Research On3-D Surface Multi-point Thermoforming For Polymer Sheet

With the growing demand for high-performance and lightweight industrial products,3-Dpolymer sheet parts are wildly used in the fields of aerospace engineering, automotiveengineering, ship engineering, construction engineering, decoration, etc. Thermoforming is ageneric term encompassing many techniques for manufacturing3-D surface polymer sheetparts from flat sheets. Owing to the high-capital investments and long cycle for developingand manufacturing forming die, the conventional thermoforming is generally utilized formass production. However, taking economic benefit into consideration, conventionalthermoforming is not suitable for manufacturing small-batch3-D surface polymer sheet parts.Therefore, the development of flexible3-D surface polymer sheet forming technique hasextensive application prospect and practical value.Multi-point thermoforming (MPTF) is a novel flexible forming method for3-D surfacepolymer sheet parts manufacturing. In MPTF, the fixed die employed in conventionalthermoforming is replaced by a multi-point die (MPD). By controlling the height of eachpunch, the forming surface of MPD can be transformed into various3-D shapes, and thepolymer sheet parts with different3-D shapes can be thermoformed rapidly and efficiently.In the present thesis, the3-D surface polymer sheet part multi-point thermoformingequipment was developed and the finite element model (FEM) was established. The causes,influence factors and suppressing methods of common forming defects in MPTF wereinvestigated through numerical simulations and experiments. The real-time adjustingmulti-point thermoforming technique was proposed and its feasibility was validated bynumerical and experimental research. Meanwhile, the causes and influence factors offorming error in MPTF were analyzed, and the forming error compensation method wasproposed and validated through experiments.The main contents and conclusions of this thesis are as follows: 1. Research on equipment and forming process of3-D surface multi-pointthermoforming for polymer sheetMaking full use of the discrete characteristic of MPD, flat polymer sheet can be formedinto various3-D shapes by MPTF. The design scheme of MPTF equipment was discussedbased on the structure and feature of MPD. Experimental MPTF equipment was developedand the principal of MPTF was introduced. The feasibility and advantage of MPTF weredemonstrated through a series of MPTF experiments conducted by using polycarbonate (PC)and poly(methyl methacrylate)(PMMA) sheets. The typical MPTF processes wereintroduced, i.e. one-time adjusting MPTF process and real-time adjusting MPTF process.2. Establishment of the finite element model of3-D surface multi-point thermoformingfor polymer sheetThe viscoelastic behavior of polymer sheet and hyperelastic behavior of elastic cushionmaterial were analyzed through stress relaxation and uniaxial tensile tests performed atelevated temperatures. The Generalized Maxwell model parameters of polymer sheets andMooney-Rivlin model parameters of elastic cushions were determined based on mechanicalproperty tests data. In addition, the friction coefficients between polymer sheets, elasticcushions and MPD were measured at MPTF processing temperatures. The basic theory andformulas of finite element analysis were described. The finite element model of MPTFprocess was established in Abaqus, and the element typeset, boundary conditions, loadingmethod of the finite element model were discussed in detail. The simulation results werecompared with the experimental results to validate the correctness of the numericalsimulations.3. Analysis and suppressing of the dimpling in multi-point thermoforming processIn MPTF, dimple on formed part induced by discontinuous forming surface of MPD is themain forming defect. According to the manifestations, dimples were distinguished as contactarea dimple and non-contact area dimple, and quantitative the characterization methods fortwo types of dimples were given. Numerical simulations were performed to analyze thecauses and influence factors of dimpling, and the dimple suppressing method was discussedas well. A series of MPTF experiments were carried out to validate the numerical analysis. Itshows that the section size of punch element mainly affects the severity of dimple, and thesurface quality of multi-point thermoformed part has been significantly improved by usingthe MPD consists of smaller punch elements. Placing an elastic cushion between polymersheet and MPD is the most effective way to prevent the multi-point thermoformed part from dimpling. The larger the punch element is, the thicker the elastic cushion is needed toeliminate dimpling in MPTF process.4. Research on real-time adjusting multi-point thermoformingTo improve the forming quality of multi-point thermoformed part, the real-timemulti-point thermoforming process was investigated. The thickness distributions of3-Dsurface polymer sheet parts formed using different real-time adjusting ways were analyzedthrough numerical simulations and experiments, and were compared with the single-timeadjusting multi-point thermoformed part. The comparison result indicates that the real-timeadjusting multi-point thermoforming process can improve the thickness distributionuniformity of the formed part. Numerical simulations were carried out to discuss theinfluence factors of wrinkling in MPTF, and the non-wrinkle limits were obtained accordingto simulation results. The deformation behaviors of polymer sheet in one-time and real-timeadjusting multi-point thermoforming process were investigated through, and the experimentsdemonstrate that wrinkling can be avoided in real-time adjusting multi-point thermoformingprocess.5. Forming accuracy analysis and forming error compensation of3-D surfacemulti-point thermoforming for polymer sheetThe forming accuracy analysis of multi-point thermoformed part indicates that theforming error of MPTF is mainly caused by the nonuniform thickness reduction of elasticcushion in forming stage and the shrinkage of polymer in cooling stage. The forming errorwas predicted through numerical simulation and was validated by experiments. A series ofnumerical simulations were performed to discuss the factors which affect the formingaccuracy of multi-point thermoformed part. Simulation results show that the formingaccuracy of MPTF decreases with the increasing of elastic cushion thickness, and the thickerthe polymer sheet is, the lower the thermal expansion coefficient and the higher elasticmodule of the polymer are, the higher the forming accuracy of the formed part will be. Aforming error compensation method for MPTF was proposed and was validated throughexperiments. The experimental results demonstrate that the forming accuracy of multi-pointthermoformed part can be significantly improved after forming error compensation.