注射成型熔接线区域流动诱导双折射行为

带有熔接线的透明聚苯乙烯平板制品通过带嵌件的模具注射成型,利用光弹测试考察了不同工艺条件下的制品双折射行为,探讨了嵌件造成的相邻熔接线区域熔体流动行为及对制品双折射分布的影响。结果表明,保压压力和熔体温度对制品双折射整体分布影响明显;嵌件后熔接缝区域与制品其他区域的双折射行为有较大差异,其变化与熔体相遇后流动行为和熔接部位分子取向分布有关;制品最大双折射值出现在嵌件后一定距离处。热收缩实验进一步验证了上述结果。     

多面体金属效果颜料的制备与应用

通过磁控溅射镀膜工艺,采用铝靶为溅射源,成功实现铝单质在微米级二氧化硅粉体表面的沉积,制备出新型多面体金属效果颜料。重点比较了多面体金属效果颜料与铝颜料在注塑制件中的流痕与熔接痕差异,同时,研究了多面体金属效果颜料对塑料基材物性的影响及加工工艺对多面体颜料所呈现表观效果的影响。最后,采用多面体金属效果颜料共混入塑料基材中成型出“无流痕”免喷涂制件。结果表明,与传统铝颜料相比,多面体金属效果颜料表现出良好的金属质感,流痕几乎完全消除,熔接痕亦大幅度减轻,并进一步从理论角度分析和解释了产生上述流痕和熔接痕差异的原因;与喷涂制件相比,“无流痕”免喷涂制件更环保,成本更低,表观效果更加独特,并得到了消费者认可,成功实现了商业化量产。     

The influence of the type of polypropylene and the length of the flow path on the structure and properties of injection molded parts with the weld lines

Weld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. In the weld line area, the V‐shaped notch is formed and its shape and size depend on injection molding conditions and properties of processed polymer. Addition of the foaming agent to the polymer can be one of the way to improve conditions of melt streams welding due to the higher velocity of the colliding streams of unfilled polypropylene (PP) and PP filled with talc. The examinations of mechanical properties showed, however, lower tensile strength of porous parts compared to solid ones, but in the microscopic observation and measurements of the geometric structure of moldings, in the weld lines area, better surface conditions were achieved for samples made of the foamed polypropylene. The size of V‐notch, determined by the total height of the raw profile Pt, depends also on the length of polymer flow path from the gate to the weld line area. The values of Pt parameter increase with the length of the flow path, but this increase is smaller for foamed polypropylene. POLYM. ENG. SCI., 59:1710–1718 2019. © 2019 Society of Plastics Engineers     

Uni‐ and biaxial impact behavior of double‐gated nanoclay‐reinforced polypropylene injection moldings

Polypopylene/nanoclay three‐dimensional parts were produced without intermediate steps by direct injection molding to explore the influence of flow features and nanoclay incorporation in their impact performance. The nanocomposite was obtained by direct compounding of commercial PP with nanoclay masterbatch. The as‐molded morphology was analyzed by X‐ray and TEM analyses in terms of skin‐core structure and nanoclay particle dispersion. The nanoclay particles induced the reduction of β‐form spherulites, a known toughener. The impact behavior was assessed in tensile and biaxial modes. The PP nanocomposite molding toughness was practically unaffected by the processing melt temperature and flow rate. Conversely the nanoclay presence is influent in the impact performance. Under biaxial stress impact, the regions close to weld lines are tougher than the bulk and the fracture develops with main crack paths along the flow direction and the weld line. Cracking along the weld line results from less macromolecular interpenetration and chain entanglement, and unfavorable nanoparticle orientation. It seems that a failure mechanism which involves nanoclay delamination and multiple matrix crazing explains the toughening of PP in the directions where the nanoparticle orientation with respect to loading is adequate. POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

Load‐carrying ability of polystyrene products with molded‐in holes

The combined influence of stress concentration, weld line, and local distribution of molecular orientation on injection molded polystyrene (PS) plates with a moldedin hole is described. Birefringence measurements were used to investigate the molecular orientation, and three‐point flexure was used to investigate the mechanical properties. Both blunt notch tests and transverse bending tests were carried out. Blunt notch tests showed a strong variation of the material resistance to fracture around the hole. The molecular orientation increased the material resistance not only at the hole equator, but also at the flow split area. In a large domain, wela line fracture was the dominant fracture mechanism. Transverse bending tests also showed a strong influence of the molecular orientation on the load‐carrying ability. The weld line was definitely the weakest region, but the reduction caused by the weld line itself appeared to be small.     

TOF‐SIMS study of injection‐molded polystyrene

The purpose of this study was to provide experimental evidence of the separation of the polymer components at different scales during conventional processing. This was achieved by characterizing the surface and the bulk (cross section) of moldings manufactured with a high‐flow grade and a low‐flow grade of commercial polystyrene by the time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) analytical technique. Owing to the geometric constraints of the mold used, a weld was also obtained. Different surface spectra were observed for the two molded polystyrenes. The surface of the high‐flow grade moldings showed the spectral features of low‐molar polyolefin (paraffin) contaminants, whereas the bulk was dominated by polystyrene. Spectra from both the surface and the bulk of the low‐flow grade moldings were characteristic of polystyrene. Mold‐filling effects on the surface composition were observed in the flow front region of molded short‐shots of the low‐flow grade. The spectral changes indicated the abundance, in the surface, of the high end of the molar‐mass distribution of the material during the mold filling process. Two‐dimensional maps of the secondary ions from the low‐flow grade also showed an occasional alkali contamination, preferentially along the notch of the weld.     

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     

Development of a noble aluminum-pigmented metallic polymer: Recommendations for visible flow and weld line mitigation

We investigated the appearance of flow and weld lines when metallic pigments are used in polymer blends and how such lines can be eliminated by improving the pigment particle shape and optimizing pigment loading. Acrylonitrile butadiene styrene copolymer and two types of aluminum flakes, lamellar and three-dimensional (3D), were blended in a twin-screw extruder with a screw diameter of 25 mm. The temperatures from the hopper to the nozzle were 140, 180, 220, 220, 220, 220, and 220°C. Weld and flow lines were observed using field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy of specially manufactured injection specimens. In the flow line region, traditional lamellar flakes were randomly oriented, while 3D flakes exhibited a distinct and stable orientation. Based on these observations, flow and weld lines in a finished metal/polymer blend can be minimized by using 3D metal particles in place of lamellar flakes. We also investigated the effects of aluminum flake loading on weld and flow line visibility. At low loading, weld lines were clearly visible due to the lack of pigmentation in the front of the polymer flow. Conversely, high loading resulted in relatively high concentrations of pigment near the weld line area, reducing weld line visibility. These findings suggest that there is an optimum metal loading level where the visibility of flow and weld lines is minimized.     

Effect of an obstacle during processing on the weld line of injection‐molded glassy polystyrene: Microhardness study

The microhardness (H) technique is used to characterize the quality of the weld line in injection‐molded glassy polystyrene by means of a cylindrical obstacle. In particular, the effect of the indentation location (closer or further from the obstacle edge parallel to the injection direction and across the weld line), both on the surface and in the bulk, was examined. Only for surface measurements close to the obstacle (up to 10 mm) a well‐pronounced decrease in H (～30%), followed by a sharp increase in a narrow distance (0.20–0.25 mm), was observed. For the bulk measurements on the same location a slight decrease in H was detected. Additional H measurements made up to 60 mm from the obstacle for both cases showed that the weld line remains undetectable. The results obtained reveal that the presence of a cylindrical obstacle causes the formation of a weld line on and near the surface only at distances not exceeding the obstacle diameter. At larger distances, because of the effective mutual interdiffusion of polymer chains, the two parallel fronts coming from the two sides of the obstacle developed a homogeneous material without any weld line according to the microhardness test. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3362–3367, 2004     

Weld line morphology of injection molded polypropylene

The weakness of plastics at weld lines provides serious difficulties for the design and long term durability of injection molded parts. The goal of this work was to identify the cause of weld line weakness in polypropylene (PP) systems. The morphology of weld lines in a high molecular weight PP has been studied. It was found that the PP contains a hindered phenolic antioxidant additive that is not soluble in the polymer at the standard processing conditions. Transmission electron microscopy (TEM) pictures reveal the additive existing as a dispersed phase in the bulk polymer. Even though very small concentrations of this additive are normally used, (0.1–0.5%) large quantities were found at weld lines in a band approximately 100 nm wide and penetrating about 10 μm into the surface of the part, hindering strength development at the weld line. X-ray photoelectron spectroscopy (XPS) results confirm enhanced concentrations of antioxidant on the flow front and mold wall surface of short shot samples. The mechanical properties (Izod impact, tensile strength) are measured for samples molded at various processing conditions, varying amounts of antioxidant additive and with and without weld lines. The results are consistent with the presence of the additive playing a key role in strength development at PP weld lines.     

Fracture toughness evaluation of adjacent flow weld line in polystyrene by the SENB method

Fracture toughness of adjacent flow weld lines, defined as weld lines that occur when two flow fronts meet and continue to flow together in the same direction (meld line or hot weld line), was evaluated by the single‐edge notched‐bend (SENB) method using three differently‐shaped obstructive pins. Although the fracture toughness varied depending upon the shapes of the pin, the values could be standardized as the distance from the meeting point of the two flow fronts flowing around the pin. The fracture toughness decreased drastically from the meeting point along the weld line and then slightly increased. These characteristic features could be explained by flow‐induced molecular orientation at the weld line interface. The molecules around the meeting point that were initially oriented parallel to the weld line due to fountain flow were able to relax, and then entanglement across the weld line interface developed because the flow stopped in the middle of the filling process, resulting in high fracture toughness. In contrast, the material at the downstream side of the weld line continued flowing during the filling process, being stretched along the flow direction. So, the molecular orientation at this area could not relax. In addition, the V‐notch shape, i.e., the depth and length at the surface of the weld line, which also varied depending on the shape of the obstacles, was considered to be identical when the meeting point was allowed to be a datum point. Thus, the meeting point was found to be a significant factor when the properties of weld lines are investigated. POLYM. ENG. SCI., 45:1059–1066, 2005. © 2005 Society of Plastics Engineers     

Evaluation of mechanical properties of adjacent flow weldline

Weldlines occur at the interface of two adjacent flows of material behind an obstructive pin in a cavity in injection molding (meldline or hot weldline). Tensile strength of such “adjacent flow weldline” in injection molded polystyrene plates was evaluated by a mechanical step‐by‐step milling technique. The strength when the milling depth was 1/5 of the thickness from each surface was about the same and independent of the distance from the pin. In contrast, the strength without milling decreased once and then increased along the flow direction. This demonstrates that the strength of a weldline is predominantly dependent on the properties of the surface layer of the weldline. The depth of the surface layer was defined as the depth of the weld, D_w. D_w reduced monotonously along the flow direction and faded away with the V‐notch, resulting in an increase of strength along the direction. On the other hand, it was considered that the farther from the pin, the flow‐induced molecular orientation in the surface layer is greater. It caused a decrease of the strength along the flow direction. The sequence of decrease and increase in tensile strength of adjacent flow weldline is due to the complex effect of these two contradictory factors. POLYM. ENG. SCI., 45:1180–1186, 2005. © 2005 Society of Plastics Engineers     

Weld‐line characteristics of polycarbonate/acrylonitrile–butadiene–styrene blends. I. Effect of the processing temperature

The effects of the processing temperature on the morphology and mechanical properties at the weld line of 60/40 (w/w) polycarbonate (PC)/acrylonitrile–butadiene–styrene (ABS) copolymer blends were investigated. The influences of the incorporation of poly(methyl methacrylate) (PMMA) as a compatibilizer and an increase in the viscosity of the dispersed ABS domain phase were also studied. The ABS domain was well dispersed in the region below the V notch, and a coarse morphology in the core region was observed. When tensile stress was applied perpendicularly to the weld line, the fracture propagated along the weak region behind the weld part; there, the domain phase coalescence was significant because of the poor compatibility between PC and styrene–acrylonitrile (SAN). Phase coalescence became severe, and so the mechanical strength of the welded specimen decreased with an increasing injection‐molding temperature. The domain morphology became stable and the mechanical strength increased as the viscosity of the domain phase increased or some SAN was replaced with PMMA. That the morphology was well distributed behind the weld line and the mechanical properties of PC/ABS/PMMA blends were improved was attributed to the compatibilizing effect of PMMA. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 689–699, 2005     

Fatigue performance of injection‐molded short e‐glass fiber reinforced polyamide‐6,6. II. Effects of melt temperature and hold pressure

Tensile and fatigue properties of an injection molded short E‐glass fiber reinforced polyamide‐6,6 have been studied as a function of two key injection molding parameters, namely melt temperature and hold pressure. It was observed that tensile and fatigue strengths of specimens normal to the flow direction were lower than that in the flow direction, indicating inherent anisotropy caused by injection molding. Tensile and fatigue strengths of specimens with weld line were significantly lower than that without weld lines. For specimens in the flow direction, normal to the flow direction and with weld line, tensile strength and fatigue strength increased with increasing melt temperature as well as increasing hold pressure. The effect of specimen orientation on the tensile and fatigue strengths is explained in terms of the difference in fiber orientation and skin‐core morphology of the specimens. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers.     

Effects of geometry and injection‐molding parameters on weld‐line strength

Polymeric flows in microchannels are found to differ significantly from those in macrogeometries. Increasing the mechanical properties of microstructures is one of the most important issues in injection‐molding processes. Weld‐line characteristics of structures with different cross‐sections are investigated in this study. The effects of process parameters and cross‐sectional dimensions on the tensile strength of a weld line are discussed. A mold was designed in such a way that specimens with and without weld lines can be developed separately. Five specimens, with different cross‐sections, are injection‐molded simultaneously. Both polypropylene (PP) and high density polyethylene (HDPE) are used in this study. With the Taguchi method, four process variables: melt temperature, mold temperature, injection speed, and packing pressure were found to be the most influential. Experimental results show that the weld‐line strength from a standard test is not applicable in microinjection molding. The microstructure of weld lines is clearly observed from the micrographs. POLYM. ENG. SCI., 45:1021–1030, 2005. © 2005 Society of Plastics Engineers     

Mechanical properties of polypropylene-low density polyethylene blends

Blends of polypropylene (PP) and low density polyethylene (LDPE) were prepared by both batch mixing followed by compression molding and extruder compounding followed by injection molding. Compression molded PP-LDPE blends were found to have very poor toughness, whereas extruded blends, injection molded without weld lines, were quite tough. Injection molded blend specimens with weld lines were found to be weaker and failed at very low elongations at break. A simple adhesion analysis is presented which explains well the weakness at the weld line expected for incompatible blends. Addition of an ethylene-propylene polymer with residual ethylene crystallinity was found to be a more effective “compatibilizer” for blends deficient in toughness than a related copolymer with less crystallinity. This effect is attributed to the more block-like character of the former which permits it to play more nearly the interfacial role required of the ideal blend compatibilizing agent.     

Effect of process conditions on the weld‐line strength and microstructure of microcellular injection molded parts

This study was aimed at understanding how the process conditions affect the weld‐line strength and microstructure of injection molded microcellular parts. A design of experiments (DOE) was performed and polycarbonate tensile test specimens were produced for tensile tests and microscopic analysis. Injection molding trials were performed by systematically adjusting four process parameters (i.e., melt temperature, shot size, supercritical fluid (SCF) level, and injection speed). For comparison, conventional solid specimens were also produced. The tensile strength was measured at the weld line and away from the weld line. The weld‐line strength of injecton molded microcellular parts was lower than that of its solid counterparts. It increased with increasing shot size, melt temperature, and injection speed, and was weakly dependent on the supercritical fluid level. The microstructure of the molded specimens at various cross sections were examined using scanning electron microscope (SEM) and a light microscope to study the variation of cell size and density with different process conditions.     

Effect of supercritical nitrogen content on the weld lines and mechanical properties of microcellular injection molded double gate PP/Al parts

Injection molding products made of aluminum flakes and polymer blends exhibit a distinctive esthetic effect. However, during the filling process, the melt flows in different directions converge and collide, resulting in the flop effect of the aluminum flake and consequent weld line formation. Herein, microcellular injection molding (MIM) was employed to fabricate polypropylene/aluminum flakes (PP/Al) composite foamed parts with distinct weld lines using supercritical nitrogen (scN₂) as the physical blowing agent. The scN₂ content has a significant effect on cell diameter and cell density. When the scN₂ content was 0.6%, the weld line width of the foamed part was 13.03 μm, while it was 30.41 μm for the solid counterpart due to the expansion and rupture of cells in the flow front during filling. Moreover, the orientation of Al flakes was mostly along the flow direction for the foamed parts, while it was generally aligned perpendicular to the flow direction for solid parts in the weld line region. In addition, the flexural modulus of foamed parts was increased by 29% compared with the solid parts, although the tensile strength was reduced by 18% due to the alignment of Al flakes and the stress concentration on the cell walls. Therefore, this work provides insight into the improvement of flexural property and the mitigation of weld lines for injection molded composite parts using MIM.     

Advanced injection molding mold and molding process for improvement of weld line strengths and isotropy of glass fiber filled aromatic polyester LCP

An advanced injection molding tool for measurement of mechanical strength and anisotropy of liquid crystal polymers (LCP)/mineral filler composites was developed. The mold produces thin‐walled LCP specimens that can be used by water cutting technique for production of an injection molded flow direction test bar, a transverse‐to‐injection molded flow direction test bar, a test bar for knit line strength measurement, and a test bar for butt weld line strength measurement. This tool and its use for molding experiments were optimized by experimental research and by computational calculations based on experimental parameters obtained by molding of several LCP test materials. Different pressure profiles and different injection speeds were tested as well as application of mold overflow phenomenon in production of test specimens. It was observed that a pressure controlled X‐melt technique and on the other hand fast injection speeds with overflow in conventional molding methods gave the best strength and isotropy properties for the test specimens. Results indicate that the mold developed is useful for determination of anisotropic and weld line strength properties of LCP composites. When developing “isotropic LCP” by different possibilities of nanotechnology this tool significantly reduces time of LCP material and process development. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Vibration welding of nano‐TiO2 filled polypropylene

Polypropylene (PP)‐based nanocomposites compounded by a twin‐screw extruder and injection molded into plates those were then joined by linear vibration welding. The mechanical performances of the welds and bulk materials were examined. While the incorporation of rigid particles slightly improves the impact strength of the bulk PP, the mechanical properties of the welds decrease with increasing nanoparticle contents. The best weld quality is obtained at low weld pressure without nanoparticles. The fracture surfaces and microstructure of the welds showed that the reduced weld quality is caused by the orientation of nanofillers parallel to the weld plane, the destruction of interphase between fillers and matrix, and the reduction of molten‐film thickness by incorporation of nanoparticles. POLYM. ENG. SCI., 55:243–250, 2015. © 2014 Society of Plastics Engineers