Optical monitoring of polypropylene crystallization during injection molding

The nonisothermal crystallization of polypropylene resins, i‐PP, during injection molding, using an optical device inserted in the injection mold cavity was monitored. The device detected the change of optical properties which occurs in polymers during their crystallization process; thus the intensity of a laser beam after it passed through the crystallizing polymer was measured during an injection molding cycle. The collected light intensity after the end of the cycle was correlated with the morphologies and final crystallinity degree of the samples. The influence of nucleating agents and the change of the parameters of the injection molding process on the morphology and optical signals were also investigated. The morphologies were analyzed by polarized light optical microscopy, PLOM. The % of crystallinity of the samples was measured by wide angle X‐rays diffraction, WAXS. It was concluded that the optical device was sensible to different polymer crystallization kinetics, morphology type, and changes in the injection molding parameters. It was also found that the mold temperature and packing pressure and time were the factors that affected most the kinetics of crystallization of these polymers in this particular disk geometry. The WAXS results showed that the lower the final light intensity the higher the % of crystallinity in the samples. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Optical monitoring of the injection molding of intercalated polypropylene nanocomposites

Intercalated polypropylene (PP)/clay nanocomposites were produced by twin screw extrusion; afterwards, the optical monitoring of their injection molding was done using a laser sensor. The transmitted light intensity as a function of molding time was measured. The mold and melt temperatures, packing pressure and flow rate were changed. The nanocomposite had higher induction times than the PP, that is, scattering structures were detected later in the nanocomposite than in the PP, which was attributed to a retardation effect promoted by the clay on the PP crystallization growth rate. The morphologies of the injection molded samples were analyzed by polarized light optical microscopy, differential scanning calorimetry and transmission electron microscopy. The nanocomposite samples showed a second core, a thicker skin layer, highly oriented nanoclay's tactoids in the skin region and average spherulites' sizes smaller than the PP. The final light intensity I_f was correlated with the spherulites' sizes: high values of I_f represented samples with large spherulites. The PP sample had average spherulites' sizes larger than the Nano samples. However, the surging of a second core with large spherulites in the Nano samples changed the expected pattern: the PP samples showed I_f lower than the Nano samples. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Optical monitoring of polyesters injection molding

An optical fiber sensor similar to the one developed by Thomas and Bur 1 was constructed for the monitoring of the crystallization of three polyesters during the injection molding process. The polyesters studied were: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene terephthalate (PET). With this optical system it was possible to obtain, in real time, some essential parameters of the polyester crystallization kinetics at different processing conditions. Thus, a study of the influence of injection molding variables on the nonisothermal crystallization kinetics of these polyesters was done. The processing variables were: mold wall and injection temperatures, T_w and T_i, respectively; flow rate, Q; and holding pressure, P_h. The experiments were done following a first order central composite design statistical analysis. The morphology of the samples was analyzed by polarized light optical microscopy, PLOM. The signal of the laser beam during the filling and the crystallization stages of the injection molding of these materials was found to be reproducible. The measurements showed that this system was sensitive to variations of the crystallization of different types of polymers under different processing conditions. The system was not able, however, to monitor the crystallization process when the crystallinity degree developed by the sample was very low, as in the PET resin. It was also observed that T_w and T_i were the most influential variables on the crystallization kinetics of PBT and PTT. Due to its slower crystallization kinetics, PTT was found to be more sensitive to changes in these parameters than the PBT. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 563–579, 2006     

Modeling/simulating the injection molding of isotactic polypropylene

A unified formulation is presented for modeling the injection molding of isotactic polypropylene (i‐PP). The crystallization kinetics are based upon the differential Nakamura equation in which the characteristic time is dependent upon temperature, pressure and flow‐induced shear stress, without any explicit need for an induction time. A Cross/WLF model is used to represent the shear viscosity in which η_O is dependent upon temperature, pressure and crystallinity. Use is made of a recent correlation for the PVT behavior of i‐PP with an explicit dependence upon crystallinity. A finite‐difference implementation of the modeling is applied to two independent molding experiments available from the literature, with notable results concerning the late‐time cavity pressure traces and time‐dependent gapwise shrinkage prior to ejection.     

In-line optical monitoring of polymer injection molding

An optical sensor, consisting of optical fibers to transmit light to and from the mold cavity, was constructed for the purpose of measuring the onset of polymer solidification during injection molding. The sensor was used to detect characteristic fluorescence radiation from a dye which had been doped into the resin at very low concentration. By measuring changes in fluorescence intensity it was possible to detect whether the state of the resin was liquid or solid. We observed that, as the resin cooled in the mold, the onset of solidification was indicated by highly characteristic and distinct changes in the fluorescence intensity/time profile. Application of the method involved the use of a calibration relationship between the fluorescence intensity and temperature of the doped polymer in order to determine the distict features which characterize the onset of solidification. Injection molding of a glass forming polymer (polystyrene) and a crystallizable polymer (polyethylene) was monitored by this technique.     

On-line ultrasonic monitoring of the injection molding process

On-line ultrasonic monitoring of injection molding of a simple polymer box is studied using pulse-echo techniques. The flow front of molten polymers inside the mold has been probed by a multiple-channel probing system with a time resolution of 2 ms. This information can be used to control the plunger movements. The gap development, because of the shrinkage of the part in the mold, is also monitored. This information, which is important for the understanding of the part's cooling process, has been found to be superior than that measured by a conventional pressure probe. The relation between the gap formation time and the packing pressure has been investigated at various part locations characterized by different thicknesses. The velocity and the amplitude variations of ultrasonic waves, in a reflection as well as in a transmission configuration, have also been measured in the part's material during its solidification. The behavior of these ultrasonic parameters contributes to the interpretation of the solidification process.     

注塑成型工艺对透明PP光学性能的影响

采用不同的工艺条件注塑成型无规共聚透明聚丙烯(PP),测试透光率、雾度和表面光泽度,分析了成型工艺参数对制品光学性能的影响。所选透明PP的最佳注塑工艺为:加工温度220～230℃,模具温度40～60℃。在保证制品顺利成型的情况下应尽量采用较小的注塑压力。注塑速率和注塑时间对制品的光学性能影响较小。     

A comparison of short glass fiber reinforced polypropylene plates made by conventional injection molding and using shear controlled injection molding

This paper describes a comparison of the fiber orientation structures and resulting elastic properties of samples of short glass fiber filled polypropylene made by conventional injection molding and by the SCORIM (Shear Controlled Orientation in Injection Molding) process developed at The University of Brunel. The 3D fiber orientation distributions of the composites were measured using a unique transputer based image analysis system developed at The University of Leeds. The mechanical properties of the samples were characterized using an ultrasonic velocity technique, which allows a full set of elastic constants to be determined for each material. The link between fiber orientation distributions and measured elastic properties was then investigated using theoretical models developed in this laboratory.     

Glass bead-filled polypropylene part II: Mold-filling studies during injection molding

Five glass bead-filled polypropylene composites, which had been rheologically characterized, were used in mold-filling studies to investigate the influence of rigid beads (0 to 26 vol percent) in complex flow fields. The two basic flows studied were simple shear flow in an edge-gated, rectangular cavity and stagnation flow leading to diverging radial flow in a center-gated, rectangular cavity. Pigmented tracer elements were used to illustrate flow patterns and to distinguish the skin-core structure which is a consequence of the injection molding process. Despite the strong dependence of the rheological properties (both viscous and elastic) and physical properties (density, thermal conductivity, etc.) on bead volume fraction, the basic character of the polymer dominated the flow behavior of each composite and the flow patterns were unaffected by the beads. Injection pressures were increased with increasing bead fraction to maintain the same, constant volumetric flow rate (2.21 in.³/min) for each composite. Relative injection forces follow closely the relative viscosit trend.     

Fluorescence monitoring of polymer injection molding: Model development

We have developed models to describe the behavior of an optical fiber sensor which was used to detect fluorescence from a polymer resin during the cooling phase of injection molding. The optical fiber sensor was positioned at the wall of the mold cavity by using the ejector pin channel as access to the cavity. The sources of fluorescence were dyes, which were chosen because of their sensitivity to temperature and which were mixed with the resin at dopant concentrations (parts per million by weight). The behaviors of a molecular rotor dye, dimethylamino diphenyl hexatriene, doped into polyethylene, and an excimer producing dye, bis-(pyrene) propane, doped into polystyrene were the subjects of the modeling calculations. The models consist of two modules: (a) a solution to the thermal diffusion equation for the resin cooling in the mold and (b) using temperature/time profiles and, in the case of polyethylene, crystallinity/time profiles obtained from the thermal diffusion equation, fluorescence intensity as a function of time was computed. Factors incorporated in the models are: adiabatic heating and cooling, light scattering due to microcrystals of polyethylene, crystallization kinetics, temperature and pressure shift factors for viscoelastic relaxation near the glass transition temperature of polystyrene, and the thermal resistance at the resin/mold interface.     

Compounding and injection molding of polybutene‐1/polypropylene blends

The effect of shear‐controlled orientation injection molding (SCORIM) was investigated for polybutene‐1/polypropylene blends. This article reports on the methods and processing conditions used for blending and injection molding. The properties of SCORIM moldings are compared with those of conventional moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated with mechanical tests, differential scanning calorimetry studies, and polarized light microscopy. The application of SCORIM improved Young's modulus and the ultimate tensile strength. The gain in stiffness was greater for higher polybutene‐1 content blends. A drastic decrease in the strain at break and toughness was observed in SCORIM moldings. The enhanced molecular orientation of SCORIM moldings resulted in a featureless appearance of the morphology. Interfacial features due to segregation were visible in the micrographs of SCORIM moldings. Both conventional and SCORIM moldings exhibited form I′ in polybutene‐1. This article explains the relationship between the mechanical properties and micromorphologies. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 806–813, 2003     

Deformation velocity dependence on tensile fracture characteristics of polypropylene injection molding parts

To clarify the mechanism of the deformation and fracture in a low‐velocity impact test on the isotactic polypropylene (i‐PP) sheet made by injection molding, the change of the style of fracture and the form of deformation was examined while changing the speed of the striker in a low‐velocity impact test. In the injection molding sheet, an oriented skin layer of some thickness is formed on the surface of the sample sheet. By the stress perpendicular to the orientation direction of the skin layer, crazes were formed easily in parallel with the orientation direction in this layer, and cracks were formed from there. Because these cracks bring the sample sheet a strong restraint of strain, a high stress concentration occurs at the end of this crack even if the formation of the oriented layer is limited on the surface of the sample sheet only, and the low‐velocity impact test leads the sample sheet to a brittle fracture. As a result, the injection molding sheet that forms oriented structure on its surface causes the ductility‐brittleness transform at a lower velocity of deformation compared with the nonoriented sheet. POLYM. ENG. SCI., 53:2659–2665, 2013. © 2013 Society of Plastics Engineers     

Alternating multilayer structure of polyethylene/polypropylene blends obtained through injection molding

The formation of multilayer structures in the high‐speed thin wall injection‐molded samples of high‐density polyethylene/isotactic polypropylene blends is reported. Based on the morphology development in injection runner and mold, a possible formation mechanism of multilayer structure was proposed in this study. Injection molding could be used as a simple and an effective method for the fabrication of multifunctional multilayer structure. This work is interesting and important for scientific research as well as several potential applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

注塑级聚丙烯专用料ZK-1240D的开发

通过选择合适的基础树脂,在挤压造粒时添加复合改性剂的方法,开发出注塑级聚丙烯专用料ZK-1240D,该专用料的性能达到国产外同类产品的水平,并已进行工业规模的生产,满足市场需要。     

热压成型和注塑成型CF/PP复合材料的力学性能对比

通过热压成型和注塑成型2种不同加工方法制备了碳纤维(CF)/聚丙烯(PP)导电复合材料,对比了2种复合材料的力学性能,结果表明注塑成型的CF/PP具有较高的拉伸强度和弹性模量。结合扫描电子显微镜观察,分析了这2种加工方法对CF/PP复合材料的微观结构和力学性能的影响。     

The study of short-shot water-assisted injection molding of short glass fiber reinforced polypropylene

water penetration length and fiber orientation (along the melt flow direction) are important indicators for water-assisted injection molding products of the fiber-reinforced polymer. The effects of melt short shot size, water injection delay time and water injection pressure on these two important indexes are analyzed theoretically and experimentally. The study found that with the increase of the melt short shot size, the extension of the water injection delay time and the increase of the water injection pressure, the water penetration length changed from 216 to 96 mm, 170 to 210 mm, and 215 to 180 mm, respectively. Therefore, it can be known that melt short shot size has the greatest influence on water penetration length, followed by water injection delay time, and finally water injection pressure. Meantime, due to the fiber orientation and change degree of water-assisted injection-molded products along the melt flow direction, the fiber orientation in the water channel layer along the melt flow direction has the highest and lowest change degree, followed by the wall layer and finally the core layer. It can be known that the melt short shot size has the greatest influence on the fiber orientation and the degree of change along the melt flow direction, followed by the water injection delay time, and finally the water injection pressure.     

Online pattern-based part quality monitoring of the injection molding process

The quality of injection molded parts is currently monitored in the plant using techniques that focus on the statistical analysis of discrete data and, in particular, peak values. This paper presents an alternative online technique for part quality monitoring that focuses on the analysis of complete data patterns. Specifically, this paper discusses the application of artificial neural networks (ANNs) as part quality monitoring tools. The method of approach is to train a back propagation network (BPN) to associate part quality with the corresponding data pattern produced during injection. In Part I of this work, the data pattern consists of a series of discrete values and the part quality measure is defined as part weight. In Part II, the data pattern is the measurement profile observed from a pressure sensor placed in the mold cavity and the part quality measure is defined as part length. Results show that ANNs are successful in predicting part quality based on data patterns when an entire sensor profile is analyzed. Furthermore, demonstrations show that the approach is superior in predicting part quality when compared to statistical techniques now widely practiced by the injection molding process industry.     

Study of the flexural modulus of natural fiber/polypropylene composites by injection molding

Effect of fiber compression on flexural modulus of the natural fiber composites was examined. The kenaf, bagasse, and polypropylene were mixed into pellets, and composites were fabricated by injection molding. To predict flexural modulus of the composites, the Young's modulus of kenaf and bagasse fiber were measured. Using the obtained Young's modulus, the flexural modulus of the composites was predicted by Cox's model that incorporates the effect of fiber compression. It was found that those fibers with high Young's modulus were more compressed than that with low Young's modulus. Moreover, the distribution of fiber length and orientation in the composites were also investigated. To calculate the orientation factor for the prediction model, the distribution function of fiber orientation was determined to a triangular function. The flexural modulus of the composites increased with increase of volume fraction. The predicted values were in good agreement with the experimental values. Furthermore, it was revealed by SEM that the porous structure of the natural fibers was compressed. The fiber compression ratio (3.6) in bagasse was higher than that in kenaf (1.4) due to the difference in porous structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 911–917, 2006     

Capacitive transducer for in‐mold monitoring of injection molding

A capacitive transducer is developed for online monitoring of the in‐mold material status for injection molding, particularly for measurements of melt‐front position and flow‐rate. This paper will show, in addition to fulfilling its design purposes, that such a sensor can also be effectively used for the detection of the start and end of mold filling, the time of gate freezing, and over‐packing. Polym. Eng. Sci. 44:1571–1578, 2004. © 2004 Society of Plastics Engineers.     

Facilitating transcrystallization of polypropylene/glass fiber composites by imposed shear during injection molding

The transcrystal plays an important role in the enhancement of mechanical and thermal performances for polymer/glass fiber composites. Shear has been found to be a very effective way for the formation of transcrystal. Our purpose of this study was to explore the possibility to obtain the transcrystal in real processing such as injection molding. We will report our recent efforts on exploring the development of microstructure of polypropylene (PP)/glass fiber composite from skin to core in the injection-molded bars obtained by so-called dynamic packing injection molding which imposed oscillatory shear on the melt during solidification stage. A clear-cut shear-facilitated transcrystallization of PP on glass fibers was observed in the injection-molding bar for the first time. We suggested that shear could facilitate the transcrystalline growth through significantly improving the fiber orientation and the interfacial adhesion between fiber and matrix.