注塑件熔接痕的研究进展

介绍了注塑件熔接痕的形成机理,分析了影响熔接痕强度的各种因素,从注塑工艺参数设定、模具结构设计等方面给出了消除熔接痕或提高熔接痕强度的措施。     

提高注射成型塑料制品熔接痕强度的研究进展

论述了熔接痕的形成原因,分析了熔接痕的微观结构,介绍了通过改善加工方法增强熔接痕强度的措施,指出了增强熔接痕强度研究的开发方向、技术动态和发展趋势。     

玻纤增强尼龙6材料熔接痕强度研究

通过选择不同种类增韧剂和改变增韧剂含量,研究了玻纤增强尼龙材料的熔接痕强度,结果表明,使用马来酸酐接枝POE(即POE-g-MAH)作为增韧剂的熔接痕拉伸强度和弯曲强度相比未改性的POE增韧体系分别提高约35%和31%;在使用POE-g-MAH作为的增韧剂的体系,增韧剂含量越高,熔接痕强度越低,即增韧剂含量从5%提高到10%,熔接痕拉伸强度保持率只有76%;同时发现,随着测试环境温度的上升,熔接痕强度下降非常明显。     

滑石粉填充ABS材料的熔接痕强度研究

采用熔融共混法制备了滑石粉填充丙烯腈-丁二烯-苯乙烯共聚物（ABS）材料,研究了滑石粉含量及粒径、复配填充体系以及润滑剂对材料熔接痕强度的影响,利用扫描电子显微镜分析了材料在熔接痕界面处的微观形貌。结果表明,材料的熔接痕强度降低,主要是由于片状滑石粉阻挡聚合物分子链在熔接痕界面处的相互扩散和熔合;通过降低滑石粉含量,与硅灰石复配,添加含有极性基团的酰胺类润滑剂可较好地提高材料的熔接痕强度;当5 %（质量分数,下同）的滑石粉与5 %的硅灰石复配,并添加1 %的改性酰胺类润滑剂时,材料的熔接痕强度达到33.4 MPa。     

挤出制品熔接痕的形成、结构分析

对挤出制品熔接痕形成的原理进行细致的分析,指出制品的熔接痕有“V形槽”及“弱连接”两种形式。熔接痕的强度及物理力学性能远远低于制品的其它部位。研究发现,熔接痕的结构与材料配方、挤出机机头模具及加工工艺参数均有关系。     

模具温度对PVC熔接痕拉伸性能的影响

针对聚氯乙烯(PVC)试样熔接痕性能的分析和评价,从模具温度出发,结合正交试验、信噪比分析和方差分析,详细研究了注射成型工艺参数对PVC试样熔接痕拉伸强度的影响,及在不同模具温度下,各工艺参数因子对试样熔接痕拉伸强度的影响程度。结果表明:注射压力是影响PVC试样熔接痕拉伸强度的关键因素;随着模具温度的升高,注射速率的影响程度升高,而PVC试样熔接痕拉伸强度先升高后降低。     

注塑件熔接痕位置预测技术的研究进展

介绍了注塑件充填过程熔接痕形成的数学模型,综述了数值模拟技术、数值模拟技术结合数学规划理论、阀式浇注塑大型注塑成型制品多浇口进料顺序控制技术等3种注塑件熔接痕位置预测、优化和控制方法,介绍了该技术的分析流程、预测方法及国内外的研究进展和应用实例,熔接痕控制技术,尤其是熔接痕消除技术的完善是今后熔接痕研究的一个重要方向。     

新型丙烯酸酯类相容剂对PC/PP合金熔接痕强度的影响

针对聚碳酸酯(PC)/聚丙烯(PP)合金材料熔接痕强度较低的问题,在传统乙烯–丙烯酸甲酯共聚物(EMA)相容剂的基础上,合作开发了一种主链段与PC相容、支链段与PP相容的含有丙烯酸酯共聚物的新型相容剂(CD型EA),通过扫描电子显微镜、差示扫描量热法和力学性能测试研究了两种相容剂对PC/PP合金分散相粒径、界面结合、熔接痕强度及拉伸与缺口冲击强度的影响,并研究了CD型EA的支化率(代表支链长度)对合金材料性能的影响。结果表明,与EMA相比,CD型EA在减小合金分散相粒径、提高界面结合及熔接痕强度方面具有更好的效果;此外,CD型EA相容剂的支化率是相容好坏的关键,当支化率为3%时,CD型EA未降低合金的拉伸强度,且使熔接痕强度和缺口冲击强度达到最大,比未加相容剂的合金分别提高了95.8%和53.2%,与EMA增容的合金相比分别提高了42%和4.2%。     

变模温控制技术改善注塑制品熔接痕的研究

阐述了变模温技术理论,借助CAE数值模拟仿真平台对翻盖手机外壳注塑成型过程进行动态模拟,对传统注射工艺和变模温注射工艺产生的熔接痕进行了详细的比较和分析。结果表明:相比传统的恒定模温控制,变模温控制技术对塑件熔接痕的改善效果更加明显,提高了熔接强度,优化了熔接痕,在不影响生产效率的基础上,达到了提升制品品质的目的。     

半透明注塑制品熔接痕分析及控制对策

借助模流分析软件Moldnow对半透明注塑制品熔接痕位置进行模拟分析,以此来优化浇口的位置和成型工艺参数,最终使熔接痕位于半透明注塑制品不重要的部位,并提高了熔接痕的强度.     

Computer simulation of weld lines in injection molded poly(methyl methacrylate)

Computer simulation packages have had success in predicting filling behavior in extremely complicated geometries. However, few packages offer the possibility of predicting the location and strength of weld lines. This work examines the sensitivity of Moldflow's weld-line prediction algorithm to variations in material properties and processing conditions. Qualitatively, the algorithm correctly predicted the effects of changes in viscosity, density, and PVT relationship on weld-line strength of a poly(methyl methacrylate). The algorithm is also successful in predicting the influence of variations in the injection time on weld line strength. However, the algorithm predictions for changes in the mold and die temperatures were at odds with the experiment. An attempt was made to correlate Moldflow's computed viscosities with the experimentally measured reduction in the strength in the weld-line area. It was shown that a one-to-one relationship existed between these two quantities. Whereas the potential of using the viscosity to predict weld-line strength has been demonstrated, further refinement of this new concept is needed and its validity for other systems has to be established.     

A modified model predictions and experimental results of weld-line strength in injection molded PS/PMMA blends

In this paper, the model based on melt diffusion and Flory-Huggins free energy theory for predicting the weld-line strength of injection molded amorphous polymers and polymer blends parts were modified by considering the diffusion thickness in the interface as a function of contact time. The modified model for weld-line strength prediction of homopolymers and polymer blends were, respectively, used to evaluate the weld-line strength of Polystyrene (PS) and Poly(methylmethacrylate) (PMMA), and that of PS/PMMA blends. The model predictions show that the theoretic predictions as a function of temperature and contact time for PS, PMMA and PS/PMMA (80/20, 70/30) are in good agreements with corresponding experimental results. However, the model predictions for PS/PMMA (20/80, 30/70) blends are much higher than experimental results. The morphology in weld-line regions for PS/PMMA (20/80, 30/70) shows lack of dispersed PS phase. Near the weld-line regions, dispersed PS phase is highly oriented along the weld-line. In theoretic prediction for polymer blends, three kinds of diffusion: Polymer A-Polymer A and Polymer B-Polymer B self-diffusions and Polymer A-Polymer B mutual diffusion were considered. This is why model predictions for PS/PMMA (20/80, 30/70) blends are higher than experimental results.     

Comparative study of weld-line strength for unfilled and glass-filled thermoplastic polyamide-6 materials

The injection molding process is widely accepted for the processing of engineering thermoplastics due to the ease of manufacturing complex designs. Weld-line is a defect occurring in injection molded parts when two flow fronts join each other. At weld-line locations, parts exhibit lower mechanical strength mainly due to inadequate intermolecular diffusion and fiber orientation anisotropy. The present work is aimed at investigating and comparing weld-line strength for unfilled and glass-filled polyamide-6 materials. To achieve this, polyamide-6 unfilled, 30% glass-filled, and 50% glass-filled materials are used to manufacture plaques. The special-purpose mold is designed to obtain plaques with and without weld-lines with help of Moldflow simulations. The specimens for tensile tests are then cut from molded plaques and experimental testing is conducted to evaluate tensile properties. Fractured surfaces of specimens are examined using a scanning electron microscope. The results demonstrated a significant drop in tensile strength and modulus for glass-filled material weld-line specimens when compared to specimens of no weld-line. However, for unfilled specimens, tensile strength and modulus are almost the same for samples with and without weld-line. A reduction in tensile strength of 13%, 49%, and 57% is observed for unfilled, 30% glass-filled, and 50% glass-filled polyamide-6 material respectively.     

Evaluation of the weld-line strength of thermoplastics by compact tension test

In order to understand the relationship between processing conditions and the properties of weld-lines on a molecular level, it is necessary to evaluate the true strength of the weld-line that is not affected by the V-shape notch on the surface of the weld-line zone. In this experiment, the weld-line strength of several brittle, ductile, or phase-separated polymers was evaluated using the compact tension test by measuring the critical stress intensity factor, K_IC, or the critical J-value, J_IC, and the results were compared with those obtained by tensile testing. For brittle polymers such as poly(methyl methacrylate) (PMMA) or styrene acrylonitrile copolymer (SAN), the value of the weld-line factor, i.e., the strength ratio between the welded and the non-welded specimen, is higher than that measured by tensile testing, because of the notch sensitivity of brittle thermoplastics and the notch dependence of tensile strength. On the other hand, in the case of ductile polymers such as polycarbonate (PC), the weld-line factor is similar for both the tensile and compact tension tests. However, the dependency of the weld-line factor on melt temperature is more obvious in the compact tension test. From these results, it seems that the compact tension test is more appropriate for measuring the interfacial adhesion strength across the weld-line, which excludes the notch effect.     

Weld-line characteristics in short fiber reinforced thermoplastics

Fibers can greatly improve the mechanical properties of polymers but may also severely weaken molded parts at their weld-line compared to their bulk strength. The tensile properties and fiber orientation of injection and compression molded fiber reinforced Noryl and polypropylene samples with and without a weld-zone were studied. Distinct differences in structure and mechanical properties of weld-containing and weld-free samples were identified. In unfilled Noryl and unfilled polypropylene, the presence of a weld-line was found to only have a small effect on the tensile strength and modulus, while in the corresponding fiber reinforced systems, orientation of the fibrous reinforcement parallel to the weld-line caused a significant reduction of the tensile strength compared to the weld-free products. The strength ratio of welded and unwelded specimens was found to decrease with increasing fiber concentration. Quantitative determination of the glass fiber orientation distribution within the weld-line region and in the bulk was carried out by analyzing photomicrographs of polished sections at desired locations.     

复合塑料的熔合缝

介绍改性、增强聚合物和聚合物混合物的熔合缝的力学性能、熔合机理和改善复合塑料熔合缝力学性能的方法。     

A fracture mechanics approach to weld-line fracture in an amorphous cellulose acetate propionate

The fracture properties of molded articles containing weld-lines are analyzed by a fracture mechanics approach, treating the weld-line as a sharp crack. The theoretical length of the weld-line crack is estimated from diffusion properties and a simple model of cooling in a mold. These estimated weld-line crack lengths are found to be in excellent agreement with weld-line crack lengths calculated from measured fracture properties. Above a critical melt temperature, this weld-line crack length will fall below a critical crack length, whereby the weld-line will no longer initiate brittle fracture. This critical crack length is a function of the material's inherent yield strength and fracture toughness. The embrittling effect of weld-lines can also be easily removed by surface treatments such as surface milling or solvent polishing. Weld-lines are, therefore, determined to be near-surface flaws, which embrittle notch sensitive materials such as most amorphous polymers.     

Visualization of the flow history contours at the cross-section of a weld-line in an injected molded part

This article presents visualization of flow history contours at the cross-section of an injection molded part along the weld-line originated by an obstacle pin in a plate-shaped cavity. The visualized flow history contours unveiled the interface of the two adjacent flows through which the penetration of one flow front to the other at the interface was observed. The penetration which was the most at the core of the part behind the obstacle pin was inferred to be originated from an eccentricity in the position of the obstacle pin. Applying image processing techniques, a sequence of the variation in the shape of the interface along the weld-line was extracted from which the amount of penetration at different distances behind the obstacle was measured. Experimental results revealed an oscillation in the direction of the penetration along the weld-line at the core of the part. A scanning electron microscope micrography was also applied to characterize the V-notch along the weld-line at the surface of the molded part. V-notch characterization was used to investigate the possible correlation between the internal and external behaviors of the molten plastic during weld-line formation by which an obstruction affected zone at the weld-line area behind the obstacle pin was distinguished. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Experimental study and computer simulation of the effect of spider shape on the weld-lines in extruded plastic pipe

Weld-lines, also referred to as spider-lines or knit-lines, are regions in the wall of an extruded plastic pipe, running the whole length of the pipe, at which there is a change in dimension and a possible reduction in mechanical properties. Weld-lines result from the flow of the polymer melt around the supports (spiders) that hold the metal core to the outer die assembly in an in-line annular die. This work examines the fatigue life and the tensile strength of weld-lines created by a conventionally shaped “torpedo” spider and four modified spider shapes. It is demonstrated that the shape of the spiders has a dramatic effect on the fatigue life of weld-lines while having a minimal effect on the tensile strength in static loading. The experimental results show that the fatigue life of the weld-line produced by one of the four new shaped spiders is significantly higher than that of a conventionally shaped spider. Numerical simulations of the flow field around this new and conventional spiders were carried out using the package FIDAP® based on the finite element method (FEM). The simulation results provide a good explanation for the experimental observations and show that the newly designed shape of spider can give rise to a higher relative fatigue life of the weld-line when compared with that of the bulk material than does the conventional spider.     

Morphology and mechanical properties of injection molded articles with weld-lines

The effect of weld-lines on the morphology and mechanical properties of injection molded articles made of neat poly(butylene terephthalate) (PBT) and glass fiber-reinforced PBT was investigated. The weld-line was introduced to a molded article by using a rectangularly shaped insert inside a mold cavity, and tensile specimens were prepared at various positions through the entire molded article. The weld-line position was further checked by a short-shot experiment. Although the maximum tensile stress for specimens of neat PBT with a weld-line is almost the same as that without a weld-line, the maximum tensile stress and the elongation at break for fiber-reinforced PBT with a weld-line were found to be about half of those without the weld-line. This is attributed to the fact that the fibers near the weld-lines are oriented parallel to the weld-line direction (or perpendicular to the tensile force direction) due to stretching flow. Finally, we compared experimental results of flow pattern and fiber orientations with numerical simulations. We found that the predictions of flow fronts and fiber orientations are in good agreement with experimental results.