Mechanical Prototyping Processes: What to Use and When
Here at Mindtribe, our product design team works with clients who have varied schedules and budgets. To best serve their individual needs, we use a variety of prototyping methods to create mechanical models for review. Sometimes the parts are used for engineering purposes, and other times the parts are purely cosmetic for interdisciplinary design reviews. Understanding the pluses and minuses of each process allows us to minimize time and budget while achieving the design objectives. Below is a short summary of the processes we use most often for small quantities of mechanical parts.
How it works: SLA is an additive prototyping process in which parts are built layer by layer from the ground up. The process begins by raising a platform up to the top of a pool of UV curable photopolymer resin. A squeegee wipes a thin layer of photopolymer across the top of the platform (about 0.004” thick). A UV laser is activated which bounces off a movable mirror, and strikes the photopolymer hardening it at the point of contact. A computer connected to the machine moves the mirror in an x-y pattern so the laser can trace out the rest of the first layer. Once the first layer is complete, the platform drops down one layer thickness (.004”) and the process begins again. Once all the layers are complete, the part is removed from the machine for cleaning and one final cure under a UV light source.
How it’s used: SLA is one of the most popular rapid prototyping processes because it produces dimensionally accurate parts in one to two days. Product designers can build the parts into assemblies to check fit and identify potential problems. For example, the clear resins give you the ability to see inside the enclosure to spot potential problems within. You can also sand and paint the parts to make cosmetic models. The downside of SLA is that the photopolymer resins only approximate the mechanical performance of ABS or polycarbonate (PC), the most common plastics for consumer electronics, so you may have to machine another set of prototypes from ABS or PC to perform reliability testing. We’ve found SLA to be most useful in the early stages of a project where risk areas must be mitigated as quickly as possible. SLA was particularly useful on a recent project for a consumer electronic device that required solutions for acoustics, wire routing, feedback to the ID team, and a complex flip-out USB connector. We quickly designed the parts, submitted the parts to an SLA shop, and had prototypes built two days later. The working prototypes validated the design, and helped us identify other risk areas to tackle in the next iteration. Once the design was stable, we machined parts from ABS for preliminary reliability testing.
Selective Laser Sintering (SLS)/ Direct Metal Laser Sintering (DMLS)
How it works: SLS is similar to SLA, but uses powder instead of photopolymer resin. In SLS, a laser cures small granules of nylon powder into any shape. The powder is available in a variety of blends, some of which are fuel resistant, heat resistant, or reinforced with glass for stiffness. The process is very fast, so big parts can be produced in one to two days. Additionally, the parts are ready to use right out of the machine which saves time as well. Another attractive feature of this process is the ability to sinter multiple parts of an assembly all together at once in its assembled state. The DMLS process is similar to SLS, but fuses small granules of metal such as stainless steel. The parts require some secondary processing, but it is possible to produce complex metal parts in a few days.
How it’s used: Product designers can use SLS to produce accurate and rugged models in days. This process is particularly useful for producing large parts, units that will get tossed around a bit, or pieces that require electroplating for EMI. SLS is less favorable for cosmetic models because the parts require multiple cycles of sanding and priming before painting. Like SLA, it may be necessary to machine a set of parts for reliability testing.
DMLS is great for fast turn metal parts with complex surfaces in low volumes. The parts can be used for fit check, to identify potential problems, and some mechanical testing. DMLS is not a replacement for 5-axis machining or casting, but can be useful when time is tight.
How it works: Like SLA, Polyjet is an additive process that builds parts in layers of UV curable resin. Unlike SLA, Polyjet uses an inkjet style head to deposit the resin in very thin layers on the platform (less than .001” thick). The extra-thin layers create parts that are dimensionally accurate, and the inkjet head is very fast. Polyjet has resins in various colors that mimic acrylic, but they also have rubber like materials that can be used in the same machine. In fact, some of the newer machines can deposit plastic and rubber in the same build making it possible to make overmolded parts, or parts with a living hinge. The Polyjet machine is designed with safety in mind, so it can be installed in an office like a copy machine.
How it’s used: Polyjet parts are fast and very accurate. Product designers can use Polyjet in place of SLA to check fit and identify potential problems. However, the finished parts are brittle and less rigid than SLA, so the designer has to analyze their geometry and testing requirements before choosing Polyjet. Given the office-friendly nature of Polyjet machines, engineers can fully integrate check models into their design process. For example, I once had two in-house Polyjet machines at my disposal. It was extremely convenient to design a part in CAD, print a 3D model, check things out, and update the CAD. A few iterations produced a design that was ready for production, and only required one round of machined prototypes for reliability testing. This reduced time to market and the machine shop workload.
How it works: Machining is the opposite of all processes discussed thus far, in that it removes material from a block instead of adding it layer by layer. Machining is the most versatile process because you can make almost any part from nearly any material in the same machine. The downside of machining is that it requires a great deal of up-front computer programming by a trained operator to tell the machine where to remove material. This typically makes the lead time longer than rapid prototyping, and the cost is higher for a hand full of parts. However, once the machine is set up, it can run identical parts over and over which reduces cost for higher volumes of parts.
How it’s used: Machining can be used in any phase of a project. Unlike the other processes, product designers can order parts in the material they plan to use in production (or very close). They can use the parts for fit check, to look for potential problems, and perform mechanical testing. Machined parts are also well suited for cosmetic models, though the rapid prototyping methods could be a better choice if a low-touch cosmetic model is the only deliverable. If the models are handled repeatedly, we recommend machined parts. For example, we recently helped a client launch a new product at a tradeshow with lots of press coverage. The models had to be beautiful and durable so the press could handle them. The final assemblies looked like perfect production units, and still look great months after fabrication.