Chris, Plastite screws are a good choice where a) only fastening is being done in plastic , and b) there is sufficient boss strength and thickness (since being thread formers, they put significant tensile stress on the screw boss). If it were me, I'd consider a thread cutter such as a standard type BT sheet metal screw. Type BF could also be considered, but there its thread cutting action is done by simple cuts in the thread, and in my experience there isn't enough space for the plastic chips to "go" as the thread is cut. The BT screw has a nice large "scallop" at the front that cuts the thread and thus offers some where to accommodate the chips. Type T is similar to type BT, but the thread is much finer, and is not really suited to use in plastic. Cracking is definitely a concern. I have had problems with this in the past when using thread forming screws with less-than-massive screw bosses. Using a thread cutting screw allowed much smaller boss diameter, while still having good holding power. The only real down-side is that one must take a little care not to cut new threads when re-inserting the screw. Conductivity of stainless steel here is of little concern. Even though the screw will have somewhat poorer conductivity than a normal hardened steel screw, you have to remember that the bulk resistance of the screw is only a VERY insignificant contributor to the overall system resistivity. Besides you are using the screw to make contact to ....even more stainless steel! Nickel plating is nicely compatible with stainless, but there are no sheet metal screws that are made of brass! So if you go the route of using a type BT thread cutting screw, your choices are either plated high carbon steel or high carbon, low-300-series stainless steel. Self tapping screws in general must be hard in order to self tap! Your suggestion of using an (externally) threaded insert is a good possibility. Of course it WILL require (obviously) a larger boss diameter than if you used only a screw alone. I have used inserts with external threads that were thread forming, and also thread cutting in the past. The latter are promoted as not creating excessive tensile stresses in smaller bosses, and in my experience they worked well. Insertion into the plastic is done using an ordinary tapping head on a drill press. I used these some years ago and can try to dig up information on them if you are interested, Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Chris Wells [mailto:[email protected]] Sent: June 4, 2002 5:11 AM To: Robert Wilson; EMC-PSTC Discussion Group Cc: [email protected] Subject: Re: Bonding to conductive plastic - best method?? - BEST SCREW Robert - Putting inserts aside for a moment - What do you see as the best screw type? The "plastite" style high/thin/coarse thread screw we presently use does not appear to cut the plastic as much as it deforms and pushes into the plastic to make the threads. I could be wrong - would a self taping style like used on metal plate have a better performance? Keeping with the plastite style - We have been using about 80% engagement of threads and I have experimented with %100 but am concerned about cracking of the screw boss. Of corse the 100% not only has the thread engagement but the base shaft of the screw as well in contact with the plastic. On plating/base matterial - It sounds like SS screws will be my move but I wonder about the conductivity of SS in general. I take it that nickle plated brass would be uncommon and perhaps would also give a corosion problem. Back to the SS inserts - I will check pricing and availability of a self tapping style screw in insert. Thanks - I think I am getting closer Chris Wells Senior Design Engineer Cutler-Hammer Pittsburgh, Pa. USA 412 490 6862 [email protected]
