Heat staking
- Holds metal, boards, and film that cannot carry a snap feature
- Permanent; removing the part breaks the head
- Adds a staking step and fixture after molding
Process guide
Heat Staking
A circuit board, a metal bracket, or a lens often has to be held to a molded housing without screws. Heat staking does it with a post molded into the housing, melted down into a rivet head.
How it works
A molded post becomes a rivet
No fastener, no adhesive, no second plastic part.
The housing is molded with a post where the fastener would have gone. The part to be held has a hole that fits over the post. Once the two are assembled, the tip of the post is softened and pressed into a head that sits on top of the held part and clamps it down.
It suits joints between plastic and something that is not the same plastic: metal brackets, circuit boards, film, fabric, and parts made from a resin that will not weld to the housing. The joint cannot come apart without breaking the head, so it is best where the assembly never needs servicing.
Methods
Ways to soften the post
They all end with a formed head. They differ in how the heat gets there and how much stress stays in the joint.
| Method | How heat is applied | Good for | Watch for |
|---|---|---|---|
| Hot tip | A heated punch shaped like the head presses down on the post | Simple joints and many posts at once | Resin sticking to the tip; outer post cold while the tip is hot |
| Hot air, then cold punch | Hot air heats the whole post, then a cool punch forms the head | Stronger heads, since the whole post softens evenly | Longer cycle; heat reaching nearby parts |
| Infrared | Infrared lamps heat the post without contact, then a punch forms it | Clean heads and resins that stick to hot metal | Dark and light colors absorb differently, so settings change with color |
| Ultrasonic staking | A vibrating horn melts the post by friction and forms the head | Fast cycles on amorphous resins | Vibration can damage nearby electronics; crystalline resins are harder |
Design
Designing the post and the hole
A good stake starts in the mold, not at the staking machine.
- Enough post above the held part to fill the head, and no more; extra material squeezes out as flash
- A rounded fillet where the post meets the wall, since a sharp corner there is where stakes crack
- A hole in the held part with some clearance, so parts go together without shaving the post
- A hollow post for larger heads, so the plastic flows outward into a ring instead of piling up
- Room around the post for the punch and any hold down tooling
Failures
When a staked joint comes loose
Most loose stakes trace back to one of these.
The head formed on a cold post and split at the rim instead of flowing
The punch pulled away before the head cooled, and it sprang back
Glass filled resin left a brittle head with fibers at the surface
The held part expanded more with heat than the plastic and worked the head loose over time
The post was not fully molded, so the head was short of material
Alternatives
Staking or a snap fit
Both keep screws out of the assembly. Only one can be undone.
Snap fit
- Both parts need to be plastic, or at least one needs a hook
- Can be designed to come apart for service
- No step after molding, but the hooks add slides or lifters to the mold
Keep reading
Related guides
Other ways to join molded parts, and the features that make them work.
Ultrasonic welding
Joining two plastic parts with vibration.
Ultrasonic weldingLaser welding plastics
Joining through a clear top part.
Laser welding plasticsCantilever snap fit
The hook that can be undone.
Cantilever snap fitInsert molding vs post assembly
Molding metal in, or adding it later.
Insert molding vs post assemblyElectronic enclosures
Housings that hold boards in place.
Electronic enclosuresAutomated assembly cells
Staking, welding, and checks in one cell.
Automated assembly cellsProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about heat staking
Is heat staking the same as heat set inserts?
No, though the equipment can look alike. Heat staking melts a plastic post over another part. A heat set insert is a metal insert pressed into a hole while hot, so the plastic flows into its knurls.
Can a staked part be removed for repair?
Only by cutting or drilling off the head. If the part must come out in the field, a screw or a snap fit is the better choice.
Which resins stake well?
Amorphous resins such as ABS and PC soften over a range and form clean heads. Crystalline resins such as nylon and PBT can be staked, but they need tighter heat control, and glass fillers make heads more brittle.
How many posts can be staked at once?
As many as the fixture and machine allow. Multi post tooling that forms every head in one stroke is common on housings and boards.
Next step
Show what has to hold to what
Upload CAD for both parts and mark the joints. The review looks at ultrasonic welding, insert installation, and assembly after molding. NetProto manufacturing is ISO 9001 certified.