Chapter 1 Aqueous Colloidal Injection Molding of Ceramics (CIMC) Based on Gelation 
Abstract In this chapter, aqueous colloidal injection molding (CIMC), which is based on gelation of monomer polymerization, like a reactive injection molding of double slurryof polyester,is discussed systematically.The solidi.cation mechanism is based on gelation of monomer polymerization. It was found that pressure can induce the polymerization of the monomer. The gelation time can be effectively controlled by both temperature and pressure. The testing equipment of gel point was developed and used to test the gel reaction of the monomer in the suspension under different pressures. A machine of for ceramic colloidal injection molding was developed. It was observed that copper accelerated the polymerization of the monomer. Therefore, copper parts are forbidden for use in a CIMC machine. 
Keywords Gelcasting ¡¤ Injection molding ¡¤ Colloidal injection molding ¡¤ Pressure-induced forming ¡¤ Fast mixing with double suspensions¡¤ Ceramic slurry 
Nowadays, high-performance ceramics are becoming increasingly more important in several .elds, such as space.ight, energy, mechanics, and bio-techniques. How-ever, the high cost and low reliability have prevented high-performance ceramics from being utilized ona large scale until now.To solve this problem, more attention is being paid to the forming process of ceramics. In the industrialization of high-performance ceramics, forming has become a bottleneck not to overstep. Colloidal formingisan importantforming technique.Itincludesslipcasting,tapecasting,direct coagulation casting (DCC), injection molding, and gelcasting. Among these tech-niques, gelcasting and injection molding are considered the two possible solutions to address the industrialization of high-performance ceramics. Though both have manyadvantages, there are still several problems to be solved in the industrialization process. 
Gelcastingisanewceramic forming technique thatis generatingworldwide atten-tion. The process is based on the casting of slurry containing powder, water, and water-soluble organic monomers. After casting, the mixture is then polymerized to form gelled parts. Drying,burning out, and sintering complete the manufacturing pro-cess.The processisgenericandcanbeusedforawiderangeof ceramicand metallic powders.Itisasuitabletechnique forthefabrication of near net shape prototypes or small series using inexpensive molds. In contrast with slip casting, the gelled parts are more homogeneous and have a much higher green strength. Gelcast parts containonlyalow percentoforganic components, thereby making binder removal much less critical compared to injection molding. The advantages of gelcasting can be summarized as follows: 
(1) 	
Capabilityof producing complex parts likeinjectionmolding. 

(2) 	
Ease of implementation, owing to its similarity with other well-established processes like slip casting. 

(3) 	
Low capital equipment cost. 

(4) 	
Possibilityto uselow-costmoldmaterials. 

(5) 	
Capabilityof implementationfor mass production. 

(6) 	
Highgreen strength. 

(7) 	
Excellent green machinability that allows machining much .ner details than in CIPed parts. 

(8) 	
Highly homogeneous material properties. 

(9) 	
Low organic content that translatesinto easybinder removal. 

(10) 	
Generic method applicablefor both ceramic and metalpowders. 


Meanwhile, there are still some disadvantages that make it dif.cult to realize theindustrializationbygelcasting. Most important of all, low automationprevents gelcasting from being used in the industrialization of high-performance ceramics (Omateteetal. 1991;Gilissenet al. 2000). 
Injectionmolding,however,hasbeenusedinthe ceramicindustryforseveralyears foritshigh automation. Ceramicinjectionmoldingisawell-established shaping tech-nique, whichinvolvesthe mixingof ceramicpowder withalarge concentrationofa melt polymer (upto 50¨C60vol%). The carrier polymer providesvery high viscosity, and sovery high pressures(10 ¡À150 MPa) and temperatures (120 ¡À200 ¡ãC) are neededforinjection.In additiontothehigh costowingtotheuseoforganics,the majorproblem arises fromthe debinderingstepthat can easily lead to defects and failureofthesinteredbody.Inthepreparationofproductswith complexshapeand bigcross section,theproblemis moreobvious.Thesedisadvantagesprevent injec-tionmolding from being usedin theindustrializationofhigh-performance ceramics (Novaketal. 2000;Wen-Chengetal.2000;Liu1999;Kruget al. 2000). 
Theaimofthisstudyistodevelopanewforming techniqueto meetthe needs of industrialization. The new technique is called Colloidal Injection Molding of Ceramics (CIMC). 
1.1 WhatisColloidal Injection Molding? 3 
1.1 What is Colloidal Injection Molding? 
1.1.1 Colloidal InjectionMoldingof Ceramics (CIMC) 
Traditional injection molding can be used to form complex ceramic parts with high dimensional precision, and can be used in the automation of large-scale produc-tionofceramic productsinindustries. Theproblem with this techniqueis that due to thehighorganic content, debindering becomesvery dif.cult. Colloidal forming processes, such as gelcasting and DCC can avoid this disadvantage, and improve the uniformityof green bodies and thereliabilityof the ceramicsproducts. These approaches, however, have shortcomings. For example, they need manual opera-tion and thus aredif.culttobeusedin ceramic industries, where automationisa necessity.An optimal approachwouldbea combinationofbothinjection molding and colloidal formingprocesses, bringing together their advantages.The problemis that injectionmoldingisaplastic formingprocess, and the colloidal formingpro-cessstarts fromslurry.Sincetheslurryisofno plasticityin colloidal forming,itis normally impossibleto applyinjectionmolding. 
Wehavedevelopedanew technique, namelyCIMC(Fig.1.1)(Wen-Chengetal. 2000), combining bothinjection molding and the colloidal formingprocess. This technique canbe usedto produce complex ceramic partsofhigh reliability,byusing a colloidal injection molding machine invented by the authors. The new approach (withour machine)provideshigh automationofinjection molding,high homogeneity of products,andloworganic contentinthe colloidal formingprocess.Itis possible to produce high-performance ceramic parts of complex shapes with high reliability bythisnewlydeveloped technique,andthusitcanbeappliedin ceramicsindustries. 


1.1.2 The Flowchart ofCIMC 
Thefast, uniform mixing and controllable colloidal formingprocessofceramicswas todividethe ceramic suspensionintotwo componentsAandB.The monomerwas addedintoA,andtheinitiatorwasaddedintoB.Inthiscase,therewasno reactionin eitherAorBbecauseofsegregationofthemonomerandtheinitiator.Thesuspensions maintained good .uidity until theywere mixedquickly and uniformly. OnceAandB were mixed together, the encounter of the monomer and initiator resulted in gel reactioninthe suspension. Theinitiator quantity and the appliedpressure were the factors dominatingthe solidi.cationreaction. Figure1.2 showsthe schematic graph of this process. The catalystinsuspensionAcould not generatea reactionwith the monomer.Its main functionwas to acceleratethe reaction when the monomer and initiator encountered. 

1.1 WhatisColloidal Injection Molding? 	5 

1.1.3 	The MachineofColloidal InjectionMolding of Ceramics 
Special equipmentwas manufactured forthe process. 
Figure1.3 showsthe .ow chartfor theworkingof the machine. Theprepared suspensions were transferred into the A and B containers. In the containers, the suspensions werevacuumized through thevacuum pumps to completelyremove the gassolvedinthem.Toavoid the depositionofthe suspensions,a stirring paddlewas used in each of the containers to continuously intermix the slurries. Furthermore, withtheuseof bellows pumps,the slurries couldcirculateintwo routes:inner circle and outer circle.Thiscircular transportationin additiontothe stirring actiononthe slurries completelyprevented the occurrence of deposition. Thesuspensions¡¯state wasmonitoredthroughthetestingvalves, whichwere positionedonthe outer circle line. 
Before injectionmolding, themetering pumps quanti.ed twoslurriesofthe same volume.The twoslurries were simultaneously and quicklyinjected into thestatic admixerwithahighpressureof5¨C8MPa.The corestatusofthe staticadmixercan be seen in Fig. 1.3. It is similar to a bridge connecting the machine and the mold. Itsmainfunctionwasto realizefast uniform mixingofAandB suspensions.The inner structureofthedevicecanbeseeninFig.1.4.Fromthischart,wecanobserve that themixingprocesswas realizedbyaseriesofmixing units with different shapes in the hollowtube. These units made the penetratingslurriesrapidlylevo-rotate and dextral-rotate alternately. Thesefrequent changesof theslurries¡¯ .owing directions could helpin obtaininga good mixingeffect, whichis calledfast, uniform mixing. 


Thesemixed suspensionswereinjectedintothemoldunderapressureof3¨C8MPa. The antisticking agentis usually pre-coated ontothewallofthestainless steel mold. Theinitiatorvolume and injectionpressure arethe dominativefactorsto controlthe solidi.cationprocess. Theseparationof the monomer and initiator guaranteed no reactioninthe separated suspensions, which hadgained enough operating timefor theprocess.By adjustingthe initiatorvolumeand injectionpressureintheprocess, thesolidi.cationspeedofthesuspensioninthemoldcouldbeeffectively controlled, whichexplainsthemeaningof ¡®controllable colloidal forming¡¯.Inthe process,the crucial impact of temperature on gel reaction was completely avoided. 
Itis well known that thepressure distributionis homogenousin .owing suspen-sions,andthefast, uniform mixing resultsinphysical uniformityinthesuspensions. The combination ofthese two points enabled us to ensure simultaneous solidi.ca-tionindifferentpartsofthesuspensions.The synchronous solidi.cation consumedly decreased theinner stressin thegreen bodies. 
1.2 Pressure-Induced Forming 
1.2.1 Effect of Hydrostatic Pressure on Solidi.cation 
Apressure inductionunitwasthenaddedtoapplyanexternal pressureafterinjection. The gelation curvesofAl2O3slurry underdifferent pressureswithamoldtemperature of 25 and 36 ¡ãC are shown in Fig. 1.5. Under increased pressures, the onset time of gelation becomes shorter and the gelationspeed increases signi.cantly.Athigher temperature,thein.uenceofthepressure becomes more signi.cantandthe gelation .nishes almost immediately. Pressure-induced solidi.cation has distinct advantages overtemperature-induced solidi.cation. 
1.2 Pressure-InducedForming 7 

1.2.2 Homogeneityof theGreen Bodies 
An alumina(Al2O3)green bodyof turbineshape withadiameterof 105 mmwas preparedby this process. Thespatialdistributionof densityis compared with that obtainedby the conventional processby using thesameinjection molding system showninFig.1.6.Thevalueineachpartrepresentsthedensitying/cm3.The relative standarddeviationof densityfor colloidal injectionmoldingis0.2% comparedto 0.7% for conventional injection molding. The results show that the colloidal injec-tion molding process signi.cantly improves the homogeneity in the density of the green body. Nevertheless, the solidi.cation in this process is associated with the surface temperature of the mold. The gradient of the temperature will inevitably cause nonuniform solidi.cationand inner stressinthegreenbody.A newstrategy hasbeendevelopedto minimizetheinnerFig.1.5.Theeffectofexternal pressure onthe gelationofAl2O3 slurrywithasolid loadingof50vol%;Tsisthestarting temperaturestressingreen bodies preparedin this process. 
1.2.3 Controllingthe InnerStressin theGreen Body 
Theinner stressin green bodiesis oftenresponsibleforthe initiationofmicrocracks duringthe subsequent drying and debinderingprocesses. During colloidal injection moldingorother gelcastingprocesses, monomers inthe ceramic suspension poly-merizeandthenformgel networkstoholdthe ceramic particles.Thesolidi.cation speed increases with increasingtemperatureforagiven composition. Nonuniform solidi.cation occurs due to thetemperaturegradient in the ceramic suspension and results in the development of inner stress in the green body. Theoretical analysis 
Fig. 1.6 Comparison of density distribution in Al2O3 green bodiesof turbineshape prepared by a conventional injection molding and b colloidalinjection molding in the same system 

has indicatedthatthemagnitudeoftheinner stress increaseswiththestiffnessofthe green body.Based on this .nding,afractionofthe monomersisreplacedbyamoder-ator,hydroxylethylacrylate,inthesuspensionduring gelcastingorcolloidal injection moldingto controlthe stiffnessofthegel network.Test barswith dimensionsof6 ¡Á 6¡Á 42 mm3 were prepared with 50 vol% solid loading and various amounts ofthe moderatorfrom0to100wt%inthetotal monomers.Thesamples weredehydrated at about 70 ¡ãC in an electrical furnace. The .exural strength and elastic modulus of thegreen body areevaluated usingthree-point bending. Theresults areshownin Fig.1.7againsttheamountofthe moderator.Boththe.exural strengthand elastic modulus of green body decrease simultaneously with the amount of the moderator. This reveals that the strength of the polymer network is reduced when the harder polymer chainisrelaxedbytheincorporationofthe shorter chain moleculesofthe moderator.Fivealumina disks,a,b,c,d,ande, were produced underthesame condi-tions.The diameterof thesamplesis about50 mm and thethicknessis about4mm. In Fig. 1.7, theidenti.ersofthe disk samplesare marked above the corresponding amountofthe moderator.Figure1.8showsthesurface patternsofthe aluminagreen bodies after drying and debindering to remove all the water and organic binders. Radial cracks werefoundonsamplesc,d,ande,whilesamplesaandbwithhigher amount of moderator were free of visible cracks. It is likely that the inner stress wasinitiatedatthe formingstage and magni.ed duringthe drying process when the 
1.2 Pressure-InducedForming 9 


preform becomes harder.The intrinsicstrengthof the green body afterdrying and debinderingis mainly determinedby the natureofthe ceramicpowder and thesolid loading. Observationsofthesurface patterns con.rmthat boththe inner stressand theelastic modulusof thegreen body decrease with theamountof the moderator. Choosinga proper amountofthemoderator caneffectivelyreducethe occurrenceof cracks duringthe subsequent processes. 

1.3 Storage Stability of Ceramic Slurries 
1.3.1 The ImportanceofStorage Stabilityof Slurry 
Inthis article,idletimeof polymerization wasusedtojudgethestore stabilityof theslurry.Since acrylamide polymerizationisanexothermicreaction, gelation can be determined by the change of temperature. The metering system(Liguo et al. 2002) is as shown in Fig. 1.9. At .rst, ceramic slurries with high solid loading were prepared by long-term. Then, the slurry was poured into the mold, and the moldwasplacedin constant-temperaturewateruntilits temperaturewasequaltothe scheduled temperature.Theinitiatorand catalyst were addedtotheslurrylater.The periodfromthentothebeginningof polymerizationisde.nedasidletime.Whenthe monomerwaspolymerized, thetemperatureofthe slurry rose becauseofexothermal polymerization. During this process, the temperature of slurry was detected by a temperature detector and recorded by a computer. Based on this temperature, the polymerizationprocess couldbe analyzed. 
1.3.2 Chemical Stability 
During theresearch on gelcasting,itwasfound that afterlong-term storage,even though theslurry had not been polymerized, thesolidi.cation properties ofthe ceramic slurry had been modi.ed greatly. Sometimes,the idle timewas prolonged, and sometimes it was shortened. This affected the repeatability and the stability of theforming process. Hence,itwasan obstaclefor industrializationofgelcasting. 
Itisknownthat acrylamideintheslurryispronetohydrolysisandtherebyacrylic acidis generated. If theslurry were stored under strong light or thereweresome