Filament beslemeli 3D yazıcının nozul bölgesinin sonlu elemanlar yöntemi ile modellenmesi ve analizi
Finite element modeling and analysis of nozzle area of filament feed 3D printer
Yazar: OĞUZHAN EMRE AKBAŞ
Danışman: DR. ÖĞR. ÜYESİ ATAKAN ALTINKAYNAK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Konstrüksiyon Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: ABS = ABS ; Akış analizi = Flow analysis ; Filament = Filament ; Polimerler = Polymers ; Üç boyutlu yazıcı = Three dimensional printer

Özet:
Eklemeli imalat diğer bir ifadeyle katmanlı üretim veya 3D baskı ilk kez 1980'li yıllarda ortaya çıkmış modern bir üretim tekniğidir. Günümüzde yaygın olarak kullanılan eklemeli imalat, imal edilecek parçanın katman katman üretilmesi prensibine dayanır. Eriyik yığma modelleme, stereolithografi, seçmeli lazer sinterleme, seçmeli lazer ergitme, lamine nesne imalatı ve multijet modelleme endüstride başlıca kullanılan eklemeli imalat teknikleridir. Eriyik yığma modelleme tekniği, polimer malzemenin ısıtılması sonucunda nozulun ucundan tablaya serilerek parçaların katman katman üretilmesidir ve birçok farklı endüstride kullanılan en yaygın eklemeli imalat teknolojisidir. Eriyik yığma modelleme teknolojisine sahip yazıcılar fabrikalarda, araştırma merkezlerinde, okullarda ve evlerde dahi kulllanılmaktadır. Bu teknolojinin yaygın olmasının nedeni yazıcılarının kullanım kolaylığına sahip olması, taşınabilir olması ve diğer eklemeli imalat teknolojileri ile çalışan yazıcılara göre ucuz olmasıdır. Ayrıca kullanılan polimer malzemenin ucuz ve tedariğinin kolay olması da bu teknolojinin yaygın olmasında önemli bir etkendir. Eriyik yığma modelleme tekniği ile çalışan yazıcıların avantajlarının yanında bazı dezavantajlara da sahiptir. Bu yazıcılarda parçaların uzun sürede basılabilmesi ve basılan parçanın düşük mekanik özelliklere sahip olması gibi dezavantajlar mevcuttur. Parçaları daha kısa sürede basabilmek amacıyla yazıcıda polimer malzemenin eritildiği nozul bölgesinin optimizasyonu gereklidir. Nozul bölgesinin optimizasyonu için eriyik polimerin nozul bölgesindeki davranışını anlamak şarttır. Polimerler, uygunluklarından dolayı eklemeli imalat teknolojisinde yaygın olarak kullanılan malzemelerdir. Eklemeli imalat teknolojisinde polimerin bir türü olan termoplastikler kullanılmaktadır. Eriyik yığma modelleme tekniğinde ise termoplastikler filament şeklinde kullanılmaktadır. ABS polimer, Dünya'da birçok farklı sektörde tüketilen bir termoplastik çeşididir ve eriyik yığma modelleme tekniğinde ABS filamentlerin kullanımı oldukça popülerdir. Bu tez çalışması kapsamında nozul bölgesinin sonlu elemanlar modeli kurularak analizi yapılmış ve polimer eriyiğin nozul bölgesindeki davranışını anlama amaçlanmıştır. Nozul bölgesinin sonlu elemanlar modeli ve analizi COMSOL paket programında gerçekleştirilmiştir. Polimer eriyiğin davranışının analizleri için izotermal olmayan akış modülü ile olaylar modülü kullanılmıştır. ABS polimer, analizde filament malzemesi olarak atanmıştır. Analizler zamana bağlı olarak sırasıyla 1, 2, ve 3 dakika süreler verilerek yapılmıştır. Analizler sonucunda farklı sürelerde polimer eriyiğin merkezindeki sıcaklık değişimi incelenmiştir. Nozul bölgesinde polimerin merkezindeki sıcaklık değişimi üç sürede de aynı olmuştur. Polimerin merkezindeki sıcaklık, lineer olmayan bir şekilde yükselmiştir ve nozulun ucunda sistem sıcaklığına ulaşmıştır. Üç sürede de analiz sonuçlarının aynı olmasından dolayı iki dakikalık analiz seçilerek sonuçlar ayrıntılı incelenmiştir. Filament, nozul bölgesinde izotermal olmayan akış gerçekleştirmektedir. Sıcaklık, kanal duvarından filament veya polimer eriyik merkezine doğru azalmaktadır. Kanal girişinden 10,4 mm sonra filament akmaya başlamıştır. Nozul bölgesinde filamentin şekil değiştirme hızı kanal merkezinden kanal duvarına doğru artmaktadır. Filamentin şekil değiştirme hızı, nozul ucunun duvarında yüksek değerler almıştır. Kanal merkezinde filamentin dinamik viskozite değeri, duvardaki dinamik viskozite değerine göre daha yüksek çıkmıştır. Nozulun ucunda dinamik viskozite düşük değerler almıştır. Filamentin viskoz yayılma değeri kanal duvarından filament merkezine doğru azalmıştır. Bu yüzden filament sıcaklığı kanal duvarından filament merkezine doğru azalmıştır. Ayrıca, viskoz yayılma nozulun ucunda yüksek değerlere ulaşmıştır. Filament hızı, duvardan kanal merkezine doğru artmaktadır ve filament nozulun ucunun merkezinde ani bir şekilde yüksek hız değerlerine ulaşmaktadır. Nozul bölgesinde filamentin yarattığı basıncın düşüşü ısı bariyeri ve ısıtıcı blokta az iken nozulda yüksektir. Bu tez çalışmasında elde edilen sonuçların nozul bölgesinin optimizasyonu konusunda yararlı olacağı umulmaktadır.

Abstract:
Additive manufacturing, in other words, 3D printing, which is introduced for the first time in the 1980s, is a modern manufacturing technique. Nowadays, commonly used additive manufacturing is based on layer-by-layer producing principle of the objects. Additive manufacturing is used in many different sectors such as automotive, white appliances, health, dentistry, aerospace and defence. Fused deposition modeling, stereolithography, selective laser sintering, selective laser melting, laminated object manufacturing and multijet modelling are mostly used additive manufacturing techniques. Fused deposition modeling technique is layer-by-layer producing of parts form polymer material. In fused deposition modelling technique, polymer material is spread over the build platform from nozzle tip as a result of heating of polymer material. Fused deposition modeling, which is used in many different industries, is the most commonly used additive manufacturing technology. Printers, which have fused deposition modeling technology, are used in factories, research centers, schools and even homes because these printers have ease of utilization, are portable and cheaper than other additive manufacturing printers. Furthermore, polymer material is cheap and supply of this material is easy so this additive manufacturing technology is widespread. Fused deposition modeling printers have some disadvantages beside their advantages. In these printers, parts can be printed in a long time and printed parts have low mechanical properties. These disadvantages are still main problem for fused deposition modelling printers. Therefore, the optimization of printer's nozzle area is necessary in order to print parts in a short time. In nozzle area, understanding behavior of melt polymer is requirement for optimization of nozzle area. Stereolithography method is based on solidification of liquid resin by photopolymerization. Selective laser sintering is a powder bed fusion technique and laser fuses powder material according to CAD model of parts. Selective laser melting is similar to selective laser sintering technique and only steel and aluminium powder are used in this technique. Laminated object manufacturing is another additive manufacturing technology and sheet metal is shaped in this method. In multijet modelling technique, photopolymer material is sprayed and solidified by ultraviolet beam. Polymer filament is melted in nozzle area, also named as liquefier area. Nozzle area consist of main six parts. These parts are heater cartridge, thermistor, heat barrier, PTFE tube, heater block and nozzle. Heater cartridge has high electric power and increases system temperature in a short time. Thermistor controls system temperature and closes heater cartridge when system temperature is above intended temperature. On the other hand, thermistor opens heater cartridge when system temperature is below intendend temperature. Heat barrier minimizes thermal conduction above nozzle area and is made from stainless steel that has a low coefficient of thermal conductivity. PTFE tube is placed in heat barrier, possess a low coefficent of thermal conductivity and prevents polymer melting early. Heater block is made from aluminium material so heater block has a high coefficent of thermal conductivity. Moreover, heater cartridge and thermistor are embedded in heater block, heat barrier and nozzle are mounted in heater block. Polymers are generally organic materials. They have low density, low cohesion, high electrical resistance, high dielectric strength and are susceptible to oxidative degradation. Polymers are commonly used materials in the additive manufacturing technology due to their suitability. Thermoplastics, which are kinds of polymers, are used in additive manufacturing. Furthermore, thermoplastics are used in the forms of filaments in fused deposion modelling technique. ABS polymer, which is the most commonly consumed in many different industries in the world, is a kind of thermoplastic. In fused deposition modelling technique, use of ABS filaments are quite popular. Polymers are non-Newtonian fluids. Polymer melts have both viscous and elastic behaviour. Shear thinning behaviour are seen in many non-Newtonian fluids, in other words, they are pseudoplastic fluids. Dynamic viscosity decreases when shear rate increases in pseudoplastic fluids. Furthermore, various viscosity models were developed for non-Newtonian fluids. Cross-WLF viscosity model is one of these model. This model is used for calculation of dynamic viscosity in extrusion and plastic injection applications. Finite elements method, which is applied for solving partial differential equation, is a numerical technique. In this method, complex geometry is divided into small geometries that are called finite elements. Finite element method is commonly used in engineering problems. Nowadays, engineering problems are more complex so various finite elements programs are developed. COMSOL, which solve various pyhsical problems such as heat transfer, fluid dynamics and structural mechanics, is a finite elements program. In this study, finite element modeling and analysis of behavior of polymer melt in nozzle area of fused deposition modelling printer were carried out. The behavior of polymer melt is highly nonlinear and complicated in nozzle area. Therefore, finite elements modeling of behavior of polymer melt is quite difficult. Finite elements modelling and analysis of behavior of polymer melt were carried out in COMSOL. In COMSOL, nonisothermal laminar flow and events module were used for analyzes of behavior of polymer melt. Nonisothermal laminar flow module solves heat transfer and laminar flow together. In addition to nonisothermal laminar flow module, events module was used for modeling of thermistor function. In this thesis, first of all, the geometry of nozzle area was supplied from OO-KUMA Company. In Siemens NX, the design of geometry was revised, and later geometry was imported to COMSOL. In COMSOL, firstly, the function of shear viscocity was defined as a piecewise function. Later, materials were assigned for filament and each component of nozzle area. ABS polymer was assigned as filament material in analysis. Moreover, material of nozzle was assigned as brass, material of heat barrier was assigned as stainless steel, material of tube was assigned as PTFE and material of heater block, thermistor and heater cartridge were assigned as aluminium. The one of most important parts of finite elements analysis is to determine boundary conditions. In analysis, incorrect boundary conditions cause incorrect results so boundary conditions must be determined accurately. In COMSOL, the boundary conditions of laminar flow, heat transfer and events modules were defined for analysis of nozzle area. Initial velocity of filament, initial pressure, wall condition, inlet velocity of filament and outlet pressure boundary conditions were defined in laminar flow module. In heat transfer module, initial temperature of system, entry temperature of filament, heat flux and power of heater catridge boundary conditions were defined. Also, initial condition of heater cartridge, intended temperature range, condition of heater cartridge at low temperature and high temperature were defined in events module. After boundary conditions were defined, mesh was assigned to the geometry of nozzle area and flow channel. Free tetrahedral mesh type was assigned to the geometry of nozzle area and flow channel. Also, finer mesh was chosen as mesh density in analysis programme. Analyzes was performed as time dependent and 1, 2 and 3 minutes were given as time. In results of analyzes, temperature variation of center of polymer melt was investigated at different times. In nozzle area, temperature variation of the center of polymer melt was same at three different times. The temperature of the center of polymer melt rose nonlinearly and reached system temperature in nozzle tip. Moreover, 2-minute analysis was chosen and results of this analysis were examined in detail because results of analysis were same at three different times. Filament carried out nonisothermal flow in nozzle area. Temperature decreased from channel wall to the center of filament or polymer melt. After 10.4 mm from inlet of channel, filament started to flow. The shear rate of filament increased from the center of channel to channel wall in nozzle area and filament had high shear rate values at the wall of nozzle tip. The dynamic viscosity of filament was higher in the center of channel than the wall of channel. Furthermore, filament had low dynamic viscosity values at nozzle tip. The viscous dissipation of filament diminished from channel wall to the center of filament. Thus, the temperature of filament decreased from channel wall to the center of filament. The viscous dissipation of filament had high values at nozzle tip. The velocity of filament increased from channel wall to the center of channel and filament had high velocity values suddenly at the center of nozzle tip. The pressure, which was generated by filament, dropped slightly in heat barrier and heater block. Also, pressure drop was high in nozzle. It is expected that the results, which are obtained from this thesis study, will be useful for optimization of nozzle area.