Is it generaly acceptable practice to produce blank holes in plastic bosses and allow a metal screw to self-thread the hole?
There is a lot of data available on the web. Here's a start:
Once you narrow down the vendor for the screw you can ask for design guideleines specific for that screw and the material you are using. It's not as straight forward as it might seem.
I would yes as it will make the self threading screw easily to go inside the boss.
Thanks for the links. The first one, from DuPont, is pretty old and doesn't include the latest fasteners. It also focused very much on the DuPont materials. The second seems to be based primarily on the first. The third is new, but focused only on Bayer materials. I liked the discussion of insertion speed and learned some helpful facts for where to begin. Between the three, they do a reasonable job of showing how to use self-tapping screws in plastic.
For the original poster, we like the Delta PT screws, from EJOT in Germany, available from Acument, Semblex, and ATF in the US. As Harold said, it is not as easy as one might think to screw pieces of plastic together, depending on what your requirements are.
Jerry - I couldn't find some of my favorite links since I haven't used them in a while (at least a couple PC's ago) but like I mentioned, there are a ton of resources out there. Google is my friend. I am rather partial to Hi-Lo threads in plastics especially when the screw needs to be re-installed a couple times. Ultimately there is nothing like actual pull tests and torque tests once parts are made. Thanks for the source references. I'll be sure to look them up and add them to my favorites before I lose them too!
I suspect that the design guides will point to this, but will make you aware of it anyway.
When designing bosses it is important to have a short relieved section of the hole in the boss larger than the OD of the thread. This only needs to be 1-2mm long. The idea is that the thread form creates a large stress concentration where it first enters the boss. The hoop stress then splits the boss starting from this point. By relieving the top of the boss, this intact hoop then withstands the hoop stress.
This is important for both thread cutting and thread forming screws. The thread cutting creates a sharp notch. The thread forming adds large hoop stress.
Thank you for your replies.
Cheers for the links Harold, I will take your advice and identify a screw then talk with the manufacture. Always good to ask questions aye! Nothing's ever as simple as it may seem.
Also thanks for the general info Craig.
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