3D Printing Technologies Explained

Three-dimensional (3D) printing is greatly enabling rapid prototyping in the product development process, and in some specific applications it is even being used in production. It is the process of making a solid object from a computer-aided-design (CAD) model and, in an additive manufacturing process, it essentially builds up successive thin layers of materials such as metals or different blends of plastic or resin laid down in different shapes or cross-sections.

3D printing technology is being used in many industries including engineering, construction, automotive, defence and aerospace, medical applications and in a number of consumer industries such as fashion. While many 3D printing methods have been used over the years, three of the most common technologies being used today are Fused Deposition Modelling (FDM), Stereolithography (SLA) and Selective Laser Sintering (SLS), each coming with their relative pros and cons.

 

Fused Deposition Modelling (FDM)

The first is Fused Deposition Modelling (FDM) – or Fused Filament Fabrication (FFF) as it is also known in the open source community – which is perhaps the most common process used today by desktop 3D printers and professional printers in the lower price bracket. In essence, a plastic filament, such as biodegradable PLA (polylactic acid) polyester-type material, is extruded through a resistively heated nozzle that melts the material and progressive layers are deposited to create a single cross-section or layer of an object. The nozzle then moves up vertically ready to print the next layer, and so on. The extruded material immediately hardens as it bonds to the layer below it. The quality of prints using FDM will depend largely on the layer height: increasingly thin layers result in a smoother printed object. In FDM, the layer resolution will typically range from 75 to 300 microns. A key advantage of FDM is that it produces mechanically strong and robust objects, however the print speed is much slower and the layers are more noticeable compared to other methods. The RepRap printers – the latest of which is the Ormerod 2 – employ this FDM/FFF technology.

FDM printers are fed by relatively inexpensive filament, usually rolled on a spool, and are mainly thermoplastics or thermoplastic/organic-material blends. Two of the most common materials used in FDM printers are PLA and ABS (Acrylonitrile butadiene styrene). PLA is perhaps most popular in the home environment partly due to its biodegradable properties, but also because it does not emit unpleasant chemical fumes during the printing process. However, FDM printers that are used in a professional environment commonly have the ability to extrude more advanced thermoplastics that offer fire retardant properties, higher abrasion resistance, higher tensile strength etc.

 

Stereolithography (SLA)

The second technology is Stereolithography (SLA). SLA-based printers use a laser to trace the cross-section pattern of an object in a liquid photopolymer resin. Exposure to an ultraviolet (UV) laser light then cures and solidifies the pattern traced on the resin. The printed layer is then repositioned to leave room for new and unhardened resin to fill the newly created space between the print and the laser, and the process is repeated a layer at a time.  The print production time depends greatly on the size of the model; in addition the trade-off for higher resolution can be made at the expense of print speed, meaning the creation of even quite small objects can take many hours. Offering a higher resolution than FDM, SLA technology typically offers layer thicknesses of less than 30 microns, and therefore can produce objects with significantly less visible layers than FDM. Overall, advantages of the SLA process include a relatively fast build time and fine detail. However, in terms of object strength, parts can be more brittle over time with increased exposure to light.

SLA printers can produce objects with various properties including flexibility, durability, stiffness, as well as resistance to water, thermal effects and high impact. The photopolymer resins have been designed to mimic ABS, polypropylene, and wax based materials making them suitable for many applications from high-quality prototyping to lost wax casting.

 

Selective Laser Sintering (SLS)

The third process is Selective Laser Sintering (SLS), which like SLA uses a laser, however the process involves the ‘sintering’ of finely powdered material rather than using resin. Again, creating objects layer by layer, the laser sinters powdered material, binding it together to create a solid structure. As SLS offers fast and accurate printing, it can be both a cost- and time-effective method, making it ideal for prototyping and even in limited-run end-use production. However, surface finish can be poor.

 

SLS typically sinters metals, but also thermoplastics, and can produce parts from a relatively wide range of commercially available powder materials including: metals such as steel, titanium and various alloy mixtures; and polymers such as nylon or polystyrene. While most SLS printers will use two-component powders, some are able to use single-component materials, which leads to less porous and higher performance products.