FASTENER + FIXING TECHNOLOGY
Tri-Step™ - A step change in the design of threaded inserts for plastics
P
SM International was challenged by one of its major OEM customers to develop a new range of heat installed inserts for plastics that would reduce weight, whilst providing a similar level of performance to the brass inserts they currently use. For good measure the
customer also requested both a product and in-place cost reduction. This request
required PSM International to adopt a ‘blank sheet’ approach to insert design and manufacture. Traditionally inserts for plastics have been made from brass
and manufactured using single spindle and multi-spindle automatic lathes. The PSM design team set the following design objectives:
• High pull-out resistance. • High direct torque resistance. • High thread strength – 70% safety margin when used with grade 8.8 bolts.
• A large insert bearing surface to prevent torque induced jack-out.
• A moulded hole in the host plastic that is easy to produce and measure.
• Excellent location properties to aid automatic installation. • Plastic flash free insertion. • Excellent inserted height tolerance capability. • Good insert orientation for auto feeding. • Insertion cycle time reduction. • Weight reduction. • Product and in-place cost reduction.
The PSM team started with material selection - titanium,
plastic and aluminium were the three materials considered. Titanium although having an excellent strength to weight ration was quickly ruled out due to material and manufacturing costs. On investigation it was found that plastic had previously been used to manufacturer threaded fasteners. However, the automotive industry had not readily adopted them, largely due to their low thread strength and thread creep limitations. Also the method of assembly had been largely limited to self tapping and plastic welding of high glass filled fasteners manufactured from the same plastic as the host moulding. PSM considered the use of thermosetting plastics due to their high strength and greater resistance to thread creep. However, high volume
48 Fastener + Fixing Technology • Issue 5 Apr - Jun 2012 Hall: 21 Stand: D53
The automotive industry is constantly looking to reduce weight to improve performance, fuel consumption and vehicle range. Previously the focus has been on large components offering significant weight reduction; however, attention has filtered through to small components, including fasteners.
manufacturing of threaded fasteners proved both complex and expensive. Recycling thermosetting plastic from the host thermoplastic host moulding was also considered to be a major drawback. Aluminium being one third of the weight of brass and steel
ticked the weight reduction objective and was available in strength grades similar to brass. PSM consulted an expert in aluminium, which turned out to be a very wise decision. Aluminium material selection is quite complex, it is available in a number of alloy types, 2000, 5000, 6000 and 7000 series being the most widely used for high strength load bearing applications. Considering tensile and yield strength, corrosion resistance, temperature resistance and very importantly method of manufacture, PSM chose both a 5000 and a 6000 series alloy for its new inserts for plastic range. The final alloy selection is dependent on the application, method of installation and method of manufacture. The PSM team then looked at alternative methods of
manufacture to bar turning to reduce cost. Die-casting, stamping, sintering and cold forming were all considered. Of the four options, cold forming was deemed to be the best alternative process due to its reduced manufacturing cycle time, high material utilisation and that both 5000 and 6000 series aluminium could be cold formed. However, cold forming has limitations compared to bar turning and these had to be factored into the new design. Also cold forming has high initial tooling costs that tend to limit the process to very high volume production. As a result, bar turning was still viewed as being a viable process for lower volume orders and complex insert geometries not ideally suited to cold forming. PSM International has been designing inserts for plastics
since the 1970s, with the previously patented Sonic-Lok design, still a widely used insert around the world today. The Sonic- Lok and the majority of the PSM insert range have parallel diameters designed to suit a mating parallel hole with typically a 1° draft angle taper. The key benefit of this combination is good performance without creating plastic flash or insert ‘spring back’ after installation. An alternative tapered design has the benefit of improved location when inserts are automatically installed, however if does suffer from plastic flash and spring back. The PSM
design team wanted to create a new insert that combined the benefits
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