Introduction
Micro 3D printing or µAM is the process of creating the part with very small features measuring in microns. Current available commercial machines create parts with resolution max up to 25~30 microns. However, the µAM machines provide resolutions of less than 5 microns. Few specialized AM technologies create incredibly tiny parts in nanometers that is 1000 times smaller than micron size. The expanding requirements for microfabricated parts across a wide spectrum of industry such as medical, electronics, automotive etc. has led to the accelerated development of micro-AM technologies. Micro AM creates parts and shapes so tiny in nature that cannot be made by the conventional manufacturing set-up.
There are some specialized conventional manufacturing technologies (micro injection molding) available that can make parts at microscale but not widely available due to high costs & complexity and expertise available with very few companies. On the other hand, µAM provides a viable and cost-effective option to companies to manufacture prototypes and parts at micro/nanoscale from one to a few thousand in the shortest possible time. Currently, most µAM parts are made by resin-based SLA process means photopolymerization by exposing the resin to light. Few companies have developed the capability of making micro-AM parts in metals such as aluminum, copper, steel etc. At present, µAM is a relatively new development and acts as an additional option w.r.t current AM technologies.
Micro AM Technologies
- Micro SLS
- Micro SLA
- Projection MicroStereolithography (PµSL)
- Two-photon polymerization (TPP)
- Lithography-based Metal Manufacturing (LMM)
- Electrochemical deposition
- Nano ink-jetting
Micro SLS
It is the same as typical powder bed fusion technology where a high-intensity laser beam scan in 2D format across the powder bed layer by layer creates the final shape. µSLS usually refer to the metal SLS process. The powder particle size is much smaller than the standard SLS and ultra-thin laser beam ultimately creates the part with very high surface detail with sub 5micron resolution and build speed of more than 60mm3/hr.
In this process, a layer of metal nanoparticle ink is first overlayed onto a substrate and then dried to produce a consistent layer. Then a precise positioning system locates the substrate under the laser scanning system. The laser scanning system scans the laser light across the length and width of the substrate and the digital micromirror array is used to heat and sinter the nanoparticles into the required patterns. This process is repeated to get the final 3D shape.
µSLS merges the benefits of SLM AM and the micromachining process permitting the creation of parts with unbelievable details and accuracy. This process also offers very high dimensional accuracy, incredible resolution and surface finish. Complex internal cavities and channels could be easily created even the complicated assemblies could be created in µSLS.
µSLS made flow channel & other parts. Photo credits: 3D MicroPrint
Micro SLA
This process is the same as general stereolithography which involves the polymerization of photo resin by a light source in a vat. However, µSLA is a modified version of other commercially available SLA machines to make micro-SLA parts. The modification involves the specially designed lenses which emit incredibly small diameter laser beams creating very small size cure resin parts. Also, special resins are developed for µSLA.
Projection MicroStereolithography (PµSL)
PµSL is another high potential additive manufacturing process due to its low cost, speed and accuracy. Also, a wide range of materials such as ceramics, polymers and biomaterials with applications ranging from micro-opticals, metamaterials, micro fluids, tissue engineering and micro biomedical devices. PµSL uses a projector to project the entire 2D cross-section onto the surface which cures the liquid layer in one go. The process is repeated to generate a 3D part. PµSL works on DLP (Digital Light Processing) principal which share commonality with other commercially available DLP printers. This process was able to achieve a layer thickness of 1µm.
PµSL made 5µm turbine. Photo credits: BMF3D
PµSL made Micro medical device. Photo credits: BMF3D
Two-photon polymerization (TPP)
TPP or 2PP is perhaps the most precise AM technology to create nanoscale complex shapes and structures. This used a highly focused Femto scale laser along with scanning mirrors. 2 photons are absorbed by the photo resin initiating the local polymerisation process to create a solid element known as a voxel. The process is repeated layer by layer to generate a 3D part. Applications are in the field of material engineering, microdevices etc.
Lithography-based Metal Manufacturing (LMM)
In this process, the tiny metal particles are suspended in a photosensitive resin-based solution up to 50% of the total volume and then cured by a light source layer by layer creating a ‘green part’. The curing process is very similar to any other vat polymerisation process. Later, the green part is sintered inside a furnace to obtain the final shape. The sintering process is like the MIM process (Metal Injection Molding). This process can create highly complex metal structures with good dimensional accuracy and low surface roughness. Available materials are stainless steel, titanium, brass, copper etc.