Note: Descriptions are shown in the official language in which they were submitted.
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Expendable d~e -cast~ng sand core
The ~nvent~on relates to the art of die casting
such metals as aluminum, zinc, magnesium, copper and their
alloys and to a solution to a long standing problem therein;
i.e., the lack of a commercially feasible die casting
technique to produce castings having undercut regions.
Traditional pressure die casting requires molds or dies which
are a~le to withstand the high temperatures and pressures to
which they are subjected. Thus, ferrous materials are commonly
used for d~e casting molds. Because these die materials are
not easily collapsible, complex undercuts and relie~s are not
possible because the castiny cannot be removed f~sm the mold.
Other common casting techniques, such as and ancl semi-permanent
mold casting, ha~e utilized expendable or di~posable cores to
produce castings having undercuts. Thi~ has h~en pos~ible
because the pressure requirementæ for such techni~ues are
usually under the order of 30 psia as compared with at least
about several thou~and psia required during high pres.sure die
casting.
A typical core is composed of foundry sand mixed
with a binder or resin. Through the use o~ heat, a catalyst,
or chemical react.~on, the sand grains are ~onded together into
i a discrete shape, and can then he us~d in the casting process.
The heat evolved during solidification and cooling of the
actual cast parts evaporates moisture contained in the core
or results in
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chemical breakdown of the binder. Relatively eas~ removel
of the core from the casting is thereby facilitated.
Prior attempts to utilize sand cores for die casting
- have included the use of glass and soluble sal-t cores. Such
-~ 5 techniques are discussed in detail in British Patent Number
l;l79~241n These systems are considered to be unsatisfactory
from the standpoints of process control, economics, handling~
and the corrosive characteristics of the sal~s.
Binding systems of the boronated aluminum phosphate type
have been proposed for use in making expendable sand cores for
die casting. Such systems are illustrated in United States
Patent Number 4,127,157 and in my United States Patent Number
4,298,051, issued November 3, 1981. This application, however,
relates to the use of a different binding system than that of
this invention. In general, the system of the invention is
considered to represent an improvement of the boronated alum-
inum phosphate system because, although both systems have
excellent shakeout properties, the system of the invention
produces cores having higher strength, especially immediately
following coremaking. Moreover, coremaking prvcedures are
simplified with use of the binding system o~ the invention
because this system has higher blowabilLty than boronAted
aluminum phosphate binding systems and thus denser cores can
be obtained.
A major problem in the development of a satisfactory
expendable core for use in high pressure die casting has been
the inability of a single core/binder system to simultaneously
meet four primary core characteristics. They are good shake-
out, good washout resistance, freedom from surface penetration,
and core strength~ Good shakeout is necessary to facilitate
core removal from the casting. Washout resistance is the
ability of the core to withstand erosion from the high metal
velocities that occur during die casting. not only does
washout adversely affect the tolerances on the finished part
but the sand physically
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removed from -the core becomes embedded in the casting.
Surface penetration i5 caused by the comb~na-tion o~ high
heat and pressure tha-t breaks down the core surEace and permits
the metal to pene~ra-te between the sand grains thus causing
- 5 a sand/metal mixture interface at the surface of the casting.
This condition is extremely detrimental to subsequent machining
and tool li~e. Moreover, should the sand become sepa~ated
from the surface after component installation, damage to
related parts, such as automobile parts, could result. High
strength Eor cores is desirable in that the cores are more
resistant to breakage during handling and are also more
resistant to breakage during the rigors of the casting. This
` invention is considered to solve such problems with use of a
core system that is capable of attaininy the requisite balance
of the four properties.
The Figure is a sectional view of a mold portion of
a die casting machine and is useful for illustrating regions
inthe casting in which the three problems discussed above
occur. Plunger 11 is used to inject molten metal 12 into -the
die casting mold formed by steel members 13 and 14 and sand
core 15. Note that the final die casting shape includes an
undercut region. Surface penetration of metal 12 into sand
core 15 occurs along the dark shaded region identified as 16
in -the Figure. Washout usually occurs at areas such as denoted
by 17. Shakeou-t refers to the ability to remove core 15 upon
solidificatlon Oe the die casting, its removal Erom the die
castiny dle, and subsecIuent cooling to amb:ienk temperature.
It h~s been discovered tha-t expendable sand cores
can be used to produce d.ie castings having undercut regions
provided that a binding agent comprising an acid curable resin
in an amount from about 0.25% to 5%, based upon weight of
foundry sand, is used. ~owever, it is preferred to employ
between about 1% to 2%. The choice of a specific binder
level will be dependent upon core shape,
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core thickness, complexity, the manner in which the core is
secured within the casting die, and casting conditions. The
binder, mixed with foundry sand and an appropriate amount of
oxidizing agent forms the core. The core can also be coated
5 to provide improved resistance to penetration and washout.
The acid curable resin binding agent mentioned above is
more fully described in United States Patent Number 3,879,339.
ThiS agent has proven to be beneficial to shakeout properties
in die casting applications when present in amounts from about
0.25% to 5% by weight of foundry sand. The lower limit is
re~uired to provide sufficient core strength to withstand
handling while the upper limit should not be exceeded due to
blowing problems during coremaking caused by a lack of uniform
density related to variations in sand flow and unacceptable
decreases in shake-out efficiency. When using typical silica
foundry sands of a fineness of AFS No. 65, it is preferred
- to employ from about 1.0% to 2.0%. Should heavier foundry
sands such as zircon be used, less binder is required, i.e.,
preferably on the order of from about 0.5% to 1.0%. The
respective preferred upper and lower limits are chosen for
the same reasons as for the more general range. Of course,
the use of other commonly used foundry sands having diferent
densities than the above mentioned sands is wikhin khe scope
of the invention. Such other sands would preferably require
the use of binder amounts consistent with density.
; Acid curable resin binding systems that are suitable
for use in the invention include urea/formaldehyde, phenol/
: formaldehydel furane, and copolymer resinsO It is preferred
to use furane because this system is more resistant to
distortion immediately following coremaking than the other
systems. It is also possible to use copolymers of these
resins with epoxidised compounds or with unsaturated compounds.
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The silianization of the resins of the invention through
the addition of from about 1% to 10% by weight based on resin
of a silane such as gamma-amino-propyl-triethoxysilane is
optional. Such additions function to strengthen the core.
An oxidiæing agent such as methyl ethyl ketone peroxide,
should be present in the system in quantities ranging from
about 20~ to 70%, based upon weight of the resin. Amounts
from 30% to 50~ are preferred to ensure complete curing. The
oxidizing agent functions to react with gaseous sulfur dioxide
to form sulfuric acid, which, in turn, cures the resin. Other
suitable oxidizing agents include but are not limited to
peroxide, hydroperoxide, hydroxyhydroperoxide, chlorate,
perchlorate, chlorite, hydrochloride, perbenzoate, metal
` oxide, permanganate, monoperiphthalic acid, and hydrogen
peroxide. Such oxidizing agents are normally added as a
liquid to the resin to facilitate mixing although the use of
solid or gaseous agents is not outside the scope of this
invention.
To typically prepare the expendable core of the invention,
sand and an acid curable resin are mixed, then silane may be
optionally added to the mixture, and then the oxidizing agent
is added to the mixture. The mixture may then be air blown or
hand packed into a core box having the desired shape oE the
core. A curing ga5 such as sulfur dioxide is passed through
the core box to cure the resin. The core is removed from the
box as a solid mass and utiliæed in the die casting process.
Other techniques and curing gases are disclosed in United
States Patent Number 3,879,339 and 3,639,654.
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Followiny ~ts preparation, the core may be coated
- -to ~urther impro~e perfo~mance with respect to washout and
surface penetration. Core coatings generally comprise a
suspending agent, a refractory material, a binding agent,
and a solvent.
Core coatings for die castings axe more critical
than core coatings suitable fox other casking methods. The
core coating should possess the capability of being able to
substantially seal the pores on the surface of the core.
Because die casting places molten metal under pressure, any
porosity at the surface of the cores will lead to penetration
of the molten metal and thus trap sand on the surface of the
as-cast part. An application of -the proper core coating to
the core will provide a die like finish with no resultant
penetration of the molten metal into the sand core.
Suspending agents are usuall~ clay or clay
derivatives. These materials should be present in amounts
sufficient to per~orm the function of maintaining the refractory
- material in suspension. Based upon total solids weight, such
agents may be present in amounts ranging from about 4% to 30%.
Typical particulate refractory materials that are
useful in the cOAting ~ormulation include but are not limited
~- to graphite/ silica, aluminum oxide, magnesium oxide, zircon,and mica. These materials are present in amou~ts generally
- 25 ranging from about 60% to 95~ based UpOII total solids weight. The mass o~ pa~ticles is bound together through
use o;E binding agents sllch as thermoplastic resins. Binding
agents use~ul in the practice o~ the invention generally
comprise from about 1% to lO~ by total solids weight of the
coating composition. The binding and suspending agents
` should be compatible with the particular solvent which may
~ be an organic liquid. The solvent should
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be included in an amount which is efective to obtain the nec-
essary viscosity to control coating thickness and uniformity.
A typical suitable core coating comprises, based upon total
solids weight, from 4~ to 30~ of an amine treated bentonite
suspending agent, from 1% to 10% of a thermoplastic resin
binding agent, and from 60% to 95% of a refractory such as
silica or the like. The above constituents, in powder form,
are mixed with a sufficient quantity of organic liquid vehicle
to attain the necessary viscosity to produce, upon drying, the
desired coating thickness and serve to seal the pores on the
surface of the core~
Other suitable core coatings include those shown in United
States Patent Number 4,001,468. The patent discloses coating
compositions comprising an organic liquid solvent having a
: 15 kauri-butanol value of at least 36, such as liquid 1,1,1~
trichloroethane; a suspending agent; a powdered refractory
material such as graphite, coke, mica, silica, aluminum oxide,
magnesium oxide, talc or zircon flour; and an organic polymer
such as vinyl toluene/butadiene copolymer, styrene/butadiene
copolymer, vinyl toluene/acrylate copolymer, styrene/acetylene
copolymers, or acrylate homopolymers. The ratio by weight
of organic polymer to organic liquid solvent is maintalned
between about 1:50 and about 1:200 and the ratio by weight of
powdered refractory to organic liquid solvent is maintained
25 between about 1:2.5 and 1:3.5.
An additional core coating that has proven to be satis-
factory for use in combination with the binding system of the
invention is that described in United States Patent Number
. 4,096,293. The coating material has a viscosity sufficient
to substantially seal surface porosity of the core and is
suitable to obtain
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; a coating thickness and uni~or~ity that leads to ~ood
resistance to washout and penetration duriny die castin~.
The coating cons~sts of from about 5% to gQ~ o~ an organic
liquid solvent, from about 0.1% to 2% of a suspending agent,
from about 5~ to 80% of calcium aluminate particles having
an average particle size of 20 to 25 microns and no particles
being larger t~an about 70 microns, and a hard resin which is
the reaction product of fumaric acid, gum rosin, and pen-
taerythr`;tol, the resin is within the ratio by weight between
about 0.5 and 5 parts per 100 parts of composition, all
percentages expressed by weight of composition. A wetting
agent may opt;onally be added in amounts ranging from about
0.01% and 2%.
Following manufacture of the core in a core box
and its removal, the core is sufficiently strong enough to
be handled. A core coating is then applied by brushing,
dipping, spraying or an equivalent method. Once the coating
is dry, the core is placed into a die located on a casting
machine. The steel portion of the die form~ the surface
shape of the metal part that is not formed by the core. The
core is placed in this die and is located by pins,
impressions or other methods commonly known to those skilled
in the art. The die is then closed, thus trappiny the core
in a fixed location and molten metal is then injected into
the die.
Duriny the process o solidification in the die,
heat is emitted from the casting. A portion of the heat flows
into the core and increases its temperature. This flow breaks
down the binder and drives off moisture and gaseous materials.
3~ Once the molten metal has solidified in the dle, the machine
is opened and the resultant casting and expendable core are
removed. Upon cooling to ambient temperature, the core may
be shaken out mechanically.
The following example illustrates an embodiment
of the invention:
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An aluminum alloy was die cast i.nto the shape
shown ~n th.e Fi,gure w~th use of a core contai~ing s;lica
' foundry sand (~FS Fineness No. 65). Furane in an amount
of 1.5%, ~ased upvn we;ght of the foundry sand was used
as the binding agent and 40%, based upon weight of the
resin, of the oxid~z~ng agent methyl ethyl ketone pero~ide,
and 3%, based upon weigh.t of the resin, of silane. A core
coating as set forth previously was applied to the core
prior to die casting. Core strength was good. Good
shakeout properties were noted following mechanical separa-
tion of the core from the casting upon cooling to ambient
temperature. The casting exhibited good resistance to
surface penetration and washout resistance was good.
