side 
7a of the mold 7 to this end. 
The secondary runner includes a main runner portion 64a and a plurality of 
branch runner portions 64b each communicating with a respective ingate 16. 
The ingates 16 (pin gates) preferably have a major dimension (e.g., 
diameter for the cylindrical ingates shown) not exceeding about 0.50 inch, 
preferably not exceeding about 0.25 inch (e.g., about 0.22 inch) for 
purposes set forth in U.S. Pat. No. 4,340,108 and hereinafter described. A 
particular pattern of the primary and secondary runners 62,64 formed in 
the bottom side 7a of the mold 7 is shown best in FIG. 2 wherein the 
pattern is superimposed in phantom lines atop the upper side 11b of the 
drag slab 11. 
Referring to FIGS. 1-3, countergravity casting of the melt 4 into the 
casting mold assembly 6 is effected by relatively moving the mold assembly 
6 and the pool 2 to immerse the underside 11a of the drag slab 11 in the 
melt 4. Typically, the casting mold assembly 6 is lowered toward the pool 
2 using a hydraulic power cylinder 61 (shown schematically) actuating a 
movable support arm 63 (shown schematically) that is connected to the 
vacuum box 20. The vacuum chamber 22 is then evacuated to draw the melt 4 
upwardly through the sprue 50 and through reaction chamber 52 where the 
melt 4 so contacts the alloyant 60 as to have the alloyant 60 introduced 
(e.g., dissolved) therein above a predetermined effective concentration. 
The treated melt 4 (i.e., the melt containing the alloyant) is drawn 
through the outlet 52d of the reaction chamber 52 through the runners 
62,64 and the pin gates 16 into the mold cavity 14 to fill it with the 
treated melt 4. 
After filling of each mold cavity 14 with the treated melt 4 and initial 
solidification of the treated melt in the pin gates 16, the vacuum box 22 
is raised by hydraulic power cylinder 60 to withdraw the underside 11a of 
the drag slab 11 out of the pool 2. The number and size of the narrow pin 
gates 16 to achieve melt solidification initially at the pin gates 16 can 
be selected in accordance with the teachings of U.S. Pat. No. 4,340,108. 
Alternatively, the treated melt 4 can be allowed to solidify in both the 
pin gates 16 and the mold cavity 14 before raising the vacuum box 22 to 
withdraw the mold assembly 6 out of the pool 2. The vacuum box 22 and the 
melt-filled mold assembly 6 are then separated. 
Those skilled in the art will appreciate that the size and shape of the 
sprue 50, reaction chamber 52 and inlet chamber 54 formed in the drag slab 
11 are dependent upon the size and shape of the part to be cast as well as 
on the composition of the specific melt 4 and alloyant 60 employed. 
Moreover, the size and shape of the primary and secondary runners 62,64 
and pin gates 16 formed in the mold drag 10 are similarly dependent. These 
features of the mold assembly 6 are selected to provide a desired melt 
flow rate and residence time in the reaction chamber 52 and melt flow rate 
into the mold cavity 12. 
By way of illustration and not limitation, a casting mold 7 and drag slab 
11 similar to those described hereinabove (i.e., having a similar sprue 
50, reaction chamber 52, inlet chamber 54, runners 62,64 and pin gates 16) 
were used to countergravity cast an automobile engine manifold of nodular 
iron. The cast exhaust manifold weighed about 21 lbs. including the 
solidified metal in the reaction chamber 52, runners 62,64 and pin gates 
16. The reaction chamber 52 included an upper square (about 4.47 
inch.times.about 4.47 inch) cross-section tapering down (5.degree.) to a 
depth of about 2.52 inches to provide a lower square (about 4.05 
inch.times.about 4.05 inch) cross-section at the bottom wall 52a. The 
sprue 50 had a length of about 3.70 inches and a maximum diameter of about 
1.40 inch at its outlet adjacent the recessed inlet chamber 54 and minimum 
diameter of about 0.79 inch at its inlet 50a. The recessed chamber 54 was 
about 0.30 inch in depth and about 1.62 inch in width where it intersected 
the reaction chamber 52 to provide a reaction chamber inlet area of about 
0.49 square inches. The longitudinal axis of the sprue 50 was offset from 
the reaction chamber inlet 52c by about 1.25 inch. 
The primary and secondary runners 62,64 were formed in the bottom side 7a 
of the casting mold 7 in a pattern similar to that illustrated in FIG. 2. 
The primary runner 62 had an overall length of about 5.12 inches. The 
riser portion 62a of the primary runner 62 was rectangular in 
cross-section (about 1.68 inch major width.times.about 0.413 minor 
width.times.about 1.00 inch height) and intersected the reaction chamber 
52 to provide a reaction chamber outlet area of about 0.42 square inches. 
The area of the reaction chamber outlet 52d was selected to maintain the 
reaction chamber 52 metallostatically pressurized and filled with the melt 
4 during the entire casting process (i.e., until the mold cavity 14 was 
filled with the melt 4). 
The smaller portions 62b of the primary runner 62 had a square 
cross-section (about 0.413 inch.times.about 0.413 inch). 
The main portion 64a of the secondary runners 64 had a square cross-section 
(about 0.413 inch.times.about 0.413 inch) and an overall length of about 
25.62 inches (the left hand segment in FIG. 2 being about 14.50 inches in 
length and the right hand segment being about 11.12 inches in length). The 
smaller branch portions 64b of the secondary runners 64 each had a 
generally square cross-section (about 0.220 inch.times.about 0.220 inch). 
The branch portions 64b were formed with cylindrical wells 64c (about 
0.375 inch diameter) in communication with each cylindrical pin gate 16 
(0.220 inch diameter) formed in the mold drag 10. A total of fifteen pin 
gates 16 were used to supply treated molten iron to the manifold-shaped 
mold cavity 14. Molten metal filter (not shown) may be used in the runner 
system 62,64. 
Prior to assembly of the casting mold 7 and the drag slab 11, about 120 
grams (0.264 lbs.) of an inoculant alloy 60 in particulate form (e.g., 
5.times.18 mesh) was positioned in the reaction chamber 52 to a depth of 
about 0.25 inches. The inoculant alloy 60 has a nominal composition in 
weight percent (w/o) of 4.06 w/o Mg, 46.04 w/o Si, 1.14 w/o total rare 
earths, 0.48 w/o Ca, 0.71 w/o Al, balance Fe and is available under the 
trademark INMOLD II inoculant alloy owned by Material & Methods Limited, 
Surrey, England. 
A gray iron melt 4 devoid of any carbon nodularizing agent was maintained 
at about 2640.degree. F. in the underlying casting vessel 3. The melt 4 
was drawn upwardly from the vessel 3 by establishing a suitable vacuum in 
the vacuum chamber 22 (e.g., about 140 inches of water) when the lower 
side 11a of the drag slab 11 was immersed in the melt 4. The iron melt 4 
was drawn upwardly through the sprue 50 and passed through the reaction 
chamber 52 where it reacted (dissolved) the INMOLD II inoculant alloy 60. 
The treated iron melt 4 was supplied to the mold cavity 14 via the runners 
62,64 and the pin gates 16. The solidified exhaust manifold casting was 
sectioned, examined and found to contain nodularizied carbon (graphite) 
throughout the casting. 
Although the illustrative embodiment of the invention is described 
hereinabove with respect to the introduction of a nodularizing agent to an 
iron melt 4 to spherodize the carbon therein, those skilled in the art 
will appreciate that the invention is not so limited. For example, 
alloyants such as copper, chromium, manganese, molybdenum, silicon as well 
as others that are soluble in the melt 4 may be introduced into the melt 4 
during countergravity casting in accordance with the invention. 
By way of further illustration and not limitation, the alloyant 60 in the 
reaction chamber 52 may comprise copper in particulate or other form. A 
ferritic nodular iron melt 4 (corresponding in composition to the known 
ferritic nodular iron grade 4010) is drawn upwardly from the casting 
vessel 3 through the sprue 50, through the reaction chamber 52 and then 
into the mold cavity 14 in the manner as described hereinabove. The copper 
is introduced (i.e., dissolved) into the melt 4 as it passes through the 
reaction chamber 52 in a sufficient amount (e.g., about 0.4 w/o minimum to 
about 0.5 w/o maximum) to impart a microstructure and mechanical 
properties to the resultant casting corresponding to the known pearlitic 
nodular iron grade 5203. 
The present invention thus envisions producing castings having different 
compositions/microstructures and resultant mechanical properties from a 
common underlying melt 4 by successively countergravity casting a 
plurality of mold assemblies 6 having different alloyants 60 in their 
reaction chambers 52 from the common pool 2. A "universal" cupola melt 
thus can be used to supply the common pool 2. The need to prepare and 
handle different base melts in one or more melting vessels/ladles is 
thereby eliminated. Moreover, the flexibility of the vacuum countergravity 
casting process in meeting ever changing production schedule variations is 
tremendously improved. 
Furthermore, those skilled in the art will recognize that the invention is 
not limited to the casting of cast irons and may also be used in the 
differential pressure, countergravity casting of other metal/alloys where 
selective introduction of one or more alloyants is desired for some 
purpose. For example, the present invention may be used to introduce 
(dissolve) known degassing, desulfurizing, deslagging and similar treating 
agents into aluminum and steel during the vacuum countergravity casting 
thereof. 
While the invention has been described in terms of specific embodiments 
thereof, it is not intended to be limited thereto but rather only to the 
extent set forth hereafter in the claims which follow.