Additive Manufacturing Technologies PDF

Tecnologias de Manufatura Aditiva 30 set 2024 AM is design! Challenge for now is understand why these 2 things are so connected and much more important than every other tech!! Also understand that designs and designers are associated with creativity p.e. Ferrari in Modena, kind of a museum → each c...

Tecnologias de Manufatura Aditiva 30 set 2024 AM is design! Challenge for now is understand why these 2 things are so connected and much more important than every other tech!! Also understand that designs and designers are associated with creativity p.e. Ferrari in Modena, kind of a museum → each cars have 6 engineers, see the evolution of the cars and the Ferraris is not only in the motor or in the wheels / breaks BUT in the design of the car!! Immediately after Senna’s death they modified cars in order to be faster and safer! Design has been always associated w/ engineering, today more than ever. Design is the tool which promote creativity 1  AM → design → creativity The only way to learn a tech is to use it obv Create the feeling of the big advantages is that you can create anything you want!! Having an idea and being creative, then w/ AM you can do whatever you want! Who change the world are those who have an idea! When I’m more creative? Sm says when they’re bored, or even when in an empty space, where you can stay alone and study → looking for inspiration. Design boom: designers are now using AM to construct houses, furniture… w/ different materials → opportunity for these techs!! You will find design boom on the internet, a lot of inspiring information p.e. volumetric AM These techs are bond almost everyday: lot of tech coming w/ variations The very base of AM is create layer by layer, atom by atom almost!! You could be able to put atoms exactly where you want them to be and create everything you can imagine. Connection w/ AI!! The original data is digital and AI works w/ digital → digital and AI are connected, sooner or later. CAP 1 1. Introduction and basic principles 1.1. What is AM? Cartoon: 3D printing, designation evolved: we are not using 3d printing as we used it at the beginning. All’inizio, infatti, una delle maggiori applicaizone della stampa 3D era nell’ambito del Rapid Prototyping, mentre ora pezzi stampati in 3D vengono direttamente immessi sul mercato e venditi al cliente o utilizzati come pezzi di ricambio. Is like a chain starting from raw materials, which needs to be circular, otherwise no sustainability!! Design for AM is one of the most important chapter of the course, it can allows you to do things never experimented before You need a simple printer and you can print whatever! P.e. prototypes, you can easily and fast design and redesign items in order to be better. → but today is not possible anymore to say that 3D methods are just used to create prototypes!!! Better to talk about ADDITIVEMANUFACTURING in a more generic way. Of course there are very expensive printer, but the basic ones are super cheap!! w/ a little amount of money you can have a lot of opportunities Even if AM is still a baby technology! Ok AM: plenty of definitions about the tech → definition can define where we are. One is: “A process of joining materials to make objects from 3D (three dimensional) model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies”. From ASTM international, organization for standardization. Standardization is key in Europe p.e. how can you do a part of a car if it’s not standard? This association manages all of these standards, even in different metrics (English ones are different from European ones) The norm is F2792 − 12a and it’s been redone a lot of time! Currently ongoing The world now is universal, you can sell everywhere in the world! Ok layer by layer is really important!! Really different from the traditional tech, where I’m removing, not adding!!! Benefit → reduce amount of waste, very good in companies today, since the relevance of sustainability and circular business models. Patents (brevetto): you have the data in digital form, using the data you produce and you need a computer to do it → more than 20 years old the tech, but earlier there were no tools!! Now you have Pc and tablets to create a digital twin. We are now spending too much producing in the traditional way → not sustainable anymore! AM can save raw materials and energy and everything important in the world and nature. Ideally in the future we will add atoms by atoms to create layers! Now you can use cells to construct tissue!! Even if they’re not vascularized p.e. artificial skin w/ the exact shape and thickness you desire is possiply created with this technologies. The tech is going to be everywhere in the future. OSSOk it’s not zero waste obv but compared to other tech the waste is minimum 2 AM Synonyms: additive fabrication, additive processes, additive techniques, additive layer manufacturing, layer manufacturing, and freeform fabrication → the last one is important: you don’t’ need a mold!! Unlike the metal production, when you pour liquid metal in a mold, w/ AM you don’t need a mold. The problem w/ molds is that they’re super expensive!! Also everything you need in the environment where you use the mold p.e. cold water has an impact on the total cost. You must have several molds also, just in case → high investment in general! BUT you print some materials, not every kind of material → one of the biggest limitations at the day. Principle: You have an object, you take a digital photo of that / a scan and you place the scan on a PC → then you transform the data in a file that your machine is able to read (usually you first have to convert the data in something a CAD software can read) CAD = computer aided design STL = is a file format native to the stereolithography CAD software created by 3d systems Ok recall: you need digital data, transform it in a STL file to produce the final product. Ok not always final item actually, sometimes you have to refine it, post- processing. OSS “3 Fs” of Form, Fit, and Function 1. The initial models were used to help fully appreciate the shape and general purpose of a design (Form) 2. Improved accuracy in the process meant that components were capable of being built to the tolerances required for assembly purposes (Fit) 3. Improved material properties meant that parts could be properly handled so that they could be assessed according to how they would eventually work (Function) Most AM processes involve, to some degree at least, the following eight steps as illustrated: 1. CAD: the output must be a 3D solid or surface representation; 2. STL convert: nearly every AM machine accepts the STL file format, which has become a de facto standard → this file describes the eternal closed surface of the original CAD model and forms the basis for calculation of the slices. OSS STL = stereolithography, an interchangeable file format that represents 3D surface geometry. 3. File transfer to machine 4. Machine setup: related to build parameters such as material constraints, energy source, layer thickness, timing… 5. Build: mainly an automated process, carried out without supervision 6. Remove from the AM room 7. Post-process: p.e. cleaning up, remotion of support parts → manual manipulation (cost and time, expertise) 8. Application OSS: AM machines require maintenance and a specific schedule of these actions; most processes use materials that can be reused for more than one build, maintaining for some cylces the original properties → ci piace, va incontro a quei principi di circolarità di cui sopra. OSS oltre al termine Additive manufacturing, come già detto, è possibile utilizzare dei sinonimi, ciascuno dei quali mette l’accento su una delle caratteristiche del processo e della tecnologia. Fra queste Freeform Fabrication: non ho bisogno di uno stampo e sono in grado di creare qualsiasi forma desideri (certo, ogni tanto mi serviranno dei supporti ma va bene lo stesso…). In questi casi si dice che AM is providing complexity for free. OSS attenzione che, in linea dii massima, 3D printing e Additive Manufacturing NON sono sinonimi: 3D priniting indica una specifica tecnologia / metodo di realizzazione dell’oggetto desiderato; invece, con il termine AM si indende una gamma molto più vasta di tecnologie. Ad oggi, cmq, il termine 3D printing è comunemente accettato come rappresentativo della tecnologia che si basa sull’aggiunta di materiali, layer by layer, piuttosto che sulola rimozione di materiale. Okay let’s classify the AM processes. AM processes shar ethe following commonalities: 3  - Use of a computer to store and process geometric information and to drive the fabrication - Deposition of feedstock, which is processed as points, lines or areas to create a part o Deposition volume element or voxel is small compared to critical features of the part o As such, AM processes do not require part-specific tooling There are also a lot of elements that can diversity the different AM processes → let’s have a look at the criteria that can be used for the classification. There are numerous ways to classify AM technologies. The most popular approach is to classify according to baseline technology: process uses lasers, printer technology, extrusion technology, etc. Another approach is to collect processes together according to type of raw material input. In general, these classifications simply di not work!!! a. Pham uses 4 separate material classifications: liquid polymer, discrete particles, molten material and laminated sheets: 1. Liquid polymers appear to be a popular material, like photo polymer liquid ones. 2. discrete particle systems (=powders graded into a relatively uniform size and shape and narrow distribution): the finer the particles, the better BUT there can be problems if the dimension gets too small in terms of controlling the distribution and dispersion 3. molten material systems: characterized by a pre-heating chamber that raises the material temperature to melting point so that it can flow through a delivery system; you can use 2 different nozzles to extrude both the material you want to create you object and the support structures needed for the creation of it. 4. solid sheet systems: the result is very like a wooden block (paper by paper) b. Hopkinson and Dickens proponed another classification: Not a standard classification, we are using this one cause it’s easier to understand, it makes sense, simple to explain. By machine, by material… different drivers you can use to classify the AM technologies. Ok this is not universally recognized but it’s fine for the course. The tech is so simple! 4 OSS Polymere vs metal vs ceramic vs glass: different structure, different atoms, different mechanical properties and applications, different chemical bonds, different processing… → all the materials used in AM are based on atoms of the periodic table. Glass has no crystal structure, atoms are randomly placed (vs gorilla glass, made to be unbreakable) Ceramics are made of silicon and aluminum → brittle, hard to shape… Metals are also very different from them! Chemicals bondings are one of the driver you can use to understand the differences between those materials. Ceramics, metal and glasses have a common thing: all of them requires very high temperatures p.e. 1300 C° → difficult to do AM at these temperatures → we can usually use polymers, easier to melt. Very advanced the tech w/ polymers, a little behind w/ others materials The final classification we are actually going to use during the course can be seen on the table below → 7 different processes families, with the corresponsive description, material used and specific names of the technologies within each family. There’s a paper on the Moodle, w/ a table describing the technologies AM uses → very important also when you have to choose which machine you should buy in your company: SLS and SLM are the 2 most wanted techs in the metal sector, belonging to the same group of processing → the way you process materials is the real driver in the table! Robocasting is ideal for ceramics. Unique and you should explore the uniqueness. 7 October 2024 HW: Identify the biggest fairs/ events for the industry in the world of AM → when they launch a new car they usually go to a fair or exhibition in which they show the last thing produced and they compete thanks to this. If it’s something relevant, than it’s a big business! In Europe we have Frankfurt, very practical cause the hub and he airport it has; in Italy depending for the type of industry we have districts, and the same happens in Spain, Boston, Texas, Brazil… Travel virtually and find which are the most famous fairs about AM. In 2025, which are the most relevant fairs? Where are the key places? Who is really being present and dominating the market? P.e. Reninshaw, AmGE are two of the most important actor → you should talk to them and get to know them. Design boom is a very interesting site → look at it! Last time we were talking about the classes of AM technologies: not a mess but very complicated → valid today, eventually there will be a revision, an update of that! 5 As we mentioned, this is her classification. We may classify AM techs p.e. by source, number of layers of deposits, materials, treatment of the layers after the deposit… there is a nomenclature to have in mind, the one you see on the tab. There are techs that use liquids, powder and others that use solids. Everything which is a resin is liquid, but also glass is included in this class. I can have plastic and ceramics in powder, instead. → very different techs!! They allow you to produce different type of products. The correct classification is a complex table: there are a lot of imaginative people in the world, they tempt to create names for techs already existing. So we really need to talk about patents!! When you have smt new p.e. design and you pay to put your name and especially to protect you and the thing you invented!! p.e. ABS: when we brake, and the soil is humid you could lose control → they developed ABS in order not to slip on wet soil; now is compulsory and common to have them. The company who developed ABS wanted to make money out of it → they wrote a lot of pages explaining the idea and then said ok this is my idea and none can use it for 20 years. You have to pay an office in order to be protected. BIG BUSINESS During these 20 years nobody in the world will do the same thing, but they will pay a lot to use the technology → uniqueness. AM was discovered 25 years ago, but it’s becoming common just now due to patents! When the patent expires you can do whatever you want w/ the technology. Patents are usually very complex, often modified by companies. Patents are relevant in this business → you need to pick up patents and understand them. Ok patents are public, in Europe we are ruled by the European patent office: if you google you will find a lot of patents falling into public domain. Reading patents can give you a lot of information, even if you can’t use the technology yet!! P.e. if you want to create a specific glass you can take a look at the gorilla glass’ patent and search for information here. Number of patents produced is always increasing in the USA, but constant in Europe → industry is not doing anything new!! Really bad. Ok table is organized is this way: we have the processes vs the technologies (be careful not to switch them) NB they tell you the form of the material, paste, filament… but also the type of material OSS there are always limitations we should try to solve. Benefits of AM What is it about AM that enthuses and inspires some to make these kinds of statements? AM is considered as a disruptive technology to produce limited number of high value components with topologically optimized complex geometries and functionalities that is not achievable by traditional manufacturing. In general: - You will save on materials waste and energy - Increased supply chain proficiency - You can also consolidate an assemble into a - Sustainable manufacturing initiatives single part - Manufacturing parts on demand - It is easy to change or revise versions of a - Quality improvements product - Increased supply chain proficiency - Demand continues to rise - Industrial efficiency - Immediate design revisions - Printing complete systems - Reduced LT - Component manufacturing - Full control over your design - Mass customization - Modifications and redesigns without - On-demand manufacturing penalties - Decentralized manufacturing A lezione sono emersi questi vantaggi dell’uso di una tecnologia additiva al posto di quelle tradizionali: - Design complex schemes - Minimal waste - Use of different materials - Achieve of different properties - No molds needed → freeform technology - Easy manufacture compared to the traditional manufacture (sni in realtà…) 6  - customization - rapid prototyping… Le slide mettono in evidenza nello specific questi 2 elementi. 1. Rapid character of this technology: speed advantage not just in terms of time taken to build parts speeding up on the whole product development process since we are using computers throughout since 3D CAD is being used much. Less concern over data conversion or implementation building within an AM machine is generally performed in a single step just as 3D CAD:what you see is what you get → what you see is what you build 2. Reduced resources: Number of processes and resources required significantly reduced when using AM, especially compared with moulding, carving, CNC… Legacy (eredità): many companies have made investments in AM, to change tech you need a big drive. When there’s a change there’s also always risk: we are naturally averse to risk. I have to know how to mitigate the risk: what I’m going to do to minimize the impact of the risk? Nothing is sure! OK SO THERE’S A RISK AND IT’S IMPORTANT TO HAVE MITIGATION TOOLS. What is so special w/ this technology?  this image is super important: the complexity increases w/ axes: the more complex the part is and the more it costs if we are using traditional methods. AM costs the same even if the parts are complex, instead for TM the cost is exponential! Ok in better words: this graph represents the complexity of a part vs the cost → if we use TM p.e. injection molds, as the complexity increases, the cost increases exponentially too! There are also some kind of complexity that you cannot reach w/ TM (e.g. cavità) If you want to produce a cube w/ Am the cost is enormous (machine, tech, material). But if you do a sponge inside of the cube then you have complexity for free!!! You can never do such a thing w/ TM!! Some machines are so sophisticated that can cost even millions of euro → optimize!!! This is the most important graph: if you produce parts very complex, w/ AM complexity is for free! What is rapid prototyping? Fast testing, you can directly create the model! 3D thing in seconds. You can test immediately what you have created! The car industry is very important in the AM world: they use a lot this tech, also for customization of cars! Easy customization is extremely important too! Key 5 benefits of AM compared to traditional manufacturing (according to Wholer’s report). AM is changing also the supply chain, reducing time of transformation in different industries. 7  We are now into and towards the circular economy, but there are also a lot of disadvantages w / AM: - Slow build rates - Questionable accuracy - Support material removal - Considerable effort required for application design and for setting process parameters - Cost: materials and equipment - Material properties / limited choice of materials - Intellectual property issues - Limitations of size - Discontinuous production process - Limited component size - High initial investment and required maintenance expertise Applications Applications are endless, huge amount of them p.e. blades (turbine) of airplanes OSS the design of Am can give you properties TM can never! P.e. density and porosity, the last one is now an excellent asset! Even in metals, if you simulate you can avoid creeps and fractures in metal. Metal AM is the future of the technology: you can simulate, understand the property, and create the object. Also the construction sector is developing techniques to use AM technology in order to make houses / buildings. P.e. you can now see some houses in Ethiopia made w/ 3D printers by Italian companies. Another huge application is associated w/ body and health: implants exactly fitting, some dentists now print teeth! Crazy Americans are printing hearts from a particular material than allows vascularization! Printable electronics are the future: everything flexible is easier to make w/ AM.  we have expectations and we are learning about the reality: this graph is valid for everything new → time and visibility on the axes. When something is new it becomes super visible, people really talk about it, becoming particular relevant. When everybody starts having it and talking about it you realize it’s not so important / relevant → the visibility starts to fall down. Many techs die here! People were expecting something which is not real. When you resolve problems and see opportunity the tech can increase again it’s visibility → that’s exactly what happened w/ 3D printing We are now in the line of illumination, even ifwe have a lot of problems to solve still. 8 p.e. look for the history of airplanes and see how they follow the line of the graph. Many many companies are now investing in the technology and understanding the power of AM and its opportunities. Ok but why AM is not fully adopted yet? A lot of advantages, as we mention, however AM is a tool that is allowing you to develop parts, object.. w/ new material, shapes and application, some not possible today w/ traditional tech → design for AM, not only a technological thing. The quality of what you produce can be not the best, since you produce layer by layer: you can see the connection between layers, so you can have some mechanical problems. 14 October 2024 If we do not have people who design, update software, materials and so on WE DON’T EVOLVE!! We really need to improve the technology to evolve, but it usually goes with very high costs! Everything has a life cycle and you need to know everything about it and it’s LCA. Functional graded materials: a combination of materials used would serve the purpose of a thermal barrier capable of withstanding a surface temperature of 2000K and a temperature gradient of 1000K across a 10 mm section. Uniqueness of this technology! The uniqueness is something we should also find in ourselves. Oaky we discussed about an important CURVE, AYTHING NEW has a huge expectation and then suddenly we realize it’s not exactly like that and the tech faces a crisis and sometimes it dies. Some other times you can overcome the disillusion and improve the performance of the tech, also reducing its costs. Best way to test if you know about a topic is talking with other people about that topic and do the exercises you find at the end of the chapter. → do them also for the exam, I think something will be in the test. Try to find video about AM technologies (for example the ones in the tab). Mapping to structure ideas is very important: you post the idea, writing simple sentences → our mind is structured in small and simple sentences. Mind mapping is not a structured exercise, but chaotic at the beginning → you just list information related to the topics you are facing with and after a first moment of chaos you make order in the ideas. 9 CAP 2 & 3– Development of Additive Manufacturing technology In order to be able to understand why Ramingshaw is using 6 lasers we are going to introduce something we already know: we need lasers to have SLS/SLM technologies. Laser tech can cut a lot of things but also burn! In the middle between cutting and burning there’s exactly what we use to produce 3D items. Computers have a huge control onto these machines: they cost millions of euros because they’re very complex!! The most expensive part of the technology is probably the computer who have control over sensors constantly. Lasers are not only used to melt but also used to control parts! They can redirect mirrors and so on → laser tech is a very important one. Oss rarely use the word 3d printing, but only printing in a general sense Without computers there would be no capability yo display 3D graphic images. Without 3D graphics, there would be no Computer-Aided design. It is safe to day that without the computers we have today, we would not have seen AM develop. → computers can perform tasks in real-time! AM technology requires precise positioning of equipment similar to a computer numerical controlled machining center, or even a high-end photocopy machine or laser printer. Controllers that take information from sensors for determining status and actuators for positioning and other output functions. Computation is required to determine the control requirements: tasks in real-time, positioning of motors, lenses, etc. Computers oversee the communication to an from these controllers and pass data related to the part build function. AM primarily makes use of the output from mechanical engineering, 3D solid modelling CAD software; CAD ensures that al models made have a volume and, therefore, by definition are fully enclosed surfaces. It is necessary to manipulate the CAD file to generate the slice data that will drive an AM machine → direct slicing The generic file format is called STL. We need very advanced computers and a processing power, we need to be connected to internet to elaborate those big big files! We also need graphic capabilities: eventually the degree of accuracy of the printed part are not what we want. Sometimes the graphs / design is not perfect and we need to control it by controlling the machines. This techs are sophisticated Understand that we have a very top technology, and then a lot of small modification of it to create something that could be seen as something completely different (but it’s actually not). Everything is integrated: integration also between traditional tech and AM! Are we going to have any hybrid tech? a mix between AM and traditional? STL means stereolithography: first tech invented → standard tesselation Language. The CAD converts a design in something the computer can print. ? what is a pixel: usually linked with 2d world. What is the designation of pixel in 3d world? Voxel Okay we are talking about voxels in this course. This is what differentiates techniques, you can have a best or worse precision based on the number of voxel. You need always to move x, y and z; not always in the smallest movement possible anymore: you can have largest movements when you print. Materials for AM Critical to the selection requirements for AM is the need for appropriate materials. Materials requirements for AM: - Ability to produce the feedstock - Amenable to the specific AM process - Suitable processing of the material by AM - Capability to be acceptably post-processed to enhance geometry and properties, and manifestation of necessary performance characteristics in service. As AM matured, specific classes of material have become associated with specific AM processes and applications_ 1. Polymers 2. Metals 3. Ceramics and composites OSS microstructural features affecting AM part properties are relevant We can print more or less all the materials; metals, polymers, ceramic, glasses… not a single class of materials is missing BUT the tech you use for each of them are different! 10  Many people englobe glasses into the ceramics and composites class but it’s an open discussion. This is an important graph: it’s unfair to compare AM with traditional technologies; but both produce things so they’re in the same graph here. Also unfair cause IM is a mature tech!! it started with plastic and moved to metal and ceramics → you can now produce both small and big pieces, with a various of complexity. Probably the red ball will grow a lot! Questo diagramma può guidare le decisioni per scegliere la tecnologia più appropriata in base al progetto specifico. Ad esempio: - Se hai bisogno di pochi pezzi complessi, AM è la scelta migliore. - Se devi produrre componenti microscopici, opta per MI. - Per produzioni di massa di grandi quantità, IM è imbattibile Polymers in AM are already good friend of the technology (ABS, Acrylics, Celulose…). We can also print metals, using different techniques as you can see in the graph below: OSS with investment casting we produce parts and then we assemble them → costs With AM you can directly produce the whole item! Why I can not print all the material? For example why not every metal available? Do we need a metal who don’t oxydite? Or with particular specs? Patterns are still coming out and try to produce new materials for AM. Ceramics are a relatively new group of materials that can be used for AM with various levels of success. The particular thing to note with these materials is that, post AM, the ceramic parts need to undergo the same processes as any ceramic part made using traditional methods of production – namely firing and finishing. AM of ceramics has been demonstrated to be successful, through direct and indirect methods. Still, the growth of ceramics is not comparable with the growth of metals in the years!! The ceramic is not melted, we have a powder in a liquid / melted form. If we look we have a lot of printing attempts and the majority of the good ones are indirect methods! Sintering can be very interesting. Okay so we can recapitulate: There’s different ways of classification of AM processes: based on the material form, the ASTM table and so on (the last one is what we should use). 21 oct 2024 We mention many times what is the classification we are using for AM technologies. To facilitate the norm Is really important: 4/ 5 years ago we were not talking about the exact same thing → we can classify tech in many different ways p.e. based on the form of the material we are using. Companies make decision based on something we eventually don’t know p.e. if it’s not profitable we are not doing it! Market and competitors are essential to be understood: some companies look at the landscape and see where 11 competitors are working to decide in which segment to work p.e. if everyone is producing equipment for plastic AM I’m probably going to produce other technologies!! Everyday there’s something new, new ideas and new technology →huge EVOLUTION! If you look at the usual table: group of technologies with the specific terminology, information associated with the description of the tech and the materials we are using for each of them. Let’s go through the most important of those technologies. Liquid polymer systems Mainly based on polymers; relevant because one of the first tech. Sometime called stereolithography, invented by an American professor able to create a machine for this and patent the idea → much money!! We have a liquid polymer (vat polymerization), that liquid became solid thanks to a laser, which is guided to be in the exact place we want the piece to be formed. We can polymerize the exact shape we want!! Still one of the tech who gives us the best surface accuracy (finishing) → one of the uniqueness of the tech!! But we should talk also about the toxicity of the resin used. If I have liquid, I have more freedom to put the particles where I want (more than if you think about solid materials): manipulating the object we are constructing → the surface is more perfect because I’m starting with a liquid!!! Usually, we use optical wavelength optical curling methods: although resins can be toxic, the wavelengths we are using is very simple to use → advantage. Companies really like simplicity! Main thing is light, used at room temperature, pressure and humidity; easy and simple Ok you have a container, you put inside the liquid and use a beam to polymerize; the beam have a dimension and a direction (thanks to a mirror) and design within the liquid the object. Extremely simple tech! Companies are able to do even similar things that slightly change the equipment needed p.e. vertical vs horizontal construction BUT those are simple commercial details → we need to understand the physical principle. Many times we need support structures while we are printing the object, also because when the liquid became solid its density increase Plenty of movies on the internet explaining how SLA works Usually this vat polymerization is for relative small objects, cause it takes a lot of time (pensa quanto te ne servirebbe se fossero oggetti enormi!). in general, there are no many tech that can allow us to print very big objects. Discrete particle systems (powders) Every kind of powder can be used (metallic, ceramic, polymeric…) Powders, why? Even better than resins cause they’re easy at transportation than a lquid solution → easy to mix and obtain an homogeneous mix (si e no questa utimissima cosa): we here just need simple big bags to transport powder, while we need specific containers to transport liquids. But you can also easy mix different powders to make a composite and use it. This composite does not usually occupy much space, and we like this specs. Stock Space = money!! That’s why we prefer this tech o the VAT polymerization, when possible. Also pallets are considered powders When we use this tech? SLS is very used Sintering is a consolidation process, that requires very high temperature (>1000°C) → we do it only if it’s fundamental due to the amount of energy required. With this technique we guarantee that powder grains are really together, so the object will be hard enough. The temperature of the laser is very very high There are different kinds of laver, you should choose the best one for the powder material you are using. Combines the advantage of powder BUT you need very expensive lasers to do that We have again a container with powder, we collect the excess after the laser melt the part we need to construct the object Not exactly a zero waste tech but surely not many of the powder are waste. The problem is normally the fact that it’s one of the most expensive tech existing! Limitations? This tech seems very similar to the first one if we just see the videos! We have a pc and a beam that designs something in liquid or powder, layer by layer. 12 The beam can be light or can be a laser!! The power of the light is much different between the two techs. Also the container is different: when I have liquid I need a close container, while when I’m using powder I can also have just a table. The laser has a temperature, so also the particles not directly colpite dal laser, ma appena vicine queste ultime non saranno più a temperature ambiente → non puoi pensare di usarle ancora !! ecco perché comunque un po’ di sprechi ce li hai. In the electron gun tech we have a chamber, closed and under vacuumed to control the electrons Electron gun don’t need lasers or a mirror to reflect, if you have a very accurate beam the contour of the object are super defined, precise. They can define the accuracy in a super precise way with this kind of electron gun. Usually you need also a minimum cleaning process after the printing okay maybe it’s just a small cleaning but you have to do it. Binder jetting tech is simple: there is a binder and it’s jetting. This is very interesting p.e. for the ceramic world We have a bed with powders, some rollers and a head which is dispersing drop by drop a binder. It’s different from sintering. This is also called 3DP (3D printing) Let’s compare those 3 powders technologies. Common things: they’re made layer by layer; set-up is similar: we have rolls! A table and a roller who pushes the powder on the table; than you can have a laser or a binder jetting. Okay the common is the starting point, the powder bed. We can also decide the thickness of the layer depending on the dimension of the grain. Obv the finer the grain and the better the surface accuracy will be → you can manipulate everything in the machine, also the velocity of the rolls. Differences between high and low-cost car: one is the noise inside the car. Usually in very expensive cars the noise problem is better controlled: the same is here → if you control everything since the very beginning you can have an excellent result in the end. Molten material system (FDM and FFF) We melt material: the tech is super easy and also cheap We melt and we do a sort of extrusion Very common for polymers but also for glasses nowadays! FDM of glasses is a new way in industry. Surface will not be smooth at all!!! The thickness of our layer is defined and the porosity is also very high Okay very very simple tech but limited to the type of material we can use. One of the tech we can use quit every type of materiasl, a lot of polymers But no post- processing! And it’s strange, cause the surface finiture is very low. Teher are companies try to use a tech that allows you to smooth the surface with ultrasounds. 13 28 October 2024 Chapter 4 – VAT photopolymerization / Stereolithography (SLA) In the last class we were presenting some general technologies, based on the classification we use We have for example, solid sheets systems. In LOM (cerca quale sia il pdf corretto, questa parte Non è nel cap 4; sono nella parte finale del cap 3) tech the material is sheet (usually metallic sheet, very thin): the system is very sophisticated and cut the material exactly the shape we want → after printing another sheet come and you obtain layers. Although these techs are based on the same approach, you can have fresh views on the processes and problems of the topic, since we never worked with those techniques before. In those techs, by the way, you have a lot of waste!! You have huge sheets, with a laser you cut the shape you desire and throw away everything else → big waste, bigger than the one in other techs we already spoke about. Okay let’s talk about photopolymerization processes. Vat photopolymerization / SLA process includes: - SLA (Steriolithography) - Micro – SLA - DPL (Digital Light Processing) - CLIP (Continuous Liquid Interface Production) Photopolymerization processes use liquid, radiation curable resins or photopolymers as their primary materials. Most photopolymers react to radiation in the UV range of wavelengths, but some visible light systems are used as well. Upon irradiation, these materials undergo a chemical reaction to become solid. This reaction Is called photopolymerization, is typically complex, involving many chemical participants. SLA is one of the oldest forms of 3D printing. Using a laser, it delivers a concentrated amount of light to induce photopolymerization of resin monomers in a given spot. By sweeping the laser across a vat of resin, a layer of the print is drawn in accordance to the design specifications. The laser scanning of the part is the phase that actually solidifies each slice in the SLA machine! After building the part, the part must be cleaned, post-cured and finished. During either the cleaning and the finishing phase, the SL machine operator may remove support structures. During finishing, the operator may spend considerable time sanding and filling the part to provide the desired surface finishes. There are 2 basic designs of SLA printers: 1. The laser is positioned above the vat and points down into the resin 2. More frequently, however, the laser sits below the vat and points upward into the resin Typically, the laser does not shine directly onto the resin, but is instead deflected off a rapidly moving mirror galvanometer that directs the beam to the appropriate point. When the laser finishes printing a layer, the platform holding the printer structure moves away from the laser by one layer height, allowing the next layer to be cured. Concept and definitions Vat polymerization, the oldest of the commercial AM processes, involves photolithographic crosslinking of liquid thermoset polymer resins to form a solid In vat polymerization and photopolymer-based material jetting, the feedstock must be a liquid thermoset plastic monomer that will crosslink when exposed to the appropriate electromagnetic radiation. OSS vat polymerization is limited to photosensitive thermosets. 14 Various types of radiation may be used to cure commercial photopolymers: - Gamma rays - X- rays - Electron beams - UV - Visible light In SL systems, UV and visible light radiation are the most commonly types several configurations were developed for photopolymerization processes: 1. Type 1 – vector scan, or point-wise, approaches typical of commercial SL machines 2. Type 2 – mask protection, or layer – wise, approaches that irradiate the entire layers at one time 3. Type 3 – two-photon approaches that are essentially high resolution point-by-point approaches. What is a photopolymer? A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the. Material occurs as a result of cross linking when exposed to light. SL photopolymers are composed of several types of ingredients: - Photo initiators - Reactive diluents - Flexibilizers - Stabilizers - Liquid monomers When UV radiation impinges on SL resin, the photo initiators undergo a chemical transformation and become “reactive” with the liquid monomers. A “reactive” photo initiators reacts with a monomer molecule to start a polymer chain. Subsequent reactions occur to build polymer chains and then to cross – link (creation of strong covalent bonds between polymer chains). OSS polymerization of SL monomers is an exothermic reaction, with heats of reaction around 85kJ/mol. Despit ehigh heats of reaction, a catalyst is necessary to initiate the reaction (photo initiator). SLA vs DLP Common philosophy - both rely on the use of light, typically in the UV region of the spectrum, to cure a photosensitive viscous resin - typically composed of epoxy or acrylic and methacrylic monomers, will polymerize and harden when exposed to light - both are resin-based technologies. The differences: - DLP processing projects each layer, creating an illuminated plane where photopolymerization will occur - The moment the light hits the resin, it’s not restricted to a single spot as SLA, instead, the whole layer is formed at once - Here, patterning of the illumination is critical to achieve the desired shape for each layer - This is achieved with a mask produced by a digital micromirror device (DMD): a dynamic mask composed of an array of rotating micrometer-size mirrors that reflect the light into or away from the resin. This allows for the differential illumination and polymerization of the resin at different locations within the layer. - Modern DPL projectors typically have thousands of micrometer-size LEDs as ligh sources. Their “on” and “off” states are individually controlled and allow for increased XY resolution. 15 Photo means light → We use light or an electromagnetic ray close to the light length They have a very impressive surface accuracy, and it’s easy to understand why if you think about the process of printing. VAT photopolymerization / SLA is one of the commonest. We have layers of resin, sensible to light. Do we have waste issues in VAT? If resin is affected by light yes → we can reuse it or should throw it away after one use?? This is a good question for people who work with AM. If my laser shakes this will affect in a bad way my final product → mirror systems guarantees that laser don’t shake and it’s super precise, impossible to do it by hand obv! The most use light is UV but it can be also something else, it depends from the target Why the object is coming up and not down in the VAT? It’ s a question of density, in this way you can remove the excess of material, while using gravity will make it harder. The construction of the machine accords with physical principles. Ci sono anche un po’ di diagrammi che ha senso mettere qui Why the process is so costly? What part does cost the most? The mirror is absolutely fundamental: I’m not complicating the system for nothing, the. Mirror guarantees that the laser moves exactly in the direction we need to shape the resin; you will not able to focus with the same precision the laser without it!! Conclusions Photopolymerization processes make use of liquid, radiation-curable resins called photopolymers to fabricate parts. Upon irradiation, these materials undergo a chemical reaction to become solid Several methods of illuminating photopolymers for part fabrication are available: - Vactor scan point - wise processing, used with UV lasers in the SL process; - Mask projection layer – wise processing, DLP micromirror array chips are commonly used for mask projection technologies; - Two-photon approaches, which have the highest resolution, remain of research interest only. Photopolymerization processes lend themselves to accurate analytical modeling due to the well-defined interactions between radiation and photopolymers. SLA fabricates 3D objects by selectively solidifying the resin trough photopolymerization initiated by absorbing light energy. Currently there are 4 generations of SLA. SLA is able to build 3D objects by curing each layer simultaneously instead of tracking the contour profile of each layer, while DMD brings high resolution and production rates to stereolithography. Although evolving the current and future needs include: high resolution, high print speed, bigger sizes One of the major constraints are materials: specific materials with the required properties are needed. 16 Cap5 – Powder bed fusion (PBF) Powder Bed Fusion process includes: - SLS (Selective Laser Sintering) - SLM (Selective Laser Melting) - EBM (Electron Beam Melting) General information Powder bed fusion (PBF) entails surface exposure of a powder bed to a heat source to enable binding. The heat source is typically a laser or electron beam, although intensive light sources have been used with physical or chemical “masks” to allow all voxels on an entire surface to be processed simultaneously; Powder bed fusion PBF, due to minimal constraints on feedstock type for manufacturing, is popular for part manufacture for service applications. Developed at the University of Texas at Austin, USA, Selective Laser Sintering (SLS) was the first commercialized powder bed fusion process. Powder bed fusion of polymers is mostly limited to semicrystalline materials, but a few amorphous polymers such as polystyrene are processable. The difference in melt temperature on heating and the crystallization temperature on cooling should be large For metal powder bed fusion, the general rule is that metallic alloys which are suitable for casting or welding can be processed successfully using AM Question: why is PBF most limited to semicrystalline materials? Basic method of operation is shown beside. All other PBF processes modify this basic approach in one or more ways to enhance machine productivity, enable different materials to be processed and / or to avoid specific patented features. Powder bed fusion processes in laser-based AM have received significant attention for advanced engineering materials because of their higher cooling rate and better surface finish compared with other additive manufacturing processes. All PBF processes share a basic set of characteristics, these include: - one or more thermal sources for inducing fusion between powder particles - a method for controlling powder fusion to a prescribed region of each layer - mechanisms for adding and smoothing powder layers The SLS process was originally developed for producing plastic prototypes using a point-wise laser scanning technique. This approach has been extended to metal and ceramic powders. Additional thermal sources have been utilized and variants for layer-wise fusion of powdered materials now exist. 17 PBF processes are widely used world-wide, have a broad range of materials: polymers, metals, ceramics and composites. Increasingly being used for direct digital manufacturing of end- use products, as the material properties are comparable to many engineering grade polymers, metals, and ceramics. Working Principles The laser-based powder bed fusion process is schematically illustrated in the following Figures: 1. A laser beam scans at a controlled speed the selected locations of the powder bed and fuses the powder to the solid material underneath by either full melting [selective laser melting (SLM)] or partial melting [selective laser sintering (SLS)] 2. The powder bed is lowered by the defined layer thickness and a new layer of powder is dropped and levelled after laser radiation in one layer is completed 3. The process repeats until the part is completely built 4. The laser scanning path in each layer is defined by the part geometry at the corresponding z location and the selected scanning strategy The built chamber is protected with the flow of inert gas (argon, nitrogen) during processing to prevent oxidation. Because the gas flow also helps to remove the condensate that is produced by melting the powder, homogeneous gas flow across the build area plays an important role in the quality and properties of SLM-processed parts. Based on the process description, it is important to achieve a homogeneous thickness of powder in each layer to control the quality of the part built by powder bed processes. Various methods have been developed to provide and level the powder in each layer before laser scanning. Sufficient powder for building each layer can be supplied by dropping a controlled volume through a recoater or a hopper, or by lifting the feed cartridge to a controlled height. The powder is then spread uniformly over the build platform by a counter- rotating roller, wiper or doctor blade. A powder delivery system should maximize the ability of the powder to flow, minimize the formation of a particle cloud and minimize the shear forces over the previous layer of the build. 18 SLS (Selective Laser Sintering) SLS fuses thin layers of powder (~0.1 mm thick) which have been spread across the build area using a counter-rotating powder leveling roller. The part building process takes place inside an enclosed chamber filled with nitrogen gas to minimize oxidation and degradation of the powdered material. The powder in the build platform is maintained at an elevated temperature just below the melting point and/or glass transition temperature of the powdered material. Infrared heaters are placed above the build platform to maintain an elevated temperature around the part being formed; as well as above the feed cartridges to pre- heat the powder prior to spreading over the build area. In some cases, the build platform is also heated using resistive heaters around the build platform. This pre-heating of powder and maintenance of an elevated, uniform temperature within the build platform minimizes the laser power requirements and to prevent warping of the part during the build due to nonuniform thermal expansion and contraction (curling). Once an appropriate powder layer has been formed and preheated, a focused CO2 laser beam is directed onto the powder bed and is moved using galvanometers in such a way that it thermally fuses the material to form the slice cross-section. Surrounding powder remains loose and serves as support for subsequent layers, thus eliminating the need for the secondary supports which are necessary for photopolymer vat processes. After completing a layer, the build platform is lowered by one layer thickness and a new layer of powder is laid and leveled using the counter-rotating roller. The beam scans the subsequent slice cross-section. This process repeats until the complete part is built. OSS A cool-down period is typically required to allow the parts to uniformly come to a low-enough temperature that they can be handled and exposed to ambient temperature and atmosphere. If the parts and/or powder bed are prematurely exposed to ambient temperature and atmosphere, the powders may degrade in the presence of oxygen and parts may warp due to uneven thermal contraction. Finally, the parts are removed from the powder bed, loose powder is cleaned off the parts, and further finishing operations, if necessary, are performed. Powder fusion mechanisms 4 different “fusion” mechanisms which are present in PBF processes: 1. Solid-state sintering 2. Chemically-induced binding 3. Liquid-phase sintering 4. Full melting OSS most commercial processes utilize primarily liquid-phase sintering and melting. 1. Solid-state sintering Sintering, in its classical sense, indicates the fusion of powder particles without melting (i.e. in their “solid state”) at elevate temperatures. This occurs at temperatures between one half of the absolute melting temperature and the melting temperature. 19 The driving force for solid-state sintering is the minimization of total free energy (Es) of the powder particles. The mechanism of sintering is primarily diffusion between powder particles. 2. Chemically induced binding Chemically induced sintering involves the use of thermally activated chemical reactions between two types of powders or between powders and atmospheric gases to form a by-product which binds the powders together. This fusion mechanism is primarily utilized for ceramic materials!! Examples of reactions between powders and atmosphertic gases include: - laser processing of SiC in the presence of oxygen, whereby SiO2 forms and binds together a composite of SiC and SiO2 - laser processing of ZrB2 in the presence of oxygen, whereby ZrO2 forms and binds together a composite of ZrB2 and ZrO2 - laser processing of Al in the presence of N2, whereby AlN forms and binds together the Al and AlN particles. One common characteristic of the chemically-induced sintering is part porosity. As a result, post-processing infiltration or high temperature furnace sintering to higher densities is often needed to achieve properties useful for most applications. This post-process infiltration may involve other reactive elements, forming new chemical compounds after infiltration. NB The cost and time associated with post-processing have limited the adoption of chemically-induced sintering in commercial machines. 3. Liquid phase sintering and partial melting LPS is arguably the most versatile mechanism for PBF! LPS is a term used extensively in the powder processing industry to refer to the fusion of powder particles when a portion of constituents within a collection of powder particles become molten, while other portions remain solid. In LPS, the molten constituents act as the glue which binds the solid particles together. As a result, high-temperature particles can be bound together without needing to melt or sinter those particles directly. LPS is used in traditional powder metallurgy to form, for instance, cemented carbide cutting tools where CO is used as the lower melting-point constituent to glue together particles of WC. 4. Full melting Full melting id the mechanism most commonly used associated with powder bed fusion processing of engineering metal alloys and semicrystalline polymers. In these materials, the entire region of material subjected to impinging heat energy is melted to a depth exceeding the layer thickness. Thermal energy of subsequent scans of laser or electron beam is typically sufficient to re—melt a portion of the previously solidified solid structure; and thus, this type of full melting is very effective at creating well-bonded, high- density structures from engineering metals and polymers. Processing parameters in the PBF process - Layer thickness t - Laser power P - Laser scanning speed v and scanning path strategy - Hatching phase h and laser spot size d - Particle size and distribution - Platform pre-heating temperature - Laser beam scanning strategy 20 As we can see in the figure below: Almost all metals can be processed by SLM. However, processing windows and, consequently, their difficulty can vary because of differences in chemical composition, laser absorption, surface tension and viscosity of the melt and thermal conductivity. 1. Definition of the combined processing parameters: a number of key variables defined by the processing parameters can be used to characterize the process (p.e. volumetric energy density, linear input energy density,…) 2. Morphology and size of particles: the morphology and size of feedstock (MP) powders are important factors for PBF processes because they affect powder flowability, laser energy absorption and the thermal conductivity of the powder bed. A higher packing density of the powder bed is preferred for powder bed fusion processes because of the lower internal stresses, part distortion, porosity and surface roughness in the built part. The packing density of the powder bed is significantly affected by the particle morphology, size and distribution. A high powder bed packing density can be achieved with a wider distribution of particle size, as the gaps between larger particles can be filled with smaller particles. A spherical particle morphology improves flowability of powders to achieve a high packing density in powder bed, which improves final quality of SLM processed parts! On the other hand, flow of powder with a non-spherical shape is obstructed because particles tend to interlock mechanically and entangle with each other. Hence, non – homogeneous layers of powder with a varying packing densities form on the top of the previously built solid surface, which may lead to the formation of defects such as porosity and / or incomplete melting. A reduction in particle sieze results in: An increase in surface area, which favors the absorption of laser energy to increase the melt pool temperature An increase in the gap in the powder bed, which may lead to high porosity in the consolidated part if the gap is too large An increase in the tendency for particles agglomeration A reduction of powder flowability The thermal conductivity of loose powder is primarily determined by the packing density of the powder bed. In general, the thermal conductivity of powder beds formed by irregular particles or by those with a wide size distribution is higher than that formed by mono-disperse spherical powders at the same density. The incident laser power is absorbed by the powder layer by multiple reflections through the gaps among the particles. Hence the absorption of laser energy by the powder bed is significantly higher than the absorption by a flat surface of a solid metal with the same composition. The effect of multiple reflection on absorption is more significant for highly reflective metals (aluminum and copper) than for moderately absorptive metals (iron and titanium). The dependence of absorptivity on powder array and powder feed system is more pronounced for highly reflective metals. The percentage of the incident energy absorbed by the substrate of the previously built solid underneath the powder layer decreases with an increase in the optical thickness of the powder layer and decrease in particle size. 3. Characteristics of the melt pool A very small melt pool id produced as a result of laser radiation on the surface of the powder bed during SLM.it ahs various characteristics: a. Melt pool temperature: maximum temperature significantly increases with a general increase in laser power or linear energy density, but it slightly decreases with an increase in laser scanning speed; b. Temperature gradient: temperature gradient in the melt pool increases linearly with an increase in the applied laser power. The temperature gradient is more pronounced in materials with low thermal conductivity 21  c. Melt lifetime: liquid lifetime is the duration from the time power particles in the local region start to melt until they eventually solidify. It increases with an increase in the laser power and a decrease in the laser scanning speed; d. Melt pool dimensions: including length, width and depth increase with increase laser power e. Melt viscosity: decreases with the increase in energy density and increase in melt pool temperature. Low viscosity for melt pool be spread properly on formerly processed layer and high enough to prevent balling; f. Melt flow: mass transfer in melt pool occurs as a result of the thermocapillary flow g. Stability of the malt pool: a melt pool is stable at a range of scanning speeds for a given laser power; the range of stable zone increases with an increase in laser power, and it becomes narrower for materials with a higher thermal conductivity at a given laser power and layer thickness. The melt pool stability is critical for the quality of SLM fabricated parts!!! Melt pool instability results in an irregular and / or discontinuous track, which leads to high surface roughness and volumetric porosity due to balling in the fabricated parts. h. Balling: is the phenomenon where the molten track shrinks and breaks up into a row of spheres to reduce the surface energy by the surface tension if the molten material does not wet the underlying substrate. The balling effect can lead to high surface roughness and porosity in the as-built parts, and it might even jeopardize the powder-laying process if the size of the balls is large enough to obstruct the movement of the paving roller or recoater. i. Sputtering: is caused by overheating the melt pool, and the intensity of sputtering increases with an increase in input energy density. Liquid droplet and non-melted power particles around the melt pool are expulsed by the recoil pressure generated on the melt pool as the result of the melt evaporating. Some figures on the slide Approach to metal and ceramic part creation Thera re four common approaches for using powder bed fusion processes in the creation of complex metal components: 1. Full melting: metallic powder material is fully melted using a high-power laser or electron beam; 2. Liquid-phase sintering: a mixture of 2 metal powders or metal alloy is used where a higher melting temperature constituent remains solid and a lower melting temperature constituent melts; 3. Indirect processing: a polymer coated metallic powder or a mixture of metallic and polymer powders are used for part construction. The steps are shown in the figure below: During indirect processing the polymer binder is melted and binds the particles together; the metal powder remains solid, the metallic powder particles remain large, unaffected by the heath of the laser. The parts produced are generally porous (sometimes exceeding 50% volume porosity!!) The polymer bound green parts are subsequently furnace processed: Furnace processing occurs in 2 stages (1) debinding; (2) infiltration / consolidation. - During debinding, polymer binder is vaporized to be removed from green part. Typically, temperature s raised to the extent that a small degree of necking (sintering) occurs between metal particles; subsequently, remaining porosity is either filled by infiltration of a lower melting point metal to produce a full dense metallic part, or by further sintering and densification to reduce the part porosity; - Infiltration is easier to control, dimensionally, as overall shrinkage is much less than during consolidation. However, infiltrated structures are always composite in nature whereas consolidated structures are of single material type! 22  4. Pattern methods: in the previous 3 approaches, metal powder is utilized in the PBF process; in this approach, the part created in the PBF process is a pattern used to create the metal part. Two most common ways PBF created parts are utilized as patterns for metal part creation are: a. As investment casting patterns (polystyrene or wax-based powders) b. As sand-casting molds (mixtures of sand and thermosetting binder are directly processed in the machine to form a sand-casting core, cavity or insert) In the case of ceramics, similar to metal parts, there are a number of ways that PBF processes are utilized to create ceramic parts. These include: 1. Direct sintering: a high temperature is maintained in the powder bed and a laser is utilized to accelerate sintering of the powder bed in the prescribed location of each layer; the resultant ceramic parts will be quite porous and thus are often post-processed in a furnace to achieve higher density! This high porosity is also seen in chemically-induced sintering of ceramics. 2. Chemically-induced sintering 3. Indirect processing: identical to indirect processing of metal powders. After debinding, the ceramic brown part is consolidated to reduce porosity or is infiltrated. In the case of infiltration. When metaal powders are used as the infiltrant then a ceramical / metal composite structure can be formed! In some cases, such as when creating SiC structures, a polymer binder can be selected, which leaves behind a significant amount of carbon residue within the brown part Infiltration with molten Si will result in a reaction between the molten Si and the remaining carbon to produce more SiC, thus increasing the overall SiC content and reducing the fraction of metal Si in the final part. These and related approaches have been used to form interesting ceramic-matrix composites and ceramic-metal structures for a number of different applications. 4. Pattern methods. Variants of powder bed fusion processes Powder bed fusion is one of the most important techniques within the AM world. A large variety of powder bed fusion processes have been developed. Some types are: Laser-based systems for low- temperature processing; Laser-based systems for metals and ceramics; Electron Beam Melting. To understand the practical differences between these processes, it is important to know: - How the powder delivery method - Heating process - Energy input type - Atmospheric conditions - Optics - Ther features vary with respect to one another. There’s a variant of the basis tech: electron beam melting. To have a fusion we need heat obv (so we introduce laser or electron beam): lasers are less expensive than electron beams, which heat by electron transfer. Both of them heat at very high temperature, because we need them to create a manufacture from powder. Laser based systems are commoner but from a scientific point of view electron beam is super interesting, even if not very common in industry. Electron Beam Melting (EBM) has become a successful approach to PBF. In contrast to laser-based systems, EBM uses a high-energy electron beam to induce fusion between metal powder particles. This process was developed at Chalmers University of Technology, Sweden, and was commercialized by Arcam AB, Sweden in 2001. Okay so we are using an electron beam (very different from a laser), immediately start to be more expensive since more complicated as a tech! In the EBM process, a focused electron beam scans across a thin layer of pre- laid powder, causing localized melting and resolidification as per the slice cross-section. Everything is controlled by a vacuum chamber in EBM. 23 → Since the source of energy in EBM is electrons, there are a number of differences between EBM and SLM and how SLM and EBM are typically practiced, summarized in the next Table: THIS SLIDE IS SUPER IMPORTANT! electron beams are inherently different from laser beams: 1. as electron beams are made up of a stream of electrons moving near the speed of light, whereas, laser beams are made up of photons moving at the speed of light; 2. when an electron beam is passed through a gas at atmospheric pressure, for instance, the electrons interact with the atoms in the gas and are deflected; in contrast, a laser beam can pass through a gas unaffected as long as the gas is transparent at the laser wavelength; 3. EBM is practiced in a low-partial-pressure vacuum environment; whereas SLM is practiced in an inert gas atmosphere at atmospheric pressure; 4. electrons have a negative charge and are focused and deflected magnetically; whereas photons are optically focused and deflected using mirrors attached to motors; 5. an electron beam can be moved instantaneously from one location to another without needing to traverse the area in-between; in contrast, galvanometers are mirrors attached to motors; 6. for a laser beam focal spot to move from point A to point B, the galvanometer motors have to move the mirrors accordingly. Thus, virtually instantaneous motion is not possible and the scan speed is determined by mass of mirrors, characteristics of motors, and distance from mirrors to powder bed; 7. Laser beams heat the powder when photons are absorbed by powder particles; electron beams, however, heat powder by transfer of kinetic energy from incoming electrons into powder particles; 8. As powder particles absorb electrons they gain an increasingly negative charge;tThis has two potentially detrimental effects: (1) if the repulsive force of neighbouring negatively charged particles overcomes the gravitational and frictional forces holding them in place, there will be a rapid expulsion of powder particles from the powder bed, creating a powder cloud; (2) increasing negative charges in the powder particles will tend to repel the incoming negatively charged electrons, thus creating a more diffuse beam. 9. There are no such complimentary phenomena with photons; 10. The conductivity of the powder bed in EBM must be high enough that powder particles do not become highly negatively charged, and scan strategies must be used to avoid build-up of regions of negatively charged particles; 11. electron beam energy is more diffuse; as a result, the effective melt pool size increases, creating a larger heat- affected zone; consequently, the minimum feature size, resolution and surface finish of an EBM process is typically larger than an SLM process. 24  12. in EBM the powder bed must be conductive; thus, EBM can only be used to process conductive materials (e.g., metals). Whereas, lasers can be used with any material that absorbs energy at the laser wavelength (e.g., metals, polymers and ceramics); electron beam generation is typically a much more efficient process than laser beam generation; 13. when a voltage difference is applied to the heated filament in an electron beam system, most of the electrical energy is converted into the electron beam, and higher beam energies (above 1 kW) are available at a moderate cost; 14. only 10–20% of the total electrical energy input for laser systems is converted into beam energy, with the remaining energy lost in the form of heat; in addition, lasers with beam energies above 1 kW are typically much more expensive than comparable electron beams with similar energies; thus, electron beams are a less costly high energy source than laser beams. OSS lasers are not so efficient due to the electric lacks. Because they’re different, the 2 techs have different fields of application p.e. you can’t use EBM with plastic! EBM powder beds are maintained at a higher temperature than SLM powder beds: 1. the higher energy input of the beam used in the EBM system naturally heats the surrounding loose powder to a higher temperature than the lower energy laser beams; 2. in order to maintain a steady-state uniform temperature throughout the build the EBM process uses the electron beam to heat the metal substrate at the bottom of the build platform before laying a powder bed. NB: Electrons need to find nothing (no air, no CO2, no hydrogen…) on their way to go exactly where I want them to go → that’s why I need to use a vacuum really strong! We pump all the air inside our room but keeping the vacuum and seal the room is very difficult!! The room tends to go in a sort of equilibrium with extern environment, and we don’t want that to happen, we want our room to be completely empty! → cost Okay you don’t need to know everything you see in the list, just the fundamental information you can also see on the table. The other important thing is associated with temperature: we need a fast cooling generally. Metal are very good on transferring energy and cool faster. OSS The microstructure is more related to the material or to the process we use to print? People with knowledge can do an educated guess and say is a 50-50 situation. Really? The process plays a very important role, a big % in every case is dominated by the material BUT this really depends on the specific case (sometimes more process, sometimes more material). By maintaining the powder bed at an elevated temperature, however, the resulting microstructure of a typical EBM part is significantly different from a typical SLM part. How is this going to impact the properties? Mechanical properties particularly. On the left: material more resistant to pressure On the right: cooling is faster and not as good under pressure as the other one. The one on the left will be delaminated when torsion is applied due to the shape of its structure but can be good under extension. Probably vale l’opposto per il materiale sulla destra. Cmq we need to do the measurements to say it with a certain sure. On the right we can identify a pattern but just in areas → good and bad p.e. probably the material will not be so resistant if fractures emerge. In general we can say: - Powder bed at an elevated temperature, resulting microstructure of a typical EBM part is significantly different from a typical SLM part 25  - Rapid cooling in SLM creates smaller grain sizes; and subsequent layer deposits only partially re-melt the previously deposited layer. The powder bed is held at a low enough temperature that elevated temperature grain growth does not erase the layering effects. - In EBM, the higher temperature of the powder bed, and the larger and more diffuse heat input result in a contiguous grain pattern that is more representative of a cast microstructure, with less porosity than an SLM microstructure. Melting is a very complicated topic in PBF: elements like cobalt and antimuonium are associated also to environmental toxicity → we need to educate people! How do we reach the level of truth closer to reality? As deep as you go, you get more close to the reality. If we confront the information, we go really close to the reality. Okay so material rules 50% and process the other 50% → sni, it really depends by the specific case (specific material and specific process we are dealing with). Conclusions: - Powder bed fusion processes were one of the earliest AM processes, and continue to be one of the most popular; - Polymer-based laser sintering is commonly used for prototyping and end-use applications in many industries, competing with injection molding and other polymer traditional manufacturing processes; - PBF processes are particularly competitive for low-to- medium volume geometrically complex parts; - Metal-based processes, including laser and electron beam, are growing in popularity and are widely available from manufacturers around the world; - Metal PBF processes are becoming increasingly common for aerospace and biomedical applications, due to their geometric complexity benefits and excellent material properties when compared to traditional metal manufacturing techniques!! - As methods for moving from point-wise to line-wise to layer- wise PBF are improved and commercialized, build times and cost will decrease → PBF processing will be even more competitive - The future for PBF remains bright: likely PBF processes will remain one of the most common types of AM technologies for the foreseeable future. OSS when we deal with powder, companies don’t like to change that much: PBF has an opportunity in the industry cause its really efficient (some companies are now using 6 lasers!). 26 Cap 6 – Extrusion-Based Systems Extrusion based systems includes: FDM (Fused deposition Modelling) and FFF (Fused Filament Fabrication) are the most common within this area, because printers are very cheap (even 50€!). Robocasting and Bioploting are different methods within the area (but we are not going to see them) FDM = Fused deposition Modelling https://www.youtube.com/watch?v=PcNCyHSKK1w How does this work? We have thermoplastics in some containers, then pulled up; they go to the extrusion, where usually there is a needle that heat them to become more plastic and easy to extrude. Then a nozzle deposit layer by layer the filaments obtained. OSS. Feedstock = raw material Oss in here you do not have either powder or liquid (like other techs)! But you deposit material. The surface finish is not that good: tra le 3 famiglie di tech viste fino ad ora (liquid, polvere e filamenti), quella che tiene la finitura superficiale migliore è PBF (when i melt and synter powder i can have a really good finish). While using filaments I will always have a fingerprint very “seeable” (unless I use a very small nozzle to extrude filaments!). General information: these technologies can be visualized as material contained in a reservoir is forced out through a nozzle when pressure is applied; If the pressure remains constant, then the resulting extruded material (commonly referred to as “roads”) will flow at a constant rate and will remain a constant cross-sectional diameter; this diameter will remain constant if the travel of the nozzle across a depositing surface is also kept at a constant speed that corresponds to the flow rate; the material that is being extruded must be in a semi-solid state when it comes out of the nozzle; this material must fully solidify while remaining in that shape; furthermore, the material must bond to material that has already been extruded so that a solid structure can result since material is extruded, the AM machine must be capable of scanning in a horizontal plane as well as starting and stopping the flow of material while scanning; once a layer is completed, the machine must index upwards, or move the part downwards, so that a further layer can be produced. 27 Two primary approaches when using extrusion process: 1. Temperature aided most commonly used approach is to use temperature as a way of controlling the material state; molten material is liquefied inside a reservoir so that it can flow out through the nozzle and bond with adjacent material before solidifying; similar to conventional polymer extrusion processes, except the extruder is vertically mounted on a plotting system rather than remaining in a fixed horizontal position 2. Chemically aided use a chemical change to cause solidification; a curing agent, residual solvent, reaction with air, or simply drying of a “wet” material permits bonding to occur; parts may therefore cure or dry out to become fully stable; this approach may be more applicable to biochemical applications where materials must have biocompatibility with living cells and so choice of material is very restricted; industrial applications may also exist. OSS You deposit a layer and this need to solidify rapidly to avoid deformation. Pressure and temperature are really important to control viscosity. Material extrusion is the most popular AM technology; Feedstock is forced through a nozzle which defines the voxel (unità di misura del volume - Wikipedia) size In some instances, the feedstock is heated and melted prior to deposition In other instances, the feedstock is deposited at room temperature, and a combination of solvent curing/drying and high-viscosity nature of the slurry serves to preserve part geometry Something about basic principles and working principles There are a number of key features that are common to any extrusion-based system: - Loading of material - Liquification of the material - Application of pressure to move material through nozzle - Extrusion - Plotting according predefined path in a controlled manner - Bonding material to itself or secondary build materials to form a coherent solid structure - Inclusion of support structures to enable complex geometrical features - The basic science involves extrusion of highly viscous materials through a nozzle - It is reasonable to assume that the material flows as a Newtonian fluid in most cases. In the figure below you can see a schematic representation of the method: OSS Versatility is an advantage of this technology. 28 In construction sector this tech is now commonly used. We can eventually change the dimension and type of nozzle to adapt the manufacture to our requests p.e. ridurre la dimensione dell’augello per avere una migliore finitura superficiale. 2 December 2024 Last Monday we were talking about one of the most important tech: extrusion based systems → FFF, FDM, gel casting… It requires filaments with some plasticity: using a ceramic paste is important here. The majority of these techs are very good for plastics, ceramics and composites; in general, everything that has a gel status and can flow through a nozzle. Okay you have a filament, you heat it and you deposit it through a nozzle. The size of the filament depends by the size of the nozzle. The smallest the nozzle, the better the superficial roughness will be → questo ci paice, uno degli svantaggi principali dell’utilizzo di qusat tecnologia è proprio la scarsa finitura superficiale! Usually to do things with the FDM we use something that is plastic and not rigid. Also among ceramics the method it’s interesting and relevant. → Advantage: very good for ceramics, traditionally prepared using paste We need to be circular → industrial symbiosis initiatives are meant to be more and more popular in the future, also associated with AM technology. If you have 2 nozzle you can use contemporary different materials → potential is enormous for this tech Before talking about limitations and critic points of this tech, let’s do a summary using some slides: The most common extrusion-based AM technology is Fused Deposition Modelling (FDM), produced and developed by Stratasys, USA; FDM uses a heating chamber to liquefy polymer that is fed into the system as a filament: the filament is pushed into the chamber by a tractor wheel arrangement and it is this pushing that generates the extrusion pressure; the major strength of FDM is in the range of materials and the effective mechanical properties of resulting parts made using this technology; parts made using FDM are amongst the strongest for any polymer-based additive manufacturing process; main drawback of this technology is the build speed: inertia of the plotting heads means that the maximum speeds and accelerations that can be obtained are somewhat smaller than other systems; FDM requires material to be plotted in a point-wise, vector fashion that involves many changes in direction; The most popular material is the ABSplus material, which can be used on all current Stratasys FDM machines. Let’s talk now about common defects in FDM There are a few drawbacks in FDM. These drawbacks directly affect the strength and appearance of the printed part Generally, these defects can be summarized as: - shape distortion that occurs due to residual stresses, caused by non-uniform temperature gradients - micro voids in the matrix and filament