Overall numerical simulation of extrusion blow molding process

This paper focuses on the overall numerical simulation of the parison formation and inflation process of extrusion blow molding. The competing effects due to swell and drawdown in the parison formation process were analyzed by a Lagrangian Eulerian (LE) finite element method (FEM) using an automatic remeshing technique. The parison extruded through an annular die was modeled as an axisymmetric unsteady nonisothermal flow with free surfaces and its viscoelastic properties were described by a K‐BKZ integral constitutive equation. An unsteady die‐swell simulation was performed to predict the time course of the extrudate parison shape under the influence of gravity and the parison controller. In addition, an unsteady large deformation analysis of the parison inflation process was also carried out using a three‐dimensional membrane FEM for viscoelastic material. The inflation sequence for the parison molded into a complex‐shaped mold cavity was analyzed. The numerical results were verified using experimental data from each of the sub‐processes. The greatest advantage of the overall simulation is that the variation in the parison dimension caused by the swell and drawdown effect can be incorporated into the inflation analysis, and consequently, the accuracy of the numerical prediction can be enhanced. The overall simulation technique provides a rational means to assist the mold design and the determination of the optimal process conditions.     

An experimental investigation of capillary extrudate swell in relation to parison swell behavior in blow molding

An experimental study was carried out to study and characterize the capillary extrudate swell and parison swell behavior in extrusion blow molding of two commercial blow molding grade high density polyethylene resins. The capillary extrudate swell behavior of these resins were determined employing a capillary rheometer and a special thermostatting chamber. Parison swell behavior was determined using an Impco A13-R12 reciprocating screw blow molding machine in conjunction with cinematography and pinch-off. The experimental conditions under which capillary extrudate and parison swell data can be related are elucidated. Excellent agreement is found between the area swell values determined on the basis of capillary and parison swell experiments.     

Measurement and calculation of parison dimensions and bottle thickness distribution during blow molding

Experimental data are reported regarding the dynamics of the blow molding process, including parison formation, growth, and inflation. These data have been obtained with the aid of high speed cinematography and pinch mold experiments, in conjunction with two commercial blow molding polyethylene resins. It is shown that pinch mold experiments alone do not yield accurate data regarding thickness and diameter swell. Furthermore, the inflation process involves decreasing rates of inflation with time, as a result of the rise in viscosity due to the cooling of the parison during inflation. Mathematical procedures are proposed for a first-order estimation of parison length and swell as a function of time and the inflation behavior after clamping. In the absence of more dependable basic procedures, the proposed treatment is employed to estimate the effective transient swell functions for the parison using experimental data obtained under the specified conditions. The mathematical treatment is extended to determine the thickness distribution of the bottle. Good agreement is obtained between experimental and calculated results.     

The dynamics of parison development in blow molding

An experimental program was carried out to study the dynamics of parison swell and development in extrusion-blow molding. Two commercial blow molding grade polyethylene resins were employed in conjunction with an Impco, Model A13-R12 reciprocating screw blow molding machine equipped with a cylindrical bottle mold. Parison weight swell was measured with the aid of a parison pinch-off mold. In order to obtain more reliable and useful information regarding diameter and thickness swell of the parison and the dynamics of parison formation and development, high speed cinematography was employed. Data obtained by this technique are more reliable than results obtained with the pinch-off mold alone. They also give further insight into the phenomena of swell, sag, and parison spring back or recovery.     

Integrated numerical modeling of the blow molding process

The numerical modeling of the extrusion blow molding of a fuel tank is considered in this work. The integrated process phases are consecutively simulated, namely, parison formation, clamping, and inflation, as well as part solidification, part deformation (warpage), and the buildup of residual stresses. The parison formation is modeled with an integral type viscoelastic constitutive equation for the sag behavior and a semi-empirical equation for the swell behavior. A nonisothermal viscoelastic formulation is employed for the clamping and inflation simulation, since parison cooling during extrusion strongly affects the inflation behavior. Once the parison is inflated, it solidifies while in the mold and after part ejection. Warpage and residual stress development of the part are modeled with a linear viscoelastic solid model. Numerical predictions are compared with experimental results obtained on an industrial scale blow molding machine. Good agreement is observed. A process optimization based on a desired objective function, such as uniform part thickness distribution and/or minimal part weight, is performed. The integrated clamping, inflation, and cooling stages of the process are considered. The optimization is done by the systematic manipulation of the parison thickness distribution. Iterations are performed employing a gradient based updating scheme for the parison thickness programming, until the desired objective of uniform part thickness is obtained.     

Numerical simulation of blow molding—Viscoelastic flow analysis of parison formation

The simulation of the parison formation process in blow molding has been studied. The flow field was divided into two regions, namely, the extrudate swell region near the die lip and the parison formation region after the exit swell. In the swell region, we predicted the swelling ratio and residual stress distribution for high Weissenberg numbers for steady planar well using the 1-mode Giesekus model. In the parison formation region, the flow is assumed to be an unsteady unaxial elongational flow including drawdown and recoverable swell and is modeled using the 10-mode Giesekus model. We calculated the time course of parison length and thickness distribution, and compare the calculation results of parison length with experimental data. It was found that the predicted values agreed rather well with the experimental values. The calculation results could especially predict the shrink-back, which is the phenomenon where the parison length becomes shorter after the cessation of extrusion, and it was found tat this was caused by the recoverable swell of the parison, which depends on the tensile stress generation in the die. Various flow rates and die geometries were studied and confirmed the reliability and usefulness of the method.     

A comprehensive experimental study and numerical modeling of parison formation in extrusion blow molding

Parison dimensions in extrusion blow molding are affected by two phenomena, swell due to stress relaxation and sag drawdown due to gravity. It is well established that the parison swell and sag are strongly dependent on the die geometry and the operating conditions. The availability of a modeling technique ensures a more accurate prediction of the entire blow molding process, as the proper prediction of the parison formation is the input for the remaining process phases. This study considers both the simulated and the experimental effects of the die geometry, the operating conditions, and the resin properties on the parison dimensions using high density polyethylene. Parison programming with a moving mandrel and the flow rate evolution in intermittent extrusion are also considered. The parison dimensions are measured experimentally by using the pinch-off mold technique on two industrial scale machines. The finite element software BlowParison® developed at IMI is used to predict the parison formation, taking into account the swell, sag, and nonisothermal effects. The comparison between the predicted parison/part dimensions and the corresponding experimental data demonstrates the efficiency of numerical tools in the prediction of the final part thickness and weight distributions. POLYM. ENG. SCI., 47:1–13, 2007. © 2006 Society of Plastics Engineers     

Numerical simulation of blow molding—prediction of parison diameter and thickness distributions in the parison formation process

In our previous study, we calculated the time course of parison length in the parison formation stage, but it could predict only the parison area swell. The next target in our study is to calculate the parison diameter and thickness swell. Annular extrudate swell simulation is necessary for the understanding of various kinds of swelling ratios in blow molding. We have examined three kinds of swells (outer diameter, thickness, and area swells) obtained from simulation results of annular extrudate swell, using the Giesekus model, and have developed a method of predicting parison outer diameter and thickness swell values. The predicted values of parison outer diameters are discussed in comparison with experimental data, and reasonable results are obtained by the proposed method. This prediction method could also be applied to the parison formation process using a parison controller. As a result, it is possible to predict approximately the whole process of parison formation by numerical simulation.     

An investigation of the kinematics of stretch blow molding poly(ethylene terephthalate) bottles

An experimental study of the kinematics of inflation of poly(ethylene terephthalate) (PET) parisons in a Corpoplast stretch blow molding process is reported. LVDT displacement transducers have been placed at various positions along the length and at the top of a specially designed and built mold. A six-channel LVDT demodular circuit was built and attached to a minicomputer. Studies were conducted at 90 and 100°C at various pressures. Kinematic models were developed for the parison inflation, and stress fields along the deforming surface of the parison were computed using membrane theory.     

挤出吹塑型坯吹胀的CAD/CAE技术

对挤出吹塑型坯吹胀过程的CAD／CAE技术进行了初步研究。此技术可通过对吹塑CAD几何造型，CAD／CAE间信息传递，以及CAE分析的集成，实现对挤出吹塑型坯吹胀成型工艺过程的模拟和分析。最后用实例验证了此技术的可行性，为塑料制品的设计、材料选择、模具设计、吹塑成型工艺的制定及吹塑成型工艺过程的控制提供了科学依据。     

塑料挤出吹塑的机理问题

采用不同的方法对挤出吹塑过程的型坯成型、型坯吹胀与制品冷却三阶段的机理问题进行了研究.采用人工神经网络方法预测了受模口温度和挤出流率影响的型坯成型阶段的膨胀.利用建立起来的神经网络模型预示的膨胀与实验结果很吻合,且可在一定范围内,预示不同工艺条件下型坯的直径膨胀和壁厚膨胀,为型坯的直径和壁厚的在线控制提供了理论依据.基于薄膜近似和neo-Hookean本构关系,建立了描述型坯自由吹胀的数学模型,并通过实验方法获得了型坯吹胀的瞬态图象.     

Influence of glass fibers on the extrusion and injection molding characteristics of polyethylene and polystyrene melts

A basic study is reported of extrusion and injection molding for melts of polyethylene and polystyrene filled with glass fibers. It is shown that glass fibers cause large increases in end pressure losses and decreases in extrudate swell. Flow visualization of injection molding studies indicates that polymer melts containing high loadings of glass fibers exhibit jetting in end gated molds. This is attributed to the reduction in extrudate swell. To avoid jetting, special care is required in design of gating.     

Variables affecting parison diameter swell and their correlation with rheological parameters

An important factor in the selection of blow molding resins for producing handled bottles is the effective diameter swell of the parison. Ideally, the diameter swell is directly related to the weight swell and would require no separate consideration. In actual practice, the existence of gravity, the finite parison drop time and the anisotropic aspects of the blow molding operation prevent reliable prediction of the parison diameter swell directly from the weight swell. The parison diameter swell is a complex function of the weight swell, the rate of swell and the melt strength. Elements of this function are presented which show the effect of extrusion rate, parison drop time and parison weight. A technique is presented which allows the estimation of local weight and diameter swell ratios. Their direct relationship is confirmed by data obtained on several blow molding resins. The relationship between weight swell and diameter swell is definitely anisotropic. A mathematical model for swell is proposed which incorporates experimentally determined rate constants and swell coefficients. Correlations are given which suggest fundamental relationships between these derived coefficients and basic variables such as resin properties or process conditions. The model's predictive capability is demonstrated by using it to back calculate parison dimensions.     

Swell of extrudate from an annular die

Diameter and thickness swells have been measured as functions of time and wall shear rate for three high density polyethylenes at 170°C and one polypropylene at 190°C. By extruding into an oil having the same temperature and density as the extrudate, it was possible to measure isothermal swell in the absence of drawdown. Seventy to 80 percent of the swell occurs in the first one or two seconds, while several minutes are required to reach an equilibrium state. Relationships between various swell parameters, including parison weight swell and capillary extrudate swell, are examined. Important differences between the behavior of the polyethylenes and that of the polypropylene are noted.     

Parison material distribution: Viscoelastic and programming effects

Blow molding of high performance bottles—including carbonated beverage bottles—requires close control of material usage and distribution. Two parameters—polymer viscoelasticity and mechanical/electronic programming—are investigated to determine their influence on weight distribution within the extruded parison. Barex® 210 Resin is utilized in a study of polymer swell and drawdown forces and the changes in material distribution that occur due to melt temperature, extrusion time, parison length, and weight. A system for multipoint mechanical/electrical parison programming is described and its influence on material distribution determined. This technique enables the blow molder to vary the parison material distribution for high performance and economical resin usage.     

Effect of acrylic processing aids on PVC die swell

The die swell behavior of PVC melts is a manifestation of melt elasticity and is of considerable commercial as well as fundamental importance. This behavior is a critical issue in extrusion blow molding application where die swell (i.e. parison thickness) needs to be controlled. Advantageously, the addition of high molecular weight acrylic processing aids to PVC provides better die swell control, thus, improving dramatically the processability of PVC. Hence, knowledge of molecular weight variables of such acrylic processing aids is important from both the commercial and rheological point of view. Various acrylic processing aids were prepared by polymerization designed to provide systematic variation of molecular parameters. Molecular weight distribution of the polymers was characterized by GPC, and their die swell behavior in a typical PVC blow molding formulation was determined at 200°C over various range of residence times using different L/D capillary dies. The results are presented showing effects of specific molecular variables.     

Mathematical modeling of the blow-molding process

A mathematical simulation of the blow-molding cycle has been developed by combining general conservation principles along with appropriate constitutive relations for the material. A model of the parison formation stage has been devised by considering the competing effects due to swell and drawdown. A more rigorous numerical analysis of parison formation is also discussed. A theoretical treatment of parison inflation is described for both inelastic and viscoelastic materials by assuming uniform radial growth, Comparisons are made with experimental data for all phases of the molding cycle. The mathematical model is in reasonable quantitative agreement with experimental results and is capable of elucidating the influence of material properties and process conditions on the dynamics and performance of the blow-molding process.     

聚碳酸酯的吹塑成型

简介聚碳酸酯吹塑容器的特性与用途,着重分析其吹塑(包括挤出吹塑、共挤吹塑和注射吹塑)成型的机械(螺杆、机筒、型坯机头与吹塑模具)设计要点及工艺条件。     

挤出吹塑中型坯自由吹胀的动力学分析

聚合物挤出吹疗程 型坯的自由吹胀受到多方面因素的影响。采用动力学方法对此进行了研究。模拟了史胀过程听型坯轮廓变化， 吹胀压力，材料模量，型坯初始壁厚对此过程的影响。     

The dynamics of parison free inflation in extrusion blow molding

Parison free inflation behavior, associated with the extrusion blow molding process, is considered both experimentally and theoretically. Experimental observations indicate1 that the parison assumes a rather complex shape under conditions of unrestricted inflation. In particular, the time-dependent shape is markedly ellipsoidal rather than cylindrical in nature. This nonuniform behavior, however, becomes more prominent in relation to the entire length as the parison-length-to-diameter ratio is decreased. Based on the experimental observations, a simplified analytical treatment of the free inflation of a viscoelastic parison is presented. The theoretical results illuminate the influence of material properties and process conditions on the inflation process. Expectedly, inflation is enhanced by an increase in the pressure driving force as well as by a decrease in viscosity. However, melt elasticity is also found to exert a significant influence on the inflation behavior. Moreover, the theoretical analysis suggests that the initial parison dimensions play a central role in controlling the inflation process.