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Journal of Materials Processing Technology 187188 (2007) 690693Adaptive system for electrically driven thermoregulationof moulds for injection mouldingB. Nardina, B.Zagara, A. Glojeka, D. Kri zajbaTECOS, Tool and Die Development Centre of Slovenia, Kidri ceva Cesta 25, 3000 Celje, SloveniabFaculty of Electrical Engineering, Ljubljana, SloveniaAbstractOne of the basic problems in the development and production process of moulds for injection moulding is the control of temperature con-ditions in the mould. Precise study of thermodynamic processes in moulds showed, that heat exchange can be manipulated by thermoelectricalmeans. Such system upgrades conventional cooling systems within the mould or can be a stand alone application for heat manipulation withinit.Inthepaper,theauthorswillpresentresultsoftheresearchproject,whichwascarriedoutinthreephasesanditsresultsarepatentedinA6862006patent. The testing stage, the prototype stage and the industrialization phase will be presented. The main results of the project were total and rapidon-line thermoregulation of the mould over the cycle time and overall influence on quality of plastic product with emphasis on deformationcontrol.Presentedapplicationcanpresentamilestoneinthefieldofmouldtemperatureandproductqualitycontrolduringtheinjectionmouldingprocess. 2006 Elsevier B.V. All rights reserved.Keywords: Injection moulding; Mould cooling; Thermoelectric modules; FEM simulations1. Introduction, definition of problemDevelopment of technology of cooling moulds via thermo-electrical (TEM) means derives out of the industrial praxis andproblems, i.e. at design, tool making and exploitation of tools.Current cooling technologies have technological limitations.Their limitations can be located and predicted in advance withfiniteelementanalyses(FEA)simulationpackagesbutnotcom-pletely avoided. Results of a diverse state of the art analysesrevealed that all existing cooling systems do not provide con-trollable heat transfer capabilities adequate to fit into demand-ing technological windows of current polymer processingtechnologies.Polymer processing is nowadays limited (in term of short-ening the production cycle time and within that reducing costs)onlywithheatcapacitymanipulationcapabilities.Otherproduc-tion optimization capabilities are already driven to mechanicaland polymer processing limitations 3.Corresponding authors. Tel.: +386 3 490920; fax: +386 3 4264612.E-mail address: Blaz.Nardintecos.si (B. Nardin).1.1. Thermal processes in injection moulding plasticprocessingPlastic processing is based on heat transfer between plasticmaterial and mould cavity. Within calculation of heat transferone should consider two major facts: first is all used energywhich is based on first law of thermodynamicslaw of energyconservation 1, second is velocity of heat transfer. Basic taskat heat transfer analyses is temperature calculation over timeand its distribution inside studied system. That last depends onvelocity of heat transfer between the system and surroundingsand velocity of heat transfer inside the system. Heat transfer canbe based as heat conduction, convection and radiation 1.1.2. Cooling timeComplete injection moulding process cycle comprises ofmouldclosingphase,injectionofmeltintocavity,packingpres-sure phase for compensating shrinkage effect, cooling phase,mould opening phase and part ejection phase. In most cases, thelongest time of all phases described above is cooling time.Cooling time in injection moulding process is defined astime needed to cool down the plastic part down to ejectiontemperature 1.0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2006.11.052B. Nardin et al. / Journal of Materials Processing Technology 187188 (2007) 690693691Fig. 1. Mould temperature variation across one cycle 2.The main aim of a cooling process is to lower additionalcoolingtimewhichistheoreticallyneedless;inpraxis,itextendsfrom 45 up to 67% of the whole cycle time 1,4.From literature and experiments 1,4, it can be seen, that themould temperature has enormous influence on the ejection timeand therefore the cooling time (costs).Injection moulding process is a cyclic process where mouldtemperature varies as shown in Fig. 1 where temperature variesfrom average value through whole cycle time.2. Cooling technology for plastic injection mouldsAs it was already described, there are already several differ-ent technologies, enabling the users to cool the moulds 5. Themost conventional is the method with the drilling technology,i.e. producing holes in the mould. Through these holes (coolinglines),thecoolingmediaisflowing,removingthegeneratedandaccumulatedheatfromthemould1,2.Itisalsoveryconvenientto build in different materials, with different thermal conductiv-ity with the aim to enhance control over temperature conditionsin the mould. Such approaches are so called passive approachestowards the mould temperature control.The challenging task is to make an active system, which canalter the thermal conditions, regarding to the desired aspects,like product quality or cycles time. One of such approaches isintegrating thermal electrical modules (TEM), which can alterthe thermal conditions in the mould, regarding the desired prop-erties. With such approach, the one can control the heat transferwith the time and space variable, what means, that the temper-ature can be regulated throughout the injection moulding cycle,independent of the position in the mould. The heat control isdone by the control unit, where the input variables are receivedfrom the manual input or the input from the injection mouldingsimulation. With the output values, the control unit monitors theTEM module behaviour.2.1. Thermoelectric modules (TEM)For the needs of the thermal manipulation, the TEM modulewasintegratedintomould.Interactionbetweentheheatandelec-trical variables for heat exchange is based on the Peltier effect.The phenomenon of Peltier effect is well known, but it was untilFig. 2. TEM block diagram.now never used in the injection moulding applications. TEMmodule (see Fig. 2) is a device composed of properly arrangedpairsofPandNtypesemiconductorsthatarepositionedbetweentwo ceramic plates forming the hot and the cold thermoelectriccooler sites. Power of a heat transfer can be easily controlledthrough the magnitude and the polarity of the supplied electriccurrent.2.2. Application for mould coolingThe main idea of the application is inserting TEM moduleinto walls of the mould cavity serving as a primary heat transferunit.Such basic assembly can be seen in Fig. 3. Secondary heattransfer is realized via conventional fluid cooling system thatallows heat flows in and out from mould cavity thermodynamicsystem.Device presented in Fig. 3 comprises of thermoelectricmodules (A) that enable primarily heat transfer from or to tem-perature controllable surface of mould cavity (B). Secondaryheat transfer is enabled via cooling channels (C) that deliverconstant temperature conditions inside the mould. Thermoelec-tric modules (A) operate as heat pump and as such manipulatewith heat derived to or from the mould by fluid cooling sys-tem (C). System for secondary heat manipulation with coolingchannels work as heat exchanger. To reduce heat capacity ofcontrollable area thermal insulation (D) is installed between themould cavity (F) and the mould structure plates (E).Fig. 3. Structure of TEM cooling assembly.692B. Nardin et al. / Journal of Materials Processing Technology 187188 (2007) 690693Fig. 4. Structure for temperature detection and regulation.The whole application consists of TEM modules, a temper-ature sensor and an electronic unit that controls the completesystem. The system is described in Fig. 4 and comprises of aninput unit (input interface) and a supply unit (unit for electronicand power electronic supplyH bridge unit).The input and supply units with the temperature sensor loopinformation are attached to a control unit that acts as an exe-cution unit trying to impose predefined temperate/time/positionrelations.UsingthePeltiereffect,theunitcanbeusedforheatingor cooling purposes.The secondary heat removal is realized via fluid coolingmedia seen as heat exchanger in Fig. 4. That unit is based oncurrent cooling technologies and serves as a sink or a sourceof a heat. This enables complete control of processes in termsof temperature, time and position through the whole cycle.Furthermore, it allows various temperature/time/position pro-files within the cycle also for starting and ending procedures.Described technology can be used for various industrial andresearchpurposeswhereprecisetemperature/time/positioncon-trol is required.ThepresentedsystemsinFigs.3and4wereanalysedfromthetheoretical,aswellasthepracticalpointofview.ThetheoreticalaspectwasanalysedbytheFEMsimulations,whilethepracticalonebythedevelopmentandtheimplementationoftheprototypeinto real application testing.3. FEM analysis of mould coolingCurrent development of designing moulds for injectionmoulding comprises of several phases 3. Among them is alsodesign and optimization of a cooling system. This is nowa-daysperformedbysimulationsusingcustomizedFEMpackages(Moldflow 4) that can predict cooling system capabilities andespeciallyitsinfluenceonplastic.Withsuchsimulations,moulddesigners gather information on product rheology and deforma-tionduetoshrinkageasellasproductiontimecycleinformation.This thermal information is usually accurate but can still beunreliable in cases of insufficient rheological material informa-tion. For the high quality input for the thermal regulation ofTEM, it is needed to get a picture about the temperature distri-bution during the cycle time and throughout the mould surfaceandthroughoutthemouldthickness.Therefore,differentprocesssimulations are needed.Fig. 5. Cross-section of a prototype in FEM environment.3.1. Physical model, FEM analysisImplementation of FEM analyses into development projectwas done due to authors long experiences with such packages4 and possibility to perform different test in the virtual envi-ronment.WholeprototypecoolingsystemwasdesignedinFEMenvironment(seeFig.5)throughwhichtemperaturedistributionin each part of prototype cooling system and contacts betweenthem were explored. For simulating physical properties inside adeveloped prototype, a simulation model was constructed usingCOMSOL Multiphysics software. Result was a FEM modelidentical to real prototype (see Fig. 7) through which it waspossible to compare and evaluate results.FEM model was explored in term of heat transfer physicstaking into account two heat sources: a water exchanger withfluid physics and a thermoelectric module with heat transferphysics(onlyconductionandconvectionwasanalysed,radiationwas ignored due to low relative temperature and therefore lowimpact on temperature).Boundary conditions for FEM analyses were set with thegoal to achieve identical working conditions as in real test-ing. Surrounding air and the water exchanger were set at stabletemperature of 20C.Fig. 6. Temperature distribution according to FEM analysis.B. Nardin et al. / Journal of Materials Processing Technology 187188 (2007) 690693693Fig. 7. Prototype in real environment.ResultsoftheFEManalysiscanbeseeninFig.6,i.e.temper-ature distribution through the simulation area shown in Fig. 5.Fig. 6 represents steady state analysis which was very accuratein comparison to prototype tests. In order to simulate the timeresponse also the transient simulation was performed, showingverypositiveresultsforfuturework.Itwaspossibletoachieveatemperature difference of 200C in a short period of time (5s),what could cause several problems in the TEM structure. Thoseproblems were solved by several solutions, such as adequatemounting, choosing appropriate TEM material and applyingintelligent electronic regulation.3.2. Laboratory testingAs it was already described, the prototype was produced andtested (see Fig. 7). The results are showing, that the set assump-tions were confirmed. With the TEM module it is possible tocontrol the temperature distribution on different parts of themould throughout the cycle time. With the laboratory tests, itwas proven, that the heat manipulation can be practically regu-lated with TEM modules. The test were made in the laboratory,simulating the real industrial environment, with the injectionmoulding machine Krauss Maffei KM 60 C, temperature sen-sors, infrared cameras and the prototype TEM modules. Thetemperature response in 1.8s varied form +5 up to 80C, whatrepresents a wide area for the heat control within the injectionmoulding cycle.4. ConclusionsUse of thermoelectric module with its straightforward con-nection between the input and output relations represents amilestone in cooling applications. Its introduction into mouldsforinjectionmouldingwithitsproblematiccoolingconstructionand problematic processing of precise and high quality plasticparts represents high expectations.The authors were assuming that the use of the Peltier effectcan be used for the temperature control in moulds for injectionmoulding. With the approach based on the simulation work andtherealproductionoflaboratoryequipmentproved,theassump-tions were confirmed. Simulation results showed a wide area ofpossible application of TEM module in the injection mouldingprocess.With mentioned functionality of a temperature profile acrosscycle time, injection moulding process can be fully controlled.Industrial problems, such as uniform cooling of problematicA class surfaces and its consequence of plastic part appear-ance can be solved. Problems of filling thin long walls can besolvedwithoverheatingsomesurfacesatinjectiontime.Further-more, with such application control over rheological propertiesof plastic materials can be gained. With the proper thermalregulation of TEM it was possib