The what and why of rapid prototyping
Would you believe me if I told you that by using state-of-the-art rapid prototyping techniques, one could print a fully functional racecar? What about a five-story building? What about an object so small it can fit on an ant’s forehead? Well you should, because all of those are actually possible .
Rapid prototyping is the use of specialised manufacturing hardware and software to quickly produce scale models. Not only is rapid prototyping 70-90% faster than traditional prototyping techniques, but rapid prototyping also permits greater freedom of design, can reduce overall costs, and allows for a more flexible design process .
So what are your options?
To figure out which rapid prototyping technique is the best fit with your process, a variety of questions should be considered:
- What is the budget?
- How many prototypes are needed?
- What material characteristics are required?
- At what stage in the development process are you?
- Are you creating a conceptual prototype or a functional prototype?
This article examines three prevalent rapid prototyping techniques: Fused Deposition Modeling, Selective Laser Sintering, and Stereolithography.
All share general advantages over traditional prototyping, but differ in their manufacturing technology, the physical properties of their models, and thus, their overall fit in your design process.
Additive vs. subtractive processes
The rapid prototyping techniques discussed in this article are additive processes, meaning that material is gradually added, layer-by-layer, until the desired form is created. This can be contrasted with traditional subtractive processes, which start with a solid block and remove material.
Additive processes are an attractive alternative to traditional processes because they waste less material, can produce complex forms quickly, and can reduce time to market .
Fused Deposition Modeling (FDM)
The most common rapid prototyping technique is FDM . This is primarily due the low cost, both in terms of printer hardware and material.
In FDM, a solid material is heated above its melting point and pushed through a nozzle of specific dimensions. The deposited layer hardens on the printing platform and the platform is lowered. This process repeats itself until the desired three-dimensional object is formed.
Although metal can be used, the most common starting material is thermoplastic, of which a wide range is already commercially available . FDM models are commonly used for concept prototyping, where their purpose is to physically convey a design idea. Due to their low thermal stability and low resolution, they are less relied upon for precise functional prototyping.
Melt extrusion processes (like FDM) produce objects with ridged surfaces, which may require chemical or mechanical finishing to meet your aesthetic standards . Additionally, FDM may require support structures to design certain forms, because a new layer of material cannot be deposited on air .
SLA could be used to produce either your concept or functional modeling prototypes, but only if you’re ok with not being able to create metal, glass, and ceramic models. SLA printers only print with a photo-reactive polymer resin as feed . SLA is an interesting alternative to FDM for intricate thermoplastic modeling, but requires a more expensive printer and photopolymer resin.
SLA creates models by using light radiation to bond polymer chains in a liquid resin into a solid form. Light radiation, typically UV light, is directed at specific points on the resin surface per computer instructions. As the light moves across the resin surface, it irreversibly crossed-links the polymer fragments that it contacts .
SLA prototypes have a higher feature resolution, fewer printing imperfections, and better physical properties (i.e. strength, thermal stability) than FDM prototypes . Sensitivity to sunlight prevents SLA to being suitable to certain applications. SLA printers create a form from the top-down, with the printing platform raising up with each added layer. This limits the height of a printed object to the size of the resin bath .
Selective Laser Sintering (SLS)
If you need to produce an intricate metal model, SLS is the rapid prototyping technique for you. SLS works by exposing a powdered solid to a high-powered laser, which compacts and fuses the powder into a “new” solid with different physical properties and behaviour .
Similar to FDM, a printing platform is lowered with each subsequent layer, but the SLS printing platform is covered in powdered solid that gets compacted with a laser. In FDM, remember, the starting material is melted and extruded onto the printing platform. Thus, SLS does not require additional supports for overhangs and can print structures with greater geometric complexity .
SLS is versatile in terms of starting material, allowing for not only printing with metal, but also with thermoplastic, ceramic, or glass. The drawback is the high cost of the SLS printer and specialised SLS powder feed. Although pore size and density can be controlled to a degree, SLS prototypes have inherently porous surfaces, meaning that a further coating may be necessary, depending on the prototype’s purpose .
Which technique is right for you?
All three rapid prototyping techniques outlined in this article will play a role in tomorrow’s design environment.
FDM is an excellent choice for simple iterative prototype production, where the low production costs are of primary importance.
If your end goal is prototype with complex geometry, however, SLA and SLS are likely more suitable.
SLA is the cheaper of the two options, but SLA prototypes can only be made from polymer resin that can degrade in the sunlight. The ability of SLS to print intricate prototypes out of metal fills prototyping niche that SLA and FDM cannot.
Rapid prototyping technique
The ability to prototype quickly, cheaply, and efficiently is an important factor for a well-developed design process. The speed and ease of modification that rapid prototyping techniques enable suggest it will continue playing a central role in shaping the prototyping landscape.
For many development processes, cost is the determining factor and FDM remains the only viable rapid prototyping option. In terms of speed, precision, and material flexibility, SLS is arguably the superior technique and will likely experience a surge in popularity if technological advancements can bring the price down.
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