Effect of gate size on the melt filling behavior and residual stress of injection moldedpartsPengcheng Xie a ,Fengxia Guo a ,Zhiwei Jiao a ,Yumei Ding a ,Weimin Yang a ,b ,⇑a College of Mechanical and Electrical Engineering,Beijing University of Chemical Technology,Beijing 100029,ChinabState Key Laboratory of Organic–Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,Chinaa r t i c l e i n f o Article history:Received 8March 2013Accepted 28June 2013Available online 20July 2013Keywords:MoldingVisualization Flow behavior Residual stressa b s t r a c tThis paper studies the effects of gate size on the cavity filling pattern and residual stress of injection molded parts.A total of three rectangular gates with different sizes were used.Experiments were carried out by using a dynamic visualization system.A flow visualization mold was specially designed and made for this study.A high-speed video camera was used to record the mold filling phenomena of cavities with different gate size and different processing parameters.In addition,a Stress Viewer was used to charac-terize the residual stress of molded samples.It was found that the undersized gate has many adverse effects on the filling behavior and residual stress of molded parts.With a larger gate,the cavity will be filled faster and residual stress of parts may be smaller.The result of the study also indicates that nozzle temperature and injection rate can significantly affect the above two aspects.Ó2013Elsevier Ltd.All rights reserved.1.IntroductionGating system design is a key link in the process of injection mold design,because as a channel to connect the runner and the cavity,the gate plays a very important role.The design of gate not only affects the melt filling process,but also concerns the demolding process and separation of products and waste,thus affecting the production costs and benefits [1].Gate size as an essential aspect of gate design has very important influence on the quality of part.A gate with suitable dimension should be able to ensure the plastic filling with fast speed and good liquidity [2].And at the packing stage,gate must remain open long enough to inject additional material into the cavity for shrinkage-compensating.Generally,the gate size is established by experience.Cross-sec-tion of gate is typically smaller than that of the runner and parts,thus parts can be easily separated from the runner without leaving a visible scar on the part.In addition,when the material in the gate drops below the freeze temperature,there is the end of packing,therefore,the gate dimension controls the packing time.From these points of view,the overlarge gate is not desirable.In recent years,a large number of studies on gate design were carried out.But there are limited published works on studies relat-ing to design of gate size:Tor et al.[2]used five rectangular gateswith different ratio of width and depth to the impacts of gate size on the quality of powder injection molding.By performing the analysis of weight and density on the samples molded for each of the five gates,they evaluated the impact of different gate size.Shen et al.[3]analyzed the optimal gate design of thin-walled injection molding by using control volume finite element method.Xie and Ziegmann [4]investigated the effect of gate dimension on micro injection molded weld line strength with polypropylene (PP)and high-density polyethylene (HDPE)and found out some relation-ship between gate size and the quality of micro-molded part.This paper aims to intuitively display the influence of gate size on melt flow behavior in cavity,as well as the relationship between gate size and residual stress of parts.As moldings are used in wider areas,higher requirements for precision of products have been constantly put forward.How to suppress the generation of product defects (e.g.jet,weld lines,air bubbles,flash,crazing,etc.)and maximize the dimensional accuracy of products has been an important subject for researchers.Observing the flow behavior as an effective method could help people know the generation princi-ple of defects,and further to find out the causes and even the solu-tions [5].The emergence of visualization injection molding method applied an effective way for observing the phenomenon of melt flow in the mold.Injection molding visualization technology is a technology that the injection molding process can be directly ob-served.It is essentially adding a system that can real-time monitor-ing the filling process and reproducing the melt flow behavior in a conventional injection molding process,thus making the injection molding process from the traditional sense of ‘‘invisible’’becomes ‘‘visualization’’and ‘‘repeatable’’.Up to now,visualization method0261-3069/$-see front matter Ó2013Elsevier Ltd.All rights reserved./10.1016/j.matdes.2013.06.071⇑Corresponding author at:College of Mechanical and Electrical Engineering,Beijing University of Chemical Technology,Beijing 100029,China.Tel./fax:+861064434734.E-mail address:yangwm@ (W.M Yang).has been widely used in multiple studies of injection.Yokoi has carried out a lot of research by this technique,for example,the study of molding process of two-color products,observation of flow front behavior duringfilling process with two-axis tracking system,analysis of thermoset phenolic resinflow behavior by Gate-Magnetization Method,and the study of meltfilling disci-pline of ultra-high speed injection molding[6–10],etc.In addition, by this method,Liu and Wu[11]compared the difference offilling process and molding parts between water-assisted and gas-as-sisted injection molding.Mehdi et al.[12]studied the bubbledynamics in foam injection molding.However,the visualization technology being applied to the gate size has not yet been found, in view of the successful application of it in above-mentioned areas,as well as the significant advantage it had shown,this study will use visualization techniques as one of the main means of research.Besides,defects in the products,such as warping and shrinkage, are detrimental to the quality and accuracy of the products[13]. Actually,an important factor causing these defects is residual stress.In general,the residual stress of injection molded products are divided into two Categories.One is the thermal residual stress, which is resulting during cooling period in the mold and after demolding[14].The other one is referred as residualflow stresses, which is due to the shear and normal stresses duringfilling and packing.Flow-induced stress is smaller than the former,but it could induce anisotropy of optical and some mechanical properties because of different molecular orientation in the directions of par-allel and perpendicular to theflow direction[15].Residual stress is not only the main cause of dimensional and shape inaccuracies of molded parts,but also responsible for environmental stress crack-ing[16,17].The dimension of gate influences the orientation of polymer molecule,fibers,and the mechanical and physical proper-ties of molding parts[18,19].Therefore,through comparing the residual stress of the products can provide the basis for the choice of gate size.Residual birefringence could be a valid measuring method for the polymer molecular orientation and residual stress [20],but it also can reflect a microscopic morphological structure of polymer products[21].Friedl[22]considered that the refractive index essentially contains all the information ofstatus characteristics of transparent injection products.In this paper,the visualization method was used toflow behavior in the case of different gate size.Throughthe differences offlow behavior with three gates underof injection parameters,the relationship between gatemeltfilling process will be drawn.And then,be used to measure the residual stresses in the molding experiment will be performed by Photo-elastic,whicheffect that the induced stresses inside a material willincoming light and form an interaction pattern.Thisbe related to the stress level and distribution inside the2.Experimental procedureThe injection experiment was conducted by the means injection molding.The emergence of visualizationing method applied an effective way for observing thenon of meltflow in the mold[5].The visual systemstudy including a injection molding machine,a visual mold,a high speed camera,a light and a data acquisition device.2.1.MaterialMaterial used in this study is an injection molding graded Polypropylene(PP,ST868M,from Chemical LCY,Taiwan),which is a random copolymer with ultra-high transparency,and its properties are listed in Table1.The recommended processing temperature ranges of190–270°C and the mold temperature is recommended between20and50°C.2.2.Part geometry and mold designThe mold cavity used in this paper is a tensile specimen with single gate.The geometries and dimensions of the tensile speci-mens were shown in Fig.1,which were designed and manufac-tured according to ISO527-2:2012[23].The gate of cavity is replaceable,three gates with different dimensions have been adopted in the experiment.Concretely,those gates have the same length(2mm)but different width and depth.According to the cross section size,they were respectively named as gate S(small), gate M(middle)and gate L(large),the actual sizes are shown in Ta-ble2.The middle gate size was selected based on experience val-ues,and then as the standard.The length and width of small gate and large gate were proportional changed and rounded.In order to facilitate the comparison,the small gate was made as small as possible,the cross-sectional edge length was as1/3times as the middle gate and into an integer of1Â1.Meanwhile,the large gate was expanded by the ratio of4/3times as the gate M,andfinal rounded to4Â3.Fig.2shows the photo of cavity plate used in the study,the cav-ity is processed on a removable patch,which could be tightly pressed on thefixed mold plate by positive pressure.When replac-ing the mold,people just need to loosen the screw then the embed-Table1General property of PP used in this study.Property Unit Globalene ST868M Density g/cm30.899MFI(meltflow rate)g/10cm18(230/2.16)Shrinkage% 1.3Tensile stress at yield MPa28Tensile strain at yield%12Heat deflection temperature°C88Table2Gate dimensions used in the experimental mold.Gate Small Middle Large Width(mm)134Depth(mm)1 2.53Length(mm)222Fig.1.Dimensions of tensile specimen.P.Xie et al./Materials and Design53(2014)366–372367the plastic molding processing is carried out in a closed flow chan-nel and cavity,the process of melt,mold filling,solidification and cooling are all invisible.Different to the conventional mold whose cavity is surrounded by metal,visual mold changed the cavity wall on the moving platen into a transparent quartz glass,and placed a mirror at a 45°angle in the other side of the glass.By the reflection of light,the phenomenon in the cavity can be observed from the outside.The rectangle in Fig.2shows the area that could be seen through the monitoring window.As shown in the figure,the length of monitoring window is smaller than the length of the article,so it to see the complete filling process through study.Fig.3shows the mold schematic.the melt filling process is nearly a transient the process to be seen by the naked eye This paper used a high-speed camera whose reach 17,500frames per second,and each moment of the filling process could be recorded clearly by it.The devices of visualization experiment are shown in Fig.4.2.3.Injection moldingAll specimens were prepared on an electric injection molding machine (GSK AE80).The maximum clamping force is 80tons,screw diameter is 32mm.The maximum injection velocity and volume can be provided is 300mm/s and 101cm 3.During the course of the experiment,corresponding to each set of experimen-tal parameters with different gates,more than 10shots were made before shooting to ensure that the process was stable.If no signifi-cation variation was observed during these runs,high-speed cam-era would be used to capture the melt filling process.Each set of parameters was shot five times,and the five specimens were col-lected for internal stress test.Table 3shows the experimental parameters and corresponding number.The packing pressure was always set to 80%of injection pressure,and packing time was 5s.The melting temperature was 240°C and mold tempera-ture was constantly at 35°C,original nozzle temperature was 190°C.3.Results discussion and analysis 3.1.Filling behaviorFig.5shows the flow behavior of melt front in the case of parameter (5)with gate S.In the initial stage of melt into the cav-ity,a small amount of material was straightly injected into the cav-ity and jetting occurred.And because of the decrease of temperature,the viscosity increased and the fluidity is reduced after the melt had flown through channel to the gate,a temporary filling hysteresis generated.Until a large enough pressure had been gradually built up,the low-temperature melt would be promoted into the cavity and moved forward,then the subsequent melt would flow smoothly through the gate and fill the cavity at a high-er speed.During the experiment process,it was found that sometimes the products obtained with gate S may be not fully formed,and increasing the rate and injection pressure can not completely solve the problem.The main reason is that the material in gate S had fro-zen before the cavity was filled completely.In order to obtain the full filled specimen,the method of improving the nozzle tempera-ture was tried,and the results showed that the nozzle temperature was a significant impact factor on the filling volume of small gate cavity.Fig.6is the filling volume of the parts shaped in the case of different injection pressure and nozzle temperature,it is obvious that the higher temperature of the nozzle,the greater the filling volume.The reason is that if the nozzle temperature is increased,the temperature of melt near the small gate will rise,thereby extending the gate solidification time,so that the melt filling quan-tity increases.Besides,increasing the injection pressure can also help to improve the filling volume.When using gate M and gate L,the melt fronts were smooth arcs,and there was no jetting or filling hysteresis occurred in the filling process.As shown in Fig.7,solid lines shows the regionFig.4.The equipments of visualization experiment.Fig.2.Photo of cavity plate.Table 3Experimental data and corresponding number.Inj.P (MPa)Inj.V (mm/s)408012010(1)(4)(7)30(2)(5)(8)50(3)(6)(9)Fig.3.Schematic of the visual mold.Design 53(2014)366–372can be directly observed,the wave front curve was derived directly from tracing the video capture,the contour shown in dashed line was inferred according to the flow pattern of the preceding para-graph.Through comparing between three schematic diagrams,it can be found that under the same processing parameters,a larger gate may cause a higher filling speed and there was hardly any short shot parts generated with gate M and gate L when the nozzle temperature was 190°C.The reason is that the volume flow rate of a large gate is generally higher than a small gate when the molding process is conducted in the same condition.As shown in Fig.8,through comparing the filling processes under different conditions,it can be concluded that the larger set of injection speed resulted in the faster filling rate.And there was no obvious relationship be-tween filling rate with injection pressure (Fig.9).On the whole,undersized gate like gate S will cause jetting and low filling speed.If molding process is conducted in condition of low injection rate or high injection pressure,the defect of shortshot will be able to generate.Therefore,for the filling behavior,undersized gate is disadvantageous.3.2.Residual stressThe residual stress in the specimens was examined by Stress Viewer R5.1(by Moldex3D,Fig.10),which is an instrument that could non-destructively and qualitatively observe the internal stress of transparent plastic parts.The instrument works by Brew-ster’s law of photo-elasticity.It uses the photo-elastic properties of plastic under stress to observe the variations of material birefrin-gence.When placing a transparent plastic sample between two polarized sheets and shining polarized light on it,the components of the light wave that are parallel and perpendicular to the direc-tion of the stress will propagate through the plastic with different speeds.Color fringes can be observed correspond to different speeds at that point,which in turn correspond to stress level,theFig.7.Filling process with three gates in the case of parameters (5).Fig.5.Filling process with gate S in the case of parameter (5).Fig.6.Specimens shaped by gate S with different nozzle temperature and injection pressure.principle is shown in Fig.11.From the color fringes patterns,the areas with higher density of color fringe lines higher stress inside can be learned.For a polymer material which has been subjected to stress and generated stress deformation,its refractive index of the light in the space will have a directional difference,in another words,the stress components of the plastic material in each directions are dif-ferent.As a result,the refractive index in these directions will also be different,and the difference will be proportional to the the for-mer.Therefore,by observing the light and dark fringes that were presented due to the different refractive index,the distribution and magnitude of residual stress can be known directly.With theoretic analysis and visualization techniques,Du et al.[24]once took rectangular plate cavity for the study,and observed the dynamic evolution process of residual stress during melt filling.They concluded that the residual stress near the point gate was sig-nificantly more than the fan gate.With the reduction of melt tem-perature or the increase of the holding pressure,the residual stress became larger.In this paper,the effects of various gate size andFig.10.Residual stress Viewer.Fig.11.Schematic diagram of birefringence.12.Residual stress distributions of specimens for three gates when injection rate was 30mm/s.9.Melt-flow-length of gate M with the injection speed of 30mms.(T-time,melt flow length)Melt-flow-length of gate M with the injection pressure of 40MPa.(T-time,flow length)parameters(including temperature,injection speed and injection pressure)on the residual stress were studied,and the discussion of the experimental results were as follows.As shown in Fig.12,when the injection rate was30mm/s and the nozzle temperature was190°C,the residual stress of speci-mens for three gates were significantly different.For ease of com-parison,the stress region had been divided into four parts according to the shape of the specimen.Stress of specimens shaped with gate S distributed much wider than specimens of the other two gates,the stress concentrations in the region of part2and part 3were particularly pared to gate S,residual stress of specimens shaped with gate M and L was much less than the for-mer.Difference between gate M and gate L was not obvious,but residual orientation,namely frozen in orientation[25,26].The chains with frozen orientation always have a development trend from high energy state to lower energy state,it means they may tend to curl,wound,or recrystallization,this will lead to inconsis-tent alignment direction and further generate internal stresses[26]. Therefore,when the meltflow rate is slower,the cooling rate will be faster,then the frozen in orientation will be more serious,thus the residual stress of products will be larger.That is why articles of gate S have the maximum residual stress and L has the minimum.Another set of contrasting results further demonstrate the influ-ence of temperature on the residual stress.As shown in Fig.14,in the case of injection rate of10mm/s,injection pressure of 120MPa,different nozzle temperature caused different residualFig.14.Residual stress distributions of specimens for gate S under different nozzle temperature.Fig.15.Residual stress distributions of specimens molded by gate M.Fig.13.Fig.13.Sketch of chains of a polymeric in different state[27]P.Xie et al./Materials and Design53(2014)366–372371the wall and in the center is larger than when the temperature was low,the cooling rate is uneven,made the difference of segment ori-entation became greater and thus the phenomenon occurred.In general,the residual stress will decline as the temperature rising, which is consistent with the conclusion of Du[24].In addition to the effect of gate size on residual stress,which also can be seen from Fig.12is that injection pressure has no sig-nificantly influence on the residual stress in this experiment.Fig.15shows the residual stress distribution of specimens molded by gate M.It can be found that the injection rate has a great influence on the residual stress.When the rate was10mm/ s,the residual stress distribution was the biggest and the difference with each other mainly lied in part1.The smallest stress was gen-erated in the case of50mm/s.This result suggests that a large speed is helpful to reducing the residual stress.Reason is that most of the polymer molecular chains are arranged along theflow direc-tion in thefilling process.Due to the faster injection rate resulted in the higher shear rate of the melt,it will lead to the higher orien-tation degree of the segment.Therefore,the preference consistency of products segment will be higher,while theflow residual stress will be small.From another point of view,due to the high injection rate,the temperature of melt in the cavity is relatively uniformly and high,according to the previous conclusions,it can be known that the thermal residual stress will also be small.Therefore,the total residual stress will be lower when the injection rate is higher.4.ConclusionsAccording to the results of injection molding visualization experiments and observations on residual stress,the following conclusions could be drawn.(1)Gate size is an important factor affecting thefilling behavior.The undersized gate will cause jetting and lowfilling speed, and is likely to produce short shot products.If the gate is appropriately enlarged,thefilling speed,flow stability and integrity of products will all be improved.Moreover,these effects are coupled with processing conditions,altering the injection speed and nozzle temperature will cause the change offilling behavior.(2)Gate can significantly impact the magnitude and distribu-tion of residual stress:A larger gate may generate smaller stress.Besides,the effect of injection speed and temperature on residual stress should not be ignored.The residual stress will be likely to reduce when the melt isfilling in a higher speed or a higher nozzle temperature.Through comparing between these three gates,it can be identi-fied in this paper that the undersized gate has many adverse effects on thefilling process and the residual stress.However,based on the traditional experience,the gate is not the larger the better.Fur-ther study is needed to be carried out in this aspect. AcknowledgementsThe authors are supported by the Laboratory of Advanced Poly-mer Processing.We gratefully acknowledge CoreTech System Co.,Ltd.(Moldex3D)and GSK CNC Equipment Co.,Ltd.for their gener-ous supply of the devices.Funding was provided by the National Natural Science Foundation of China(Grant Nos.51203009and 21174015).References[1]Pye RGW.Injection Mould Design.Harlow(Longman Scientific&Technical);1989.p.358.[2]Tor SB,Loh NH,Khor KA,Yoshida H.The effects of gate size in powder injectionmolding.Mater Manuf Processes1997;12(4):629–40.[3]Shen YK,Wu CW,Yuc YF,Chungc HW.Analysis for optimal gate design of thin-walled injection molding.Int Commun Heat Mass Trans2008;35(6):728–34.[4]Xie L,Ziegmann G.Effect of gate dimension on micro injection molded weldline strength with polypropylene(PP)and high-density polyethylene(HDPE).Int J Adv Manuf Technol2010;48(1–4):71–81.[5]Xie PC,Du B,Yan ZY,Ding YM,Yang WM.Visual experiment study on theinfluence of mold structure design on injection molding product’s defects.Adv Mater Res2010;87–88:31–5.[6]Yokoi H.Recent development of visualization analysis techniques in injectionmolding.Denso Tech Rev2006;11(2):3–13.[7]Yokoi H,Masuda N,Mitsuhata H.Visualization analysis offlow front behaviorduringfilling process of injection mold cavity by two-axis tracking system.J Mater Process Technol2002;130–131:328–33.[8]Ohta T,Yokoi H.Visual analysis of cavityfilling and packing process ininjection molding of thermoset phenolic resin by the gate-magnetization method.Polym Eng Sci2001;41(5):806–19.[9]Yokoi H.Visualization and measurement technologies for ultra-high-speedinjection molding phenomena.Prod Res2007;59:483–91.[10]Yoshimura Y,Endo M,Yokoi H.Visualization analysis of meltfilling behaviorfrom submarine-gate in ultra-high-speed injection molding.Prod Res 2009;61:985–8.[11]Liu SJ,Wu YC.Dynamic visualization of cavity-filling process influid-assistedinjection molding-gas versus water.Polym Test2007;26(2):232–42.[12]Mehdi M,Amir HB,Mohammad Rezavand SA,Amir P.Visualization of bubbledynamics in foam injection molding.J Appl Polym Sci2010;116(6):3346–55.[13]Demirer A,Soydan Y,Kapti AO.An experimental investigation of the effects ofhot runner system on injection moulding process in comparison with conventional runner system.Mater Des2007;28:1467–76.[14]Wang TH,Young WB.Study on residual stresses of thin-walled injectionmolding.Euro Polym J2005;41:2511–7.[15]Zoetelief WF,Douven LFA,Ingen Housz AJ.Residual thermal stresses ininjection molded products.Polym Eng Sci1996;36(14):1886–96.[16]Kamal MR,Lai-Fook RA,Hernandez-Aguilar JR.Residual thermal stresses ininjection moldings of thermoplastics:a theoretical and experimental study.Polym Eng Sci2002;42(5):1098–114.[17]Wimberger-Friedl R,de Bruin JG,Schoo HFM.Residual birefringence inmodified polycarbonates.Polym Eng Sci2003;43(1):62–70.[18]Fiske T,Gokturk HS,Yazici R,Kalyon DM.Effects offlow induced orientation offerromagnetic particles on relative magnetic permeability of injection molded composites.Polym Eng Sci1997;37(5):826.[19]Yamada K,Tomari K,Ishiaku US,Hamada H.Fracture toughness evaluation ofadjacentflow weld line in polystyrene by the SENB method.Polym Eng Sci 2005;45(8):1059–66.[20]Tumbull A,Maxwell AS,Pillai S.Residual stress in polymers evaluation ofmeasurement techniques.J Mater Sci1999;34(3):451–9.[21]Neves NM,Pouzada AS.The use of birefringence for predicting the stiffness ofinjection molded polycarbonate discs.Polym Eng Sci1998;38(10):1770–7. [22]Wimberger-Friedl R.The assessment of orientation,stress and densitydistributions in injection-molded amorphous polymers by optical techniques.Prog Polym Sci1995;20(3):369–99.[23]ISO527-2.Plastics—determination of tensile properties—Part2:Testconditions for moulding and extrusion plastics;2012.[24]Du B.Dynamic visualization experimental study of internal stress on theoptical products.Beijing University of Chemical Technology;2011.[25]Jansen KMB,Flaman AAM.The influence of surface heating on thebirefringence distribution in injection molded parts.Polym Eng Sci 1994;34(11):898–904.[26]Xu QJ,Yu SW.Calculation of residual stress in injection molded productionmolded products for polymer materials.Chinese J Theor Appl Mech 1998;30:157–67.[27]ten Grotenhuis SM,Piazolo S,Pakula T,Passchier CW,Bons PD.Are polymerssuitable rock analogs?Tectonophysics2002;350(1):35–47.372P.Xie et al./Materials and Design53(2014)366–372。
