Validating injection stretch‐blow molding simulation through free blow trials

A 2D isothermal finite element simulation of the injection stretch‐blow molding (ISBM) process for polyethylene terephthalate (PET) containers has been developed through the commercial finite element package ABAQUS/standard. In this work, the blowing air to inflate the PET preform was modeled through two different approaches: a direct pressure input (as measured in the blowing machine) and a constant mass flow rate input (based on a pressure–volume–time relationship). The results from these two approaches were validated against free blow and free stretch‐blow experiments, which were instrumented and monitored through high‐speed video. Results show that simulation using a constant mass flow rate approach gave a better prediction of volume vs. time curve and preform shape evolution when compared with the direct pressure approach and hence is more appropriate in modeling the preblowing stage in the injection stretch‐blow molding process. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Experimental study and finite element analysis of the injection blow molding process

A finite element numerical analysis of preform inflation associated with the injection blow molding process has been developed using a neo-Hookean constitutive model. The analysis is capable of predicting final wall thickness distributions for axisymmetric mold geometries. Experimental studies were conducted on a Uniloy injection blow molding machine (Model 189-3 and Model 122). A twelve ounce (355 mL) cylindrical bottle mold was instrumented with contact sensors, thermocouples, and pressure transducers. Visualization studies of the inflation process were performed using specialized tooling and high-speed video cameras. The experimental studies provide justification for analyzing the deformation by means of a static elastic approach. The predicted wall thickness distribution is in reasonable agreement with the experimental data. Nonuniformities in the temperature distribution in the preform were found to have the most significant impact on the inflation behavior and the resulting wall thickness.     

A 3‐D finite element model for gas‐assisted injection molding: Simulations and experiments

To gain a better understanding of the gas‐assisted injection molding process, we have developed a computational model for the gas assisted injection molding (GAIM) process. This model has been set up to deal with (non‐isothermal) three‐dimensional flow, in order to correctly predict the gas distribution in GAIM products. It employs a pseudo‐concentration method, in which the governing equations are solved on a fixed grid that covers the entire mold. Both the air downstream of the polymer front and the gas are represented by a fictitious fluid that does not contributeto the pressure drop in the mold. The model has been validated against both isothermal and non‐isothermal gas injected experiments. In contrast to other models that have been reported in the literature, our model yields the gas penetration from the actual process physics (not from a presupposed gas distribution). Consequently, it is able to deal with the 3‐D character of the process, as well as with primary (end of gas filling) and secondary (end of packing) gas penetration, including temperature effects and generalized Newtonian viscosity behavior.     

A simulation of the non‐isothermal resin transfer molding process

A simulation of the non‐isothermal resin transfer molding manufacturing process accounting for both the filling and the consolidation stage has been developed. The flow of an exothermally reactive resin through a porous medium has been analyzed with reference to the Darcy law, allowing for the chemorheological properties of the reacting resin. Thermal profile calculations have been extended to a three phase domain, namely the mold, the dry preform and the filled preform. The mold has been included in order to evaluate the thermal inertial effects. The energy balance equation includes the reaction term together with the conductive and convective terms, and particular attention has been devoted to setting the thermal boundary condition at the flow front surface. The moving boundary condition has been derived by a jump equation. The simulation performance has been tested by comparing the predicted temperature profiles with experimental data from literature. Further numerical analysis assessed the relevance of using the jump equation at the flow front position for both filling time and thermal profile determination.     

A slightly compressible transient finite element model of the packing phase in injection molding

A 2-D finite element model of the packing phase using an Augmented Lagrangian approach is described. The model is tested for various geometrical configurations. The influence of the cavity thickness, length, and shape on the packing time and flow patterns is investigated, showing a good agreement with published data.     

Non‐isothermal crystallization kinetics of cork‐polymer composites for injection molding

The non‐isothermal crystallization behavior of cork–polymer composites (CPC) based on polypropylene (PP) matrix was studied. Using differential scanning calorimetry (DSC), the crystallization behavior of CPC with 15 wt % cork powder at different cooling rates (5, 10, 15, and 20 °C/min) was studied. The effect of a coupling agent based on maleic anhydride was also analyzed. A composite (PPg) containing polypropylene grafted maleic anhydride (PPgMA) and PP was prepared for comparison purposes. Crystallization kinetic behavior was studied by Avrami, Ozawa, Liu, and Kissinger methods. The Ozawa method fails to describe the behavior of these composites. Results show that cork powder surface acts as a nucleating agent during non‐isothermal crystallization, while the addition of PPgMA decreases the crystallization rate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44124.     

Isothermal and non‐isothermal crystallization behavior of PET nanocomposites

Polyethylene terephthalate (PET)/clay nanocomposites (PCNs) containing 1 wt% Cloisite 30B (C30B) were prepared via melt compounding. Modulated differential scanning calorimetry (MDSC) for isothermally crystallized samples revealed that the third endotherm at the highest temperature may be attributed to the recrystalization and melting of crystals, reorganized during heating. The first and second endotherms may be associated with melting of the secondary and primary crystals, respectively. The overall isothermal crystallization rate in PCNs was faster than in the neat resin. Growth kinetics revealed that the work required for chain folding and the equilibrium melting temperature in PCNs were somewhat higher than for neat PET. During isothermal crystallization, the steric hurdles introduced by clay layers lead to a reduction in the transport of the PET chains into crystallites. The effective non‐isothermal activation energy for the PCNs was higher than for PET, possibly leading to less perfect crystals in the PCNs. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

A fast computational model to the simulation of non‐isothermal mold filling process in resin transfer molding

The numerical simulation of mass and heat transfer model for the curing stage of the resin transfer molding (RTM) process is known as a useful method to analyze the process before the mold is actually built. Despite the intense interest in the modeling and simulation of this process, the relevant work is currently limited to development of flow models during filling stage. Optimization of non‐isothermal mold filling simulation time without losing the efficiency remains an important challenge in RTM process. These were some reasons that motivate our work; namely the interested on the amelioration of the performance of RTM simulation code in term of execution time and memory space occupation. Our approach is accomplished in two steps; first by the modification of the control volume/ finite element method (CV/FEM) and second by the implementation in the modified code of an adapted conjugate gradient algorithm to the compressed sparse row storage scheme. The validity of our approach is evaluated with analytical results and excellent agreement was found. The results show that our optimization strategy leads to maximum reduction in time and space memory. This allows one to deal with problems with great and complex dimensions mostly encountered in RTM application field, without interesting in the constraint of space or time. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Simulation of non‐isothermal melt densification of polyethylene in rotational molding

Rotational molding involves powder mixing, heating and melting of powder particles to form a homogeneous polymer melt, as well as cooling and solidification. The densification of a loose powder compact into a homogeneous melt occurs over a wide range of conditions as the material passes from a solid state into a melt state. The numerical simulation of the non‐isothermal melt densification in the rotational molding process is presented in this work. The simulation combines heat transfer, polymer sintering and bubble dissolution models, and is based on an idealized packing arrangement of powder particles. The predictions are in general agreement with experimental observations presented in the literature for the rotational molding of polyethylenes. The simulation allows for systematic and quantitative studies on the effect of molding conditions and material properties on the molding cycle and molded part density. Results indicate that the densification process is primarily affected by the powder characteristics, which are accounted for in terms of the particle size and the particle packing arrangement. The material rheological properties become increasingly important as the powder characteristics lessen in quality. The simulation demonstrated that while certain combinations of processing conditions help reduce the molding cycle, they have a detrimental effect on the densification process.     

Experimental study of preform reheat temperature in two‐stage injection stretch blow molding

Plastic bottles used for carbonated soft drink (CSD) packages are most commonly made from poly(ethylene terephthalate) (PET) by injection stretch blow molding (ISBM). The required bottle performance criteria vary with its application but typically include top‐load strength, burst strength, optical clarity, thermal stability, and barrier properties. An experimental study of the preform reheat temperature was carried out for a 1.5‐l PET bottle produced by a two‐stage ISBM machine. The overall temperature of the preform was changed by controlling the reheat temperature of the preform; all the other process variables and preform dimensions were kept constant. Performance of the PET bottles for differing preform reheat temperatures was measured experimentally in terms of top‐load strength, burst pressure resistance, environmental stress cracking resistance (ESCR), and thermal stability. It was observed that the ESCR values and the burst strength decreased with the increasing reheat temperature, whereas the top‐load strength increased. Thermal stability tests confirmed that high‐preform reheat temperatures had a detrimental effect on the self‐standing feature of the bottles. Decreasing the reheat temperature as low as possible, while maintaining a certain preform temperature profile, ensured high ESCR and burst strength values and prevented the concaveness at the bottom of the bottle. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Stationary bootstrapping for non‐parametric estimator of nonlinear autoregressive model

We consider stationary bootstrap approximation of the non‐parametric kernel estimator in a general kth‐order nonlinear autoregressive model under the conditions ensuring that the nonlinear autoregressive process is a geometrically Harris ergodic stationary Markov process. We show that the stationary bootstrap procedure properly estimates the distribution of the non‐parametric kernel estimator. A simulation study is provided to illustrate the theory and to construct confidence intervals, which compares the proposed method favorably with some other bootstrap methods.     

The role of fiber bundle interaction forces in non‐isothermal compression molding of thermoplastic composites

The non‐isothermal flow during compression molding of thermoplastic‐based composites having a planar, randomly oriented fiber structure has been investigated. Local interactions at the bundle‐bundle level were measured and a constitutive equation was derived to describe the shear and squeeze flow of the composites. This equation contains only micro‐mechanics‐related variables, and accounts for the temperature dependence of the interaction forces. The flow of the material under industrial processing conditions can thus be related to the deformation mechanisms of the reinforcement structure at the scale of the individual fibers Good agreement was obtained between the predicted and experimentally observed theological behavior of the composites in both shear and squeeze flow. The use of this approach to analyze the industrial molding of complex‐shaped parts is dis cussed.     

Three‐dimensional numerical simulation of injection molding filling of optical lens and multiscale geometry using finite element method

This article presents the development, verification, and validation of three‐dimensional (3‐D) numerical simulation for injection molding filling of 3‐D parts and parts with microsurface features. For purpose of verification and comparison, two numerical models, the mixed model and the equal‐order model, were used to solve the Stokes equations with three different tetrahedral elements (Taylor‐Hood, MINI, and equal‐order). The control volume scheme with tetrahedral finite element mesh was used for tracking advancing melt fronts and the operator splitting method was selected to solve the energy equation. A new, simple memory management procedure was introduced to deal with the large sparse matrix system without using a huge amount of storage space. The numerical simulation was validated for mold filling of a 3‐D optical lens. The numerical simulation agreed very well with the experimental results and was useful in suggesting a better processing condition. As a new application area, a two‐step macro–micro filling approach was adopted for the filling analysis of a part with a micro‐surface feature to handle both macro and micro dimensions while avoiding an excessive number of elements. POLYM. ENG. SCI., 46:1263–1274, 2006. © 2006 Society of Plastics Engineers     

A real‐time process optimization system for injection molding

The proposed real‐time intelligent model for the injection molding process control consists of the initial parameters setting and online defects correction. First, preliminary optimization based on a simplified simulation model is used for the initial setting. This simplified model adopts a geometric approximation of the original part by a rectangular edge‐gated plate. Then, the molding trial will be run on the molding machine by using the initial process parameters. And a fuzzy inference model based on expert knowledge is developed for correcting defects during the molding trial. This defects correction procedure will be repeated until the part quality is found satisfactory. A corresponding intelligent system has been developed that is integrated with the injection machine by communicating with the controller. The system can be used to optimize process parameters real time. Experimental studies have been carried out for verification. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers.     

3D finite element method for the simulation of the filling stage in injection molding

During the molding of industrial parts using injection molding, the molten polymer flow through converging and diverging sections as well as in areas presenting thickness and flow direction changes. A good understanding of the flow behavior and thermal history is important in order to optimize the part design and molding conditions. This is particularly true in the case of automotive and electronic applications where the coupled phenomena of fluid flow and heat transfer determine to a large extent the final properties of the part. This paper presents a 3D finite element model capable of predicting the velocity, pressure, and temperature fields, as well as the position of the flow fronts. The velocity and pressure fields are governed by the generalized Stokes equations. The fluid behavior is predicted through the Carreau Law and Arrhenius constitutive models. These equations are solved using a Galerkin formulation. A mixed formulation is used to satisfy the continuity equation. The tracking of the flow front is modeled by using a pseudo-concentration method and the model equations are solved using a Petrov-Galerkin formulation. The validity of the method has been tested through the analysis of the flow in simple geometries. Its practical relevance has been proven through the analysis of an industrial part.     

Non‐isothermal crystallization kinetics of PET

The crystallization kinetics of poly(ethylene terephthalate) was studied using constant cooling rate, isothermal and quenching experiments. A non‐isothermal crystallization kinetics equation based on a single mechanism was used to analyze the data. Different mechanisms of crystallization at low, intermediate, and high cooling rates were hypothesized based on deviation of the experimental data from the single mechanism model.     

Co‐injection molding of immiscible polymers: Skin‐core structure and adhesion studies

In this article, the microstructure and the adhesion developed in co‐injected specimens obtained with polypropylene (PP; core) and polystyrene (PS; skin) were studied as a function of process conditions and additives used. The study shows that the incorporation of low amounts of fillers such as Nanoclays and styrene‐ethylene‐butadiene‐styrene (SEBS) copolymer to the core material, working as compatibilizers, improves the adhesion at lower and higher polymer melt temperatures, respectively. The authors concluded as well that the use of such fillers, also improves the reproducibility of the process. The adhesion was assessed by shear tests using double lap shear specimens. A data acquisition system was attached to the mold to evaluate the pressure inside the cavity. Results of the in‐mold pressure profiles corresponded well when compared with MoldFlow predictions, and demonstrated that the adhesion of both materials is also related to their behavior and shrinkage inside the mold. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers.     

A full 3D finite element analysis of the powder injection molding filling process including slip phenomena

A full 3D finite element analysis system has been developed to simulate a Powder Injection Molding (PIM) filling process for general three‐dimensional parts. The most important features of the analysis system developed in this study are i) to incorporate the slip phenomena, the most notable rheological characteristics of PIM feedstock, into the finite element formulation based on a nonlinear penalty‐like parameter and ii) to simulate the transient flow during the filling process with a predetermined finite element mesh with the help of a volume fill factor and a melt front smoothing scheme. The treatment of the nonlinear slip boundary condition was successfully validated via a steady state pipe flow. For the purpose of comparisons, not only the numerical simulations but also experimental short‐shot experiments were performed with two 3D mold geometries using two typical materials of slip and no‐slip cases. The good agreements between the numerical and experimental results indicate that the melt front tracking scheme successfully simulates the transient filling process.     

Three‐dimensional simulation of multi‐material injection molding: Application to gas‐assisted and co‐injection molding

This paper presents an overview of the results obtained at the Industrial Materials Institute (IMI) on the numerical simulation of the gas‐assisted injection molding and co‐injection molding. For this work, the IMI's three‐dimensional (3D) finite element flow analysis code was used. Non‐Newtonian, non‐isothermal flow solutions are obtained by solving the momentum, mass and energy equations. Two additional transport equations are solved to track polymer/air and skin/core materials interfaces. Solutions are shown for different thin parts and then for thick three‐dimensional geometries. Different operating conditions are considered and the influence of various processing parameters is analyzed.