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Int J Adv Manuf Technol (2006) 30: 10491075DOI 10.1007/s00170-005-0152-4ORIGINAL ARTICLERegina M. Gouker.Satyandra K. Gupta.Hugh A. Bruck.Tobias HolzschuhManufacturing of multi-material compliantmechanisms using multi-material moldingReceived: 12 January 2005 / Accepted: 22 April 2005 / Published online: 22 February 2006# Springer-Verlag London Limited 2006Abstract Multi-material compliant mechanisms enablemany new design possibilities. Significant progress hasbeen made in the area of design and analysis of multi-material compliant mechanisms. What is now needed is amethodtomass-producesuchmechanismseconomically.Afeasibleandpracticalwayofproducingsuchmechanismsisthrough multi-material molding. Devices based on compli-ant mechanisms usually consist of compliant joints. Com-pliant joints in turn are created by carefully engineeringinterfaces between a compliant and a rigid material. Thispaper presents an overview of multi-material moldingtechnologyanddescribes feasiblemolddesignsfor creatingdifferent types of compliant joints found in multi-materialcompliant mechanisms. It also describes guidelines essen-tial tosuccessfully utilizingthemulti-material moldingpro-cess for creating compliant mechanisms. Finally, practicalapplications for the use of multi-material molding to createcompliant mechanisms are demonstrated.Keywords Compliant mechanisms.Compliant joints.Multi-material molding.Mold design1 IntroductionSynthetic and natural structures have several distinctdifferences. Perhaps the greatest difference between thesetwo types of structures is the use of compliance. Compli-ance, in relation to structures and mechanisms, is the abilityor process of elastically deforming in response to changesin force without disruption of structure or function. It hasbeen well recognized that man-made objects are primarilymade up of relatively rigid materials, while natural objectsare made with soft compliant materials, using rigidmaterials such as bones and teeth only in select places19. It has been suggested that this distinct differenceoccurs from the assembly and production methods for eachof the products. The fundamental requirement for naturalstructures is the absence of assembly. The entire systemmust be grown from a source or cell as a single entity 1.An example of the type of unique performance that canbe achieved through the use of compliance in naturalstructures is the flapping of bird wings to achieve flight(Fig. 1). It has been shown that birds are designed with anintegrated flexibility to promote draft and lift, caused by theair, to rotate the wings throughout the cyclic process 19.This enables the bird to achieve much higher cyclicflapping rates at lower energy consumption.A common misconception is that an object has to berigid to be strong 19. However, nature shows us thatobjects can be strong and compliant. Throughout humanexistence, nature has proven to be a fertile source forinspiration. Until recent years, many of these ideas couldnever be fully understood and reconstructed. With animproved understanding of the structure-property-functionrelationship in biological materials and systems and theinvention of new materials and manufacturing techniques,bio-inspired designs are rapidly increasing in popularity6.In a step towards bio-inspired design, the behavior ofcompliant mechanisms has been studied in order to developdesign rules for realizing actual mechanisms. A compliantmechanism is a flexible structure that elastically deforms toproduce desired force or displacement. Similar to rigidmechanisms, compliant mechanisms will still transfer ortransform motion, force, or energy, but they will also storestrain energy in the flexible members as well 9. Com-pliant mechanisms mainly consist of rigid objects orfeatures with added compliant or soft materials at strategiclocations. There are several different types of compliantmechanisms currently being researched and developed.Some of the mechanisms developed include bistablemechanisms, orthoplanar flat spring, centrifugal clutches,overrunning pawl clutches, near-constant-force compres-sion mechanisms, near-constant-force electrical connec-R. M. Gouker.S. K. Gupta (*).H. A. Bruck.T. HolzschuhMechanical Engineering Department,University of Maryland,College Park, MD 20742, USAe-mail: skguptators, bicycle brakes, bicycle derailleur, compliant grippers,and microelectromechanical systems 9.Compliant designs have many advantages over tradi-tional designs that employ articulated joints. Included inthese advantages is the reduction of wear between jointmembers, reduction in backlash, and high potential energystored in deflected members 10. In addition, absence ofassembly in production methods, reduction of weight, andpart count found in compliant mechanisms all lead to thereduction of cost to manufacture the product. Severalstudies have indicated that assembly costs make up 4050% of the manufacturing costs to produce a product 1.Part reduction reduces the assembly labor, purchasing, in-specting, warehousing, capitol requirements, and piece partcosts of a product 17. Therefore any decrease in partcount and manufacturing cost, no matter how minimal, willhave a drastic effect on the total cost of the product.The use of multiple different materials in compliantmechanisms opens up the design space considerably. Untilrecently, a method did not exist to mass-produce multi-material compliant mechanisms. In the past, layered pro-cesses, such as stereolithography, selective laser sintering,shape deposition modeling, and 3D printing were utilizedfor making multi-material compliant mechanisms 2, 3,11. While these processes can create intricate and complexgeometries as well as material distributions, they are notsuitable for mass production in most cases. A more suitabletechnique that has emerged for the manufacturing of multi-material compliant mechanisms is multi-material molding.Multi-material molding creates a part with one or morematerials via a multi-stage mold.Multi-materialcompliantmechanismsareabletoachievethe same performance as homogeneous mechanisms withcomplex geometries because they employ compliant joints.The compliant joints are created by carefully engineeringinterfaces between two materials: one that is compliant andonethatisrigid.Thispaperdescribesseveraldifferenttypesofinterfaces thatcanexist incompliant joints.Furthermore,this paper presents results from experimental characteriza-tion of these interfaces to show that these interfaces areviable for creating compliant joints with the adequate mo-tion ranges. A detailed description for designing severaldifferent compliant joints that can provide from 1 to 3degrees of freedom motions in mechanisms is outlined. Foreach joint design, we discuss possible mold design optionsand present a feasible mold design to realize the joint.Finally, we present two examples of devices consisting ofcompliant joints.The material presented in this paper will assist readers indesigning and experimentally analyzing molded multi-material compliant mechanisms. Furthermore, it will helpthem in designing molds for manufacturing compliantmechanisms. Weexpectthatthisinturnwillresultinamorewidespread use of multi-material compliant mechanisms.2 Multi-material molding2.1 Process overviewMulti-material molding (MMM) involves molding multi-material assemblies from various polymers including ther-mosets, thermoplastics, and polyurethanes 8, 1315. Thegeneral molding process entails introducing the liquidmaterial into a cavity shaped like the desired object, thenallowing the liquid to solidify through cooling or chemicalreactions. The hardened object can then be removed fromthe mold, where it may require finishing operations beforeit is complete. The multi-material molding process is verysimilar to the standard molding process, except the multi-material process makes use of more than one cavity con-figuration. Examples of some products molded with MMMtechniques are shown in Fig. 2.There are many different types of design options forcavity change. Out of these, we have determined that thefollowing three are most relevant for making compliantmechanisms: cavity transfer, removable core, and slidingcore methods. The cavity transfer method involves trans-ferring the first stage part into another mold cavity (Fig. 3).This method simplifies mold design and many complexinterfaces can easily be created. It also requires the mostmanipulation between stages. In the removable coremethod, all that is needed to transfer from one stage tothe next is to remove the core (Fig. 4). This type ofapproach requires little to no manipulation between moldstages but is also very limited in the type of interfaces thatcan be created. In the sliding core method the total cavity ofthe part is extracted from the initial mold volume andsliding cores are strategically placed within the mold torestrict or enable flows into certain sections (Fig. 5). Thismethod requires minimal manipulation between moldstages, since the part remains in the same mold cavitythroughout the process. It also enables more complex in-terface options, but is more difficult to design because ofthe need to accommodate the motion of the sliding cores.Multi-material (MM) products possess several benefi-cial qualities over traditionally molded products including:(1) multicolor appearance, (2) skin/core configurations, (3)in-mold assembly, (4) selective compliance, and (5) soft-touch portions. Additionally, it could cost less to produce aFig. 1 Bird flapping its wings in flight1050MM product compared to a similar assembly of multiplecomponents.There are two broad manufacturing technologies com-monly used to make MM objects: injection molding androom temperature molding. Injection molding is commonlyused for large-scale production processes, while roomtemperature molding is primarily used for small productionruns. Injection molding involves injecting molten plasticinto a mold where it rapidly solidifies and is then ejected asthe desired object 12. Injection molding is ideal for manyapplications because it is amenable to a variety of poly-meric materials, has a wide range of mold capabilitiesincluding complex structuresand is extremely control-lable. Additionally, the parts produced by injection mold-ing have a very low cycle time. Fundamentally, there arethree different types of MM injection molding processes.Multi-component molding is the simplest and most com-mon form of MM injection molding. It involves thesimultaneous (or sometimes sequential) injection of twodifferent materials through either the same or different gatelocations in a single mold. Multi-shot molding (MSM) isthe most complex and versatile MM injection moldingprocess. It involves injecting the different materials into themold in a specified sequence, where the mold cavitygeometry may partially or completely change between se-quences. Overmolding and insert molding simply involvemolding a resin around a preformed part, either metal (as ininsert molding) or a previously made injection moldedplastic part (as in overmolding). Each of the three classes ofMM injection molding processes is unique and can bedistinguished using the taxonomy shown in Fig. 6.Room temperature molding utilizes polymers that areliquids at room temperature. Instead of injecting a hotliquid into the mold and then allowing the plastic to coolinto a solid, room temperature molding utilizes reactivecompounds that polymerize (i.e., cure) at ambient condi-tions. This involves mixing two compounds, a resin and ahardener, to activate the polymer, then pouring the mixtureinto the mold. The material then polymerizes in the shapeof the cavity. The hardening time will be considerablylonger than that of injection molded plastics. Because MMobjects have geometric features consisting of differingmaterials, the molds must be filled in stages. This meansthat the molds are assembled in an initial configuration andthe first material stage is poured. After a specified hard-Fig. 3 Cavity transfer moldingmethod. a Mold for first stage.b Remove part from first stageand insert into second stage.c Pour second material intoremaining cavity. d ResultingpartFig. 2 Various examples of compliant injection molded parts. (a)e-slimcase by ejector. (b) crest toothbrush1051Fig. 5 Sliding core moldingmethod. a Core position for firststage. b Molded part first stageand core position for secondstage. c Molded part for secondstage and core position forthird stage. d Resulting partFig. 4 Removable core mold-ing method. (a) Mold with coreinserted for first stage. (b)Resulting part from first stage.(c) Remove core and pour sec-ond stage. (d) Resulting part1052ening time, the molds are partially disassembled and somemold pieces are added, removed, or replaced with differentones. The second material stage can then be poured. Thisprocess is continued until all of the materials have beenpoured.There are several considerations when designing MMmolds for room temperature molding. As mentioned pre-viously, they must be designed to incorporate severalmolding stages. Within each of these molding stages themolds must be easy and simple to manufacture, containingno undercuts in them. The absence of undercuts and theinclusion of draft angles ensures ease in demolding.Finally, since there will be no pressure exerted on themolds to hold them together the parting surfaces must bedesigned with pins or grooves so that proper alignmentmay be obtained. The absence of pressure when pouringthe part also has a tendency to permit the existence of airbubbles and air pockets in the mold. The strategic locationof air holes can effectively eliminate a majority of theseproblems. Thus, the biggest difference between MMinjection molding and room temperature molding is theproperties of the polymers that will be employed in each ofthe processes. This means that the specific MMM processthat is chosen for a particular MM compliant mechanismwill depend on the set of material properties that aredesired.2.2 Characteristics governing the MMM processThis section discusses some of the characteristics that mustbe considered for MMM.Interfacial adhesion: The phenomenon of adhesionbetween two materials is complex, and consequently, itis difficult to predict the nature and exact quality of theresulting interface. In particular, the strength of inter-molecular bonding and mechanical interlocking willcontrol the mechanical properties attributed to ad-hesion, such as interfacial strength and fracturetoughness. Adhesion is a characteristic that can be en-gineered in the MMM process using three types ofinterfaces:Chemicalinterfaces:Chemicalinterfacesareformed as a result of the intermolecular bonding(i.e., cross-polymerization) alongthe mating surfacebetween two compatible materials. The extent andstrength of the chemical interface will be governedFig. 7 Example of a chemical interface. a CAD model. b MoldedpartFig. 8 Geometrically locked interface. a CAD model. b MoldedpartMulti-Material Molding ProcessesMulti-Component MoldingMulti-Shot MoldingInsert/Over MoldingBi-InjectionMoldingSkin/CoreMoldingIntervalMoldingRotaryPlatenIndexPlateCoreToggleCo-InjectionMoldingSandwichMoldingFig. 6 Multi-material moldingprocess tree1053Fig. 9 Two types of physicaltesting specimens (source:Multishot)Fig. 10 Test specimen geometryFig. 11 Tensile test with multi-ple interface types1054by the curing time and the similarity of thesolubilityparametersfortwopolymers18.Figure 7 shows an example of a typical moldedchemical interface.Mechanical interfaces: Mechanical interfaces areformed by geometrically interlocking two incom-patible materials (e.g., polystyrene and polypropyl-ene) together. Additionally, interlocking interfacescan be used to selectively control the relativemovement (i.e., degrees of freedom) between thematerials. Figure 8 shows a schematic of a bar witha mechanical interface that does not permit anyrelative movement between the two materials.Combination interfaces: Using compatible materi-als at an interface with geometrically interlockingfeatures allows components to be designed withcombined interfaces that exhibit both chemicalbonding and interlocking. Previous research showsthat combination interfaces performed much betterin tensile loading over flat bonded interfaces, whichin turn performed much better than non-bondedinterlocking interfaces 4. This suggests wheninterface strength is an important considerationcombined interfaces should be used.Rheology: Fortunately, for multi-shot molding andovermolding processes, different materials flow fromseparate nozzles or into separate sections of the mold,meaning the actual flows never combine. In thesecases, the only concern becomes whether the materialswill meet along the desired interfaces and form thedesired bonds or mechanical locking. This usuallyresults in a series of equivalent standard single materialinjection molding flow problems that can be solvedwith any of the widely available mold flow simulatorson the market. However, certain MMM process, suchas co-injection and sandwich molding, utilize flowsthat are actually embedded inside of each other. Inthese cases, flows of multiple materials must becarefully understood in order to produce suitableMM objects. However, these processes are not suitablefor making multi-material compliant mechanisms.Hence, from the rheological point of view we can treatMMM process as a sequence of single material flowproblems and use established techniques from injectionmolding area to perform mold flow simulations.Fig. 12 Schematic diagram of the tensile test variablesFig. 13 Graph of 90A tensiletest results10553 Design and characterization of moldedmulti-material interfaces3.1 Processing variablesThere are many variables that have an affect, either director indirect, on the MMM process. These parameters can bebroken down into four main categories: temperature,pressure, time, and distance 5. A key parameter inMMM is the cooling time between molding stages, whichaffects the degree of cross-polymerization at an interface.Typically, for cross-polymerization to occur the secondmold stage must be performed before the material hascompletely hardened from the first mold stage. However,the material from the first mold stage must be hardenedenough so that it can be taken out from the mold. Therefore,the desire to maintain geometrically complex interfacialfeatures directly contradicts the desire to maintain the bestcross-polymerization. Due to the sensitivity of the materialatthiscriticaltime,itisessentialtolimit eitherthemanualorrobotic manipulations required between mold stages whenengineering the interface. It is also necessary to design amolding schedule that balances the degree of cross-polymerization with the desire to maintain the geometricallycomplex interfacial features.3.2 Interfacial strength characterizationTo determine the effects of processing variables on theengineered interface, it is necessary to characterize thestrength and deformation response of the interface. The in-terfacial strength will determine the critical loads for MMcompliant mechanisms. To characterize the interfacialstrength, it is necessary to conduct experiments to determinethe critical stresses at which the interface begins to debond.For example, a recent study explored the use of complexgeometry to enhance interfacial tension strength by con-Fig. 15 Graph of multi-material90A and 72DC tensile testresultsFig. 14 Graph of 72DC tensiletest results1056Fig. 16 Flat-end connectionflexure testFig. 17 Graph of multi-material90A and 72DC flexure testresultsFig. 18 Calculation parametersof the flexure tests1057ducting tensile tests on two-dimensional interfaces. Thisstudy found that “geometric complexity can increase theinterfacial strength 2025%.” Among the geometricallycomplexinterfacesthatweretested,itwasfoundthatcirculargeometries exhibited approximately 5% greater strengththan rectangular geometries 4.There are many choices for mechanical testing of in-terfacial strength. However, actually conducting these testson complete assemblies under a variety of loading con-ditions would be cost prohibitive. In order to circumventthis difficulty,interfacial strength can be simply determinedon chemically bonded flat interfaces under four differentloading conditions that are locally dominant at differentlocations in the assembly. The four loading conditions are:tension, shear, peeling, and torsion.Shear and tension strength tests measure how well thechemical interfaces resist uniform shear and normalstresses, respectively. The shear strength tests are con-ducted on simple lap shear specimens, shown in Fig. 9. Forthe tension strength tests, specimens, as illustrated inFig. 10, contain flat interfaces with circular cross sections.In both tests, specimens are pulled apart under tensileloads, and the strength determined from the critical loadand interfacial area. While some may argue that the twophysical test specimens discussed above are adequate tocompletely determine the two materials compatibility, itcan also be necessary to conduct the peel and torsionstrength tests in order to determine the chemical interfacestrength under gradients of normal and shear stress. Thepeeling test specimen is similar to the shear test specimen(Fig. 9b), but is loaded with a tensile force perpendicular tothe interface. The torsion strength tests utilize the samespecimen testing configuration as the tensile tests, exceptspecimens are twisted rather than pulled apart. In bothcases, the geometry of the interface is used to compute thestress gradient in order to determine conditions for failure.Thus, both stress and stress gradients are used to determineinterfacial failure.A series of tension tests were conducted on the flatinterface specimens seen in Fig. 7 and the mechanicallylocking interfaces seen in Fig. 8. The materials used in thetest specimen are IE-70DC, a hard material, and IE-90A, asoft material (Appendix 1). Typical load-displacementcurves from these tests are shown in Fig. 11. From theseresults, it can be seen that both the combination interfaceand the flat chemical interface have comparable mechan-ical responses, with the flat interface exhibiting 15%greater strength. However, when there is no chemicalbonding alongthe interface only the mechanical interface iscapable of bearing load with a strength that is 50% of thelevel that would be experienced with chemical bonding.Therefore, when designing with chemically compatiblematerials, either type of interface is suitable. However, onlythe mechanical interface can be used when the materials areincompatible. Both interfacial types will be employed inthe case studies below to demonstrate their viability usingthe measured strengths to set the design limits for thecompliant mechanisms.3.3 Characterizing deformation response of interfacesIn addition to interfacial strength, it is also desirable tocharacterize the deformation response of the interface tosubcritical loads. Because of geometric complexity, it canbe difficult to predict interfacial response using analyticalformulas, or even finite element analysis (FEA) software.In such situations, experimental characterization is thesimplest and most accurate means of understanding theeffects of geometric complexity on the mechanical re-sponse. Once these measurements are obtained, it may bepossible to replace the geometric complexity with surro-gate models containing flat interfaces of finite thicknesswith appropriate material properties. This will make it moreFig. 20 Graph comparing the multi-material 90A and 72DC flexuretest and the theoretical flexure test responseFig. 19 Graph comparing the multi-material 90A and 72DC tensiletest and the theoretical tensile test response1058Fig. 23 1 DOF combinationinterfaceFig. 24 2 DOF motion attainedwith a flat interfaceFig. 25 2 DOF motion attainedwith spherical interfaceFig. 22 1 DOF motion attainedwith a cylindrical interfaceFig. 21 1 DOF motion attainedwith flat interface1059reasonable to analyze assemblies with MM compliantmechanisms using commercial FEA software packages.An example of a three-dimensional geometrically com-plex interface consisting of a cylindrical end with fivehemispheres attached to the end is shown in Fig. 8. Thisgeometry was chosen based on a two-dimensional heterog-eneous circular interface, fabricated with the multi-materialmolding process, that had been previously characterized 4.Theanalysiscontainsthefollowingtwotests:(1)tensiontestand (2) flexure test (e.g., cantilever bending test). Thematerials used in the test specimen and in both of the casestudies are IE-70DC, hard material, and IE-90A, softmaterial (Appendix 1).The tension test is used to determine the Youngsmodulus of each of the materials. First the specimen isstretched uniaxially, then the load and extension aremeasured. The axial stress is determined by dividing theload value by the specimens original cross-sectional area.The appropriate slope is then calculated from the stress-strain curve in the elastic region. The value of Youngsmodulus is used to calculate compliance of structuralmaterials that follow Hookes law when subjected touniaxial load (that is, the strain is proportional to theapplied stress). A schematic diagram of the tensile testvariables is shown in Fig. 12. The results of the IE-90Atension tests are shown in Fig. 13. A best fit linearcorrelation is used to determine the slope of the tensiletests; this result indicates the Youngs modulus for the softmaterial as 17.83 N/mm2. The result of the IE-72DC tensiletests are shown in Fig. 14. A best fit linear correlation isused to determine the slope of the tensile tests; this resultindicates the Youngs modulus for the hard material as2493.4 N/mm2. The force versus the displacement plot forthe multi-material specimen, IE-90A and IE-72DC isshown in Fig. 15. The result from the tensile test for multi-material MM-90A72DC using the best fit straight lineshows a slope of F/ = 70.203 N/mm.This result has to be compared with the theoreticalresults for the flat interface under tension loading. Inputtingthe value of the Youngs modulus for the hard and softmaterial and the dimensions of the specimens, the com-parison and calculation is developed using Hookes lawFig. 26 2 DOF combinationinterfaceFig. 27 Molding method tocreate 1 DOF spherical inter-face. a Mold for first stage.b Remove part from first stageand insert into second stage.c Pour second material intoremaining cavity. d Resultingpart1060and the definitions of stress and strain. F/ is defined asfollows:F? A,L1E1L2E2?(1)WhereA=cross-sectionalarea,E1=modulusofelasticityforIE-72DC, and E2= modulus of elasticity for IE-90A; seeFig. 12 for definition of dimensional parameters L1and L2.By substituting geometric parameters A= 38.159 mm2, L1=10.795 mm, and L2= 10.795 mm, a value ofF? 62:579 N/mm is obtained.The flexure test determines the modulus of elasticity inbending using the relationship between the load and thesubsequent deflection. The specimen is loaded as a simplecantilever beam, and the deflection is measured at setincrements of the load. The modulus of elasticity in bend-ing is determined from the bending stress-strain curve inthe elastic region using a procedure similar to the tensiletest stress-strain curve. A schematic for the flexure test isshown in Fig. 16. The force versus the displacement plotfor the multi-material specimen, IE-90A and IE-72DC isshown in Fig. 17. Using the slope of the best fit straight linefor the multi-material (MM-90A72DC) tests the follow-ing value is obtained F/y = 2.7072 N/mm.Fig. 28 3 DOF motion attainedwith a circular cross sectionand flat interfaceFig. 29 Examples of complexcombination interfaces withcircular cross section andextended lengthFig. 30 Molding method tocreate 3 DOF flat interface withcircular cross section. a Coreposition for first stage.b Molded part first stage andcore position for second stage.c Resulting part1061This result has to be compared with the theoreticalresults for the flat interface under flexure loading. Inputtingthe value of the Youngs modulus for the hard and softmaterial and the dimensions of the specimens, the com-parison and calculation is shown below. I and F/y(x) aredefined as follows (see Appendix 2 for derivation):Fy x I1E1L313L21L22?1E2L1 L233?L313? L21L2? L1L22 !#I H4112?H4212?D464,(2)Fig. 31 Example of a configu-ration where large distance canbe obtained with a small amountof rotational motionFig. 32 Varying degrees of di-mensional stability of the inter-face interstage, (a) highlydistorted to (c) no distortionTable 1 Interface type vs molding methodInterface type Flat interface CylindricalinterfaceComplex interfaceMoldingmethodsCavitytransferCavity transferCavity transferRemovablecoreRemovable core Sliding core(sometimes)Sliding coreFig. 33 Example of traditional molded clip1062Where E1= modulus of elasticity for IE-72DC and E2=modulus of elasticity for IE-90A; see Figs. 16 and 18 fordefinition of dimensional parameters L1, L2, H1, H2, and D.BysubstitutingthegeometricparametersL1=30.5mm,L2=13.75, H1= 6.35 mm, H2= 3.175 mm, and D = 3.172 mm,values of I = 132.011 mm4andFy x 2:40N/mm areobtained.In both graphs (Figs. 19 and 20), the test results are verysimilar to the calculated results of the theoretical model.However, it is necessary to distinguish that the theoreticalresults of the flexure test are very sensitive to the values ofL2(soft material length) and I (moment of inertia). If thevalues for L2(soft material length) and I (moment ofinertia) are between 1314 mm and 120140 mm4, thetolerance will be 10%. The difference between themeasured and predicted values are within this level oftolerance; therefore, the above results indicate that thecomplex interfacial geometry can be accurately modeled asa simple flat-end connection amenable to a more complexball-end connection when designing compliant mechanisms.4 Design and manufacturing of compliant joints4.1 Joint type A (1 DOF rotation)4.1.1 DesignWhile the geometry of the interface is essentially up to thedesigners specifications, several basic characteristics orguidelines can be used when determining what type ofmotion one wants to enable or inhibit. Since all of thepossible choices of interfaces cannot possibly be described,three different types of interfaces will be focused on in thispaper: flat, cylindrical, and complex. In all of theseinterfaces it is important to make sure that the soft materialis not fully encased by the hard material so that is canexpand and contract in the necessary manner. In thissection some simple examples will be broken down intodesign characteristics and manufacturing methods. Thefirst example is a 1 degree of freedom (DOF) rotationalmotion. There are essentially two common ways toincorporate this into the designed system. In the firstexample, a rectangular cross section along the interfacerestricts the motion. The small dimension along the z-axiswill rotate much easier than the larger dimension along they-axis. Seen in Fig. 21, the bar is only able to rotate aboutthe y-axis.A similar method to achieve this desired constrainedmotion is shown in Fig. 22. This method contains acylindrical interface, in which the soft material takes theform of a small cylinder and is surrounded by hardmaterial. It is important to remember that in each of theseexamples flat interfaces can be replaced with morecomplex interfaces as long as the neighboring homoge-neous cross section remains the same. Figure 23 showswhat a combination interface of this type would look like.4.1.2 Manufacturing methodOne method to create the joint seen in Fig. 21 involves asliding core arrangement. Refer to “Process overview” andFig. 5. The best method to create a cylindrical core, seen inFig. 22, is to incorporate a removable core into the mold ofthe first stage seen in Fig. 4. The cross section of the coremust be designed in such a way that it can easily be re-moved. This criterion will require the cross-sectional areaFig. 35 Molds for compliantclipFig. 34 CAD drawing of compliant clip1063of the core to be smaller in the center section and stay thesame or increase in size towards the edges of the part. Themolding method to create the complex interface, seen inFig. 23, is the cavity transfer method seen in Fig. 3.As shown, there are three possible methods for creating a1 DOF joint. If two materials tend to produce goodbonding, then the best method for creating this joint is theremovable core method which contains the cylindricalcore, as seen in Fig. 4. This method is preferred becausethere is very little manipulation between mold stages;therefore, the timing between the moldstages is minimized.The surface area of the joint interface is also maximizedpromoting more cross-linking between the materials.Finally, the cylindrical interface is minimally restrictiveon the cross-sectional area of the surrounding jointmembers.4.2 Joint type B (2 DOF rotation)4.2.1 DesignThe next example is the design of a joint with 2 DOFrotational motion. In Fig. 24, a flat interface with a squareFig. 36 Molding sequence forcompliant clip: 2 mold volumesand 1 removable core are com-bined (a), IE-72DC is shot intothe mold producing part (b), 2mold volumes are recombinedwithout removable core (c),IE-90A is shot into the moldproducing part (d)Fig. 37 Molded compliant clip1064cross section can be used to achieve this motion. Thisgeometric configuration allows rotation about the y-axisand z-axis, but is more restrictive in the rotation about acombination of these axes. An alternate method to achievethis movement is shown in Fig. 25. A spherical interface isused for this rotational motion. Due to the geometry of thesphere the bar is now free to rotate about the y-axis and z-axis and any combination of these two axes in between.Likewise, a more complex interface can be used to create 2DOF rotation, shown in Fig. Manufacturing methodThe sliding core or removable core manufacturing method,similar to the methods shown in “Process overview” can beFig. 38 Graph of the force vsdisplacement for the moldedcompliant clip and the as-sembled clipFig. 39 Piccolo Micro ElectricRC helicopterFig. 40 CAD version of theoriginal helicopter rotor head1065used to create the joint with a flat interface. For a morecomplex interface, such as the spherical interface (Fig. 25),the cavity transfer method must be used. The sphere mustbe created in the first stage, then manually or roboticallytransferred into the second mold stage (Fig. 27). To create apart similar to Fig. 26 the cavity transfer method seen inFig. 3 is used.For a 2 DOF joint with only rotation about the y-axis andz-axis (not a combination of the axes), the sliding coremethod is the optimum method to achieve this motion.With this method there is a minimal amount of manipu-lation between molding stages and the timing betweenstages can be minimized. However, to achieve rotationabout a combination of these axes, it would be better to usethe cavity transfer method. Although this method requiresmore manual manipulation, this type of motion causesadditional stresses to occur at the interface. Both geometricinterlocking as well as chemical bonding are desired tohandle this type of loading.4.3 Joint type C (3 DOF rotation)4.3.1 DesignA fourth method to achieve a combination of the y-axis andz-axis with a flat or complex interface would be to use acircular cross-sectional area shown in Fig. 28. This con-figuration can also be used to obtain the third DOF: rotationabout the x-axis created through the use of twisting ortorsional motion. This joint is achieved by a cylindricalinterface similar to that of the first example, but is orientedin a different fashion. The circular interface is the leastrestrictive geometry for the twisting action along the x-axis. In addition it does not introduce many restrictiveforces for the rotation about the y-axis and z-axis. There isa general rule in compliant mechanisms: the greater theamount of soft material introduced into the system, themore susceptible the part will be to bending deformations.Therefore, a longer compliant section will tend to bend andtwist more easily than a short one. The flat interface inFig. 28 can be replaced by a complex interface shown inFig. Manufacturing methodIn Fig. 30, the sliding core method is used to create the flatinterface with a circular cross section. The cavity transfermethodmust be used tocreatethe complex 3 DOFinterface.Mold designs for this method are identical to those used forthe 1 DOF combination interface, shown in Fig. 3.The 3 DOF joint will have multiple forces acting on it atany given time. It must be able to withstand torsion, ten-sion, and flexure loading all at the same time. Due to thenature andprobable use of this joint type it is recommendedthat the joint be configured to promote geometric lockingFig. 41 Exploded view of the original designFig. 42 Redesigned compliantsystem1066as well as chemical bonding. Therefore, the recommendedmethod for creating this type of joint is the cavity transfermethod, using a complex geometry for the interface.4.4 General design guidelines for compliant jointsThere is a limit to rotational motion along all axes ofrotation in compliant joints. For the examples shown, thisrotation is limited to 180 of rotation for the y- and z-axes,provided the elastic properties of the material are not ex-ceeded causing the part to collide with the base structure.For the twisting rotation about the x-axis this is limited tothe elastic properties of the material. However, in rota-tional joints, a small limit in rotation does not alwaysequate to a small amount of movement. The movementattained through rotational motion is directly related to thelength of the moment arm, shown in Fig. 31. Therefore, asmall amount of rotational capabilities can introduce a lotof movement in the structure. This means that rotationalcompliant joints can be designed to be far more useful thantranslational compliant joints.The critical timing characteristics of each stage arelargely influenced by the molding method being used tocreate the part. Minimal manipulation operations, such asremovable and sliding core methods, require less hardeningtime for the first stage. Less hardening time enables thesecond stage to be poured sooner creating more cross-linking between the two materials at the interface. On theother hand, operations that require large amounts of manip-ulation between stages, such as the cavity transfer method,require a longer amount of hardening time between moldingstages and enable less material adhesion at the interface. Thischaracteristicleadsustoageneralguideline:ifalargeamountof manual manipulation is required between molding stages,the interface must be designed to incorporate geometricinterlocking at the interface.Before deciding on which strategy to use, it is importanttoexperimentwiththegivenmaterialstodeterminetrade-offparameters. These trade-off parameters include: (a) measur-ing the dimensional stability of the interface during theintermediate stage, (b) the overall dimensional stability ofthe part, and (c) the strength of the interface at the end ofeach stage. The dimensional stability of the interface at theintermediate stage refers to how malleable and flexible thefirst stage partwill be whenit is manipulated into the secondmolding stage and the second material is shot. If the part istoo malleable at this time, surface flaws may occur fromboth the manipulation and the pressure from the secondmaterial. To test this parameter one would have to mold thefirst stage at different time intervals and measure thedeformation of the first stage geometry. Figure 32 shows anexample of this deformation after different time intervals.The overall dimensional stability refers to the amount ofFig. 43 Exploded view of redesignFig. 44 Flybar rotation com-parison, flybar angle 15, com-pliant system (left), originalmodel (right)1067geometric locking integrated into the interface. To measurethe dimensional stability of the part one must mold the partfree from chemical bonding.It is also important to consider three-dimensional fac-tors when determining which mold transfer method to use:(a) the shape of the soft and hard material sections, (b) theshape of the interface, and (c) the size of each of theseparts. For example, the drop core method lends itselfeasily to a part that requires a small amount of soft mate-rial and has a relatively simple interface. The sliding coremethod is generally used when a part requires equalamounts of soft and hard materials and has relativelysimple interfaces. For the first method, manual manipu-lation is generally used for more complex interfaces. Table 1shows which type of molding methods can be used de-pending on the joint interface type. In the following caseFig. 45 Rotor head rotationcomparison, compliant system(left), original model (right)Fig. 46 Molds for compliant jointsFig. 47 Molds for second stage of rotor part 1. a Exploded view.b Enlarged view1068studies the drop core method and the cavity transfer moldingmethods have been used.5 Case study 1: clip5.1 DesignThe first case study, a clip, demonstrates how the use ofcompliance can reduce the number of parts and assemblyprocesses. First, it is necessary to describe how the partsand joints are designed. This part is modeled after ahomogeneous three-part clip shown in Fig. 33. The firststep in a redesign is to sketch the geometry of a finishedproduct, then determine the type of magnitude and durationof the forces and how and where they act on the product7. In this design, the geometry of the finished productmimics the geometry of the original design. This willensure that all functional parameters influenced by thegeometry are maintained. The desired motion for this partis for the grips to move back and forth with some res-toration force so that the steady state position will be theclosed position.Fig. 48 Molds for second stage of rotor part 2, exploded viewFig. 49 a Original mold designfor rotor part 2. b New molddesign with nonlinear partingplane1069To make this part, a 1 DOF rotational joint in the shapeof a cylinder was incorporated into the base structure of thepart. As seen in Fig. 34,for this type of joint a pin made of asofter material is placed at a critical section where relativemotion is required. The part pivots along a combination ofthe x and y plane due to the elastic nature of the softmaterial, but is constrained in the z direction via cross-polymerization or compliant properties. In this design, thecompressive force output attained by the clip is directlyrelated to the elasticity of the soft material. Using this typeof interface, it is very easy to control the amount ofclamping force through the choice of soft material. Thecompliant design reduces the amount of parts from threeparts in the original assembly to one part in the redesignedcompliant assembly, while streamlining the design byremoving the metal spring.5.2 ManufacturingWhen designing the pin connection, a removable core ordrop core arrangement was selected because there was asmall amount of soft material required for this part and theinterface required was very simple. Seen in Fig. 35, thispart requires two mold pieces and one removable core.First the mold pieces are combined with the core insertedinto the proper position. Then IE-72DC is shot into themold for the first stage. After 1.5 h, the core is removed andIE-60A is shot into the remaining mold cavity. After 10more hours the part is then demolded. The molding stepsare shown in Fig. 36 and the final molded part is shown inFig. 37.5.3 AnalysisOnce the part has been created, it is necessary to measurethe load-displacement response of the mechanism to de-termine if the design is viable. Identical tests have also beenconducted on an assembled clip to verify the similarity inthe loading responses for each design. Results obtainedfrom loading and then unloading both of the clips can beseen in Fig. 38. The force required to open the assembly isabout 250% greater than the compliant design, indicatingthat the assembled clamp has greater clamping force. Thiscan be partially attributed to the difference in the size of thetwo clips, with the remainder of the difference due to theproperties of the soft material chosen for the compliant clip.However, the response of the compliant clip is much morelinear than the assembled clip with reduced hysteresis. Thisis most probably due to the lack of end constraint for themetal spring in the assembly and the tolerance in the hinge.These results indicate that it will be much easier toprecisely control mechanical response of the compliant clipat intermediate loads.6 Case study 2: rotor system6.1 DesignIn the second case study, a rotor system was designed withcompliance that is capable of mimicking the degrees ofFig. 50 a, b First mold stagefor rotor part 1. c First moldstage to rotor part 2Fig. 51 Finished part1070freedom and motion range of a model helicopter rotorsystem shown in Fig. 39. Therefore, the rotor system can beconsidered kinematically equivalent to the model system.There was no attempt made to match aerodynamic char-acteristics of the rotor. As shown in Fig. 40, the rotorsystem actuation point, a center axis of rotation, and theservo plate are all kept consistent between the compliantand model system. The goals of the redesigned compliantsystem will be to reduce the number of parts and eliminateassembly processes while still maintaining joint kinemat-ics. Shown in Fig. 41 is an exploded view of the originaldesign.To preserve functionality, the new design mimics theoriginal design in shape and relative size. The two verticalcompliantsectionsreplacethetwopinjointsatthetopoftheoriginal model and control the flybar angle. The bendingcompliant joint replaces two linked pin connections andcontrols the rotor head angle (Fig. 42). The compliantdesign reduces the number of parts by using complexmicroscopicinterfaceswithacircularcross-sectionalareaatneighboring homogeneous sections. The original designconsists of 14 assembled parts, the new design consists of 2parts: rotor part 1 and rotor part 2 (Fig. 43). These parts areconnected with one snap fit connection. The complete rotorsystem is connected (via linkages) to the servo plate.Figures 44 and 45 illustrate the relative joint movementsattained by the redesign compared to the original designrequirements.6.2 ManufacturingThe compliant rotor design provides many unique moldingchallenges. Due to the complexity of this mold, the cavitytransfer method is the easiest possible molding method touse. The first step to solving this complex mold challengeis to insert all required cores needed to eliminate undercuts.After all undercuts are removed we can begin to breakdown the mold into joint types. This part, Fig. 43, has threecompliant pieces. Seen in Fig. 46, the mold can be brokendown into two simple cavity transfer problems. This firstjoint type, seen in Fig. 46a is very similar to the bar moldshown in Fig. 29 and uses the cavity transfer method seenin Fig. 3. This joint is used in two places connecting rotorpart 1 to the servo plate. The second joint type, Fig. 46b, isa little harder to visualize as a simple bar joint because it isencased by a complex shape. However, this joint type canalso be produced in the same fashion as the cavity transfermethod seen in Fig. 3. After two cores are inserted into themold to remove undercuts, the section just breaks downinto essentially two complex joint configurations with cir-cular cross sections linked together. To reduce the numberof transfer operations we have designed this to be a singlejoint embedded in hard material. Now that we have a goodunderstanding of the molding methods being used we willaddress the challenges that arise when you integrate com-plex shapes and multiple cores.Fig. 53 Dimensions and prop-erties of the flybarFig. 52 Schematic of desired applied load and correspondingmotion of the flybar1071When designing a mold for complex structures, such asthe rotor head example seen in case study 2, it is importantto align joints in such a way that they will be easilymanufacturable. For example if there is a joint alignedalong the main parting plane of the mold it will be easier todemold the part. The first step in this alignment process isto determine the main parting plane of the mold. Toaccomplish this, it is necessary to take into account themold volumes of the final products. Various parting planescan then be manually tried out with a CAD program.Instead of manually creating the parting planes it will bemuch easier to use an automated mold design program,such as Multi-Piece Mold Designer (MPMD), to produce avariety of possible mold configurations 16.After the main parting plane is selected the joint shouldbe aligned along that plane if at all possible. Seen inFig. 47, the helicopter rotor joints and cores are aligned insuch a way that two parting planes are required.After cores are inserted into rotor part 2, this joint typecan be seen as a complex combination interface with acircular cross section. This joint is unique in that thecompliant section of the joint is curved making the partingplane creation more difficult. In Fig. 48, it is seen that thegeometrical configuration required for the functionality ofrotor part 2 makes it difficult to limit the number of partingplanes. In order to help minimize the number of moldpieces a nonlinear parting plane is used. Seen in Fig. 49, thenumber of molds is reduced from six mold pieces to four.For the new compliant rotor system, each rotor part canbe manufactured in two mold stages. In the initial moldstages for rotor parts 1 and 2, the soft compliant materialIE-90A is shot (see Fig. 50). The soft material is left toharden until it reaches a point where the material is still softenough to cross-polymerize with the second shot whilebeing hard enough to be taken out of the mold. For this partthe first stage is removed from the mold after 5 h.Force vs. Angle of Deflection0510150.1150.230.346Force (N)Angle (Deg)0102030400.1150.230.346Force (N)Angle (Deg)Length =20.32 (mm)Length =25.4 (mm)Length =30.48 (mm)Force vs. Angle of DeflectionDiameter =3.81 (mm)Diameter =5.08 (mm)Diameter =6.35 (mm)Force vs. Angle of Deflection02468101214160.1150.230.346Angle (Deg)Elastic Modulus = 2000N/mm2 (hard material), 15N/mm2 (soft material)Elastic Modulus = 2200N/mm2 (hard material), 20N/mm2 (soft material)Elastic Modulus = 2400N/mm2 (hard material), 25N/mm2 (soft material)bForce (N)caFig. 54 Flybar test results force vs angle of deflection: length varied (a), diameter varied (b), elastic modulus varied (c)Table 2 Varied parameters usedin joint analysisL (mm)Ehard(N/mm2)Esoft(N/mm2)D (mm)Model 1 (LmEmDm)25.402200205.08Model 2 (LlEmDm)20.322200205.08Model 3 (LhEmDm)30.482200205.08Model 4 (LmEmDl)25.402200203.81Model 5 (LmEmDh)25.402200206.35Model 6 (LmElDm)25.402000155.08Model 7 (LmEhDm)25.402400255.081072The hard material IE-72DC is used in the second moldstage. For this stage, the soft material is inserted into themolds relying on a secure mold fit to keep them in place.The molds are assembled and the second stage is shot(Figs. 47 and 48). After 5 more hours the part is demolded.The servo plate and second mold stages can be manufac-tured simultaneously. The compliant rotor system can beassembled with one snap fit connection. The bearings ofthe rotor part 2 snap in the rotor part 1. The completecompliant rotor system is connected with the servo platevia linkages (Fig. 51).6.3 AnalysisThis example was quite complex and creating force vsdisplacement required development of custom test fixtures.So in this case we analyzed the mechanism using finiteelement-based analysis tool Pro/Mechanica.In order to better understand the relationships betweenthe size and shape of the compliant mechanism and itscorresponding joint properties a series of models wereevaluated. Seven models are created with varying thedimensions and the value of Youngs modulus of the flybar.For each model, different loads F are applied in Pro/Mechanica and the displacements of a specified line aremeasured Fig. 52. The parameters used in the models areshown in Fig. 53, and the values for each model are shownin Table 2.After analyzing the kinematics of the flybar displace-ment in each model, the values for the rotation angles canbe determined from the deformation in various joints.Figure 54 shows how rotation angle changes as a functionof length, diameter, and Youngs modulus. We also testedthis device by manually applying forces on it. The devicesuccessfully exhibited the full motion range.Based on the above test, we believe that if one wanted tooptimize bending conditions, to require minimal appliedforce and largedeflections, the length should be maximizedand the diameter and the Youngs modulus should beminimized, which is consistent with beam bending theory.The trade-off with this geometry would be an increasedprobability of failure at the interface due to greater sen-sitivity to axial loading. If it was required to design a jointthat was less sensitive to small changes in forces, a shortcompliant section with a large diameter and a high modulusof elasticity would be required. This geometry would alsohave the advantage of being less sensitive to axial loadingat the interface.7 ConclusionsThis paper shows that multi-material molding is apromising manufacturing process for creating multi-mate-rial compliant mechanisms. Several different types of in-terfaces that can be used in compliant mechanisms aredescribed. Experimental characterization results for theseinterfaces show that these interfaces have the requiredmotion range and do not break under the loads needed toproduce the motion. Furthermore, experimental character-ization results show that certain geometrically complexinterfaces can be modeled as simple interfaces withouthaving any serious affect on the accuracy of an assemblyanalysis. This paper also describes design of severaldifferent compliant joints that provide 13 degrees offreedom. For each joint design, we also present a feasiblemold design to realize that joint. Finally, we show how acomplex device consisting of multiple different compliantjoints can be fabricated by combining mold pieces forindividual joints into an overall mold.The case studies clearly show that compliant joints canbe used to reduce part count and eliminate assemblyoperations. In the clip assembly the part count is reducedby 66.6% and in the rotor assembly the part count isredu