Thermoforming shrinkage prediction

Most thermoforming product development processes rely on costly and time‐consuming forming trials to determine the adequacy of the mold and process. In this paper, an analytical method is developed for shrinkage predictions on the basis of a visco‐elastic constitutive material model with initial conditions from a commercial thermoforming simulation. The theoretical analysis has been developed to accommodate different sets of materials, process conditions, and mold geometry. The shrinkage model consists of a transient thermal analysis for temperature solution; a stretching phase analysis for Inflation‐induced stress estimation; a post‐contact analysis for thermal stress and relaxation; and a post‐molding strain analysis based on stress solution. The shrinkage prediction analysis has been developed and validated with a complex geometry thermoforming application. The results indicate that the shrinkage estimates provided by the analysis were within the objective tolerances of 0.1%, as measured in terms of absolute prediction error of part dimensions.     

Interlayer adhesion of coextruded films: Effect of biaxial stretching

In order to understand the effect of thermoforming on the interlayer adhesion of coextruded films, a peel test was performed for coextruded films after being stretched. To simulate the non‐uniaxial stretching nature of actual thermoforming processes, planar stretching and biaxial stretching were applied to the coextruded films prior to the peel test. Both the planar stretching and biaxial stretching were performed at an optimum thermoforming temperature under well‐controlled stretch rates to a predetermined stretch ratio. It was found that there was a significant amount of reduction of interlayer adhesion due to stretching. Furthermore, the loss of interlayer adhesion at the optimum thermoforming temperature was linearly related to thickness drawdown as a result of stretching regardless of stretching modes. Therefore, it is suggested that the effect of thermoforming on interlayer adhesion of coextruded films can be easily estimated from the thickness distribution of thermoformed parts. Polym. Eng. Sci. 44:948–954, 2004. © 2004 Society of Plastics Engineers.     

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.     

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.     

Thermoforming triangular troughs

This analysis for thermoforming triangular troughs focuses on the manufacturing process speed and follows the method of Kershner and Giacomin for thermoforming cones. We distinguish between what happens before and after (free versus constrained forming) the melt touches the prismatic mold. Neither free nor constrained forming yields analytical solutions for the required forming time. Our analysis is restricted to the fabrication of triangular troughs from nearly Newtonian melts, the second simplest relevant problem in commercial thermoforming. The simplest relevant problem, thermoforming cones, yielded analytical solutions for the forming time. Whenever we thermoform straight edges into rigid packaging, the problem of a melt stretching into a triangular trough arises. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Crystal transition behavior of odd–odd nylon 11,11 under annealing and stretching

Nylon 11,11 is a new odd–odd polyamide with multiple crystalline structures. The wide‐angle X‐ray scattering indicated that the structure of quenched sample was not amorphous but a γ‐form crystal with a relatively low crystallinity. The α‐form and the γ‐form crystal of nylon 11, 11 could be obtained by annealing the quenched samples at high and low temperature, respectively. No crystal transition happened for the α‐form sample when annealed at any temperature before melting. However, the γ‐form would quickly transform into the α‐form when annealed above 145°C. Under the stretching conditions, the α‐form rapidly transformed into the γ‐form at low temperature, while the γ‐form changed into the α‐form only at high temperature. These results indicated that the stretching inducement was beneficial to produce the γ‐form, and the thermal inducement was favorable to forming the α‐form. POLYM. ENG. SCI., 54:2785–2790, 2014. © 2013 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.     

Biaxial characterization of poly(ether‐ether‐ketone) for thermoforming: A comparison between bulge and in‐plane biaxial testing

The biaxial response of extruded PEEK films at conditions relevant to thermoforming has been investigated extensively using a combination of load‐controlled (bulge test) and displacement‐controlled (biaxial stretcher) experiments. Results from bulge testing yielded average and maximum strain rate ranges of 2.5–5 and 5–18 s^?1, respectively, across the forming temperature range. In‐plane biaxial characterization highlighted the anisotropic nonlinear viscoelastic behavior of the films with strong dependence on the yield and strain hardening behavior on the temperature and strain history at conditions equivalent to the forming process. The combined approach to material characterization highlights the pros and cons of each test method, the complementary nature of the data generated, and the need to use both methods to have a complete data set for developing accurate material models and validated numerical simulations of thermoforming. POLYM. ENG. SCI., 59:1853–1865, 2019. © 2019 Society of Plastics Engineers     

Processing and characterization of a biaxially oriented conducting polymer

Two methods have been developed for the preparation of biaxially oriented poly(phenylene vinylene) from its poly(sulfonium salt) precursor. A two-stage stretching process permits non-equibiaxial stretching over a wide range of deformation ratios and a bubble expansion technique allows equibiaxial planar extension. The resulting films were examined using X-ray diffraction prior to chemical doping with SbF₅ vapor. Biaxial orientation was shown to exert considerable influence over the attainable electrical conductivity.     

Thermo‐mechanical behavior of stretch‐broken carbon fiber and thermoplastic resin composites during manufacturing

Stretch‐broken fiber reinforcements and thermoplastic resin commingled prepregs are interesting for manufacturing composite parts in aeronautic and automobile industries. With these materials it is possible to produce composite parts with complex geometries, and high curvatures. On the other hand the length of the fibers leads to mechanical properties of the final composite that are close to those of the composite with continuous fibers. This paper analyzes the thermo‐mechanical properties of Stretch Broken Carbon Fiber (SBCF) / PPS and PEEK commingled prepregs during manufacturing. Tensile and in‐plane shear tests at different temperatures are analyzed. The experiments are realized in an isothermal oven. The range of temperature is those of the part during a thermoforming process. The experimental data allow to analyze the differences on the tensile and in‐plane shear behaviors at different temperatures between thermoplastic prepregs with continuous fibers and thermoplastic prepregs with stretch‐broken fibers. Forming simulations show that wrinkling can be avoided with SBCF prepregs while these wrinkles develop during continuous fibers prepreg forming. POLYM. COMPOS., 36:694–703, 2015. © 2014 Society of Plastics Engineers     

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.     

Real‐time monitoring of fluorescence anisotropy and temperature during processing of biaxially stretched polypropylene film

An optical sensor based on fiber optics has been developed to measure fluorescence anisotropy and temperature during processing of biaxially stretched polypropylene films. The sensor, containing optical fibers, polarizing elements and lenses, was mounted above the polypropylene film as it was processed in a tenter frame oven stretching machine. Fluorescence observations were made using the fluorescent dye, bis (di‐tert butylphenyl) perylenedicarboximide (BTBP), which was doped into the resin at very low concentrations. To monitor biaxial stretching, fluorescence anisotropy measurements were carried out with light polarized in the machine and the transverse directions corresponding to the directions of biaxial stretching. Fluorescence based temperature measurements were obtained from the ratio of fluorescence intensities at 544 nm and 577 nm. A matrix of experiments involving three levels of stretch ratio in both the machine and transverse directions was undertaken. We observed significant differences between anisotropy in the machine and transverse directions that we attributed to the sequential stretching operation, i.e., the film was stretched in the machine direction first, followed by stretching in the transverse direction, and to film temperature and strain rate for each stretching operation. The result was uniformly higher anisotropies in the machine direction. Film temperature obtained from fluorescence corresponded to oven thermocouple measurements within 2°C. Polym. Eng. Sci. 44:805–813, 2004. © 2004 Society of Plastics Engineers.     

Stress‐induced crystallization of biaxially oriented polypropylene

The hot stretching of thick, extruded sheets at high temperatures is a very important process in the production of finished biaxially oriented polypropylene (BOPP) films with special inner structures. Through a simulation of hot stretching in the machine direction (MD) of the processing of BOPP films, it was found that at high temperatures, the stretching ratio greatly influenced the obtained crystalline structure, as observed by differential scanning calorimetry (DSC). Also, in MD hot stretching, the crystallinity increased by an average of 20%. Wide‐angle X‐ray diffraction patterns of extruded sheet samples with and without stretching confirmed the structural changes shown by DSC, and the results proved that β‐crystal modification did not occur during the MD hot‐stretching process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 686–690, 2003     

The use of 3D in-mold decoration technology to form a film with printed circuits

In-mold decoration is an integrated process for plastic processes, such as printing, hot pressing, and injection molding. The process has simpler steps, the production time is shorter, products are more durable, and manufacturing costs are less than traditional methods. In-mold decoration is affected by shape, mold design and the parameters for forming, and cracking, wrinkling or stress concentration can occur. A large curvature can produce cracks and wrinkles. This study designs a series of experimental methods for film forming, hot press forming, and injection molding. The effects of various features on the quality of the hot press forming and injection molding processes are determined. A three-dimensional film forming process uses pressure thermoforming technology to form either without pretensioning, with pretensioning, and with local heating. The local heating method gives the best results. For injection molding processes, the mold temperature, the melt temperature, the injection speed, and the holding pressure are the parameters. The injection molding process is optimized using the Taguchi method, and the factor with the highest contribution is determined to be the holding pressure. The conductive line of the finished in-mold decoration product can be formed smoothly by hot pressing using local heating, and the measured resistors are almost the same after each process.     

Preparation and characterization of biaxially oriented polypropylene film with high molecular orientation in the machine direction by sequential biaxial stretching

Novel approach of applying the ternary polymer blend of long‐chain branched polypropylene (LCB‐PP), conventional polypropylene (PP), and hydrogenated polydicyclopentadiene (hDCPD) has been employed to tensilize biaxially oriented polypropylene (BOPP) film in the machine direction (MD) by successive sequential biaxial stretching method. It is found that the addition of LCB‐PP improves the MD stretchability of the BOPP film of PP/hDCPD blend. Depending on the content of LCB‐PP, LCB‐PP/PP/hDCPD ternary blend could be biaxially stretched up to the MD stretching ratio (MDX) of 12 without film breakage whereas that of PP (conventional BOPP film) resulted in the MDX up to 6. This excellent MD stretchability enabled to tensilize the BOPP film in the MD, where Young's modulus in the MD could be increased up to 4.9 GPa, twice higher than that of conventional BOPP film. The orientation of total molecular chains and that of crystalline molecular chains were evaluated by in‐plane distribution of refractive indices and wide‐angle X‐ray diffraction, respectively. The results are discussed from the viewpoint of deformation behavior during stretching process. Moreover, the resultant film had a dimensional stability substantially equivalent to that of conventional one, in spite of the higher stretching ratio, and an improved moisture barrier property. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

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.     

BOPP料的结晶行为与高温拉伸试样的熔融特性

采用示差扫描量热仪(DSC)研究了双向拉伸聚丙烯薄膜专用料PD382的非等温结晶特性与不同的高温热拉伸比试样的熔融特性。结果表明：PD382在冷却结晶过程中，受到冷却速率相当大的影响，降温速率增加，材料的开始结晶温度和最大结晶温度降低，结晶能力提高，结晶速率加快；经过拉伸作用的试样的结晶结构趋于规整，有序，结晶结构中的缺陷减少，当拉伸比达到200％时，试样的结晶结构已经完成了从较为无序到有序规整状态的转变，进一步提高拉伸比则只是强化这种有序规整的结构。     

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

可生物降解双向拉伸聚乳酸薄膜成型技术研究进展

介绍了国内外高功能性双向拉伸聚乳酸（BOPLA）薄膜材料成型技术的研究现状，重点综述了基础型、功能型两大类可生物降解BOPLA薄膜的研究进展，包括通过优化基体配方、熔融共挤、拉伸温度、拉伸倍率、拉伸速度及热定型温度等双向拉伸成型技术和基于表层可金属化技术的方式制备可生物降解BOPLA薄膜。最后，对可生物降解BOPLA薄膜的研究趋势进行了展望。