Transient heat transfer phenomena in the rotational molding of plastic parts are modeled in this study. Natural convection and radiation from the furnace and flue gases to the mold housing are analyzed. Other models include transient heat transfer through the mold, single-phase conduction through the particulate plastic material prior to phase change, melting of the plastic and heating of the liquid pool. Subsequent staged cooling of the mold and solidification of the plastic using a combination of free and forced convection and radiation, are also modeled. The mold wall, melt, and solidified plastic regions are divided into a number of finite segments to track the temperature variation with time during the molding process. The corresponding variations in masses and thicknesses of the melt and solidified plastic regions are estimated. This information is used to estimate the energy consumption rates for various phases of the process. The model is applied to a specific molding process in a commercial rotational molding plant. Parametric studies of the effect of heating and cooling durations on the plastic temperatures and the energy consumption rates are conducted. These analyses provide insights about opportunities for optimization of the heating and cooling schedules to reduce overall energy consumption and improve throughput. The overall energy and gas consumption for the rotational molding process, taking into consideration the thermal mass of the auxiliary housing (steel) required to hold the molds, is estimated on a per-batch basis. In addition, a preliminary design for an alternative system for heating and cooling the molds using a high temperature heat transfer fluid (HTF) flowing through jackets integral to the molds is proposed.

Thesis (M.S.)--Brigham Young University. Dept. of Chemical Engineering, 2003. / Includes bibliographical references (p. 69-74).

Thesis (M.S.)--Mechanical Engineering, Georgia Institute of Technology, 2005. Directed by Jonathan Colton. / Dr. Jonathan Colton, Committee Member ; Dr. Shelson Jeter, Committee Member ; Dr. Srinivas Garimella, Committee Chair. Includes bibliographical references.

Thermotropic liquid crystalline polymers (TLCPs) possess a number of physical and mechanical properties such as: excellent chemical resistance, low permeability, low coefficient of thermal expansion, high tensile strength and modulus, and good impact resistance, which make them desirable for use in the storage of cryogenic fluids. Rotational molding was selected as the processing method for these containers because it is convenient for manufacturing large storage vessels from thermoplastics. Unfortunately, there are no reports of successful TLCP rotational molding in the technical literature. The only related work reported involved the static coalescence of two TLCP powders, where three key results were reported that were expected to present problems that preclude the rotational molding process. The first result was that conventional grinding methods produced powders that were composed of high aspect ratio particles. Secondly, coalescence was observed to be either slow or incomplete and speculated that the observed difficulties with coalescence may be due to large values of the shear viscosity at low deformation rates. Finally, complete densification was not observed for the high aspect ratio particles. However, the nature of these problems were not evaluated to determine if they did, in fact, create processing difficulties for rotational molding or if it was possible to develop solutions to the problems to achieve successful rotational molding. This work is concerned with developing a resin selection method to identify viable TLCP candidates and establish processing conditions for successful rotational molding. This was accomplished by individually investigating each of the phenomenological steps of rotational molding to determine the requirements for acceptable performance in, or successful completion of, each step. The fundamental steps were: the characteristics and behavior of the powder in solids flow, the coalescence behavior of isolated particles, and the coalescence behavior of the bulk powder. The conditions identified in each step were then evaluated in a single-axis, laboratory scale, rotational molding unit. Finally, the rotationally molded product was evaluated by measuring several physical and mechanical properties to establish the effectiveness of the selection method. In addition to the development and verification of the proposed TLCP selection method, several significant results that pertain to the storage of cryogenic fluids were identified as the result of this work. The first, and argueably the most significant, was that the selection method led to the successful extension of the rotational molding process to include TLCPs. Also, the established mechanical properties were found to be similar to rotationally molded flexible chain polymers. The biaxial rotationally molded container was capable of performing to the specified requirements for cryogenic storage: withstand pressures up to 34 psi at both cryogenic and room temperatures, retain nitrogen as a gas and as a cryogenic liquid, the mechanical preform retaining nitrogen, as both a gas and as a cryogenic liquid, and resist the development of micro-cracks during thermal cycling to cryogenic conditions. / Ph. D.

Extrusion compounding addresses such problems as the non-homogeneity of the mixture and separation of fiber from the polymer during rotational molding, which consequently affect the mechanical and physical properties of the resulting composites. <p>Using triethoxyvinylsilane as chemical pre-treatment on flax fibers and linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) as polymer matrices, this study focused on the effects of flax fiber content (0%, 12.5% or 25%) and extrusion parameters such as barrel zone temperatures (75-110-120-130-140°C or 75-120-130-140-150°C) and screw speed (110 or 150 rpm) on the extrudate and composite properties (extrudate color, extrudate density, extrudate melt flow index, extrudate morphology, composite color, composite density, composite morphology, composite tensile strength and composite water absorption). <p>A mixture of chemically pre-treated flax fibers and powdered polyethylene matrices underwent extrusion compounding using a twin-screw extruder. The extrudates were then pelletized, ground, rotationally molded and cut into test specimens (composites). The mechanical and physical properties of both the extrudates and the composites from different treatments were then measured and compared. <p>Using multiple linear regression, models were generated to show quantitatively the significant effects of the process variables on the response variables. Finally, using response surface methodology and superposition surface methodology on the preceding data, the following optimum values for fiber content and extrusion parameters were determined: for LLDPE composites, fiber content = 6.25%, temperatures = 75-117.3-127.3-137.3-147.3°C, screw speed = 117.5 rpm; for HDPE composites, fiber content = 5.02%, temperatures = 75-118.1-128.1-138.1-148.1°C, screw speed = 125.56 rpm.

Extrusion compounding addresses such problems as the non-homogeneity of the mixture and separation of fiber from the polymer during rotational molding, which consequently affect the mechanical and physical properties of the resulting composites. <p>Using triethoxyvinylsilane as chemical pre-treatment on flax fibers and linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) as polymer matrices, this study focused on the effects of flax fiber content (0%, 12.5% or 25%) and extrusion parameters such as barrel zone temperatures (75-110-120-130-140°C or 75-120-130-140-150°C) and screw speed (110 or 150 rpm) on the extrudate and composite properties (extrudate color, extrudate density, extrudate melt flow index, extrudate morphology, composite color, composite density, composite morphology, composite tensile strength and composite water absorption). <p>A mixture of chemically pre-treated flax fibers and powdered polyethylene matrices underwent extrusion compounding using a twin-screw extruder. The extrudates were then pelletized, ground, rotationally molded and cut into test specimens (composites). The mechanical and physical properties of both the extrudates and the composites from different treatments were then measured and compared. <p>Using multiple linear regression, models were generated to show quantitatively the significant effects of the process variables on the response variables. Finally, using response surface methodology and superposition surface methodology on the preceding data, the following optimum values for fiber content and extrusion parameters were determined: for LLDPE composites, fiber content = 6.25%, temperatures = 75-117.3-127.3-137.3-147.3°C, screw speed = 117.5 rpm; for HDPE composites, fiber content = 5.02%, temperatures = 75-118.1-128.1-138.1-148.1°C, screw speed = 125.56 rpm.

Conformity MENDONÃA [10], the investments in sanitation are four times more effective than health. There are currently answering sewage deficit in Brazil, reaching 52.9 percent of the population. The infrastructure of sewage systems of large cities has a huge variety of materials, ranging from concrete to composite materials. Disorders caused by premature failure of materials, applied in sewage systems cause environmental pollution, depreciate image utility/company and even accidents. The manholes represent 15.5 percent of the costs of implementing the sewage systems. This study aims to assess the effects of the degradation of manholes in polyethylene (PE) recycled. The accelerated degradation test simulates the etching with sulfuric acid, present in sewage systems (pH 1.1) of the test specimens for mechanical tensile and impact. The PE was produced by the rotational molding process, from virgin and recycled raw material for comparative purposes. Before the attack the materials were characterized by thermal analysis tests thermogravimetry (TG) and differential scanning colorimetric (DSC), X-ray diffraction, melt index, ash and loads. Data from the thermography images compared with assays term analysis revealed that machining by computer numerical control (CNC) did not change the mechanical properties of specimens for tensile and impact tests . Analyses term perspective and stereopsis revealed flaws in the rotational molding process (bubbles, sink marks and surface irregularity). The crystallinity of recycled PE was greater than virgin PE, this property being obtained by DSC and X-ray. The resistance of the recycled PE fracture was higher, ratified by the crystallinity and moisture present (Technical TG). The melt flow index and mechanical tests revealed that the virgin PE is more ductile than recycled. After the attack the specimens increased their residual mass being 3 times more recycled PE. Destructive of the specimens after chemical attack, trials there were no significant changes in the mechanical properties because of the high standard deviation data, arising from the failure of the material rotational molding process. / Segundo MendonÃa [10], os investimentos realizados em saneamento bÃsico sÃo quatro vezes mais eficazes que na saÃde. Atualmente, hà dÃficit de atendimento de esgotamento sanitÃrio no Brasil, atingindo 52,9% da populaÃÃo. A infraestrutura de redes coletoras de esgoto (RCEs) de grandes cidades possui uma imensa diversidade de materiais, que vÃo desde o concreto atà os materiais compÃsitos. Transtornos causados pela falha prematura dos materiais, aplicados nas redes coletoras de esgoto causam poluiÃÃo ambiental, desgaste da imagem da concessionÃria/empresa e atà acidentes. Os poÃos de visita (PVs) representam 15,5% dos custos para implantaÃÃo de RCEs. Este trabalho tem como objetivo a avaliaÃÃo dos efeitos da degradaÃÃo dos PVs em polietileno (PE) reciclado. O ensaio de degradaÃÃo acelerada simula o ataque quÃmico com Ãcido sulfÃrico, presente nas RCEs (pH de 1,1) dos corpos de prova para ensaios mecÃnicos de traÃÃo e impacto. O PE foi produzido pelo processo de rotomoldagem, de matÃria prima virgem e reciclada, para fins comparativos. Antes do ataque os materiais foram caracterizados por ensaios de anÃlises tÃrmicas de termogravimetria (TG) e colorimetria diferencial de varredura (DSC); difraÃÃo de raios X (DRX); Ãndice de fluidez; teor de cinzas e cargas. Os dados das imagens termogrÃficas comparativamente com os ensaios de termoanÃlises revelaram que a usinagem por meio do controle numÃrico computadorizado (CNC) nÃo alterou as propriedades mecÃnicas dos corpos para os ensaios traÃÃo e impacto. As termoanÃlises e a estereoscopia Ãtica revelaram as falhas do processo de rotomoldagem (bolhas, rechupes e irregularidade na superfÃcie). O grau de cristalinidade do PE reciclado foi maior do que o PE virgem, sendo esta propriedade obtida pelas tÃcnicas de DSC e Raios X. A resistÃncia a fratura do PE reciclado foi superior, ratificada pelo grau de cristalinidade e umidade presente (tÃcnica TG). O Ãndice de fluidez, juntamente com ensaios mecÃnicos, revelou que o PE virgem à mais dÃctil que o reciclado. ApÃs o ataque, os corpos de prova aumentaram sua massa residual, sendo trÃs vezes mais que o PE reciclado. Os ensaios destrutivos dos corpos de prova apÃs o ataque quÃmico, nÃo houve alteraÃÃes significativas das propriedades mecÃnicas, visto o alto desvio padrÃo dos dados, oriundo das falhas do material provenientes do processo de rotomoldagem.

The development of acrylonitrile-butadiene-styrene (ABS) resins for use in rotational molding would provide a medium performance material, thus opening doors to new markets for the rotational molding industry. Unfortunately, ABS resins have shown serious problems during the rotational molding process, namely discoloration, bridging, and poor impact strength. It is believed that these effects are due to degradation of the carbon-carbon double bond in the butadiene, through attack by either oxygen or heat. Previous efforts have shown some success in addressing these issues. However, additional improvements are necessary to make ABS resins commercially viable to rotational molders. This study, fourth in a series of similar projects conducted though Brigham Young University, was focused on remediation of the ABS difficulties via two different approaches. First, a survey of several additives was performed with the intent of investigating four different strategies: increased protection from oxygen, decreased butadiene concentration, increased butadiene concentration, and promotion of flow. The best formulation was achieved when 15 wt % of a benzoate ester (XP-2280 available though ChemPoint) was blended into MAGNUM 342 EZ, an ABS resin (The Dow Chemical Company). This formulation showed the best balance between increased impact strength and improvement of cosmetic properties. Second, optimization of several rotational molding processing parameters was executed. These included particle size distribution of the resin, drying of the resin, internal mold atmosphere, and oven temperature. It was found that using coarse particle sizes (ground at 20-mesh rather than the industry standard of 35-mesh) increased the impact strength by about 19%. None of the other parameters proved to have a significant effect upon the system, except for the use of a nitrogen atmosphere, which lowered the impact strength. Final properties testing of this best formulation at the optimal processing conditions showed increased impact strength from 2 ft-lbs (the previous best value) to 8 ft-lbs. There was also a marginal decrease in surface hardness (95 to 78 on the Rockwell R scale) and yield tensile strength (3,900 psi to 3,300 psi). Larger differences were observed in flexural modulus (200,000 psi to 110,000 psi) and heat distortion temperature (95°C to 61°C). Therefore, these formulation and processing changes show a trade-off where stiffness and thermal stability (i.e. flexural modulus and heat distortion temperature) can be sacrificed for an increase in toughness and aesthetics, made manifest by increased impact strength, elimination of bridging, and eradication of discoloration.

This thesis focuses on data-driven modeling and control of batch and batch-like processes. These processes are highly nonlinear and time-varying which, unlike continuous operations, are characterized by the finite duration of operation and absence of equilibrium conditions. This makes the modeling and control approaches available for continuous processes not readily applicable and requires appropriate adaptations of the available approaches to handle a) batch data structure for modeling and b) a control objective different than that of maintaining a steady-state operation as often encountered in a continuous process. With these considerations, this work adapted the batch subspace identification for modeling and control of a variety of batch and batch-like processes. A particular focus of this work was on the application of the proposed ideas on real engineering systems along with simulated case studies. The applications considered in this work are batch crystallization, a hydrogen plant startup dynamics in a collaboration with Praxair Inc. and a rotational molding process in collaboration with the polymer research group at McMaster University. For the seeded batch crystallization process, subspace identification techniques are adapted to identify a linear time invariant model for the, otherwise, infinite dimensional process. The identified model is then deployed in a linear model predictive control (MPC) strategy to achieve crystal size distribution (CSD) with desired characteristics subject to both manipulated input and product quality constraints. The proposed MPC is shown to achieve superior performance and the ability to respect tighter product quality constraints as well as robustness to uncertainty in comparison to an open loop policy as well as a traditional trajectory tracking policy using classical control. In another contribution, merits of handling data variety in a subspace identification framework was demonstrated on the crystallization process. The proposed approach facilitates the specification of a desired shape of the particle size distribution required at the termination of the batch process. Further, novel model validity constraints are proposed for the subspace identification based control framework. In the collaborative work on hydrogen plant startup, it is recognized as a batch-like process due to its similarity to batch processes. Firstly, in this work a high fidelity model of the Hydrogen unit was developed with relevant startup and shutdown mechanisms. This setup is used to mimic the trends in the key process variables during the startup/shutdown operation. The simulated data is used to identify a state-space model of the process and validated on new simulated startup. Further, the approach was demonstrated on real plant data from one of the Praxair's plants. The predictive capabilities of the model provide ample handle for the plant operator for averting failures and abrupt shutdown of the entire plant. This is expected to have immense economic advantages. Finally, the subspace identification based modeling and control approach was applied to a lab-scale rotational modeling (RM) process. It is a polymer processing technique that is characterized by the placement of a polymer resin inside a mold, subsequent closure of the mold, followed by the simultaneous application of uni-axial (as is the case in the present work) or bi-axial rotation and heat. The resin is deposited on the mold wall where it forms a dense unified layer following which, the mold is cooled while still rotating the mold. Once demolding temperatures are achieved, the finished part is removed from the mold. Its potential as a manufacturing process for polymeric components is limited by a number of concerns including difficulties in process control, in particular, determining efficiently the process operation to yield the desired product consistently, and produce new products. This work has contributed by developing optimal control strategies for the process to achieve user-specified product quality and reject variability across batches. The results obtained demonstrate the merits of the proposed approach. / Thesis / Doctor of Philosophy (PhD)

Made available in DSpace on 2017-07-21T20:42:31Z (GMT). No. of bitstreams: 1 Claudiacanova.pdf: 1038020 bytes, checksum: c13369757b0e9c72adfb63dbe23d85e4 (MD5) Previous issue date: 2004-12-15 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The purpose of the present work was to simulate the cooling phase of the rotational molding in order to evaluate the influence of some variables in the cooling time, that is the longest one in the process. ANSYSTM software was used to perform the simulation using the finite element method, thus it will be possible to work with complex geometries in future works. However, in the present simulation a spherical mold was used. The effect of the crystallization process of polymer during the cooling phase was evaluated by the incorporation of the heat of crystallization in the curve of enthalpy. Using this procedure it was possible to observe the effect of the crystallization process on the temperature profile as a function of time for the polymeric part. It was possible to compare this profile with the correspondent theoretical curve for a similar amorphous polymer. It was evaluated also the effect of different values of convection heat transfer coefficients for the external air and the effect of using a polymer with a higher thermal conductivity, associated to the use of fillers as the aluminum. The validation of the simulated results of the cooling phase of rotational molding was done by comparison with experimental and simulated results from literature, being obtained good agreement. In present work it was performed also a validation of the ANSYSTM simulation of solidification process of polymers involving crystallization by comparison with data from quenched slabs experiments. In this case, low density polyethylene samples and low density polyethylene with aluminum, from recycled Tetra Pak packages, were used in disc shapes and submitted to quenching, being recorded the temperature profile as a function of time in different cooling rates. The simulation in the software ANSYSTM allowed a good agreement between experimental and simulated curves. It was vii observed that the convection heat transfer coefficient for the external air is the main variable that control the process and that the thermal conductivity of polymer does not play an important role in the process. It was observed also that the heat released in the crystallization process presents a significant role and that it is important to include this phenomena in the simulation. / A proposta do presente trabalho foi simular a fase de resfriamento do processo de moldagem rotacional para avaliar a influência de algumas variáveis no tempo de resfriamento, que é o mais longo no processo. O software ANSYSTM foi usado para realizar a simulação pelo método dos elementos finitos, o que permitirá em trabalhos futuros se trabalhar com geometrias complexas. No entanto, na presente simulação foi usado molde com geometria esférica. O efeito do processo de cristalização de polímeros na fase de resfriamento foi avaliado pela incorporação do calor de cristalização na curva de entalpia. Usando este procedimento, foi possível observar o efeito da cristalização no perfil de temperatura em função do tempo, comparando com a correspondente curva teórica para um polímero amorfo similar. Foi avaliado também, o efeito de diferentes valores de coeficientes de transferência de calor por convecção do ar externo e o efeito de se usar um polímero com condutividade térmica maior, devido ao uso de cargas como o alumínio. A validação dos resultados de rotomoldagem simulados via ANSYSTM foi feita por comparação com dados experimentais e simulados de literatura, tendo sido verificada uma boa concordância. No presente trabalho também foi feita uma validação da simulação via ANSYSTM da solidificação de polímeros envolvendo transformação de fase através de comparação com experimentos de choque térmico. Neste caso, amostras de polietileno de baixa densidade e polietileno de baixa densidade com alumínio, oriundos da reciclagem de embalagens Tetra Pak, foram utilizadas na forma de discos e submetidas a choque térmico, onde se registrou o perfil de temperatura em função do tempo para diferentes velocidades de resfriamento, indicando diferentes valores de coeficiente de transferência de calor por convecção. A simulação via ANSYSTM, possibilitou um bom ajuste às curvas experimentais. As simulações indicaram que o coeficiente de transferência de calor por convecção do ar externo é o fator que mais afeta o perfil de resfriamento. Verificou-se também que a condutividade térmica do polímero não afeta de maneira significativa a velocidade de resfriamento e que o calor liberado no processo de cristalização apresenta influência significativa, devendo necessariamente ser incorporado na simulação do perfil de temperatura. Estes comportamentos se mostraram coerentes com os observados na simulação do perfil de temperatura da rotomoldagem.