Prototyping Materials and Processes for Automotive Lightweighting

Everybody in the auto industry is looking for ways to shed a few pounds. Well, make that a lot of pounds. There are myriad material options to make it happen. The question is, how can you make the best choices and get them into test quickly? The design and modeling software tools available today are a real boon to the early stages of the product development process. But there’s still a need for physical, functional prototypes to prove out design assumptions. That needs to be fast too.

Protolabs specializes in rapid prototyping for exactly this requirement. In many cases, you can upload a solid model to its website and literally get a part back the next day. They don’t just know materials; they also know manufacturing processes including 3D printing, CNC machining and injection molding. Here are some tips from Protolabs on how to get to lighter parts faster.

Magnesium Instead of Steel

One thing to remember before embarking on any lightweighting project is to take small bites. Shaving ounces and even fractions of ounces out of each component will end up making a significantly lighter car. The trick is to develop products that fulfill cost and duty requirements but use alternate materials and clever designs to reduce weight. Fortunately for designers and engineers, today’s array of prototyping materials and advanced manufacturing technologies are creating new opportunities for iterative, even parallel-path design testing.

Magnesium is a good place to start. With a density of 106 lb. per cubic foot, magnesium is the lightest of all structural metals, and has the highest strength-to-weight ratio as well. It carries a proven track record in the automotive, aerospace, medical and electronics industries, and is used in everything from fuel tanks to gearboxes. For example, BMW started using magnesium for its N52 six-cylinder crankcases and cylinder head covers in 2005.

Magnesium is routinely milled into a variety of prototype parts. Compared to aluminum, the lightweighting runner-up, it is more expensive per pound, but that cost delta is offset somewhat by magnesium’s 33-percent lighter weight and comparable strength. It’s also easily machined, although some care must be taken to control fine chips and metal particles, as these can be flammable in oxygen-rich environments.

The AZ31 and AZ91 grades of magnesium alloy used at Protolabs are even weldable with melting points of roughly 900° F (482°C). Unless you’re designing a lightweight furnace liner, magnesium is an excellent choice for many different components.

Plastic Instead of Metal

Magnesium and aluminum are excellent alternatives to steel for lightweighting, but thermoplastic and thermoset materials are robust possibilities as well. An extensive selection ion of glass-, metal- or, ceramic-filled polymers as well as liquid silicone rubber (LSR) can also be used to replace metal parts, thus reducing product cost and weight while improving durability. Some of the best alternatives include:

Polypropylene is a flexible, fatigue resistant family of thermoplastics commonly used in automotive interiors, battery cases, boat hulls, prosthetics and other products requiring toughness and light weight. They feature superior strength-to-weight ratios and good impact resistance even at cold temperatures.

Polyethylene has mechanical properties similar to polypropylene but is more rigid and offers greater resistance to warping. Because of its low cost and relatively high strength, polyethylene is well suited for the interior of a glove box, perhaps, or a cold air intake.

ABS is another thermoplastic with exceptional impact resistance and toughness. It is a lightweight alternative to metal used in dashboard trim, electronics enclosures, hubcap covers and other such automotive applications. Injection-molded ABS is also available in either flame-retardant or anti-static grades in a rainbow of colors. Chrome-plated ABS is used on hubcaps, grills and fender flares.

An extensive selection of glass-, metal- or, ceramic-filled polymers as well as liquid silicone rubber (LSR) can also be used to replace metal parts.

Polycarbonate is a transparent material often used for thermoformed parts where glass is unsuitable, due to weight or breakage concerns. It has 250 times the impact resistance but only half the weight of regular glass, which is why “bulletproof glass” and aircraft windows are actually made of polycarbonate or its slightly more flexible cousin, acrylic. Protolabs 3D prints this material with 10-percent glass-filled polycarbonate for functional prototypes. Another grade can be used for high-temp applications. Similar grades of polycarbonate are available for machining or injection molding.

Nylon is one of the strongest plastics available at Protolabs and is an excellent candidate for sprockets, fan blades, gears, latches, manifolds and bearing surfaces. It’s also very light, with 15-percent the weight of steel and 40-percent of aluminum. Protolabs offers selective laser sintering (SLS) of several engineering-grade nylons, which can be used for functional testing of prototypes prior to machining or injection molding. One of these is Nylon 11, a material that works well for living hinge designs as used in hose and wire clips, washer fluid caps, and other automobile components.

Acetal, more commonly known by its trade name Delrin, is a regular go-to material for machined prototypes. It is strong and stiff and regularly called upon to replace precision metal parts in a range of industrial and consumer products. Electrical and fuel system components, power transmission parts such as gears, bushings, and bearings, and other high-performance parts can be milled or injection molded from different grades of acetal copolymer or homopolymers stocked at Protolabs.

Liquid silicone rubber (LSR), is a versatile material for many molding applications. Upon curing, LSR becomes strong yet flexible, and is suitable for gaskets, lenses, connectors, and other parts that require excellent thermal, chemical and electrical resistance. Wiring harnesses, panel buttons, spark plug boots—these are but of few of the places LSR can be found in modern vehicles.

Liquid silicone rubber is strong but flexible.

A new material at Protolabs worthy of mention is CoolPoly, a unique polymer moldable in hardness levels ranging from Shore A 40 (soft like an eraser) to Shore D 80 (hard like a bowling ball). It was created as a replacement for heatsinks, lighting shrouds and other thermally conductive parts normally made of aluminum.

Machining, Molding or Additive Manufacturing?

A good starting point is to understand which materials best lend themselves to each of the manufacturing processes. The old days of taking weeks to get a machined or molded prototype are long gone, and with a good plan you can often get a prototype manufactured in a day made from nearly every material previously mentioned.

Protolabs’ CNC machining centers mill prototypes from solid blocks of metal and plastic roughly the size of a thick encyclopedia, and lathes with live tooling cut parts about as large as a flower vase. Both processes hold tolerances to +/- 0.005 in. (0.13 mm) or better, depending on part geometry.

Another option for magnesium parts is a form of injection molding known as thixomolding. Here, chips of magnesium feedstock are loaded into the hopper of a molding press. Heat and agitation are then applied, bringing the magnesium payload to a semisolid state, whereupon it is shot into a mold cavity. The result is that fully functional magnesium components can be produced in low volumes at a fraction the cost of “production-tooled” parts. Thixomolding is essentially a “cold” process, operating just short of magnesium’s melting point. Because of this, there is less shrinkage and warp compared to die-cast parts, and the mechanical properties of thixomolded parts are generally better.

Feedstock for the thixomolding process.

Additive Manufacturing Processes

3D printing offers a great option because it’s so quick to go from solid model to part. Moreover, 3D printing has become much more adept at working with a variety of materials. The processes supported at Protolabs include:

Stereolithography is the grandfather of all 3D printing technologies. Protolabs uses it to print parts from nine grades of polymer across three primary groups: ABS, polycarbonate, and polypropylene. It’s important to note that these materials mimic plastics and are not rated for functional product use. For testing the form and fit of products destined for die casting, Protolabs offers SLArmor, a nickel-plated, ceramic-filled additive material that is very light yet still strong enough to pinch-hit for metal in certain cases, and an ideal solution for many lightweighting applications.

Selective laser sintering employs a laser to draw each part layer. Part features are somewhat less accurate than those produced by SL, but still plenty good for functional testing. Of all the plastic materials available at Protolabs, glass-filled nylon is one of the most popular materials with automotive manufacturers, largely due to its low cost and toughness.

Direct metal laser sintering (DMLS) melts layers of metal powder, as thin as 0.0008 in. (20 microns) and is used at Protolabs to create complex, 98-percent dense part shapes that are often impossible to manufacture otherwise. It is highly accurate with tolerances of +/- 0.003 in. plus an additional 0.001 in./in. that is typically achieved on well-designed parts.

DMLS works with aluminum and titanium, so it is an obvious contender for manufacturing lightweight parts, but can be used with 316L and 17-4 PH stainless steel, cobalt chrome alloy and Inconel. DMLS can fabricate hollow metal parts as well as parts with ultra-thin walls and spider web-like lattices. It allows consolidation of multi-part assemblies into a single, sintered component.

Final Considerations

Sorting through all the different possibilities is one of the biggest challenges with lightweighting. That’s because improving product design in the automotive world isn’t a matter of grabbing whatever material weighs the least and replacing the legacy steel or iron used previously. For example, plastic parts that will eventually be mass-produced via injection molding must be designed with the correct draft angles and wall thicknesses up front. Ejector pins must be considered, as should areas with undercuts, tight internal radii, and a host of other details that can make or break your lightweight part.

The good news is that, considering the array of prototyping and low-volume production services available at Protolabs—along with its in-house expertise—you have many different resources to help reduce part weight. If you have any questions regarding materials, manufacturing processes or design, please contact a Protolabs applications engineer at customerservice@protolabs.com or 877-479-3680.