Screw inserts for plastics may not seem all that expensive. Unless, that is, your application requires hundreds of thousands or millions of them. For these high-volume jobs, PennEngineering Fastening Technologies has developed a patented insert that costs 50 percent less than standard inserts.
A new manufacturing process makes these low-cost DeltaForm™ inserts possible. Whereas threaded inserts have traditionally been produced on screw machines, Penn produces the DeltaForm in a progressive stamping operation. Made from 0.020-inch-thick brass, the insert starts out as a flat blank. On the last die, it "folds up" into its finished triangular shape and the faces lock together in a keystone arrangement. Penn stamps the threads too—onto the three faces of the flat blank. These stamped threads accept standard machine screws just as cut threads would.
Despite its triangular cross-section, the DeltaForm still installs in round holes, thanks to barb-like features on its outer surfaces. Flanges on either end of the DeltaForm aid installation too; lead flanges orient the insert in the hole, while the ones at the insert's trailing end form a flush surface with the plastic component.
As a very conservative rule-of-thumb, the DeltaForm achieves roughly 75 percent the pullout and torque strength of standard inserts. A 6-32 DeltaForm in ABS, for instance offers a 70-lb pullout strength and a 17 inch-lb torque-out. For most applications, this trade-off shouldn't pose any problems because the thread strength on the DeltaForm exceeds the strength of the surrounding plastic. When strength is paramount, though, the company continues to recommend standard inserts. For this reason, DeltaForm applications will mostly involve smaller sizes under 8-32 and M4.
Get a grip: Cost-conscious engineers sometimes choose to replace a machine screw and insert combination with thread-forming screw alone, but they run the risk of cross-threading the plastic. The DeltaForm insert provides twice the torque-out strength of a thread-forming screw at half the price of a screw-machined insert.
CONTACT: Jay McKenna,PennEngineering Fastening Technologies
Tel (215) 766-8853 x1229; Fax (215) 766-3633; e-mail: