It is just over two decades since air bags started to appear widely in vehicles and nearly two decades since metal injection moulding was first used in air bag applications. Air bags were placed into steering wheels and dashboards giving the driver and, in some cases, the front-seat passenger some extra degree of safety to seat belts in the event of a collision. Over the past 10 years or so most governments have required frontal air bags to be fitted in all cars and light vehicles, but increasingly car producers are offering up to eight or ten airbags in many 2008 car models in order to appeal to safety conscious consumers. These include side, knee and seat mounted air bags, and already more than half of all cars sold offer at least a side air bag or side air curtain option. In the USA alone the usage of air bags in vehicles is expected to soar to over 100 million by 2012 which is nearly double that of the 57.6 million used in US vehicles in 2006.
Major global producers of air bags include: Sweden-based Autoliv (ca 35% market share), TRW Inc. (USA) and Takata Corp. (Japan) each with ca 25% market share, and Key Safety Systems Inc. (USA) – known as Breed Technologies Inc until it filed for bankruptcy in 1999. It was the late Allen K. Breed who was responsible for inventing the automotive air bag, and he was posthumously induced into the National Inventors Hall of Fame in Washington in 2007 joining luminaries such as Thomas Edison, Velcro inventor George de Mestral, and Charles Goodyear, developer of vulcanised rubber.
There are basically two competing air bag systems in use today. The ‘Electronic Sensor’ system where the air bag is triggered electrically via crash sensors, and the ‘Mechanical Air Bag’ sensor system. In the latter system an inertial ball actuates the D-shaft when sufficient jolt is created on impact , which then releases the ‘firing pin’ which is preloaded in a tube. The firing pin with its pointed tip ignites a primer which sets off the sodium azide pellets to inflate the air bag and cushion the impact. The mechanical system eliminates the need for a power supply as well as any control electronics or external crash sensors. The cycle takes only around 40 milliseconds and the air bag deflates in approximately 2 seconds.
In 1991 an assembly of three metal injection moulded (MIM) parts used in a mechanical air bag sensor system produced by Breed Technologies and used in Jaguar cars, won a Grand Prize in the Metal Powder Industries Federation Powder Metallurgy Parts Competition. The parts were developed in conjunction with Breed Technologies by MIM producer Parmatech Corp of Petaluma, Calif., and involved a D-shaft, tab insert and firing pin – all made from 17-4 precipitation hardened sintered stainless steel. The MIM parts were produced to a sintered density of 7.6 to 7.68 g/cm3 and were said to offer a combination of rigidity, high strength (1170 N/mm2 tensile), plus wear and corrosion resistance. Tolerance control and repeatability of dimensions in manufacturing were also critical to the success of the MIM parts.
An example of a MIM ‘firing pin’ is shown in Fig.1. The thin-wall thickness of this part together with its shape (small tab and pointed front end) makes it an ideal MIM part. Similarly, the relatively thin arms and the different cross-sectional thicknesses on the D-shaft also makes this an ideal MIM part (Fig.2). Smaller versions of these MIM parts known as ‘Mini Firing Pins’ and ‘Mini D-shafts’ have been made from Fe-2%Ni feedstock material.
MIM parts for air bags are still in production today, including in China, where leading metal injection moulding producer Eversun Technology Corp in Qinqdao offers a selection of MIM components for mechanical air bags to the Chinese and global automotive industry (Fig.3).