7. 3D printing techniques
Additive manufacturing, often referred to as 3D printing, is a collective term for techniques used to manufacture objects in layers, based on a three-dimensional computer model.
There are several ways of making 3D prints, depending on the material and desired properties of the final model. The most common techniques used in consumer-grade 3D printers are FDM (Fused Deposition Modeling) and Polyjet.
In addition to additive manufacturing, other digitally controlled manufacturing processes such as CNC milling and turning can also be used to create physical models from digital models.
FDM (Fused Deposition Modeling) is the technology used in almost all consumer-grade 3D printers. It is also referred to as FFF (Fused Filament Fabrication) or MPD (Molten Polymer Deposition). The technology is reminiscent of inkjet technology, with printheads building the in layers with thin filaments of melted, liquid thermoplastic being sprayed on to a base and then left to harden as it cools. The filament, which is normally between 1.75 mm and 3 mm thick, is pushed through a nozzle which is between 0.25 mm and 0.8 mm, and fuses with the underlying layer. Once the layer is done, the base is lowered so that the next layer can be added on top. Usually, the material used is plastic filament stored on rolls.
The range of materials for this technique is limited, and the most common materials are the plastics ABS (Acrylonitrile-Butadiene-Styrene), PLA (Polylactide or polylactic acid), and PC (Polycarbonate).
PLA is a lactic acid-based bio-polyester and is made from renewable raw materials such as corn and sugar starch. PLA can be approved for human consumption, and is often used in food packaging.
ABS is a strong, tough, and fairly cheap plastic. It is also easy to tint and chrome to create a metal surface. ABS is commonly used in the automotive and toy industries; most non-transparent LEGO blocks are made from ABS.
PC is a transparent thermoplastic with exceptional durability. It is mainly used for shop windows and door windows which need to withstand shocks, kicks, and punches, and airplane windows, visors, machine protection and other products which must be both durable and transparent. CDs are also made from PC. Polycarbonate should not be confused with plexiglass, an acrylic plastic which is more brittle than the tough polycarbonate. You can tell the materials apart by bending them. The plexiglass will break while the polycarbonate will bend. Polycarbonate can burn in air but is self-extinguishing and will not keep burning on its own.
The advantages of FDM-technology is that it is simple and cheap, while capable of printing in exceptionally durable materials which are also available in numerous colours and which can be coated to achieve a metallic surface. The drawbacks include a less appealing surface structure and poor detailing due to low resolution and varying accuracy in simple consumer printers. Certain models may also need substructures to be printed using this technology.
Polyjet is a technique where small drops of two different polymers are put on a plate and then hardened using UV radiation. One is the actual printing material, while the other is a support material which can easily be removed once the printing is done. The end result is a more even surface with better details than FDM models, where the layers are often visible.
There are thousands of materials available which can be combined in a single model. The materials can be tinted or transparent, rubber-like and bendy. However, for models that need to withstand major physical strains, the strong thermoplastics used in FDM can be a better alternative. Thermosets, which are used in polyjet models, can unlike thermoplastics not be melted down and reworked into new products.
SLA is short for Stereolithography, and the printer creates models by hardening liquid plastic, layer by layer, using a UV laser. There are consumer-grade printers which use this technology, but they are less common than FDM and Polyjet printers.
The advantages of SLA include very high resolution and level of detail, and the ability to create transparent models. Drawbacks include difficult handling of liquid when filling up the printer and finishing the printout, some forts require support materials, and the model requires hardening once done in the printer.
DLP (Digital Light Processing) is similar to SLA, but the plastic liquid is instead hardened using a projector. The final results are of equal quality, but the process is faster and has less material waste and lower operating costs.
SLS (Selective Laser Sintering) is in Swedish referred to simply as “lasersintring”. Just as in SLA, a laser is used to fuse the material, which in this case is in powder form. All materials which have a melting point and which can be made into powder can (at least in theory) be used to produce models using this technique.
Common materials in addition to plastics include metals, ceramics, and glass. No support materials are needed, the material properties are good, and it is possible to create very complex geometries. This technique is not used for consumer-grade machines, and the handling of powder materials is complicated and the material costs are high.
CJP (ColorJet Printing) is a powder-based technique where a layer of powder is applied, after which a print head, much like an inkjet printer, is used to apply a liquid that binds the powder in the right places into a solid surface. The surface is then lowered by the width of one layer, and the process is repeated until the model is finished. As with SLS, this technique does not require any support material, and it is therefore easy to create complex geometries. A CJP printer can also use multiple colours at the same time. The big drawback of this technique is that ceramic models become very brittle.