Key takeaways
- An overmould has to protect the winding, locate it, seal the housing and pass heat, and those jobs compete.
- Sealing fails at interfaces: the lead exit, the housing face and the insert joint, not through the material.
- Injection pressure and melt flow can move turns or break enamel, so gate and support the winding before tuning the process.
- Compare inductance before and after moulding, then again after thermal cycling, to prove the shell has not moved the core.
What the overmould has to do
An overmoulded coil is a winding, a bobbin, a core and a lead exit enclosed in a moulded shell. The shell protects the enamel from handling and moisture, fixes the winding in position, provides the mounting interface and often forms the seal against the housing. It can also conduct heat away from the winding, or trap it, depending on the material and the wall.
Those jobs compete. A thick wall protects and seals but adds thermal resistance and cost. A thin wall saves space and heat but is easier to damage and harder to fill without a void. The design has to decide which job the shell is doing before the material and the wall are chosen.
Sealing at three interfaces
A sealed coil leaks at interfaces, not through the material. The three that matter are the lead exit, the face where the coil meets the housing and the joint between the overmould and any insert or terminal. Each needs a controlled compression or a bonded surface.
An O ring in a gland is a defined seal, but it adds parts and a groove. A moulded lip seals by compression and is cheaper at volume, but the lip geometry and the shut-off have to be right. A bonded seal depends on the material pair and the surface preparation. Flash across a sealing face is a leak path, so the tool needs a clean shut-off and enough pack pressure to fill the seal without overpacking the cavity.
Melt flow and fine wire
The winding is a delicate insert. Injection pressure, melt temperature and flow speed all act on it. A flow front that hits the winding directly can displace turns, break enamel or move the bobbin. A gate placed to fill the cavity around the winding, rather than through it, is the first control.
Fine wire is worse. A wire that is a fraction of a millimetre in diameter has little resistance to a pressure wave and less enamel to spare. Support the winding with a bobbin wall, a vane or a moulded rib, and keep the melt viscosity high enough to avoid flashing but low enough to fill. The process window is narrower than for a solid insert.
Pressure is not the only risk
Melt temperature can soften the bobbin, and the flow can carry a loose turn. Check the winding after moulding, not only the outside of the part.
The process route
The sequence is short, but each step can damage the coil or move the value that the coil was wound to hit. The checks belong inside the route, not only at the end.
- 01
Wind and terminate
Wind to the inductance target, terminate the leads and check continuity before the coil leaves the winding station.
- 02
Preheat and load
Dry the bobbin and preheat where the material needs it, then load into the tool without stressing the leads.
- 03
Close the tool
Locate on the bobbin, not on the winding, and confirm the shut-off before the first shot.
- 04
Inject and hold
Fill at a controlled profile and hold pressure long enough to pack without moving the winding.
- 05
Cool and eject
Cool to a stable shape and eject without loading the lead exit or the thin wall.
- 06
Test the coil
Measure inductance, resistance, hi-pot and the seal before the part is packed.
Routes for protecting a coil
Overmoulding is not the only way to protect a winding. The table compares it with the routes that appear in the same enquiries.
| Route | Best for | Sealing | Process control | Note |
|---|---|---|---|---|
| Insert overmould | Medium to high volume | Good at the tool shut-off | Melt pressure and winding support | Shortest cycle |
| Two shot | Housing and soft seal in one cycle | Good, bond formed in the tool | Two materials and tool rotation | Higher tool cost |
| Potting | Low volume and prototypes | Good if the shell is sealed | Fill, cure and voids | Easy to change |
| Encapsulation cup | Simple sensor bodies | Depends on the cup joint | Fill level and cure | Simple tooling |
The choice depends on volume, sealing level and how much the winding can tolerate. The table is a starting point, not a ranking.
Strain relief at the lead exit
The lead exit is where a cable or a lead frame leaves the shell, and it is where a bending load concentrates. A straight, stiff exit creates a hard edge at the end of the overmould, and the conductor bends there. A tapered exit spreads the bend over a longer length.
Anchor the lead inside the shell so the load goes into the overmould and not into the winding. A knot, a crimp or a moulded-in strain relief does that. Then check the pull and the bend: a pull test proves the anchor, and a bend test proves the taper. Neither test alone covers the exit.
Materials and stress
The overmould material has to bond or seal to the bobbin, survive the temperature and resist the chemicals in the application. PA, PBT and PPS are common for the housing, with a thermoplastic elastomer for a soft seal. The pair matters more than either material alone.
Shrinkage and thermal expansion differ between the shell and the winding. A material that shrinks hard onto a fragile bobbin can crush it or shift the core, which changes the inductance. A material with a high expansion can open a seal at temperature. Check the inductance and the seal after thermal cycling, not only after moulding.
Inspection and acceptance
The inspection plan should cover the coil and the shell. Measure inductance and resistance before and after moulding to see the shift, run a hi-pot and insulation resistance test, and check the seal by dye penetrant, pressure decay or a water test at the agreed level.
Add a cross-section at the lead exit and at the thinnest wall, and a pull test on the lead. For a sensor, a functional check against a target after thermal cycling is the best proof that the overmould has not moved the winding. Keep the pre-mould and post-mould inductance readings with the batch, because the difference is the process signature.
Frequently asked questions
What is the difference between overmoulding and potting a coil?
Overmoulding injects a thermoplastic shell around the coil in a tool. Potting fills a shell or a cup with a liquid resin that cures in place. Overmoulding gives a shorter cycle and a defined shape, while potting is easier to change at low volume.
How do you stop an overmould from damaging the winding?
Support the winding with the bobbin, gate away from the coil, control the fill profile and keep the melt temperature inside the bobbin's limit. Then inspect the winding after moulding, because external defects do not show a displaced turn.
What IP rating can an overmoulded coil achieve?
The rating depends on the seal design and the test, not on the material alone. It has to be demonstrated on the finished part at the interfaces. An overmould that looks closed can still leak at the lead exit or the housing face.
Why does inductance change after overmoulding?
The shell can move the core, compress the bobbin or change the dielectric around the winding. Shrinkage and thermal expansion add to the effect. Compare inductance before and after moulding, then after thermal cycling, to separate a process shift from a temperature effect.
Sources and further reading
- IEC 60529:1989+AMD1:2013, Degrees of protection provided by enclosures (IP code)
- IEC 60068-2-14:2009, Environmental testing, change of temperature
- Qumek overmoulding process notes for coils and sensors
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Have a coil that changes value after overmoulding?
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