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.     

Elastomer biaxial characterization using bubble inflation technique. I: Experimental investigations

The biaxial rheological behavior of materials such as elastomers or polymers can be obtained using a bubble‐inflation‐technique. A circular membrane is clamped at the rim and expanded under gas pressure. The inflation of the circular membrane is recorded using a CCD video camera and the blowing pressure by a pressure sensor. Then, from elongation and curvature radius measurement at the pole of the bubble, one can deduce equibixial stress‐strain data. This study describes the optimization of a bubble‐inflation rheometer. The most sensitive point of the technique is the estimation of the elongation at the bubble pole, deduced from video camera measurements. A direct measurement of the bubble thickness was performed using a magnetic probe in order to validate rheometer results. Such a validation has evidently never been carried out before. Results of quasi‐static equibiaxial characterisation of elastomers are presented and analyzed.     

Flow‐induced warpage of injection‐molded TLCP fiber‐reinforced polypropylene composites

The most common belief is that warpage in injection‐molded fiber‐reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow‐induced stresses generated during mold filling do not play a significant role. Injection‐molded plaques of polypropylene (PP) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) microfibrils were generated in order to investigate the role of residual flow‐induced stresses relative to that of thermal stresses on the warpage. In an effort to relate the material parameters to warpage, the rheological behavior of these fiber‐filled systems was investigated. The shrinkage and the thermal expansion of the TLCP/PP composites, and hence, the thermally induced stresses decreased with an increase in fiber loading while the flow‐induced stresses increased. The increase in the flow‐induced stresses was attributed to increased relaxation times (this is not the only cause, but is a significant factor) with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber‐reinforced thermoplastics, the flow‐induced residual stresses must be accounted for. It is expected that the results reported here can be extended to glass‐reinforced PP composites as well. POLYM. COMPOS., 27:239–248, 2006. © 2006 Society of Plastics Engineers     

Shear tests of glulam at elevated temperatures

Experimental investigations are conducted to characterize the evolution with temperature of the shear strength of glulam wood. To realize the tests, an original specimen with cylindrical shape has been developed and justified by a numerical study. The geometry allows obtaining thermal gradient within material to represent the real combustion of timber members, while keeping constant the temperature of sheared section. The experimental programs consider various parameters such as the presence or absence of moisture and the thermal gradient within the specimen. The experimental results are discussed and analyzed. They show the correlation between the density of the material and the reduction of its strength at high temperatures. The experimental failure loads are used to evaluate the reduction factors for wood strength depending on the temperature. These factors are compared with those given by EN1995‐1‐2 for the advanced calculations methods in fire situation. The comparisons show that the reduction factors given by EN1995‐1‐2 are conservative in comparison with the experimental results. Copyright © 2014 John Wiley & Sons, Ltd.     

Optimization and numerical simulation analysis for molded thin‐walled parts fabricated using wood‐filled polypropylene composites via plastic injection molding

Plastic injection molding is discontinuous and a complicated process involving the interaction of several variables for control the quality of the molded parts. The goal of this research was to investigate the optimal parameter selection, the significant parameters, and the effect of the injection‐molding parameters during the post‐filling stage (packing pressure, packing time, mold temperature, and cooling time) with respect to in‐cavity residual stresses, volumetric shrinkage and warpage properties. The PP + 60 wt% wood material is not suitable for molded thin‐walled parts. In contrast, the PP + 50 wt% material was found to be the preferred type of lignocellulosic polymer composite for molded thin‐walled parts. The results showed the lower residual stresses approximately at 20.10 MPa and have minimum overpacking in the ranges of ?0.709% to ?0.174% with the volumetric shrinkage spread better over the part surface. The research found that the packing pressure and mold temperature are important parameters for the reduction of residual stresses and volumetric shrinkage, while for the reduction of warpage, the important processing parameters are the packing pressure, packing time, and cooling time for molded thin‐walled parts that are fabricated using lignocellulosic polymer composites. POLYM. ENG. SCI., 55:1082–1095, 2015. © 2014 Society of Plastics Engineers     

Modeling particle inflation from poly(amic acid) powdered precursors. III. Experimental determination of kinetic parameters

Several concurrent phenomena occur during the thermal inflation of poly(amic acid) precursor particles leading to polyimide foams as part of the solid‐state powder foaming process. The precursor experiences bubble growth from within while volatiles desorb and the polymer itself increases its molecular weight and changes its backbone structure. These changes affect the transport properties of the material by modifying significantly the effective glass transition temperature, T_g. By studying the chemical transformations that take place during the inflation process (amidation and imidization reactions), a complete understanding of the material's molecular changes can be obtained and corresponding property changes can be followed. This article is the third of a series where the inflation of precursor materials for polyimide foams has been studied. The two previous articles in the series present numerical models that simulate the inflation process from first principles. In this article, the authors discuss the experimental and analytical methodologies employed to accurately characterize and incorporate the changes in material and transport properties as a function of the glass transition temperature. POLYM. ENG SCI., 2008. © 2008 Society of Plastics Engineers     

Warpage and countermeasure for injection‐molded in‐mold labeling parts

Decades ago, the production of packaging with the injection in‐mold labeling (IML) has been established. With this manufacturing technique, label and packaging, both are of the same polymeric material, become inseparably connected during the injection molding process. Because thermal conductivity of the polymeric label material is clearly smaller than that of the metal mold wall, thermal‐induced warpage of injected IML parts or part surface deformation could occur. In this study, structure and warpage behavior of IML parts, which are different from those of conventional molded parts without labels were intensively investigated. It was found that it is the volume contraction difference between label and substrate that forces IML parts to warp to the opposite side of the label. In addition, IML part warpage problem can be coped by varying the mold temperature on the stationary and moving mold platen. By increasing the mold temperature on the label side, the degree of IML part warpage can be reduced with acceptable reduction in mechanical properties. The optimum mold temperature range for particular substrate material, however, was found to be more decisive in maintaining the modulus of elasticity of IML parts than the magnitude of mold temperature difference. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

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.     

Increasing nano‐particle dispersion ratio in the bubble stretching‐based technique

We present a new method to increase nano‐particle migration rate in bubble stretching‐based technique. Vibration created by the inflation and shrinking process of bubbles is used. Process parameters can be adjusted to increase the probability of collision between the nano‐particles and the bubble wall. In effect, particles sufficiently migrate to the bubble wall, increasing both particle migration rate and dispersion ratio. Our measurement show that: (1) particle diameter, initial bubble radius, and initial bubble pressure strongly influence the migration of particles; (2) with appropriate parameters, nano‐particles can quickly and efficiently migrate to the bubble wall through this new method. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Thermal Decomposition and Ignition of PBXN‐110 Plastic‐Bonded Explosive

Due to safety requirements, insensitive behavior in slow or fast thermal heating (cook‐off) conditions is a desired behavior for today’s munitions. The ignition time of munitions under slow or fast cook‐off conditions is an important parameter in the design of insensitive munitions. The critical temperature, which mainly depends on the chemical, physical, and the geometrical properties of the energetic material, is the determining factor whether the material will end up with thermal initiation or not, when it is exposed to an external heat source. In this study a slow cook‐off test setup is designed and developed and the tests for a generic munition containing PBXN‐110 plastic‐bonded explosive are performed in order to obtain temperature distribution in the test item, ignition time, ignition temperature, and ignition location. In this paper the development procedure and the experimental results of the slow cook‐off tests are explained. Moreover, the kinetic parameters such as activation energy and pre‐exponential factor for the plastic‐bonded explosive obtained from the TGA tests are presented.     

微孔注塑制品表面质量改善和翘曲降低的技术

为拓宽微孔注塑制品的应用范围,需要改善其表面质量,降低其翘曲变形。模腔气体反压技术、模腔被动控温技术、微孔共注塑技术、泡孔成核速率控制技术、微孔注射压缩成型技术和快速热循环微孔注塑技术等可有效改善微孔注塑制品的表面质量。模腔气体反压技术、微孔共注塑技术和调整加工参数可进一步降低微孔注塑制品的收缩率和翘曲,提高制品的尺寸精度。本文较系统介的绍了这些技术和方法。     

Modeling particle inflation from poly(amic acid) powdered precursors. II. Morphological development during bubble growth

The morphological development of cellular polyimide microstructures from poly(amic acid) powders has been shown to depend on the processing conditions throughout the inflation process and the morphological characteristics of the precursor particles. In an earlier publication the authors presented a numerical study of the preliminary stages prior to particle inflation when the processing temperature is below the glass transition temperature, T_g. In the present article, a second numerical scheme is presented for behavior above T_g in which bubble growth is modeled to account for the effect of multiple phenomena in the final stages of morphological development. The bubble growth kinematics and subsequent cessation of growth are predicted as a function of process parameters and material properties. Morphological characteristics of the precursor particles have also been shown to influence the kinematics of inflation. These results provide a clearer understanding of the solid‐state foaming processes for polyimide cellular materials. POLYM. ENG. SCI., 47:572–581, 2007. © 2007 Society of Plastics Engineers.     

Analyse Expérimentale et Numérique du Comportement de Membranes Thermoplastiques en ABS et en HIPS dans le Procédé de Thermoformage

In this work, we are interested, on the one hand in the characterization of circular polymeric ABS and HIPS membrane under biaxial deformation using the bubble inflation technique, on the other hand in modelling and numerical simulation of the thermoforming of ABS and HIPS materials using the dynamic finite element method. Hyperelastic models (Mooney‐Rivlin, Ogden) are 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 models used. For numerical simulation, the lagrangian formulation together with the assumption of the membrane theory is used. Moreover, the influence of the hyperalastic model on the thickness and on the stress distribution in the thermoforming sheet are analysed for ABS and HIPS materials.     

Elastomer biaxial characterization using bubble inflation technique. II: Numerical investigation of some constitutive models

An elastomeric material was investigated with a bubble inflation rheometer, and its mechanical behavior was modeled as a rubbeer‐like solid. Classical strain energy functions were considered and the hyper‐elastic were calculated by a direct identification procedure from simple uniaxial and equibiaxial extension test data, and the reults are compared against those obtained by an inverse method from matching the meaured response to a finite element to a finite element analysis solution, which dependent on the unknown material parameters. The optimised employed the Levenberg‐Marquardt algorithm and Abaqus software to compute the cost function and its gradients. The constants so obtained were further used in finite element analysis, and the numerical results were compared with experiments. This study showed that the inverse method, used to estimate the material parameters, is a good alternative to the direct identification, especially since the latter often requires homogeneous strain state, which is very difficult to obtain.     

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.     

Experimental investigation of the cure‐dependent response of vinyl ester resin

The objective of this research is to understand the influence of the thermochemical and thermomechanical material response of low temperature cured vinyl ester resin (Dow Derakane 411‐C‐50) on the development of residual stress and warpage during processing. The primary experimental technique is the bimaterial specimen experiment, in which the warpage of a bimaterial beam is used as a measure of residual stress. The bimaterial specimen experiment was developed to isolate the chemical and thermal contributions to curvature. Existing material models for shrinkage, modulus, and glass transition temperature as a function of cure were evaluated. These material models were used as input into the bimaterial equation for curvature prediction. The predicted curvatures were used along with the experimental curvatures to evaluate the material models and their ability to accurately describe the material response of the vinyl ester resin. Results showed that the model captured the overall experimental trend in curvature buildup during processing but overestimated the curvature from chemical effects during isothermal cure. Improved correlation was achieved by incorporating a time shift in the model to account for viscoelastic stress relaxation of the resin.     

Effect of Temperature on the Transport Properties and Morphology of Polymeric Asymmetric Membranes

Abstract

Polymeric membranes have not been practical for application to high temperature processes, due to the thermal instability of most polymeers. New materials are being developed with higher glass transition temperatures and a greater degree of thermal stability. Use of these polymers to perform gas separations at higher temperatures is promising; however, the performance of the membrane as the process approaches the polymer's glass transition temperature is unknown. This study was conducted to explore this issue. Gas flux and helium/nitrogen ideal selectivities through heat treated integrally-skinned asymmetric polysulfone membranes were measured. Membranne morphology was also evaluated through bubble point tests and SEM micrographs. Experiments demonstrated that as the polymer is exposed to temperatures approaching the polymer glass transition temperature, internal pores begin to collapse, causing both the gas flux and selectivity to decrease.     

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACT

Effect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.     

Volume‐Dependent Self‐Ignition Temperatures for Explosive Materials

In this paper the differential equation of thermal explosion in the steady‐state approximation is established and solved. This solution is based on Fourier’s heat equation along with the Arrhenius heat source. The theory allows to draw conclusions on the thermal behaviour of any substance. The critical ambient temperature appears as a function of the apparent activation energy, Arrhenius temperature rate, temperature diffusivity, and the volume. The thermal stability of a chemically reactive material may be determined by means of these dependencies. Different volumes of five explosive materials were exposed to hot storage tests. These experiments generate appropriate kinetic parameters for these materials that are compared with known values from literature. Once these parameters are known, self‐ignition temperatures can be calculated for any arbitrary volume and boundary conditions. This is of major importance in the safe transportation and storage of explosives, munitions, and weapons.