Residual orientation in injection‐moulded plaques of atactic‐polystyrene II: The effect of molecular weight

Injection moulding of polymers is a well‐established technique for the production of increasingly complex articles, and residual molecular orientation is a matter of considerable interest. A single matrix of commercial atactic polystyrene was systematically blended with deuterated polystyrene chains of five different weight fractions and the orientation investigated using small angle neutron scattering (SANS), optical birefringence and thermal shrinkage. The strengths and limitations of the experimental techniques used to measure residual orientation are explored and highlighted the unique contribution of SANS to observe ensemble averaged molecular orientation using deuterated polymers. The residual molecular orientation was found to vary with the molecular weight of the deuterated chains with an increase with increasing molecular weight. The observed correlations lead to a better understanding of the processing parameters that influence the degree of residual orientation on the molecular scale. Such information is valuable in the selection of the polymer matrix, mould design, and processing conditions. POLYM. ENG. SCI., 58:1332–1341, 2018. © 2017 Society of Plastics Engineers     

Influence of Mould Materials on the Morphological and Mechanical Properties of Injection‐Moulded Prototypes

The use of prototype moulds made with rapid tooling (RT)‐techniques offers the possibility to manufacture prototypes in the series process. Although the series material is used during injection moulding, the properties of the prototype are not identical with the product because of the varying surface qualities of the RT‐moulds and the different thermal conditions in the process. In this work, different RT‐techniques (resin casting, metal spraying, selective laser sintering, investment casting and keltooling) are investigated according to the morphological as well as the mechanical properties of injection‐moulded prototypes. The specimen are compared with parts that were injection‐moulded in a steel mould to realise a comparison between prototype and product. The thermal diffusivity of the analysed RT‐mould materials differs from one tenth of the value of steel up to seven times higher compared to steel. This leads to a different morphology of the prototype concerning the boundary layers and the spherulite size. A good heat conducting material (e. g. aluminium investment casting mould) causes thick boundary layers and small spherulites. In contrast, a rather insulating mould material (e. g. resin casting mould) results in thin boundary layers and large spherulites. The investigations show that the effect of a good heat conducting material can be achieved by using a lower mould temperature for badly conducting materials in the same way. The yield stress of the prototypes differs up to 20% from the mechanical properties of the product and is influenced by the spherulite size and the mould's surface quality. The prototype, moulded in a keltool mould, shows the best resemblance with the product.     

Properties of bio‐based polymer nylon 11 reinforced with short carbon fiber composites

In this work, micro‐composite materials were produced by incorporating 3‐mm long reclaimed short carbon fibers into bio‐based nylon 11 via melt compounding. A systematic fiber length distribution analysis was performed after the masterbatching, compounding and an injection moulding processes using optical microscopy images. It was found that the large majority of the fibers were within the 200–300 μm in length range after the injection moulding process. The mechanical (flexural and tensile), thermo‐mechanical, and creep properties of the injection moulded materials are reported. We found that an enhancement in flexural and Young's modulus of 25% and 14%, respectively, could be attained with 2 wt% carbon fiber loading whilst no significant drawback on the ductility and toughness of the matrix was observed. The creep resistance and recovery of the nylon 11, tested using dynamic mechanical thermal analysis at room temperature and 65°C, was significantly improved by up to 30% and 14%, respectively, after loading with carbon fiber. This work provides an insight into the property improvement of the bio‐based polymer nylon 11 using a small amount of a reclaimed engineered material. POLYM. COMPOS., 36:668–674, 2015. © 2014 Society of Plastics Engineers     

Effects of recycling on morphology and mechanical properties of a semi‐aromatic Rodrun LC‐5000 liquid crystalline polymer

In this work, a virgin Rodrun LC‐5000 liquid crystalline polymer was recycled several times by an injection moulding process. The results showed that there is a significant decrease in the Young's modulus and the tensile strength after the recycling operation, while there is almost no change in the tensile strain. A power law relationship was established between the melt flow rate of the recycled resin and the number of recycling stages. The characteristic three‐layer, skin‐transition‐core structure of the injection moulded specimens was also found to be altered by the number of recycling stages. © 2000 Society of Chemical Industry     

Influence of high injection moulding pressures on the engineering properties of linear polyethylene

Two grades of high density polyethylene, one injection moulding grade and another with a substantially higher molecular weight (melt index $\text{0.1}\text{g}\text{10}\text{min}$) were injection moulded at pressures ranging from 100 to 500 MPa using a modified conventional injection moulding machine. For the high molecular weight grade, improvements were observed in the elastic modulus, the tensile strength at rupture measured in the flow direction, and the unnotched impact strength. These improvements were accompanied by a second high temperature (137°C) melting peak in d.s.c. diagrams. For both grades it was also found that the mould shrinkage decreased and the crystallinity increased with injection pressure.     

Yield stress distribution in injection‐moulded glassy polymers

A methodology for structural analysis simulations is presented that incorporates the distribution of mechanical properties along the geometrical dimensions of injection‐moulded amorphous polymer products. It is based on a previously developed modelling approach, where the thermomechanical history experienced during processing was used to determine the yield stress at the end of an injection‐moulding cycle. Comparison between experimental data and simulation results showed an excellent quantitative agreement, both for short‐term tensile tests as well as long‐term creep experiments over a range of strain rates, applied stresses, and testing temperatures. Changes in mould temperature and component wall thickness, which directly affect the cooling profiles and, hence, the mechanical properties, were well captured by the methodology presented. Furthermore, it turns out that the distribution of the yield stress along a tensile bar is one of the triggers for the onset of the (strong) localization generally observed in experiments. © 2015 Society of Chemical Industry     

多型芯径向抽芯注塑模设计

轴套塑件上有12个周向均布的圆孔,注塑生产时需要多型芯径向抽芯。为此设计了一种特殊的轴套注塑模具,论述了模具结构特点和工作原理。该模具利用异型弯销、转盘及滑块完成多型芯径向抽芯,而未采用一般的齿轮-齿条机构或连杆机构,简化了模具结构。采用三脚推管推出塑件,解决了轴套塑件脱模难的问题。模具结构简单紧凑,工作可靠,操作方便,成型塑件质量好。     

Experimental study of weldline caused by flow lead effect in gas assisted injection moulded parts

Abstract

Gas assisted injection moulding has become one of the most important methods for the manufacture of plastic products. However, there are several unsolved problems that confound the overall success of this technique. The weldline caused by the flow lead effect in the polymer melt is one of them. In this report, an L′18 orthogonal array design based on the Taguchi method has been conducted to investigate the flow lead induced weldlines in gas assisted injection moulded parts. A plate cavity with a gas channel on the side was used for moulding. Experiments were carried out on an 80 t reciprocating screw injection moulding machine equipped with a high pressure gas injection unit. After moulding, the depth of the weldline was measured. For the factors selected in the main experiments, melt injection temperature and mould temperature were found to be the principal factors affecting the weldline depth of gas assisted injection moulded parts. Experimental investigation of a gas assisted injection moulding problem can help in better understanding the weldlines caused by flow lead effects, so that steps can be taken to optimise the surface quality of moulded parts. PRC/1755     

Process–property–structure relationship in miniaturized injection moulded polyoxymethylene samples

Micro‐injection moulding is one of the key manufacturing technologies for thermoplastic polymeric materials for the mass production of high value miniaturized components. However, this process is not just a straightforward down scaling of the conventional injection moulding technique. Indeed, during the micro‐injection the polymer melt is forced to flow at high strain rates through very small channels in nonisothermal conditions; this can lead to complex microstructures and to parts with unexpected performances. In this work, the relationships among the processing conditions, the mechanical properties and the microstructural characteristics of polyoxymethylene miniaturized specimens obtained by injection moulding were investigated. The attention was focused on the influence of the process temperatures on the mechanical behavior, examined by uniaxial tensile tests, and on the microstructural characteristics of the specimens, examined by differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized light microscopy, and dynamic‐mechanical thermal analysis. The results highlighted that material ductility in the miniaturized specimens is significantly affected by the mould temperature, because of the sample microstructure. Different degrees of orientation of polymer crystallites and different morphologies of the skin/core transition region were observed in dependence on the process temperatures. POLYM. ENG. SCI., 54:512–521, 2014. © 2013 Society of Plastics Engineers     

Variations in surface gloss on parts made by gas assisted injection moulding

Abstract

Gas assisted injection moulding has proved to be a breakthrough in moulding technology for thermoplastic materials. However, there are still unsolved problems that limit the overall success of this technique. The aim of this work was to study the phenomenon of gloss variations occurring across the surfaces of gas assisted injection moulded parts. Experiments were carried out on an 80 t injection moulding machine equipped with a high pressure, nitrogen gas injection unit. The materials used were pigmented acrylonitrile/butadiene/ styrene and polypropylene. A plate cavity with a gas channel across its centre was used to mould the parts. Various processing parameters were varied: melt temperature; mould temperature; melt filling speed; short shot size; gas pressure; and gas injection delay time. After moulding, a glossmeter was used to determine the effects of these processing parameters on the surface gloss profiles of the parts. A roughness meter and scanning electronic microscope were also employed to characterise the surface quality of moulded parts. In addition, a numerical analysis of the filling process was carried out to help better understand the mechanisms responsible for the phenomenon of surface gloss variations. It was found that the surface gloss difference occurs mainly in the transition area between channel and plate in the moulded parts, which might be the result of the shear stress gradient in the polymer melt during the filling process. Surface roughness of moulded parts might also be another factor resulting in the gloss difference problem. PRC/1720     

Bulk and surface molecular orientation distribution in injection‐molded liquid crystalline polymers: Experiment and simulation

Bulk and surface distributions of molecular orientation in injection‐molded plaques of thermotropic liquid crystalline polymers (TLCPs) have been studied using a combination of techniques, coordinated with process simulations using the Larson‐Doi “polydomain” model. Wide‐angle X‐ray scattering was used to map out the bulk orientation distribution. Fourier Transform Infrared Attenuated Total Reflectance (FTIR‐ATR) and Near‐Edge X‐ray Absorption Fine Structure (NEXAFS) were utilized to probe the molecular orientation states to within about ～5 μm and ～2 nm, respectively, of the sample surface. These noninvasive, surface‐sensitive techniques yield reasonable self‐consistency, providing complementary validation of the robustness of these methods. An analogy between Larson‐Doi and fiber orientation models has allowed the first simulations of TLCP injection molding. The simulations capture many fine details in the bulk orientation distribution across the sample plaque. Direct simulation of surface orientation at the level probed by FTIR‐ATR and NEXAFS was not possible due to the limited spatial resolution of the simulations. However, simulation results extracted from the shear‐dominant skin region are found to provide a qualitatively accurate indicator of surface orientation. Finally, simulations capture the relation between bulk and surface orientation states across the different regions of the sample plaque. POLYM. ENG. SCI., 50:1864–1877, 2010. © 2010 Society of Plastics Engineers     

注塑成型保压参数对塑件翘曲变形的影响

以阿基米德螺旋线塑件为例,应用Moldflow软件模拟塑件保压过程,研究了保压压力和保压方式对塑件翘曲变形的影响,并利用模腔压力测量仪测定不同保压参数下的模腔压力,得到模腔压力曲线。结果表明:保压压力对塑件的翘曲变形和体积收缩率均有显著影响;与恒压保压方式相比,分段保压使模腔压力均匀,塑件表面质量高,塑件体积收缩均匀。     

Characteristics of mould separation and its effects on qualities of thin wall injection moulded parts

Abstract

Precision injection moulding of thin walled parts has become an important concern in the computer, communication, and consumer electronics plastics industry. Previous studies in precision injection moulding control focus on the injection screw and the associated operations. In the present study, the influence of relevant parameters including injection speed, melt temperature, mould temperature, filling-packing switchover, and packing pressure on the mould plate separation under different clamping pressure were investigated as part of precision moulding control. A two cavity tensile test specimen mould equipped with four linear variable displacement transducers across the parting surfaces of the mould was used. A computer based monitoring system was built to detect the mould separation signals. Mould separation can also be identified from part weight and thickness variation and exhibits relevant correspondence with them. It was found that owing to the high injection speed required for thin wall moulding, mould separation is not negligible. In all situations, mould separation decreases with increasing clamping pressure. As melttemperature andmouldtemperatureincrease,mouldseparationincreases, resulting in an increase in part weight and thickness. Similarly, when packing pressure and injection speed increase, mould separation also increases. Earlier switchover from filling to packing can decrease mould separation as well as part weight and thickness. Among all the parameters studied, packing pressure exhibits the greatest influence on mould separation and on the associated weight and thickness change. This influence also becomes larger when the moulded part becomes thinner, owing to the larger injection moulding pressure. PRC/1746     

3D‐Flow Analysis by Means of Streamline Calculation

Over the last several years simulation software has become more and more important for mould design and process optimisation in polymer processing. Due to the mainly thin‐walled nature of most injection moulding parts, the currently used simulation programs are predominantly based on shell‐type elements with a two‐dimensional flow field in each element. Because of some assumptions, related to this specific calculation method, these so‐called 2.5D‐programs reach their limits in the simulation of complex‐shaped, thick‐walled mouldings. The fully three‐dimensional calculation of the injection moulding process offers a high potential which leads to improvement in result quality. This paper is a report of an investigation of streamlines in three‐dimensional flow fields, which typically occur in injection moulding. The streamlines were investigated by numerical simulation using 3D‐simulation software and by experiments. For the experiments a special mould has been designed, which enables pigments to be injected into the runner system of the mould. The injection can be done at different positions over the flow channel cross section. Thus the path of different coloured particles can be directly observed. The mould can be equipped with different inserts in order to allow an investigation of different geometries. In general, simulated and observed streamlines are in good agreement. Minor differences are due to special features in mould design or the calculation algorithm. The insights gained from this investigation can be applied to find appropriate simplifications in numerical 3D‐simulation in order to cut down computing times.     

基于CAE软件的液晶电视前面框翘曲变形诊断分析

针对液晶电视前面框在生产实际中出现的翘曲变形问题,采用CAE分析软件进行计算机数值模拟,分析冷却温度、分子取向、收缩率等对翘曲变形的影响,提出了针对性的模具优化方案。利用优化方案改进的模具进行注塑生产,得到了合格塑件,说明CAE技术在注塑模具优化设计中的可靠性与实用性。     

双色排钮注塑模具设计

介绍了双色注塑成型原理,以双色排钮制品为例,分析了双色注塑模具的结构特点、模具工作过程和设计要点。     

应用Moldflow软件优化哈夫模具结构设计

采用注塑CAE技术,利用Moldflow软件,对电热水壶塑料外壳的成型过程进行分析。结果表明:浇口数量及形式、温度、温度调节系统对塑件质量均有影响。根据上述影响因素,改进模具设计方案,找出最佳工艺参数分布范围,从而起到优化哈夫模具结构设计的作用。     

Melting and crystallization behaviors of injection‐molded polypropylene

The melting and crystallization behaviors of the skin layer in an injection‐molded isotactic polypropylene (PP) have been studied, mainly in comparison with those of the core layer and subsidiarily in comparison with those of a compression‐molded PP and a nucleator (talc)–added PP. The skin layer contains about 5% crystals, which have a high melting point of up to 184°C. They thermally vanish by melting once. The subsequent melting history will scarcely affect the melting behaviors. On the other hand, crystallization behaviors are strongly affected by the melting history. The skin layer crystallizes in a wide temperature range at high temperature. This tendency weakens with increasing melting temperature, approaching a constant and that of the core layer above 230°C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230°C. In explaining these experimental results, it is assumed that the residual structure substance is a melt orientation of molecular chains that works as crystallization nuclei and that the vanishing of the residual structure is nothing but a relaxation of the melt orientation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1751–1762, 2000     

Orientation effects during the flow of short-fibre reinforced thermoplastics

A study has been carried out to investigate molecular orientation in injection moulded bars of short glass fibre reinforced polypropylene and polyethylene. For the range of fibre concentrations encountered commercially, the fibres do not appear to have any direct effect on the matrix orientation. As the fibre concentration increases, however, the matrix orientation becomes dominated by the orientation of the fibres. These effects are interpreted in terms of current ideas of the rheology of polymer melts during injection moulding and the crystallization of polymers at fibre surfaces.     

Studies of Injection‐Moulded Isotactic Poly(propylene) by Synchrotron WAXD/SAXS: Effects of Nucleating Agent on Morphological Distribution

The morphological distribution of injection‐moulded isotactic poly(propylene) (iPP) plates in the presence of nucleating agents was extensively investigated using synchrotron radiation. The commercial PP compound was injection‐moulded under a variety of different conditions in order to explore the effects of shear flow and temperature on the morphology and morphological distribution. The iPP structures obtained were characterized using the degree of crystallinity, α‐phase orientation index, β‐phase index, long spacing of lamellae, and the thickness of both crystalline and amorphous lamellae. These parameters were plotted as a function of position through the plate depth for the injection‐moulding conditions. Unlike relatively pure iPP, the distributions of crystallinity and α‐phase orientation index in this commercial iPP are independent of position through the plate depth. The “skin‐core” structure that is generally found for injection‐moulded iPP is not present because of the addition of nucleating agents. The β‐phase of iPP has the same distribution through the plate depth as that expected for iPP without nucleating agents. Additionally, the lamellar dimension is found to be independent of position through the plate depth and the fraction of noncrystalline materials residing outside the lamellar stacks can be up to about 30%. The results indicate that the properties of different injection‐moulded iPP grades should be investigated individually.

Typical WAXS patterns of the sample S9 at different labeled distances from the surface. The patterns are vertically shifted for clarity.