Threaded Inserts in 3D Printed Parts
A threaded insert is a metal fastener installed in a plastic hole so a bolt can be removed and reinstalled without cutting new threads into the plastic itself.
How big should the hole and boss be for a threaded insert?
Tappex publishes a hole and boss size table for its MULTISERT heat-set inserts, but Tappex's own brochure calls the figures approximate and not for production tooling, and the table describes a molded plastic boss, not a printed one. Applying a molded-boss figure to an FDM boss is an extrapolation Tappex itself does not make.
Tappex's own caveat sits next to the numbers in its brochure: the figures are approximate and not intended for production tooling. Our own added caveat is separate from Tappex's: these are molded-boss dimensions, and no source in this audit tests them against a printed boss.
| Thread size | Hole diameter (mm) | Boss OD (mm) | Provenance |
|---|---|---|---|
| M2 | 3.0 to 3.3 | 6.0 | Tappex MULTISERT brochure, approximate, molded boss |
| M3 | 3.7 to 4.0 | 7.4 | Tappex MULTISERT brochure, approximate, molded boss |
| M4 | 5.4 to 5.9 | 10.9 | Tappex MULTISERT brochure, approximate, molded boss |
| M5 | 6.0 to 6.5 | 12.2 | Tappex MULTISERT brochure, approximate, molded boss |
| M6 | 7.7 to 8.2 | 14.6 | Tappex MULTISERT brochure, approximate, molded boss |
| M8 | 10.2 to 10.6 | 19.1 | Tappex MULTISERT brochure, approximate, molded boss |
How deep should the insert hole be, and does filler change that?
SPIROL recommends a hole depth of at least the insert length plus two thread pitches, and adds 0.08 mm to that depth for a plastic filled with 15 percent filler or 0.15 mm for one filled with 35 percent filler. SPIROL also states a molded hole is preferred over a drilled one, guidance written for molding, not FDM.
What is the only published pull-out data for a threaded insert in an FDM part?
Vasic and coauthors printed the only published FDM insert pull-out study found, measuring heat-staked brass inserts in PLA holding 624 N when new and falling to 179 N after aggressive aging. A low-infill sample fell from 433 N to 55 N over the same aging, hole diameter was not reported, and this is a single study, not a generalizable design number.
| Sample | Pull-out force, new (N) | Pull-out force, aged (N) | Provenance |
|---|---|---|---|
| Reported maximum | 624 | 179 | Vasic and coauthors, Materials 2025, 18(11):2591, PMC12156079, one test setup |
| Lowest-infill sample | 433 | 55 | Vasic and coauthors, Materials 2025, 18(11):2591, PMC12156079, one test setup |
How does that compare with an insert in SLS nylon, not FDM?
In a separate SLS PA12 study, explicitly not FDM, a printed thread pulled out at 3.409 kN, a heat-set insert at 3.827 kN, and a steel Helicoil insert at 4.779 kN. The same comparison found the heat-set insert outperforming the printed thread by about 12.3 percent and the Helicoil outperforming it by about 40.2 percent, all in SLS PA12.
So what should you actually do before specifying an insert in a printed part?
Published pull-out data for a threaded insert in an FDM part is limited to one aging study in one material, so a vendor table or a single study is not a safe substitute for testing your own part. Print a coupon in the material, infill, and hole size you plan to use, and pull it to failure before committing a design to an insert's holding force.
What this page does not tell you
- No OEM or bureau reviewed for this page publishes a hole or boss table written for an FDM boss. The Tappex and SPIROL figures describe a molded boss, and applying them to a printed one is an extrapolation neither vendor makes.
- The one FDM pull-out study covers heat-staked brass inserts in PLA only, at the infill levels and aging condition that one research team chose. It does not cover other materials, other insert types, other hole diameters, or normal use without aggressive aging.
- The SLS PA12 comparison is a different process on a different material. It shows the same insert types in a different context, not an FDM number, and should not be substituted for one.
When to print a test coupon instead
Print a coupon with the exact hole diameter, depth, material, and infill you plan to use, install the insert the way you plan to install it, and pull or torque it to failure before committing a design to that insert's holding force. Send the file through a review request and tell us which insert and material worry you. A person reviews your request and follows up by email.
Sources
- Tappex Brochure 2016, Tappex
Page 16, hole and boss dimensions for M2 through M8 MULTISERT heat-set inserts, for a molded plastic boss. Tappex's own caveat calls the figures approximate and not for production tooling. Host www.tappex.co.uk is not on this file's reachability allowlist as of 2026-09-05.
- How to Design the Proper Hole for Heat and Ultrasonic Inserts, SPIROL
States minimum hole depth as insert length plus two thread pitches, with 0.08 mm added for plastic with 15 percent filler and 0.15 mm added for plastic with 35 percent filler, and states a molded hole is preferred over a drilled one. Host www.spirol.com is not on this file's reachability allowlist as of 2026-09-05.
- Materials 2025, 18(11):2591 (PMC12156079), MDPI Materials, PMC open access
One test setup, the only FDM-specific insert pull-out study found in this audit. Heat-staked brass inserts in FDM PLA measured a maximum pull-out of 624 N new falling to 179 N after aggressive aging, and a lowest-infill sample falling from 433 N to 55 N over the same aging. Hole diameter was not reported.
- SLS PA12 insert pull-out comparison (PMC12787388), PMC (peer-reviewed)
SLS PA12, not FDM. Measured printed-thread pull-out at 3.409 kN, a heat-set insert at 3.827 kN (plus 12.3 percent over the printed thread), and a steel Helicoil insert at 4.779 kN (plus 40.2 percent over the printed thread).
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Published 5 September 2026.