How To: Threaded Inserts welded into your plastic parts

by EricYoung, published

How To: Threaded Inserts welded into your plastic parts by EricYoung Sep 16, 2012

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Threaded inserts rock. Hard. If you don't know what they are:

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.


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:

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.

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NICE! I am taking this handy method straight to my Rapid Prototyping class. The students will LOVE THIS!

Thank you for posting this! I just got an opportunity to model and print a prototype part for a local inventor, but he wants threads in some of the holes and that is beyond my abilities. This is fantastic and I'm almost certain it'd be preferable to printed threads anyway.

I was to print a "fork" to hold a blind rivet nut screwed on a bowden tube -- see, e.g, http://www.thingiverse.com/thing:33379http://www.thingiverse.com/thi... , but your idea could prove better if the weld is really strong in the longitudinal direction. Do you have feedback on pull-out resistance in the long term?

Bowden clamp with a blind rivet nut (not printed)

I'm no pro with these inserts, but here's my 2 cents. Hope it helps:
I think with the type of 'blind rivet nut' shown in your pics wouldn't have as good a pull-out resistance as a threaded insert with grooves and/or barbs. This because there aren't really any features on your rivet nut to prevent movement in the longitudinal direction - there are only knurl features to prevent the nut from turning/torquing. These 'Heat Set Inserts" from Mcmaster would have a better pull-out resistance I'd think: http://www.mcmaster.com/#standard-threaded-inserts/=k2srdgIhttp://www.mcmaster.com/#stand... guess this is kind of obvious, but your blind rivet nut will probably work just fine if the longitudinal forces aren't large or cyclical. Cyclic/repeated loading is what I'd be most worried about with that kind of nut pulling out.You can get more detailed info from the Penn Engineering website if you really want to get into the technical specs of different types of insert nuts.

+1 for Eric's point. There are special kind of inserts for heat insertions. Sometimes they are called 'heat-set', sometimes 'heatfit'. These inserts will give the best pull resistance. Make sure to have the hole at the exact dimensions specified for the inserts you use. See example at this McMaster insert http://www.mcmaster.com/#94180a353/=ypzo9p .

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Well I was to hold the blind rivet nut by its shoulder, not by the knurled surface which most probably would fail quickly indeed. Of course the bigger the shoulder, the better.Now, you're right at highlighting cyclical longitudinal movements that could wear the plastic out quickly, while welded rivets like yours would prevent any free play in the first place.

Oh okay, I didn't really understand the application apparently. Hope it works out well for you :)

I thought this was common knowledge as this is how we insert nuts into pulleys to accept set screws.

Yeah I guess it might be. I certainly thought it was, but after looking at lots of things on this website I never saw it being eomployed, even on things where it would be pretty useful. So I figured I'd throw it up for the heck of it.

But lately I use a heat gun.

Great Job! Like you I had used these years ago. With injection molded parts, the tol was such that we had machines that "press fit" all of the pem's at one time, but during pilot builds, we would use the method you define here. Also, in that we work with nylon materials, we also use eyelets as the nylon can be printed thin, very strong and pliable.

Hey thanks Taulman. We also used this method for prototypes - mostly in place of Plastite trilobular screws since the proto's always get dis-assembled and re-assembled constantly during the design process.

I've never heard of using eyelets before - that sounds interesting for strengthening the holes on thin parts though :-$

I've always called those "pem inserts". I suppose it's because I'm familiar with Penn engineering


Haha I actually had to catch myself and not use the term PEM because that's what I'm used to calling them as well.

Now I'm going to blow my nose with some Kleenex while I make a Xerox copy of some different flavor Coke cans :-P

Check http://Rockler.comRockler.com, http://woodcraft.comwoodcraft.com, and online woodworking stores for threaded inserts in different styles and metals. ;)