My RC car suspension arms were snapping. The obvious suspect: heat inserts. They require a 3.2mm hole instead of 1.8mm — less plastic around a high-stress joint. I built a break test rig to prove it. I was wrong.

This article covers the complete results of a controlled strength test comparing heat inserts vs direct screws in FDM structural parts, including pull-out tests, real lateral break tests on printed suspension arms, and the slicer behavior that explains why the results came out the way they did.

Why I Suspected Threaded Inserts

The MadBaxRC uses M3 heat inserts in its suspension arms. A heat insert requires a 3.2mm hole. A direct screw into plastic needs only 1.8mm. That's 1.4mm less material around every fastener hole in a part that takes impact loads.

Several people reported arm failures. The geometry looked suspicious. Rather than guess, I built a simple test rig — a load scale attached to a roller — and ran controlled destructions on identical printed parts.

Test Methodology

Two variables were tested: joint type (heat insert vs direct screw) and material (PLA vs PETG). All arms were printed on the same printer with identical settings. Fastener size: M3 throughout. Two test types were run.

Test 1 — Pull-Out Strength (Axial Load)

A single screw joint was pulled straight out of a printed block — simulating a joint being ripped apart along the screw axis. This is the load type most commonly cited when people argue for heat inserts.

Test 2 — Lateral Break Test (Bending Load)

Full suspension arms were mounted in the rig and pulled from both ends — mount point to wheel hub — replicating the lateral bending load that occurs during an RC car crash. This is how arms actually fail in use. Three samples per configuration were tested in PLA; one each in PETG.

Note: n=3 per PLA condition; interpret PETG single-run results as indicative, not conclusive.

Results

Pull-Out Strength (Single Screw, Axial)

Heat inserts hold nearly twice the axial load of a direct plastic thread. At this test type, the result is unambiguous.

Lateral Break Strength (Full Suspension Arms)

Under lateral bending load — the direction that matters in a crash — heat inserts made no measurable difference. Both joint types failed at statistically identical loads across PLA and PETG.

*Single run — indicative only. PETG results consistent with PLA trend.

Why the Results Are Identical — The Slicer Explanation

The counterintuitive result has a clear mechanical explanation rooted in how FDM slicers generate toolpaths around holes.

When a slicer encounters any hole in a model — regardless of diameter — it automatically generates concentric perimeter walls around the opening. This behavior exists to reinforce hole edges and ensure accurate dimensions, and it applies equally to a 1.8mm hole and a 3.2mm hole.

In practice, this means both joint types receive the same filament ring reinforcement around the fastener location. The larger hole created by the insert is compensated automatically by the slicer's toolpath algorithm. The net structural geometry at the joint — the filament orientation and density that determines bending strength — is nearly identical in both cases.

This is consistent with published research on FDM part anisotropy and failure modes, which consistently shows that inter-layer bonding and print orientation are the dominant structural variables — not small changes in hole diameter.

The break points confirmed this visually: both insert and direct-screw arms failed away from the fastener hole, at the weakest cross-section of the arm geometry — not at the joint itself.

When to Use Heat Inserts (and When Not To)

The test data makes the decision straightforward. Use heat inserts when the load on the joint is axial (pull-out) or when the part needs to survive repeated assembly cycles. Skip them when the only load is lateral bending and the part is not reassembled frequently.

Use heat inserts for:

• Pull-out resistance — any joint that must stay clamped under axial load. 55–59 kg vs 22–32 kg is a real and significant difference.
• Reassembly durability — direct plastic threads strip after approximately 10 assembly cycles. A heat insert survives around 50 cycles before degradation. For parts you service regularly, this is the most important benefit.
• High-clamping joints — motor mounts, gearbox covers, anything tightened and retightened under torque.

Heat inserts do NOT improve:

• Lateral bending strength — the slicer compensates, results are identical
• Impact resistance of arms or structural links
• Fatigue life under cyclic lateral load (no data either way, but the static test suggests no gain)

If maximum pull-out strength is the priority and the part allows for it, nut inserts are in a different category entirely — the nut insert in this test exceeded 85 kg without failure.

What Actually Breaks RC Arms

If heat inserts aren't the weak point, what is?

The break tests showed consistent failure away from the fastener location — at the minimum cross-section of the arm geometry. This points to two real variables:

• Arm geometry — the thinnest section of the arm, regardless of joint type, is where failure initiates. Adding perimeter walls in the slicer (not just around holes, but throughout the part) is the most effective structural change available without redesigning the part.
• Material choice — PETG showed a modest improvement over PLA (~75 kg vs ~68 kg average), consistent with its higher impact resistance. For RC parts that take repeated hits, PETG or a blend is worth the tradeoff in print difficulty.

The insert choice is largely irrelevant to arm survival in a crash. Geometry and material are the levers.

Frequently Asked Questions

Do heat inserts make 3D printed parts stronger?

It depends on load direction. Heat inserts nearly double pull-out (axial) strength — 55 kg vs 22 kg for an M3 in PLA. Under lateral bending load, which is how most structural parts actually fail, heat inserts make no measurable difference.

Why do heat inserts not improve lateral strength in FDM parts?

FDM slicers automatically wrap concentric perimeter walls around any hole, regardless of diameter. A 1.8mm hole and a 3.2mm hole both get the same ring reinforcement. The slicer compensates for the larger void, leaving both joint types with equivalent structural geometry under bending load.

What is the pull-out strength of a heat insert vs a direct screw in PLA?

In M3 tests: direct screw into plastic held 22–32 kg. Heat insert held 55–59 kg. Nut insert exceeded 85 kg without failure. Heat inserts provide roughly 2× the pull-out resistance of a direct plastic thread.

When should I use heat inserts in 3D printed parts?

Use them when pull-out resistance matters (joints under axial clamping load) and when the part will be disassembled repeatedly. Direct plastic threads strip after ~10 cycles; heat inserts last ~50. For parts under lateral load only, inserts add no structural benefit.

Does PETG hold heat inserts better than PLA?

In lateral break tests, PETG performed marginally better than PLA regardless of joint type — 75 kg vs ~68 kg average. The difference between insert and direct screw within PETG was negligible: 75 kg vs 76 kg. Material gain exists; insert gain under lateral load does not.

What is the MadBax Problem Series?

The MadBax Problem Series is a YouTube series documenting real engineering problems encountered during the MadBaxRC build — a 3D-printed 1/18 scale 4WD RC car with independent suspension. Each episode covers one problem with real tests and data. No fluff.