Comparison of Advantages and Disadvantages of Prototyping Process

1. SLA

SLA is an industrial 3D printing or additive manufacturing process that uses computer-controlled lasers to manufacture parts in a pool of UV-curable photopolymeric resins. The laser is used to outline and solidify the cross section of the part design on the surface of the liquid resin. The cured layer is then lowered directly 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 completed.

Advantages: For conceptual models, appearance prototypes and complex designs, SLA can produce parts with complex geometry and excellent surface finish compared to other additive processes. The cost is competitive and the technology is available from multiple sources.

Disadvantages: Prototype parts may not be as strong as parts made with engineering grade resin, so parts made with SLA have limited use in functional testing. In addition, when parts are cycled through ultraviolet light to cure the outer surface of the part, the parts built into the SLA should be used with minimal exposure to ultraviolet light and humidity to prevent degradation.

2. SLS

In the SLS process, a computer-controlled laser draws on a hot bed of nylon-based powder from bottom to top, gently sinting (fusing) the powder into a solid. After each layer, rollers lay a new layer of powder over the top of the bed, and the process is repeated. SLS uses rigid nylon or elastic TPU powder, similar to actual engineering thermoplastics, so the parts have higher toughness and accuracy, but the surface is rough and lacks fine details. SLS offers a large build volume and can produce parts with highly complex geometry and create durable prototypes.

Advantages: SLS parts tend to be more accurate and durable than SLA parts. The process can manufacture durable parts with complex geometries and is suitable for some functional tests.

Disadvantages: Parts have a granular or sandy texture and limited process resin options.

3. DMLS

DMLS is an add-on manufacturing technology that produces metal prototypes and functional end-use parts. DMLS uses a laser system to attract atomized metal powders to the surface. At the suction point, 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, giving the prototype full-strength, functional hardware made from the same materials as the production components. If designed with manufacturability in mind, it also has the potential to transition to metal injection molding when increased production is needed.

Advantages: DMLS produces robust (typically 97% density) prototypes from a variety of metals 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 those of conventional formed parts.

Disadvantages: Costs may rise if multiple DMLS parts are produced. Due to the powder metal source of the direct metal process, the surface finish of these parts is slightly rough. The process itself is relatively slow and often requires expensive post-processing.

4. FDM

FDM uses an extrusion method to melt and re-cure 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 binder spray method, and its use in functional testing may be limited.

Advantages: The price of FDM parts is moderate, relatively strong, and can be used for some functional tests. The process can manufacture parts with complex geometries.

Disadvantages: The surface finish of these parts is poor and has a noticeable ripple effect. It is also a slower addition process compared to SLAs or SLS, and has limited applicability for functional testing.

5. MJF

MJF uses an inkjet array to selectively apply a fusing and refiner to a nylon powder layer, which is then fused into a solid layer by a heating element. After each layer, the powder is distributed on the top of the bed and the process is repeated until the part is finished. When the build is complete, the entire powder bed and packaged parts are moved to a 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 the appearance.

Advantages: MJF is fast - functional nylon prototypes and end-use production parts can be produced in as little as a day. Compared to processes such as SLS, the final part has a high-quality surface finish, fine feature resolution and more consistent mechanical properties.

Disadvantages: Currently MJF is limited to PA12 nylon, SLS has better small feature accuracy (small feature tolerance).

6. PolyJet

PolyJet uses a printhead to spray layer after layer of a photopolymeric resin cured by ultraviolet light. These layers are very thin, allowing for high-quality resolution. The material is supported by a gel matrix, which is removed after the part is finished. PolyJet offers flexible parts.

Advantages: The process is affordable, can prototype wraparound molded parts from both flexible and rigid materials, can produce parts in a variety of colors, and can easily reproduce complex geometry.

Disadvantages: PolyJet parts have limited strength (comparable to SLA) and are not suitable for functional testing. While PolyJet can manufacture parts with complex geometrics, it doesn't have a deep understanding of the ultimate manufacturability of a design. In addition, the color exposed to light may turn yellow over time.

7. CNC

In machining, solid blocks (or bars) of plastic or metal are clamped on a CNC milling machine or lathe respectively and cut into finished products by subtraction processing. This method generally produces higher strength and surface finish than any additive manufacturing process. It also has the complete, homogenous properties of plastic, as it is made from solid blocks of extruded or compressed molded thermoplastic resin, as opposed to most addition processes, which use plastic-like materials and are constructed in layers. The range of material options allows parts to have the desired material characteristics, such as: tensile strength, impact resistance, thermal deformation temperature, chemical resistance, and biocompatibility. Good tolerances produce parts, jigs and fixtures suitable for fit and functional testing, as well as functional parts for end use.

Advantages: 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 vendors.

Disadvantages: CNC machining can have some geometric limitations, and it is sometimes more expensive to do this in-house than the 3D printing process. Because the process removes material rather than adding it, milling the edge can sometimes be difficult.

8. Injection molding

Rapid injection molding works by injecting a thermoplastic resin into a mold, just like in production injection molding. What makes the process "fast" is the technology used to produce the molds, which are typically made of aluminum rather than the traditional steel used to produce the molds. Molded parts are strong and have an excellent surface finish. This is also the industry standard production process for plastic parts, so there are inherent advantages to prototyping in the same process if the situation permits. Virtually any engineering grade plastic or liquid silicone rubber (LSR) can be used, so designers are not constrained by the material limitations of the prototyping process.

Advantages: Molded parts are made from a range of engineering grade materials with excellent surface finish and are an excellent predictor of manufacturability at the production stage.

Disadvantages: The initial mold costs associated with rapid injection molding are not incurred in any additional process or CNC machining. Therefore, in most cases, it makes sense to do one or two rounds of rapid prototyping (subtraction or addition) to check fit and function before moving into injection molding.