Note: Descriptions are shown in the official language in which they were submitted.
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This application is a division of application 336,736 filed
October 1, 1979.
This application relates to metal casting in gas permeable molds.
Although the techniques disclosed in United States Patent Nos.
3,863,706 and 3,900,064 have been in successful commercial use for several
years, we have discovered the existence of certain problems in their use with
gas permeable molds of the low temperature bonded sand grain type rather than
the high temperature resistant ceramic type with which they were primarily
intended to be used.
These problems occur because low temperature bonded sand grain shell
molds, such as those of the Croning type, in which sand grains or similar
particles are bonded together with a small proportion of an inorganic or
organic plastic thermal or chemical setting resin or equivalent material,
although much less expensive to produce than ceramic molds, have two major
deficiencies as compared to ceramic molds, in that they have relatively soft
interior mold cavity surfaces and also fail rapidly at high temperatures be-
cause their low temperature bonding materials decompose at low temperatures
so that the mold fails rapidly at temperatures lower than that of the molten
casting metal, particularly with ferrous metals.
Insofar as the first deficiency is concerned, under the high vacuum
required with the techniques of those patents in order to lift the molten
metal up the single long vertical central riser from which it flows into the
multiple mold cavities through vertically spaced gate passages, the molten
metal frequently penetrates the soft mold surface of a low temperature bonded
sand grain mold to the extent that casting quality is so reduced as to be
unacceptable.
Insofar as the second deficiency is concerned, since the effective
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life before failure of a low temperature bonded sand grain mold is measured in
seconds in the presence of molten ferrous metals, the time required to solidify
the castings in the molds of those patents is frequently of such duration that
the low temperature bonded sand grain mold fails before the molten metal in
the mold cavities is sufficiently solidified.
Because of these problems, under many circumstances, particularly when
casting parts of ferrous metals, Iow temperature bonded sand-grain molds ca~not
be utilized with the techniques of those patents, so that the much more
expensive ceramic shell molds must be substituted in order to provide acceptable
castings.
According to the present invention there is provided a rigid self
supporting gas permeable low temperature bonded sand grain mold havirlg side
surfaces extending between vertically spaced upper and lower surfaces with a
plurality of mold cavities spaced therebetween in a generally horizontal area
and horizontally spaced from one another, said mold cavities having gate pass-
ages with portions having a maximum width of 0.75 inches extending from said
cavities with their lower open ends spaced from one another and t~rminating
in a generally horizontal plane at the lower surface of said mold.
In use of the mold, the lower surface and the open end of the gate
passages are submerged beneath an underlying surface of molten metal while the
upper surface and at least a portion of the side surfaces are maintained
thereabove. A reduced pressure is applied to the upper surface of the mold
to fill the mold cavities with molten metal. The molds are unheated and at
ambient room temperature, so that the thin sections of molten metal in the
relatively narrow gate passage portions quickly solidify, but only for a short
period of time before they remelt due to the heat provided by the underlying
molten metal in the container. This brief period of gate passage solidifica-
tion makes it possible quickly to move the mold vertically upwardly out of
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contact with the underlying surface of molten metal~ even though the molten
metal in the mold cavities may not yet have entirely solidified, before the
solidified metal in the narrow gate passage portions remelts and allows the
molten metal in the mold cavities to drain back into the container of molten
metal. Particularly with high melting point metals such as ferrous metals
cast at temperatures of 2000 degrees ~ or higher, we have found that by quick-
ly moving the mold out of contact with the underlying surface of molten metal,
after the initial occurrence of solidification of metal in the narrow gate
passage portions, further heat input into the mold is prevented and mold
failure time is extended sufficiently for the castings in the mold cavities
to solidify. It also makes possible an unusually short casting cycle time,
which reduces production cost.
With molds having relatively small cavities, such as those having
internal thicknesses of less than 0.50 inches, we have found that filling
and solidification of the molten metal both in the mold cavity and the adja-
cent narrow gate passage or portion thereof will occur rapidly enough so that
the mold may be removed before it fails. With larger mold cavities, at least
with metals which do not shrink upon solidification, more than a single narrow
gate passage may be used for more rapid mold cavity filling so that the mold
may be removed before it fails.
With metals which shrink upon solidification and with large mold
cavities, such as those having internal thicknesses of greater than 0.50 inches
which cannot be filled through the narrow gate passage portions of the in-
vention before mold failure occurs, a blind riser may be used between one or
re vertical gate passages and a mold cavity, so that at least a portion of
the metal in the blind riser and in the mold cavity will remain in molten
condition for flow into the mold cavity after removing the mold from contact
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with the underlying surface of molten metal.
When using multiple cavity molds, since the lower open ends of the
gate passages are spaced from one another, a plurality of unconnected cast
metal parts or groups of parts are automatically provided.
With the conventional rigid, self supporting, low temperature bonded
sand grain mold as used in the methods of the present invention, we have dis-
covered that the maximum permissible submergence times, that is, maximum
length of time that the mold may be in contact with the underlying surface of
molten metal before the solidified metal in the narrow portions of the gate
passages remelts or the mold begins to fail, is largely determined by the
temperature at which the underlying molten metal must be maintained.
In the case of ferrous metal, such as cast iron and steel, which
are cast at temperatures greater than 2000 degrees F, the time is relatively
short, a maximum of about 30 seconds; so that submergence times of no more
than about 5 to 15 seconds have been found to be desirable. Also, in order
to prevent mold cavity surface penetration, reduced pressures of only about
-1.0 to -3.0 psig (13.7 to 11.7 psia) should be used to raise the molten
ferrous metal into mold cavities to a level no higher than about 6 to 8 inches
above the surface of the molten metal in the container. With lower melting
point metals, such as copper and alur.~inum and their alloys, longer times and
higher mold cavity heights may be used.
Thc mold cavities may extend to or across a mold parting plane and
may be arranged in a generally horizontal plane, preferably distributed both
lengthwise and widthwise thereof, and horizontally spaced from one another.
The maximum width of each of the gate passage portions is prefer-
ably no more than about 0.5 inches. With multiple cavity molds, the vertical
portions of the gate passages are generally perpendicular to the parting plane
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and their open ends are spaced from one another and distributed in a horizontal
plane.
For castings having wall thicknesses of less than about 0.50 inch,
the narrow gate passage portions may be adja.cent the mold cavity, with a
larger central vertical gate passage. For larger castings having greater wall
thicknesses, more than one narrow gate passage portion may be used if shrink-
age is not a problem; otherwise, a blind riser may be interposed between one
or more gate passages having a narrow vertical portion and one or more part
cavities.
The vacuum means for relatively varying the pressure within the
chamber preferably provide the sole support for the mold against the chamber.
In the accompanying drawings, which illustrate exemplary embodiments
of the present invention:
Figure 1 is a diagrammatic side view, partly in section, of a mold
and apparatus according to the invention;
Figure 2 is a detail side cross-sectional view of the chamber por-
tion of the apparatus of Figure l;
Figure 3 is a top view of the mold of Figure l;
Figure 4 is a detail side partial cross-sectional view of the mold
of Figure 3;
Figure 5 is a detail side partial cross-sectional view of the mold
of Figures 3 and 4 mounted on the chamber of the apparatus, with the lower
surface of the mold submerged beneath the underlying surface of molten metal
in the container;
Figure 6 is a cross-sectional side view of a molded metal part;
Figure 7 is a detail side partial cross-sectional view of a modifi-
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cation of the mold of Figure l;
Figure 8 is a detail top partial cross-scctional view of the mold
of Figure 7, taken along line 8-8 of Figure 7;
Figure 9 is a detail side partial cross-sectional view of another
modification of the mold of Figure l; and
Figure 10 is a detail side partial cross-sectional view of a further
modification of the mold of Figure 1.
Referring to Figure 1, the apparatus in general includes a base 12
having mounted thereon a post 14 on which is mounted, for vertical sliding
movement by power piston and cylinder 16, a horizontally extending arm 18.
Chamber 20, hereinafter more fully described, is mounted on support member
19 which extends downwardly from the free end of arm 18 above a container 22
for holding molten metal.
Referring to Figures 3 and 4, the rigid, self supporting, gas perm-
eable, low temperature bonded sand grain mold, generally designated 30,
commonly referred to as a Croning shell mold, is made by techniques and equip-
ment well known in the art, of sand grains or equivalent particles and in-
organic or organic thermal or chemical setting plastic or equivalent low
temperature bonding material, with a minor percentage, usually about 5 %, of .
low temperature bonding material, by distributing the loose sand and bonding
material mixture over heated metallic half patterns on a metal base plate which
forms the parting plane, over which it hardens into a rigid, self supporting
mold half shell which is then removed from the metallic half patterns and
base plate for use.
- As shown in Figure 4, the mold 30 is constructed of two such half
shells, upper and lower, which are then adhesively secured together along
horizontal mold parting plane 29 to provide a unitary, disposable, rigid, self
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supporting mold 30. Mold 30 has peripherally extending side surfaces 32
extending vertically between vertically spaced upper surface 31 and lower sur-
face 33 which are generally parallel to mold parting plane 29. Surfaces 31
and 33 are irregular and have a rough outer surface since they wereformed of
generally uniform thickness on the irregular contour of the heated pattern.
To provide for the support of mold 30 and for the application of re-
duced pressu~e to its upper surface 31, said upper surface is formed at its
outer edge, as by pressing it while still in plastic condition, to form a con-
tinuous peripheral horizontal flat sealing surface portion 38 suitable for
sealing against chamber 20, as hereinafter more fully explained.
A plurality of single part mold cavities are provided spaced between
the upper and lower surfaces, extending across mold parting plane 29, as shown
in Figures 3 and 4, of which two are shown in Figure 4. Multiple part cavities
may also be so provided, as explained in more detail hereinafter. In commercial
practice, the number of such mold cavities would generally fall between six and
twenty, seventeen being shown in Figure 3. Such single or multiple part mold
cavities are distributed within the horizontal area within the periphery of
mold 30, with a plurality thereof extending across the length and width of the
mold 30 between its upper and lower surface 31 and 33. Cavities 34 are hori-
zontally spaced from one another generally in a horizontal plane and extend
across parting plane 29. Each mold cavityJ such as is shown in connection with
cavities 34, has an individual vertical gate passage 35, generally perpendicular
to parting plane 29, extending from its lower side, with the lower open ends
of such vertical gate passages 35 being spaced from one another both widthwise
and lengthwise and terminating in a generally horizontal plane parallel to
parting plane 29 at the lower surface 33 of mold 30.
As explained above, at least a portion of each of gate passages 35
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should be relatively narrow in at least one dimension, at most not greater
than 0.75 inch, and preferably not more than 0.5 inch. Conveniently, these
narrow gate passages or portions thereof are vertical and of circular cross
section, although other configurations may be used.
Referring to Figures 1, 2 and 5, chamber 20 provides the sole sup-
port for holding mold 30 against chamber 20 and for applying reduced pressure
from vacuum pump 24 through a suitable valve 26 and hose 28 to its upper sur-
face 31. As seen in Figure 2, chamber upper wall 44 is connected to the
lower end of support 19 and is provided with an access port 58 to which vacuum
hose 28 is connected for applying a reduced pressure to the interior of cham-
ber 20 and to the upper surface 31 of mold 30 when desired.
In addition, chamber 20 has a bottom opening defined by its down-
wardly extending peripheral outer wall 40 which extends downwardly from the
outer periphery of its upper wall 44 to define the interior of chamber 20.
As best seen in Figures 2, 4 and S, outer wall 40 may be provided about its
lower end with a horizontal sealing surface 42 for sealing against the hori-
zontal upper sealing surface 38 of mold 30 around the periphery thereof and
generally coextensive with the horizontal area of mold 30 containing the
mold cavities, with a portion of the peripheral side surface 32 and bottom
surface 33 of mold 30 extending downwardly beyond chamber 20.
In operation, with chamber 20 in raised position as shown in Figure
1, mold 30 is manually or automatically positioned with its peripheral sealing
surface 38 against sealing surface 42 of chamber 20. Valve 26 is then operated
to provide the sole force to hold mold 30 into operating position against
chamber 20 and to apply throughout upper surface 31 of mold 30 a reduced pres-
sure, preferably only of about -1.0 to -3.0 psig (13.7 to 11.7 psia), through
chamber port 58 to the interior of chamber 20 and the upper surface 31 of mold
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30 within the periphery of sealing surface 38 and coextensive with the mold area
containing the mold cavities.
Power piston and cylinder 16 are then operated to move chamber 20
carrying mold 30 therebeneath downwardly toward container 22 to lower the lower
surface 33 of mold 30 with the lower open ends of all of the vertical gate
passages beneath the surface 60 of molten metal in container 22.
The reduced pressure applied to the upper surface 31 of mold 30
causes molten metal to rise into the gate passages and fill all the mold cavi-
tives simultaneously.
The power piston and cylinder 16 are operated shortly af*er sub-
mergence, as soon as the mold cavities have been filled and molten metal extend-
ing across at least a portion of each of the gate passages has solidified, to
raise chamber 20 and mold 30, whereupon a portion of molten metal remaining in
the gate passages adjacent their lower ends below the solidified portion drains
back into container 22, leaving unconnected metal parts, such as shown in
Figure 6, in mold 30.
After chamber 20 has been raised to its inoperative position, as shown
in Figure 1, valve 26 may be operated to disconnect the vacuum pump 24 and to
release mold 30 so that a new mold can be substituted.
The unconnected metal parts 62, Wit]l a short portion of gate passage
metal 64 connected to them, as shown in Figure 6, may then be separated from
the decomposed mold 30 in the usual manner.
In Figures 7 through 10 are shown molds having multi-part cavities
and multiple vertical gate passages.
Thus, in Figures 7 and 8 is shown a portion of a multi-cavity mold,
generally designated 65 and constructed as explained above, having spaced
between its upper surface 67 and its lower surface 69 and inwardly of its peri-
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pheral side surface 71, a plurality of multi-part mold cavities, of which one
is shown in Figures 7 and 8.
Each multi-part mold cavity includes two part cavities 73 and 75
having horizontal riser ingate passages 77 and 79, respectively, both connect-
ed to a central blind riser 78, which is in turn connected to a narrow vertical
gate passage 80. The shape, quantity and size of the riser ingate passages
77 and 79 and of blind riser 78 may be varied to suit the particular castin~
shape and size. The transverse dimension of vertical gate passage 80 is about
0.25 to 0.50 inches in diameter. More than one such vertical gate passage
may be needed in certain circumstances.
Molds of the type illustrated in Figures 7 and 8 are particularly
useful when large parts, having mold cavity dimensions in excess of 0.50
inches, for example, are to be molded since otherwise there may be insuffi-
cient time available to completely solidify the molten metal in the mold part
cavities before mold failure occurs, particularly with ferrous metals. Also,
with metals which shrink upon solidification, the blind riser acts as a
source of supply of molten metal during solidification of the metal in the
part cavities.
In operation, mold 65 is filled as described abave and the mold
~0 removed from contact with the molten metal in the container as soon as molten
metal has filled mold cavities 73 and 75 and blind riser 78 and has solidified
in vertical gate passage 80. However, the metal in blind riser 78 remains
molten for a sufficieint period of time after the removal of mold 65 from
contact with the molten metal in the container to continue to feed mold
cavities 73 and 75 through their riser ingate passages 77 and 79 to compen-
sate for shrinkage during solidification of the meta-l in the mold cavities
73 and 75. This arrangement allows the mold cycle time to be reduced so that
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premature mold failure is avoided. After solidification is complete, uncon-
nected groups of metal parts, including their connecting riscr ingates and
portions of the blind riser and the vertical gate, remain in the decomposed
mold 65.
In Figllre 9 is shown a multi-cavity mold 81 having, between its upper
surface 82 and lower surface 83, a plurality of mold cavities 84, of which
two are shown in Figure 9, clustered around a central vertical gate passage
85 'naving narrow horizontal gate passage portions 86 according to the inventionconnecting the mold cavities 84 to vertical gate passage 85. This arrangement
is satisfactory for casting parts having thicknesses of no more than about
0.5 inch, since solidification will immediately occur both in the mold cavities
84 and the narrow gate passage portions 86, with the molten metal draining
~rom vertical gate passage 85 upon removal of mold 81 from contact with the
underlying surface of molten metal to provide unconnected cast parts.
In Figure 10 is shown a multi-cavity mold 90 having, between its
upper surface 92 and its lower surface 94, a plurality of mold cavities 95,
each having two vertical gate passages 97 and 98, for more rapid filling of the
relatively large mold cavities 95 through narrow vertical gate passages in
accordance with our invention in order to fill the mold cavities and remove
the mold as soon as the metal in the vertical gate passages solidifies and
before mold failure occurs. This type of mold is particularly useful when
casting metals in which shrinkage compensation is not required, in molds having
large part cavities which cannot be filled through a single narrow vertical
gate passage before mold failure occurs.
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