Fair Fight for the Light Weights
By Shannon Wetzel, MetalCastingDesign.com
If you’re producing a structural part out of an engineered plastic because it’s inexpensive, it might be time to rethink your material choice.
Magnesium is a competitive option for many typically plastic applications, and the growing amount of conversions from plastic to magnesium point to the metal’s optimal physical properties, as well as design and cost advantages.
The plastic used in structural applications is most often an engineered plastic, such as ABS, reinforced nylon or a polycarbonate. Added reinforcements help give plastic the strength needed to meet the application’s requirements, but they also add density. The density of magnesium is quite similar to that of reinforced plastic, so the two materials generally weigh the same. The strengths of both materials also are on par, but magnesium has superior stiffness, which is essential in many structural
“The Achilles heel of plastics is that while it can be strong, it doesn’t offer stiffness,” said Thomas Ruden, director of global business development for Quay Magnesium Ltd., Sydney, Australia. “Any plastic application that is considered a structural part is a good candidate for conversion to magnesium, especially in applications that need a lot of strength and stiffness.”
Poor stiffness means plastic parts vibrate more than magnesium parts, making magnesium castings a good choice for automotive applications. Plastic is more prone to bend, as well. According to Ruden, magnesium alloys offer an elastic modulus of around 6.5 million psi, whereas the stiffest plastic offers a modulus of 1-2 million psi.
A few specialized magnesium alloys offer elongation properties that are competitive to plastic. Many applications that make good conversion candidates can be found in the sporting goods (tennis frames, snowboard bindings, brackets), automotive (cam covers, valve covers, air inlet manifolds) and electronics (cell phones, cameras, computer cases) industries. Magnesium cases for rugged, high quality laptops are becoming standard because of the durability and sleek look of the material.
Material for a Better Design
With equal strength, superior stiffness and competitive elongation, magnesium already shows itself as a viable alternative to plastic, but its design and process advantages are compelling, as well.
Magnesium is more easily machined than other commonly used structural metals and has excellent dimensional stability. It is high impact and dent resistant, and its damping capacity and low inertia make it a good choice for parts that will undergo frequent and sudden changes in its direction of motion.
The structural reinforcements used in plastic to beef up its strength keep the material from achieving walls any thinner than 0.12 in. (3 mm). Magnesium is routinely cast to 0.12 in. wall thickness and in some cases to as thin as 0.03 in. (0.76 mm) wall thickness. Plastic’s properties also work best when the wall thicknesses are uniform. Varying thicknesses could require extra molding steps or eliminate plastic as an alternative altogether. Magnesium, however, is easily cast into varying thicknesses and shapes. Thinner walls contribute to lower-weight designs than plastic. Eliminating extra steps to achieve a thin wall keeps manufacturing costs low.
According to Ruden, the price of magnesium also is competitive with plastic, particularly in today’s market, where magnesium can be obtained for about $1.10 per pound, compared to $1.50-2.25 per pound for plastic. Ruden said this is owed largely to the mass quantities of magnesium production coming out of China in the last few years.
One of the industries with the most opportunities for plastic to magnesium conversions is the electronics industry, as trends for smaller, thinner components continue.
As a group, metals have several advantages over plastic for electronic applications, including:
• Inherent conductivity and EMI shielding capability;
• Low raw material costs;
• Structural strength in thin-wall designs;
• Durability in service.
Unlike plastic, magnesium has the capability to achieve the desired thinness for applications such as cell phones, mp3 players, laptops and stereo units. Because these applications are less sophisticated designs than those for automotive applications, tooling can be set up much quicker. Electronic applications tend to have production lives of six months to one year, so quick tooling is important to keep up with the constantly changing technologies.
As the design of electronics moves toward more power in smaller components, the ease of manufacturing, heat resistance of metals, and the ability to include heat sinks in the castings also become more significant factors. You might have noticed the amount of heat that laptops can generate on a long plane ride. Magnesium cases keep cooler longer than typical plastic-framed laptops.
Electronic devices that carry an electric current emit electromagnetic radiation, and this emission can interfere with the function of nearby electronic devices. The critical need to shield against EMI gives magnesium an advantage over plastic.
An EMI shield prevents the emissions from entering or leaving an electronic device. Radiation hitting a shielding barrier is either absorbed, reflected or transmitted. Because metals are conductive, they reflect and absorb the electromagnetic radiation to enough of a degree to avoid interference. Most of the energy is usually reflected with metal shields, although if the magnetic field is dominant, the energy will be absorbed.
Plastics, however, are insulative and transparent to electromagnetic radiation. In order to accommodate plastic’s insulative properties, engineers will modify the material by adding metal particles to help meet the shielding requirements. This modification to the material is costly and takes away the economic edge plastics may have had over magnesium diecasting’s manufacturing costs.