Effects of rheological properties and processing parameters on ABS thermoforming

Understanding effects of material and processing parameters on the thermoforming process is critical to the optimization of processing conditions and the development of better materials for high quality products. In this study we investigated the influence of both rheological properties and processing parameters on the part thickness distribution of a vacuum snap‐back forming process. Rheological properties included uniaxial and biaxial elongational viscosity and strain hardening and/or softening while processing parameters included friction coefficient, heat transfer coefficient, and sheet and mold temperatures. The Wagner two parameter nonlinear viscoelastic constitutive model was used to describe rheological behavior and was fit to shear and elongational experimental data. The linear viscoelastic properties along with the Wagner model were utilized for numerical simulation of the thermoforming operation. Simulations of pre‐stretched vacuum thermoforming with a relatively complex mold for a commercial refrigerator liner were conducted. The effects of nonlinear rheological behavior were determined by arbitrarily changing model parameters. This allows determination of which rheological features (i.e., elongational mode, viscosity, and strain hardening and/or softening) are most critical to the vacuum snap‐back thermoforming operation. We found that rheological and friction properties showed a predominant role over other processing parameters for uniform thickness distribution.     

The effects of radiation cross‐linking and process parameters on the behavior of polyamide 12 in vacuum thermoforming

Compared to amorphous thermoplastics, semi‐crystalline thermoplastics usually have a smaller processing range for thermoforming, due to their narrow temperature window for the transition from viscoelastic to viscous material behavior. On the other hand, semi‐crystalline thermoplastics offer superior properties for applications like ductility or chemical resistance. Within this article, modification of semi‐crystalline polyamide 12 by radiation cross‐linking with respect to its suitability for vacuum thermoforming as well as the effects of processing parameters and sheet thickness on the resulting strain distributions in thermoformed parts are shown. Experimental thermoforming processing studies in combination with digital image correlation measurements, thermo‐mechanical and elongational rheometry were performed to characterize the behavior of cross‐linked semi‐crystalline thermoplastics in the vacuum thermoforming process. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers     

Three‐dimensional simulation of thermoforming process and its comparison with experiments

Three‐dimensional solid element analysis and the membrane approximated analysis employing the hyperelastic material model have been developed for the simulation of the thermoforming process. For the free inflation test of a rectangular sheet, these two analyses showed the same behavior when the sheet thickness was thin, and they deviated more and more as the sheet thickness increased. In this research, we made a guideline for the accuracy range of sheet thickness for the membrane analysis to be applied. The simulations were performed for both vacuum forming and the plug‐assisted forming process. To compare the simulation results with experiments, laboratory scale thermoforming experiments were performed with acrylonitrile‐butadiene‐styrene (ABS). The material parameters of the hyperelastic model were obtained by uni‐directional hot tensile tests, and the thickness distributions obtained from experiments corresponded well with the numerical results. Non‐isothermal analysis that took into account the sheet, temperature distribution measured directly from the experiments was also performed. It was found that the non‐isothermal analysis greatly improved the predictability of the numerical simulation, and it is important to take into account the sheet temperature distribution for a more reliable simulation of the thermoforming process.     

Caractérisation viscoélastique du comportement d'une membrane thermoplastique et modélisation numérique de thermoformage

In this work, we are interested, on the one hand in the characterization of circular polymeric ABS membrane under biaxial deformation using the bubble inflation technique, on the other hand in modelling and numerical simulation of the thermoforming of ABS materials using the dynamic finite element method. The viscoelastic behaviour of the Lodge model is considered. First, the governing equations for the inflation of a flat circular membrane are solved using a variable‐step‐size‐finite difference method and a modified Levenberg‐Marquardt algorithm to minimize the difference between the calculated and measured inflation pressure. This will determine the material constants embedded within the model used. For dynamic finite elements method, we consider a nonlinear load in air flow which obeys the Redlich‐Kwong equation of state of the real gases. For numerical simulation, the lagrangian formulation together with the assumption of the membrane theory is used. Moreover, the influence of the viscoelastic model on the thickness and on the stress distribution in the thermoforming sheet are analysed for ABS material.     

Thermoforming process of amorphous polymeric sheets: Modeling and finite element simulations

A temperature and strain rate dependent model for the thermoforming process of amorphous polymer materials is proposed. The polymeric sheet is heated at a temperature above the glass transition temperature then deformed to take the mold shape by the means of an applied pressure. The applied process temperature is taken uniform throughout the sheet and its variation is due only to the adiabatic heating. The behavior of the polymeric material is described by a micromechanically‐based elastic‐viscoplastic model. The simulations are conducted for the poly(methyl methacrylate) using the finite element method. The polymer sheet thickness and the orientation of the polymer molecular chains show an important dependence on the process temperature, the applied pressure profile, and the contact forces with the mold surface. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Experimental study on the dynamics of thermoforming of polystyrene

An experimental study of the dynamics of thermoforming a high-impact polystyrene sheet was undertaken to evaluate the effect of evacuation rate and temperature on the rate of sheet deformation and the wall thickness distribution of the molded part. The studies were conducted using an instrumented cylindrical mold having an adjustable bottom insert to vary the depth of draw. The evaculation rate was varied by introducing a flow restriction in the form of an orifice plate in the base of the mold. The deformation rate of the sheet was determined by means of fiber-optic infrared detectors located at various depths within the mold. Mold contact sensors also provided information with regard to the deformation process after contact with the mold surface had been achieved. The evaluation characteristics were monitored by a pressure transducer. A three-level, two-variable factorial design was conducted to provide information of the influence of the principal operating conditions and their interactions on the measured response variables. Reliable predictive expressions were derived for the minimum pressure, time to reach minimum pressure, overall forming time, and wall thickness near the lip of the mold. The wall thickness at other locations around the periphery of the mold contour was found to be relatively insensitive to the rate of evacuation and material temperature.     

The role of process parameters in determining wall thickness distribution in plug‐assisted thermoforming

This article describes the results of a comprehensive investigation to determine the link between process parameters and observed wall thickness output for the plug‐assisted thermoforming process. The overall objective of the work was to systematically investigate the process parameters that may be adjusted during production to control the wall thickness distribution of parts manufactured by plug‐assisted thermoforming. The parameters investigated were the sheet temperature, plug temperature, plug speed, plug displacement, plug shape, and air pressure. As well as quantifying the effects of each parameter on the wall thickness distribution, a further aim of the work was to improve the understanding of the physical mechanisms of deformation of the sheet during the different stages of the process. The process parameters shown to have the greatest effect on experimentally determined wall thickness distribution were the plug displacement, sheet temperature, plug temperature, and plug shape. It is proposed that during the plug‐assisted thermoforming of polystyrene the temperature dependent friction between the plug and sheet surface was the most important factor in determining product wall thickness distribution, whereas heat transfer was shown to play a less important role. POLYM. ENG. SCI., 50:1923–1934, 2010. © 2010 Society of Plastics Engineers     

Wall thickness distribution in thermoformed food containers produced by a Benco aseptic packaging machine

Effects of process parameters such as forming temperature, forming air pressure and heating time on wall thickness distribution in plug‐assist thermoformed food containers using multilayered material were investigated. Multilayered rollstockbase material formed into containers by thermoforming process using a Benco aseptic packaging machine. Forming temperatures in the range of 131–170°C, airforming pressures of 2, 3, 3. 5 and 4 bars, and heating times of 66, 74, 84, 97 and 114 seconds were used in the thermoforming process. Analysis of wall thickness data obtained for the thermoforming parameters used in this study showed that wall thickness was significantly affected by forming temperature, pressure and heating time at 0.05 significance level. Besides the processing parameters, wall location, container side, and their interactions significantly affected wall thickness. Forming temperature was found to be the principle parameter influencing wall thickness distribution in a plug‐assist thermoforming operation. The optimum operating conditions of the packaging machine for the thermoforming process are: 146–156°C for forming temperature, 2–4 bars for air‐forming pressure and 74–97 seconds for heating time.     

Rheological modeling of plug‐assist thermoforming

Solving problems for thermoforming processes in the production of axisymmetric thin walled plastics is investigated in this research work. A nonlinear viscoelastic rheological model with a new strain energy function is suggested for improvement of physical properties of final product. For model validation, a quantitative relation between stress and technical parameters of plug‐assist thermoforming is determined by comparison of theoretical and experimental results. This process with the proposed rheological model could be suggested for prevention from some technical defects such as wall thickness variations, physical instability during inflation‐shrinkage, and warpage exhibited in the final part of a polymeric sheet thermoforming. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4148–4152, 2006     

A new hybrid approach using the explicit dynamic finite element method and thermodynamic law for the analysis of the thermoforming and blow molding processes for polymer materials

In this work, the problem of the modeling of thermoforming and blow molding processes for viscoelastic sheet are considered. To take account of the enclosed gas volume, responsible for inflation of the thermoplastic membrane (which contributes significantly to the strength and stiffness of a thermoplastic structure), we considered thermodynamic approach to express external work in terms of a closed volume. The pressure load is thus deduced from the thermodynamic law of ideal gases. The viscoelastic behavior of the K‐BKZ model is considered. The Lagrangian formulation together with the assumption of the membrane theory is used in the explicit dynamic finite element implementation. The numerical validation is performed by comparing the theoretical bubble free inflation with numerical results. Moreover, the influence of the K‐BKZ constitutive model on the thickness and stress distribution in the thermoforming of containers is presented. POLYM. ENG. SCI., 46:1554–1564, 2006. © 2006 Society of Plastics Engineers     

Wall thickness distribution in plug-assist vacuum formed strawberry containers

An experimental study on the influence of processing parameters namely film temperature, plug velocity, and temperature on wall thickness distribution in plug-assist vacuum thermoformed fresh strawberry container using high-impact polystyrene is presented. Film temperatures of 118, 125, 136, 150, and 165°C, plug velocities of 0.15, 0.20, and 0.27 m/s, and plug temperatures of 25, 60, 100, 123, and 135°C were used in the thermoforming. Increasing the plug velocity resulted in improved wall thickness distribution due to elastic deformation of the plastic sheet during thermoforming. Decreasing the stretching time and the temperature difference between the plastic film and plug was important for good wall thickness distribution. Better wall thickness was obtained at the plug velocity of 0.27 m/s and the plug temperature of 123°C.     

Thermodynamic approach of inflation process of K‐BKZ polymer sheet with respect to thermoforming

The problem of modeling and the dynamic finite element simulation of thermoforming process for viscoelastic sheet are considered. The pressure load used in modeling is thus deduced from the thermodynamic law of ideal gases. The viscoelastic behavior of the K‐BKZ model is considered. The Lagrangian formulation together with the assumption of the membrane theory is used in the finite element implementation. The numerical validation is performed by comparing the theoretical solution for the uniaxial and equibiaxial hencky deformation with numerical results. Moreover, the influence of the K‐BKZ constitutive model, for three linear time distribution of airflow rate loading, on the thickness and on the stress distribution in thermoforming of containers made of HDPE are analyzed. POLYM. ENG. SCI., 45:1319–1335, 2005. © 2005 Society of Plastics Engineers     

A viscoelastic–viscoplastic modeling approach for amorphous polymers in vacuum forming simulation

In this study, the simulation of a vacuum forming process employing a micromechanical inspired viscoelastic–viscoplastic model is investigated. In the vacuum forming process, a plastic sheet is heated above the glass transition temperature and subsequently forced into a mold by applying a vacuum. The model consists of a generalized Maxwell model combined with an dissipative element in series. Each Maxwell element incorporates a hyperelastic element in series with a viscous element based on a hyperbolical law. While the generalized Maxwell model considers the relaxation due to molecular alignment, the additional viscous element is a modification based on the approach of Bergström and thus considers molecular chain reptation. The model is designed with the aim to converge to the generalized linear Maxwell model in the limit of small deformation. Furthermore, the viscous modeling is temperature activated and follows the Williams–Landel–Ferry approach in the limit of linear viscoelasticity. To simulate rheological standard experiments, a physical-network-based implementation into Simscape is presented. To validate the performance of the model in thermoforming, it is implemented into Fortran programming language for finite element simulation with Abaqus/Explicit. It can be shown that the simulation is able to predict the thickness in high correlation with experimental results.     

Experimental and numerical investigations of the vacuum-forming process

This paper exmines the influence of process variables on final thickness distributions for vacuum-formed thermoplastic parts. The process variables investigated include evacuation rate, sheet surface temperature, mold temperature, and material slip over the mold surface. The experimental data presented include, in addition to thicknesses, sheet surface temperature obtained via infrared thermography. A finite element program to model the vacuum-forming process is discussed, and the wall thickness distribution predicted by this program for a vacuum-formed part is compared with the results of the experiments.     

Effects of ABS rubber particles on rheology,melt failure,and thermoforming

The rubber particles included in rubber modified polymeric materials such as acrylonitrile‐butadiene‐styrene (ABS) polymer and impact modified polymers play an important role in determining their rheological properties, processing behavior, and mechanical properties. In this study both small strain oscillatory shear viscosity in the frequency range from 10^?2 to 10² s^?1 and uniaxial elongational viscosity behavior at two elongation rates ( = 0.1 and 1.0 s^?1) over the range of temperatures from 140°C to 200°C were measured for commercial ABS polymers with different contents and deformability of rubber particles. The influences of rubber content and deformability on rheological properties such as melt elasticity, elongational viscosity, strain hardening and/or softening, the onset of nonuniform deformation, and thermoforming performance were investigated. The Wagner two‐parameter nonlinear viscoelastic constitutive model was used to describe strain hardening behavior, while the Considère criterion was used to determine the onset point of nonuniform deformation. The part thickness distribution obtained through use of a vacuum snap‐back forming process was simulated to investigate the effects of rheological changes associated with different rubber particles on the thermoforming performance. It was found that ABS polymers with larger contents of hard rubber particles exhibited more melt elasticity, stronger strain hardening, a maximum of biaxial elongational viscosity, onset of nonuniform deformation at later time, and better thermoforming performance. Strain hardening and the Considère criterion provide simple, reliable indicators of the thermoforming performance of ABS polymers.     

A Robust Experimental Methodology for Polymer Bubble Inflation

In thermoforming technique thermoplastic sheets are heated up well above their glass transition temperature and formed to the required shape by using an appropriate mold. Characterization of thermoplastic materials for thermoforming can be accomplished by employing polymer bubble inflation and rheology tests instead of undertaking expensive biaxial tensile testing. Polymer bubble inflation technique is very sensitive to process condition variations, so a robust experimental methodology is essential. Design and development of one such experimental system was undertaken by carrying out a variety of preliminary tests. This paper presents the experimental methodology developed for polymer bubble inflation. The developed experimental system demonstrates highly repeatable polymer bubble inflations. Bubble inflations were conducted at different temperatures and different diameter circular clamping using acrylonitrile butadiene styrene (ABS) thermoplastic. Polymer sheet initial sag due to heating and its influence on bubble inflation have been captured by using the experimental system.     

Coupled thermo‐mechanical analysis for plastic thermoforming

An FEM software ARVIP‐3D was developed to simulate the process of 3‐D plastic thermoforming. The coupled thermo‐mechanical analysis, thermal stress and warpage analysis for plastic thermoforming was carried out by means of this software. Rigid visco‐plastic formula was adopted to simulate the deforming process. During this process, the method of comparing velocity, time and area was adopted as the contact algorithm at different nodes and triangular elements. Sticking contact was assumed when the nodes become in contact with tool surface. The Arrhenius equation and the Williams equation were employed to ascertain the temperature dependence of material properties. In order to analyze the temperature field of plastic thermoforming, the Galerkin FEM code and the dynamic heat conduction boundary condition were adopted; latent heat and deformation heat were treated as dynamic internal heat sources. Based on the above, the model of coupled thermomechanical analysis was established. Assuming that the thermal deformation occurs under elastic conditions, the thermal stress and the warpage following the cooling stage were estimated. Experiments of plastic thermoforming were made for high‐density polyethylene (HDPE). An infrared thermometer was used to record the temperature field and a spiral micrometer was used to measure the thickness of the part. Results of numerical calculation for thickness distribution, temperature field and warpage were in good agreement with experimental results.     

Numerical investigation on flow through porous media in the post‐infusion process

During the vacuum‐assisted resin transfer molding (VARTM) processing, the post‐infusion behavior after complete wet‐out and before gelation of the resin is critical for the development of the thickness and fiber volume fraction distribution in the cured composite part. The pressure gradient developed during infusion results in a thickness gradient due to the flexible nature of the bagging approach. After full infusion, the resin typically bleeds into a vacuum trap, allowing redistribution of pressure and preform thickness. In this study, a non‐rigid control volume is used to formulate a set of governing equations for analysis of the post‐infusion process. The model is used to investigate the effects of processing parameters and different processing scenarios on resin flow, resin pressure, and thickness variation of the composite laminate. This work provides a tool for optimization of the VARTM process to reduce final part variability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Study of the thermoformability of ethylene‐vinyl alcohol copolymer based barrier blends of interest in food packaging applications

The thermoforming capacity of a number of blends of an ethylene‐vinyl alcohol copolymer (EVOH‐32, with 32 mol % ethylene) with amorphous polyamide (aPA) and/or Nylon‐containing ionomer with interest in multilayer food packaging structures have been studied. These blends were vacuum‐thermoformed between 100 and 150°C onto male molds of different shapes and areal draw ratios. It was found that EVOH/aPA extruded blends did not improve the inherently poor formability of EVOH alone. In contrast, significant improvements in thermoformability were achieved by blending EVOH with a compatibilized‐ionomer. Optimum forming capacity was achieved in a ternary blend by addition of a compatibilized‐ionomer to EVOH/aPA blends in the range of 140–150°C. Analysis of wall thickness data obtained in the thermoformed parts showed that wall thickness was significantly affected by the ionomer and amorphous polyamide content in the blend. The ternary blend showed a lower thickness reduction in the critical areas, as well as a higher uniformity in the part. A finite element analysis was used to evaluate the wall thickness distribution and the modeling results were compared with the thermoforming experiments. The simulations were performed for the vacuum‐forming process employing a nonlinear elastic material model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 96: 3851–3855, 2004