Question

1. The additive manufacturing process contained eight steps; one of them is to have the design in a Standard Triangle Language format (use your own words) to explain all about it?
2. The additive manufacturing can be Classified by the technology used such as FDM and SLA, write the difference between those two in term of the process, the material used, cost, efficiency then give two industrial examples for each

Answer 1

Additive Manufacturing (AM) processes, which are also known as Rapid Prototyping (RP), refer to an evolutionary type of fabrication that utilize a 3D CAD file and slice it to different thicknesses. A computer uses the sliced files as geometry of each layer and orders the fabrication setup to deposit a layer regarding that geometry . The layer-by-layer deposition continues to the last layer in order to fabricate a complete 3D component. There are various deposition methods that are working on different basis. However, these processes are similar in the thermal, chemical and mechanical ways they fabricate parts. The most common processes are Stereolithography (SLA), Liquid Polymerization (LP), Fused Deposition Modelling (FDM), Ballistic Particle Manufacturing (BPM), Selective Laser Melting (SLM), Laser-Engineered Net-Shaping (LENS) and Binder Jet Printing (BJP).

The Eight Steps in Additive Manufacture

1. Conceptualization and CAD
2. Conversion to STL
3. Transfer and manipulation of STL file on AM machine
4. Machine setup
5. Build
6. Part removal and clean-up
7. Post-processing of part
8. Application

Conceptualization and CAD

Producing a digital model is the first step in the additive manufacturing process. The most common method for producing a digital model is computer-aided design (CAD). There are a large range of free and professional CAD programs that are compatible with additive manufacture. Reverse engineering can also be used to generate a digital model via 3D scanning.

The generic AM process start with 3D CAD information. There may be a many of ways as to how the 3D source data can be created. The model description could be generated by a computer.Most 3D CAD systems are solid modeling systems with some surface modeling components.

There are several design considerations that must be evaluated when designing for additive manufacturing. These generally focus on feature geometry limitations and support or escape hole requirements and vary by technology.

Conversion to STL

A critical stage in the additive manufacturing process that varies from traditional manufacturing methodology is the requirement to convert a CAD model into an STL (stereolithography) file. STL uses triangles (polygons) to describe the surfaces of an object. A guide on how to convert a CAD model to an STL file can be found here. There are several model limitations that should be considered before converting a model to an STL file including physical size, watertightness and polygon count.

STL is a simple way of describing a CAD model in terms of its geometry alone.It works by removing any construction data, modeling history, etc., and approximating the surfaces of the model with a series of triangular facets. The minimum size of these triangles can be set within most CAD software and the objective is to ensure the models created do not show any obvious triangles on the surface.

The process of converting to STL is automatic within most CAD systems. STL file repair software is used when there are problems with the file generated by the CAD system that may prevent the part from being built correctly. With complex geometries, it may be difficult to detect such problems while inspecting the CAD or the subsequently generated STL data.If the errors are small then they may even go unnoticed until after the part has been built.

STL is essentially a surface description, the corresponding triangles in the files must be pointing in the correct direction; (in other words, the surface normal vector associated with the triangle must indicate which side of the triangle is outside vs. inside the part).While most errors can be detected and rectified automatically, there may also be a requirement for manual intervention.

Transfer to AM Machine and STL File Manipulation

Once the STL file has been created, it can be sent directly to the target AM machine.Ideally, it should be possible to press a “print” button and the machine should build. However there may be a number of actions required prior to building the part. The first task would be to verify that the part is correct. AM system software normally has a visualization tool that allows the user to view and manipulate the part.

The user may wish to reposition the part or even change the orientation to allow it to be built at a specific location within the machine.It is quite common to build more than one part in an AM machine at a time.This may be multiples of the same part (thus requiring a copy function) or completely different STL files.

Machine Setup

All AM machines will have at least some setup parameters that are specific to that machine or process.Some machines are only designed to run perhaps one or two different materials and with no variation in layer thickness or other build parameters.In the more complex cases to have default settings or save files from previously defined setups to help speed up the machine setup process and to prevent mistakes.Normally, an incorrect setup procedure will still result in a part being built.

Build Setup

The first few stages of the AM process are semi-automated tasks that may require considerable manual control, interaction, and decision making. Once these steps are completed, the process switches to the computercontrolled building phase. All AM machines will have a similar sequence of layer control, using a height adjustable platform, material deposition, and layer cross-section formation. All machines will repeat the process until either the build is complete or there is no source material remaining.

Removal and Cleanup

The output from the AM machine should be ready for use.More often the parts still require a significant amount of manual finishing before they are ready for use.The part must be either separated from a build platform on which the part was produced or removed from excess build material surrounding the part.Some AM processes use additional material other than that used to make the part itself (secondary support materials).

For some additive manufacturing technologies removal of the print is as simple as separating the printed part from the build platform. For other more industrial 3D printing methods the removal of a print is a highly technical process involving precise extraction of the print while it is still encased in the build material or attached to the build plate. These methods require complicated removal procedures and highly skilled machine operators along with safety equipment and controlled environments.

Post Process

Post-processing refers to the (usually manual) stages of finishing the parts for application purposes. This may involve abrasive finishing, like polishing and sandpapering, or application of coatings.

Post processing procedures again vary by printer technology. SLA requires a component to cure under UV before handling, metal parts often need to be stress relieved in an oven while FDM parts can be handled right away. For technologies that utilize support, this is also removed at the post-processing stage. Most 3D printing materials are able to be sanded and other post-processing techniques including tumbling, high-pressure air cleaning, polishing, and coloring are implemented to prepare a print for end use.

Application

Following post-processing, parts are ready for use. Although parts may be made from similar materials to those available from other manufacturing processes (like molding and casting), parts may not behave according to standard material specifications. Some AM processes create parts with small voids or bubbles trapped inside them, which could be the source for part failure under mechanical stress. Some processes may cause the material to degrade during build or for materials not to bond, link, or crystallize in an optimum way.

2)

Stereo lithography (SLA)

Stereo lithography (SLA) is one of the most common types of AM. in this process, the feedstock are in liquid form and after impact of a ray of light (laser, UV, etc.), they solidify and form each layer. This process is called curing step of SLA. As the materials are solidified by light curing, the thickness of each layer cannot pass a specific value that depends on feedstock material properties and intensity of light emitted. Also, reflectivity of materials plays an important role in fabrication process. Similar to the concept of all AM processes, after solidification of a layer, the ram of sample holder descends and a next layer is deposited and solidified, consecutively. However, based on complexity of the geometry, there might be some modifications and considerations needed to have a solid 3D component. These features are known as supports, overhangs and undercuts [21]. As mentioned before, the steps of SLA AM process are: a CAD model is used as a reference and a software slices the model. A processing unit obtains the slices and send them to a CNC machine equipped with deposition head. Each layer solidifies and at the final step a 3D component is fabricated. Figure 1 represents a schematic SLA setup. Benefits of this process are that there is flexibility in materials selection and a high accuracy can be achieved but all materials cannot be used and the fabricated parts must undergo a curing process after completion.

Laser Scanner system Solidified resin Liquid resin Figure 1: Schematic Stereolithography setup

Fused Deposition Modeling (FDM)

Fused deposition modeling (FDM) is a type of additive manufacturing of polymers that does not utilize laser. This setup is equipped with a computer-controlled nozzle head that deposits semisolid materials on a surface on order to form a layer. This process is widely used in deposition of hard polymers as they are in semisolid form prior to deposition. The deposition materials exit the nozzle head in a filament mode with variable diameters and geometries depending on nozzle head condition. The fabrication takes place in a layer-by-layer mode again. Similar to LTP, after deposition of each layer, a milling head cuts the surface to adjust surface finish and thickness of the layers . In order to facilitate material deposition, the process of FDM takes place in a temperature close to melting point of the polymeric materials. In addition, a second nozzle can be added to the setup that are able to deposit secondary materials in order to have a composite and modify properties of the fabricated component. A benefit of using this process is obtaining a finished part at the end without needs for post-processing and machining. Besides, it can be used in office scale, as well.Materials such as acrylonitrile butadiene styrene (ABS), medical grade ABS and E20 can be used in FDM. As it is known, ABS is a tough polymer with high wear resistance. However, there are some modifications in feedstock materials to enhance properties of FDM parts. On small pieces, there is no need of supports while like other AM processes, when the parts are complicated, a set of supports must be assigned to the design. Although this process offers many benefits, it is unable to use a wide range of materials and provide a high surface quality and geometrical accuracy. Figure 2 shows a schematic FDM process setup.

Design Top view Design Top view PP/TCP composite filament Speel of filaments! X-Y Direction Liquifier Porous PP/TCP Scaffold Prototype Design z-Direction Platform with foam base Figure 2 Schematic overview of FDM process

FDM vs. SLA

FDM 3D printers form layers by depositing lines of molten material. With this process, the resolution of the part is defined by the size of the extrusion nozzle and there are voids in between the rounded lines as the nozzle deposits them. As a result, layers may not fully adhere to one another, layers are generally clearly visible on the surface, and the process lacks the ability to reproduce intricate details that other technologies can offer.

In SLA 3D printing, liquid resin is cured by a highly-precise laser to form each layer, which can achieve much finer details and is more reliable to repeatedly achieve high-quality results. As a result, SLA 3D printing is known for its fine features, smooth surface finish, ultimate part precision, and accuracy.

The use of light instead of heat for printing is another way SLA printers guarantee reliability. By 3D printing parts at close to room temperature, they don't suffer from thermal expansion and contraction artifacts, which can happen during the FDM printing process.

While FDM printers produce a mechanical bond between layers, SLA 3D printers create chemical bonds by cross-linking photopolymers across layers, resulting in fully dense parts that are water and airtight. These bonds provide high degrees of lateral strength, resulting in isotropic parts, meaning that the strength of the parts does not change with orientation. This makes SLA 3D printing especially ideal for engineering and manufacturing applications where material properties matter.

Plastic extrusion 3D printers work with a range of standard thermoplastic filaments, such as ABS, PLA, and their various blends. The popularity of FDM 3D printing in the hobbyist space has led to an abundance of color options. Various experimental plastic filaments blends also exist to create parts with wood- or metal-like surface.

Engineering materials, such as Nylon, PETG, PA, or TPU and high-performance thermoplastics like PEEK or PEI are also available, but often limited to selected professional FDM printers that support them.