Measurement of polymer‐to‐polymer contact friction in thermoforming

Contact friction plays a critical role in all the major thermoforming processes for polymers. However, these effects are very difficult to measure in practice and, as a result, have received little scientific investigation. In this work, two independently developed test methods for the measurement of elevated temperature polymer‐to‐polymer contact friction are presented, and their results are compared in detail for the first time. One is based on a modified moving sled friction test, whereas the other uses a rotational rheometer. In each case, friction tests were conducted between two plug and two sheet materials. The results show that broadly similar coefficients of friction were obtained from the two test methods. The measured values were quite low (<0.3) at lower temperatures and typically were higher for polypropylene (PP) sheet than for polystyrene (PS). On approaching the glass transition temperature for PS (95°C) and the crystalline melting point for PP (165°C), the friction coefficients rose very sharply, and both test techniques became increasingly unreliable. It was concluded that despite their physical differences, both test techniques were able to capture the highly temperature sensitive nature of friction between polymer materials used in thermoforming. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

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     

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.     

An in‐process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts

The orientation and relaxation behaviors of a low‐density polyethylene melt and polypropylene melts with different melt indices undergoing a shear flow in a restricted channel were investigated by using ultrasound. A capillary rheometer was used to force the polymer melt through a slit die equipped with pressure, temperature, and ultrasound sensors, and the variation of ultrasound velocity traversing the melt was measured. Experimental results revealed that due to different mechanisms involved, the relaxations of orientation and disorientation processes show different dependences of ultrasound velocity on shear rate, temperature, and melt index. POLYM. ENG. SCI., 2008. Published 2008 Society of Plastics Engineers     

Thermodynamic approach to interfacial transport phenomena: Single‐component systems

Balancing extensive quantities at interfaces in terms of excess densities is a subtle problem because these densities change with the precise location of the dividing surface within the interfacial region. We propose to handle such ambiguities for moving interfaces by introducing a gauge degree of freedom associated with the location of the dividing surface and by studying all the normal velocities associated with different excess densities. Unambiguous interfacial balance equations can then be obtained directly from the differences between normal velocities. By assuming local equilibrium, considering the interfacial entropy balance, and identifying the entropy production rate at the interface, we are naturally led to thermodynamically consistent constitutive assumptions for the fluxes characterizing transport in the interface and for the boundary conditions characterizing transport across the interface. The usefulness and generality of the proposed approach is illustrated in the context of several examples. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1424–1433, 2014     

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     

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     

Statistical analysis of product variability associated with continuous and cut sheet thermoforming operations

Extensive published data exist with regard to the variation of part weight and dimensions for products manufactured by injection molding and blow molding. Relatively little information is available with regard to thermoformed parts. Such data are of considerable practical and theoretical value in establishing realistic processing targets in commercial processing operations. Experimental data have been systematically collected over an extended time from three continuous roll-fed thermoforming processes, as well as a cut sheet forming process. Two of the continuous processes studied were high-volume production lines. One of the operations produced 16-ounce drinking cups from a blend of a styrene-butadiene block copolymer with polystyrene. The other commercial continuous process manufactured tubs from high-density polyethylene. The third process studied involved a pilot plant test former used to produce 10-ounce drinking cups from the styrenebutadiene/polystyrene blend material. All three continuous processes employ plug assist in conjunction with a multi-cavity mold. The measured parameters included the weight of individual parts, and the wall thickness measured at several different locations. The cut sheet forming process employed a plug assist with a single-cavity mold for the production of a scanner cover. The polymer used consisted of a blend of an acrylic and a poly(vinyl chloride) resin. The deformation and wall thickness of the scanner cover were measured at several different locations.     

Axial‐feed thermoforming of oriented polypropylene tubes

The effect of newly developed axial feed thermoforming process (AFTF) on mechanical properties and morphology were investigated to gain a better understanding of structure–properties–process relationship. The starting material for AFTF is an oriented polypropylene tube (OPP) produced in uniaxial direction by using solid state extrusion process. Morphological changes from solid‐state extrusion are briefly reported. A die‐less bulge testing system was designed to bulge OPP tube at a high temperature in biaxial direction. Wide‐angle X‐ray diffraction, field emission scanning electron microscope, and optical microscopy were used to characterize the microstructure of the extruded and bulged samples of OPP. In addition, tensile tests were carried out at room temperature of samples machined from the extruded and bulged tubes along the axial and hoop directions. The results show that tensile strength increases with draw ratio in extruded samples as well as improvements in ductility were obtained in die‐less bulge tests with an increase in axial feed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Failure of multimaterial fusion bonding interface generated during over‐injection molding/thermoforming hybrid process

The effects of processing parameters on the strength of the fusion bonding interface have been investigated. The interface was generated when an isotactic polypropylene homopolymer was injection molded on a solid self‐reinforced polypropylene substrate. The interface strength was measured in shear configuration, and the melting behavior of substrate was studied using differential scanning calorimetry. The results show that strong bonding interface can be achieved when the correct processing parameters are chosen. The interfacial strength is largely improved if the interface temperature is higher than the melting temperature of the substrate layer, and in these specimens failure does not take place at the interface. Furthermore, for a fixed interface temperature, interface strength increases with thermal gradient. Finally, in the analyzed holding pressure range, pressure apparently has no effect on strength. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 261–265, 2006     

In‐process rheological monitoring of extrusion‐based polymer processes

Since rheology is very sensitive to the structure and macromolecular conformation of polymer systems, rheological measurements performed in situ during extrusion are attractive for monitoring the process. After introducing the concepts of in‐process monitoring during extrusion operations, the benefits of rheology for assessing filler dispersion in polymer composites, morphology development in polymer blends or the extent of chemical reaction in reactive extrusion are briefly reviewed. Then, in‐process rheological tools are reviewed. For each device, the information conveyed is detailed. © 2020 Society of Industrial Chemistry     

Synthesis and characterization of nanoclay–polymer composites from soil clay with respect to their water‐holding capacities and nutrient‐release behavior

Water and nutrients are two important inputs to agriculture that need to be used judiciously with higher efficiency to save these limited resources. For these purposes, a series of nanoclay–polymer composite (NCPC) superabsorbent nutrient carriers were prepared. These NCPCs were based on the reactions of different types of nanoclays (10 wt %) with partially neutralized acrylic acid and acryl amide by a free‐radical aqueous solution copolymerization reaction with N,N′‐methylene bisacrylamide as a crosslinker and ammonium persulfate as an initiator. The nanoclays isolated from three different types of soils were dominant in kaolinite (clay I ), mica (clay II ), and montmorillonite (clay III ), and a portion of each was freed from amorphous aluminosilicate. Thus, there were six different types of nanoclays used, namely, those dominated by kaolinite, mica, and smectite with and without amorphous aluminosilicate. Fourier transform infrared spectroscopy and X‐ray diffraction (XRD) investigations showed evidence of interaction between the clays and polymer. XRD investigation also showed that the reaction between the polymer and clays I and II occurred on the surface of various clay particles without intercalating into the stacked silicate galleries, whereas in the case of clay III (the smectite‐dominated clay), evidence indicated the intercalation of polymer into the stacked silicate galleries of the clay and the exfoliation of the clay. The water absorbency decreased in the NCPCs compared to that of the pure polymeric hydrogel. In case of the pure polymer, the entire amount of nutrient loading released within 15 h of incubation; this was higher than that of the NCPCs. In the initial stage (up to 15 h), no significant differences in nutrient release were observed among the different polymer/clay composites, but there were differences in later stages. Among the different NCPCs, the percentage release of nutrients at 48 h ranged from around 70% in the polymer/clay III composite to 90% in the polymer/clay I composite. The presence of amorphous aluminosilicates in clay did not make any difference in the nutrient‐release rate. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39951.     

Rheological modeling of fracture in plug‐assisted vacuum thermoforming

The fracture of polymeric sheets is one of the practical problems occurring during plug‐assisted vacuum thermoforming. This defect can occur during both the plug‐assist and vacuum‐forming stages. This article focuses on two issues: (1) the origins of fracture creation and (2) the determination of the process parameters needed for removal of the defect. The results of our work not only lead to an understanding of the cause of this problem but also enable us to calculate the parameters that affect the fracture of polymeric sheets during plug‐assisted vacuum thermoforming. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Profile control of plastic sheet in an industrial polymer processing process

A control system was developed using a two-degree of freedom control structure with four input variables in the feedforward, and two inputs, the screw rotation speed and the gap of the forming roller, in the feedback loop. The feedforward signals were predetermined from first principles and the operators' heuristic rules for each changeover operation and prepared in a database. At the end of each changeover operation, the values in the database were rewritten by evaluating control performance. The magnitude of the true dead time changed as the production rate was changed. The rate of sampling and control action in the feedback control was adapted in each changeover operation so that the changing time delay was always expressed by a one-sampling-step time delay in the discrete models and dealt with by the controller robustness against parameter uncertainty. The developed control system was applied to an actual industrial plastic sheet production line and allowed one to easily perform the changeover operation and reduce material consumption.     

Measurements on the biaxial extension viscosity of bulk polymers: The inflation of a thin polymer sheet

An experimental study of the inflation of a thin polymer sheet has been conducted to determine whether this technique can be used to measure the biaxial extensional viscosity of bulk polymers. Viscosities were determined at various extensional strain rates using two undiluted polyisobutylene samples having different molecular weights. Advantages, limitations, and errors associated with the method are discussed.     

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     

Structure and properties of new thermoforming bionanocomposites based on chitin whisker‐graft‐polycaprolactone

Thermoformable bionanocomposites of chitin whisker‐graft‐polycaprolactone (CHW‐g‐PCL) were synthesized by initiating the ring‐opening polymerization of caprolactone monomer onto the CHW surface under microwave radiation. In this case, the “graft from” strategy contributed to long and dense “plasticizing” PCL tails onto the CHW surface as the key of thermoforming, and, therefore, such bionanocomposites were injection‐molded as the sheets with a structure of cocontinuous phase mediated with the entanglement of grafted PCL chains. The structure and properties of the molded CHW‐g‐PCL sheets were investigated by FTIR, XRD, SEM, DSC, DMTA, contact angle measurement, and tensile test. With an increase of the PCL content in CHW‐g‐PCL, the strength and elongation as well as the hydrophobicity of the nanocomposites increased at one time. This is the first report on the thermoformable polymer‐grafted nanocrystal derived from natural polysaccharide. Moreover, such new bionanocomposites with good mechanical performances could have great potential applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dynamic optimization of the methylmethacrylate cell‐cast process for plastic sheet production

Traditionally, the methylmethacrylate (MMA) polymerization reaction process for plastic sheet production has been carried out using warming baths. However, it has been observed that the manufactured polymer tends to feature poor homogeneity characteristics measured in terms of properties like molecular weight distribution. Nonhomogeneous polymer properties should be avoided because they give rise to a product with undesired wide quality characteristics. To improve homogeneity properties force‐circulated warm air reactors have been proposed, such reactors are normally operated under isothermal air temperature conditions. However, we demonstrate that dynamic optimal warming temperature profiles lead to a polymer sheet with better homogeneity characteristics, especially when compared against simple isothermal operating policies. In this work, the dynamic optimization of a heating and polymerization reaction process for plastic sheet production in a force‐circulated warm air reactor is addressed. The optimization formulation is based on the dynamic representation of the two‐directional heating and reaction process taking place within the system, and includes kinetic equations for the bulk free radical polymerization reactions of MMA. The mathematical model is cast as a time dependent partial differential equation (PDE) system, the optimal heating profile calculation turns out to be a dynamic optimization problem embedded in a distributed parameter system. A simultaneous optimization approach is selected to solve the dynamic optimization problem. Trough full discretization of all decision variables, a nonlinear programming (NLP) model is obtained and solved by using the IPOPT optimization solver. The results are presented about the dynamic optimization for two plastic sheets of different thickness and compared them against simple operating policies. © 2009 American Institute of Chemical Engineers AIChE J, 2009