Sooner or later, almost every functional part needs a screw. Enclosures need lids, brackets need to bolt onto frames, and fixtures need knobs that can be tightened again and again. The question we hear constantly at our Glendale shop is simple: what is the best way to add threads to 3D printed parts? The answer depends on the material, the load, and how many times the joint will be assembled. Below we compare the three main options (printed threads, tapped holes, and heat-set inserts) and explain when each one makes sense.
Option 1: Modeling Threads Directly Into 3D Printed Parts
The most obvious approach is to model the thread in CAD and print it as part of the geometry. It costs nothing extra and requires no secondary operations, which makes it attractive for prototypes.
It works best for large, coarse threads: jar lids, bottle caps, pipe-style connections, and threads roughly M8 and up. At those sizes, FDM, SLA, SLS, and MJF can all reproduce a usable thread profile. Small machine-screw threads (M2 to M5) are a different story. Layer lines, nozzle width, and resin shrinkage blur the profile, and the result is often a thread that binds, strips, or feels gritty.
A few tips if you go this route:
- Add clearance. Offsetting the internal thread outward by roughly 0.2–0.4 mm (depending on the process) keeps mating parts from seizing.
- Print threaded features vertically whenever possible so the thread forms from stacked layers rather than steps on a sloped wall.
- Use trapezoidal or rounded thread forms for printed plastic-to-plastic threads; they tolerate small inaccuracies better than sharp V-threads.
Option 2: Tapping Threads After Printing
Tapping means printing a pilot hole sized to the tap drill diameter, then cutting the thread with a standard hand tap. This produces a cleaner, more accurate thread than printing it, especially at small sizes.
Tapping works well in solid, tough materials such as SLS or MJF nylon, tough SLA resins, and FDM parts printed with enough perimeter walls around the hole. We usually recommend designing extra wall material around any hole you plan to tap so the threads are cut into solid plastic instead of sparse infill.
The limitation is durability. Plastic threads wear every time a steel screw is driven in and out. For a cover that gets opened once or twice, tapped threads are fine. For a service panel that gets removed every week, they will eventually strip.
Option 3: Heat-Set Inserts for 3D Printed Parts
Heat-set inserts are small knurled brass (or sometimes stainless) sleeves with internal machine threads. A heated tip presses the insert into a printed hole; the surrounding plastic melts, flows into the knurling, and locks the insert in place as it cools. The result is a metal thread anchored in plastic.
For most functional thermoplastic parts, this is the method we recommend. Heat-set inserts:
- Hold up to repeated assembly and disassembly without wearing out
- Handle much higher clamping torque than plastic threads
- Come in standard metric and imperial sizes, so you can use off-the-shelf screws
- Add very little cost per insert
They work with thermoplastics such as PLA, PETG, ABS, ASA, nylon, and polycarbonate, including FDM, SLS, and MJF parts. They are not a good fit for most SLA and PolyJet resins, which are thermosets and do not re-melt. For resin parts we typically use press-fit or bonded inserts instead, or switch to a nylon process when threads are critical.
Designing Holes for Heat-Set Inserts
Follow the insert manufacturer’s recommended hole diameter and depth, since they vary by brand and style. In general, the hole should be slightly smaller than the insert’s outer diameter, a little deeper than the insert length to give displaced plastic somewhere to go, and surrounded by enough wall thickness (a common rule of thumb is at least the insert’s diameter) so the boss does not crack.
Other Options: Captive Nuts and Self-Tapping Screws
Two more approaches are worth knowing. Captive nut pockets are hexagonal cavities modeled into the part so a standard nut can be dropped in or slid in from the side. They are cheap, strong, and work with any printing process, including resin. Self-tapping screws designed for plastics cut their own threads into a properly sized pilot hole and are a fast option for enclosures that will rarely be opened.
Quick Comparison: Which Thread Method Should You Use?
- Printed threads: large, coarse threads; caps and lids; quick prototypes
- Tapped holes: small threads in tough material; joints assembled only a few times
- Heat-set inserts: functional thermoplastic parts; repeated assembly; higher torque
- Captive nuts: resin parts, high loads, or when inserts aren’t practical
- Self-tapping screws: low-cost enclosures that are rarely serviced
Choosing the method early matters, because it changes the CAD: hole sizes, boss thickness, and print orientation all depend on it. If you’re not sure, send us your model and the screws you plan to use, and we’ll suggest the right approach before anything is printed. Our product design team can also add bosses, pockets, and insert holes to an existing model.
Get Threaded 3D Printed Parts Made in Los Angeles
With more than 22 years of experience, eCadCam prints functional parts in FDM, SLA, SLS, MJF, PolyJet, and metal, and we can help you plan threads, inserts, and fasteners before your part is printed. Learn more about our 3D printing services, or request a quote today. Call us at 213.489.1173 or visit our studio at 4212 San Fernando Rd, Glendale, CA 91204.