effect of changing water powder ratio on ji

7/21/2019 Effect of Changing Water Powder Ratio on JI

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Effects Of Changing The Water-To-Powder Ratio On Jewelry Investments

Ralph Carter

Materials Engineer

Ransom &Randolph

Mamee! Ohio! "#$

Introdction

In any artistic endeavor or manfactring process! different techni%es are sometimes

tried in order to enhance certain reslts& #ometimes these techni%es havepredicta'le

reslts! while others are %ite e(perimental& $ good e(ample of this is sing higher

water-to-powder ratios for some castings and lower ratios for others& In fact! some

investment manfactrers provide for this fle(i'ility 'y recommending a range of water

to powder ratios& )ower water to powder ratios are advised for large! heavy pieces and

higher ratios are recommended for smaller and more detailed pieces&

It is commonly *nown that the investment strength! wor*ing time and flidity are

dependant on the water-to-powder ratio& +owever! the degree of dependence might not 'e

as well *nown& $lso! while ta*ing action to enhance one property! another lesser-*nown

property may 'e overloo*ed and changed ndesira'ly& This may lead to casting defects&,ecase of this! investments are designed to 'e ro'st enogh to provide some sta'ility!

despite changes in the water-to-powder ratio& ,t! in order to avoid large adverse

changes! the recommended water-to-powder ratio range is limited&

#omeewelers have sed this techni%e of changing the water-to-powder ratio %ite often

and from their e(perience can easily predict the reslts& Others may not have the l(ry

of e(perience! ths this techni%e may 'e more e(perimental& .or 'oth the e(perienced

and novice caster! it wold 'e helpfl to nderstand what investment properties are

affected 'y the water-to-powder ratio and more importantly to what degree&

This paper e(plores how the varios properties of a gypsm-'onded investment change

as a fnction of the water-to-powder ratio& This paper also discsses how these changes

can affect mold and casting attri'tes and reinforces the importance of sing the water

to-powder ratio range set 'y investment manfactrers&

,ac*grond

The ratio of the amont of water to the amont of investinent powder is commonly

e(pressed as the water-to-powder ratio /w0p1! having nits of milliliters of water per

233grams ofpowder& $ w0p of 43ml water per 233grams powder is sometimes written as40/100 or simply as 43 or even 0040. #ome investment manfactrers recommend that

their gypsm-'onded ewelry investments 'e sed at a w0p in the range of56 to 47 829&

Others may direct only a specific w0p li*e 5:! instead of a range&

;ypsm-'onded investments contain varios types of silica as well as plaster and many

controlling chemicals& The silicas act as a refractory material& Plaster! calcim slfate

hemihydrate /Ca#34&J 12H20), is sed to prodce a 'onding material& Chemicals are sed


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to control the properties of wor*ing time and flidity as well as to accentate varios

other investment properties&

plaster is an interesting component to investments in that it can 'e mi(ed with water to

create a flid and will react with this water to harden into a solid mass& $s a flid it can

'e pored in and arond patterns to fill in detailed designs& $s a solid it will hold its form

and withstand the rigors of metal casting& When mi(ed with the proper refractories andchemicals! plaster retains these characteristics and is ths well sited for ewelry

investments&

The primary prodct of the reaction 'etween plaster and water is gypsm! calcim slfate

dihydrate /Ca#34&7+731& ;ypsm acts as the 'inding agent! holding the refractory

material together in the desired shape& This reaction is represented in the following

'alanced chemical e%ation&

The do'le arrow indicates that this reaction is reversi'le& It is dring the investment

'rnot that this reaction is reversed& $t a'ot 79&

$nother reason is to assre the proper mold strength& It is thoght that a wea*

investment cases a rogh srface and possi'le 0fins on the castings 8>9& Other reasons

given are to


3/17

assre proper wor*ing time! permea'ility! setting e(pansion! thermal e(pansion! smooth

casting srface! no fins! no watermar*s and ease of removing investment from casting

pieces 8>!


4/17

the investment strength is only appro(imately 77G of its flly cred strength 849& The

investment wold not 'e strong enogh for frther processing for another 2-7hors&

#lmp /.lidity1

The investment flidity is measred sing the RFR procedre for slmp& The investment

is mi(ed in the same manner as it is for the por time test& Itis then pored into a cylinder

that is resting on a glass plate& The cylinder is 7H tall and has an inner diameter of 25@:H&"sing a spatla! the investment is Hstrc*-offD level with the top of the cylinder& $te(actly the 7-minte mar*! the cylinder is lifted off the glass& The investment will spreadot on the glass plate in the shape of a dis*& Bery thin investments will reslt in a largedis* and conversely! thic* investments reslt in a small dis*& The diameter of the dis* ismeasred in inches and reported as the slmp& ,ecase ewelry investments e(hi'itthi(otropic characteristics! this test is measred after a specified amont of mi(ing andat a specified time&

#et E(pansion

$s the investment sets! it will e(pand& This e(pansion is commonly referred to as theset e(pansion! The mechanism for this stems from interfering otgrowths from themany nclei of crystalliAation of gypsm 849& #et e(pansion can 'e controlled 'ychemical additions to the investment& This property is important to those who areconcerned with dimensions& The actal set e(pansion realiAed is dependant on thegeometries of the investmentDs rigid 'ondaries! as allded to 'y Ott 8>9& In otherwords! the investment will e(pand most in the direction of least resistance& This cancase distortions in the pattern! particlarly ifit is made of very soft wa( 849&

There are a few ways to measre set e(pansion 84! >9& .or this stdy the set e(pansionwas measred according to the RFR I#O 6337 %ality procedres& .igre 2 shows the set

e(pansion apparats sed& Each side ofthe apparats is 'ond! e(cept the side of thedilatometer /left in .igre 21 and the top&


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Figure 1

Mi(ed investment ispored into the apparats! the dial is set to Aero and one hor after

the investment sets the reslt is read& #ince the length of the sample cavity is

lOOmmlong and the dial has O&Olmmdivisions! the dial can 'e read aspercent linear

change orpercent linear set e(pansion&

Green Compressive Strength

;reen compressive strength refers to the compressive strength 7hors after the mi(edinvestment is considered set 'y the vicat test method& Cylinders! with a cross sectionalarea of IirrD! are made sing the formspictred in .igre 7& $t the appropriate time! theforce re%ired to destrctively compress each cylinder is measred sing the devicepictred in .igre 5& This device measres the force in ponds and given the crosssectional area of the cylinders! apressre in pondsper s%are inch /psi1 is o'tained&

The mean offive cylinders is reported&

Figure 2


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.igre 3=. />233C1 when cast& $ 'rass alloy 38=. /2362=C1& The same pattern was sed for each w0p&

Reslts and iscssion


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It shold not 'e assmed that all gypsm investments 'ehave in the e(act same manner as

the one sed here& Other gypsm investments shold! however! trend in at least a similar

manner&

Por Time

Chart 2 shows the dependence of por time on w0p& Inalmost a linear fashion! the por

time is increased as the w0p is increased& In the recommended w0p range of56 to 47 the

por time changed a'ot 2&7>mintes& This shold 'e enogh fle(i'ility for the investor!

withot ris*ing other properties! 'y going otside the recommended w0p range&

In applications where this range ofpor times is not enogh! then possi'ly other process

parameters shold 'e loo*ed into& These inclde mi(ing time! vacm times and the

nm'er of flas*s invested at a time& The por time can also 'e slightly increased 'y

decreasing the temperatre of the investment and water 8?9&

#et Time

The set time reslts are shown in Chart 7& This relationship is almost linear as well& The

slope is a little greater than the por timeDs& Therefore! as the w0p increases! the

difference 'etween the set time and por time increases slightly&


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$fter the flas*s are set aside for hardening! it is desira'le to have the set time occr as

%ic*ly as possi'le& This is for the prpose of decreasing the chance of water separating

from the investment and casing watermar*s& This data shows that ifthe w0p is increased!

the set time will 'e delayed& Ths! the pro'a'ility of water separating is increased& It is

important to note that some investments will not case watermar*s regardless ofthe set

time&

#lmp

Chart 5 shows the slmp verss w0p& $s e(pected! the investment gets thinner with

increased w0pDs& What is more interesting is the magnitde of change within the

recommended w0p range& In the recommended range of56 to 47! the slmp changes 'y

more than GH& This is certainly more than enogh fle(i'ility to satisfy the investorDs

preferences in flidity&

,elow the recommended w0p range! the investment 'ecomes thic* and is not easy towor* with& It is easy to see how air cold 'ecome trapped ne(t to the wa( patterns and

case casting defects& $'ove the recommended w0p range! the investment 'ecomes so


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thin it is difficlt to mi( withot it splashing ot of the mi(er& Thinner mi(es canalso lead to wea*er molds! as will 'e discssed later&

#et E(pansion

Chart 4 shows the dependence of the set e(pansion on w0p& The chart demonstrates thatset e(pansion is inverselyproportional to w0p& $t higher w0pDs! there is less sete(pansion! 'ecase the nclei of crystalliAation are more spread ot& This allows for thecrystals to progress throgh mch of its growth 'efore pshing against a neigh'oringcrysta) In contrast! at lower w0pD s there are more nclei of crystalliAation per nitvolme! reslting in more growth interaction& This in trn reslts in greater net sete(pansion 849& &

$s mentioned earlier! thisproperty is important for the dimensionalprecision of the

casting& When contemplating changing the w0p! it may 'e prdent to consider thatthe dimensions of the finished piece cold'e adversely affected&

;reen Compressive #trength

$s shown in Chart >! the green compressive strength measred at two hors is inverselyproportional to w0p& In the recommended w0p range of56 to 47! there is modest change in

the green strength! in fact a'ot 5?psi& This sta'ility can 'e helpfl ifa higher w0p is

desiredfor something li*e flidity!'t losing mold strength is a concern& Otside the

recommended range! the strength changes more dramatically& $ high w0p cold wea*enthe investment too mch& This increases the pro'a'ility of small crac*s forming in the

mold! which cold reslt in fins on the casting& $ rogh srface may also'e o'servedif the investment is wea*ened too mch 8>9&


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.ired Compressive #trength

Chart < shows the fired compressive strength as a fnction ofw0p& ,ecase fired

compressive strength is measred after firing and cooling! it is more of an indication

of the strength after casting& Therefore! it may impart some information on the ease ofremoving the casting from the mold& $s the w0p is increased! the fired compressive

strength decreases&

The slope ofthis relationship is fairly consistent! e(cept for the lower fringe w0p! wherethere is a large increase in fired strength& This will reslt in more energy'eing re%ired

to remove the casting& This is! of corse! not the only factor affecting the ease of casting

removal& There are formlation factors as well as raw material %ality factors! for theinvestment manfactrers to consider! which affect the casting removal process and @ or

a'ility to %ench&


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rometer Type $

$s with fired compressive strength! the rometer readings may indicate ease of casting

removal& Chart ? shows how the rometer relates to changing w0pDs& It has a fairly

linear dependence on the w0p! at least in the range stdied&This

does not show a largeincrease in strength at 54! as did the fired compressive strength& This shows the

limitations of the rometer reading& .or harder materials there are other types of

rometers& Perhaps a rometer type C wold have revealed the pswing that firedcompressive strength showed&

Investing I Casting

$t w0pDs a'ove the recommended range! the investment was so flid it splashed ot of the

+o'art mi(er! when mi(ed at normal speed& ring the vacming steps! the investment

rose mch higher than normal& $fter the flas* was set aside for hardening! water did notseparate ot or pool& The mold withstood the forces of centrifgal casting& The castings

loo*ed fine& The srface appeared to 'e as smooth as the other castings& ,ecase thewater did not separate! there were no watermar*s& There were no fins& +owever! therewere a few air '''les&

$t w0pDs 'elow the recommended range! the investment was very thic*! Itwas even

difficlt to por into the flas*& ring vacming! the investment did not rise p mch

at all& These castings loo*ed fine as well& They filled completely and there were no air'''les& The lac* of air '''les is contrary to what was e(pected& Conventional wisdom

sggested that at lower w0pDs there wold 'e more air '''les 8>9& +owever! air

entrapment is also inflenced 'y the %ality of vacm&

The castings did not have the defects that investment manfactrers and indstry e(perts

associate with e(ceedingly high or e(ceedingly low w0pD s 82! 4! >!


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'e increased& These castings simply show that in a careflly controlled environment it

might 'e possi'le to avoid defects&

Two possi'le e(planations for the sccess of the castings otside the recommended w0p

range are as follows& The pattern sed was not large and heavy enogh to really stress the

mold& #ee .igre >& Ifa larger! more fll tree was sed! then possi'ly the mold wold fail

at the higher w0pDs& $nother possi'le reason cold 'e this 'rand of premim investmentis fairly ro'st and as these reslts show! it is capa'le of withstanding some mishandling&

Figure 5

To confirm this! a non-premim 'rand of investment was sed to ma*e castings and

compare& This second 'rand of investment was sed in the same manner and with the

same patterns& $t w0pDs a'ove the recommended range severe watermar*s reslted& It

might 'e possi'le to see them on the casting on the right in .igre >& The molds held p

for the most part! e(cept for the watermar*s& Air '''les were present to a'ot the same

degree on each casting made with this investment&

These reslts show that there is some difference in sta'ility 'etween the two investments&

+owever! the fact that there were no fins on the castings indicates that the pattern sed

may not have stressed the molds&

Summary

There are several varia'les that have to 'e considered dring the manfactring of

ewelry& +aving a clear nderstanding of what affects these varia'les and to what degree

is important& This *nowledge can save time having to salvage poor castings or even

having to remanfactre them& To help investors o'tain consistent reslts! investment

manfactrers stress the importance of sing a w0p in a specified range& Within this

range! some properties are fairly sta'le& Other properties change! ths giving theewelersome fle(i'ility&

Ithas 'een shown that the por time! set time and slmp each increase with increasing

w0p& The slmp is more responsive than the others& This allows the investor to change the


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flidity withot changing the other properties as mch& The set e(pansion and strength

decreased with increasing w0p&

$ccording to investment manfactrers and indstry e(perts! the changes realiAed in

these properties are! at times! the case of certain defects& $ long por time and set time

increase the pro'a'ility of watermar*s in some investments! especially ifthey are not

mi(ed long enogh& The set e(pansion can case changes in dimensions& $ wea*investment can case fins to form and as well as a rogh srface to reslt on heavycastings& ,ecase these properties are dependent on w0p and they affect casting reslts!

the w0p is very important to control&

Appendix

Pour SetSlump Set Green Fired

Dur.

W:P Time(min

Time(min

(in !xp. (p"i (p"i Type

$

54 :&>3 2 7G 2&55G >4< 273 ??

5< 23&7> 2:&33 56@2< 2&7:G 4:< :4 ?2

5: 27&33 73&7> 4 3&66G 43: :2 77&?> 5 3/8 3&?:G 5

11/163&G 73 < 3&


15/17


16/17

Pengarh Meng'ah $ir-To-Powder Rasio On Investasi Perhiasan

Ralph Carter Materials Engineer Ransom &

Randolph Mamee! Ohio! "#$

Pengantar

alam setiap paya artisti* ata proses manfa*tr! te*ni* yang 'er'eda *adang-*adang menco'a nt* mening*at*an hasil t

Kadang-*adang te*ni* ini memili*i hasil yang 'isa dipredi*si! sementara yang lain c*p e*sperimental& #e'ah contoh yang 'ai* daradalah menggna*an le'ih tinggi rasio air-to-powder nt* 'e'erapa coran dan rasio yang le'ih rendah 'agi orang lain& ,ah*an! 'e'er

prodsen investasi menyedia*an fle*si'ilitas ini dengan mere*omendasi*an 'er'agai airrasio ''*& $ir yang le'ih rendah nt* rasio ''* disaran*an nt* 'esar! potongan 'erat dan rasio yang le'ih tinggi yang

dire*omendasi*an nt* yang le'ih *ecil dan le'ih rinci potongan&

+al ini mmnya di*enal 'ahwa *e*atan investasi! wa*t *era dan fliditas yang

tergantng pada rasio air-to-powder& Lamn! ting*at *etergantngan mng*in tida* di*enal& Jga! saat mengam'il tinda*an

mening*at*an sat properti! properti *rang ter*enal lain dapat dia'ai*an dan 'er'ah tida* diingin*an& +al ini dapat menye'a'*an capengecoran& Karena it! investasi yang dirancang nt* menadi c*p *at nt* mem'eri*an sta'ilitas! mes*ipn per'ahan dalam

to-powder& Lamn! nt* menghindari 'esar yang mergi*an

per'ahan! *isaran rasio air-to-powder yang dire*omendasi*an ter'atas&

,e'erapa perhiasan telah menggna*an te*ni* ini nt* meng'ah rasio air-to-powder c*p sering dan dari pengalaman me

dapat dengan mdah mempredi*si hasil& Orang lain mng*in tida* memili*i *emewahan

pengalaman! sehingga te*ni* ini mng*in le'ih e*sperimental& "nt* *eda *astor 'erpengalaman dan pemla! it a*an sangmem'ant nt* memahami apa investasi properti dipengarhi oleh rasio air-to-powder dan yang le'ih penting nt* apa gelar&

Ma*alah ini mem'ahas 'agaimana 'er'agai sifat dari per'ahan investasi gipsm i*atan se'agai fngsi dari rasio air-to-powd

Ma*alah ini ga mem'ahas 'agaimana per'ahan ini dapat mempengarhi atri't ceta*an dan casting dan memper*at pentingnya

menggna*an air *isaran rasio-''* yang ditetap*an oleh prodsen investasi&

latar 'ela*ang

Rasio mlah air dengan mlah ''* investinent mmnya dinyata*an se'agai rasio air-to-powder /w0 p1! memili*i nit miper

233grams ''*& $ w0 p air 43ml per 233 gram ''* *adang-*adang ditlis se'agai

& 43@233 ata hanya 43 ata 'ah*an 3343 ,e'erapa prodsen investasi menyaran*an investasi gipsm i*atan perhiasan mere*

digna*an pada aw0 p di *isaran of56 menadi 47 829& Orang lain mng*in langsng hanya tertent w0 p seperti 5:! '*annya *isaran&

Investasi ;ypsm-'eri*at mengandng 'er'agai enis sili*a serta plester dan 'ahan *imia pengendalian& #ili*a 'ertinda* se'a

'ahan tahan api& Plester! *alsim slfat hemihydrate /Ca#34&J 27+731! digna*an nt* menghasil*an 'ahan pengi*at& ,ahan *imia y

digna*annt* mengendali*an sifat dari wa*t *era dan fliditas serta nt* menonol*an 'er'agai properti investasi lainnya&

plester merpa*an *omponen menari* nt* investasi di 'ahwa hal it dapat dicampr dengan air nt* mem'at cairan dan

'erea*si dengan air ini mengeras menadi massa padat& #e'agai cairan it 'isaditang*an di dalam dan se*itar pola nt* mengisi dalam desain rinci& #e'agai padat it a*an ters 'ent*nya

dan menahan *erasnya pengecoran logam& Keti*a dicampr dengan refra*tori yang tepat dan 'ahan *imia! plester mempertah

*ara*teristi* ini dan dengan demi*ian sangat coco* nt* investasi perhiasan&

Prod* tama dari rea*si antara plester dan air gypsm! *alsim slfat dihidrat /Ca#34&7+731& ;ypsm 'ertinda* se'agai age

pengi*at! memegang 'ahan tahan api 'ersama-sama dalam 'ent* yang diingin*an& Rea*si ini diwa*ili dalam persamaan *imia seim'a'eri*t&

Panah ganda menn**an 'ahwa rea*si ini adalah reversi'el& Ini adalah selama 'rnot investasi yang rea*si ini di'ali*& Pad

7


17/17

Jmlah air tam'ahan yang di'th*an tergantng pada enis plester digna*an! distri'si *ran parti*el terse't yang plestesili*a! adhesi antara parti*el dan 'ahan *imia pengendalian ditam'ah*an *e investasi 849& Karena fa*tor-fa*tor ini! investasi

prodsen mengha'is*an paya 'esar nt* menamin 'ahan 'a* adalah se'agai *onsisten mng*in& Tersisa sedi*it in*onsis

dalam 'ahan 'a* di*ompensasi*an dengan penggnaan pengendalian 'ahan *imia! se'elm 'ahan investasi di*irim *elar nt* dig

Prodsen investasi! serta pa*ar indstri! telah mene*an*an meng*r 'ai* air dan ''* a*rat& $lasan mere*a 'anya*& #alah

alasannya adalah nt* memasti*an 'ahwa investasi campran tida* terlal te'al& Ia telah mengem*a*an 'ahwa i*a investasi camp

terlal te'al! rincian hals tida* dapat diisi dan gelem'ng dara mng*in tida* mdah melari*an diri 8>9&

$lasan lain adalah nt* menamin *e*atan ceta*an yang tepat& iper*ira*an 'ahwa investasi yang lemah menye'a'*an per*asar dan mng*in0 sirip pada coran 8>9& $lasan lain yang di'eri*an adalah nt*

menamin wa*t *era yang tepat! permea'ilitas! pengatran e*spansi! e*spansi termal! perm*aan pengecoran hals! tida* ad

tida* ada tanda air dan *emdahan menghaps investasi dari pengecoran potongan 8>!

effect of changing water powder ratio on ji

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