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Journal of Mechanical Science and Technology 24 2010 145 148 DOI 10 1007 s12206 009 1126 5 Optimal injection molding conditions considering the core shift for a plastic battery case with thin and deep walls Dong Gyu Ahn 1 Dae Won Kim 2 and Yeol Ui Yoon 3 1 Department of Mechanical Engineering Chosun University Gwang Ju 501 751 Korea 2 Incheon Service Division KITECH Incheon 406 840 Korea 3 Chun Bok Injection Mold Inc Gwang Ju 1233 12 Korea Manuscript Received April 24 2009 Revised September 21 2009 Accepted October 12 2009 Abstract The objective of this paper is to examine the influence of injection molding parameters on the core shift to obtain the optimal injection molding conditions of a plastic battery case with thin and deep walls using numerical analyses and experiments Unlike conventional injection molding analysis the flexible parts of the mold were represented by 3 D tetrahedron meshes to consider the core shift in the numerical analysis The design of experiments DOE was used to estimate the proper molding conditions that minimize the core shift and a dominant parameter The results of the DOE showed that the dominant parameter is the injection pressure and the core shift de creases when the injection pressure decreases In addition it was shown that the initial mold temperature and the injection time hardly affect the core shift The results of the experiments showed that products without warpage are manufactured when the injection pressure is nearly 32 MPa Comparing the results of the analyses with those of the experiments optimal injection molding conditions were deter mined In addition it was shown that the core shift should be considered to simulate the injection molding process of a plastic battery case with thin and deep walls Keywords Optimal injection molding condition Core shift Thin and deep walls Battery case Design of experiments 1 Introduction One of the recent concerns in the automotive industry is the reduction of overall weight to improve fuel efficiency and re duce a vehicle s environmental impact 1 3 The injection molding process of thinner plastic components allows consid erable weight savings on the automotive a significant reduc tion in production cost and a shorter cycle time 1 Various studies are actively undertaken in an effort to develop the injec tion molding process of thin wall plastic parts 1 High injec tion pressure and high injection speed are necessary to manu facture plastic parts with thin and deep walls 4 However a high injection pressure can give rise to a core shift which is the spatial deviation of the position of the core during injection molding 5 Shepard et al found that the mold design and injection molding conditions strongly affect the core shift of the mold 6 Leo et al reported that the deflection of weak plates in the mold causes variations in the thickness of the product and the over packing of moldings 7 Bakharev et al reported that core shift effects in injection molding can be pre dicted through mold filling simulation coupled with an elastic analysis of the flexible parts of the mold 4 In this paper the influence of injection molding parameters on the core shift in the injection molding of a plastic battery case with thin and deep walls is examined to estimate an op timal injection molding condition The design of experiments is used to estimate a proper molding condition minimizing the core shift as well as to determine a dominant molding pa rameter Several experiments are carried out to obtain an op timal injection pressure Comparing the results of numerical analyses with those of the experiments the optimal injection molding condition is acquired The effects of the core shift on the quality of the product are also discussed 2 Numerical analysis and experiments In order to simulate the core shift and injection molding characteristics a three dimensional injection molding analysis was performed using the commercial code MPI V6 1 Fig 1 illustrates the analysis model The dimensions of the battery case are 164 4 mm W 251 4 mm L 184 0 mm H The maximum and minimum thicknesses of the walls are 2 7 mm and 1 8 mm respectively The depth of the walls is 168 7 mm The runner system consists of a conical sprue with an initial This paper was presented at the ICMDT 2009 Jeju Korea June 2009 This paper was recommended for publication in revised form by Guest Editors Sung Lim Ko Keiichi Watanuki Corresponding author Tel 82 62 230 7043 Fax 82 62 230 7234 E mail address smart mail chosun ac kr KSME Springer 2010 146 D G Ahn et al Journal of Mechanical Science and Technology 24 2010 145 148 Table 1 Parameters and their levels for the DOE Level Parameters 1 2 3 A Initial mold temperature o C 35 0 40 0 45 0 B injection time seconds 1 6 2 2 2 8 C Injection pressure MPa 40 0 35 0 30 0 D Holding time seconds 2 0 3 2 4 4 a b Fig 1 Design of the runner system and core a Design of the runner system and product b Meshes for the flexible parts of the core a b Fig 2 Mold for the manufacture of the plastic battery case a Design of the mold b Manufactured mold diameter of 8 mm and a final diameter of 15 mm circular runners with diameters of 8 mm and pin point gates with diameters of 2 mm In order to consider the core shift phe nomenon in the numerical analysis the flexible parts of the mold and the flow part were represented by 376 674 EA of tetrahedron meshes and 61 098 EA of shell meshes respec tively The injection material was a polypropylene resin The melting temperature of PP was set at 230 o C The design of experiments DOE was used to quantita tively examine the influence of injection molding parameters on the core shift Table 1 shows the injection molding parame ters and their levels for the L 9 3 4 orthogonal array The sig nal to noise S N ratio with the smaller the better characteris tics was calculated to estimate the proper condition for mini mizing the core shift The contribution ratio of each parameter is estimated using Analysis of Variance ANOVA to obtain the dominant parameter affecting the core shift Several experiments were performed using an injection molding machine with 600 tons of clamping force Fig 2 shows the design of the mold as well as the manufactured Table 2 Orthogonal array and results of the numerical analyses Deformation of flexible parts mm of Exp ABCD F1 F2 F3 F4 F5 F6 1 1 1 1 1 0 16 0 11 0 07 0 14 0 08 0 23 2 1 2 2 2 0 18 0 09 0 07 0 14 0 08 0 23 3 1 3 3 3 0 13 0 09 0 05 0 11 0 08 0 16 4 2 1 2 3 0 14 0 10 0 07 0 13 0 08 0 18 5 2 2 3 1 0 12 0 09 0 05 0 11 0 08 0 16 6 2 3 1 2 0 14 0 10 0 07 0 14 0 08 0 21 7 3 1 3 2 0 11 0 09 0 05 0 10 0 08 0 15 8 3 2 1 3 0 17 0 10 0 07 0 14 0 08 0 23 9 3 3 2 1 0 15 0 10 0 07 0 15 0 08 0 22 Fig 3 Results of the numerical analysis Core shift mold for the experiments The dimensions of the mold are 750 mm W 700 mm L 870 mm H The dominant parameter was varied within 10 of the proposed condition by the DOE to determine the optimal condition In order to examine the influence of core shift on the simulation of the injection molding process the results of the experiments are compared to those of the numerical analyses 3 Results and discussion 3 1 Results of injection molding analysis and DOE Fig 3 and Table 2 show the results of the injection molding analysis The flexible parts of the core were deformed in iden tical directions regardless of the combination of injection molding conditions as shown in Fig 3 Fig 3 and Table 2 show that the deformations of the F1 F5 and F6 parts of the core were greater than 0 1 mm and flexible parts of the core deformed symmetrically In addition it was noted that the shifts of the F2 and F3 parts are negligible in comparison with those of other parts From the results of the injection molding analysis the S N and contribution ratios were calculated for the F1 F5 and F6 parts of the core with relatively large core shifts Fig 4 and Table 3 show the results of the DOE In Fig 4 it can be seen that the S N ratios of the injection pressure the holding time the injection time and the initial mold tempera ture are maximized when their values are 30 MPa 1 6 seconds 4 4 seconds and 40 o C respectively The S N ratio of the in D G Ahn et al Journal of Mechanical Science and Technology 24 2010 145 148 147 Table 3 Pure contribution ratios of each injection molding parameter for different flexible parts Contribution ratio Flexible Parts Injection time Injection Pressure Holding time F1 4 1 56 5 15 8 F4 0 0 99 2 0 2 F6 3 3 73 3 6 3 Mean 2 5 76 3 7 4 a b Fig 4 Variation of signal to noise ratios according to the levels of injection molding parameters a S N ratios of injection pressure and holding time b S N ratios of initial mold temperature and injection time jection pressure also increases remarkably when the injection pressure decreases as shown in Fig 4 Table 3 shows that the mean value of the contribution ratio of the injection pressure is nearly 76 3 and that the contri bution ratio of the injection pressure is markedly higher than that of the other parameters From these results it was noted that the dominant parameter which mainly affects the core shift is the injection pressure The variation in S N ratio and the contribution ratio for the injection time and the initial mold temperature are negligible as shown in Fig 4 and Table 3 From these results it was noted that the injection time and the initial mold temperature hardly affect the core shift 3 2 Results of the experiments Using the results of the DOE the experimental conditions of injection time holding time and initial mold temperature were set at 1 6 seconds 4 4 seconds and 40 o C respectively The injection pressure was varied in the range of 27 32 MPa Fig 5 shows the results of the injection molding experi Fig 5 Molded product for different injection pressures a Injection pressure 30 MPa Condition proposed by the DOE b Injection pressure 32 MPa Optimal condition a b Fig 6 Comparison of the results of numerical analyses and those of the experiments a Thickness distribution of channels b Post deformation of the molded product ments The shortshot did not occur in all experimental condi tions as shown in Fig 5 However the warpages of the walls of the molded product occurred when the injection pressure was lower than 30 MPa as shown in Fig 5 a This results from the insufficient holding pressure The warpages of the product walls do not occur when the injection pressure is 32 MPa as shown in Fig 5 b From these results the optimal injection molding conditions of the tested plastic battery case with thin and deep walls were determined as an injection pres sure of 32 MPa an injection time of 1 6 seconds a holding 148 D G Ahn et al Journal of Mechanical Science and Technology 24 2010 145 148 time of 4 4 seconds and an initial mold temperature of 40 o C The results of the experiments were compared to those of the numerical analyses as shown in Fig 6 Fig 6 a shows that the difference in wall thickness between the analyses and the experiments is reduced from 0 15 mm 0 18 mm to 0 09 mm 0 07 mm when the core shift is considered in the nu merical analysis In addition it was noted that the thickness variation of the walls is properly predicted by the core shift analysis Fig 6 b shows that the core shift appreciably affects the post deformation pattern of the molded product and the numerical analysis accounting for the core shift can predict the post deformation of the product within 0 15 mm of computa tional accuracy The results of the numerical analyses show that the injection pressure is reduced from 50 2 MPa to 32 0 MPa when the core shift is considered This is due to in creased cavity volume induced by the elastic deformation of the core Based on the above results it is noted that injection molding analysis accounting for the core shift can properly simulate the injection molding process of the battery case with thin and deep walls 4 Conclusions The influence of injection molding parameters on the core shift in the molding of a plastic battery case with thin and deep walls was investigated using numerical analysis and the ex periment The elastic deformation of the core was considered to reflect core shift effects on the numerical analysis Through nu merical analysis and DOE it was shown that the injection pressure is the dominant process parameter affecting core shift and that the core shift decreases when the injection pressure decreases In addi tion it was noted that the injection time the holding time and the initial mold temperature hardly affect core shift It was demon strated through experiments that the molded product without war pages can be manufactured at an injection pressure of approxi mately 32 MPa Comparing the results of the numerical analyses with those of the experiments the optimal injection molding con ditions were obtained In addition it was shown that the core shift should be considered to accurately simulate the injection molding process of a plastic battery case with thin and deep walls References 1 R Spina Injection moulding of automotive component comparison between hot runner systems for a case study J Mater Process Technol 155 156 2004 1497 1504 2 K J Kim C W Sung Y N Baik Y H Lee D S Bae K H Kim and S T Won Hydroforming simulation of high strength steel cross members in an automotive rear subframe Int J Prec Eng Manuf 9 3 2008 55 58 3 K J Kim M H Rhee B I Choi C W Kim C W Sung C P Han K W Kang and S T Won Development of ap plication technique of aluminum sandwich sheets for auto motive hood Int J Prec Eng Manuf 10 4 2009 71 75 4 S C Chen R D Chien H H Tseng and J S Huang Re sponse of a sequential valve gate system used for thin wall injection molding J Appl Polym Sci 98 5 2005 1969 1977 5 A Bakharev Z Fan F Costa S Han X Jin and P Ken nedy Prediction of core shift effects using mold filing simu lation Proc of ANTEC2004 Plastics Chicago Illinois USA 2004 621 625 6 T A Shepard M O Connell K Powell and S Charwinsky Minimising coreshift in injection moulded container Plast Eng 52 2 1996 27 29 7 V Leo and C Cuvelliez The effect of the packing parame ters gate geometry and mold elasticity on the final dimen sions of a molded part Polym Eng Sci 36 15 1996 1961 1971 Dong Gyu Ahn received his B S degree in Production and Mechanical Engineer ing from Pusan National University Korea in 1992 He then received his M S and Ph D degrees from KAIST in 1994 and 2002 respectively Dr Ahn is currently a Professor at the Department of Mechanical Engineering at Chosun University in Gwang Ju Korea Dr Ahn s research interests include rapid prototyping and manufacturing lightweight sandwich panel laser material processing and molds and dies 1 優(yōu)化注塑條件考慮核心轉(zhuǎn)變 對(duì)于一個(gè)塑料電池盒與薄壁深腔 摘要 這篇文章的目的是利用數(shù)值分析和實(shí)驗(yàn)的方法來檢驗(yàn)型芯移動(dòng)時(shí)對(duì)注射模 參數(shù)的影響以及對(duì)薄壁深腔塑料電池盒的最佳注塑條件 與傳統(tǒng)的注射成型分析不同 模具的活動(dòng)部件利用三維四面體網(wǎng)格來表示以數(shù) 值分析法來考慮型芯移動(dòng) 實(shí)驗(yàn)設(shè)計(jì) DOE 是用來評(píng)估適當(dāng)?shù)某尚蜅l件下 使型 芯移動(dòng)量最小的一個(gè)重要參數(shù) 結(jié)果表明 DOE的主要參數(shù)是注射壓力 當(dāng)注射壓 力降低時(shí)型芯移動(dòng)量也在減小 此外 研究結(jié)果表明 初始模具溫度和注射時(shí)間 幾乎不影響型芯移動(dòng) 實(shí)驗(yàn)的結(jié)果表明 當(dāng)注射壓力接近32 MPa時(shí) 制造的產(chǎn)品 不發(fā)生彎曲 分析的結(jié)果與實(shí)驗(yàn)對(duì)比 確定了最佳注射成型條件 此外 結(jié)果表 明 模擬注塑一個(gè)塑料電池盒薄壁深腔的工藝應(yīng)考慮型芯移動(dòng) 關(guān)鍵詞 優(yōu)化注塑條件 型芯飄動(dòng) 薄壁深腔 試驗(yàn)設(shè)計(jì) 1 引言 介紹最近汽車行業(yè)中人們普遍關(guān)心的問題之一是用減小車輛整體重量來提 高燃油效率以及減少車輛對(duì)環(huán)境產(chǎn)生影響 1 3 薄塑料部件注射成型工藝性 可以較大的減小汽車的重量 大幅度降低生產(chǎn)工藝 1 人們都在積極努力的從 事各項(xiàng)研究來開發(fā)薄壁塑料件的注塑模成本和縮短生產(chǎn)周期 1 制造生產(chǎn)薄壁 深腔的塑料零件必須要使用高噴射壓力和高噴射速度 4 然而 一個(gè)高噴射壓 力能增加一個(gè)型芯移動(dòng) 這是型芯在注塑中空間位置的偏差形成的 5 Shepard 等人發(fā)現(xiàn)在模具設(shè)計(jì)和注塑條件強(qiáng)烈影響型芯移動(dòng) 6 Leo 等人報(bào)道 模具中 強(qiáng)度較差的模板的彎曲會(huì)使產(chǎn)品的厚度發(fā)生變化 7 Bakharev等人報(bào)道 可以 通過模具填充模擬和易變形模具的零件的彈性分析可事先知道注塑成型時(shí)型芯 移動(dòng)的作用情況 4 本文對(duì)注射成型參數(shù)影響具有薄壁深腔的塑料電池盒考慮到其型芯的飄動(dòng) 時(shí) 來評(píng)估最佳注射模型條件 實(shí)驗(yàn)設(shè)計(jì)是用來評(píng)估最小化型芯偏移時(shí)適當(dāng)?shù)?成型條件 以及確定主要成型參數(shù) 多次進(jìn)行了試驗(yàn)研究用來獲得一個(gè)理想的噴 2 射壓力 最優(yōu)注塑條件是通過對(duì)實(shí)驗(yàn)結(jié)果進(jìn)行數(shù)值分析對(duì)比獲得的 2 探討研究型芯偏移對(duì)產(chǎn)品的質(zhì)量的影響 實(shí)驗(yàn)和數(shù)值分析來模擬型芯偏移的注射成型特點(diǎn) 一個(gè)三維的注射成型進(jìn)行 了分析使用MPI V6 1 圖1模型分析說明 電池盒的尺寸是164 4毫米 W 251 4毫米 L 184 0毫米 H 最大和最小厚度的薄壁分別是2 7毫米和1 8毫 米 深腔的深度為168 7毫米 澆注系統(tǒng)由一個(gè)錐形澆道 初始直徑8毫米和最 后直徑15毫米 圓形通道的直徑8毫米 澆口直徑在2毫米 為了考慮型芯移動(dòng) 現(xiàn)象的數(shù)值分析 動(dòng)模的模具和活動(dòng)部件分別用四面體網(wǎng)格376674 EA和61098 EA殼網(wǎng)格代表 注射材料是聚丙烯樹脂 聚丙烯融化溫度設(shè)定在230 oC 實(shí)驗(yàn)設(shè)計(jì) DOE 用來定量檢查型芯移動(dòng)對(duì)注塑參數(shù)的影響 表1顯示了注塑 參數(shù)的水平幅附圖用繪 3 4 正交陣列表示 信噪比 S N 的特點(diǎn)是用計(jì)算估算 優(yōu)的條件下最小型芯移動(dòng) 用每個(gè)參數(shù)的貢獻(xiàn)比率來估算使用方差分析 ANOVA 獲得型芯移動(dòng)的主要參數(shù)影響 注射成型機(jī)用600噸的夾緊力進(jìn)行幾個(gè)實(shí)驗(yàn) 圖2顯示了模具的設(shè)計(jì) 以及模 具的制造實(shí)驗(yàn) 模具的尺寸為750毫米 700毫米 W L 870毫米 H 主要參 數(shù)變化在 10 的由DOE提出條件來確定最佳條件 為了檢驗(yàn)型芯移動(dòng)在模擬注 塑工藝中的影響 實(shí)驗(yàn)結(jié)果用數(shù)值分析進(jìn)行比較 3 討論與結(jié)果 3 1 DOE的注射成型結(jié)果分析 圖3和表2注塑分析顯示結(jié)果 在任何注塑條件的組合下動(dòng)模的型芯的變形 方向相同 見圖3圖3和表2表明 變形的F1 F5 F6部分的型芯是大于0 1毫米和動(dòng) 模型芯變形對(duì)稱 此外 他們注意到F2和F3變化的部分相比 是可以忽略不計(jì)的 其他部分 從結(jié)果的注塑成型分析 S N和貢獻(xiàn)比率計(jì)算的F1 F5 F6部分的 型芯與相對(duì)較大的型芯偏移 圖4和表3DOE顯示結(jié)果 圖4中 可以看到 S N比率的噴射壓力 保溫時(shí)間 注射時(shí)間 初始模具溫度是最大的時(shí)候 他們的標(biāo)準(zhǔn)分別是是30 MPa 1 6秒 4 4 秒 40攝氏度 當(dāng)注射壓力降低S N比率在注射壓力也增加明顯 見圖4 表3顯示 平均比例的噴射壓力是近76 3 在比率中的噴射壓力明顯高于其 他參數(shù) 從這些結(jié)果 指出主要參數(shù) 主要影響型芯偏移的 就是注射壓力 不同 的S N 注射時(shí)間和初始模具溫度都可以忽略不計(jì) 見圖4和表3 從這些結(jié)果 指 出注射時(shí)間和初始模具溫度幾乎沒有影響到型芯偏移 3 2 DOE實(shí)驗(yàn)結(jié)果使用結(jié)果 3 實(shí)驗(yàn)條件的注射時(shí)間 保溫時(shí)間 初始模具溫度分別設(shè)定在1 6秒 4 4秒 40攝氏度 注射壓力變化范圍在27 32 MPa 圖5顯示了注射成型的實(shí)驗(yàn)結(jié)果 在所有的實(shí)驗(yàn)條件中不包括短時(shí)間注射 如圖5所示 當(dāng)然 在注射壓力低于30 MPa不能制作薄壁易變形的模制產(chǎn)品 見圖 5 a 這是保持壓力不足的結(jié)果 當(dāng)注射壓力是32 MPa時(shí)薄壁易變形的產(chǎn)品也 不能生產(chǎn) 見圖5 b 從這些結(jié)果 塑料電池盒的薄壁深腔優(yōu)化注塑條件的測(cè)試 被確定為注射壓力32 MPa 注射時(shí)間1 6秒 保持時(shí)間4 4秒 初始模具溫度為40 攝氏度 實(shí)驗(yàn)的圖6 a 表明 在壁厚之間的差異分析的實(shí)驗(yàn)是減少從 0 15 mm 0 18毫米到 0 09毫米 0 07毫米當(dāng)型芯偏移被認(rèn)為是在數(shù)值分析 結(jié)果進(jìn)行了 數(shù)值分析比較 見圖6 此外 它是指出 厚度變化的深腔是正確預(yù)測(cè)的型芯偏移 分析 圖6 b 表明 偏芯偏移略微影響變形期后的模制產(chǎn)品模式 數(shù)值分析會(huì)計(jì) 為核心轉(zhuǎn)變可以預(yù)測(cè)產(chǎn)品的變形期后在0 15毫米的計(jì)算精度 數(shù)值分析的結(jié)果 表明 當(dāng)型芯偏移被認(rèn)為是從注射壓力降低到32 0 MPa從50 2 MPa 這是由于增 加了空腔體積引起的彈性變形的型芯 基于上述結(jié)果 型芯偏移注塑分析可以正 確地模擬電池外殼的薄壁深腔注塑工藝 4 結(jié)論 注射成型對(duì)型芯偏移成型的塑料電池盒薄壁深腔的影響參數(shù)采用了實(shí)驗(yàn)和 數(shù)值分析 彈性變形的型芯被認(rèn)為是反映型芯偏移影響數(shù)值分析 通過數(shù)值分 析和DOE顯示注射壓力是主要過程參數(shù)影響型芯偏移 當(dāng)注射壓力降低時(shí)型芯偏 移減小 此外 它是指出 注射時(shí)間 保壓時(shí)間 初始模具溫度幾乎沒有影響型 芯偏移 通過實(shí)驗(yàn)證明 一個(gè)注射壓力約為32 MPa是不可以制造較厚壁厚模制產(chǎn) 品 比較結(jié)果的數(shù)值分析與實(shí)驗(yàn) 獲得了最佳注塑條件 此外 結(jié)果表明 型芯偏 移應(yīng)該準(zhǔn)確地模擬一個(gè)塑料電池盒薄壁深腔注塑工藝 5 作者Ahn博士介紹 Ahn1992 年在韓國釜山國立大學(xué)獲得機(jī)械工程學(xué)士學(xué)位 他 1994 年和 2002 年在 KAIST 韓國科學(xué)技術(shù)院 又分別獲得了碩士和博士學(xué)位 Ahn 博士目前任 韓國光州的朝鮮大學(xué)機(jī)械工程系教授 Ahn 博士的研究興趣包括快速原型設(shè)計(jì) 制造 輕質(zhì)夾板 激光材料加工 及模具 機(jī)械科學(xué)與技術(shù)雜志 24 2010 145 148 作者 Dong Gyu Ahn 2009年4月24日收到手稿 修訂2009年9月21日 接受于2009年10月12日