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王波波 200705010322 机电工程学院 机械设计制造及其自动化 周明贵

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2011 年 5 月 20 日

在先进的结构发泡成型中获得一个有高间隙率方法的研究
John W. S. Lee, Jing Wang, Jae D. Yoon, and Chul B. Park 摘要:结构性泡沫提供比它们同类更多的优点,包括更大的几何准确性、最终产品的表面上没有凹痕, 较低的重量(由此延伸的需要以较低的材料),和更高的刚度与重量的比率。 用传统的结构实现一个合适 的空隙率在结构泡沫发泡成型方法已经有一些成功;这些方法允许小的控制和产量大的孔洞及非均匀 的单元结构。本文章报告使用一种先进的结构发泡成型机以一个高的空隙率,达到一个统一的单元结 构。我们研究以下方面:注塑工艺参数流量、吹气的理论容量,和熔体温度。在内部的剖面压力不同的 加工条件下的模腔内研究了塑料的成核和生长。 通过优化工艺条件,所有我们取得了一个统一的单元结 构和非常高的空隙率(40%)。

1 简介:
结构成型是塑料成型所使用的一种传统的注塑机。 一种用物理吹剂(PBA),另一种用化工吹剂(CBA), 或者两者都被选用,在这个过程中,产生一种单元(泡沫)结构。这种结构性泡沫成型的优点有缺乏凹痕 的最后一个部分的表面上,一个减了体重,低背压,更快捷的生产周期时间,具有相当高转速.因为这独 特的优势,低压预塑式结构发泡成型技术中得到了广泛的应用制造大产品,需要几何精度。 实现一个适当的空隙率在结构泡沫使用传统的注塑机并没有证明是非常成功的,但由于这些成型 方法允许小的控制和产量大的孔洞及非均匀的细胞结构。获得一种统一的单元结构具有高空隙率、机 器必须能先具有一张完全溶解和均匀的气体混合物的没有任何气体的口袋。如果一个统一的单一气体 解决方案不是达到前发泡,将很难获得一种统一的细胞结构发泡制品。在决策中, 为满足这一需求, 要 求一种先进的结构发泡成型技术与连续聚合物发展,该技术有利于均匀的离散和溶解气体的聚合物熔 体在成型过程中,从而保护的产生对难溶气体大口袋。在一个我们展示了以前的工作,用一个定制的可 行性小注塑系统组成的一个微型注射单位和发泡挤出机,基于这种新技术。然而,除了改善硬件技术, 它也是必要开发适当的处理策略以控制细胞生长成核和模具型腔内。 在此背景下,当前一些探讨处理策略需要获得一个统一的高间隙先进的结构发泡成型工艺单元结 构。我们调查了下列重要参数:吹剂含量、注入流量、熔体温度。使用我们的结构性泡沫获得先进的成 型技术进行表征方面的空隙率、细胞密度、细胞三维地形尺寸分布;x 射线用来描写的三维结构泡沫细 胞的组织形态。内部的压力剖面下模具型腔也被记录在案,为了更好的理解不同加工条件下细胞的形 核、长大的行为。

2 研究背景:
近年来,泡沫塑料注射成型的优势已经引发了改进结构发泡成型技术。 Trexel 公司开发了一种微往 复式注射成型技术的基出上,对预塑式注塑机进行了大量的工作。 以进一步改善质模板在微孔发泡过程 中使用了微结构成型。Turng,苏达权等, ,研究了改变工艺条件的影响上,特别是在当前国内外微孔结 构的例子, 混合成型用结构.何振平, 高庆宇报道的创造与微孔发泡细胞的结构和表面质量良好使用了 共聚物聚碳酸脂(PC).尹恩惠, 孙俐, 在当前国内外微孔形貌控制的聚丙烯(PP)等课程教学中存在的报 道说,有一个高庆宇甲级的表面和高空隙率可以达到通过使用一个透气通道.发泡等,综述了最近高庆 宇的微孔复合材料的新型高分子材料和钢筋与矿物填料及自然光纤。

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Shimbo 报道, 在典型的结构成型工艺另一种微孔发泡过程中注塑机, 使用了一个预塑式注塑机 被用来塑化螺柱塞聚合物,是用来注入聚合物进入模具腔,另一个替代方案泡沫注射成型工艺是在发达 的德国亚琛的一个系统,在这个系统中,气体注射在一个特别设计的喷油嘴, 它安装在塑化单元之间的, 可对喷嘴关闭的常规射出成型机。 此外,它达到更好的分散性之气, 静态混合元素被安装之间的气体喷 油嘴和关闭喷嘴。这项技术后来为商业化专利。 在 2006 年, 有人提出了一个结构,经过在先进的高庆宇发泡成型技术的基础上,预塑式注射机传 统的结构发泡技术这样就提高了注入气体会完全溶解在聚合物。由一个强化技术的齿轮油泵及附加蓄 能器使聚合物/气体混合物形成一步连续不断的成型操作。 换句话说,更新的设计完全解耦,气体溶解步 骤的注塑操作使用一个主驱动泵。这一先进的结构发泡的细节 技术概述在下一节。

3 先进的成型结构
先进的成型机。经过先进的发泡成型机器.这种技术促进统一的气体色散和完整(或实质)溶解在 聚合物熔体,尽管是稳定成型工艺。 但是它认识到连续成型行为不可避免地引起不一致的气体充填、 这 种结构使得流动但是聚合物熔体和天然气是连续的(即不停止在注射时期)。

图1

图2 2

图 3-4

图1 显示的原理图结构,经过先进的泡沫成型机在发达的Toronto 大学的这台机器包含了一主驱动 泵(例如:一个齿轮泵)和额外的蓄电池、 附于挤压桶和之间的关断阀。 (一个位于前关闭阀门柱塞,另一 种是位于喷嘴处。 )此设计完全减弱气体溶解步骤的注塑操作使用和维护主动驱动泵齿轮泵的稳态气体 溶解作用。 在注塑业务,橡胶压片机压出的螺杆转动,而生成聚合物/气体混合物收集在加时赛的蓄电池。 后两者混合遭受到注塑和收集到的,它移动通过柱塞机制进入到下一个周期。这项技术确保了压力,在 挤压桶内保持相对稳定,达到一致的气体充填是这样一个统一的聚合物/气体混合物是取得了不管压力 波动柱塞。这项技术已经成为商业专利。 均匀分布和完全溶解吹塑过程保持一致的气体充填的聚合物和替代或近乎溶解所有的气体在聚合 物熔体,螺杆必须保持相对稳定的自转时,在螺杆的优点是恒转速移动一倍。首先,一致的气体充填是 容易实现:由于压力波动的挤压桶内减至最低。第二,维持一个高压力下确保解散的注入气体进入聚合 物熔体。 一个统一的聚合物/气体混合物,其中的气体已经完全(或实质上)溶解, 为改善制品塑料结构。 就需要有一个常数溶气/重量配比提供理论依据。 表1

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图5

图6

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图 7 .瓦斯含量的影响和注入流量等泡沫的形态

一个齿轮油泵是一种最基本的组成部分,因为它提供了一份改进工艺恒体积流率对聚合物/气体混 合物;泵上的压力,从而控制的挤压,并允许一个一致的连续性桶重量比为粘性聚合物熔体,压力在挤压 酒桶保持相对稳定,因为这种积极的位移的齿轮泵。由于气体流量压力取决于在桶显著,恒气流量可以 通过保持固定的压力,在挤压桶。聚合物/气体混合物能够控制的变转速的齿轮泵。通过独立控制的流 动速率两种气体与聚合物/气体混合物,这种聚合物流量也可以被控制住。因此,既有一致的重量比”, 并获得统一流动聚合物/气体混合物可以很容易地实现与齿轮泵。这些优势不能被轻易的做到了,用一 个关闭或止回阀。背后的基本原理与装备新模型具有额外的蓄能器来源于需要适应这个混合物在每个 周期的注射期间使螺杆可以匀速旋转和煤气可以不断的注入 melt.4 不断旋转螺杆是一种重要的差异, 从以前所有的结构发泡成型技术是基于低压塑料注塑系统。 一旦是压力相对稳定的挤出桶,它会变得更 容易控制的流量,注入气体的高分子,和气体即可更为均匀散布到融化

图 8 .细胞密度测量的地点 A-C(0.3 硅油%氮气)。

当一个一致的气体聚合物量比,实现了注入氮气,有一个非常低的溶解性,可完全溶化,如果一个足 够高的压力保持在这两种挤压桶和累加器。“足够高的压力”意味着熔体压力远高于溶解性的压力进 行了给定的气体的注入聚合物熔体。此外,保持了足够高的压力后的油已经完全溶解,防止形成第二阶
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段在聚合物熔体在积累阶段。 因为溶解性的压力进行了瓦斯含量要求产生一个 fine-celled 结构[例如, 为 0.1-1.0% N2 期的 140-1400 psi 的高密度聚乙烯(HDPE)在 200°C]17 号低比压极限存在的低压预 塑式结构性泡沫成型机(最大许用压力≈3000 psi),一个足够高的压力就可以很容易地保持先进的结 构发泡成型机。

4 结果和讨论:
加工参数的影响程度,充模。 4 显示了吹剂的影响(氮气)和温度对泡沫融化程度充满了模具。 图 卒 中是用于不同的注入不同数目的 N2 为了达到不同的空泡内馏份:60,50,和 40 毫米,和 0.5 ,0.1,0.3 硅油%氮气,分别。这些注入中风占期末无效的分数占 17%,31%和 45%,分别。 很清楚,氮气含量和喷射流量中起到了至关重要的作用,在确定充填型腔的程度。充填型腔的程度 随氮气含量和注入流量而增加。因为低压结构发泡成型使用一种近程注射,在这个过程中,依靠泡沫膨 胀以填充模子腔。 一个更高的氮气含量增加的程度,从而提高了泡沫膨胀模具, 也是值得注意是由高细胞密度增加氮 气含量是另一个推动力的创作中较大的空系率。 注射充模流动速率也受到了影响。 因为在何种程度上 的不同,熔体冷却流量、 更高注射注塑流动速度下降冷却速率在注射过程中,这导致熔融粘度较低,同时, 也增加了聚合物的力学性能。 此外,因为熔体温度比较高,在高注入流量、 时间较长的细胞形核、 长大。 应该指出的是,晶核的成核和生长在模具型腔熔体温度降低会了停一下下面的结晶温度。

5 总结
在这项研究中,实验对各种材料的低压注塑成型加工条件进行了调查,注射流量和模腔平均压力 在注塑中起到了至关重要的作用,它也发现氮气的数量对形成致密的单元结构很重要。当氮气含量太 低(即,0.1 硅油%),空腔压降成核率会下降并导致制品的密度过低。另一方面,当氮气含量足够高(例 如,0.3 硅油%及以上),会导致制品密度过高。我们还发现,没有一个合适的阻力,我们不可能获得一个 统一的制品结构和较高的制品精度。 通过优化所有的压力加工条件,我们就能实现一个统一的细单元结 构和较高的制品精度(接近 40%)。

参考文献
(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;

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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
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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.

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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
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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

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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.

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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
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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
13

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 determinedbecause the relative position of the solubility (or threshold) pressure in the pressure profile during injection decides the cell nucleation rate. The gas content is also important in filling the cavity, in determining the pressure profile through the plasticating effect,23 and in determining the thermodynamic instability that causes cell nucleation.

Conclusion
I n this study, experiments were conducted to investigate the effects of various materials and processing conditions on injection-molded foams in low-pressure structural molding. The injection flow rate played a critical role in the degree of filling and the cavity pressure profile. It was also found that the amount of N2 was important for achieving a high cell density. When the N2 content was too low (i.e., 0.1 wt %), the cavity pressure drop rate governed cell nucleation and led to the production of foams with low cell densities. On the other hand, when the N2 content was high enough (i.e.0.3 wt % and above), the pressure drop rate at the gate governed cell nucleation and resulted in foams with high cell densities. We also found that a uniform cell structure and high void fractions could not be achieved without a proper gate resistance. By optimizing all of the processing conditions, we were able to achieve a uniform cell structure with a very high void fraction (close to 40%).

References
(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;

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