What is Additive Manufacturing?
Additive manufacturing is the process of building three-dimensional objects by adding layers of material. It is the opposite of subtractive manufacturing, which involves removing material from a workpiece to create a final product.
The three-dimensional object is constructed from a CAD model, or digital 3D model. A machine is used to read the instructions provided by the CAD software in order to build the object according to the dimensions specified in the design.
The Additive Manufacturing Process
A design is created using computer aided design (CAD) software. This design is a 3D model which represents in graphical format the look and dimensions of the intended physical object.
This model is then translated into .STL format. This is a triangulated representation of a 3D CAD model.
This model is then divided into layers – a process known as slicing. This is where the 3D model is converted into a sequence of 2D layers. The slicer produces a text file which the 3D printer can read.
In the manufacturing process, the design is converted into a solid object. The machine will read the instructions sent to it by the text file to build the slice.
The file is loaded into a 3D printer. The machine is prepped, including the loading of material, and, when the machine is ready, the print is started. Through a combination of layer-by-layer deposition (FDM/FFF), laser curing (SLA) or laser fusion (SLS, SLM), the three-dimensional part is created.
The product will then undergo verification and testing. During this phase, dimensions, tolerances and geometries will be verified. Validation of the product can be carried out either by using sophisticated machinery, such as a CMM machine or 3D scanners, or through manual examination.
Once the 3D printing of the object is complete, the post-processing phase begins. This involves the removal of the object from the print bed. This can be a multi-faceted process, and, depending on the 3D printed part, it may involve the removal of support structures, sintering, or improving its visual appearance, either by sanding, polishing, painting or electroplating the final object.
The post-processing phase will be different per work object but will usually involve some level of cleaning or treatment to improve the exterior of the product.
The piece is then re-examined one last time, which may include dimensional or surface finish inspections.
Advantages of Additive Manufacturing
Allows for the creation of bespoke parts with intricate shaping and complex contours
Less material wastage than with subtractive manufacturing
Digital designs can be altered quickly, making the process ideal for rapid prototyping
Shorter lead times
Smaller production runs means cost reduction for customers
Virtual inventory means there is no real requirement for warehousing and storage space
Recreating and optimising legacy parts is easier with CAD files and a printer, rather than using potentially obsolete machinery
Greater ease of assembly – components that would normally need to be assembled from various elements can be fabricated as one object. This has further cost and lead time benefits, while minimising the amount of manual labour required. This process also improves the integrity of the finished object, adding improved strength and robustness.
Types of Additive Manufacturing
There are seven main types of additive manufacturing, each with its own processes, layering and equipment.
VAT Photopolymerisation (SLA and DLP) – Also known as stereolithography. Uses a vat of liquid photopolymer resin and a laser beam draws a shape in the resin, creating a layer. The production process is quick using this method, but the post-processing phase is lengthy.
Material Jetting – The print head is above the platform, and material is deposited onto the surface in the form of droplets. These droplets create a layer. The process is repeated, resulting in the building of one layer after another. This method is often used to create models or prototypes.
Binder Jetting – A powder-based material is applied to the build platform and the print head deposits the binder on top. A binder fixes the layers together. This process is repeated to create more layers until the product is finished. This method is quick and allows for customisation. Binder jetting is sometimes used to create medical and dental devices, as well as aerospace components.
Material Extrusion (FDM or FFF) – Material is drawn through a nozzle, heated, and deposited in a continuous stream. The nozzle moves horizontally while the platform moves vertically. The material is heated when it is applied so that it fuses to the previous layer, thus creating layer-upon-layer. As with binder jetting, polymers and plastics can be used, which provide strong structural support. This method is popular in the automotive sector.
Powder Bed Fusion (PBF) – Powder is applied to the platform and a laser fuses the powder before a second layer is applied with a roller or a blade. There are different types of powder bed fusion, including Selective Laser Melting (SLM), Selective Laser Sintering (SLS), Electron Beam Melting (EBM) and Direct Metal Laser Sintering (DMLS). Metals and polymer powder materials are used for prototypes. Powder Bed Fusion is one of the more time-consuming types of additive manufacturing.
Sheet Lamination – Binds layers using ultrasonic welding or an adhesive. There are two main types: ultrasonic additive manufacturing (UAM) and laminated object manufacturing (LOM). UAM uses metal bound together with ultrasonic welding, whereas LOM uses paper bound together using an adhesive. The material is placed on a cutting bed and layers are applied and bonded to that material. The shape is cut with a knife or a laser. Low cost and quick but lacks accuracy. Used for prototypes.
Directed Energy Deposition (DED) – a four or five axis arm deposits melted material around a fixed object. The material is melted by a laser and then solidifies. This method is often very accurate, but the finish achieved varies based on the type of material used.
Next, we will be looking at the injection moulding process, understanding its key principles, characteristics and processes, before examining how it differs from additive manufacturing.