One thing that should be remembered before embarking on any lightweighting project is to take small bites. The rear view mirror used in passenger cars, for example, was once heavy enough to pound a nail. Today, most rear view mirrors are made of a magnesium frame and a plastic shell, yet retain the same strength and functionality as their corpulent predecessors. 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 mean never before possible opportunities for iterative, even parallel-path design testing. When you begin to explore material selection, magnesium might be a good place to start.
With a density of 106 lb per cubic foot, magnesium is the lightest of all structural metals. It is routinely milled into a variety of prototype parts. Compared to aluminium, the lightweighting runner-up, it is more expensive per pound, but that cost delta is offset somewhat by magnesium’s 33% 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.
BMW & the Big Three with magnesium components
BMW started using magnesium for its N52 six-cylinder crankcases and cylinder head covers in 2005. The Volkswagen Beetle sported a magnesium alloy engine block for decades, and BMW started using magnesium for its N52 six-cylinder crankcases and cylinder head covers in 2005. The AZ31 and AZ91 grades of magnesium alloy used at Protolabs are even weldable, and have melting points of roughly 900-degrees F (482 C).
The Big Three are also big on magnesium. In 2006, General Motors began using die cast magnesium for the engine cradle in its Z06 Corvette, shaving 12 pounds off the old design. Ford Motor Company’s started using magnesium in the liftgate of the 2010 Lincoln MKT, and the third row passenger seats for the 2011 Ford Explorer. In that same year, Chrysler Group introduced a magnesium instrument panel for its Jeep Grand Cherokee, helping the vehicle achieve highway fuel economy of 23 mpg. In summary, domestic and foreign automakers alike are turning to magnesium for its strength, light weight and—because it can be extracted from seawater—its abundant supply.
Magnesium and aluminium are excellent alternatives to steel for lightweighting, yet thermoplastic and thermoset materials are robust possibilities as well. 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, thus reducing product cost and weight while improving durability.
Polypropylene (PP) 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. The different grades of polypropylenes are grouped as homopolymers, impact copolymers, or random copolymers, with the latter two considered engineering-grade materials, offering superior strength-to-weight ratios, and good impact resistance even at cold temperatures. All polypropylenes are heat-resistant, with melting points in the neighborhood of 300-degrees F (149 C).
Polyethylene is offered in both high-density (HDPE) and low-density (LDPE) versions. HDPE has uses and mechanical properties similar to polypropylene, but is more rigid and offers greater resistance to warping. HDPE’s lower density, more flexible counterpart LDPE sees use in squeeze bottles, plastic wrap, and playground slides. Both materials are suitable for use where toughness and low weight is needed.
ABS is another thermoplastic with exceptional impact resistance and toughness. It is a lightweight alternative to metal in dashboard trim, electronics enclosures, hubcap covers and other such automotive applications. Injection-moulded ABS is also available in either flame-retardant or antistatic grades in a rainbow of colours. Chrome-plated ABS hubcaps are used on a large number of passenger cars to reduce vehicle weight, as are ABS grills and ABS fender flares. Finally, the chassis of the world’s first 3D-printed automobile—the STRATI—produced onsite during the 2014 IMTS show, was made of carbon-fiber reinforced ABS. It weighs just 1,100 pounds.
Polycarbonate (PC) is very thermoformable, and is frequently shaped into see-through architectural panels, 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. On the Additive Manufacturing side of the house, Protolabs offers 10% glass-filled polycarbonate-mimic Accura 60 plastic from 3D Systems for functional prototypes with properties similar to commercial-grade PC, and Accura 5530 or ceramic-filled DSM Somos NanoTool for high-temp aerospace and automotive applications. Similar grades of PC are available for machining or injection moulding.
Nylon is also frequently filled with mineral or glass fibres to improve stiffness-to-weight ratios and improve mechanical properties. This makes it one of the strongest plastics available at Protolabs and—because nylon is self-lubricating, thermally stable and very wear-resistant—it is an excellent candidate for sprockets, fan blades, gears, latches, manifolds, and bearing surfaces. It’s also very light, with 15% the weight of steel and 40% of aluminium. Nylon 11 is 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 called by its trade name Delrin, is a regular go-to material for machined prototypes. It is strong and stiff without the need for nylon’s glass-fibre reinforcement, and is 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 moulded from different grades of acetal copolymer or homopolymers stocked at Protolabs.
Liquid silicone rubber, or LSR, is a surprising but versatile material for many moulding applications. It starts out as a two-part thermoset compound, which is mixed at low temperatures and then injected into a heated mould. 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 few of the places where LSR can be found in modern vehicles.
Weighing the manufacturing options
Despite the flexibility in material options, it’s a good idea to understand what material peg fits into which manufacturing hole. Machined prototypes were once produced on hand-cranked milling machines and engine lathes. They were largely made of steel, brass, or aluminium and took weeks to deliver. Protolabs has automated that same basic process to the extent that electronic part designs can literally be uploaded one day and delivered the next, and made from nearly every material previously mentioned.
Magnesium feedstock ships for injection moulding
Feedstock used in magnesium injection moulding to create 98% dense magnesium parts. A growing method to rapidly manufacture magnesium parts is through injection moulding (also known as thixomoulding). Here, chips of magnesium feedstock are loaded into the hopper of a moulding press. Heat and agitation are then applied, thus bringing the magnesium payload to a semi-solid state, whereupon it is “shot” under pressure into a mould cavity via a feeder screw. The result is that fully functional magnesium components can be produced in low volumes at a fraction of the cost of “production-tooled” parts.
Many manufacturing engineers associate magnesium with die casting, as this has long been the traditional, high-volume method of forming this ubiquitous metal. Yet magnesium injection moulding offers a number of distinct advantages over its more mature counterpart. Thixomoulding 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 thixomoulded parts are generally better as well. The cooler process also requires less sophisticated tooling, as there is little need for cooling channels. And since the magnesium slurry is fed into the mould at very high pressure—in some cases, twice as that of die casting—very fine part details are produced. All things considered, thixomoulding is a solid choice for low-volume magnesium components.
Technician removes gating material on magnesium parts
Magnesium injection-moulded parts before overflow gating material is removed via CNC machining. Another mature prototyping process is stereolithography (SL), the grandfather of all 3D printing technologies. Protolabs uses SL to print parts from nine grades of polymer across three primary groups: ABS, polycarbonate, and polypropylene. It’s important to point out that these materials mimic plastics and are not rated for functional product use. SL does, however, produce highly accurate prototypes, and is a logical first step for an initial “touch and feel” of lightweighted concept parts. And 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.
Next on the 3D printing list is selective laser sintering, which is limited to four types of engineering-grade nylon materials at Protolabs, two of which are reinforced for high-heat applications and greater structural integrity. Like all additive processes at Protolabs, SLS 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, although glass-filled polycarbonate comes in a close second. Because of this, SL and SLS are both very suitable for prototyping lightweight parts.
DMLS aluminium design with difficult-to-machine geometry
DMLS can be used to build complex aluminium parts that are difficult to machine. Another increasingly used Additive Manufacturing process offered is DMLS, or direct metal laser sintering. DMLS melts layers of metal powder, as thin as 0.0008 in (20 microns) at Protolabs, to create complex, 98% 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 are typically achieved on well-designed parts.
At Protolabs, DMLS works with aluminium & titanium, so it is an obvious contender for manufacturing lightweight parts, but is also used with 316L and 17-4 PH stainless steel, cobalt chrome alloy, and Inconel, super strong metals known for their extreme heat resistance and durability rather than weight reduction.
Fully functional 3D-printed metal parts
DMLS is slower than other additive processes, and more expensive—if your part design can be efficiently machined or moulded, DMLS may not be the right manufacturing method. But for complex assemblies, improbable shapes, or parts where small amounts of superalloy go a long way, DMLS might be just the ticket to reduce part weight and cut manufacturing costs. Lastly, DMLS isn’t limited to prototype quantities—given a small, complex workpiece too difficult or expensive to manufacture via conventional methods, DMLS is often a viable alternative for low-volume production volumes in the thousands.
Sorting through all the different possibilities is one of the biggest challenges with lightweighting. That’s because improvements to 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. Plastic parts that will eventually be mass-produced via injection moulding 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.
Additional factors to consider
Support ribs and honeycombed sections reduce part weight while retaining structural integrity, especially with injection-molded thermoplastic parts. Magnesium parts are 33% lighter than aluminium and a whopping 75% lighter than steel. Magnesium injection moulding is a fast and easy stepping stone to high-volume die-casted magnesium products. A thorough analysis of projected part volumes early on can avoid costly re-designs when quantities increase. Even though stainless steel, cobalt chrome and Inconel weigh more than aluminium and magnesium, DMLS technology can make these “heavy” metals a strong, weight-efficient alternative to lighter materials. Don’t be scared off by the higher expense of glass-filled plastics, as their greater strength can lead to significant weight reduction. Use CoolPoly plastics for heatsinks and thermal management applications. They have half the weight of aluminium and equivalent heat dissipation.