In‐line monitoring of the injection molding process by dielectric spectroscopy

In recent years, electrical techniques like microdielectrometry have increasingly been utilized for their ability to continuously monitor, in a nondestructive way, the advancement of the reaction of thermoset resins under cure. This paper discusses an extension of this technique for the “in‐situ” monitoring of the crystallization of thermoplastics applied during an injection molding process. Electric sensors were positioned at the walls of the mold cavity so that an analysis of the volume dielectric properties of material during the filling, the post‐filling, and the cooling steps could be carried out. Poly(vinylidene fluoride) was chosen for this study. A correlation between the evolution of the dielectric parameters and the succession of the steps in this process was undertaken. The dielectric response was sufficiently sensitive to identify the steps of the closing of the mold, filling, post‐filling, cooling, and ejection of the part. In addition, information concerning the crystallization phenomenon near the wall or in the middle of the sample was collected. The gradual filling of the cavity of the mold was also identified by dielectric measurements. The temperature dependence of dielectric properties of the sample was beneficial in evaluating the increase of the temperature of the mold with the succession of injection cycles. The influence of the packing pressure has been clearly identified and confirms the usefulness of the dielectric method as a probe for detecting the shrinkage of the part during the optimization phase of the machine parameters. The dielectric method detailed herein provides a new non‐invasive technique and could be applied to a closed‐loop control of the injection molding process.     

Nature of contact between polymer and mold in injection molding. Part II: Influence of mold deflection on pressure history and shrinkage

In injection molding of thermoplastic parts, high hold pressures are set during the packing phase to generate a post‐filling, which compensates the shrinkage of polymer due to its cooling. The polymer pressure in mold cavity leads to a cavity deformation due to mold and machine compliance. Then, the increase in cavity thickness can modify the post‐filling and consequently the pressure history, the volumetric shrinkage and the part mass. The first goal of this paper is to present a simple method to locally determine mold rigidities: over‐packed slabs are injected and local deflections are determined from measurements of the local residual pressure, the local in‐plane shrinkages and the plate thickness. In the studied plate mold, which can be considered as stiff compared to some industrial molds, a rigidity of more than 1 μm/MPa has been measured close to the center of the plate. The second goal of this paper is to show the influence of mold deflection on dimensional properties. If the cavity thickness is small as for our 1‐mm‐thick plate mold, considering an infinitely rigid mold cannot do realistic predictions of polymer pressure history, volumetric shrinkages and part mass. Nevertheless, in‐plane shrinkage seems to be less affected by mold deflection. It means that the additional polymer mass due to mold deflection is mainly distributed in the part thickness.     

Modeling and simulation of fiber‐reinforced polymer mold‐filling with phase change

A gas‐solid‐liquid three‐phase model for the simulation of fiber‐reinforced composites mold‐filling with phase change is established. The influence of fluid flow on the fibers is described by Newton's law of motion, and the influence of fibers on fluid flow is described by the momentum exchange source term in the model. A revised enthalpy method that can be used for both the melt and air in the mold cavity is proposed to describe the phase change during the mold‐filling. The finite‐volume method on a non‐staggered grid coupled with a level set method for viscoelastic‐Newtonian fluid flow is used to solve the model. The “frozen skin” layers are simulated successfully. Information regarding the fiber transformation and orientation is obtained in the mold‐filling process. The results show that fibers in the cavity are divided into five layers during the mold‐filling process, which is in accordance with experimental studies. Fibers have disturbance on these physical quantities, and the disturbance increases as the slenderness ratio increases. During mold‐filling process with two injection inlets, fiber orientation around the weld line area is in accordance with the experimental results. At the same time, single fiber's trajectory in the cavity, and physical quantities such as velocity, pressure, temperature, and stresses distributions in the cavity at end of mold‐filling process are also obtained. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42881.     

Experimental analysis and numerical modeling of flow channel effects in resin transfer molding

Composite manufacturing by Liquid Composite Molding (LCM) processes such as Resin Transfer Molding involve the impregnation of a net‐shape fiber reinforcing perform a mold cavity by a polymeric resin. The success of the process and part manufacture depends on the complete impregnation of the dry fiber preform. Race tracking refers to the common phenomenon occurring near corners, bends, airgaps and other geometrical complexities involving sharp curvatures within a mold cavity creating fiber free and highly porous regions. These regions provide paths of low flow resistance to the resin filling the mold, and may drastically affect flow front advancement, injection and mold pressures. While racetracking has traditionally been viewed as an unwanted effect, pre‐determined racetracking due to flow channels can be used to enhance the mold filling process. Advantages obtained through controlled use of racetracking include, reduction of injection and mold pressures required to fill a mold, for constant flow rate injection, or shorter mold filling times for constant pressure injection. Flow channels may also allow for the resin to be channeled to areas of the mold that need to be filled early in the process. Modeling and integration of the flow channel effects in the available LCM flow simulations based on Darcian flow equations require the determination of equivalent permeabilities to define the resistance to flow through well‐defined flow channels. These permeabilities can then be applied directly within existing LCM flow simulations. The present work experimentally investigates mold filling during resin transfer molding in the presence of flow channels within a simple mold configuration. Experimental flow frot and pressure data measurements are employed to experimentally validate and demonstrate the positive effect of flow channels. Transient flow progression and pressure data obtained during the experiments are employed to investigate and validate the analytical predictions of equivalent permeability for a rectangular flow channel. Both experimental data and numerical simulations are presented to validate and characterize the equivalent permeability model and approach, while demonstrating the role of flow channels in reducing the injection and mold pressures and redistributing the flow.     

Analysis of resin injection molding in molds with preplaced fiber mats. II: Numerical simulation and experiments of mold filling

This work presents the results of numerical simulation and experimental visualization of the mold filling process in resin injection molding with preplaced fiber mats. The mold filling experiments were conducted with various mat stacks consisting of continuous random glass fiber mats and bidirectional stitched glass fiber mats. The use of two different mat types in the mat stack created porosity and permeability variations. The effect of these permeability variations was studied by taking flow pressure measurements and observing the progress of the flow front of a non-reactive fluid filling a clear acrylic mold that contained the reinforcement mat stack. Numerical simulation corresponding to each experiment was also carried out. The numerical results were compared to the experimental measurements.     

Effects of injection‐molding conditions on the gloss and color of pigmented polypropylene

The effects of processing conditions on appearance characteristics of injection‐molded mineral‐filled polypropylene (compounded with pigments giving differing intensities of a beige color) have been studied; characteristics studied included gloss, color, and texture. A mold cavity embossed with smooth, fine, and coarse surface patterns was used. In‐mold rheology and gate‐seal analysis were used to select the filling and postfilling processing parameters. Interest was focused on the effects of filling rate, holding pressure, and mold temperature on the appearance characteristics, and a significant influence of these processing conditions on the gloss and color was found. For all the surface patterns examined, a better replication of the mold texture was obtained with a low melt viscosity at a high shear rate (high injection speed or short injection time) and a high mold temperature. This gave a higher gloss in the smooth surface regions and a lower gloss in the textured regions. An increase in the holding pressure had an effect similar to but smaller than increasing the filling rate or mold temperature. The gloss (or surface topography) had a significant effect on the color; an increase in gloss was associated with an increase in the color coordinate b* and a decrease in the lightness L*. POLYM. ENG. SCI. 45:1557–1567, 2005. © 2005 Society of Plastics Engineers     

Determination of the heat transfer coefficient from short‐shots studies and precise simulation of microinjection molding

The filling process of a micro‐cavity was analyzed by modeling the compressible filling stage by using pressure‐dependent viscosity and adjusted heat transfer coefficients. Experimental filling studies were carried out at the same time on an accurately controlled microinjection molding machine. On the basis of the relationship between the injection pressure and the filling degree, essential factors for the quality of the simulation can be identified. It can be shown that the flow behavior of the melt in a micro‐cavity with a high aspect ratio is extremely dependent on the melt compressibility in the injection cylinder. This phenomenon needs to be considered in the simulation to predict an accurate flow rate. The heat transfer coefficient between the melt and the mold wall that was determined by the reverse engineering varies significantly even during the filling stage. With increasing injection speed and increasing cavity thickness, the heat transfer coefficient decreases. It is believed that the level of the cavity pressure is responsible for the resulting heat transfer between the polymer and the mold. A pressure‐dependent model for the heat transfer coefficient would be able to significantly improve the quality of the process simulation. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Curing kinetics and viscosity change of a two‐part epoxy resin during mold filling in resin‐transfer molding process

The curing kinetics and the resulting viscosity change of a two‐part epoxy/amine resin during the mold‐filling process of resin‐transfer molding (RTM) of composites was investigated. The curing kinetics of the epoxy/amine resin was analyzed in both the dynamic and the isothermal modes with differential scanning calorimetry (DSC). The dynamic viscosity of the resin at the same temperature as in the mold‐filling process was measured. The curing kinetics of the resin was described by a modified Kamal kinetic model, accounting for the autocatalytic and the diffusion‐control effect. An empirical model correlated the resin viscosity with temperature and the degree of cure was obtained. Predictions of the rate of reaction and the resulting viscosity change by the modified Kamal model and by the empirical model agreed well with the experimental data, respectively, over the temperature range 50–80°C and up to the degree of cure α = 0.4, which are suitable for the mold‐filling stage in the RTM process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2139–2148, 2000     

PA成型收缩率与注射工艺条件的关系

分析了注塑制品的收缩机理及收缩过程，并讨论了聚酰胺（PA）注射成型过程中模腔平均压力、熔体温度、模温、充模速率、成型时间等工艺条件对其收缩率的影响及制品后收缩率的因素，给出了减小制品收缩率，提高制品尺寸稳定性的方法。     

Experimental compaction characterization of unidirectional stitched noncrimp fabrics in the vacuum infusion process

In this study, experimental test equipment developed in‐house was used to study the compaction behavior of stitched quasi‐unidirectional (UD) non‐crimp fabrics (NCF) during the pre‐filling, filling, and post‐filling stages of the vacuum infusion (VI) process. The effects of the stitch pattern, stitch tension, and fiber sizing of reinforcements, as well as the effect of nesting of fiber bundles in neighboring layers, were studied. Moreover, the effects of cyclic compaction, resin viscosity, and different post‐filling strategies were studied. The developed experimental test equipment provided an applicable measuring method for characterizing the compaction behavior of both the dry and resin‐impregnated reinforcements. The effects of the stitching parameters and fiber nesting of reinforcements were found. The stitch pattern and post‐filling strategies were noted to have an effect on the preform and laminate thickness. POLYM. COMPOS., 37:2692–2704, 2016. © 2015 Society of Plastics Engineers     

Mechanical properties of glass fiber reinforced polypropylene injection molded with a rotation mold

A special mold (Rotation, Compression, and Expansion Mold) was used to impose a controlled shear action during injection molding of short glass fiber reinforced polypropylene discs. This was achieved by superimposing an external rotation to the pressure‐driven advancing flow front during the mold filling stage. Central gated discs were molded with different cavity rotation velocities, inducing distinct levels of fiber orientation through the thickness. The mechanical behavior of the moldings was assessed, in tensile and flexural modes on specimens cut at different locations along the flow path. Complete discs were also tested in four‐point flexural and in impact tests. The respective results are analyzed and discussed in terms of relationships between the developed fiber orientation level and the mechanical properties. The experimental results confirm that mechanical properties of the moldings depend strongly on fiber orientation and can thus be tailored by the imposed rotation during molding. POLYM. ENG. SCI. 46:1598–1607, 2006. © 2006 Society of Plastics Engineers.     

Edge effect in RTM processes under constant pressure injection conditions

In resin transfer molding processes, the edge effect caused by the nonuniformity of permeability between fiber preform and edge channel may disrupt resin flow patterns and often results in the incomplete wetting of fiber preform, the formation of dry spots, and other defects in final composite materials. So a numerical simulation algorithm is developed to analyze the complex mold‐filling process with edge effect. The newly modified governing equations involving the effect of mold cavity thickness on flow patterns and the volume‐averaging momentum equations containing viscous and inertia terms are adopted to describe the fluid flow in the edge area and in the fiber preform, respectively. The volume of fluid (VOF) method is applied to tracking the free interface between the two types of fluids, namely the resin and the air. Under constant pressure injection conditions, the effects of transverse permeability, edge channel width, and mold cavity thickness on flow patterns are analyzed. The results demonstrate that the transverse flow is not only affected by the transverse permeability and the edge channel width but also by the mold cavity thickness. The simulated results are in agreement with the experimental results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Modeling the chemorheological behavior of epoxy/liquid aromatic diamine for resin transfer molding applications

The analysis of the chemorheological behavior of an epoxy prepolymer based on a diglycidylether of bisphenol‐A (DGEBA) with a liquid aromatic diamine (DETDA 80) as a hardener was performed by combining the data obtained from Differential Scanning Calorimetry (DSC) with rheological measurements. The kinetics of the crosslinking reaction was analyzed at conventional injection temperatures varying from 100 to 150°C as experienced during a Resin Transfer Molding (RTM) process. A phenomenological kinetic model able to describe the cure behavior of the DGEBA/DETDA 80 system during processing is proposed. Rheological properties of this low reactive epoxy system were also measured to follow the cure evolution at the same temperatures as the mold‐filling process. An empirical model correlating the resin viscosity with temperature and the extent of reaction was obtained to carry out later a simulation of the RTM process and to prepare advanced composites. Predictions of the viscosity changes were found to be in good agreement with the experimental data at low extents of cure, i.e., in the period of time required for the mold‐filling stage in RTM process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4228–4237, 2006     

Measurement and Analysis of Cavity Pressure and Melt Filling Capacity During Injection Molding

Using an online measure system consisted of a high sensitivity pressure sensor, a spiral channel mold and a data collecting/analysis series. The cavity pressure and flow length response under different injection pressure, injection velocity, mold temperature and different vibration condition was measured and analyzed in this study. The results show that the cavity pressure and flow length increased with the increase of injection pressure, injection velocity and mold temperature. In vibration assisted injection molding (VAIM), the cavity pressure and flow length increased as a result of imposing vibration. The change of amplitude and frequency in VAIM had more distinct effect on flow length than injection pressure. The results also indicate that the measure system established in this study can accurately monitor and record the changes of cavity pressure, and could be used in the study of melt's filling capacity in injection molding process.     

Some relationships between injection molding conditions and the characteristics of vitrified molded parts

The various mold filling phenomena influencing the characteristics of fabricated parts are surveyed. The phenomena leading to jetting in injection mold filling are considered. These are associated with the magnitude of swell by the melt as it exits the gate into the mold. Special attention is given to the influence of non-isothermal runner flow. A theory of extrudate swell of polymer melts with temperature profiles is developed using Tanner's unconstrained recovery theory. In the. absence of jetting, mold filling by a simple advancing front takes place. The hydrodynamics of the advancing front and the stress fields in the flowing melt are determined. Analysis and modeling are presented based on the use of hydrodynamic lubrication theory involving a solid layer along the mold wall and a hot isothermal melt core. This theory is compared with experimental measurements of pressure losses in mold filling. The development of birefringence in injection molding processes is analyzed. Birefringence distributions are due to frozen-in flow birefringence. A new experimental study is presented and its results compared with theoretical predictions. The problem of thermal stresses in injection molded parts is considered.     

Three‐dimensional simulation of microchip encapsulation process

A finite element simulation of moving boundaries in a three‐dimensional inertiafree, incompressible flow is presented. A control volume scheme with a fixed finite element mesh is employed to predict fluid front advancement. Fluid front advancement and pressure variation in a flow domain similar to the mold cavity used for microchip encapsulation are predicted. The predicted fluid front advancement and pressure variation are in good agreement with the corresponding experimental results. As the difference in the thicknesses of mold cavities above and below the microchip is changed, the weld line location and pressure variation during mold filling are found to change significantly.     

Prediction of flow–induced process variables based on similarity analysis in the liquid molding process

In general, a numerical scheme is a widely accepted technique for estimating resin flow in the liquid molding process. A numerical mold filling analysis is essential to optimize the manufacturing process of a composite. However, finding an optimal condition from the numerical analysis requires many numerical calculations. The efforts can be greatly reduced if a similarity solution replaces the repeated numerical calculations. In this study, similarity relations are proposed to predict the flow‐induced process variables. such as resin pressure, resin velocity, and flow front evolution time, during mold filling. Numerical simulations are performed for two cases where a material property, an injection condition or a part shape is different. The model is verified by applying the similarity relation for two numerical results obtained from the thin shell structure.     

A semi-empirical algorithm for flow balancing in multi-cavity transfer molding

Multi-cavity transfer molding is an important process step in several electronic and photonic technologies. In some applications, uniform filling of the cavities of the mold, or mold balancing, is required. A semi-empirical flow model to predict mold filling patterns was developed. The algorithm is a one-dimensional network flow simulation that uses experimental pressure drop data to determine the volumetric flow rate through the gates and runners. A comprehensive experimental program was undertaken to determine these hydraulic resistances for different flow rates and mold geometries. A theoretical treatment is also described to compute hydraulic resistance from gate geometry. Uniform gate resistances provide unbalanced filling and higher velocities in the cavities. Balanced filling can significantly reduce the molding compound velocity and the flow induced stresses, but imperfect balancing compromises the benefits. Experimental filling patterns were obtained for two sets of gates. The agreement between the model and the experiments was satisfactory, and the discrepancies were attributed to correctable phenomena.     

面向家族制模具多异型腔不平衡充填的快速优化设计

由于家族制模具多异型腔结构的不平衡性,极易在注射成型过程中产生局部充填不满、迟滞效应等缺陷,很大程度上制约着制品的品质。在对塑料熔体流动分析的基础上,发现流道截面半径和长度是影响熔体体积流量和压力分布的重要因素。针对指示灯柱产品多异型腔组合结构的注射成型特征,提出将不同型腔间充填末端的最大平均压力差作为不平衡因子,集成一种基于均匀设计多维结构变量的快速优化机制,结合遗传算法全局优化获得指标最优的流道方案。模拟验证结果表明,结构参数改进后的流道系统充填平衡效果明显优于初始结果。     

RTM工艺中玻纤增强材料渗透率的测量与分析

通过对树脂传递模塑成型工艺中广泛使用的纤维增强材料——玻纤连续毡渗透率的测量和分析，建立了该增强材料在模具中的纤维体积含量与渗透率之间的关系，考察了纤维增强材料的渗透率与注模时间的关系，分析对比了纤维增强材料的结构型式对注模时间和纤维浸透性的影响。结果表明；随着纤维体积含量的增大，渗透率迅速下降，对于玻纤连续毡，其渗透率ｋ与纤维体积含量ｖｆ的关系可以表示为一个多项式；在恒定的压力下，渗透率大，注模需要的时间越短，体积含量相同的玻纤连续毡和玻纤方格布比较，玻纤连续毡的渗透率约大一倍，而所需注模时间约为玻纤方格布的１／２；玻纤方格布中存在渗透率相差特别大的两大区域是造成其浸透性差的主要原因。