Stereolithography (SLA)

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.

Cross section of SLA machine

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.

Cross section of SLS machine

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.

Polyjet

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.

Machining

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.

Machining can produce complex shapes out of a vast number of materials

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.

At my house, it’s not enough to love great products and every detail of how they were made. That fact is obvious to anyone who’s seen my less-than-interested daughter hold her ears and run out of the room screaming at the first peep of conversations involving “machining” or “part line.” Product design infatuation was clearly part of our marriage vows, along with brewing strong coffee, making soufflé, and having and holding until the end. But those who know my situation best know that a keen love of motorsport was also part of the pre-nup. So when Formula 1 starts using a new method of rapid prototyping in metal, well, the pairing of the two topics—racing + product—seems almost cause for a celebration where I live, or at least a multi-hour discussion of the method’s potential over dinner with our equally obsessive friends.

Real Metal Parts from an Astonishing Prototyping Process
(photo courtesy of 3T RPD)

Race Tech, a publication widely read among what must be tens of fans across the English speaking world, had an article in its January issue about Direct Metal Laser Sintering (DMLS) getting some traction with Formula-1 teams. This excerpt might pique your interest:

Advances in the latest sintering technology are likely to turn the whole design rulebook completely on its head and enable designs in metal that would have been completely impossible in earlier times. Design practices associated with traditional manufacturing—turning, milling, drilling and other techniques—could in the future no longer apply and components will be designed solely on their functionality.

To take simple example, holes could be positioned more accurately and in places where previously drill access was totally impossible. The possibilities are endless! Gone also is the concept of tolerance dimensioning because in theory, at least, every part made using DLSM [sic] is exactly the same as the previous one. The future, at least in design and manufacturing, starts here.

Metal parts without machining, casting or tooling! Parts that are functionally near-manufacturing durability and can be used for testing, assembly trials and final design validation!? I imagine this technology having a similar impact to when stereolithography (SLA) took off. Only in this case, the parts are durable, with finishing could emulate a final manufactured part, and can have details that were never possible with conventional fabrication methods. Just to be 100% clear, they are putting these prototype parts in actual cars and using them to test.

For comparison, consider nylon sintering, called “Selective Laser Sintering” (SLS), if you are familiar with that approach. SLS for prototyping plastic parts has been around a bit longer and is a lot more accessible than metal sintering in terms of availability and cost. Nylon sintering makes durable parts, but the parts are hard to finish relative to an SLA, for example, which can be easily sanded. Metal laser sintering, on the other hand, is made with much finer layers than its nylon cousin, which means it has a nicer surface to begin with, has greater strength in the z-axis, and—with finishing— is as cosmetically beautiful as a “real” part.

Nylon Chain Mail Part Shows How Sintering Allows Detail Not Possible with Traditional Fab Methods – This Mesh Is Constructed As All One Part, Not Put Together from Separate Rings
(photo: MindTribe)

DMLS works by melting a powdered material layer-by-very-fine-layer with a laser. The powdered substance contains a mixture of hard and soft materials. (The composition of the powders appears to be undisclosed and proprietary.) The build platform is currently 250mm x 250mm x 215mm high (10″ x 10″ x 8.5″). At each layer, the laser melts the softer substances so that the harder, higher melting point metal is held in suspension. The fusing process has evolved to the point that it almost completely melts the entire powdered mixture. And perhaps more exciting, the alloys have evolved to include not only bronze and nickel but also different steels and even titanium.

DMLS can produce parts with comparable or better properties than casting, including tensile strength, yield strength and elongation, but it cannot yet meet tightest tolerances and surface finish requirements without secondary machining, bench work and polishing.

Metal Sintered Engine Exhaust Parts Can Be Put in a Car and Tested
(photo courtesy of 3T RPD)

The DMLS machine, which is made by EOS, starts at a pricey $600K to buy, but supposedly some model shops are starting to have them. Given that the technology has only recently moved from the aerospace industry to Formula-1, that bastion of low-cost engineering prototyping (cough), most of these model makers are near the motorsport centers in Europe.

Sue Burnip from 3T RPD in the UK helped me out with all of the photos of the metal parts for this blog. 3T RPD specializes in SLS and DMLS, and is one of the largest LS providers in the UK – home of several F-1 teams. (Race enthusiasts may be interested in 3T RPD case studies, including work for the Jordan-Honda team.)

Aerospace Part Made with DMLS and Secondary Finishing
(photo courtesy of 3T RPD)

In the SF Bay Area, I found Prototypes Plus to be the only local option for sintering, but only nylon sintering, which is interesting but not METAL. Dylan Ternes showed MindTribe the EOS SLS machine and some samples of nylon sintered parts, including parts made with a small percentage of carbon, glass, and aluminum. He explained that the nylon-based parts seem best suited for prototyping plastic components that are functional pieces on the inside of a product and for not the cosmetic outer enclosure.

While Prototypes Plus does offer secondary processes for making the SLS parts cosmetic, the approach doesn’t lend itself to finishing the way SLAs do. Prototypes Plus plans to get a DMLS machine in the future. Dylan has personally checked them out, and he says the metal sintered parts are truly awesome. He adds that metal sintering is “really expensive” at this time.

Dylan at Prototypes Plus Discusses Laser Sintering

Complex Part that Prototypes Plus Made with Laser Sintering
(photos: MindTribe)

My colleague Lionel provided a sample part made with nylon sintering that might help bring home the truly marvelous flexibility of this approach. The spring hook, including the captured functional spring, rotatable strap feed, and other moveable features, was made as a “single part” using nylon laser sintering. This part is not brittle like an SLA would be, so it does not break in use.

Spring Hook with Functional Captured Features Made AS A SINGLE PART with Sintering

Nylon Laser Sintered Parts Are Not Brittle Like SLAs
(photos: MindTribe)

Just to wrap, as I consider some of the metal details of products my husband has worked on, it occurs to me that metal prototyping may have taken months of quality time away from my marriage. I am sure I watched a lot of F-1 on my own in the 2006 season, in particular. Indeed, my product-consumed partner seems to salivate over the Race Tech article, and I vow to find out more about these machines and how far they are to becoming accessible – if not for the rest of us, at least for those like Apple who can afford to take the pole position for new design methods.

Dyed Nylon Sintered Hands
(photo: MindTribe)