Tuesday, August 19, 2014

Techy Tuesday - Plastic Screws and Bolts As Strong as Steel

I'm always keep an eye out for the really cool or "how about that?" things in the stream which crosses my desk everyday.  I have gone through some periods where not much came across, but usually there's a few every week.  Tonight, thanks to Technical Editor in Materials Ann Thryft at Design News, we learn of new fasteners from Piper Plastics.  Plastics which approach - and can exceed - the strength of metal fasteners.
Kyron MAX is a new line of injection-moldable composites, with both glass and carbon fiber versions, from Piper Plastics. The material comes in three performance levels. Depending on the combination of polymer type and fillers, tensile strength can reach from up to 50,000 psi (345 MPa) to as high as 120,000 psi (827 MPa). That last figure puts it above steel, Dave Wilkinson, materials engineering manager for Piper Plastics, told Design News. Tensile modulus ranges from up to 5 million psi (35 GPa) to as high as 12 million psi (83 GPa).

The composites are almost 75% lighter than steel and about 60% lighter than titanium. Polymers include PEEK, PPS, PEI, PPA, and PA. "The strength of any fiber-filled polymer is the strength of that fiber combined with the strength of the fiber's adhesion to the polymer," Wilkinson told us. "So we've developed a stronger fiber and a new sizing technology to adhere the fiber to the polymer. Depending on the application's mechanical strength needs, we use either short or long fibers."
Pictured here, Piper Plastics used one of the highest-performing XS series Kyron MAX polymer grades to mold a standard #10-32 bolt for replacing titanium aerospace bolts. The part exceeds target minimum tensile load at 741 lb and double shear strength at 1,890 lb while reducing the weight of the titanium bolt by about 60%.   They also say it beat the cost targets but don't say if it's supposed to be cheaper than the titanium bolt; with that kind of weight reduction, it might well cost more.  (Source: Piper Plastics)

Piper presents this table of the capabilities of this plastic:
As Ann points out, Piper Plastics is a machining/injection molding company.  I think that means that these threads can't be cut by conventional means like taps or dies or single point threading on a lathe.  It would be interesting to try.  It's not uncommon to booger up the threads on a bolt while assembling things (who? me?) and the ability to clean the threads up with a tap or die would be nice.  On the other hand, if it's cheaper than metal, too, why not just pitch it and use a fresh piece of hardware?

There are still obvious questions here.  For example plastics are known for cold flow and creep, and a bolt stretching out under its intended load and getting looser isn't much of a bolt.  Still, that should have shown up by now in early development, and there's nothing else that won't be resolved by some standard tests.  An interesting, cool development.


  1. And NO corrosion worries, either!

  2. I wonder if "injection moldable" means it can be put through a 3D plastic printer.

  3. Great hoogely moogely! Think how fast your car would go with the weight reductions possible with this material. Presumably, they could do frames as well.

  4. Definitely a place for them in the aircraft manufacturing and repair business. Corrosion is always a problem on commercial aircraft with both the fastener and the fastener hole. Even with skins going to composites there are a lot of items that need fasteners and weight is an issue.

  5. Anon 0428 - I doubt they're printable. Injection molding is done under pressure and I don't think their special bonding process would work with 3D printing. That innovation will have to wait for a while.

  6. I'd suspect cutting threads might be a problem; bolts with rolled threads have higher tensile strength than bolts with cut threads because when cutting threads the grain of the steel in the threads is also cut. Chasing threads might be OK, though, because that's usually just thread edge and face cleanup.

    I'd be interested to see how they perform in sensitive torqued applications. There are a few places on aircraft - usually in or on engines - where proper torquing is done by measuring bolt stretch rather than rotating torque measurements with a torque wrench. That assumes these fasteners have sufficient heat tolerance.