3D Printing Threads and Adding Threaded Inserts to 3D Printed Parts (With Video)

Screw-to-expand inserts are cylindrical, with a slight taper and knurling on the exterior surfaces. During the design stage, incorporate a boss with a depth and diameter based on the insert’s specs into your part. Print and post-process the part as normal, following the usual steps for SLA or SLS post-processing, taking care to make sure no extra material remains inside the cavity, and install the insert with a simple press fit. Adding a screw will press the knurled surface into the surrounding printed material, creating a strong friction fit.

Tip for using screw-to-expand inserts with 3D printed parts made with SLA 3D printing: Wash the part as normal, insert the screw-to-expand insert, install a screw, and post-cure the part with the screw in place. Saving this step for last reduces the chance that the insert will crack the surrounding material when expanded.

Heat-set threaded inserts are designed to be installed into thermoplastics using a soldering iron with an installation tip. They can also be used as glue-in inserts in thermoset materials, such as SLA parts. 

To install in a thermoplastic part, like one printed with SLS Powders, follow the installation instructions for your particular hardware. The typical process is to use a soldering iron, with or without a special attachment, to heat the insert, which conducts heat into the surrounding plastic. The surrounding material softens and, by pressing down with the soldering iron, you can gently press the insert into the printed part. Be sure to allow enough time for the material to cool down and regain strength before installing a screw.

To install in a thermoset part, like one printed with SLA Resins, glue can be used to hold a heat-set insert in place. Unlike with traditional installation, make sure to design your boss to match the widest diameter of the insert, and use a bead of cyanoacrylate (CA) glue or epoxy to hold it in place when installed. Be sure to allow enough time for your glue to fully cure before installing a screw.

Note: In the SLS 3D printed part photographed for this article, the boss is sized for a press-fit, as we recommend here for thermoset plastics. This also works, with a drop of glue or epoxy, for thermoplastic parts, but won’t have as strong a hold as a true heat-set installation.

Although an additional step of soldering or gluing is required, heat-set threaded inserts for both SLS and SLA parts offer improved security and strength compared to screw-to-expand inserts With either method, these are a great option to gain a little extra security and strength compared to screw-to-expand inserts, although the additional step and equipment may be inconvenient.

Designing a pocket or boss that securely holds a nut into the part itself is another method to get metal-on-metal contact. Hexagonal or square nuts can be used, and even locking nuts are possible to accommodate. There are many design variations for this method—just make sure your pocket or boss is easily accessible (i.e. not on an interior surface) so that the nut can be installed. For extra security, a drop of cyanoacrylate (CA) glue will hold the nut in place.

Using a thread tap designed for plastic is a quick, economical way to add screw threads to 3D printed parts. It doesn’t require any extra design steps, and most shops that work with plastics will already have the equipment required.

Self-tapping screws, also called thread-forming screws, can be inserted into a negative feature with no preparation work done to the part. Follow the manufacturer’s guidelines for boss dimensions. 

It’s suggested to use these with materials that are ductile, or have high elongation. Formlabs Nylon 11 Powder or Nylon 12 Powder are both suitable for this, as are the Tough and Durable Resins in the Formlabs SLA material family. Brittle materials, or those with low elongation (such as the Rigid Resins in the Formlabs SLA material family), may crack when used with self-tapping screws, so take caution and wear eye protection when using these materials.