So you need threads within your plastic 3D printed part and you are questioning the holding strength of printed or threads, and you should. There are few applications where a 3D printed threaded hole for a piece of conventional hardware will do the trick. It is okay, we don’t need to sugar coat it, 3D printing can do some downright amazing things, but 3D printed threads are not high on the list. How about under sizing the hole and reaming it and tapping a threaded hole? Great question and a great solution in some situations. Depending on how fine the thread pitch is, the end condition of the hole, your available tap sizes and expected pulling force – cut threads might not, well…cut it. Chances are, what you need is a threaded insert.

Heat-set Threaded Inserts

The threaded insert is an amazing tool for solving one of plastic’s weak points, pulled out fasteners. The insert, typically brass, is pressed into an over-sized hole to create a metal-to-metal threaded sleeve for the desired fastener. In the case of thermoplastics like those printed on Stratasys FDM printers, heated inserts are the ticket. These are brass cylinders with a threaded inner diameter and a barbed outer diameter. While the barb designs may differ from manufacture to manufacture, the principles are the same. The insert is heated and pressed into a hole that is printed in the plastic part which is slightly smaller than the outer diameter of the brass insert. When the heated insert is pressed into the part, it melts the plastic around the insert and firmly seats itself by having the molten plastic perfectly contour to the insert’s outer barbed surface. The insert provides a much stronger thread structure than simply tapping the 3D printed plastic, and it also produces a stronger overall diameter around the hole, as the circumference of the inserts is larger than the circumference of the fastener meaning forces are distributed over a larger area of the part.

Tools and Materials for Inserting Heat-set Inserts

• Soldering Iron (40 watt minimum with a tip small enough to fit inside of the threaded insert) or heat-set insert tool (basically a soldering iron with special tips for different inserts)
• 3D printed part (with a properly sized hole for the insert)
• Heat-set inserts
• Dental pick
• Needle nose pliers

The Process

Before we heat the soldering iron or realistically before we even design our part, we need to do a dry run. We will be using the soldering iron to not just heat but to actually press the insert into our part. Now is the time to insure that the brass insert fits over the soldering irons tip, and that when the insert is being pressed into the desired hole there is enough clearance around everything without the iron touching the plastic part. After considering any possible install interference, it is time to design and print your part with a properly sized hole. The manufacturer of the heat-set insert can provide the hole dimensions for your insert.

After the part is printed and the support is removed from the part, we pre-heat our soldering iron. A 40-watt iron will have enough output to set all but the largest of inserts. Please use caution with the soldering iron, it can quickly become extremely hot and damage work surfaces and worse yet, burn the user, please respect the heat. While the iron is pre-heating, ensure the hole in your 3D printed part is clean and free of any support material. If the hole still has support material in it, it can quickly be chased with a drill bit or a reamer. Next, identify the inner and outer surfaces of your heat-set insert, they are directional and the manufacturer’s documentation can help you determine the correct orientation based on the direction of the force applied to the threaded insert to resist pull-out.

Now it is go time. Place the heat-set insert on the soldering iron tip in the proper orientation that you wish to insert it into the plastic part. When in doubt, please use a pair of needle nose pliers to minimize the risk of getting burned. The insert will become hot very fast so once the insert has touched the iron’s tip, you must not touch it with your bare skin- use needle nose pliers if it must be removed from the iron.

Allow the brass insert to heat for roughly ten seconds, you will begin to see the brass color start to get slightly darker and duller looking, this is evidence that the brass is hot enough to set into the plastic part. I want to repeat here, the insert is extremely hot, if you drop it or need to take it off the iron, it can burn you or your work space – use needle pliers to handle an already heated insert. When the insert is hot, keep it on the end of the iron and use the iron to press the insert into the plastic part. Avoid rocking the insert in, just use a steady straight-on approach so you do not wobble out the hole and cause the insert to loosely fit within the hole.

After the insert is pressed flush into the part, inspect the threaded hole and pick out any melted plastic that might have built up around the backside of the threads using a dental pick, keeping mind to not touch the still extremely hot brass insert. Finally, let it rest for three minutes to cool to a safe temperature, if you try to install a fastener in the insert too early, it will distort the hole and weaken the inserts hold as the insert and the plastic around it will be too hot. When it is cool, you will have a strong functional threaded hole in your 3D printed plastic part. Now you have a new practical skill in your tool box.

Watch our video below with all the steps explained above.

Want to learn more about other tricks for 3D printing? Make sure you check out our video library to explore all other 3D printing ideas.