SLA is an industrial 3D printing or additive manufacturing process that uses a computer-controlled laser to create parts in a pool of UV-cured photopolymerizable resin. The laser is used to outline and cure the cross-section of the part design on the surface of the liquid resin. The cured layer is then lowered to just below the surface of the liquid resin and the process is repeated. Each newly cured layer is attached to the layer below it. This process will continue until the part is complete.
Benefits: For concept models, cosmetic prototypes, and complex designs, SLA can produce parts with complex geometries and superior surface finishes compared to other additive processes. Costs are competitive and the technology is available from multiple sources.
Cons: Prototype parts may not be as strong as parts made from engineering-grade resins, so parts made with SLA are of limited use in functional testing. Additionally, parts built into the SLA should be used with minimal UV and humidity exposure to prevent degradation when the part is subjected to UV cycles to cure the outer surface of the part.
During the SLS process, a computer-controlled laser draws from bottom to top onto a heated bed of nylon-based powder, gently sintering (fusing) the powder into a solid. After each layer, the rollers lay down a new layer of powder on top of the bed, and the process is repeated. SLS uses rigid nylon or elastic TPU powder, similar to actual engineering thermoplastics, so the part has greater toughness and precision, but the surface is rough and lacks fine detail. SLS offers large build volumes, can produce parts with highly complex geometries, and create durable prototypes.
Pros: SLS parts tend to be more accurate and durable than SLA parts. The process can produce durable parts with complex geometries suitable for some functional tests.
Disadvantages: Parts have a grainy or sandy texture and limited choice of process resins.
DMLS is an additive manufacturing technology that produces metal prototypes and functional end-use parts. DMLS uses a laser system to attract atomized metal powder to the surface. Where it is sucked out, it welds the powder into a solid. After each layer, the blade adds a new layer of powder and repeats the process. DMLS can use most alloys, enabling prototypes to have full-strength, functional hardware made from the same materials as production components. If designed with manufacturability in mind, it also has the potential to transition to metal injection molding when increased throughput is required
Pros: DMLS utilizes a variety of metals to create robust (typically 97% density) prototypes that can be used for functional testing. Because components are built layer by layer, it is possible to design internal features and channels that cannot be cast or otherwise machined. The mechanical properties of the parts are the same as conventional formed parts.
Cons: Costs can go up if multiple DMLS parts are produced. These parts have a slightly rougher surface finish due to the powder metal source of the direct metal process. The process itself is relatively slow and often requires expensive post-processing.
FDM uses an extrusion method that melts and resolidifies thermoplastic resins (ABS, polycarbonate, or ABS/polycarbonate blends) in layers to form a finished prototype. Because it uses a true thermoplastic resin, it is stronger than the adhesive jet method and may have limited use in functional testing.
Pros: FDM parts are affordable, relatively strong, and can be used for some functional tests. This process can produce parts with complex geometries.
Disadvantages: These parts have a poor surface finish with a noticeable ripple effect. It is also a slower add-on process than SLA or SLS and has limited applicability to functional testing.
MJF uses an inkjet array to selectively apply fusing and refining agents on the nylon powder layer, which is then fused into a solid layer by heating elements. After each layer, the powder is distributed on top of the bed, and the process is repeated until the part is complete. When the build is complete, the entire powder bed and packaged parts are moved to the processing station, where most of the loose powder is removed by an integrated vacuum. The part is then sandblasted to remove any residual powder and finally reaches the finishing department where it is dyed black to improve its appearance.
Pros: MJF is fast – producing functional nylon prototypes and end-use production parts as fast as one day. The final part has a high-quality surface finish, fine feature resolution, and more consistent mechanical properties than processes such as SLS.
Disadvantage: Currently MJF is limited to PA12 nylon, SLS has better small feature accuracy (small feature tolerance).
PolyJet uses a printhead to spray layer after layer of UV-cured photopolymerizable resin. These layers are very thin, allowing for high-quality resolution. The material is supported by a gel matrix that is removed after the part is complete. PolyJet can provide elastic parts.
Pros: The process is affordable, can prototype overmolded parts from flexible and rigid materials, can produce parts in a variety of colors, and can easily replicate complex geometries.
Cons: PolyJet parts have limited strength (comparable to SLA) and are not suitable for functional testing. While PolyJet can manufacture parts with complex geometries, it cannot gain insight into the ultimate manufacturability of the design. Also, over time, the color may turn yellow when exposed to light.
In machining, a solid block (or bar) of plastic or metal is clamped on a CNC milling machine or lathe, respectively, and cut into finished products by subtractive machining. This approach generally yields higher strength and surface finish than any additive manufacturing process. It also has the full, homogenous properties of plastic because it is made from a solid block of extruded or compression molded thermoplastic resin, as opposed to most additive processes, which use a plastic-like material and build it in layers. The range of material choices allows parts to have desired material properties such as: tensile strength, impact resistance, heat distortion temperature, chemical resistance and biocompatibility. Good tolerances produce parts, jigs and fixtures suitable for fit and functional testing, as well as functional components for end use.
Pros: Machined parts have a good surface finish and are very strong because they use engineering-grade thermoplastics and metals. Like 3D printing, custom prototypes can be delivered within a day from some suppliers.
Cons: CNC machining can have some geometric limitations, sometimes doing this in-house is more expensive than the 3D printing process. Milling undercuts can sometimes be difficult because the machining process removes material rather than adds material.
Rapid injection molding works by injecting thermoplastic resin into a mold, just like production injection molding. What makes the process “fast” is the technology used to produce the molds, which are often made of aluminum rather than the traditional steel used to produce molds. Molded parts are strong and have an excellent surface finish. This is also an industry standard production process for plastic parts, so there are inherent advantages to prototyping in the same process if the situation warrants it. Virtually any engineering-grade plastic or liquid silicone rubber (LSR) can be used, so designers are not limited by the material of the prototyping process.
Benefits: Molded parts are made from a range of engineering-grade materials and have an excellent surface finish that is an excellent predictor of manufacturability during the production phase.
Disadvantage: The initial tooling cost associated with rapid injection molding does not occur with any additional process or CNC machining. So, in most cases, it makes sense to do one or two rounds of rapid prototyping (subtractive or additive) to check fit and function before moving into injection molding.