瓶蓋注射模具設計【注塑模具】【一模兩腔】【說明書+CAD】
瓶蓋注射模具設計【注塑模具】【一模兩腔】【說明書+CAD】,注塑模具,一模兩腔,說明書+CAD,瓶蓋注射模具設計【注塑模具】【一模兩腔】【說明書+CAD】,瓶蓋,注射,模具設計,注塑,模具,說明書,仿單,cad
湖南大學衡陽分校
畢業(yè)設計
題目 瓶蓋塑模設計說明書
學生姓名
指導教師（簽名）
2006 年 5 月
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
目 錄
一、 塑件的分析 …………………………………………… (1)
二、 塑件的形狀尺寸………………………………………… (2)
三、 形腔數(shù)目的決定及排布………………………………… (3)
四、 分型面的選擇…………………………………………… (4)
五、 澆注系統(tǒng)的設計………………………………………… (5)
六、 注射機的型號和規(guī)格校核…………………………… (6)
七、 成型零部件的工作尺寸計算…………………………… (7)
八、 導柱導向機構的設計………………………………… (11)
九、 推出機構的設計 …………………………………… (13)
十、 溫控系統(tǒng)的設計 ………………………………… (14)
十一、設計小結(jié) ………………………………………… (16)
十二、參考文獻 ………………………………………… (16)
共 16 頁 第1頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
第一部分 塑料PS
PS：聚苯乙烯
一、基本特性：
聚苯乙烯無色透明、無毒無味。落地有清脆的金屬聲、密度為1.054g/cm。聚苯乙烯的力學性能與聚合方法、相對分子質(zhì)量大小、定向度和質(zhì)量有關，相對分子質(zhì)量越大機械強度越高。它有優(yōu)良的電性能（尤其是高頻絕緣性能）和一定的化學穩(wěn)定性。它能耐堿、硫酸、磷酸、10%-30%的鹽酸、稀醋酸及其他的有機酸。但不耐硝酸及氧化劑的作用，對水、乙醇、汽油、植物油及各種鹽溶液也有足夠的抗蝕能力。但耐熱性能低，熱變形溫度一般在70-80度，只能在不高的溫度下作用。
主要用途：
在工業(yè)中做儀表、燈罩、化學儀器、零件、透明模型等。在電器方面做良好的絕緣材料、接線盒電池盒等。在日用品方面廣泛的用于包裝材料、各種容器、玩具等。
成型特性：
1. 無定形料、吸濕性小，不易分解，性脆易裂，熱膨脹系數(shù)大，易產(chǎn)生應力開裂。
2. 流動性能較好，溢邊值0.03mm左右。
3. 塑件壁后均勻，不宜有鑲件，缺口，尖角，各方面應圓滑連接。
4. 可用螺桿或柱塞式注射機加工，噴嘴可用直通式或自鎖式。
5. 宜用高料溫、模溫、低注射壓力，延長注射時間有利降低內(nèi)應力，防止縮孔，變形，但是溫高易出銀絲，料溫低或脫模劑多則透明性差。
6. 可采用各種形式進料口、進料口與塑件應圓弧連接，防止去除澆口時損壞塑件；脫模斜度宜取2度以上，頂出均勻以防止脫模不良發(fā)生開裂、變形、可用熱澆道系統(tǒng)。
綜合性能：
比熱容： 1340J/（kg.k）
熱變形溫度： 65-950oC
抗拉屈服強度: 35-63Mpa
拉伸彈性模量： 2.8-3.5%Gpa
抗彎強度： 61-98Mpa
抗壓強度： 80-112MPa
斷裂伸長率 ： 1.0%
抗拉強度： 35-63MPa
PS的注射工藝參數(shù)：
注射機類型： 柱塞式
參見《塑料
·橡膠成型
模具設計手冊》表1-2-2 P16
參見《塑
料成型工藝與模具設計》表3-1 P55
共 16 頁 第2頁
計 算 內(nèi) 容
說 明
螺桿轉(zhuǎn)速： 48（r/min）
噴嘴形式： 直通式
噴嘴溫度： 160-170 oC
料筒溫度：前段 170-190 oC
后段 140-160 oC
模具溫度： 20-60 oC
注射壓力： 60-100 Mpa
保壓力： 30-40 Mpa
注射時間： 0-3 S
保壓時間： 15-40 S
冷卻時間： 15-30 S
成型周期： 40-90 S
適用注射機類型： 螺桿、柱塞均可
第二部分 塑件的形狀尺寸
塑件圖如下所示：
塑件圖
湖南大學衡陽分校畢業(yè)設計
共 16頁 第 3 頁
計 算 內(nèi) 容
說 明
塑件的工作條件對精度要求較高，根據(jù)ABS的性能可選擇其塑件的精度等級為3級精度（查閱《塑料成型工藝與模具設計》P67表3-9）。
得塑件的體積為：V塑=9.825cm3
塑件的質(zhì)量為：W塑 =V塑×r塑=12.56(g)。
第三部分 型腔數(shù)目的決定及排布
已知的體積V塑或質(zhì)量W塑 ，又因為此產(chǎn)品屬大批量生產(chǎn)的塑件，但制件尺寸、精度、表面粗糙度較高，綜合考慮生產(chǎn)率和生產(chǎn)成本及產(chǎn)品質(zhì)量等各種因素，以及注射機的型號選擇，初步確定采用一模兩腔對稱性排布,分流道直徑可選1.5~9.5mm。由塑件的外形尺寸和機械加工的因素，確定采用側(cè)澆口，根椐塑件的材料及尺寸，澆口直徑可選1.4~1.8mm。排布圖如下圖示：
型腔數(shù)目及排布圖
第四部分 分型面的選擇
塑件冷卻時會因為收縮作用而包覆在凸模上，故從塑件脫模件精度要角度考慮，應有利于塑件滯留在動模一側(cè)，以便于脫模，而且不影響塑件的質(zhì)量和外觀形狀，以及尺寸精度。
湖南大學衡陽分校畢業(yè)設計
共 16頁 第 4 頁
計 算 內(nèi) 容
說 明
分型如下圖：
分型面圖
第五部分 澆注系統(tǒng)的初步估計
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。
澆注系統(tǒng)的設計應保證塑件熔體的流動平穩(wěn)、流程應盡量短、防止型芯變形、整修應方便、防止制品變形和翹曲、應與塑件材料品種相適用、冷料穴設計合理、盡量減少塑料的消耗。
根據(jù)塑件的形狀采用推桿推出。由于采用復式點澆口，雙分型面，分流道采用半圓形截面，分流道開設在中間板上，在定模固定板上采用澆口套, 不設置冷料穴和拉料桿。
澆注系統(tǒng)圖
注射機型
號參見《塑
料成型工藝與模具設計》表4-1
P100
湖南大學衡陽分校畢業(yè)設計
共 16頁 第 5 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
根據(jù)塑件的外形尺寸和質(zhì)量等決定影響因素，初步取值如下：
d=4mm D=6mm R=15mm h=5mm d1=1mm
H1=4.5mm l=60~70mm L=20mm a=4。
初步估算澆注系統(tǒng)的體積，V澆=8~9cm3。
其質(zhì)量約為：W澆=V澆×r塑=7.5~8.5g。
S=(n×W塑+ W澆) /0.8=23~24g。
所以，選擇用注射機型號為：XS-Z-60。
第六部分 注射機的型號和規(guī)格
注射機的技術規(guī)格如下:
型號： SX-Z-60
額定注射量(cm3)： 60
柱塞直徑(mm)： 38
注射壓力 (MPa)： 122
注射行程（mm）： 170
注射時間（s）： 2.9
注射方式： 柱塞式
最大成型面積(cm3)： 130
合模力（kN）： 500
最大開（合）模行程（mm）： 180
模具最大厚度（mm）： 200
模具最小厚度（mm）： 70
模板最大距離（mm）： 380
動、定模固定板尺寸（mm）： 330×440
合模方式： 液壓-機械
電動機功率（kw）： 11
加熱功率（kw）： 2.7
機械外型尺寸（mm）： 3160×850×1550
噴嘴圓弧半徑（mm）： 12
噴嘴孔徑（mm）： 4
噴嘴移動距離（mm） 120
收縮率見《塑料成型工藝與
模具設計》
附錄B
計算參考
《塑料成型工藝與
模具設計》
第五章第三節(jié) P151
共 16 頁 第 6 頁
計 算 內(nèi) 容
說 明
第七部分 成型零部件的工作尺寸計算
1、產(chǎn)生偏差的原因：
①．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：預定收縮率（設計算成型零部件工作尺寸所用的收縮率）與制品實際收縮率之間的誤差；成型過程中，收縮率可能在其最大值和最小值之間發(fā)生的波動。
σs=(Smax-Smin)×制品尺寸
σs ——成型收縮率波動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。
②.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。
③．成型零部件的磨損
２、本產(chǎn)品為LDPE制品，屬于大批量生產(chǎn)的小型塑件，預定的收縮率的最大值和最小值分別取0.4%和0.8％。平均收縮率sˉ 為0.6%,此產(chǎn)品采用6級精度，屬于一般精度制品。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0.5~0.75的范圍之間，凸凹模各處工作尺寸的制造公差，因一般機械加工的型腔和型芯的制造公差可達到IT７～IT８級，綜合參考，相關計算具體如下：
圖一
(LM1 )0+δz =[ (1+ sˉ )LM1s - 0.5×Δ]0+δz
=[ (1+0.6%)×60- 0.5×0.48]0 +0.48/4
= 60.640+0.12mm
湖南大學衡陽分校畢業(yè)設計
共 16 頁 第 7 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
(HM) 0+δz =[ (1+ sˉ )HMS-0.5×Δ] 0+δz
=[ (1+0.6%)×25-0.5×0.4]0+0.4/4
=25.3750+0.1 mm
(C1)±δz/2 =(1+ sˉ )C1S±δz/2
=(1+0.6%)×5±0.32/2
=5.07±0.16 mm
圖二
(hQ1) 0-δz =[(1+ sˉ )hQ1s+0.5×Δ] 0-δz
=[ (1+0.6%)×18-0.5×0.32] 0-0.32/4
=18.19 0-0.08
(lQ1) 0+δz =[ (1+ sˉ )LQ1s - 0.5×Δ]0+δz
=[ (1+0.6%)×18-0.5×0.32]+0.32/40
=18.19+0.080
共 16 頁 第 8 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
(lM1) 0-δz =[ (1+ sˉ )lM1S+0.5×Δ] 0-δz
=[ (1+0.6%)×56+0.5×0.48]0-0.48/4
=56.850-0.12
計算螺紋型芯的工作尺寸：
（1）螺紋型芯大徑： (dM大)0-δz=[(1+ sˉ )ds大+Δ中] 0-δz
螺紋型芯中徑： (dM中)0-δz=[(1+ sˉ )ds中+Δ中] 0-δz
螺紋型芯小徑： (dM小)0-δz=[(1+ sˉ )ds小+Δ中] 0-δz
dM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；
Δ中———塑件螺紋中徑公差；
δz———螺紋型芯的中徑制造公差，其值取Δ/5。
則 (dM大)0-δz =[(1+0.6%)×56+0.03]0-0.03/5
=56.580-0.006
計算參考
《塑料成型工藝與
模具設計》
第五章第三節(jié) P153
共 16 頁 第 9 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
(dM中)0-δz =[(1+0.6%)×55.401+0.03]0-0.03/5
=55.7490-0.006
(dM小)0-δz =[(1+0.6%)×54.135+0.03]0-0.03/5
=54.5610-0.006
3、成型零件的強度、剛度計算
注射模在其工作過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。如果外力過大，注射模及其成型零部件將會產(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，從而導致整個模具失效或無法達到技術質(zhì)量要求。因此，在模具設計時，成型零部件的強度和剛度計算和校核是必不可少的。
一般來說，凹模型腔的側(cè)壁厚度和底部的厚度可以利用強度計算決定，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，設計時只能對它們進行強度校核。
因在設計時采用的是鑲嵌式圓形型腔。因此，計算參考公式如下：
側(cè)壁：
按強度計算：
按剛度計算：
底部： 按強度計算：
按剛度計算：
共 16 頁 第 10 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
凸模、型芯計算公式：
按強度計算：
按剛度計算：
由公式分別計算出相應的值為：
按強度計算得：tc=4.93mm th=4.38mm r=8.52mm
按剛度計算得：tc=0.93mm th=1.91mm r=3.97mm
參數(shù)符號的意義和單位：
Pm 模腔壓力（MPa）取值范圍50~70；
E 材料的彈性模量（MPa）查得2.06×105；
σp 材料的許用應力（MPa）查得176.5；
u 材料的泊松比 查表得0.025；
δp 成型零部件的許用變形量（mm）查得0.05；
采用材料為3Gr2W8V，淬火中溫回火，≥ 46HRC。
第八部分 導柱導向機構的設計
導柱導向機構是保證動定?；蛏舷履：夏r，正確定位和導向的零件。
一、 導柱導向機構的作用：
1、 定位件用：模具閉合后，保證動定模或上下模位置正確，保證型腔的形狀和尺寸精確，在模具的裝配過程中也起定位作用，便于裝配和調(diào)整。
共 16 頁 第 11 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
2、 導向作用：合模時，首先是導向零件接觸，引導動定模或上下模準確閉合，避免型芯先進入型腔造成成型零件損壞。
3、 承受一定的側(cè)向壓力。
二、 導柱導套的選擇：
一般在注射模中，動、定模之間的導柱既可設置在動模一側(cè)，也可設置在定模一側(cè)，視具體情況而定，通常設置在型芯凸出分型面最長的那一側(cè)。根據(jù)我們所要進行的設計的 模具的要求我們所用的導柱導套如下圖所示：
模具結(jié)構圖
H——中間板的厚度（mm）。
s1=46.5mm s=49.5~51.5mm H=34.5mm L≥92mm
定端與模板間用H7/m6或H7/k6的過渡配合,導向部分通常采用H7/f7或H8/f7的間隙配合。
布局形式如圖示：
共 16 頁 第 12 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
導柱排布圖
第九部分 推出機構的設計
1、 推出機構的組成
推出機構由推出零件、推出零件固定板和推板、推出機構的導向與復位部件組成。即推件板、推件板緊固螺釘、推板固定板、推桿墊板、頂板導柱、頂板導套以及推板緊固螺釘。
2、 設計原則：
a、 推出機構應盡量設在動模一側(cè)；
b、 保證塑件不因推出而變形損壞；
c、 機構簡單動作可靠；
合模時的正確復位。
3、脫模力的計算：
根據(jù)力平衡原理，列出平衡方程式：
∑Fx=0
Ft+Fbsinα=Fcosα
Fb 塑件對型芯的包緊力；
F 脫模時型芯所受的摩擦力；
Ft 脫模力；
Α 型芯的脫模斜度。
又： F=Fbμ
于是 Ft=Fb(μcosα-sinα)
而包緊力為包容型芯的面積與單位面積上包緊力之積，即：Fb=Ap
由此可得：Ft=Ap(μcosα-sinα)
共 16 頁 第 13頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
式中： μ為塑料對鋼的摩擦系數(shù)，約為0.1~0.3；
A為塑件包容型芯的總面積；
p為塑件對型芯的單位面積上的包緊力，在一般情況下，模外冷卻的塑件p取2.4~3.9×107Pa;模內(nèi)冷卻的塑件p約取0.8~1.2×107Pa。
所以:經(jīng)計算，A=615.44mm2 ,μ取0.25,p取1×107Pa,取α=45′。
Ft=615.44×10-6×1×107(0.25×cos45′-sin45′)
=1457.91N。
因此，脫模力的大小隨塑件包容型芯的面積增加而增大，隨脫模斜度的增加而減小。由于影響脫模力大小的因素很多，如推出機構本身運動時的摩擦阻力、塑料與鋼材間的粘附力、大氣壓力及成型工藝條件的波動等等，因此要考慮到所有因素的影響較困難，而且也只能是個近似值。
4、 用推件板推出機構中，為了減少推件板與型芯的摩擦，在推件板與型芯間留0.20~0.25mm的間隙，并用錐面配合，防止推件因偏心而溢料。
5、 零件：
可以在推板上設置復位桿使能夠精確的復位。
6、 排氣系統(tǒng)：
當塑料熔體填充型腔時,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)生高溫會導致塑件局部碳化或燒焦（褐色斑紋），同時積存的氣體還會產(chǎn)生反向壓力而降低充模速度，因此設計型腔時必須考慮排氣問題。有時在注射成型過程中，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度，還需在塑料最后充填到的型腔部位開設溢流槽以容納余料，也可容納一定量的氣體。
通常中小型模具的簡單型腔，可利用推桿、活動型芯以及雙支點的固定型芯端部與模板的配合間隙進行排氣，其間隙為0.03~0.05mm。
第十部分 溫控系統(tǒng)設計
基本原則：熔體熱量95%由冷卻介質(zhì)（水）帶走，冷卻時間占成型周期的2/3。
注射模冷卻系統(tǒng)設計原則：
1.冷卻水道應盡量多、截面尺寸應盡量大 型腔表面的溫度與冷卻水道的數(shù)量、截面
尺寸及冷卻水的溫度有關。
2．冷卻水道至型腔表面距離應盡量相等 當塑件壁厚均勻時，冷卻水道到型腔表面最好距離相等，但是當塑件不均勻時，厚的地方冷卻水道到型腔表面的距離應近一些，間距也可適當小一些。一般水道孔邊至型腔表面的距離應大于10mm，常用12~15mm.
參考《塑料成型工藝與
模具設計》
第五章第七節(jié) P226
共 16 頁 第 14 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
3．澆口處加強冷卻 塑料熔體充填型腔時，澆口附近溫度最高，距澆口越遠溫度就越低，因此澆口附近應加強冷卻，通常將冷卻水道的入口處設置在澆口附近，使?jié)部诟浇哪＞咴谳^低溫度下冷卻，而遠離澆口部分的模具在經(jīng)過一定程度熱交換后的溫水作用下冷卻。
4．冷卻水道出、入口溫差應盡量小 如果冷卻水道較長，則冷卻水出、入口的溫差就比較大，易使模溫不均勻，所以在設計時應引起注意。
冷卻水道的總長度的計算可公式：Lw=Aw/π
Lw——冷卻水道總長度 Aw——熱傳導面積 Dw——冷卻水道直徑
根據(jù)模具結(jié)構要求，冷卻水道長度
5．冷卻水道應沿著塑料收縮的方向設置 聚乙烯的收縮率大，水道應盡量沿著收縮方向設置。
冷卻水道的設計必須盡量避免接近塑件的熔接部位，以免產(chǎn)生熔接痕，降低塑件強度；冷卻水道要易于加工清理一般水道孔徑為10mm左右，不小于8mm。根據(jù)此套模具結(jié)構，采用孔徑為8mm的冷卻水道。
冷卻系統(tǒng)的結(jié)構設計：
中等深度的塑件，采用點澆口進料的中等深度的殼形塑件，在凹模底部附近采用與型腔表面等距離鉆孔的形式
共 16 頁 第 15 頁
湖南大學衡陽分校畢業(yè)設計
計 算 內(nèi) 容
說 明
第十一部分 設計總結(jié)
通過這次系統(tǒng)的注射模的設計，我更進一步的了解了注射模的結(jié)構及各工作零部件的設計原則和設計要點，了解了注射模具設計的一般程序。
進行塑料產(chǎn)品的模具設計首先要對成型制品進行分析，再考慮澆注系統(tǒng)、型腔的分布、導向推出機構等后續(xù)工作。通過制品的零件圖就可以了解制品的設計要求。對形態(tài)復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性、收縮率，透明性和制品的機械強度、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)濟性。模具的結(jié)構設計要求經(jīng)濟合理，認真掌握各種注射模具的設計的普遍的規(guī)律，可以縮短模具設計周期，提高模具設計的水平。
參考資料：
1. 屈華昌主編.塑料成型工藝與模具設計.北京：機械工業(yè)出版社，1995
2. 黃毅宏、李明輝主編模具制造工藝.北京：機械工業(yè)出版社，1999.6
3. 《塑料模設計手冊》編寫組編著.塑料模設計手冊.北京：機械工業(yè)出版社，2002.7
4. 李紹林，馬長福主編.實用模具技術手冊.上海：上?？茖W技術文獻出版社，2000.6
5. 王樹勛主編.注塑模具設計與制造實用技術.廣州：華南理工大學出版社，1996.1
6. 李紹林主編.塑料·橡膠成型模具設計手冊. 北京：機械工業(yè)出版社，2000.9
共 16 頁 第 16 頁
18
編號：
畢業(yè)設計(論文)外文翻譯
（原文）
學 院： 機電工程學院
專 業(yè)： 機械設計制造及其自動化
學生姓名： 韋良華
學 號： 1000110129
指導教師單位： 機電工程學院
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 o
Article history:
Received 25 October 2010
Received in revised form
12 January 2011
Accepted 14 January 2011
Available online 21 January 2011
Keywords:
Microcellular injection molding
Plastic foaming
Swirl-free surface
a b s t r a c t
Microcellular 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 Introduction
The 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 microcellular
injection 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. Theoretical
2.1. Nucleation theory for polymeric foams
In 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 as
N=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 Boltzmann’s 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 by
?Ghom**?16πr33?P2
where g is the surface energy of the bubble interface and ?P.is
assumed to be the gas saturation pressure. More precisely,
?P=|Pr'-Pr| where Pr` is the pressure that is exerted in a high
pressure vessel and Pr is the pressure of the supersaturated vapor in
the sample [16]. That is, DP is the pressure difference between the
pressure that is applied to the sample and the pressure of the supersaturated vapor in the sample. When the pressure that saturates
the 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 as
?Ghet**=?Ghom**×f(θ) (3a)
fθ=12-34cosθ+14cosθ3 (3b)
where f(q) is a geometric factor that is dependent upon the contact
angle, θ, of the interface between the polymer and a second phase,
and has values of less than or equal to 1. For a typical wetting angle
of 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, the?P 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 [16,17]
?P=|Pr'-Pr|=2rrc （4）
where rc is the radius of a characteristic droplet, and the W.
Thomson equation
RTlnPrP∞=2r?Mr?p （5）
where P∞ is the pressure of the saturated vapor (i.e., the equilibrium
pressure), R is the universal gas constant, M is the molar mass, and p is the density. These equations yield
?P=RTρlnPrP∞M （6）
which can be alternatively expressed as
?P=ktρ1lnS (7)
whereρ1is 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 by
?G**=16πr33(kTρ1lnS)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/gas
solution becomes a metastable supersaturation solution when it is
injected 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 [22]
S@1 atm; melt temperature=S@STPexp?(-?HsR(1Tmelt-1298)) (9)
where S@STP is the solubility of the gas in the polymer at standard
temperature 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 by
S=mgS@STPexp?(-?HsR(1Tmelt-1298)) (10)
where mg is the gas dosage which can be controlled by the supercritical
fluid (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, γ. 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 can
be treated as a liquid mixture. Thus, the surface energy of the
bubble interface, g, can be expressed as [23,24]
γmix=γpolymerρmixρpolymer4(1-wgas) （11）
where γpolymer is the surface energy of the polymer, P′S are the
densities, and wgas is the weight fraction of gas.
In addition, a frequency factor for a gas molecule, f, in Eq. (1) can
be expressed as [24-26]
f=Zβ(4πrc2) (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 Zβcan be used as a correction factor and is determined experimentally.
Once the nucleation rate as a function of the degree of supersaturation
is 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. Experimental
3.1. Materials
The material used in this study was an injection molding grade
low 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 controlling
the 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 molding
In 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 methods
To 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 accelerating
voltage 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
收藏