Heat Staking and Insert Assembly Design Guide

Heat staking and thermal inserts are the workhorses of plastic assembly: they join dissimilar materials, they allow parts to be disassembled and re-assembled, and they tolerate the molding tolerances that make snap fits unreliable. They are also frequently applied after the fact, on a design that was never intended to be staked — which is why so many staked joints crack, loosen or fail in the field.

Figure 1. Heat Staking and Insert Assembly Design Guide.

This guide covers when to stake, how to design the boss and insert, and the process windows that make it repeatable.

1. Choosing the Assembly Method

How to decide:

• Reversible joint needed? → thermal insert plus screw, or a self-tapping screw.
• Permanent, high volume, fast cycle? → ultrasonic staking.
• Permanent, thicker bosses, poor ultrasonic response (e.g. high glass content, amorphous blends), or a need for a slow and controlled collapse? → heat staking.
• Load-bearing thread in plastic? → thermal insert, always. A self-tapping screw in a repeatedly serviced joint will strip the plastic.

2. Boss Design for Staking

The boss is where most staked joints fail, and the failure is almost always mechanical: the boss wall is too thin, the boss is not supported, or the boss has a sharp corner at its base.

Boss geometry rules:

The critical detail — the stake head. The material that is deformed to form the stake must come from somewhere. Design a sacrificial boss top: a boss that extends above the mating part by roughly 0.5–1.0 × the stake diameter, with a pilot hole or a small recess to guide the staking tip.

Anti-sink measures. A thick boss on a thin wall will sink on the visible face. Core the boss from the back if possible, or reduce the wall thickness locally. If neither is possible, accept the sink and put a texture on that face — do not simply add thickness.

3. Thermal Insert Design

A heat-set insert is a brass or stainless threaded insert installed by applying heat and pressure so the plastic melts and flows around it. When it cools, the plastic grips the insert's knurls and undercuts.

Insert selection:

Boss sizing for inserts:

• Hole diameter: the insert manufacturer specifies a mold hole diameter; typically the insert's minor diameter minus 0.05–0.15 mm. Too small a hole cracks the boss; too large a hole gives poor holding.
• Boss wall: 0.5–1.0 mm minimum around the insert. For a brass insert of 6 mm diameter, an 8 mm boss outer diameter is a reasonable target — but check the specific part.
• Boss depth: insert length plus 0.3–0.5 mm for melted material to flow, but with a solid floor under the insert. Do not let an insert break through the wall.
• Draft in the hole: keep the hole straight, not drafted, over the insert length.

Holding strength expectations. For a properly designed M3 brass insert in glass-filled PA66, pull-out strength is typically in the 500–900 N range and torque resistance 1.5–3 N·m. For unfilled PP or PE, expect a fraction of that — polyolefins are difficult to grip because they do not bond and flow readily. For polyolefin applications, consider a molded-in insert or a press-fit clip instead.

4. Heat Staking Process

Heat staking uses a temperature-controlled tool (a "staking tip") pressed onto the boss to soften it and form the head in one motion.

Tip design matters more than people expect:

• Rosette tip — forms a swaged flower shape; strongest, tolerant of material, and the standard choice for load-bearing joints.
• Dome tip — quick and clean, good for cosmetic or low-load joints.
• Flat tip — simplest, but produces a weaker head and a tendency to over-compress.
• Hollow tip — leaves a cavity in the head; useful for controlled collapse and for parts that must not distort.

Multiple stakes at once: heat staking can do many joints in one stroke, which is its main productivity advantage. But the pressure must be even across all stakes, or the first stake collapses before the last has touched. Design the tool with a compliant pad or a self-aligning tip holder, and check the height variation of the bosses across the part.

5. Ultrasonic Staking — the Faster Cousin

Ultrasonic staking uses high-frequency vibration to melt the boss locally, forming the head in typically 0.3–2 seconds.

Design requirements:

• A small pilot/energy director at the top of the boss so the energy concentrates there and melts predictably.
• Boss top protruding 0.5–1.0 × stake diameter as with heat staking.
• A horn contact area sized to the stake; the horn must reach the boss without touching adjacent features.
• Material suitability: amorphous resins (ABS, PS, PC/ABS) stake very well. Semi-crystalline resins (PP, PE, POM, PA) are harder — they have a sharp melting point and transmit energy poorly. Highly glass-filled resins and elastomer-blended resins are usually poor candidates.

When ultrasonic fails, heat staking works. That is the main reason to keep heat staking in the process plan: it is more tolerant of material, section variation and feature proximity.

6. Common Defects and Fixes

A note on boss height tolerance. The most common cause of inconsistent staking is not the process — it is boss height variation on the molded part. Specify and hold boss height to a reasonable tolerance (typically ±0.1 mm on small parts), because staking cannot compensate for a 0.4 mm spread across a part.

Figure 2. Quick reference: the key checks and reference values from this guide.

7. Design Checklist

• [ ] Assembly method chosen deliberately, with reversibility requirement stated
• [ ] Boss outer diameter ≥ 1.8 × insert diameter; wall ≈ 0.6 × nominal
• [ ] Boss base filleted and, where possible, tied into a wall or rib
• [ ] Boss protrusion above the mating part = 0.5–1.0 × stake diameter
• [ ] Boss height tolerance defined and achievable (±0.1 mm typical)
• [ ] Sink risk addressed (cored boss, reduced wall, or textured face)
• [ ] Insert type matched to load: barbed or diamond knurl for anything structural
• [ ] Insert hole diameter taken from the insert manufacturer's specification
• [ ] Minimum 0.5–1.0 mm of material around the insert; solid floor beneath
• [ ] Insert length plus 0.3–0.5 mm accounted for in boss depth
• [ ] Radial clearance for multiple stakes verified; compliant tooling specified
• [ ] Process window (temperature, pressure, dwell, cooling) defined with limits
• [ ] Serviceability requirement documented — staked joints are permanent, and the design should say so

Conclusion

Heat staking and thermal inserts are reliable, repeatable assembly methods — as long as the boss is designed for them. Size the boss from the insert, provide the material that becomes the stake head, keep the boss supported and the walls sensible, and define a process window with limits. Then staking becomes a controlled joining process rather than a repair operation at the end of the line.

Send us your assembly drawing and we will review boss geometry, insert selection and the staking process window before your first production run.