Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
5~;~
~BER REIN~ORCED Pl.ASTER MOLDS ~OR PAETAL CASTING
TECHNICAL }~lELD
This lnvention rel~tes to processes for ~esting non~errous metals
using plsster molds. More p~r'deularly, this invention relates to improved
compositions for producing plQster molds used in metal c~sUng and to
the resul~ng metal cas~:ing molds.
BACK~3ROVND OF THE INV~NTION
The use of pl~ster molds in casffng of nonferrous met~ls ~s long
been wel~known in the ~rt. Pl~ster m~y ~sily be oonformed to ~
desired p~ttern to be reproduced Iss a met~l product, the pl~ster being
hardened Into ~ mold, usually in ~everal æctions. Molten metal is
poured into the mold, ~llowed to ~ool and 8cjlidify, and U~e mold is
removed from the ~sting.
The recogr,ized ~dvant~ges of pl~ster c~sting of metals include
~he low ~ost of both the plsster mol~ing comp~sition ~nd the mold-
m~)ci~ pr~cess~ and ghe esse with which high definltion and accurate
reproduction of p~ttern surf~c~s may be ~chieved using a plaster
¢omposition of suit~ble ~onsistency. ~espite reasorls f~voring plaster
¢asting vf metals, Ulere ~re ~eversl recogn~zed dis2dv~ntages to ~uch
processes as ~onvent;ondlly practiced. Plas~er mold quality ~s dependent
UpOD a a~umber of vari~bles which must be controlled, including
s~onsistency of the plaster molding composition, mold pouring procedu~es,
~nd pl~ster curing techniques. 3mproper pr~edures m~y result in blow
~uts Or molten met~l from the c~sUng mold, distorted cast met~l product
~hapes due to shrinkage of the plaster mc>ld during ~ing, rolgh ~urfaces
4f ~as'dnKs due ~o excessive plaster mold poro~ity, and complete fflil~e
of U~e metal ~ssting ~ess due to cracking o~ the plHster mold during
, ~,, .~
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the met~l pouring ~tep.
The need continues in the ~rt for improvement in Ule qu31ity of
plaster o~mpositions for producing meta~ ~asting molds, so that this
convenient method of met~l c~stîng m~y be more e~sily employed., The
pl~ster oompositions of the present invention, which comprise ins,oluble
c~lcium sulfQte anhydrite whisker fibers resulting in met~l c~sting mol~
of improved quality, are a significant contribuUon to this need.
SUMMARY OF THE INVENTION
It is 8n object of the present inv~ntion to provide improved plaster
compositions for the production of mol~is for c~sting nonferrous met~ls.
It is another o~ject o this invention to provide comp~sitions for
the production of molc~; for casffng nonferrous met~ls h~ving substantially
increased strength ~nd dursbility.
It is ~n addition~l object of this invenUon to provide improved
methods of producing pl~ster mol~; Ior cssting nonferrous metals, yielding
plaster mold~; having sufficient dura~ility ~nd ~trength.
It is also an object of this invention to incorporate a reinforcing
fiber which will not present any health hazard in the use of the metal
CQsting.
It is yet ~nother object of this ~nvention to provide a metal
c~s~ng plaster ~omposition to reproduce origin~ patterns Ynth ~ ~gh
degree of ~ccur~cy ~nd definition so ~s to reduce or eliminate subsequent
clean up of the c~sting.
It i~ yet a furU)er obj2ct of the present ~nvention to provide
improved plaster mold members ~or cssting nonferrous metals having a
gPe~tly incre~sed capQbility for permitting ~cc~ate reproduction of
desired patterns in ~st met~l form.
It ls still Another objeet of this invention to provide plsster molds
for metal casting allowing ror substantS~l elimination of easting blow
outs, deformation of cast met~l shapes due to pl~ster mold shrinkage,
rough pl~ster mold sL~fAces resul'dng in rough s~face metal products~
~nd c~ ~cked molds resulting in partiA~ or complete f~ zre of the metal
c~sting process.
In accordance with this invention, ættable compositions for the
pr~duction of moldb for casting of nonferrous me~s ~re ~rovided,
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comprising pnrffcul~te c~lcium sulfate molding plaster intimately ~dmixed
with an effective arnount of insoluble c61cium 6ulfate ~nhydrite whisker
fibers.
The invention ~lso prwides a method for producing ~ mold rnember
for c~sting nonferrous metAls, comprising the steps of mia~ng w~ter and
~ settable composition of the invenUon to form a ~lurry, apply:ing the
composition to 0. mold p~ttern in an amount sufficient to cover the
~esired portion of the s~face of the pattern, ~nd c~ing the composiUon
with the pattern in place to produce ~ mold member.
The invenffon f~ther contemplates mold members for CASting
nonferrous met~ls essentially consisting of ~ cured mixture of calciurn
sulfate mol~ng pl~ster and insoluble c~lciwn ~ulfate ~nhydrite whisker
fibers.
Pl~ster molds used for metal c~sting ~re cslcined (burned) to
remove the chemicAlly combined wflter ~o thAt ~ere ~nll be no water
which could be released during the subsequent cssting of the metaL A
mineral fiber is commonly incorporated into the plaster ~o the mold
will have sufîicient strength to be handled ~fter the calcination.
Asbestos, fibrous Wc, and Wollastonite haYe all been used ~s the mineral
fi~r reinforcing in conventional met~l 5!85'drlg plasters. All of these
have been ~ssociated wigh respiratory problems. InsoluMe cfilciwn sulfate
~nhydrite hss no such limitation and9 in ~ddition, oîfers other
improvements whi~h will be~ome apparen~ in the eontext of Shis
application.
Addition~l embodiments of the invention will be made ~ear from
the detailed discussion below.
BRIEF DESCRlPrlON OF THE lilGURES
~ ig~e I is a photograph of the drag port;on of ~ match plate
mold m~de from standard metal casting plaster containing Wollastonite
fibers, togeth~r with a metal casting p~ oduced using tlhe drag and
corresponding cope.
~ ig~e 2 iS A photograph of the drag portion oî a match plate
mold produced using a metal casUng plaster formulation comprising
insoluble calcium sulfate ~nhydrite whisker fibers, tc~gether with a metal
c~sting produced using the drag and corresponding cope.
DETAILED DESCRIPTION OF THE INVENTION
Broadly, thls invention provides settable com-
positions for producing nonferrous metal casting molds,
comprising particulate calcium sulfate molding plaster
mixed with insoluble calcium sulfate anhydrite whisker
Eibers.
The primary ingredient in settable plaster
molding compositions is calcium sulfate molding plaster.
Normally, such plaster essentially consists of parti-
culate calcium sulfate hemihydrate. Calcium sulfatehemihydrate molding plaster is commonly referred to as
plaster of Paris, calcined gypsum, and stucco. Molding
plaster is, of course, well known in the art.
The insoluble calcium sulfate anhydrite whisker
fibers employed in the present invention have an average
length to diameter aspect ratio of at least about 6:1,
preferably about 10:1 to about 600.1. The length of
whisker fibers generally ranges from about 5 to about
300 micrometers. Long fibers are preferred over short
fibers. Diameters of whisker fibers generally range
from about .2 to about 5 micrometers.
Sui-table insoluble calcium sulfate anhydrite
whisker fibers are known and may be prepared by methods
known in the art (see, e.g., U.S. Patent No. 3,822,340,
U.S. Patent No. 3,961,105, U.S. Patent No. 4,029,512,
and U.S. Patent No. 4,152,408).
According to one embodiment, the insoluble calcium
sulfate anhydrite whisker fibers comprise insoluble
calcium sulfate anhydrite whisker fibers having individ-
ual lengths of about 5 to about 300 micrometers, .indi-
vidual diameters of about .2 to about 5 miCrOMeterS~
an average length-to-diamter aspect ratio of about
10:1 up to about 600:1, a surface area of at least about
15,000 cm2/g (preferably at least about 18,000 to about
35 20,000 cm2/g), a bulk density of about 7 pounds per
,.... .
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-4a-
cubic foot or less, and a normal consistency of about
500 to about 800 milliliters of water per 100 grams of
fibers.
.,
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Since the p~rticul~te calcium sulfate molding plaster is the primary
ingredient ~n the molding compositions, the insoluble ~lcium ~ulfate
enhydrite whisker fibers norm~ly constitute less ~an abcut 2D~6 oY the
dry powder mixture. Higher proportions OI whisker fibers may be used
if desired, ~ut ~onsiderations of econ~my dict~te against such pr~ctice.
Preferably, the whisker fibers consfftute ~rom about 3 to ~bout 15% of
U~e dry mixt~s, and most preferably from ~bout 5 to ~bout 10'~6.
Optional materials known to those sldlled i~ the art such as
diatomacous earth, portl~nd cement, lime, sAnd, ficcelerators ~nd retarders
may be ~dded t~ the composifforls as desired to serve their convention~l
p~poses. All materials included in the moldling ~omp~itions should be
~s devoid of soluble ~lts as is practicable. Soluble salts such as
potassium s~lts are carried to the surface of the plas$er mold during
c~ing, potentially c~using na}dng ~nd defacement of the mold s~face.
This phsnomenon, known AS efflorescence, should be ~ontrolled ~nd
preferably elimin~ted to facilitate production of acceptable ~st met~l
products. Normally, any option~l ingredients such ~s cli~tomaceous e~rth,
portl~nd cement, lime, ~nd, accelerators ~nd retardants Are present in
minor ~mounts, if at ~ ccording to con~entional plaster form~tion.
Once Ule desired proportions of particulate molding plaster,
insoluble c~lcium sulfite anhydrite whisker fibers and opffonal ingredients
h~ve been determined ~or Q p~rticul~r met~l castings pl~ster form~tion,
the ingredients are combined together and intimately admixed. Any ~y
mixing appar~tus or techniques known ts those sldlled in the art m~y
be employed. The plaster mol~ffng composiffons may be employed ~s
known in the arS to produce plsster molds for ~Qsting sny nonferrous
metal ~uch as Pluminum~ beryllium, copper, brass, bronze and their alloys.
The w~ter which is combined with the plaster molding eomposition
to initi~te 9he setting process should be as pure ~s pracUcal. Impurities
in the water employed m~y hinder U~e hydration reaction in the plaster,
resl~ting in inferior product molds. S~lts present in ~he w~ter may be
c~rried to the ~face of the mold during drying causing efnorescence
and fla)dng of the mold surfflce. Impuriffes may ~o affect the rate
of U)e s~tting reflction leading to ~desired re ;~s.
Mixing of the plaster molding composition wi~h water may generally
be carried out using ~ny ~pp~AtllS or technigue capable of IDliformly
mixing the entire b~tch, ~rnpleUng such mi2ang before ~etting of the
plaster renders it wlwor}~ble, ~d preventing excessive ~erstion of the
mixture. Suit~ble mixing Qpp~ratus ~nd techniques are known in the
6rt.
Slafficient w~ter is ~dded to ~e plester to obtAin the desired
worWng ~onsistency. Working ~onsistency is defined as pounds of water
per 100 pounds of plester added together to produce a ~tch of plRster
ailurry. Generally, about ~0 to sbout 150 pouu~ds per 100 pounds of metal
casting pl~ster is used. Prefer~bly, ~bout 100 to ~bout 120 pounds weter
is employed. Excessively high consistency ~i.e., too much weter) may
result in decreased green ~trength of plaster mol~i, decreQsed ~trength
o~ molds subsequent to burn out, decreased mold density, increesed
permiebUity OI molds, ~nd incressed time ~nd energy requirements for
removal of the water dlDring oven curingO Benefits attending use of
incre~sed ~onsistency, however, lnclude reducUon of setting e~pansion,
reduction of burn out shl ink, and ease OI pattern removal from She
cured mold~ ~enerally, the consistency that w~ll provide ~od workability,
good flow and wliform pattern coverage, ~s well ss adequ~te strength
of the oven dried molds is employed~ Work;ng consistencies ~ually
range ~rom about 100 l~o about 130. Lower consistencies may be necessary
for particular ~pplicstions, e.g., where hfgh density or green strength is
8 p~rticularly import~n- factor. The ~hoice of an appropriate consistency
is e~sily within the 8kill OI this art.
In conventional pr~cti ce, ~ pattern having the shape ~nd surface
charaeteristics of Ule desired product is prepared ~o shape the pl~ster
nnold. A parti~g compound is ~pplied 1o the pattern surfsce so that
the psttern may Ibe remo~ed from the cured pl~ster mold wit~out damage
to either. P~ttern preparation techniques and suitable psrtfng compounds
include light dl cut with mica or ~tesric flcid dissolved in kerosene.
Other p~rting compounds are ~lso known in the art.
The plaster mold is c~ed in an oven after it has set while still
~upported by the fl~slc. The curing pr~cess involves burning out all
chemic~lly combined ~nd physic~Jly b~und ~ter from the hydrated ~et
plaster mold.
Complete dryness ~f the ~ven ~ured plQstsr molds may be
determined by l~own meas~emerlt methods ~ the internal temperature
of the mol~; and by drying the molds until a const~nt weight ~s sbtained.
Measurement of intern~ mold temperatllPes may be ~rried out by
embedding a thermocouple in the center of the thickest portion of the
mold, and recording the internAl temper~tureO Once the internPl
t~mperature has exceeded the boiling point ~ water, 212D F., drying hs
been oompleted. These test methods and others like them are well
lu~own to those sldlled in the ~rt.
One p~rticular Imown method for metal cesting involves the use
of match plate plaster molds. Illat~h plste mol~; comprise two plsster
mold h~lves which fit together itorming {I mold c~vity, the mold hslves
being ~ep~rated readily for preparatiorl for the ~asting of the metsl.
~ig~es 1 ~nd 2 illustrate match plate mol~;. The complimentsry mstch
plates for each of the~;e mol~; h~ve been removed to ~how the finished
metal castings. The raised cleats in ~ ~ect~ngular p~ttern on the flst
s~face of the m~tch pl~e interlocks with ~ corresponding pattern on
the match pl~te not shown for proper ali~ment between the two mQtch
plates.
After the plaster h~s ~et, the bottom of the surf~ce facing the
p~ttern is removed ~nd the fl~3k is h~rned upside down. The fl~sk is
extended upward by adding ~nother flssk section to hold ~dditional
pl~ster, the p~ttern is ret~ined in the first portion of the mold, and
~dditionQI plaster is poured on top of the exposed ~ide of the p~ttern
2nd allowed to set. The resulffng sssembly is then c~ed in ~n oven,
and Ule two plsster ~ections, termed the d~ag ~nd the cope in order
of pouring, are g;eparated from e~ch other to constitute Q tw~piece
plaster mold
The met~l casting mol~; produced using the pl~ster composiffons
containing whisker fibers have increased strength ~s coJnp~red with metal
casting molds produced frDm conventional plRster form~atiols. The
whisker fibers themselves possess high tensile strength which r~ults in
incre~sed dimensional stability and shock resistance. The pl~ster molds
produced in accordance with the psesent invention exhibit greaUy reduced
~ncidence of thermRI ~ck c~cking which res~ n a ~ignificant
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improvement in the quality ~nd clean up OI the ~st metal products.
In ~ddition, the molds for metal ~asting produced according to this
invention ~re perme~ble to permit the exit of gases emitted during the
casting o~ the met~L
The metal c~sting process itself is well-known to those skilled in
the ~rt ~nd need not be discussed in det~il here. The metal c~sting
mold Is usually heated immediately beI3re ~dertaking the metal ~!asting
in order to minimize therm~l shock caused by the difference in
tempernture between the molten metal and the plsster ~nd also to
remove residual water.
The inYention is further ~llustr~ted by the following Examples.
These Examples ~re merely illustrative and ~re not intended to limit
~e scope of the invention, which is defined in ~nd limited only by the
~ppended elAims.
EXAMPLE
A typical formul~ for ~ p~qor ~rt metal casting plaster is ~s
follows:
I~redi~nt Pounds Percent
Molding Pluster 1650 81.95
Wollastonite F 1 lOU 4.97
Celite ~C 250 12.42
Type I Por~nd Cement 12 . o60
~igh Cslcium Lime 1.1 .OS
Gypsum Accelerator .,5 .03
Pi~e 1 shows a metal C8Stirlg prodllced from ~ m~tch plate mold made
from Ule above formulation.
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EXAMPLE 2
A formula of the present invention includlng gypsum
fihers is as follows:
In~redient _unds Percent
Molding Plaster 1758 87.3
Insoluble Calcium
Anhydrite Fibers 242 12
Type I Portland Cement 12 .60
High Calcium Lime 1.0 .05
Gypsum Accelerator .5 .03
The insoluble calcium anhydrite whisker fibers
employed above had a normal consistency of about 220 milli-
liters water per 100 grams of fibers. They were prepared
by dehydration of dihydrate gypsum (Terra Alba) to hemi-
hydrate fibers which were then heated to provide insolublefibers. Figure 2 shows a metal casting produced from a
match plate mold made from the above formulation.
Comparing the casting of Figure 1 with the casting
of Figure 2, it can be readily seen that the gypsum fiber
formulations of the present invention produced a superior
metal casting.
EXAMPLE 3
A formula of the present invention employing the
novel gypsum fibers is as follows:
Ingredient Pounds Percent
Molding Plaster 1875 93.1
Insoluble Calcium
Anhydrite Fibers 125 6.2
5~
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Type I Portland 12 .60
Cement
High Calcium Lime Lû .05
Gypsum A~eelerator ,5 .0'3
The whisker fibers employed ebove had ~ normal eonsistancy of about
700 milliliters water per lûO grams of fibers.
While preferred embodiments of this invention have been discussed
sbove ~nd illustrated by the Examples, it is to be unde~stood that
modifieAtions in the invention may be made as known to those skilled
in the are.
