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1、Journal of Materials Processing Technology 171 (2006) 259267Design and thermal analysis of plastic injection mouldS.H. Tang, Y.M. Kong, S.M. Sapuan, R. Samin, S. SulaimanDepartment of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, MalaysiaReceived 3 Sep
2、tember 2004; accepted 21 June 2005AbstractThis paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis forthe mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as conside
3、ration neededin designing of plastic injection mould are presented. Design of mould was carried out using commercial computer aided design softwareUnigraphics, Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solvedusing a commercia
4、l finite element analysis software called LUSAS Analyst, Version 13.5. The software provides contour plot of temperaturedistribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves. Theresults show that shrinkage is likely to
5、occur in the region near the cooling channels as compared to other regions. This uneven cooling effectat different regions of mould contributed to warpage. 2005 Elsevier B.V. All rights reserved.Keywords: Plastic Injection mould; Design; Thermal analysis1. IntroductionPlastic industry is one of the
6、worlds fastest growingindustries, ranked as one of the few billion-dollar industries.Almost every product that is used in daily life involves theusage of plastic and most of these products can be producedby plastic injection molding method 1. Plastic injectionmolding process is well known as the man
7、ufacturing processtocreateproductswithvariousshapesandcomplexgeometryat low cost 2.The plastic injection molding process is a cyclic process.There are four significant stages in the process. These stagesare filling, packing, cooling and ejection. The plastic injec-tion molding process begins with fe
8、eding the resin and theappropriateadditivesfromthehoppertotheheating/injectionsystemoftheinjectionplasticinjectionmoldingmachine3.This is the “filling stage” in which the mould cavity is filledwithhotpolymermeltatinjectiontemperature.Afterthecav-ityisfilled,inthe“packingstage”,additionalpolymermelti
9、spacked into the cavity at a higher pressure to compensate theexpectedshrinkageasthepolymersolidifies.ThisisfollowedCorresponding author.E-mail address: saihongeng.upm.edu.my (S.H. Tang).by “cooling stage” where the mould is cooled until the part issufficiently rigid to be ejected. The last step is
10、the “ejectionstage” in which the mould is opened and the part is ejected,after which the mould is closed again to begin the next cycle4.The design and manufacture of injection molded poly-meric parts with desired properties is a costly process domi-nated by empiricism, including the repeated modific
11、ation ofactual tooling. Among the task of mould design, designingthe mould specific supplementary geometry, usually on thecore side, is quite complicated by the inclusion of projectionand depression 5.In order to design a mould, many important designingfactors must be taken into consideration. These
12、 factors aremouldsize,numberofcavity,cavitylayouts,runnersystems,gating systems, shrinkage and ejection system 6.In thermal analysis of the mould, the main objective isto analyze the effect of thermal residual stress or molded-instresses on product dimension. Thermally induced stressesdevelop princi
13、pally during the cooling stage of an injectionmolded part, mainly as a consequence of its low thermalconductivity and the difference in temperature between themolten resin and the mould. An uneven temperature fieldexists around product cavity during cooling 7.0924-0136/$ see front matter 2005 Elsevi
14、er B.V. All rights reserved.doi:10.1016/j.jmatprotec.2005.06.075260S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267During cooling, location near the cooling channel experi-ences more cooling than location far away from the coolingchannel. This different temperature cau
15、ses the material toexperience differential shrinkage causing thermal stresses.Significantthermalstresscancausewarpageproblem.There-fore,itisimportanttosimulatethethermalresidualstressfieldof the injection-molded part during the cooling stage 8. Byunderstanding the characteristics of thermal stress d
16、istribu-tion, deformation caused by the thermal residual stress canbe predicted.In this paper the design of a plastic injection mould forproducingwarpagetestingspecimenandforperformingther-mal analysis for the mould to access on the effect of thermalresidual stress in the mould is presented.2. Metho
17、dology2.1. Design of warpage testing specimenThis section illustrates the design of the warpage testingspecimen to be used in plastic injection mould. It is clearthat warpage is the main problem that exists in product withthin shell feature. Therefore, the main purpose of the prod-uct development is
18、 to design a plastic part for determiningthe effective factors in the warpage problem of an injection-moulded part with a thin shell.The warpage testing specimen is developed from thinshell plastics. The overall dimensions of the specimen were120mminlength,50mminwidthand1mminthickness.Thematerial us
19、ed for producing the warpage testing specimenwas acrylonitrile butadiene stylene (ABS) and the injectiontemperature, time and pressure were 210C, 3s and 60MPa,respectively. Fig. 1 shows the warpage testing specimen pro-duced.2.2. Design of plastic injection mould for warpagetesting specimenThissecti
20、ondescribesthedesignaspectsandotherconsid-erationsinvolvedindesigningthemouldtoproducewarpagetestingspecimen.ThematerialusedforproducingtheplasticFig. 1. Warpage testing specimen produced.injectionmouldforwarpagetestingspecimenwasAISI1050carbon steel.Four design concepts had been considered in desig
21、ning ofthe mould including:i. Three-plate mould (Concept 1) having two parting linewith single cavity. Not applicable due to high cost.ii. Two-platemould(Concept2)havingonepartinglinewithsingle cavity without gating system. Not applicable dueto low production quantity per injection.iii. Two-plate mo
22、uld (Concept 3) having one parting linewithdoublecavitieswithgatingandejectionsystem.Notapplicable as ejector pins might damage the product asthe product is too thin.iv. Two-platemould(Concept4)havingonepartinglinewithdoublecavitieswithgatingsystem,onlyusedspruepulleract as ejector to avoid product
23、damage during ejection.In designing of the mould for the warpage testing spec-imen, the fourth design concept had been applied. Variousdesign considerations had been applied in the design.Firstly,themouldwasdesignedbasedontheplatendimen-sion of the plastic injection machine used (BOY 22D). Thereis a
24、 limitation of the machine, which is the maximum area ofmachine platen is given by the distance between two tie bars.The distance between tie bars of the machine is 254mm.Therefore, the maximum width of the mould plate shouldnot exceed this distance. Furthermore, 4mm space had beenreserved between t
25、he two tie bars and the mould for mouldsetting-up and handling purposes. This gives the final max-imum width of the mould as 250mm. The standard mouldbasewith250mm250mmisemployed.ThemouldbaseisfittedtothemachineusingMatexclampattheupperrightandlower left corner of the mould base or mould platen. Dim
26、en-sions of other related mould plates are shown in Table 1.The mould had been designed with clamping pressurehaving clamping force higher than the internal cavity force(reaction force) to avoid flashing from happening.Based on the dimensions provided by standard mould set,thewidthandtheheightofthec
27、oreplateare200and250mm,respectively.Thesedimensionsenableddesignoftwocavitieson core plate to be placed horizontally as there is enoughspace while the cavity plate is left empty and it is only fixedwithspruebushingforthepurposeoffeedingmoltenplastics.Therefore,itisonlyonestandardpartinglinewasdesign
28、edatTable 1Mould plates dimensions.ComponentsSize (mm)widthheightthicknessTop clamping plate25025025Cavity plate20025040Core plate20025040Side plate/support plate3725070Ejector-retainer plate12025015Ejector plate12025020Bottom clamping plate25025025S.H. Tang et al. / Journal of Materials Processing
29、Technology 171 (2006) 259267261the surface of the product. The product and the runner werereleased in a plane through the parting line during mouldopening.Standardorsidegatewasdesignedforthismould.Thegateis located between the runner and the product. The bottomland of the gate was designed to have 2
30、0slanting and hasonly 0.5mm thickness for easy de-gating purpose. The gatewas also designed to have 4mm width and 0.5mm thicknessfor the entrance of molten plastic.In the mould design, the parabolic cross section type ofrunnerwasselectedasithastheadvantageofsimplermachin-ing in one mould half only,
31、which is the core plate in thiscase. However, this type of runner has disadvantages such asmoreheatlossandscrapcomparedwithcircularcrosssectiontype. This might cause the molten plastic to solidify faster.This problem was reduced by designing in such a way thatthe runner is short and has larger diame
32、ter, which is 6mm indiameter.Itisimportantthattherunnerdesigneddistributesmaterialor molten plastic into cavities at the same time under thesame pressure and with the same temperature. Due to this,the cavity layout had been designed in symmetrical form.Another design aspect that is taken into consid
33、eration wasair vent design. The mating surface between the core plateand the cavity plate has very fine finishing in order to preventflashingfromtakingplace.However,thiscancauseairtotrapin the cavity when the mould is closed and cause short shotor incomplete part. Sufficient air vent was designed to
34、 ensurethat air trap can be released to avoid incomplete part fromoccurring.The cooling system was drilled along the length of thecavities and was located horizontally to the mould to alloweven cooling. These cooling channels were drilled on bothcavity and core plates. The cooling channels provided
35、suffi-cientcoolingofthemouldinthecaseofturbulentflow.Fig.2shows cavity layout with air vents and cooling channels oncore plate.In this mould design, the ejection system only consists ofthe ejector retainer plate, sprue puller and also the ejectorFig. 2. Cavity layout with air vents and cooling chann
36、els.plate. The sprue puller located at the center of core plate notonly functions as the puller to hold the product in positionwhen the mould is opened but it also acts as ejector to pushthe product out of the mould during ejection stage. No addi-tional ejector is used or located at product cavities
37、 becausethe product produced is very thin, i.e. 1mm. Additional ejec-tor in the product cavity area might create hole and damageto the product during ejection.Finally, enough tolerance of dimensions is given consid-eration to compensate for shrinkage of materials.Fig. 3 shows 3D solid modeling as we
38、ll as the wireframemodeling of the mould developed using Unigraphics.3. Results and discussion3.1. Results of product production and modificationFrom the mould designed and fabricated, the warpagetesting specimens produced have some defects during trialrun. The defects are short shot, flashing and w
39、arpage. Theshortshotissubsequentlyeliminatedbymillingofadditionalair vents at corners of the cavities to allow air trapped toFig. 3. 3D solid modeling and wireframe modeling of the mould.262S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267Fig. 4. Extra air vents to avoi
40、d short shot.escape. Meanwhile, flashing was reduced by reducing thepacking pressure of the machine. Warpage can be controlledby controlling various parameters such as the injection time,injection temperature and melting temperature.After these modifications, the mould produced high qual-ity warpage
41、 testing specimen with low cost and requiredlittle finishing by de-gating. Fig. 4 shows modifications ofthe mould, which is machining of extra air vents that caneliminate short shot.3.2. Detail analysis of mould and productAfterthemouldandproductsweredeveloped,theanalysisofmouldandtheproductwascarri
42、edout.Intheplasticinjec-tionmouldingprocess,moltenABSat210Cisinjectedintothe mould through the sprue bushing on the cavity plate anddirected into the product cavity. After cooling takes place,the product is formed. One cycle of the product takes about35s including 20s of cooling time.The material us
43、ed for producing warpage testing speci-men was ABS and the injection temperature, time and pres-sure were 210C, 3s and 60MPa respectively. The materialselected for the mould was AISI 1050 carbon steel.Properties of these materials were important in determin-ing temperature distribution in the mould
44、carried out usingfinite element analysis. Table 2 shows the properties for ABSand AISI 1050 carbon steel.The critical part of analysis for mould is on the cavity andcore plate because these are the place where the product isformed. Therefore, thermal analysis to study the temperatureFig. 5. Model fo
45、r thermal analysis.distribution and temperature at through different times areperformedusingcommercialfiniteelementanalysissoftwarecalled LUSAS Analyst, Version 13.5. A two-dimensional(2D) thermal analysis is carried out for to study the effectof thermal residual stress on the mould at different reg
46、ions.Due to symmetry, the thermal analysis was performed bymodeling only the top half of the vertical cross section orside view of both the cavity and core plate that were clampedtogether during injection. Fig. 5 shows the model of thermalanalysis analyzed with irregular meshing.Modelingforthemodela
47、lsoinvolvesassigningpropertiesandprocessorcycletimetothemodel.Thisallowedthefiniteelement solver to analyze the mould modeled and plot timeresponse graphs to show temperature variation over a certainduration and at different regions.For the product analysis, a two dimensional tensile stressanalysis
48、was carried using LUSAS Analyst, Version 13.5.Basically the product was loaded in tension on one end whilethe other end is clamped. Load increments were applied untilthe model reaches plasticity. Fig. 6 shows loaded model ofthe analysis.3.3. Result and discussion for mould and productanalysisFor mou
49、ld analysis, the thermal distribution at differenttime intervals was observed. Fig. 7 shows the 2D analysisTable 2Material properties for mould and productCarbon Steel (AISI 1050), mouldABS Polymer, productDensity, 7860kg/m3Density, 1050kg/m3Youngs modulus, E208GPaYoungs modulus, E2.519GPaPoissons r
50、atio, 0.297Poissons ratio, 0.4Yield strength, SY365.4MPaYield strength, SY65MPaTensile strength, SUTS636MPaThermal expansion, 65106K1Thermal expansion, 11.65106K1Conductivity, k0.135W/(mK)Conductivity, k49.4W/(mK)Specific heat, c1250J/(kgK)Specific heat, c477J/(kgK)S.H. Tang et al. / Journal of Mate
51、rials Processing Technology 171 (2006) 259267263Fig. 6. Loaded model for analysis of product.contour plots of thermal or heat distribution at different timeintervals in one complete cycle of plastic injection molding.For the 2D analysis of the mould, time response graphsare plotted to analyze the ef
52、fect of thermal residual stress onthe products. Fig. 8 shows nodes selected for plotting timeresponse graphs.Figs. 917 show temperature distribution curves for dif-ferent nodes as indicated in Fig. 8.From the temperature distribution graphs plotted inFigs. 917, it is clear that every node selected f
53、or the graphplotted experiencing increased in temperature, i.e. from theambient temperature to a certain temperature higher thanthe ambient temperature and then remained constant at thistemperatureforacertainperiodoftime.Thisincreaseintem-perature was caused by the injection of molten plastic intoth
54、e cavity of the product.After a certain period of time, the temperature is thenfurther increased to achieve the highest temperature andremained constant at that temperature. Increase in temper-ature was due to packing stages that involved high pressure,Fig. 7. Contour plots of heat distribution at d
55、ifferent time intervals.264S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267Fig. 8. Selected nodals near product region for time response graph plots.Fig. 9. Temperature distribution graph for Node 284.Fig. 10. Temperature distribution graph for Node 213.Fig. 11. Temper
56、ature distribution graph for Node 302.Fig. 12. Temperature distribution graph for Node 290.which caused the temperature to increase. This temperatureremains constant until the cooling stage starts, which causesreduction in mould temperature to a lower value and remainsat this value. The graphs plott
57、ed were not smooth due to theabsence of function of inputting filling rate of the moltenplastic as well as the cooling rate of the coolant. The graphsplotted only show maximum value of temperature that canbe achieved in the cycle.The most critical stage in the thermal residual stress anal-ysis is du
58、ring the cooling stage. This is because the coolingFig. 13. Temperature distribution graph for Node 278.S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267265Fig. 14. Temperature distribution graph for Node 1838.Fig. 15. Temperature distribution graph for Node 1904.stage
59、causes the material to cool from above to below theglasstransitiontemperature.Thematerialexperiencesdiffer-ential shrinkage that causes thermal stress that might resultin warpage.From the temperature after the cooling stage as shown inFigs. 917, it is clear that the area (node) located near thecooli
60、ng channel experienced more cooling effect due to fur-Fig. 16. Temperature distribution graph for Node 1853.Fig. 17. Temperature distribution graph for Node 1866.ther decreasing in temperature and the region away from thecooling channel experienced less cooling effect. More cool-ing effect with quit
61、e fast cooling rate means more shrinkageisoccurringattheregion.However,thefarthestregion,Node284experiencemorecoolingalthoughfarawayfromcoolingchannel due to heat loss to environment.Asaresult,thecoolingchannellocatedatthecenteroftheproduct cavity caused the temperature difference around themiddle o
62、f the part higher than other locations. Compressivestress was developed at the middle area of the part due tomoreshrinkageandcausedwarpageduetounevenshrinkagethat happened. However, the temperature differences aftercooling for different nodes are small and the warpage effectisnotverysignificant.Itis
63、importantforadesignertodesignamouldthathaslessthermalresidualstresseffectwithefficientcooling system.Fortheproductanalysis,fromthestepsbeingcarriedouttoanalyze the plastic injection product, the stress distributionon product at different load factor is observed in the twodimensional analysis. Figs.
64、1821 show the contour plots ofequivalent stress at different load increments.A critical point, Node 127, where the product experiencesmaximum tensile stress was selected for analysis. The stressversus strain curve and the load case versus stress curves atthis point were plotted in Figs. 22 and 23.Fr
65、omtheloadcaseversusstresscurvesatthispointplottedin Fig. 23, it is clear that the product experiencing increasedin tensile load until it reached the load factor of 23, whichis 1150N. This means that the product can withstand tensileload until 1150N. Load higher than this value causes failuretothepro
66、duct.BasedonFig.23,thefailureislikelytooccurattheregionneartothefixedendoftheproductwithmaximumstress of 3.27107Pa.The product stress analysis reveals very limited informa-tion since the product produced was for warpage testingpurposes and had no relation with tensile loading analy-sis. In future, h
67、owever, it is suggested that the product ser-vice condition should be determined so that further analysismay be carried out for other behaviors under various otherloading.266S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267Fig. 18. Equivalent stress plot at load increme
68、nt 1.Fig. 19. Equivalent stress plot at load increment 14.Fig. 20. Equivalent stress plot at load increment 16.S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259267267Fig. 21. Equivalent stress plot at load increment 23.Fig. 22. Stress versus strain curve for ABS.Fig. 23. S
69、tress versus load increment curve for ABS.4. ConclusionsThe mould designed has made it possible to produce highquality warpage testing specimen to determine parametersthat affect warpage. The testing specimen was produced atlow cost and involves only little finishing that is de-gating.The thermal an
70、alysis of plastic injection mould has pro-videdanunderstandingoftheeffectofthermalresidualstressondeformedshapeofthespecimenandthetensilestressanal-ysis of product managed to predict the tensile load that thewarpagetestingspecimencanwithstandbeforeexperiencingfailure.AcknowledgementThe authors wish
71、to thank the Faculty of Engineering,UniversitiPutraMalaysiaforinitiatingthepublicationofthispaper.References1 R.J. Crawford, Rubber and Plastic Engineering Design and Applica-tion, Applied Publisher Ltd., 1987, p. 110.2 B.H. Min, A study on quality monitoring of injection-molded parts,J. Mater. Proc
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