CA1200053A - Fiber reinforced plaster molds for metal casting - Google Patents

Abstract

ABSTRACT
An improved composition and process for preparing plaster molds used in casting nonferrous metals is provided wherein plaster molds are prepared from particulate calcium sulfate molding plaster admixed with insoluble calcium sulfate anhydrite whisker fibers.

Description

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~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, ~13~3 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 ,.... .

J~
-4a-cubic foot or less, and a normal consistency of about 500 to about 800 milliliters of water per 100 grams of fibers.

., .

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 ~i~q~ 3 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.

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.

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~

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.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of producing a mold member for casting nonferrous metals, comprising the steps of:
(a) mixing water and a composition comprising water-settable calcium sulfate molding plaster intimately admixed with at least about 3% by weight of the dry weight of the composition of insoluble calcium sulfate anhydrite whisker fibers to form a slurry, the insoluble calcium sulfate anhydrite whisker fibers having an average length-to-diameter aspect ratio of at least about 6:1, (b) introducing the composition into a molding flask containing a mold pattern in an amount sufficient to partially cover the surface of the pattern in the flask, and (c) curing the composition with the pattern in place in the flask to produce a mold member.

2. A method of producing a mold member for casting nonferrous metals, comprising the steps of:
(a) mixing water and a composition comprising par-ticulate calcium sulfate molding plaster intimately admixed with insoluble calcium sulfate whisker fibers in an amount of from about 3 to about 20%, based on the dry weight of the mixture, to form a slurry having a consistency of from about 90 to about 150 pounds of water per 100 pounds of the composition, the insoluble calcium sulfate anhydrite whisker fibers having an average length-to-diameter aspect ratio of at least about 6:1, (b) introducing the composition into a molding flask containing a mold pattern in an amount sufficient to partially cover the surface of the pattern in the flask, and (c) curing the composition with the pattern in place in the flask to produce a mold member.

3. The method of claim 1 or 2 in which the insoluble calcium sulfate anhydrite whisker fibers have a surface area of at least about 18,000 cm2/g.

4. The method of claim 1 or 2 in which the pattern is removed from the mold member.

5. A mold member useful for the casting of non-ferrous metals comprising a cured mixture of calcium sulfate molding plaster and at least about 3% by weight insoluble calcium sulfate anhydrite whisker fibers based on the dry weight of the mixture, the insoluble calcium sulfate anhydrite whisker fibers having an average length-to-diameter aspect ratio of at least about 6:1.

6. The mold member of claim 5 in which the insoluble calcium sulfate anhydrite whisker fibers have an average length-to-diameter aspect ratio of at least about 10:1.

7. The mold member of claim 5 in which the insoluble calcium sulfate anhydrite whisker fibers have a surface area of at least about 15'000 cm2/g.

8. The mold member of claim 5 in which the insoluble calcium sulfate anhydrite whisker fibers have a surface area of at least about 18,000 cm2/g.

9. The mold member of claim 5 additionally comprising at least one ingredient selected from the group consisting of diatomaceous earth, portland cement, lime, sand, accelerators and retarders.

10. The method of claim 1 or 2 in which the insoluble calcium sulfate anhydrite whisker fibers have an average length-to-diameter aspect ratio of at least about 10:1.

11. The method of claim 1 or 2 in which the insoluble calcium sulfate anhydrite whisker fibers have a surface area of at least about 15,000 cm2/g.