微孔泡沫注射成型

概述了多种可行的注射成型微孔泡沫新方法,各种加工方法各有特色,工艺设计重点应在热力学、模具几何结构以及充模阶段。     

塑料注射成型新技术——微孔发泡技术的应用

主要介绍微孔发泡技术的工艺过程、应用特性及加工条件、气体条件对微孔发泡过程的影响     

微孔注射成型技术的应用

论述热塑性塑料微孔注射成型技术在电视机外壳上的应用，针对微孔注射成型的技术难点，如制品易出现的缺料、拉白、拉裂、表面凹凸不平，烧焦等问题，从产品设计，模具设计，工艺技术等方面提出了综合解决措施，以提高产品合格率，降低生产成本。     

微孔塑料的注射成型研究进展

在简要回顾了微孔塑料基本特征的基础上,重点讨论了微孔发泡塑料的注射成型机理、工艺特点及主要影响因素;并特别介绍了几种国内外微孔塑料注射成型加工工艺的最新技术动态以及其主要的工作原理。最后,就微孔塑料注射成型技术的工艺特点,以及性能优点介绍了几种典型应用。     

微孔塑料的注射成型研究新进展

1981年美国麻省理工学院的J．E．Martini、J．Colton以及N．PSuh等以CO2、N2等惰性气体作为发泡剂，研制出泡孔直径为微米级的泡沫塑料，并将泡孔直径为1～10μm，泡孔密度为10^9～10^12个／cm^3的泡沫塑料定义为微孔塑料（Microcellular Plastics）。这种微孔泡沫塑料比普通塑料中原有的缺陷或微细裂缝小，因此微孔的存在不会降低塑料的强度，相反它能使原来存在的裂缝尖端钝化，从而改善塑料泡体的力学性能，而且还能提高耐冲击强度、韧性和耐疲劳寿命，还有隔热、隔音、吸震等功能。     

微孔泡沫注塑成型

简介塑料发泡成型,微孔泡沫注塑成型原理、工艺过程,成型特点,发展趋势。     

注射成型技术进展

介绍了注射成型工艺,注射成型机的一些新进展,重点介绍了气体辅助注射成型,微孔泡沫塑料注射成型,挤出和注射成型组合的直接成型技术及多组分注射成型的特点,同时介绍了电动注射成型机,无拉杆注射成型机和PET注射成型机。     

微孔泡沫塑料成型研究进展

介绍了微孔泡沫塑料的性能、成型原理和成型技术的研究进展。     

微孔泡沫塑料成型技术

详细介绍了微孔泡沫塑料的性能、成型原理和成型技术，具体分析了微孔泡沫塑料成型的技术难点。     

微孔注射成型PPS及其性能研究

以超临界氮气(SC N2)作为发泡剂,采用注射成型法制备了微孔化聚苯硫醚(PPS)泡沫塑料,研究了模具流道、SC N2含量、PPS熔胶量位置对微孔化PPS泡沫塑料泡孔特性、相对密度、力学性能及介电性能的影响。结果表明,随着模具流道的延长,微孔化PPS泡沫塑料的泡孔孔径逐渐变大,泡孔密度降低;SC N2含量对泡孔孔径、力学性能及介电性能影响不大,但泡孔密度随SC N2含量的增大而增大;随着PPS熔胶量位置的降低,微孔化PPS泡沫塑料的泡孔孔径增大,泡孔密度降低,力学性能及介电常数也相应逐渐降低。     

A novel method for improving the surface quality of microcellular injection molded parts

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.     

Microstructure and mechanical properties of microcellular injection molded polyamide-6 nanocomposites

The microstructure and mechanical properties of microcellular injection molded polyamide-6 (PA6) nanocomposites were studied. Cell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, were based on the material system and molding conditions. The correlation between cell density and cell size of the materials studied followed an exponential relationship. Supercritical fluid (SCF) facilitated the intercalation and exfoliation of nanoclays in the microcellular injection molding process. The orientation of nanoclays near the surface of microcells and between microcells was examined and a preferential orientation around the microcells was observed. Nanoclays in the microcellular injection molding process promoted the γ-form and suppressed the α-form crystalline structure of PA6. Both nanoclays and SCF lowered the crystallinity of the parts. Microcells improved the normalized toughness of the nanocomposites. Both microcells and nanoclay had a significant influence on the mechanical properties of parts depending on the molding conditions.     

Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process

Use of supercritical carbon dioxide (scCO₂) as a blowing agent to generate microcellular polymer foams (MPFs) has recently received considerable attention due to environmental concerns associated with conventional organic blowing agents. While such foams derived from amorphous thermoplastics have been previously realized, semicrystalline MPFs have not yet been produced in a continuous scCO₂ process. This work describes the foaming of highly crystalline poly(vinylidene fluoride) (PVDF) and its blends with amorphous polymers during extrusion. Foams composed of neat PVDF and immiscible blends of PVDF with polystyrene exhibit poor cell characteristics, whereas miscible blends of PVDF with poly(methyl methacrylate) (PMMA) yield foams possessing vastly improved morphologies. The results reported herein illustrate the effects of blend composition and scCO₂ solubility on PVDF/PMMA melt viscosity, which decreases markedly with increasing PMMA content and scCO₂ concentration. Morphological characterization of microcellular PVDF/PMMA foams reveals that the cell density increases as the PMMA fraction is increased and the foaming temperature is decreased. This study confirms that novel MPFs derived continuously from semicrystalline polymers in the presence of scCO₂ can be achieved through judicious polymer blending.     

Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams

Microcellular polycarbonate/nano-silica nanocomposites (PCSN) were prepared by temperature rising process using supercritical CO₂ as the blowing agent. Neat PC foam showed a quite broad distribution of cell sizes. Under the same foaming conditions, the addition of nano-silica resulted in PCSN foams having uniform cell size distribution, reduced cell size of 0.3-0.5 μm and increased cell density of 10¹¹-10¹³ cells/cm³. The underlying nucleation mechanism was semi-quantitatively analyzed by the classical nucleation theory. The results indicate that the energy-barrier for heterogeneous nucleation was three orders of magnitude lower than that of homogeneous one. The heterogeneous nucleation of nano-silica aggregates dramatically increased the nucleation rate, decreased the nucleation time interval, and hence facilitated the almost instantaneous growth of cell size. Combined with the well-dispersed nucleation sites, resulted from the uniform dispersion of nano-silica aggregates, the narrow-distributed cell size was obtained in PCSN foams.     

Layered-silicate based polystyrene nanocomposite microcellular foam using supercritical carbon dioxide as blowing agent

Exfoliated layered-silicate in the polystyrene (PS) block copolymer with different molecular weights was employed as a model material to investigate the PS nanocomposite microcellular foams expanded by supercritical carbon dioxide. Using a well-controlled foaming procedure, we investigated the influence of molecular weight of PS, dispersion and loading of layered-silicate and pressure drop rate of a blowing agent on the cell size and cell density. Our experimental results indicate that only exfoliated layered-silicate can inhibit the cell expansion and has high nucleation efficiency during foaming. The average cell diameter can be reduced from 6 μm to 1.4 μm and the cell density can be increased from 7.6 × 10⁹ cells/cm³ to 5.0 × 10¹¹ cells/cm³. On the contrary, aggregated layered-silicate in PS did not show any effect on the cell morphology of PS foam.     

Effect of solvent plasticization on polypropylene microcellular foaming process and foam characteristics

In this research, the effect of crystalline fraction of polypropylene (PP) on cell nucleation behavior was overcome by an introduction of solvent‐plasticized step to the microcellular foaming in a solid‐state batch‐foaming process. Utilizing the plasticization performance of the solvent facilitated the PP to be foamed at the temperatures lower than its melting point with the dramatic development in the cellular morphology of the final foams. In consequence of the heterogeneous cell nucleation sites induction and the crystalline loss, which were induced by solvent, a high cell density (i.e., 10⁹–10¹⁰ cells/cm³) was promoted without the cell sacrificing at the elevated temperatures (155 and 165°C) and favorable PP microcellular foams were accomplished. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Low-stress over-molding of media-tight electronics using thermoplastic foam injection molding

Due to increasing automation and the associated rising demands on electronic assemblies, a suitable manufacturing process for large-scale production is needed to protect such products. The big challenge in this context is the low-stress encapsulation of the assemblies to protect them from external influences. In this study, the foam injection molding process was used to encapsulate FR4 (epoxy-based PCB) with Polyamid66 (PA66). The focus was on the production of a good assembly in terms of the quality of the bond and the media tightness. These parameters can be used to evaluate the protective effect against the surrounding. In the tests, a leakage rate of 0.025 m/min and shear stress of 6.5 MPa was achieved at low-foaming rates. This leakage is below the maximum acceptable threshold of 0.5 ml/min. The shear stress reaches values comparable to those in injection molding In addition to the requirements for leakage and composite quality, it could be shown that the internal mold pressure is reduced from 450 bar to below 10 bar by foaming. This can be used as the first indication of a reduced shear load on electronic components during over-molding. The suitability of the new solution concept is demonstrated.     

Ultrasonic irradiation enhanced cell nucleation: An effective approach to microcellular foams of both high cell density and expansion ratio

In this work, ultrasonic irradiation (UI) was used as the external energy source to assist polystyrene foaming process by using supercritical CO₂ as the physical blowing agent. It is shown that by introducing the UI at the very start of foaming, the resultant polymer foam exhibited significant and concurrent increase in cell density, i.e., three orders of magnitude, and expansion ratio, i.e., 1–3 times, compared to those without UI. Further experiments indicate that the enhanced cell nucleation induced by UI was the main reason for this unique phenomenon. This method also provided new insight into the mechanism of cell nucleation.     

Label behavior property attached on microcellular foamed parts

In comparison with the conventional foaming process, microcellular foaming by injection molding has the advantage of forming small bubbles of consistent size. Because of the reduction in the cycle time, the removal of sink marks, scale reliability, and weight lightening, microcellular foaming by injection molding is widely applied to electrical products, such as automotive parts, office automation equipment, and laptops. When microcellular foaming by injection molding is used with a resin such as polycarbonate, acrylonitrile butadiene styrene, or PC/ABS, microbubbles form. This enables the manufacture of cell phones, notebooks, and personal digital assistants (PDAs), which are impossible to produce with the conventional foaming technique because these products require a thin wall. For most thin‐wall products, spray and labeling processes are added. Therefore, research into the spray and labeling characteristics of injected foamed parts should come before applications. In this article, we analyze the swelling phenomenon that results from labeling on microcellular foamed parts. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 289–293, 2005     

Numerical analysis on reaction injection molding of polyurethane foam by using a finite volume method

Foam reaction injection molding (FRIM) is one of the most popular and useful processes for producing polyurethane foam with a complex geometry. A theoretical model which includes chemical reactions, foaming, and mold filling was developed to analyze FRIM. Energy balance equation was derived by considering polyurethane reaction, water-isocyanate reaction, and evaporation of physical blowing agents. Density and viscosity model was proposed for the bubble suspension, which was assumed to be a homogeneous phase. Based on the theoretical model, three-dimensional numerical simulation for mold filling of the polyurethane foam was carried out to predict flow field, flow front advancement, and density distribution during mold filling. Mold filling of a refrigerator cavity was investigated numerically. The density and thermal conductivity of the foam in the flow front was higher than those in the initially filled region.