Study on mechanical properties and material distribution of sandwich plaques molded by co‐injection

In the sandwich injection molding process (co‐injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co‐injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.     

夹芯注塑成型过程的计算机可视化模拟分析--材料粘度与工艺参数对芯层分布及均匀性的影响

为了了解夹芯注塑的成型过程、探悉其成型机理，采用Moldflow公司MPI软件中的Co-injection分析模块，对夹芯注塑成型过程进行动态模拟分析，揭示材料粘度以及工艺参数对夹芯注塑成型过程中芯层分布均匀性的影响规律。结果表明，芯层物料分布均匀性随芯／壳层熔体粘度比R值的减小而提高，这主要与芯层和壳层熔体的相对流动能力有关。此外，在工艺参数中，改变熔体注射速度对芯层物料分布均匀性的影响较为突出，而模温和熔体温度对芯层物料分布均匀性的影响却相对较弱。     

A basic experimental study of sandwich injection molding with sequential injection

An experimental study of sandwich injection molding is reported which involves sequential injection of polymer melts with differing melt viscosity into a mold. In isothermal injection molding the relative viscosity of the two melts is the primary variable determining the phase distribution in the mold. Generally the most uniform skin-core structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. Large viscosity inequalities lead to nonuniform skin thicknesses. The influence of blowing agents and non-uniform temperature fields on the extent of encapsulation is described. Temperature fields are very important especially if the first polymer melt injected has a greater activation energy of viscous flow (or a greater temperature dependence of the viscosity function).     

夹芯注塑成型过程的计算机可视化模拟分析--材料粘度与工艺参数对芯层熔体穿透深度的影响

采用Moldflow公司MPI软件中的Co-injection分析模块，对夹芯注塑成型过程进行动态模拟分析；以揭示材料粘度以及工艺参数对夹芯注塑成型过程中芯层熔体穿透深度的影响规律。结果发现，芯层熔体穿透深度值随芯／壳层熔体粘度比R值的减小而增大，这主要与芯层和壳层熔体的相对流动能力有关；此外，在工艺参数中，改变熔体注射速度对芯层熔体穿透深度的影响较为突出，而模温和熔体温度对芯层熔体穿透深度的影响相对较弱。     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

注射工艺条件对抗冲共聚聚丙烯力学性能的影响

用注塑成型方法制备了抗冲嵌段共聚聚丙(烯PP-B)试样,研究了工艺条件的改变对PP-B力学性能的影响。结果表明:注射速率和模具温度与PP-B弯曲强度、弯曲模量成正相关,而与冲击强度、拉伸强度的关系较为复杂;模具温度的升高有利于结晶度的提高,PP-B刚性上升,韧性下降;注射速率改变引起的剪切、拉伸流动使熔体发生取向流动,从而使沿取向方向上的强度升高,垂直取向方向上的强度降低;保压压力对PP-B性能的影响主要是通过熔体非晶部分在保压阶段的分子定向作用来实现的,随着保压时间的延长,PP-B塑件密度提高缺,陷减少拉,伸强度弯、曲模量呈增大趋势。     

夹芯注射成型研究进展

为了了解夹芯注射的成型过程及内部结构、探悉其成型机理,研究者主要对芯层熔体前缘的冲破现象、芯壳层物料的分布情况以及夹芯注塑件的力学性能进行了研究。文献显示,物料的性能尤其是粘度、加工工艺参数如注射速度、模温、熔融温度等以及模具尺寸对夹芯注射的充模过程及其制品最终的性能影响最为突出。     

Comparison of structure development in injection molding of isotactic and syndiotactic polypropylenes

A comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X‐ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well‐defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α‐form and the hexagonal β‐form. Considerable amounts of β‐form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen‐in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold‐filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers.     

Co‐injection molding: Effect of processing on material distribution and mechanical properties of a sandwich molded plate

The effect of molding parameters on material distribution and mechanical properties of co‐injection molded plates has been studied using experimental design. The plates were molded with a polyamide 6 (PA 6) as skin and a 20% glass fiber‐reinforced polybutyleneterephtalate (PBTP) as core. Five molding parameters—injection velocity, mold temperature, skin and core temperature, and core content—were varied in two levels. The statistical analysis of the results showed that three parameters—Injection velocity, core temperature, and core content—were the most significant in affecting skin/core distribution. A high core temperature was the most significant variable promoting a constant core thickness, while core content was the most significant factor influencing a breakthrough of the core. Mechanical properties, such as flexural and impact strength showed a high correlation with the skin/core distribution. The slight increase in falling weight impact strength of the sandwich molded plates, compared to similar plates molded from PBTP only, could be explained from the failure process, which initiates in the brittle core and propagates through the ductile skins.     

Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume fraction, and the extent of resin pre-cure. The fiber orientation, forming discrete skin-core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre-cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequentially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.     

Simulation and Experimental Investigations of Material Distribution in the Sandwich Injection Molding Process

In sandwich injection molding, two polymeric materials are sequentially injected into a mold to form a multilayer product with a skin and core structure. Different properties of these polymers and their distribution in the cavity greatly affect the applications of the moldings. In an ideal situation, the core material should be entirely encapsulated in the skin material. When the flow front of the core material overtakes that of the skin material, breakthrough occurs, resulting in a defective part. The focus of this study is to determine the effect of molding parameters on the skin/core material distribution. The commercial simulation package (Moldflow) has been extensively compared with experiments. Both simulated and measured results suggest that in order to obtain the optimum encapsulated skin/core structure in the sandwich injection molded parts, it is necessary to select a proper core volume fraction and suitable processing parameters. A good agreement between simulation and experimental results indicates that the Moldflow program can be used as a valuable tool for the prediction of melt-flow behavior during the sandwich injection process.     

Morphological development during injection molding of self‐reinforcing composites. I: Experimental results

The correlation between structure development during injection molding and the modulus of injection molded PET/LCP blends were studied. Process parameters such as injection speed and mold and melt temperatures were varied to determine the effect of these parameters on the tensile modulus and structure development of the blends. The skin/core structure in the cross section of injection molded samples was observed with both optical and scanning electron microscopy techniques. Injection molding experiments show that the thickness of the skin layer increases with decreasing injection speed and decreasing melt and mold temperatures. The trends in morphological developments in the injection molded specimens correlate with the measured tensile moduli.     

微孔发泡模内表面装饰复合成型工艺参数对泡孔结构与力学性能影响

微孔发泡模内表面装饰复合成型工艺是高表观质量、轻量化塑料制品的重要成型方法,成型制品的泡孔结构与力学性能对其最终质量具有决定性影响.以典型拉伸样条为例,采用数值模拟的方法,通过对比分析不同工艺参数条件下平均气泡半径、密度和力学性能的变化规律,研究该工艺过程中主要工艺参数对泡孔结构及其力学性能的影响.结果表明,注射速率、...     

Processing of polyolefin blends

The morphology of binary and ternary polyolefin blends of polypropylene (PP), ethylene-propylene-diene terpolymer (EPDM) and polyethylene (PE) following processing by injection and compression molding has been examined by optical and scanning electron microscopy. Internal surfaces were generated by low temperature fracture and etching with cyclohexane. In binary blends, droplets of EPDM are elongated in the flow direction within 400μm of the mold surface in injection molding, yielding a skin region which is distinct from an isotropic core containing spherical EPDM inclusions. Spherical droplets of EPDM or PE in binary blends with PP increase in size with increasing compression molding time. In ternary blends, spherical inclusions containing both EPDM and PE are dispersed in PP. With increasing compression molding time, EPDM separates from PE and concentrates at the outer edges of the PE inclusion, effectively isolating PE from the PP matrix.     

Study of the effect of processing conditions on the co‐injection of PBS/PBAT and PTT/PBT blends for parts with increased bio‐content

This work studies the effect of processing parameters on mechanical properties and material distribution of co‐injected polymer blends within a complex mold shape. A partially bio‐sourced blend of poly(butylene terephthalate) and poly(trimethylene terephthalate) PTT/PBT was used for the core, with a tough biodegradable blend of poly (butylene succinate) and poly (butylene adipate‐co‐terephthalate) PBS/PBAT for the skin. A ½ factorial design of experiments is used to identify significant processing parameters from skin and core melt temperatures, injection speed and pressure, and mold temperature. Interactions between the processing effects are considered, and the resulting statistical data produced accurate linear models indicating that the co‐injection of the two blends can be controlled. Impact strength of the normally brittle PTT/PBT blend is shown to increase significantly with co‐injection and variations in core to skin volume ratios to have a determining role in the overall impact strength. Scanning electron microscope images were taken of co‐injected tensile samples with the PBS/PBAT skin dissolved displaying variations of mechanical interlocking occurring between the two blends. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41278.     

Structure and properties of polyblends of a thermotropic liquid crystalline polymer with an alloy of polyamide-6 and ABS

The relationship between the microstructure and corresponding mechanical properties developed during injection molding of blends containing a liquid crystalline polymer (LCP) as the minor component and an engineering polymer system has been studied. A wholly aromatic copolyester LCP (Vectra A950) was melt blended at different compositions with a thermoplastic matrix consisting of a commercial compatibilized blend of polyamide-6 and ABS (Triax 1180). These blends were prepared under two different sets of injection molding conditions. In the first case, a higher melt temperature, higher barrel temperature, lower injection pressure, lower mold temperature, and shorter residence time in the mold were used during injection molding, as compared with the second case. The mechanical properties of the blends were superior to those of the base polymer. In the second case, the resulting injection-molded specimens had a distinct skin–core morphology where elongated fibrils of LCP constituted the skin layer. The mechanical properties of the blends processed under the second set of processing conditions were superior to those of the first, though the trends in both cases were the same. To study the effects of process variables the 15% LCP blend and the second set of processing conditions were taken as the base. Samples were injection-molded by varying one parameter at a time. It was seen that the properties of the blend were increased by maintaining a lower barrel temperature, greater injection pressure, lower injection speed, higher mold temperature, and a greater residence time in the heated mold. Thus it was found that the processing conditions played a vital role in determining the mechanical properties and morphology of the polyblends. © 1996 John Wiley & Sons, Inc.     

聚丙烯注塑成型条件的优化

注塑条件的选择对注塑件质量及力学性能有着很大的影响。通过改变注塑速率、注塑压力、保压压力、模具温度及注射时间,找出了最佳的注塑条件,对于正确测定聚丙烯冲击强度和弹性模量等力学指标,客观反映该材料的力学性能具有重要的意义。     

Hierarchy structure in injection molded polypropylene/ethylene–octane copolymer blends

In this article, the phase morphology and mechanical properties of polypropylene (PP)/ethylene–octane copolymer (POE) blends with fixed ratio (60/40) obtained via different processing conditions, including barrel temperature, injection speed, and mold temperature, have been investigated. SEM was carried out for detailed characterization of phase morphology from the skin to the core, layer by layer. It was interesting that for all the processing conditions no dispersed POE elastomer was observed in the skin layer but elongated POE particles with large size were observed in the subskin layer. From the transition zone to the core layer, an increased phase separation was observed, which could lead to a formation of cocontinuous morphology, depending on the processing condition used. Higher barrel temperature, lower mold temperature, and higher injection speed could result in a smaller size of POE phase. The tensile strength and impact strength were found not sensitive to barrel temperature and mold temperature but to the low injection speed, both tensile strength and impact strength had a higher value for specimen obtained via low injection speed. The formation of the skin‐core morphology and the effect of processing conditions on the phase morphology were discussed based on crystallization kinetics of PP matrix, rheology, and shear induced phase mixing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

仿真研究聚丙烯流变性能对气辅注塑的影响

应用计算机仿真手段研究了不同聚丙烯在充模速度相同条件下的压力及锁模力变化规律。结果表明，气辅注癃民传统注塑相比，所需压力，锁模力均有显著降低，且聚合物熔体流动速率越小，气体注射后产生的压力降赵大，表明在生产中应尽可能选用高ＭＦＲ树脂以利于气辅注塑工艺。     

智能热水壶主体复杂抽芯及随形水路注塑模设计

根据智能热水壶主体结构特点和技术要求设计了一副复杂侧向抽芯及随形水路注塑模具。模具采用顺序侧向抽芯及延时侧向抽芯机构,有效改善了包紧力大、脱模困难塑件的注塑模具结构。定模型芯和动模型芯采用3D打印随形水路,有效改善了熔体热量多、成型零件温差大、温度高注塑模具的温度控制系统。模具采用了多项先进的创新技术,成型零件温差降低了50 %,注射成型周期缩短了约18 %,塑件变形量减小了75 %,尺寸精度提高了2级,达到了MT3（GB/T 14486—2008）。试模一次成功,成型塑件的外观质量和尺寸精度均达到了设计要求。