A Solution for the Rupture of Polymeric Sheets in Plug-Assist Thermoforming

The rupture of polymeric sheets is one of the practical problems occurring during plug-assist thermoforming. This defect may occur both at the stage of mechanical stretching with a plug, as well as at the vacuum or pressure thermoforming stage. The results shown in our work not only lead to the understanding of the cause of this problem, but they also enable us to formulate a calculation of parameters that affect the rupture of polymeric sheets during plug-assist thermoforming for the production of axisymmetric polymeric articles.     

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     

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     

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.     

Dynamic characteristics of plug‐assist thermoforming process

Plug‐assist thermoforming is a well‐known technique in polymer processing because of its interesting features. The dynamic value of driving‐force for the stretching process is determined based on equilibrium equation. This amount of force is required for applying to a plug to stretch a sheet. It is used for calculation of the required theoretical work and power of a plug‐assist thermoforming process. By using a nonlinear viscoelastic rheological model in the proposed mathematical model, its validity was examined by performing experimental tests on ABS sheets. POLYM. ENG. SCI., 2009. © 2008 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.     

Sensitivity of operating conditions and material properties for thermoforming process

Abstract

The thermoforming process involves three stages: sheet reheat; forming; and solidification. A polymeric sheet is heated in an oven to the desired forming temperature distribution. The sheet is then deformed to take the shape of the mould cavity and subsequently solidified. The deformation of the sheet is assisted by the application of a pressure differential and/or the use of a moving plug.     

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.     

Derivation of optimal processing parameters of polypropylene foam thermoforming by an artificial neural network

The effects of processing parameters on the thermoforming of polymeric foam sheets are highly nonlinear and fully coupled. The complex interconnection of these dominant processing parameters makes the process design a difficult task. In this study, the optimal processing parameters of polypropylene foam thermoforming are obtained by the use of an artificial neural network. Data from tests carried out on a lab‐scale thermoforming machine were used to train an artificial neural network, which serves as an inverse model of the process. The inverse model has the desired product dimensions as inputs and the corresponding processing parameters as outputs. The structure, together with the training methods, of the artificial neural network is also investigated. The feasibility of the proposed method is demonstrated by experimental manufacturing of cups with optimal geometry derived from the finite element method. Except the dimension deviation at one location, which amounts to 17.14%, deviations of the other locations are all below 3.5%. POLYM. ENG. SCI., 45:375–384, 2005. © 2005 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.     

Wrinkling in polymer film‐polymer substrate systems and a technique to minimize these surface distortions

Thermally induced wrinkling during thermoforming of a commercial multi‐layered polymer film/substrate laminate has been reported. The differential thermal expansion of component layers coupled with phase transition of the substrate with increasing temperature, determined the critical conditions for wrinkling with a specific wavelength and amplitude. An effective technique to minimize wrinkling by biaxially stretching the samples at high temperature before the forming operation, has been proposed. The samples were biaxially stretched by inflating the samples using a specially designed blowing unit retrofitted to a conventional vacuum thermoformer. This method involved heating, inflation and forming, together to provide stretch‐assisted thermoforming. During biaxial stretching the stored compressive stresses in a wrinkled sample were relieved before the forming step, producing a decorative part without losses in surface appearance. POLYM. ENG. SCI., 57:31–43, 2017. © 2016 Society of Plastics Engineers     

Modeling of Deformation Processes in Vacuum Thermoforming of a Pre-stretched Sheet

Modeling of deformation processes in vacuum thermoforming for a preliminary stretched thermoplastic sheet (plug-assist vacuum thermoforming) is investigated in this paper. The model can be used for production of polymeric articles with minor wall-thickness variation. A nonlinear rheological model is implemented for developing the process model. It describes deformation process of a prestretched sheet at any phase of vacuum thermoforming process. This process is described by a set of deformation processes, each specified by appropriate boundary conditions. For model validation, a comparative analysis of the theoretical and experimental data is presented. The wall-thickness distributions obtained from modeling results corresponded well with experiments. A method for prediction and enhancement of the quality of the final products on the basis of wall-thickness distribution criterion has been established.     

Investigation into thermoformability of hot compacted polypropylene sheet

Abstract

This paper describes an investigation into the thermoformability of a new class of oriented polymeric material recently developed, namely hot compacted polypropylene sheet. Exploitation of any new material requires an intimate understanding of a whole range of factors, amongst which thermoformability is pre-eminent. This is particularly true for oriented polymeric materials, for while the preferred molecular alignment gives enhanced properties such as stiffness, strength, and resistance to impact, the downside is that the stretched molecular chains tend to limit further flow under stress, making thermoforming difficult. The aim of the present study was to establish the critical parameters for successful thermoforming of hot compacted polypropylene sheet.

Elevated temperature tensile tests were used to investigate the stress–strain behaviour of the compacted materials. The crucial parameters were found to be the post-yield modulus, which gives a measure of the resistance of the material to large scale deformation, and the strain to failure, which gives the upper limit on deformation. The post-yield modulus was found to be significantly affected by the test temperature and the high strain hardening behaviour of the material confirmed that significant force is required to thermoform the compacted polypropylene sheets. A hemispherical mould, with built-in gripping plate, was used to carry out a study of the thermoforming behaviour of the compacted sheets, and the results were found broadly to confirm the conclusions of the tensile tests. A linear relationship was found between the tensile force and the postforming force, reinforcing the synergy between the two tests. In addition the forming tests showed that the best temperatures to use were either side of the melting point of the melted and recrystallised phase, depending on the amount of postforming deformation required. Different gripping arrangements were investigated both in which the sheet was fully gripped and in which the sheet was allowed to flow into the mould during forming. The different schemes were found to control whether a successful component could be produced under different conditions and at different ultimate strains. Finally, the tests with the hemispherical mould showed that thermoforming this shape requires significant interlaminar shear deformation, and above 15% strain this resulted in destruction of the interlayer bond. For strains greater than this, successful thermoforming could only be achieved by allowing the material to flow into the mould.     

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     

Influence of initial sheet temperature on ABS thermoforming

Understanding the 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 initial temperature distribution over the sheet on the part thickness distribution of a vacuum snap‐back forming process. The linear viscoelastic properties along with the Wagner two parameter nonlinear viscoelastic constitutive 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 temperature distribution over the sheet on the part thickness distribution were determined to examine process sensitivity and optimization. Effects of the temperature distribution on the material rheology and polymer/mold friction coefficient are primarily responsible for the changes in the thickness distribution. We found that even small temperature differences over the sheet greatly influenced bubble shape and pole position during the bubble growth stage and played a critical role in determining the part thickness distribution. These results are discussed in terms of rheological properties of polymers such as elongational viscosity and strain hardening.     

Development of a multivariable online monitoring system for the thermoforming process

The thermoforming industry has been relatively slow to embrace modern measurement technologies. As a result researchers have struggled to develop accurate thermoforming simulations as some of the key aspects of the process remain poorly understood. For the first time, this work reports the development of a prototype multivariable instrumentation system for use in thermoforming. The system contains sensors for plug force, plug displacement, air pressure and temperature, plug temperature, and sheet temperature. Initially, it was developed to fit the tooling on a laboratory thermoforming machine, but later its performance was validated by installing it on a similar industrial tool. Throughout its development, providing access for the various sensors and their cabling was the most challenging task. In testing, all of the sensors performed well and the data collected has given a powerful insight into the operation of the process. In particular, it has shown that both the air and plug temperatures stabilize at more than 80°C during the continuous thermoforming of amorphous polyethylene terephthalate (aPET) sheet at 110°C. The work also highlighted significant differences in the timing and magnitude of the cavity pressures reached in the two thermoforming machines. The prototype system has considerable potential for further development. POLYM. ENG. SCI., 54:2815–2823, 2014. © 2014 Society of Plastics Engineers     

Bubble development in a polymeric resin under vacuum

Understanding the gas diffusion and evaporation behavior in a polymeric resin under vacuum is of great importance because many types of polymer and composite products are manufactured by applying a vacuum to the production system. This article proposes a theoretical model that can describe bubble growth under vacuum by combining the mechanisms of gas diffusion and evaporation. To confirm such a model, we carried out experimental analyses including evaporation experiments with a bell jar or a tube as well as vacuum‐assisted resin transfer molding (VARTM). Particularly in the VARTM process, it was identified that many bubbles were nucleated and grew at the fiber‐matrix interface due to the applied vacuum pressure. Those results suggest that more attention should be paid to vacuum‐assisted material processing to prevent bubbles from existing in the final products. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Self‐healing and interfacially toughened carbon fibre‐epoxy composites based on electrospun core–shell nanofibres

Dual components of a self‐healing epoxy system comprising a low viscosity epoxy resin, along with its amine based curing agent, were separately encapsulated in a polyacrylonitrile shell via coaxial electrospinning. These nanofiber layers were then incorporated between sheets of carbon fiber fabric during the wet layup process followed by vacuum‐assisted resin transfer molding to fabricate self‐healing carbon fiber composites. Mechanical analysis of the nanofiber toughened composites demonstrated an 11% improvement in tensile strength, 19% increase in short beam shear strength, 14% greater flexural strength, and a 4% gain in impact energy absorption compared to the control composite without nanofibers. Three point bending tests affirmed the spontaneous, room temperature healing characteristics of the nanofiber containing composites, with a 96% recovery in flexural strength observed 24 h after the initial bending fracture, and a 102% recovery recorded 24 h after the successive bending fracture. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44956.     

Surface roughness and foam morphology of cellulose acetate sheets foamed with 1,3,3,3‐tetrafluoropropene

Cellulose acetate (CA) is a bio‐based polymer suitable to replace foamed polystyrene (PS) in packaging applications. Foam trays can be produced by thermoforming of extruded sheets foamed with physical blowing agents. In this paper, the effects of various process settings and the calibration of the sheet on foam morphology and surface quality of extruded CA sheets are presented. Different contact cooling options were applied in order to investigate their influence on surface roughness, density, and morphology of the sheets. By adjusting cooling parameters, blowing agent formulation, and process settings, smooth foam sheets with a surface roughness below 10 µm and a density in the range of 150 kg m^?3 were produced. POLYM. ENG. SCI., 57:441–449, 2017. © 2016 Society of Plastics Engineers