Staking
Heat staking softens a molded plastic boss with hot air or ultrasonic energy and reforms it into a head that traps the mating part.
- Part
- Joining
- Variants
- 2
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- Formed head height controlled to about ±0.005 in (±0.13 mm) by tool stop; hole-to-boss clearance 0.002–0.010 in (0.05–0.25 mm)
- Typical volumes
- 1,000 to 10,000,000+ per year
- Lead time
- 0.2–0.8 s per ultrasonic stake, 4–15 s per hot air cycle; 2–5 weeks for multi-tip forming tooling
- Materials
- Plastic
What it is
Heat staking softens the tip of a molded plastic boss and reforms it into a head that traps a mating part underneath — the plastic equivalent of riveting, using material the part already has. No fastener, no adhesive, no added part number.
Two heat sources dominate. Hot air / cold stake blows hot air at 500–900 °F (260–480 °C) onto the boss and then consolidates the softened material with an unheated forming tool, which produces a clean head with no sticking or stringing. Ultrasonic staking uses a contoured horn at 20 kHz and low amplitude to melt and form the head in under a second.
The characterizing dimension: a boss must protrude above the mating part by roughly 1.5–2x its own diameter to supply enough material for a standard dome head. Staking is a shear-and-retention joint, not a structural weld — it holds parts in place, resists rattle and locates assemblies, but it is not sized to carry significant tension.
How it works
- Assemble. The mating part — a PCB, a metal bracket, a decorative trim panel — drops over the molded boss through a clearance hole, typically 0.002–0.010 in (0.05–0.25 mm) larger than the boss.
- Soften. Heat is applied only to the protruding boss tip. Hot air staking heats to a depth of a few thousandths of an inch in 2–8 s; ultrasonic staking melts the tip in 0.2–0.8 s through friction from a low-amplitude horn.
- Form. A tool with the head profile machined into it descends and displaces the softened material sideways into a head. In hot air / cold stake the tool is deliberately cold so it chills and sets the head instantly.
- Hold and release. Pressure is held for 1–3 s while the head solidifies, then the tool retracts. Multiple bosses are almost always staked simultaneously from one multi-tip head.
Choosing the head profile
- Dome — the general-purpose profile, for bosses up to about 1/16 in (1.5 mm) diameter.
- Flush / rosette — sits within a counterbore, used when the head cannot stand proud of the mating surface.
- Hollow — for bosses over about 5/32 in (4 mm) diameter, where a solid head would need far too much material and would sink; the tool forms only the wall of the boss outward.
- Knurled — spreads a wide, textured head to distribute load on soft or thin mating parts.
Hot air versus ultrasonic
Hot air staking is slower (4–15 s per cycle) but produces the best cosmetic head, generates no particulate, transmits no vibration into the assembly, and works well on glass-filled grades where the abrasive filler destroys ultrasonic horns. It is the standard for staking populated PCBs and visible trim.
Ultrasonic staking is faster (under 1 s) and uses cheaper tooling, but the horn contacts the melt, can leave marks and fines, and puts 20 kHz vibration into whatever is being retained — a real concern for crystals, relays and MEMS devices.
Design guidelines
Size the boss protrusion to the head volume
The head is formed from the material that stands above the mating part, so protrusion is the controlling dimension. For a standard dome head, allow a protrusion of roughly 1.5–2x the boss diameter; the exact figure follows from equating the protruding volume to the head volume, so confirm it against the specific head profile before cutting the mold. Too little protrusion gives a thin, weak head that has not filled the tool; too much squeezes out around the tool as flash.
Head and hole proportions
Aim for a formed head diameter of roughly 1.5–2x the boss diameter and a head height of about 0.5x the boss diameter. Keep the clearance hole in the mating part 0.002–0.010 in (0.05–0.25 mm) over the boss diameter — enough for assembly, tight enough that the head has material to bear against all the way around.
Use a hollow head above 5/32 in (4 mm)
A solid dome on a large boss requires so much displaced material that the head sinks, voids and takes far too long to cool. Above roughly 5/32 in (4 mm) boss diameter, switch to a hollow profile that forms only the boss wall outward, or to a knurled head.
Support the boss and design for the load
Put a radius at the base of the boss (0.010–0.020 in / 0.25–0.5 mm) to avoid a stress riser where the retention load lands, and support the underside of the mating part so the stake is not the only thing carrying the assembly load. Staked joints resist shear and rattle well; they are poor in tension and should never be the sole retention for a safety-critical or heavily loaded part.
Material suitability
| Resin | Hot air staking | Ultrasonic staking | Notes |
|---|---|---|---|
| ABS, PC, PC/ABS, PS | Excellent | Excellent | Amorphous resins form clean heads |
| PP, PE | Good | Good | Wide melt range, forgiving |
| PA (nylon) | Good | Fair | Dry before staking — absorbed moisture foams the head |
| POM (acetal) | Fair | Fair | Narrow melt window; head can crack on cooling |
| 20–50% glass filled | Good | Poor | Abrasive fill destroys horns; hot air is the practical route |
| Thermosets | No | No | Cannot be reformed once cured |
| Boss already stressed or knit-lined | Avoid | Avoid | Staking loads the boss base — a knit line there will split |
Boss diameter, wall thickness and base radius are molded features governed by the rules on the injection molding design guidelines chart.
Inspection
Formed head height and diameter are gauged, often with a simple go/no-go or a vision check on a multi-stake assembly, and head height is controlled to about ±0.005 in (±0.13 mm) by tool stop. Pull-out and torque-out testing on lot samples confirms retention. Cross-sections through the head reveal voids and unfused material. Ultrasonic stakers log energy and collapse per cycle; hot air stations log air temperature, dwell and forming force.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Boss protrusion above the mating part | 1.5–2x boss diameter | Profile dependent | Supplies the volume the head is formed from |
| Formed head diameter | 1.5–2x boss diameter | — | Bearing area against the mating part |
| Formed head height | ~0.5x boss diameter | — | Enough section to carry the retention load |
| Hole clearance over boss | 0.004 in (0.1 mm) | 0.002–0.010 in (0.05–0.25 mm) | Assembly fit without losing bearing area |
| Boss diameter for a solid dome | Under 5/32 in (4 mm) | Use hollow head above | Solid heads sink and void on large bosses |
| Boss base radius | 0.010–0.020 in (0.25–0.5 mm) | — | The retention load reacts at the boss base |
| Joint loading | Shear and retention | Not primary tension | The head is a formed cap, not a fastener |
Cost drivers
Staking is one of the cheapest joining operations available to a molded assembly, because the fastener is already part of the molding. There is no purchased hardware, no adhesive, no consumable at all — only machine time and a forming tool.
Tooling cost tracks the number of stakes and how they are grouped: a multi-tip head that forms twelve stakes in one stroke costs little more than a single-tip head and turns twelve operations into one. That grouping decision, made at design time, is the dominant cost lever.
Hot air stations cost more than ultrasonic ones and run longer cycles, but avoid horn wear on filled resins and avoid vibration damage to what is being retained — which usually shows up as scrap avoided rather than cost saved.
- Group every stake in an assembly onto one multi-tip head. One stroke instead of twelve is the single biggest saving.
- Standardize boss diameters across a product family so the same tips and profiles are reused.
- Use staking to delete screws and their bosses. Each deleted screw removes a purchased part, a driver station and a torque verification step.
- Choose hot air for glass-filled resin. Horn replacement on abrasive fill quickly outweighs the cheaper ultrasonic machine.
- Do not stake what needs to come apart. A staked head must be drilled out for rework; use a screw or a threaded insert where serviceability matters.
Variants
2 named
Hot Air Staking
Ultrasonic Staking
Questions
5 questionsHow tall does the boss need to be for heat staking?
For a standard dome head, allow the boss to protrude above the mating part by roughly 1.5–2x its own diameter. The head is formed entirely from that protruding material, so the exact figure comes from matching protruding volume to head volume for the chosen profile. Too little gives a thin unfilled head; too much extrudes as flash around the tool.
Hot air or ultrasonic staking — which should I specify?
Hot air / cold stake for glass-filled resins, for cosmetic heads, and whenever what is being retained cannot take 20 kHz vibration — populated PCBs, crystals, relays. Ultrasonic staking for speed (under 1 s versus 4–15 s) and cheaper tooling on unfilled resins where a small horn mark and some particulate are acceptable.
How much load can a heat-staked joint carry?
Enough to retain and locate a part and resist rattle, but it is not a structural fastener. Staked joints work well in shear and poorly in tension, because the head is a formed cap of the same resin rather than a separate high-strength element. Qualify retention with pull-out testing on lot samples rather than calculating it.
Why use a hollow head instead of a dome?
Above about 5/32 in (4 mm) boss diameter a solid dome needs so much displaced material that the head sinks, voids internally and takes a long time to cool. A hollow profile forms only the boss wall outward into an annular head, using far less material, cooling faster and giving a flatter, more uniform bearing surface.
Can you heat stake glass-filled plastic?
Yes, with hot air staking. Glass-filled grades up to about 50% form good heads under a cold forming tool. Ultrasonic staking is a poor choice on filled resin because the abrasive fill erodes the horn rapidly, and because the fibers at the head surface can leave a rough, weak-looking formed head.