螺紋瓶蓋注射模具設(shè)計(jì)【注塑模具】【一模四腔】【說(shuō)明書(shū)+CAD】
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湖南大學(xué)衡陽(yáng)分校畢 業(yè) 設(shè) 計(jì)課 題 瓶蓋注射模設(shè)計(jì)專 業(yè) 模具設(shè)計(jì)與制造班 級(jí) 學(xué)生姓名 指導(dǎo)老師 2006年5月20日目 錄一、 塑件的分析 (3) 二、 型腔數(shù)目的確定及排布 (5)三、 注射機(jī)的初步選擇 (6)四、 澆注系統(tǒng)的設(shè)計(jì)(7)五、 分型面與排氣槽設(shè)計(jì)(9)六、 成形零件的設(shè)計(jì) (11)七、 導(dǎo)向機(jī)構(gòu)的設(shè)計(jì) (17)八、 推出機(jī)構(gòu)的設(shè)計(jì) (19)九、 溫控系統(tǒng)的設(shè)計(jì)(19)十、 注射機(jī)的參數(shù)校核 (22)十一、設(shè)計(jì)小結(jié) (23)十二、參考文獻(xiàn) (24) 第一部分 塑件的分析一、塑件的使用要求耐用,耐磨,可以承受較大的沖擊力,不易摔壞;好看,有光澤表面較光滑;化學(xué)性質(zhì)穩(wěn)定,可以耐高溫(一般低于100oC),耐化學(xué)腐蝕。 二、塑件的材料選擇及其材料的介紹根據(jù)塑件的用途及其使用要求,選用ABS塑料。ABS的介紹:1.名稱 中文名:丙烯腈-丁二烯-苯乙烯共聚物英文名:Acrylonitrile-Butadiene-Styrene copolymer2.基本特性 無(wú)毒無(wú)味,呈微黃色,成型的塑件有較好的光澤,密度在1.021.05g/cm3,其收縮率為0.30.8%。ABS 吸濕性很強(qiáng),成型前需要充分干燥,要求含水量小于0.3%。流動(dòng)性一般,溢料間隙約在0.04mm。ABS有極好的抗沖擊強(qiáng)度,且在低溫下也不迅速下降。有良好的機(jī)械強(qiáng)度和一定的耐磨性、耐寒性、耐油性、耐水性、化學(xué)穩(wěn)定性和電氣性能。3.成型特點(diǎn) ABS在升溫時(shí)粘度增高,所以成型壓力較高,塑料上的脫模斜度宜稍大;易產(chǎn)生熔接痕,模具設(shè)計(jì)時(shí)應(yīng)注意盡量減小澆注系統(tǒng)對(duì)料流的陰力;在正常的成型條件下,壁厚、熔料溫度及收縮率影響極小。要求塑件精度高時(shí),模具溫度可控制在5060oC,要求塑件光澤和耐熱時(shí),應(yīng)控制在6080 oC。4.主要技術(shù)指標(biāo)比容:0.860.98cm3/g。 熔點(diǎn):130160oC吸水性:0.20.4% (24h)熱變形溫度:4.6105Pa- 90108oC 18.0105Pa- 83103oC屈服強(qiáng)度: 50MPa 拉伸彈性模量:1.8GPa 抗彎強(qiáng)度:80MPa5.ABS的注射工藝參數(shù) 注射機(jī)類型: 螺桿式 螺桿轉(zhuǎn)速(r/min): 3060 噴嘴形式: 直通式 噴嘴溫度(oC): 180190 料筒溫度(oC): 前 200210 中 210230 后 180200模溫(oC): 5070注射壓力(MPa): 7090保壓力(MPa): 5070注射時(shí)間(s): 35保壓時(shí)間(s): 1530冷卻時(shí)間(s): 1530成型周期(s): 4070三、 塑件的形狀及其尺寸塑件的工作條件對(duì)精度要求較低,根據(jù)ABS的性能可選擇其塑件的精度等級(jí)為5級(jí)精度(查閱塑料成型工藝與模具設(shè)計(jì)P66表3-8)。經(jīng)計(jì)算得塑件的底面積為:S塑=530.66mm2得塑件的體積為:V塑=2.49cm3塑件的質(zhì)量為:W塑 =V塑r塑=2.62(g)。塑件圖: 第二部分 型腔數(shù)目的決定及排布已知的體積V塑或質(zhì)量W塑 ,又因?yàn)榇水a(chǎn)品屬大批量生產(chǎn)的小型塑件,綜合考慮生產(chǎn)率和生產(chǎn)成本等各種因素,初步確定采用一模四腔對(duì)稱性排布。排布圖如下圖示: 型腔數(shù)目及排布圖第三部分 注射機(jī)的初步選擇一、注射量的計(jì)算:Q=42.62=10.48 (g)二、初步選擇:XS-ZY-125型注射機(jī)三、XS-ZY-125 型注射機(jī)的主要參數(shù)額定注射量(cm3): 125螺桿直徑 (mm): 42注射壓力 (MPa): 120注射行程(mm): 115注射時(shí)間(s) : 1.5注射方式 : 螺桿式 合模力 kN : 900最大注射面積(cm2): 320最大開(kāi)(合)模行程(mm): 300模具最大厚度(mm): 300模具最小厚度(mm): 200噴嘴圓弧半徑(mm): 12噴嘴孔徑(mm): 4第四部分 澆注系統(tǒng)的設(shè)計(jì)澆注系統(tǒng)的設(shè)計(jì)是注射模設(shè)計(jì)的一個(gè)重要環(huán)節(jié),它對(duì)注射成形周期和塑件質(zhì)量(如外觀,物理性能,尺寸精度等)都直接影響。一、設(shè)計(jì)時(shí)須遵循如下原則1.結(jié)合型腔布局考慮;2.熱量及壓力損失要??;3.確定均衡進(jìn)料;4.塑料耗量要少;5.消除冷料;6.排氣良好。二、澆注系統(tǒng)的組成普通流道澆注系統(tǒng)一般由主流道,分流道,澆口和冷料穴等四部分組成。三、澆注系統(tǒng)設(shè)計(jì)為使塑件去掉澆口方便,并結(jié)合物料特性,以及塑件的形狀,以采用潛伏式點(diǎn)澆口為宜。1. 主流道尺寸 根據(jù)該塑件體積及表3-10,可得體積流率Q=(42.49)/1.66.55cm3/s,取主流道中熔體流動(dòng)rs=5103s-1,由圖3-56r-Q-Rn關(guān)系曲線圖,可得Rn=4.50mm,故得主流道大端尺寸D=2Rn=9. mm,小端尺寸由注射機(jī)噴嘴尺寸,取d=4mm,SR=12+2=14mm。主流道的形狀和尺寸如圖所示:.2. 澆口套圖: 2.分流道尺寸 為使四澆口能同時(shí)進(jìn)料,各分流道按平衡式布置,故熔體在各分流道中的流速,為使分流道易于加工和頂出凝料系統(tǒng)容易,采用設(shè)在模具一邊的梯形形分流道。梯形大底邊寬度: B=0.26544.833.9=5 梯形高度: H=0.67B=3.353.為了塑件表面質(zhì)量及美觀采用側(cè)澆澆口,其尺寸 d=0.156.7=1 4.冷料穴 底部設(shè)計(jì)成帶有球頭形拉料桿的冷料穴,目的是捕集料流前鋒的“冷料”,防止“冷料”進(jìn)入型腔而影響塑件質(zhì)量。該模具澆注系統(tǒng)的尺寸如圖所示模具澆注系統(tǒng)圖第五部分 分型面與排氣槽設(shè)計(jì)分型面為定模與動(dòng)模的分界面。合理地選擇分型面是使塑件能完好的成形的先決條件。一、分型面的選擇原則1.使塑件在開(kāi)模后留在有動(dòng)模上;2.分型面的痕跡不影響塑件的外觀;3.澆注系統(tǒng),特別是澆口能合理的安排;4.使推桿痕跡不露在塑件外觀表面上;5.使塑件易于脫模。二、分型面的設(shè)計(jì) 如下圖所示: 分型面圖三、排氣槽設(shè)計(jì)當(dāng)塑料熔體填充型腔時(shí),必須順序排出型腔及澆注系統(tǒng)內(nèi)的空氣及塑料受熱或凝固產(chǎn)生的低分子揮發(fā)氣體。如果型腔內(nèi)因各種原因而產(chǎn)生的氣體不被排除干凈,一方面將會(huì)在塑件上形成氣泡、接縫、表面輪廓不清及充填缺料等成型缺陷,另一方面氣體受壓,體積縮小而產(chǎn)生高溫會(huì)導(dǎo)致塑件局部碳化或燒焦(褐色斑紋),同時(shí)積存的氣體還會(huì)產(chǎn)生反向壓力而降低充模速度,因此設(shè)計(jì)型腔時(shí)必須考慮排氣問(wèn)題。有時(shí)在注射成型過(guò)程中,為保證型腔充填量的均勻合適及增加塑料熔體匯合處的熔接強(qiáng)度,還需在塑料最后充填到的型腔部位開(kāi)設(shè)溢流槽以容納余料,也可容納一定量的氣體。通常中小型模具的簡(jiǎn)單型腔,可利用推桿、活動(dòng)型芯以及雙支點(diǎn)的固定型芯端部與模板的配合間隙進(jìn)行排氣,其間隙為0.030.05mm。第六部分 成形零件設(shè)計(jì)一、成型零件的結(jié)構(gòu)設(shè)計(jì)1.凹模 采用整體式凹模2.凸模 由于塑件帶有螺紋,為了簡(jiǎn)化模具的加工工藝,凸模設(shè)計(jì)成活動(dòng)鑲塊的形式,活動(dòng)鑲塊的固定方式如下圖所示: 活動(dòng)鑲塊的固定方式二、成型零件工作尺寸的計(jì)算1.產(chǎn)生偏差的原因.塑料的成型收縮成型收縮引起制品產(chǎn)生尺寸偏差的原因有:預(yù)定收縮率(設(shè)計(jì)算成型零部件工作尺寸所用的收縮率)與制品實(shí)際收縮率之間的誤差;成型過(guò)程中,收縮率可能在其最大值和最小值之間發(fā)生的波動(dòng)。s=(Smax-Smin)制品尺寸s成型收縮率波動(dòng)引起的制品的尺寸偏差。Smax、Smin 分別是制品的最大收縮率和最小收縮率。 .成型零部件的制造偏差工作尺寸的制造偏差包括加工偏差和裝配偏差。.成型零部件的磨損.本產(chǎn)品為抗沖制品,屬于大批量生產(chǎn)的小型塑件,預(yù)定的收縮率的最大值和最小值分別取.6%和.3。此產(chǎn)品采用級(jí)精度,屬于低精度制品。因此,凸凹模徑向尺寸、高度尺寸及深度尺寸的制造與作用修正系數(shù)x取值可在0.50.75的范圍之間,凸凹模各處工作尺寸的制造公差,因一般機(jī)械加工的型腔和型芯的制造公差可達(dá)到ITIT級(jí),綜合參考,相關(guān)計(jì)算具體如下:型腔徑向尺寸的計(jì)算:LM1=(1+(Smax+Smin)/2)sl-0.5+Z0 = (1+0.45%)26-0.50+0/30 =37.4 mmLM2=(1+(Smax+Smin)/2)s2-0.5 +Z0 = (1+0.45%)x10.2-0.50.20+0030 =14.78+0.030 mm 型腔深度尺寸的計(jì)算: 圖2:HM1=(1+(Smax+Smin)/2)Hsl-0.5 +Z0 +Z0= (1+0.45%)8.5-0.50 =12.325 mm圖1 HM2=(1+(Smax+Smin)/2)Hsl-0.5 +Z0 = (1+0.45%)4.5-0.50.15 +0.050 =6.45 +0.050 mm 型芯的徑向尺寸的計(jì)算:lM1=(1+(Smax+Smin)/2)lsl+0.50-Z = (1+0.45%)6.99+0.50.400-0.15/ =10.330-005 mmlM2=(1+(Smax+Smin)/2)lsl+0.50-Z = (1+0.45%)6.05+0.50.400-0.15/ =8.970-005 mm 型芯深度尺寸的計(jì)算: hM=(1+(Smax+Smin)/2)hsl+0.50-Z = (1+0.45%)4+0.50.150-0.15/3 =5.8750-0.08 mmCmz/2 =(1+045%)15z/2 =21.750.075 mm圖1 圖2羅紋型芯工作尺寸圖計(jì)算螺紋型芯的工作尺寸:螺紋型芯大徑: (dM大)0-z=(1+ s)ds大+中 0-z 螺紋型芯中徑: (dM中)0-z=(1+ s)ds中+中 0-z 螺紋型芯小徑: (dM小)0-z=(1+ s)ds小+中 0-zdM大, dM中, dM小 分別為螺紋型芯的大,中,小徑;ds大, ds中,ds小 分別為塑件內(nèi)螺紋大,中,小徑基本尺寸;中塑件螺紋中徑公差;z螺紋型芯的中徑制造公差,其值取/5。則 (dM大)0-z =(1+0.45%)20+0.150-0.03 =29.150-0.03 (dM中)0-z =(1+0.45%)19.7+0.150-0.03 =28.7150-0.03 (dM小)0-z =(1+0.45%)19.4+0.150-0.03 =28.280-0.03三、成型零件的強(qiáng)度、剛度計(jì)算注射模在其工作過(guò)程需要承受多種外力,如注射壓力、保壓力、合模力和脫模力等。如果外力過(guò)大,注射模及其成型零部件將會(huì)產(chǎn)生塑性變形或斷裂破壞,或產(chǎn)生較大的彈性彎曲變形,引起成型零部件在它們的對(duì)接面或貼合面處出現(xiàn)較大的間隙,由此而發(fā)生溢料及飛邊現(xiàn)象,從而導(dǎo)致整個(gè)模具失效或無(wú)法達(dá)到技術(shù)質(zhì)量要求。因此,在模具設(shè)計(jì)時(shí),成型零部件的強(qiáng)度和剛度計(jì)算和較核是必不可少的。一般來(lái)說(shuō),凹模型腔的側(cè)壁厚度和底部的厚度可以利用強(qiáng)度計(jì)算決定,但凸模和型芯通常都是由制品內(nèi)形或制品上的孔型決定,設(shè)計(jì)時(shí)只能對(duì)它們進(jìn)行強(qiáng)度校核。因在設(shè)計(jì)時(shí)采用的是整體式圓形型腔。因此,計(jì)算參考公式如下:側(cè)壁:按強(qiáng)度計(jì)算:按剛度計(jì)算:底部:按強(qiáng)度計(jì)算:按剛度計(jì)算:凸模、型芯計(jì)算公式:按強(qiáng)度計(jì)算:按剛度計(jì)算:由公式分別計(jì)算出相應(yīng)的值為:按強(qiáng)度計(jì)算得:tc=4.5mm th=6.1mm r=7mm按剛度計(jì)算得:tc=1.18mm th=0.79mm r=1.65mm 參數(shù)符號(hào)的意義和單位: Pm 模腔壓力(MPa)E 材料的彈性模量(MPa)查得2.06105; 材料的許用應(yīng)力(MPa)查得176.5;u 材料的泊松比 查表得0. 25; 成型零部件的許用變形量(mm)查得0.05;采用材料為45,調(diào)質(zhì),200HBS。第七部分 導(dǎo)向機(jī)構(gòu)的設(shè)計(jì)導(dǎo)柱導(dǎo)向機(jī)構(gòu)是保證動(dòng)定?;蛏舷履:夏r(shí),正確定位和導(dǎo)向的零件。一、 導(dǎo)柱導(dǎo)向機(jī)構(gòu)的作用1.定位件用 模具閉合后,保證動(dòng)定?;蛏舷履N恢谜_,保證型腔的形狀和尺寸精確,在模具的裝配過(guò)程中也起定位作用,便于裝配和調(diào)整。2.導(dǎo)向作用 合模時(shí),首先是導(dǎo)向零件接觸,引導(dǎo)動(dòng)定?;蛏舷履?zhǔn)確閉合,避免型芯先進(jìn)入型腔造成成型零件損壞。3.承受一定的側(cè)向壓力。二、 導(dǎo)柱導(dǎo)套的選擇1. 導(dǎo)柱導(dǎo)套結(jié)約形式及尺寸如下圖:導(dǎo)柱的結(jié)構(gòu)形式其材料采用T8A經(jīng)淬火處理,硬度為5055HRC。導(dǎo)柱、導(dǎo)套固定部分表面粗糙度Ra為08m,導(dǎo)向部分表面粗糙度Ra為0.80.4m。具體尺寸如上圖所示。導(dǎo)柱、導(dǎo)套用H7/k6配合鑲?cè)肽0濉?.導(dǎo)柱的布置采用等徑導(dǎo)柱不對(duì)稱布置,如圖所示。 導(dǎo)柱的布置形式第八部分 推出機(jī)構(gòu)的設(shè)計(jì) 一、推出機(jī)構(gòu)的組成推出機(jī)構(gòu)由推出零件、推出零件固定板和推板、推出機(jī)構(gòu)的導(dǎo)向與復(fù)位部件組成。即推件板、推件板緊固螺釘、推板固定板、推桿墊板、頂板導(dǎo)柱、頂板導(dǎo)套以及推板緊固螺釘。二、設(shè)計(jì)原則1.推出機(jī)構(gòu)應(yīng)盡量設(shè)在動(dòng)模一側(cè);2.保證塑件不因推出而變形損壞;3.機(jī)構(gòu)簡(jiǎn)單動(dòng)作可靠;4.合模時(shí)的正確復(fù)位。三、推出機(jī)構(gòu)的設(shè)計(jì)此塑件帶有螺紋,它的型芯是設(shè)計(jì)成活動(dòng)鑲塊的形式,所以可以利用活動(dòng)鑲塊來(lái)推塑件,開(kāi)模時(shí),塑件和鑲塊一起脫模,在模外,用手將塑件旋出。四、復(fù)位零件利用彈簧的彈力使推出機(jī)構(gòu)復(fù)位第九部分 冷卻系統(tǒng)設(shè)計(jì)一、注射模冷卻系統(tǒng)設(shè)計(jì)基本原則:熔體熱量95%由冷卻介質(zhì)(水)帶走,冷卻時(shí)間占成型周期的2/3。A、 冷卻系統(tǒng)從模具中帶走熱量:Q=KATt/3600 (J)A 冷卻介質(zhì)傳熱面積(m2):A=dL。 冷卻管道直徑(m); L 冷卻管長(zhǎng)度(m); K 冷卻管壁與介質(zhì)間的傳熱膜系數(shù)J/(m2hC) K=0.032x/d(vd/)0.8(gC/)0.4 . 冷卻水的平均導(dǎo)熱系數(shù)(w(/mk);f 與冷卻介質(zhì)溫度有關(guān)的物理系數(shù); g 重力加速度(m/s) v 冷卻介質(zhì)在管中流速(m/s); 冷卻介質(zhì)在該溫度下的密度kg/m3,水在30時(shí)取為0.996103kg/m3。d. 冷卻管直徑;T 模溫與冷卻介質(zhì)的平均溫差();t 冷卻時(shí)間; 水溫與f關(guān)系平均水溫202530354045f6.456.847.227.607.988.31二、冷卻管尺寸(直徑d,長(zhǎng)度L、面積A=dL)1.忽略其他散熱,冷卻介質(zhì)流量。V=WQ/(t-t) (m/min)C 介質(zhì)比熱J/kg.C,水為4.187x10;W 單位時(shí)間內(nèi)注入模具中塑料重量(kg/min);Q 塑料熔體的單位熱量(J/kg); 冷卻介質(zhì)密度(kg/ m),水為10kg/ m直徑流速流量81.665.0x103101.326.2 x10121.107.4 x10 150.879.2 x10200.6612.4 x102.經(jīng)驗(yàn)確實(shí)管道直徑:3.冷卻水對(duì)其通道表壁傳熱系數(shù)的簡(jiǎn)化公式:當(dāng)冷卻水平均溫度在0C以上,e=6 x10時(shí),其計(jì)算結(jié)果產(chǎn)生誤差在以內(nèi):K=2041x(1-0.015)v/d 為冷卻水平均溫度;4.計(jì)算流量 V=W Q/( =2.62x10 初步確定冷卻水道 d=8mm;流速為 =Q/d =1.31x10m/s管子的長(zhǎng)度L=60WQ/k 熱傳導(dǎo)面的平均溫度與冷卻水平均溫度的差值,其中冷卻的平均溫度為冷卻水在進(jìn)口處和出口處溫度的平均值。第十部分 注射機(jī)的參數(shù)校核一、塑件在分型面上的投影面積與鎖模力校核注射成型時(shí),塑件在模分型面的投影面積是影響鎖模力的主要因素,其數(shù)值越大,需鎖模力也就越大,若超過(guò)注射機(jī)的允許最大成型面積,則在成型過(guò)程中會(huì)出現(xiàn)漲模溢料現(xiàn)象。因此有:塑件總的投影面積nA與澆注系統(tǒng)的投影面積之和要小于最大成型面積 A。 nA+AA4x5.3+4x0.5x4=29.2320cm2 滿足要求應(yīng)使塑料熔體對(duì)型腔的成型壓力與塑件和澆注系統(tǒng)在分型面上的投影面積之和的乘積小于注射機(jī)額定鎖模力:(nA+A)PFT=29.2x3.5=102.2kN900kN 滿足要求二、 模具厚度校核模具厚度H必須滿足:HminHHmax式中 Hmin注射機(jī)允許的最小模厚,即動(dòng),定模板之間的最小開(kāi)距;Hmax注射機(jī)允許的最大模厚。H=200mm,H=60mm,M=300mm。符合條件。三、 開(kāi)模行程校核由于注射模最大開(kāi)模行程S與模厚無(wú)關(guān),因此有:SH1+H2 +(510)mm式中 H1推出距離(脫模距離)(mm);H2包括澆注系統(tǒng)凝料在內(nèi)的塑件高度(mm);H1=23mm, H2=55mm,所以s=88mm,遠(yuǎn)小于注射機(jī)的最大開(kāi)模行程300mm,合適。綜上所述,所選擇的注射機(jī)滿足注射要求。第十一部分 設(shè)計(jì)總結(jié)通過(guò)這次系統(tǒng)的注射模的設(shè)計(jì),我更進(jìn)一步的了解了注射模的結(jié)構(gòu)及各工作零部件的設(shè)計(jì)原則和設(shè)計(jì)要點(diǎn),了解了注射模具設(shè)計(jì)的一般程序。進(jìn)行塑料產(chǎn)品的模具設(shè)計(jì)首先要對(duì)成型制品進(jìn)行分析,再考慮澆注系統(tǒng)、型腔的分布、導(dǎo)向推出機(jī)構(gòu)等后續(xù)工作。通過(guò)制品的零件圖就可以了解制品的設(shè)計(jì)要求。對(duì)形態(tài)復(fù)雜和精度要求較高的制品,有必要了解制品的使用目的、外觀及裝配要求,以便從塑料品種的流動(dòng)性、收縮率,透明性和制品的機(jī)械強(qiáng)度、尺寸公差、表面粗糙度、嵌件形式等各方面考慮注射成型工藝的可行性和經(jīng)濟(jì)性。模具的結(jié)構(gòu)設(shè)計(jì)要求經(jīng)濟(jì)合理,認(rèn)真掌握各種注射模具的設(shè)計(jì)的普遍的規(guī)律,可以縮短模具設(shè)計(jì)周期,提高模具設(shè)計(jì)的水平。第十二部分 參考文獻(xiàn)參考資料:1. 塑料成型工藝與模具設(shè)計(jì)/屈華昌主編.北京:機(jī)械工業(yè)出版社,19962.模具制造工藝/黃毅宏、李明輝主編.北京:機(jī)械工業(yè)出版社,1999.6 3. 典型零件模具圖冊(cè)/何忠保,陳曉華,王秀英主編.北京:機(jī)械工業(yè)出版社,2000.94. 塑料模具技術(shù)手冊(cè)/塑料模具技術(shù)手冊(cè)編委會(huì)編.北京:機(jī)械工業(yè)出版社,1997.65. 塑料模具設(shè)計(jì)手冊(cè)/塑料模具設(shè)計(jì)手冊(cè)編寫組編著.第二版.北京:機(jī)械工業(yè)出版社199425編號(hào): 畢業(yè)設(shè)計(jì)(論文)外文翻譯(原文)學(xué) 院: 機(jī)電工程學(xué)院 專 業(yè): 機(jī)械設(shè)計(jì)制造及其自動(dòng)化 學(xué)生姓名: 韋良華 學(xué) 號(hào): 1000110129 指導(dǎo)教師單位: 機(jī)電工程學(xué)院 姓 名: 陳虎城 職 稱: 助教 2014年 5 月 26 日a r t i c l e i n f oArticle history:Received 25 October 2010Received in revised form12 January 2011Accepted 14 January 2011Available online 21 January 2011Keywords:Microcellular injection moldingPlastic foamingSwirl-free surfacea b s t r a c tMicrocellular injection molding is the manufacturing method used for producing foamed plastic parts.Microcellular injection molding has many advantages including material, energy, and cost savings as well as enhanced dimensional stability. In spite of these advantages, this technique has been limited by its propensity to create parts with surface defects such as a rough surface or gas flow marks. Methods for improving the surface quality of microcellular plastic parts have been investigated by several researchers. This paper describes a novel method for achieving swirl-free foamed plastic parts using the microcellular injection molding process. By controlling the cell nucleation rate of the polymer/gas solution through material formulation and gas concentration, microcellular injection molded parts free of surface defects were achieved. This paper presents the theoretical background of this approach as well as the experimental results in terms of surface roughness and profile, microstructures, mechanical properties, and dimensional stability.l IntroductionThe commercially available microcellular injection molding process (a.k.a. the MuCell Process) consists of four distinctive steps, namely, gas dissolution, nucleation, cell growth, and shaping 1. In the gas dissolution stage, polymer in the injection barrel is mixed with supercritical fluid (SCF) nitrogen, carbon dioxide, or another type of gas using a special screw which is designed to maximize the mixing and dissolving of the gas in the polymer melt. During injection, a large number of nucleation sites (orders of magnitude higher than conventional foaming processes) are formed by a rapid and substantial pressure drop as the polymer/gas solution is injected into the mold cavity, thus causing the formation of cells (bubbles). During the rest of the injection molding cycle, cells continue to grow to fill and pack out the mold and subsequently compensate for the polymer shrinkage as the material cools inside the mold. The cell growth is driven by the amount and spatial distribution of the dissolved gas. The cell growth is also controlled by processing conditions such as melt pressure and temperature as well as material properties such as melt strength and gas solubility. Finally, the shaping of the part takes place inside the mold until the mold opens allowing the part to be ejected.Since the microcellular injection molding process was invented, there have been numerous studies on process, material, and technical developments aimed at materializing the full process potential. According to previous studies 1-5, microcellular injection molding offers a number of advantages such as cost savings, weight reduction, ease in processing due to low viscosity, and outstanding dimensional accuracy. Due to these advantages, the microcellular injection molding process has been used in many industries such as automotive, electrical goods, and home appliances using a broad range of thermoplastics. Despite these advantages, however, the surface imperfections associated with microcellular injection molded partsdsuch as unique gas flow marks, referred to as swirl marks throughout this paper, and a lack of smoothnessdstill remain one of the main drawbacks surrounding microcellular injection molding. In order to eliminate or reduce these surface imperfections there have been several studies attempted, as reported in Refs. 6-14. Some researchers have focused on temperature modification of the mold surface to improve the surface quality of microcellular injection molded parts 6-8. With polymeric foam, it was found that bubbles forming at the advancing melt front are first stretched by the fountain flow behavior toward the mold surface and subsequently dragged against the mold wall causing swirl marks 9. During the filling stage of polymer melts, keeping the mold wall temperature high enough for bubbles at the mold surface to beeliminated improves the surface quality of microcellular injection molded parts. By controlling the mold temperature rapidly and precisely using mold temperature control units or other kinds of thermal or surface heating devices, microcellular foamed plastics with glossy and swirl-free surfaces can be produced. There have also been efforts to eliminate the swirl marks on microcellular injection molded parts without any mold temperature controller. In particular, it was proposed that inserting an insulator onto the mold wall might help keeping the interface temperature between the mold and the polymer melt high. This technique basically yields the same result as temperature modification of the mold 10. Thermal analysis and experimental results prove that the addition of an insulator layer on the mold can improve the surface quality of microcellular injection parts 11. Another method of producing parts with an improved surface quality leads to a microcellular co-injection molding process 12. In this technique, a proper amount of solid skin material is injected prior to the injection of a foaming core material. This can yield a sandwiched (solid skinefoamed coreesolid skin) structure with a surface finish similar to a conventionally molded component while partially maintaining the advantages of microcellular injection molding.Another approach for improving the surface quality of microcellularinjection molded parts is the gas counter pressure process 13,14. In this process, a high-pressure gas is injected into the mold prior to the polymer/gas solution to suppress cell nucleation and bubble growth while the polymer/gas solution is being injected into the mold cavity. Toward the end of injection, counter gas pressure is released and bubbles begin to form within the cavity. Since a majority of the part surface is already solidified, gas flow marks are eliminated. In spite of these efforts to improve the surface quality, there have been difficulties in applying the microcellular injection molding process in industries requiring parts with high surface qualities because these techniques entail additional equipment which results in high costs or maintenance. There have been no reported studies on improving the surface quality of microcellular injection molded parts without any additional equipment or modification to existing equipment.This paper proposes a novel approach to improve the surface quality of microcellular injection molded parts by controlling the cell nucleation rate. In this study, the cell nucleation rate was dramatically lowered or delayed by controlling the degree of supersaturation so that cell nucleation was delayed during the filling stage. After the polymer/gas solution volumetrically filled the mold cavity, intentionally delayed nucleation occurred and bubbles formed in the polymer matrix, except on the surface where the material had already solidified upon touching the mold surface. Theoretical background and experimental results are described in this paper. Microstructure, surface profile, surface roughness,mechanical properties, and dimensional stability are also investigated in this study.2. Theoretical2.1. Nucleation theory for polymeric foamsIn polymeric foams, nucleation refers to the initial stage of the formation of gas bubbles in the polymeregas solution. For nucleation,gas bubbles must overcome the free energy barrier before they can survive and grow to macroscopic size 15. According to classical nucleation theories 16-18, the nucleation rate is controlled by the macroscopic properties and states of the polymer and gas such as solubility, diffusivity, surface tension, gas concentration, temperature, and the degree of super saturation. One representative equation for the nucleation rate of polymeric foams was reported by Colton and Suh 19,20. In addition to the mathematical representation, they also verified their nucleation theory experimentally for a batch foaming process using a high pressure vessel. The nucleation equation for microcellular foams dominated by the classical nucleation theory 16e18 can be expressed asN=fCex(-G*/kT)where N is the nucleation rate, f is the frequency of atomic molecular lattice vibration, C is the concentration of gas molecules, k is the Boltzmanns constant, T is the absolute temperature, and G*is the activation energy barrier for nucleation.According to previous studies 19,20, the nucleation rate of polymeric foams is composed of two components: a homogeneous term and a heterogeneous term. The activation energy for homogeneous nucleation is given byGhom*16r33P2where g is the surface energy of the bubble interface and P.isassumed to be the gas saturation pressure. More precisely,P=|Pr-Pr| where Pr is the pressure that is exerted in a highpressure vessel and Pr is the pressure of the supersaturated vapor inthe sample 16. That is, DP is the pressure difference between thepressure that is applied to the sample and the pressure of the supersaturated vapor in the sample. When the pressure that saturatesthe gas in a high pressure vessel is suddenly released to trigger the so-called thermodynamic instability by rendering the sample into the supersaturated state, Pr becomes 1 bardso low compared to Pr that DP can be approximated as Pr. On the other hand, the activation energy for heterogeneous nucleation is affected by a geometric factor that depends on the contact (wetting) angle between the polymer and the particle and can be expressed asGhet*=Ghom*f() (3a) f=12-34cos+14cos3 (3b)where f(q) is a geometric factor that is dependent upon the contactangle, , of the interface between the polymer and a second phase,and has values of less than or equal to 1. For a typical wetting angleof around 200 on the interface between a solid particle and the polymer melt, the geometric factor is 2.7X10-3, suggesting that the energy barrier for heterogeneous nucleation can be reduced by three orders of magnitude with the presence of an interface 20,21.l 2.2. Nucleation theory for microcellular injection molding In the batch foaming process, the theory of Colton and Suh was verified by their experiments. Due to the large difference between the pressure exerted in a high pressure vessel and the pressure of the supersaturated vapor in the sample, the gas pressure dissolved in the polymer, theP in the Gibbs free energy equation, can be approximately assumed to be the saturation gas pressure. The assumption that P is the gas saturation pressure is fairly reasonable in a batch foaming process although the Pcan still have an error of about 30-40% due to overestimation as reported in a previous study 15. The nucleation theory by Colton and Suh is a simplified form derived and modified from classic nucleation theories 16-18 and is generally adequate for the batch foaming process. However, there is a need for this theory to be modified in cases of microcellular injection molding and extrusion systems because P cannot be directly controlled and measured. To predict nucleation in microcellular injection molding and extrusion processes more precisely, this paper proposes a cell nucleation theory of a different form, which includes a term for the degree of supersaturation because it is a directly controllable factor. To avoid misestimating P, and to consider the degree of supersaturation, a more proper activation energy equation for nucleation can be derived from the following equation 16,17P=|Pr-Pr|=2rrc (4)where rc is the radius of a characteristic droplet, and the W.Thomson equationRTlnPrP=2rMrp (5)where P is the pressure of the saturated vapor (i.e., the equilibriumpressure), R is the universal gas constant, M is the molar mass, and p is the density. These equations yieldP=RTlnPrPM (6)which can be alternatively expressed asP=kt1lnS (7)where1is the molecular density of the bulk liquid, and S(=PrP)is defined as the degree of supersaturation.Thus, the activation energy equation (cf. Equation (2) for nucleation in the microcellular injection molding process can be given byG*=16r33(kT1lnS)2 (8)Hence it can be stated that the activation energy for nucleation is inversely proportional to the square of the natural logarithm of the supersaturation degree.In the microcellular injection molding process, the polymer/gassolution becomes a metastable supersaturation solution when it isinjected into the mold cavity. This is because the amount of gas able to be dissolved in the polymer in the presence of a rapid pressure drop is less than the gas amount originally dissolved in polymer melts. In particular, assuming the air in the cavity is properly vented, the pressure at the advancing melt front is at the atmospheric pressure. The solubility of a gas in a polymer at atmospheric pressure and processing temperature can be obtained by an Arrhenius-type expression with regard to temperature 22S1 atm; melt temperature=SSTPexp(-HsR(1Tmelt-1298) (9)where SSTP is the solubility of the gas in the polymer at standardtemperature and pressure conditions (298 K and 1 atm). The parameter DHs is the molar heat of sorption, and Tmelt is the polymer melt temperature. Thus, the degree of supersaturation is given byS=mgSSTPexp(-HsR(1Tmelt-1298) (10)where mg is the gas dosage which can be controlled by the supercriticalfluid (SCF) supply system. The heat of sorption, HsRg, of various polymer/gas systems at standard temperature has been studied and summarized in many previously published studies. In order to obtain the degree of supersaturation for a polymer/gas solution in the microcellular injection molding process, one has to either measure the solubility of the gas in the polymer at standard temperature and pressure or consult published data on the solubility of the gas in the polymer. Then, the activation energy barrier for nucleation in Equation (8), G*, can be obtained based on the calculated degree of supersaturation and the surface energy of the bubble interface, . Given the activation energy barrier and the frequency factor, f, the nucleation rate (expressed in Equation (1) can then be calculated.The estimate of the surface energy of the bubble interface and the frequency factor is discussed below. In microcellular injection molding, the polymer/gas solution canbe treated as a liquid mixture. Thus, the surface energy of thebubble interface, g, can be expressed as 23,24mix=polymermixpolymer4(1-wgas) (11)where polymer is the surface energy of the polymer, PS are thedensities, and wgas is the weight fraction of gas. In addition, a frequency factor for a gas molecule, f, in Eq. (1) canbe expressed as 24-26f=Z(4rc2) (12)where z is the Zeldovich factor, which accounts for the many clusters that have reached the critical size, rc., but are still unable to grow to sustainable bubbles. The parameter b is the impingement rate at which gas molecules collide with the wall of a cluster. The parameter Zcan be used as a correction factor and is determined experimentally. Once the nucleation rate as a function of the degree of supersaturationis obtained, one can control the gas (SCF) content in the polymer melt to control or delay the onset of cell nucleation so that no bubble will form at the advancing melt front during the injection filling stage, thus, allowing microcellular parts with solid, swirl-free surface to be injection molded.3. Experimental3.1. MaterialsThe material used in this study was an injection molding gradelow density polyethylene, LDPE (Chevron Phillips Chemical Company, Texas, USA). It has a melt index of 25 g/10 min and a density of 0.925 g/cm3. To confirm the theory for improving surface quality by controllingthe degree of supersaturation, a random copolymer polypropylene (PP)was also used in this study. The PP used in this study was Titanpro SM668 (Titan Chemicals Corp., Malaysia), with a melt flow index of 20 g/10 min and a density of 0.9 g/cm3. Both materials were used as received without any colorant, fillers, or additives.Commercial grade nitrogen was used as a physical blowing agent for the microcellular injection molding trials.3.2. Microcellular injection moldingIn this study, an Arburg 320S injection molding machine (Arburg,Germany) was used for both the solid conventional and microcellular injection molding experiments. The supercritical fluid (SCF) supply system used in this study was the S11-TR3 model (Trexel, Woburn,MA, USA). The total gas dosagewas controlled by adjusting the gas injection time, t, and the gas injection flowrate,m_ g. A tensile test mold, which produces tensile test specimens that meet the ASTM D638 Type I standards, was used for this experiment.For injectionmolding of both LDPE and PP tensile test specimens,nozzle and mold temperatures were set at 221 。C and 25 。C, respectively. The cycle time was 40 s. An injection speed of 80 cm3/s was employed. In this study, six different gas dosages (concentrations) were used for injection molding of LDPE as shown in Table 1. Also, four different gas dosages were employed for microcellular injection molding of PP. The supercritical fluid was injected into the injection barrel at 140 bar pressure to be mixed with the polymer melts in this experiment. The weight reduction of foamed versus solid plastic partswas targeted at 8 _ 0.5% for each specimen. For the conventional injectionmolding experiment, the shot size of 20.2 cm3 and a packing pressure of 800 bars were employed for 6 s. For the microcellular injection molding experiments, the shot size of the polymer melt was 19.2 cm3 and the packing stage was eliminated.3.3. Analysis methodsTo analyze the surface roughness of the molded tensile bar specimens, a Federal Surfanalyzer 4000 (Federal Product Corporation, RI, USA)was used. The surface roughnesses of conventional and microcellular injection molded parts were evaluated at three locations shown in Fig. 1 and the averaged surface roughness based on measurementsdone at all three locationswas recordedandreported. The cutoff, drive speed, and drive length for the test were 0.75 mm, 2.5 mm/s, and 25 mm, respectively. For each process condition, ten specimens and three points on each specimen were tested.In addition to the surface roughness, swirl marks commonly observed in microcellular injection molded samples can also be clearly revealed by a 3-D surface profiler. Zygo NewView (Zygo Corporation, CT, USA), a non-contact 3-D surface profiler, was employed to examine the surface profile of injection molded parts in this study using a scan distance of 10 mm. A JEOL JSM-6100 scanning electron microscope with an acceleratingvoltage of 15 kV was employed for observing the microstructures of the foamed parts. To observe the cross section of the microcellular injection molded parts, test specimens were frozen by liquid nitrogen and subsequently fractured. Representative images of each process condition were selected and cell sizes and densities were analyzed. A UTHSCSA Image Tool was employed as the ima
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