How To: Threaded Inserts welded into your plastic parts
by EricYoung, published
Threaded inserts rock. Hard. If you don't know what they are: http://en.wikipedia.org/wiki/Threaded_insert
I was lucky enough to learn this threaded insert installation method at work years ago. After perusing Thingiverse for a while and not seeing anyone else using threaded inserts (after looking over maybe 500 or so things) I thought I'd share this tasty morsel of design goodness with your knowledge-noshing neurons.
This method provides for good torque and pull-out resistance, especially if you use the inserts specifically designed for this soldering-iron insertion method. I've seen lots of people using 'captive hex-nut' designs in their parts here on Thingiverse, and threaded inserts can often be used in a similar manner to provide an alternative and sometimes "better" solution.
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It's so easy - just use a soldering iron (heated up) to press the insert into a hole in your plastic part. The tip of the soldering iron should fit into/onto the top of the insert so that you can apply a steady downward force when installing it. If you have multiple types of soldering iron tips, choose the one that seems best for the particular size threaded insert you're using.
Don't push too hard because it's easy to sink them too far into the part if you're not careful and the plastic gets too hot - just ease it in there nice and steady. If the plastic starts melting too quickly then remove the soldering iron and let the plastic cool off a few seconds, then try again. Aligning the axis of the insert properly with the hole can be difficult at times so pay attention to make sure it's not at too much of an angle. After the insert has been installed there may be some melted plastic around the hole that needs to be filed or sanded off.
The holes in your parts should be designed a little undersized so that there is some plastic that can melt around the grippy exterior surface of the insert. I typically make my hole diameter somewhere between .25mm and .5mm (.010"-.020") undersized from the diameter of the insert I want to use.
There are types of inserts specifically designed for inserting into plastics using a soldering iron, but I've found that for most applications the push-in kind will work just fine with this soldering-iron installation method. At my local ACE hardware store they only sell the push-in type, so that's what I've been using. Mcmaster has a wider variety of threaded insert types, including the type for thermoplastics. Here is their main menu page for inserts: http://www.mcmaster.com/#standard-threaded-inserts/=jbhnkg
I've also found that threaded inserts for wood can be used in plastics if you have a soldering iron tip that is similar to a flathead screwdriver and can torque the inserts into the part - see that last attached picture.
The attached files are reference CAD model libraries for all press-in style inserts available from PENN Engineering. The PDF drawings show all PN's and their respective dimensions. The ZIP files contain the STL and Solidworks files for each insert shown in the drawings.