Comparison of the thermoformability of a PPE/PP blend with thermoformable ABS. Part II: Large deformation methods

Thermoforming involves stretching of a heat‐softened polymer sheet to large strains. This was simulated by uniaxial tensile stretching (UTS) experiments at different temperatures and strain rates. These experiments were used to identify an optimum window of thermoforming temperature and strain rate and to compare the thermoformabilities of a blend of polyphenylene ether (PPE) and polypropylene (PP) with thermoformable acrylonitrile butadiene styrene (ABS) resin. It was shown that the PPE/PP blend generally has a wider thermoforming window than ABS. Using Considere construction, it was shown that the PPE/PP blend underwent more homogeneous deformation than ABS. The higher homogeneity in deformation of the PPE/PP blend resulted in a higher uniformity in the distribution of draw ratios, which implies more uniform thickness distribution in thermoformed parts. Draw ratio and crystallization studies ruled out the possibility of postforming shrinkage in the PPE/PP blend under ambient temperature conditions. The PPE/PP blend was also found to have a higher shrinkage onset temperature than that of ABS. An attempt was also made to relate the small deformation viscoelastic properties with the large deformation UTS properties through the use of Deborah numbers. These results bring out the importance of studying both the small deformation and large deformation properties in conjunction, while comparing the thermoformabilities of different polymers. POLYM. ENG. SCI., 45:1377–1384, 2005. © 2005 Society of Plastics Engineers     

Uniaxial tensile behavior and thermoforming characteristics of high barrier EVOH‐based blends of interest in food packaging

The uniaxial tensile characteristics of blends of an ethylene‐vinyl alcohol copolymer (EVOH‐32 mol% ethylene) with an amorphous PA and/or a nylon‐containing ionomer, used as barrier layer in multilayer food packaging structures, was assessed in this paper. The stress‐strain behavior of these materials at elevated temperatures and at different strain rates was examined. The stress‐strain curves obtained were used to understand the influence of temperature and strain rate on the uniaxial deformation process of the materials, these being of general importance during typical processing steps including thermoforming. A male mold for deep‐draw was used to assess the thermoforming (biaxial deformation in nature) behavior of extruded sheets at 100, 120, 140 and 150°C, and the results were broadly found to be in agreement with results from simple uniaxial tensile tests. From the preliminary thermoforming results, it was found that EVOH/aPA extruded blends did not improve the poor formability of EVOH alone. In contrast, significant improvement in thermoformability was achieved by blending EVOH with a compatibilized ionomer. Optimum forming capacity was achieved in a ternary blend by addition of a compatibilized ionomer to an EVOH/aPA blend in the range of 140°C–150°C. The ternary blend showed a lower reduction of thickness in the sidewalls, as well as a higher dimensional uniformity in the thermoformed part. Polym. Eng. Sci. 44:598–608, 2004. © 2004 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.     

Investigation of the thermoformability of various D-Lactide content poly(lactic acid) films by ball burst test

In this article, we investigated the thermoformability of poly(lactic acid) (PLA) films with various D -Lactide contents and therefore different crystallization properties, performing tensile and ball burst tests at various temperatures and testing rates. We found that the behavior of the PLA films tested above the glass transition temperature significantly differs due to the difference in D -Lactide content, and thus crystallinity. During tensile testing, elevated temperatures and mechanical stress caused the crystallization temperature to decrease and thus highly induced crystallization. At the same time, as testing speed was increased, the ability of the polymer to crystallize decreased. In ball burst tests, the PLA films crystallized more than during tensile testing. We described the differences found between tensile testing and ball burst testing, which latter better represents the conditions of thermoforming through inducing biaxial deformation.     

A comparison of the thermoformability of a PPE/PP blend with thermoformable ABS. Part I: Small deformation methods

Different factors important in ascertaining the thermoformability of polymeric materials are identified and defined. These include resistance to sag, ease of flow, mold replication, deep draw capability, sensitivity to thermoforming temperature and speed, uniformity of thickness distribution, and post‐forming shrinkage and dimensional stability. Methods to study these properties can be classified into small deformation and large deformation methods. The small deformation methods, which are the subject of this paper, include dynamic temperature sweep tests, dynamic frequency sweep tests, stress relaxation time, and creep recovery tests. These tests were used to compare the thermoformabilities of a blend of polyphenylene ether (PPE) and polypropylene (PP) and thermoformable acrylonitrile butadiene styrene (ABS) resin. The dynamic temperature and frequency tests showed that the PPE/PP blend generally has a better viscoelastic balance than ABS implying a better balance between resistance to sag and ease of flow. Creep recovery tests suggested that the PPE/PP blend may offer better mold replication during thermoforming. Studies based on the stress relaxation time showed a lower residual stress build‐up in thermoformed PPE/PP blend than ABS implying better dimensional stability and a higher in‐service temperature window for the thermoformed PPE/PP blend than ABS. POLYM. ENG. SCI., 45:1369–1376, 2005. © 2005 Society of Plastics Engineers     

Post yield phenomena in tensile tests on poly(vinyl chloride)

Tensile tests have been made on poly(vinyl chloride) to find the conditions under which uniform extension, diffuse necking, localized necking or thermal fracture occur. These observations have been compared with the predictions of a finite element model of the tensile test specimen to ascertain the causes of the changes in post-yield behaviour with increased crosshead speed. The major effect of increased crosshead speed is to reduce the dissipation of heat generated by plastic deformation. The effect of thermal pre-treatment of the PVC is to change the extent of strain softening after yield, and thereby control the range of post-yield phenomena that are possible.     

Molecular orientation of polypropylene by rolling-drawing

Rolling-drawing is a simple, effective, solid state processing technique for manufacturing high strength and high modulus oriented polymer sheet products. The process is capable of increasing the tensile modulus and strength of polypropylene by more, than an order of magnitude with inexpensive equipment and straight forward controllable techniques. This paper gives an overview of the rolling-drawing of polypropylene. It is intended to answer the following questions. What is rolling-drawing? What changes in tensile properties can be expected as a result of this process? What deformation processes occur during rolling-drawing? What are the processing variables and how do these variables relate to the deformation ratio achieved by rolling-drawing? A tensile flow stress relationship was formulated from experimental data for oriented polypropylene. This constitutive equation, estimates the flow stress (or yield stress) of the polymer as a function of deformation ratio, strain rate and temperature. Since stretching was found to provide a significant portion of the deformation during the rolling-drawing process a Hoffman-Sachs computer analysis was written to predict plastic deformation and drawing forces in the stretch zone. The results of experimentation and the analysis are briefly compared in this paper.     

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.     

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.     

Modeling the uniaxial rate and temperature dependent behavior of amorphous and semicrystalline polymers

Strain rate and temperature dependent constitutive equations are proposed for polymer materials based on existing isotropic formulations of viscoplasticity. The proposed formulations are capable of simulating some of the important features of deformation behavior of amorphous and semicrystalline polymers. The materials model is based on the assumption that the evolution of flow stress is dependent on the rate of deformation, temperature, and an appropriate set of internal variables. The proposed theory is capable of modeling yielding, strain softening, and the orientation hardening exhibited by amorphous polymers. It is also possible to model the initial viscoplastic and subsequent nonlinear hardening behavior shown by semicrystalline polymers at large strains. Uniaxial tensile tests with uniform and hourglass specimens are made at temperatures ranging from 23 to 100°C and under various crosshead speeds. Both amorphous polycarbonate and semicrystalline polypropylene sheet materials are tested to characterize the stress and strain behavior of these materials and to determine their appropriate material constants. Load relaxation experiments are also conducted to obtain the necessary material constants describing the rate and temperature dependent flow stress behavior of polypropylene. Simulation results compare favorably against experimental data for these polymer materials.     

玄武岩-粗聚丙烯纤维干硬性混凝土拉压性能试验研究

混杂纤维增强干硬性混凝土在国内外已有广泛的应用,纤维配比是影响其拉压性能的主要因素之一。为研究玄武岩纤维与粗聚丙烯纤维配比对干硬性混凝土拉压性能的影响,将玄武岩纤维与粗聚丙烯纤维单掺或按不同比例混合掺入干硬性混凝土中,开展不同养护龄期下纤维混凝土的抗压、劈裂抗拉试验,分析纤维混杂增强效应,并基于成熟度理论修正养护龄期,优化玄武岩-粗聚丙烯纤维干硬性混凝土的劈裂抗拉强度预测模型。结果表明:玄武岩纤维与粗聚丙烯纤维的掺入不仅提升了干硬性混凝土抗压、劈裂抗拉性能,而且纤维的桥接作用能明显改善混凝土的脆性破坏特征,其中玄武岩纤维与粗聚丙烯纤维混掺配比为1 ∶2(质量比)时最为明显,表现出了最优的纤维混杂正效应。根据等效龄期-抗压强度关系式计算得到的混凝土抗压强度与劈裂抗拉强度具有更好的幂函数关系,该模型便于计算及预测不同养护温度条件下玄武岩-粗聚丙烯纤维干硬性混凝土的拉压性能。     

Uni- and biaxial stretching of chlorinated pvc sheets. A fundamental study of thermoformability

Chlorinated PVC is superior to unmodified PVC as a thermoplastic for use in thermoforming, especially if improved heat resistance and dimensional stability are required. In the present report, results tire given of a fundamental experimental study on the thermoformability of CPVC sheets obtained by calendering various formulations based on CPVC resins with at least 65 percent chlorine content. Extensibility as well as the relationship between stress and strain in uni- and biaxial stretching have been determined as a function of temperature and rate of stretching by means of specially devised, highly instrumented laboratory equipment. Stress-strain relations under isothermal conditions and at constant strain rate are compared for the two modes of stretching, and the difference in behavior between PVC and CPVC, particularly with regard to the effect of temperature, is emphasized. Internal stresses frozen in during cooling, following rapid stretching at appropriate thermoforming temperatures, have been determined by means of a detailed analysis of retractive force measurements. The relationship between internal stress and molecular orientation is discussed as well as the effect of the latter parameters on various properties of technological interest: dimensional stability, impact resistance, and gas permeability.     

PET片材热成型技术

介绍目前我国PET片材热成型加工技术中成型设备、加工原理、模具制作、加工工艺，为PET片材加工业的发展及开发应用提供参考。     

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.     

Importance of elongational properties of polymer melts for film blowing and thermoforming

Two polyethylene and two polypropylene melts were characterized in uniaxial elongational flow. They exhibit significant differences with respect to strain hardening. For the polypropylenes it was shown that the elongational behavior found in uniaxial elongation is qualitatively reflected in biaxial deformation too. From the polyethylenes, films were blown using laboratory equipment, and the polypropylenes were processed into beakers by thermoforming. For both materials it could be shown that strain hardening is of advantage for the geometrical uniformity of the processed items. POLYM. ENG. SCI., 46: 1190–1195, 2006. © 2006 Society of Plastics Engineers     

The modelling of large deformations of pre-oriented polyethylene

High temperature reversion tests have revealed a state of pre-existing molecular orientation in extruded polyethylene sheet. This state is related to differences in stress-deformation behaviour when specimens of the sheet are stretched along different angles with respect to the extrusion direction. An established large deformation, rate-dependent constitutive equation has been developed to model this material, by incorporating the pre-orientation by the addition of a strained Gaussian network. The level of pre-orientation is deduced from the dimensional changes on shrinkage. The constitutive equation is incorporated into the finite element package abaqus, and the shapes and drawing forces of tensile specimens extended at various angles to the extrusion direction are modelled.     

An analysis of stretch forming of thermoplastic composites

Thermoplastic composite sheet materials have found an increasing number of applications through the use of matched‐die compression molding. The combination of high strength, low weight and moderate manufacturing costs makes the material an attractive alternative to sheet metal stampings in a number of applications. However, a significant range of conventional sheet forming techniques may also prove suitable for these materials, provided the effects of strain rate and temperature can be properly understood and exploited, leading to a further reduction of manufacturing costs and an increasing number of potential applications. In this work, limiting strains in biaxial stretch forming were explored using the hemispherical stretch forming test. The materials tested contain 20, 35, and 40 percent by weight of randomly oriented glass fiber in a polypropylene matrix. The forming tests were performed at temperatures ranging from 75°C to 150°C and at punch speeds of 0.01cm/sec, 0.1cm/sec, and 1cm/sec. A rotationally symmetric, anisotropic material model with rate sensitivity was developed and incorporated into an axisymmetric finite element model of the stretch‐forming process. The model parameters were temperature‐dependent, though the temperature distribution in the formed part was assumed to be uniform. Strain distributions in the formed parts are compared to finite element method results, and the results are good up to the point when localized necking begins to dominate the strain distribution. These forming limit strains are compared with predictions based on Marciniak's imperfection theory, with good results.     

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.