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     

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     

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     

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     

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     

The influence of extrusion parameters on the mechanical properties of polypropylene sheet

Extruded thermoplastic sheet is widely used in the production of thin-gauge tubs and containers for the food and beverage industry using the thermoforming process. The production of high quality thermoformed parts is critically dependent on the standard of extruded sheet feedstock used. The extrusion process itself imparts a thermal history to the sheet, and this in turn partly dictates its subsequent thermoformability. This paper assesses the influence of various extrusion parameters on the mechanical and morphological properties of polypropylene sheet, with a view to defining the optimum extrusion conditions for polypropylene. The extrusion parameters under consideration are chill-roll temperature, line speed, sheet thickness and melt temperature.     

Infrared sheet heating in roll fed thermoforming: Part 2 - Factors influencing inverse heating solution

Abstract

Two methodologies for solving the inverse heating problem in thermoforming, i.e. setting the temperature of the heaters that provide a prescribed temperature of the sheet to be formed against the mould, are compared in terms of temperature gradients across the thickness and sensitivity to the most important process parameters. The influences of sheet thickness, sheet emissivity, room temperature, and distance between the heater bank and the forming station on the thermal homogeneity of the sheet are also discussed.     

热成型过程中的冷却过程分析

建立了热成型冷却过程分析所用的物理模型和数学模型，并给出了速算图。     

Thermoforming of liquid crystalline polymer sheets

Liquid crystalline polymer (LCP) extruded sheets were further processed by the conventional thermoforming method. The available processing temperature range was defined through the structural, thermal, and elevated temperature mechanical characterization of the extruded sheet. This temperature range was found for LCP to be quite narrow, in the proximity of the crystal-mesophase transition. The structural changes imposed on the LCP sheet during forming and its thermal stability were investigated using wide angle X-ray diffraction, mainly for the determination of the chain orientation distribution, DSC, and dynamic mechanical analysis. Thermoforming onto a symmetrical male mold was found to enhance the orientation in the extrusion machine direction and even change the preferred orientation in the extrusion transverse direction to orientation along the thermoforming direction. Annealing at the thermoforming temperature range results in a more ordered and thermally stable structure accompanied by just a slight orientation loss.     

Improved mathematical modeling for the sheet reheat phase during thermoforming

In any thermoforming process, plastic sheet heating is the most important phase as it is responsible for final part quality as well as overall process efficiency and productivity. The goal of the study reported here was to improve existing mathematical models to accurately predict the temperature profile inside a heated sheet, where the model could be used to better control the overall thermoforming process. A mathematical model with temperature dependent, variable sheet material properties including density, thermal diffusivity, specific heat, and thermal conductivity was developed and validated against experimental data. Models with constant and variable plastic sheet properties were created, simulated, and compared in Matlab. The models were validated by experiments which obtained temperature profiles at different depths within a plastic sheet by inserting thermocouples and recording temperatures. Further, the effect of sheet color on heating was investigated by considering two extreme cases: white (transparent) and black (opaque) colored sheets, and the effect of oven air temperature and velocity on sheet heating was also investigated. Results indicated that a variable properties model was needed to control sheet reheating especially with narrow forming windows, and that the heating profiles required for colored and noncolored sheets were very different. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Instrumented thermoforming of advanced thermoplastic composites. II: Dynamics of double curvature part formation and structure development from PEEK/carbon fiber prepreg tapes

Poly(ether ether ketone) (PEEK) carbon fiber prepreg tapes (APC-2) have been thermoformed into a hemispherical double curvature part under a variety of processing conditions. Conventional matched die molding using aluminum molds (at 200°C) were not successful in thermoforming acceptable parts. Parts with severe wrinkling and folding were obtained. A novel three-piece (steel) mold with built-in sheet clamping arrangement was, therefore, designed and fabricated. This mold was used at 400°C temperature to thermoform parts from preheated preconsolidated laminates. More interestingly, using the above conditions, 8- and 16-ply unconsolidated laminates could be directly thermoformed into parts that were microstructurally sound and exhibited good shape conformity. Results suggest a cycle time of 15 min, with scope for further reduction, if mold cooling is employed. Notwithstanding the simplicity of the thermoforming process, such a short cycle time compares quite favorably with cycle times of several hours for conventional thermosetting resin based composites.     

Role of additives in influencing heating and melting of polymers during thermoforming

The heating stage in thermoforming of amorphous and semi-crystalline polymers was analyzed for constant heat flux conditions using an energy balance model; candidate polymers were high impact polystyrene and isotactic polypropylene. Using an analytical solution, temperature differences as large as 100°C were predicted to arise between the surface and the interior of the sheet being thermoformed for conditions chosen in this work, and these can limit the heat flux being used. A Matlab program was used to compute temperature and crystallinity profiles for crystal melting. Melting took almost as much time as required to heat the surface of the film to the crystal melting point. High thermal conductivity additives, such as calcium carbonate and graphene, can provide temperature uniformity, and the additive uniformity can be verified using thermogravimetric analysis. The ability of these additives to provide temperature uniformity and to reduce energy consumption and heating time is determined in a quantitative manner. Both additives improve heat transfer, and, at the same added volume fraction, graphene is more effective. However, calcium carbonate has a lower cost. The role of density, specific heat, thermal conductivity, and amount of the polymers and additives in influencing temperature and crystallinity profiles was explored, and methods of carrying out thermoforming in an energy efficient manner are proposed.     

Improved modeling for the reheat phase in thermoforming through an uncertainty treatment of the key parameters

This work focuses on the treatment of parameter uncertainty in the simulation of the sheet reheat phase of the thermoforming process. The approach aims to improve the quality of predictions through more accurate evaluation of the input parameters. First, the modeling approach is employed to perform a sensitivity analysis on the reheat phase. Then, a series of specialized experiments with heat flux and temperature sensors are performed on a thermoforming machine. The key parameters identified through the sensitivity analysis are the subject of these experiments. The natural convective heat transfer coefficients are evaluated by two different approaches. Through treatment of the uncertainty associated with the input parameters, the prediction of sheet reheat phase is significantly improved.     

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.     

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.     

轿车塑料复合地毯热成型中传热过程的建模与研究

结合传热学中非稳态导热理论,对轿车地毯热成型加热和冷却阶段中的传热现象进行了理论研究,分别建立了加热和冷却过程的物理模型和数学模型,并推导出了加热和冷却时间的理论计算公式。最后将理论模型应用到实际的轿车地毯生产工艺中,论证了模型的实用性和精度。     

Effects of material properties and contact conditions in modelling of plug assisted thermoforming

Abstract

Thin walled food packaging is commonly manufactured using the plug assisted thermoforming process. In this paper the development of finite element models of the process is described. Work has concentrated on understanding the effects of material properties and contact conditions on the output from these simulations. The results have shown that a viscoelastic model must be used to simulate the deformation behaviour of the plastics. Contact conditions must also be accounted for in the models by including the effects of friction and heat transfer between the sheet and tool surfaces. For improved model accuracy, it is recommended that further experimental work should be carried out to enhance the viscoelastic material models and to provide better understanding of actual contact conditions.     

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.