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外文原文及翻譯
在先進(jìn)的結(jié)構(gòu)發(fā)泡成型中獲得一個(gè)有高間隙率方法的研究
John W. S. Lee, Jing Wang, Jae D. Yoon, and Chul B. Park
摘要：結(jié)構(gòu)性泡沫提供比它們同類更多的優(yōu)點(diǎn),包括更大的幾何準(zhǔn)確性、最終產(chǎn)品的表面上沒(méi)有凹痕,較低的重量(由此延伸的需要以較低的材料),和更高的剛度與重量的比率。用傳統(tǒng)的結(jié)構(gòu)實(shí)現(xiàn)一個(gè)合適的空隙率在結(jié)構(gòu)泡沫發(fā)泡成型方法已經(jīng)有一些成功;這些方法允許小的控制和產(chǎn)量大的孔洞及非均勻的單元結(jié)構(gòu)。本文章報(bào)告使用一種先進(jìn)的結(jié)構(gòu)發(fā)泡成型機(jī)以一個(gè)高的空隙率，達(dá)到一個(gè)統(tǒng)一的單元結(jié)構(gòu)。我們研究以下方面:注塑工藝參數(shù)流量、吹氣的理論容量,和熔體溫度。在內(nèi)部的剖面壓力不同的加工條件下的模腔內(nèi)研究了塑料的成核和生長(zhǎng)。通過(guò)優(yōu)化工藝條件,所有我們?nèi)〉昧艘粋€(gè)統(tǒng)一的單元結(jié)構(gòu)和非常高的空隙率(40%)。
1.簡(jiǎn)介：
結(jié)構(gòu)成型是塑料成型所使用的一種傳統(tǒng)的注塑機(jī)。一種用物理吹劑(PBA),另一種用化工吹劑(CBA),或者兩者都被選用,在這個(gè)過(guò)程中,產(chǎn)生一種單元(泡沫)結(jié)構(gòu)。這種結(jié)構(gòu)性泡沫成型的優(yōu)點(diǎn)有缺乏凹痕的最后一個(gè)部分的表面上,一個(gè)減了體重,低背壓,更快捷的生產(chǎn)周期時(shí)間,具有相當(dāng)高轉(zhuǎn)速.因?yàn)檫@獨(dú)特的優(yōu)勢(shì),低壓預(yù)塑式結(jié)構(gòu)發(fā)泡成型技術(shù)中得到了廣泛的應(yīng)用制造大產(chǎn)品,需要幾何精度。
實(shí)現(xiàn)一個(gè)適當(dāng)?shù)目障堵试诮Y(jié)構(gòu)泡沫使用傳統(tǒng)的注塑機(jī)并沒(méi)有證明是非常成功的,但由于這些成型方法允許小的控制和產(chǎn)量大的孔洞及非均勻的細(xì)胞結(jié)構(gòu)。獲得一種統(tǒng)一的單元結(jié)構(gòu)具有高空隙率、機(jī)器必須能先具有一張完全溶解和均勻的氣體混合物的沒(méi)有任何氣體的口袋。如果一個(gè)統(tǒng)一的單一氣體解決方案不是達(dá)到前發(fā)泡,將很難獲得一種統(tǒng)一的細(xì)胞結(jié)構(gòu)發(fā)泡制品。在決策中，為滿足這一需求，要求一種先進(jìn)的結(jié)構(gòu)發(fā)泡成型技術(shù)與連續(xù)聚合物發(fā)展,該技術(shù)有利于均勻的離散和溶解氣體的聚合物熔體在成型過(guò)程中,從而保護(hù)的產(chǎn)生對(duì)難溶氣體大口袋。在一個(gè)我們展示了以前的工作,用一個(gè)定制的可行性小注塑系統(tǒng)組成的一個(gè)微型注射單位和發(fā)泡擠出機(jī)，基于這種新技術(shù)。然而,除了改善硬件技術(shù),它也是必要開(kāi)發(fā)適當(dāng)?shù)奶幚聿呗砸钥刂萍?xì)胞生長(zhǎng)成核和模具型腔內(nèi)。
在此背景下,當(dāng)前一些探討處理策略需要獲得一個(gè)統(tǒng)一的高間隙先進(jìn)的結(jié)構(gòu)發(fā)泡成型工藝單元結(jié)構(gòu)。我們調(diào)查了下列重要參數(shù):吹劑含量、注入流量、熔體溫度。使用我們的結(jié)構(gòu)性泡沫獲得先進(jìn)的成型技術(shù)進(jìn)行表征方面的空隙率、細(xì)胞密度、細(xì)胞三維地形尺寸分布;x射線用來(lái)描寫的三維結(jié)構(gòu)泡沫細(xì)胞的組織形態(tài)。內(nèi)部的壓力剖面下模具型腔也被記錄在案，為了更好的理解不同加工條件下細(xì)胞的形核、長(zhǎng)大的行為。
2.研究背景：
近年來(lái),泡沫塑料注射成型的優(yōu)勢(shì)已經(jīng)引發(fā)了改進(jìn)結(jié)構(gòu)發(fā)泡成型技術(shù)。Trexel公司開(kāi)發(fā)了一種微往復(fù)式注射成型技術(shù)的基出上,對(duì)預(yù)塑式注塑機(jī)進(jìn)行了大量的工作。以進(jìn)一步改善質(zhì)模板在微孔發(fā)泡過(guò)程中使用了微結(jié)構(gòu)成型。Turng，蘇達(dá)權(quán)等, ,研究了改變工藝條件的影響上,特別是在當(dāng)前國(guó)內(nèi)外微孔結(jié)構(gòu)的例子, 混合成型用結(jié)構(gòu).何振平，高慶宇報(bào)道的創(chuàng)造與微孔發(fā)泡細(xì)胞的結(jié)構(gòu)和表面質(zhì)量良好使用了共聚物聚碳酸脂(PC).尹恩惠，孫俐，在當(dāng)前國(guó)內(nèi)外微孔形貌控制的聚丙烯(PP)等課程教學(xué)中存在的報(bào)道說(shuō),有一個(gè)高慶宇甲級(jí)的表面和高空隙率可以達(dá)到通過(guò)使用一個(gè)透氣通道.發(fā)泡等,綜述了最近高慶宇的微孔復(fù)合材料的新型高分子材料和鋼筋與礦物填料及自然光纖。
Shimbo報(bào)道, 在典型的結(jié)構(gòu)成型工藝另一種微孔發(fā)泡過(guò)程中注塑機(jī)，使用了一個(gè)預(yù)塑式注塑機(jī)被用來(lái)塑化螺柱塞聚合物,是用來(lái)注入聚合物進(jìn)入模具腔,另一個(gè)替代方案泡沫注射成型工藝是在發(fā)達(dá)的德國(guó)亞琛的一個(gè)系統(tǒng),在這個(gè)系統(tǒng)中,氣體注射在一個(gè)特別設(shè)計(jì)的噴油嘴，它安裝在塑化單元之間的，可對(duì)噴嘴關(guān)閉的常規(guī)射出成型機(jī)。此外,它達(dá)到更好的分散性之氣, 靜態(tài)混合元素被安裝之間的氣體噴油嘴和關(guān)閉噴嘴。這項(xiàng)技術(shù)后來(lái)為商業(yè)化專利。
在2006年, 有人提出了一個(gè)結(jié)構(gòu),經(jīng)過(guò)在先進(jìn)的高慶宇發(fā)泡成型技術(shù)的基礎(chǔ)上,預(yù)塑式注射機(jī)傳統(tǒng)的結(jié)構(gòu)發(fā)泡技術(shù)這樣就提高了注入氣體會(huì)完全溶解在聚合物。由一個(gè)強(qiáng)化技術(shù)的齒輪油泵及附加蓄能器使聚合物/氣體混合物形成一步連續(xù)不斷的成型操作。換句話說(shuō),更新的設(shè)計(jì)完全解耦,氣體溶解步驟的注塑操作使用一個(gè)主驅(qū)動(dòng)泵。這一先進(jìn)的結(jié)構(gòu)發(fā)泡的細(xì)節(jié) 技術(shù)概述在下一節(jié)。
3.先進(jìn)的成型結(jié)構(gòu):
先進(jìn)的成型機(jī)。經(jīng)過(guò)先進(jìn)的發(fā)泡成型機(jī)器.這種技術(shù)促進(jìn)統(tǒng)一的氣體色散和完整(或?qū)嵸|(zhì))溶解在聚合物熔體,盡管是穩(wěn)定成型工藝。但是它認(rèn)識(shí)到連續(xù)成型行為不可避免地引起不一致的氣體充填、這種結(jié)構(gòu)使得流動(dòng)但是聚合物熔體和天然氣是連續(xù)的(即不停止在注射時(shí)期)。
圖1
圖3-4
圖1顯示的原理圖結(jié)構(gòu),經(jīng)過(guò)先進(jìn)的泡沫成型機(jī)在發(fā)達(dá)的Toronto大學(xué)的這臺(tái)機(jī)器包含了一主驅(qū)動(dòng)泵(例如:一個(gè)齒輪泵)和額外的蓄電池、附于擠壓桶和之間的關(guān)斷閥。(一個(gè)位于前關(guān)閉閥門柱塞,另一種是位于噴嘴處。)此設(shè)計(jì)完全減弱氣體溶解步驟的注塑操作使用和維護(hù)主動(dòng)驅(qū)動(dòng)泵齒輪泵的穩(wěn)態(tài)氣體溶解作用。在注塑業(yè)務(wù),橡膠壓片機(jī)壓出的螺桿轉(zhuǎn)動(dòng),而生成聚合物/氣體混合物收集在加時(shí)賽的蓄電池。后兩者混合遭受到注塑和收集到的,它移動(dòng)通過(guò)柱塞機(jī)制進(jìn)入到下一個(gè)周期。這項(xiàng)技術(shù)確保了壓力,在擠壓桶內(nèi)保持相對(duì)穩(wěn)定,達(dá)到一致的氣體充填是這樣一個(gè)統(tǒng)一的聚合物/氣體混合物是取得了不管壓力波動(dòng)柱塞。這項(xiàng)技術(shù)已經(jīng)成為商業(yè)專利。
均勻分布和完全溶解吹塑過(guò)程保持一致的氣體充填的聚合物和替代或近乎溶解所有的氣體在聚合物熔體,螺桿必須保持相對(duì)穩(wěn)定的自轉(zhuǎn)時(shí)，在螺桿的優(yōu)點(diǎn)是恒轉(zhuǎn)速移動(dòng)一倍。首先,一致的氣體充填是容易實(shí)現(xiàn):由于壓力波動(dòng)的擠壓桶內(nèi)減至最低。第二,維持一個(gè)高壓力下確保解散的注入氣體進(jìn)入聚合物熔體。一個(gè)統(tǒng)一的聚合物/氣體混合物,其中的氣體已經(jīng)完全(或?qū)嵸|(zhì)上)溶解, 為改善制品塑料結(jié)構(gòu)。
就需要有一個(gè)常數(shù)溶氣/重量配比提供理論依據(jù)。
表1
圖5
圖6
圖7 .瓦斯含量的影響和注入流量等泡沫的形態(tài)
一個(gè)齒輪油泵是一種最基本的組成部分,因?yàn)樗峁┝艘环莞倪M(jìn)工藝恒體積流率對(duì)聚合物/氣體混合物;泵上的壓力,從而控制的擠壓,并允許一個(gè)一致的連續(xù)性桶重量比為粘性聚合物熔體,壓力在擠壓酒桶保持相對(duì)穩(wěn)定,因?yàn)檫@種積極的位移的齒輪泵。由于氣體流量壓力取決于在桶顯著,恒氣流量可以通過(guò)保持固定的壓力,在擠壓桶。聚合物/氣體混合物能夠控制的變轉(zhuǎn)速的齒輪泵。通過(guò)獨(dú)立控制的流動(dòng)速率兩種氣體與聚合物/氣體混合物,這種聚合物流量也可以被控制住。因此,既有一致的重量比”,并獲得統(tǒng)一流動(dòng)聚合物/氣體混合物可以很容易地實(shí)現(xiàn)與齒輪泵。這些優(yōu)勢(shì)不能被輕易的做到了,用一個(gè)關(guān)閉或止回閥。背后的基本原理與裝備新模型具有額外的蓄能器來(lái)源于需要適應(yīng)這個(gè)混合物在每個(gè)周期的注射期間使螺桿可以勻速旋轉(zhuǎn)和煤氣可以不斷的注入melt.4不斷旋轉(zhuǎn)螺桿是一種重要的差異,從以前所有的結(jié)構(gòu)發(fā)泡成型技術(shù)是基于低壓塑料注塑系統(tǒng)。一旦是壓力相對(duì)穩(wěn)定的擠出桶,它會(huì)變得更容易控制的流量,注入氣體的高分子,和氣體即可更為均勻散布到融化
圖8 .細(xì)胞密度測(cè)量的地點(diǎn)A-C(0.3硅油%氮?dú)?。
當(dāng)一個(gè)一致的氣體聚合物量比,實(shí)現(xiàn)了注入氮?dú)?有一個(gè)非常低的溶解性,可完全溶化,如果一個(gè)足夠高的壓力保持在這兩種擠壓桶和累加器。“足夠高的壓力”意味著熔體壓力遠(yuǎn)高于溶解性的壓力進(jìn)行了給定的氣體的注入聚合物熔體。此外,保持了足夠高的壓力后的油已經(jīng)完全溶解,防止形成第二階段在聚合物熔體在積累階段。因?yàn)槿芙庑缘膲毫M(jìn)行了瓦斯含量要求產(chǎn)生一個(gè)fine-celled結(jié)構(gòu)[例如,為0.1-1.0% N2期的140-1400 psi的高密度聚乙烯(HDPE)在200°C]17號(hào)低比壓極限存在的低壓預(yù)塑式結(jié)構(gòu)性泡沫成型機(jī)(最大許用壓力≈3000 psi),一個(gè)足夠高的壓力就可以很容易地保持先進(jìn)的結(jié)構(gòu)發(fā)泡成型機(jī)。
4.結(jié)果和討論：
加工參數(shù)的影響程度,充模。圖4顯示了吹劑的影響(氮?dú)?和溫度對(duì)泡沫融化程度充滿了模具。卒中是用于不同的注入不同數(shù)目的N2為了達(dá)到不同的空泡內(nèi)餾份:60,50,和40毫米,和0.5 ，0.1,0.3硅油%氮?dú)?分別。這些注入中風(fēng)占期末無(wú)效的分?jǐn)?shù)占17%,31%和45%,分別。
很清楚,氮?dú)夂亢蛧娚淞髁恐衅鸬搅酥陵P(guān)重要的作用,在確定充填型腔的程度。充填型腔的程度隨氮?dú)夂亢妥⑷肓髁慷黾?。因?yàn)榈蛪航Y(jié)構(gòu)發(fā)泡成型使用一種近程注射,在這個(gè)過(guò)程中,依靠泡沫膨脹以填充模子腔。
一個(gè)更高的氮?dú)夂吭黾拥某潭?從而提高了泡沫膨脹模具，也是值得注意是由高細(xì)胞密度增加氮?dú)夂渴橇硪粋€(gè)推動(dòng)力的創(chuàng)作中較大的空系率。 注射充模流動(dòng)速率也受到了影響。因?yàn)樵诤畏N程度上的不同,熔體冷卻流量、更高注射注塑流動(dòng)速度下降冷卻速率在注射過(guò)程中,這導(dǎo)致熔融粘度較低,同時(shí),也增加了聚合物的力學(xué)性能。此外,因?yàn)槿垠w溫度比較高,在高注入流量、時(shí)間較長(zhǎng)的細(xì)胞形核、長(zhǎng)大。應(yīng)該指出的是,晶核的成核和生長(zhǎng)在模具型腔熔體溫度降低會(huì)了停一下下面的結(jié)晶溫度。
5.總結(jié):
在這項(xiàng)研究中，實(shí)驗(yàn)對(duì)各種材料的低壓注塑成型加工條件進(jìn)行了調(diào)查，注射流量和模腔平均壓力在注塑中起到了至關(guān)重要的作用，它也發(fā)現(xiàn)氮?dú)獾臄?shù)量對(duì)形成致密的單元結(jié)構(gòu)很重要。當(dāng)?shù)獨(dú)夂刻?即,0.1硅油%),空腔壓降成核率會(huì)下降并導(dǎo)致制品的密度過(guò)低。另一方面,當(dāng)?shù)獨(dú)夂孔銐蚋?例如,0.3硅油%及以上),會(huì)導(dǎo)致制品密度過(guò)高。我們還發(fā)現(xiàn),沒(méi)有一個(gè)合適的阻力，我們不可能獲得一個(gè)統(tǒng)一的制品結(jié)構(gòu)和較高的制品精度。通過(guò)優(yōu)化所有的壓力加工條件,我們就能實(shí)現(xiàn)一個(gè)統(tǒng)一的細(xì)單元結(jié)構(gòu)和較高的制品精度(接近40%)。
參考文獻(xiàn)
(1) Hornsby, P. R. Thermoplastics Structural Foams: Part 2 Properties and Application. Mater. Eng. 1982, 3, 443.
(2) Ahmadi, A. A.; Hornsby, P. R. Moulding and Characterization Studies with Polypropylene Structural Foam, Part 1: Structure-Property Interrelationships. Plast. Rubber Process. Appl. 1985, 5, 35.
(3) Hikita, K. Development of Weight Reduction Technology for Door Trip Using Foamed PP. JSAE ReV. 2002, 23, 239.
(4) Park, C. B.; Xu, X. Apparatus and Method for Advanced Structural Foam Molding. U.S. Patent Application 11/219,309, filed Sep 2, 2005;
Strategies to Achieve a Uniform Cell Structure with a High Void Fraction in Advanced Structural Foam Molding
ABSTRACT：Structural foams offer numerous advantages over their solid counterparts, including greater geometrical accuracy, the absence of sink marks on the final product’s surface, lower weight (and, by extension, the need for less material), and a higher stiffness-to-weight ratio. The possibility of achieving a suitable void fraction in structural foams using conventional structural foam molding methods, however, has been of limited success;these methods allow for little control and typically yield large voids and a nonuniform cell structure. This article reports on our use of an advanced structural foam molding machine to achieve a uniform cell structure with a high void fraction. We studied the following processing parameters: injection flow rate, blowing agent content, and melt temperature. The pressure profile inside the mold cavity under various processing conditions was also investigated to elucidate cell nucleation and growth behaviors. By optimizing all processing conditions, we achieved a uniform cell structure and a very high void fraction (over 40%).
Introduction
Structural foams are plastic foams manufactured using ，conventional preplasticating-type injection-molding machines. A physical blowing agent (PBA), chemical blowing agent，(CBA), or both are employed in the process to produce a cellular (foam) structure. The advantages of structural foam molding，include the absence of sink marks on the final part’s surface, a reduced weight, a low back pressure, a faster production cycle ，time, and a high stiffness-to-weight ratio.1-3 Because of this unique set of advantages, a low-pressure preplasticating-type，structural foam molding technology has been used widely for manufacturing large products that require geometric accuracy. Achieving a suitable void fraction in structural foams using conventional structural foam molding has not proven to be successful, however, as these molding methods allow for little control and yield large voids and a nonuniform cell structure.To obtain a uniform cell structure with a high void fraction, the machine must be capable of first producing a completely dissolved and uniform gas/polymer mixture without any gas pockets. If a uniform single-phase polymer/gas solution is not achieved before foaming, it would be very difficult to attain a uniform cell structure in the final foam products. To meet this requirement, an advanced structural foam molding technology with continuous polymer/gas mixture formation was developed at the University of Toronto.4,5 This technology facilitates the uniform dispersion and dissolution of gas in the polymer melt during the structural foam molding process, thereby safe guarding against the creation of large, undissolved gas pockets. In a previous work,5 we demonstrated the feasibility of using a customized small injection molding system consisting of a miniinjection unit and a foaming extruder based on this new technology. However, in addition to improved hardware technology, it is also required to develop appropriate processing strategies to control cell nucleation and growth inside the mold cavity. In this context, the current article discusses some processing strategies required to obtain a uniform cell structure with a high void fraction in an advanced structural foam molding process. We investigated the following critical parameters: blowing agent content, injection flow rate, and melt temperature. The structural foams obtained using our advanced molding technology were characterized in terms of void fraction, cell density, and cell size distribution; three-dimensional X-ray topography was used to show the 3-D cell morphologies of the structural foams. The pressure profile inside the mold cavity was also recorded under various
Background
In recent years, the advantages of foam injection molding have prompted improvements in structural foam molding technologies. Trexel Inc. developed a microcellular injection molding technology (MuCell technology) based on a reciprocating-type injection molding machine.6,7 A great deal of work has been carried out to further improve the quality of the microcellular foams produced using the MuCell process. Turng et al., for example, investigated the impact of changing processing conditions on the microcellular foam structures, especially in cases of coinjection molding with nanocomposites Kanai et al. reported the creation of microcellular foams with a good cell structure and surface quality using copolymer polycarbonate reported the use of CaCO3 for controlling the microcellular foam morphology of polypropylene (PP). Sporrer et al. reported that a class-A surface and a high void fraction could be achieved in foaming by using a breathing mold.12 Recently, Bledzki et al. reviewed microcellular polymer materials and microcellular composites reinforced with mineral fillers and natural fibers.
In 2000, Shimbo reported an alternative microcellular foam process that employed a preplasticating-type injection molding machine.14 A screw was used to plasticate the polymer, and a plunger was used to inject the polymer into the mold cavity as in typical structural molding. Another alternative foam injection molding process was developed at IKV, Aachen, Germany.In this system, gas was injected in a specially designed injection nozzle mounted between the plasticizing unit and the shut-off nozzle of a conventional injection molding machine. Furthe rmore,to achieve better dispersion of the gas, static mixing ，elements were mounted between the gas injection nozzle and the shut-off nozzle.
This technology was later commercialized by Sulzer Chemtech.
In 2006, Park et al. presented an advanced structural foam molding technology based on a preplasticating-type injection molding machine.4,5 The conventional structural foaming technology was improved such that the injected gas would completely dissolve into the polymer. The enhanced technology consisted of a gear pump and an additional accumulator to make the polymer/gas mixture formation step continuous regardless of the stop-and-flow molding operations. In other words, the newer design completely decoupled the gas dissolution step from the injection and molding operations using a positive-displacement pump. The details of this advanced structural foaming technology are outlined in the next section.
This technology4 promotes uniform gas dispersion and complete (or substantial) dissolution in the polymer melt, despite the non -steady molding process. Recognizing that stop and-flow molding behavior inevitably causes inconsistent gas dosing, this design allows the flows of the polymer melt and gas to be continuous (i.e., not to stop during the injection period
Figure 1 shows a schematic of the advanced structural foam molding machine developed at the University of Toronto.4 This machine comprises a positive-displacement pump (i.e., a gear pump) and an additional accumulator, which is attached between the extrusion barrel and the shut-off valves. (One shut-off valve is located before the plunger, and the other is located at the nozzle.) The design completely decouples the gas dissolution step from the injection and molding operations using the positive-displacement gear pump and maintains steady-state gas dissolution. During the injection and molding operations, the plasticating screw rotates, and the generated polymer/gas mixture collects in the extra accumulator. After the mixture has been subjected to both injection and molding and has been collected,it moves through the plunger mechanism to be injected into the next cycle. This technology ensures that the pressure in the extrusion barrel is relatively constant and that consistent gas dosing is attained so that a uniform polymer/gas mixture is achieved regardless of the pressure fluctuations in the plunger. This technology has been patented
Homogeneous Distribution and Complete Dissolution of Blowing Agent.
To maintain consistent gas dosing of the polymer and to completely or near-completely dissolve all of the gas in the polymer melt, the screw must rotate at a relatively constant speed.4 The advantages of having the screw move ata constant rotational speed are two-fold. First, consistent gas dosing is easily realized because the pressure fluctuations inside the extrusion barrel are minimized. Second, maintaining a high pressure guarantees the dissolution of the injected gas into the polymer melt. A uniform polymer/gas mixture, in which the gas has been completely (or substantially) dissolved, that has a constant gas-to-polymer weight ratio provides the basis for improved uniform, fine-celled foam structures
A gear pump is an essential part of the improved process because it provides a constant volume flow rate for the polymer gas mixture; the pump thereby controls the pressure in the extrusion barrel and allows a consistent polymer-to-gas weight ratio to be maintained.4 For viscous polymer melts, the pressure in the extrusion barrel is relatively constant because of the positive displacement of the gear pump. Because the gas flow rate depends significantly on the barrel pressure, a constant gas flow rate can be obtained by maintaining a constant pressure in the extrusion barrel. The flow rate of the polymer/gas mixture can be controlled by varying the rotational speed of the gear pump. By independently controlling the flow rates of both the gas and the polymer/gas mixture, the polymer flow rate can also be controlled. Thus, both a consistent polymer-to-gas weight ratio and a uniform polymer/gas mixture can be easily achieved with a gear pump. These advantages could not be easily achieved with a shut-off or nonreturnable check valve alone.
The rationale behind having outfitted the new model with an additional accumulator derives from the need to accommodate the mixture during each cycle’s injection period so that the screw can rotate at a constant speed and the gas can be continuously injected into the melt.4 The constantly rotating screw represents a significant difference from all previous structural foam molding technologies that are based on the low-pressure preplasticating-type system. Once the pressure in the extrusion barrel is relatively stable, it becomes easier to control the flow rate of the injected gas into the polymer, and the gas can be more uniformly dispersed into the melt. When a consistent gas-to-polymer weight ratio is achieved,the injected N2, which has a very low solubility, can dissolve completely if a sufficiently high pressure is maintained in boththe extrusion barrel and the accumulators. A “sufficiently high pressure” means that the melt pressure is much higher than the solubility pressure for the given amount of gas injected into the polymer melt. In addition, maintaining a sufficiently high pressure after the gas has been completely dissolved prevents the formation of a second phase in the polymer melt during the accumulation stage. Because the solubility pressure for the gas content necessary to produce a fine-celled structure [e.g.140-1400 psi for 0.1-1.0% N2 in high-density polyethylene (HDPE) at 200 °C]17 is low compared to the pressure limit of the existing low-pressure preplasticating-type structural foam molding machines (maximum allowable pressure ≈ 3000 psi),a sufficiently high pressure can easily be maintained in the advanced structural foam molding machines，
Although the advanced structural molding machine features modifications that allow for the complete dissolution of gas into a polymer melt while a constant gas-to-polymer weight ratio is maintained,4,5 this system design does not automatically guarantee the production of high-quality foams. To produce high quality foams with uniform cell structures and a large void fraction, a set of overall conditions must be satisfied; these conditions are described below.
In addition to the formation of a foamable polymer/gas mixture with a uniform and constant polymer/gas weight ratio, the mold geometry including the gate shape should be designed properly.
Once the hardware machinery has been properly designed and constructed, appropriate material compositions should be selected and fed into the system. Both the molecular weight and structure variation of the plastic resin and the type and content of added materials, such as the nucleating agent, the blowing agent, and any other additives or fillers, should be prudently selected because all of these materials and their compositions affect the cell nucleation and growth behaviors.
Results And Discussion.
It should be also noted that the measured void fractions inFigure 4 were higher than the set void fraction. If the void fractions of the sprue, runner, and injection-molded parts had been uniform, the measured void fraction from the molded part would be the same as the set void fraction. However, in reality, the void fractions of the spure and runner were observed to be lower than that of injection-molded part. This must have been caused by the higher pressure in the sprue and runner compared to the pressure in the mold cavity. Consequently, the measured void fraction of the injection-molded parts became higher than the set void fraction
Some large bubbles were observed in the foam, however, when 0.5 wt % N2 was used. There might have been several reasons for this, as discussed earlier, but most likely, a content of 0.5 wt % was too high because of N2’s low solubility The cavity pressure of a foaming mold has a significant influence on cell nucleation. If the cavity pressure is lower than the solubility pressure (or the threshold pressure22) of the injected gas and if the pressure before the gate is high enough, cell nucleation occurs at the gate with a high pressure drop rate. In such cases, the cell density will be high. However, if the cavity pressure is higher than the solubility pressure (or the threshold pressure), cell nucleation occurs along the mold cavity with a low pressure drop rate, resulting in a low cell density. Therefore, it is desirable to induce cell nucleation at the gate by reducing the cavity pressure in order to have a large number of cells.
To achieve a high cell density and uniform cell structures in low-pressure structural foam molding, several requirements should be met with respect to the mold pressure profile. Figure 13 shows the proper pressure profiles in low-pressure structural foam molding. First, the pressure before the gate should be kept higher than the solubility (or threshold) pressure to prevent premature cell nucleation and growth. This pressure can be controlled by properly choosing the resistance of the gate and the injection flow rate. Second, the cavity pressure should be kept lower than the solubility (or threshold) pressure during injection to induce cell nucleation immediately after the gate. This can be achieved by regulating the melt temperature, the mold temperature, and the injection flow rate. Third, the gate should be designed properly so that a high pressure drop rate can be induced to nucleate a large number of bubbles. Finally, the blowing agent amount should be carefully determine
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