What pull-out should an M4 case threaded insert meet?

Answer: 900 N minimum against 1200 N measured, a 25% margin. Self-tapping joints are accepted at 300 N because they are not built for reuse.

How often can a self-tapping screw be reused?

Answer: Three passes cleanly, 40% torque loss by the fifth, 152 N left at the eighth. Above 10 cycles, use an insert.

Is an 8 mm boss mandatory?

Answer: For M4, yes. At 7 mm the ring wall thins to 0.35 mm and cracking probability in assembly rises to 30%.

Which process gives the highest pull-out?

Answer: Moulded-in at 1300 N, ultrasonic 1200 N, heat-set 1100 N. Ultrasonic needs a 6.2 mm hole and a 0.5 mm counterbore.

What does the hardware add in cost?

Answer: About 0.09 USD per M4 insert, 0.72 USD across eight joints, while the return rate falls from 6% to under 1%.

Case Threaded Insert: Threaded Inserts vs Self-Tapping

Quick answer: An M4 case threaded insert pulls out at 1200 N against 380 N for a self-tapping screw, a 3.2x gap that only holds with an 8 mm boss.

Pull-out load: M4 insert at 1200 N vs 380 N
Wall thickness and boss geometry
Heat-set, ultrasonic and moulded-in
Failure modes: pull-out 46%, spin-out 22%
Cycle count and selection
FAQ

1. Pull-out load: M4 insert at 1200 N vs 380 N

Pulled at 5 mm/min, the brass insert leaves the boss whole at 1200 N; the self-tapping screw strips at 380 N, with polymer failing before hardware in both cases.

The insert carries load on a knurled shear band. At 6.3 mm diameter and 8 mm depth it presents 158 mm², turning 1200 N into 7.6 MPa, well under the 18 MPa glass-filled PP sustains.

A screw has far less area. The ST4.2 thread is 0.6 mm high, so 8 mm of engagement leaves roughly 26 mm², and 380 N already means 15 MPa, close to the ceiling.

Size	Brass insert, N	Self-tapping, N	Ratio
M3	780	220	3.5
M4	1200	380	3.2
M5	1750	520	3.4
M6	2400	700	3.4

The ratio holds between 3.2 and 3.5 because insert shear area scales with diameter while thread height does not; at M6 the screw gains 3 mm².

Fixture stiffness skews readings more than specimens do: a flexing clamp adds 80 N to 120 N at 800 N, enough to separate two labs by 150 N.

2. Wall thickness and boss geometry

The boss runs 1.25 times insert diameter: 6.3 mm becomes 8 mm with 0.85 mm of ring wall, and 1.5 mm of polymer stays under the hole.

Case threaded insert seating station, a worker holding a claw hammer beside an open black case on a bench with red air hoses overhead

On a 2.5 mm wall the boss is where sink appears first. A 3 mm root leaves a 0.08 mm dish visible under raking light; moving the boss 6 mm inboard or coring 0.3 mm at the root drops it below 0.03 mm.

Weld lines are harder. A boss at mid-edge forces melt fronts to rejoin behind it, and that line faces the impact load, cutting drop energy 30% where it opens. Shifting the boss 15 mm, or widening it to 9 mm, moves the line into a lower-stress zone.

Holes split by process: 6.4 mm heat-set, 6.2 mm ultrasonic, 6.3 mm moulded-in. The clearance gives displaced melt room, the tight hole transmits amplitude, and the core pin is the hole.

3. Heat-set, ultrasonic and moulded-in

Heat-set is cheapest. A 270 ℃ tip presses 3 s and holds 5 s, eight seconds a joint, giving 1100 N and 4.5 N·m from a 400 USD hand tool.

Ultrasonic is fast but demanding. At 20 kHz and 28 μm the cycle takes 0.5 s and pull-out reaches 1200 N, the best of the three; the cost is a 0.3 mm flash ring needing a 0.5 mm counterbore.

Moulded-in is strongest. Pins located to 0.02 mm let melt encapsulate the knurl for 1300 N and 5.2 N·m, but tooling rises 12% and 1.2% of parts are scrapped.

Process	Time per joint	Pull-out, N	Torque, N·m	Capital
Heat-set	8 s	1100	4.5	400 USD tool
Ultrasonic	0.5 s	1200	4.8	8,000 USD welder
Moulded-in	In cycle	1300	5.2	Tooling up 12%

Volume decides, not strength. Under 1,000 units heat-set allows late changes; above 10,000 units moulded-in cuts piece cost to 0.13 USD; ultrasonic suits a frozen design.

4. Failure modes: pull-out 46%, spin-out 22%

Across 200 warranty returns: pull-out 46%, spin-out 22%, boss cracking 19%, thread stripping 13%, each tracing back to a different dimension.

Case threaded insert service kit in an open black case, lid egg-crate foam, cut foam holding crimpers, meters and wrenches, two latches closed

If an insert has moved 0.3 mm at 800 N, engagement is under 6 mm and half the knurl sits in air; raising boss height from 8 mm to 10 mm restored 1150 N.

Spin-out appears above 3.2 N·m because straight knurling cannot grip tangentially. Three ribs 0.4 mm deep, or a diamond knurl, push spin torque to 5 N·m.

Cracking is a design fault. Below 1.0 mm of ring wall, assembly expansion splits the boss along a 45-degree path; a R0.5 fillet and 0.85 mm of wall remove about 80% of cases.

Stripping is the screw's alone, since the thread is cut not moulded. Torque is down 40% by the fifth pass and 1.1 N·m by the eighth. Field repairs go one size up, reaming 3.6 mm to 4.1 mm and thinning the wall 0.25 mm, trading spin-out risk for cracking risk.

5. Cycle count and selection

An M4 screw holds 380 N on the first pass, 228 N on the fifth and 152 N by the eighth, torque falling from 2.5 to 1.1 N·m; a brass insert still holds 1056 N at pass 25 and 840 N at pass 50.

Planned cycles	M4 screw, N	M4 insert, N	Recommendation
1–3	340	1180	Screw is fine
4–10	190	1120	Insert or sleeved screw
11–25	Stripped	1056	Insert
26–50	Failed	840	Insert, diamond knurl

Two thresholds settle the drawing: 10 or more planned cycles, or 800 N of required pull-out, mean inserts; fail both and a screw saves 0.09 USD a joint.

Cordless drivers swing torque ±20% and strip a screw by pass four, while a station limited to 4.5 N·m lets an insert reach 50.

Conclusion

Write both thresholds on the drawing and the case threaded insert choice stops being a debate: 10 or more assembly cycles, or 800 N of pull-out, means brass inserts; below both, self-tapping is cheaper. The M4 floors are an 8 mm boss, 0.85 mm of ring wall and 8 mm of depth, and losing any one costs roughly 200 N. Then pull three samples at 5 mm/min and reject any batch peaking under 900 N.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.