Using Heat Set Inserts in 3D Prints

In today's article, I wanted to cover some design considerations and watch-outs for working with heat set inserts. Personally, I'm not a fan of printed-in-place threads. Especially when used with smaller hardware, there's such a small amount of plastic for the threads that they tend to strip out easily. I prefer making parts that can come apart more than a couple of times, and I'd rather not have to re-print parts that I've stripped later on. Heat Set Inserts have a higher pull-out strength vs. printed-in-place threads, and are more reliable for repeated use.  

Design Considerations

1. Avoid Thin Walls. 

Heat Set Inserts are installed by melting the perimeter walls and bonding the plastic directly to the teeth on the insert. If you have thin walls, the effective wall thickness that's left will be very thin. I use a wall thickness minimum of 2.4mm (3 walls x 0.8mm nozzle) on parts that will get heat-set inserts installed. If you are under-sizing the holes to drill them later, keep in mind that you will need to add additional perimeter walls to make up for the material that will be removed during drilling. 

2. To Drill or Not to Drill?

I typically under-size the holes and drill them to size after the part is printed. This is totally optional. There's some variability on 3D Printers accurately printing small holes, and not having enough bite can reduce the overall strength of the insert. It only takes a couple seconds, and gives me peace of mind. McMaster-Carr has specifications for their inserts to make sure they're sized appropriately. This is especially important for holes that are created off-axis, or perpendicular to the print bed. Plenty of people get by without drilling, it's just my personal preference at the moment.

3. Depth.

When inserts are being installed, excess melted plastic will be pushed down the hole. If the hole is a through hole, meaning there isn't a bottom, this excess plastic will simply come out the other side and can be cleaned up with a razor blade. If the hole does have a bottom (a blind hole), it's important to provide additional room for that plastic to pool at the bottom. If you don't, that plastic can come back up through the threaded center and make it difficult to install screws later on. If space is tight, I try to keep at least 1.5mm of extra depth for the hole. If I have room to work with, I will add 4-5mm below the insert, and fillet the bottom of the hole. Inserts can be shortened if necessary, but this will reduce the overall strength of the insert. If excess material ends up above the insert, it can be cleaned up with a small utility knife.

4. Neighboring Walls.

It's important to remember that installing these inserts is a hot process, and that the sides of the soldering iron are going to be just as hot as the installation tip. While it's tempting to put your hardware as close as possible to neighboring walls for packaging, this can add a risk of melting those walls during installation. Unless you're using a specialty installation tool, give yourself some wiggle room and give the iron some space. Depending on the geometry of the part and where the insert is being installed, you might consider adding the soldering iron tip into your model to have an idea of how much room you have to work with. 

5. Should I Taper the Hole?

I have not used tapered inserts, and I do not add a taper to the holes. I will sometimes add a 1.5mm fillet to the outside edge of the hole to help align the insert, but I've found that under-sizing them and drilling them works a little better in my experience.

All Right, so What Tools do I use?

Most any soldering iron will work, but personally, I like having the ability to control the temperature of the iron. This gives better repeatability and control over how quickly the inserts melt into the plastic. I use a Weller WES51, which has been replaced with the Weller WE1010NA soldering iron. I've paired the Weller with HanTof Heat Set Insert Tips.  These tips help center the insert to the iron and distribute the heat into the insert more evenly. The tips also make it easier to install straight. I typically use flanged inserts sourced from McMaster-Carr, because they have a nice, finished appearance, and the installation specs are listed clearly for each one. 

Filaments that require a higher temperature to print will require a higher temperature to install the insert. I'd recommend doing some insertion tests to find a temperature you're comfortable with. I tend to use a lower temperature that slowly inserts into the plastic over 3-5 seconds. Just because you've sized everything correctly doesn't guarantee it'll go in straight. You'll need to take care that the iron stays perpendicular to the hole as it's being set in place. You can make small adjustments after it's seated, but try to keep this to a minimum. If inserts are being installed on angled or difficult surfaces, consider printing a fixture to hold the part in a better orientation for installing them reliably. 

I hope this helped. Be sure to Subscribe to my YouTube channel for future projects and updates!