3D Printing Processes and Printing Materials

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1 3D Printing Processes and Printing Materials Introduction to 3D Printing Three-dimensional (3D) printing in recent years has become the main focus of public and media attention as a technology has at last approached the quality necessary for direct production of end-use devices. 3D printing is a method of manufacturing that materials such as metal or plastic are deposited onto one another in layers to produce a threedimensional product and are offered with different options in terms of feature detail, materials, and costs. 3D printing is a game changing that allows for the manufacturing of complex geometric shapes that can be mass-customized compare to traditional manufacturing technologies such as forging, machining, casting, and injection molding. 3D printing has already been proved workable in many medical applications including the manufacture of custom prosthetic devices and dental implants. The most popular methods of standard 3D printing include vat photopolymerization, powder bed fusion, material extrusion, sheet lamination, directed energy deposition, material jetting, and binder jetting. Industry has realized the manufacturing advantages of these method and is investing in production systems to create complex structures for rocket engines, airplane parts, customized parts for cars. Although 3D printing technologies have advanced that several materials can be used to manufacture including polymers, ceramics, and metals, the products are still generally limited to a number of compatible materials. This paper will explain some popular 3D printing methods as well as different kind of filaments that printers use. Figure 1: First ever 3D printer invented by Charles (Chuck) Hull in 1983

2 3D Printing Processes It is really a big challenge for people that are new to the world of 3D printing. What is the difference between types of 3D printing like SLA and SLS, for example? With so many different acronyms, it can be pretty confusing. Although there are many different printers available, only 9 basic types of 3D printing method that currently exist. This section will explain 3 main methods that have significantly effective uses for consumers, professionals and businesses. Fused Deposition Modeling - FDM Material extrusion is the most common device and the cheapest one in the consumer market. Its system includes a spool of filament that is injected into the printer and is ready to be fed into a platform extrusion nozzle. The nozzle then later will be heated, and one motor will push the filament through the nozzle, causing filament to melt. Extrusion head then moves along designed coordinates, laying down the melted filament layer by layer onto the base. Filament stays on the base will cool down and solidify. This method sometimes requires a necessary support depending on the complex of geometry, such as a design has a overhanging parts. Some basic filaments for the FDM method are Polylactic Acid (PLA), and Acrylonitrile Butadiene Styrene (ABS) which will their details will be given in next section. Figure 2: FDM 3D printer from LONGER

3 Stereolithography - SLA Stereolithography is a 3D printing process that uses a UV laser to treat liquid resin into hardened plastic. The most common SLA-system is upside-down, or, inverted, stereolithography. Resin is either poured into a tank by users, or dispensed automatically from a cartridge depending on the machine. At the beginning of the print, a build platform is lowered into the resin, leaving only a thin layer of liquid in between the platform and the bottom of the tank. Then galvanometers direct the UV laser through a transparent window at the bottom of the resin tank, drawing a cross section of the 3D model and selectively hardening the material. The model is built in layer by layer with each layer less than a hundred microns thick. When machine completes one layer, the part is peeled from the bottom of the tank, allowing fresh resin flow beneath, and the platform is lowered once again. It builds the part from top to bottom, the build platform lifts the part upwards, out of the resin. SLA has a wide range of materials with diverse physical properties that engineers, product designers, sculptors, jewelers, or dentists can apply. SLA printers consistently produce higher resolution objects and higher accuracy than FDM printers. Figure 3: SLA 3D printer from LONGER

4 Selective Laser Sintering - SLS Selective laser sintering, also known as powder bed method, is a rapid prototyping method that build a physical part from 3D Computer-aided design (CAD) data in a matter of hours. Materials such as powdered NylonX, and powdered NylonXI are mainly used in SLS method. This method creates a light-weight, heat and chemical resistant, highly durable parts, so it is a perfect choice for manufacturing without the expenses to tooling. The process will first divide the CAD data into thin layers, then transfer them to manufacturing equipment. One leveling roller will spread a thin layer of powdered material across on the powder bed. From above, a CO2 laser traces the cross-section on the materials, making them heated and fused together. When one layer is complete, the powder bed is lowered to make space for the next layer. New powder material will be introduced from the powder cartridge and rolled out smooth. The unused materials will also be recycled. The process will repeat layer by layer until the part is finish. While building one layer, an un-inserted powder will act as extra elements to support the structures. Complex designs or assemblies once thought impossible are now being redesigned with SLS. Figure 4: SLS 3D process Filaments Getting the right printing filament can be confused as much as understanding the printing method. Although there are hundred different materials that can be used as filament, they divide into 3 main categories: standard, flexible, and composite filaments.

5 Standard Filaments Standard filaments are best 3D printing materials for beginners due to their ease of printing, minimal warp. They are also best used for cosmetic prints, prototypes, toys, or low-stress applications. Two of the most common materials for standard filaments are PLA and ABS. PLA - Polylactic Acid PLA is a default recommended material for many printers because of its odorless, low warp, and available in a wide range of colors. It is a stiff but brittle material, and also an eco-friendly material which is made from renewable resources and requires much less energy to process than traditional plastics. Besides using as 3D printing material, PLA plastic is often used for candy wrappers, biodegradable medical implants ABS - Acrylonitrile Butadiene Styrene ABS is another commonly 3D printing material that best used for creating durable parts that can withstand high temperature. It is a strong, ductile material with wear resistance and heat tolerance. It is best to build parts such as gear, parts exposed to UV and heat like car cup holder. It can also be post-processes with acetone to have a glossy finish. Flexible Filaments Flexible filaments are materials that feel and acts like flexible rubber. They are best to create parts that are rubbery, elastic, and impact resistant such as caps, belts, stoppers, bumpers, and phone cases. The less you fill in when printing, the more flexible your finished print will be. Composite Filaments Composite Filaments are materials that ideal for faux-stone, model train scenery, and architectural models. Parts that built with composite filaments most of the time can be painted and sanded after. Liquid resin A curable photopolymer - a liquid resin - are used as a main material for SLA printers. The resin is hardened by applying focused light or UV light. Liquid resin helps the model has a fine details and smooth finishing surfaces. Different combinations of various additives that comprise a resin in different material properties. Some main benefits of using resins are smooth, fine features, high detail, high stiffness, good finishing surface. However, resins also have some limitations such as relatively brittle, not suitable for outdoors use, and susceptible to creep. There are 4 main groups of resins based on their properties

6 Standard resins: Best use for rapid prototyping, concept modeling, or art models applications due to low-cost Figure 5: Standard resin from LONGER Engineering resins: All engineering resins require a post curing under UV light to achieve their maximum mechanical properties. Engineering resins provide many mechanism options for the parts such as durable, tough, heat resistant, rubber-like, or ceramic filled. Dental and medical resins: Biocompatible resins are used to make customized medical equipment. The parts using dental and medical resins later can be steam sterilized for a direct use in operating rooms. These resins also have high resistance to fracture and wear make it ideal to create custom hard splints or retainers. Castable resins: This kind of resin are ideal for parts with sharp details and smooth finish. When burned, it leaves out no ashes or residue. Castable resins are best for jewelry and other small and intricate components. While most FDM filaments are interchangeable, 3D printing resins are more specialized products. Therefore, depending on what objects are being built, some common aspects to pay attention are: Durability: Although objects were made by layer to layer of cured resins, they are still not exactly known for their toughness. Most of resins are brittle and easy to fail by cracking or shattering. In this case, engineering resin is a solution due to its customized to simulate the toughness of injection-molded plastics Flexibility: There are many resins have high flexibility that can build rubber-like products. They are known for low tensile modulus, high impact resistance and

7 elongation. However, being so soft can make them hard to print. These soft resins will require supportive structures during the printing process. Price: Printing with resins is often more expensive than printing with filaments. A liter of high-quality 3D printing resin will cost 50$ compare to an entire roll of filament of 20$. Moreover, SLA often printers require enough resin in the tank that the build platform need to submerge completely. Making resins with the Three-Roll Mill Any kind of resin is a result of good mix from combinations of the monomers, oligomers, photo initiators, and various other additives. Good resin requires a good dispersion of all additives, elements, and liquid base. These requirements are achievable using the three-roll mill machines. Three-roll mill machine has three horizontal rollers that 2 rollers next to each other will rotate in opposite direction plus at different speed. While rotating, any material that is poured in between a tiny gap of feeder roller and center roller (Figure 5) will experience a large amount of shear force which will give a fine dispersion. Therefore, three roll mills are ideal for blending and dispersion of viscous materials like resin. Figure 6: How a three-roll mill works, picture from Post-process of 3D printing resins Resin 3D printing is an optimal option to produce high details and fine feature products. However, resin 3D printing often requires supportive structures to be attached to the

8 models, and the models need to be arranged at specific angles. These supportive structures later will leave marks and create rough surfaces. Luckily, with proper tools and cautiousness, 3D printing resins are easy material to post process. Some basic post processes for 3D printing resins are Cleaning: It is important to clean all uncured resin from the print. Alcohol or water is used to clean since resin is a liquid material. Supportive structures removal: Break off the supportive structures will leave bumpy surfaces on the model. Therefore, it is recommended adding extra thickness to the model and then sanded afterwards to achieve dimensional accuracy. Sanding: This process is to remove any debris left over after the printing. It can be done with dry or wet sanding tools. Wet sanding often provides a smoother surface to the print. Painting: Painting can be done as special requirements. Variety of colors now are available on the market. Polishing: This process can help bring back the transparency after sanding. With the use of various compounds, polishing can make the surface even more smooth and clear. All these post processes can be done manually or with special devices that are now available for all range from small to big 3D printers. Advantages and limitations of 3D printing resins Every technology has their advantages and limitations. Depend on the capability of the printer, the properties of resins using, resin 3D printing also has some advantages and limitations. Advantages: High details: Since the model is built by loading layers of liquid, resin 3D printing will provide more complex geometries and filaments Smooth surface: Although printing of layer by layer with create some layer lines, these lines are extremely small. On some colors of resin, these lines nearly invisible. Various material: Nowadays, a wide range of resin materials can manage to provide all common applications and needs. Moreover, specific designs and product can be manufactured with combinations of different resins. High speed: Resin printing is much faster than compare to other prototyping methods. High detail features can be produced with hours and still keep the same quality.

9 Limitations: Many supportive structures: Supportive structures will help the models avoid from touching the plate. These structures after being curved, won t be able to recycle into fresh resin. They become solid and useless. Necessary post process: Post processes are required to do with resin prints before they are ready. Without post process, model possibly can be ruined and won t deliver the best quality. Conclusion As 3D printers become more affordable, 3D printing will be the fastest growing manufacturing technology. 3D printing technology is a chance for businesses to increase their competitiveness and innovation. The uses of 3D printing for manufacture, prototype, and especially for medical is beyond astonishing. It is still new and continuously improving and adjusting. With a wide range from manufacturing processes to materials, the industry still has a lot more to achieve in the future. If a picture is worth a thousand words, a prototype is worth a thousand pictures.

10 References All 10 Types of 3D Printing Technology in All3DP, 29 Nov. 2018, all3dp.com/1/types-of-3d-printers-3d-printing-technology/. FlyntI, Joseph. Best 3D Printing Resin Materials: How They Work and Top Types. 3D Insider, 3 Oct. 2018, 3dinsider.com/3d-printer-resin/. Types of 3D Printing. MakeShaper, SLA 3D Printing Materials Compared. 3D Hubs, 3D Hubs Blog,