Key takeaways
- An overmoulded assembly is a system of connector, termination, cable jacket and mould compound, not a connector with plastic added.
- Strain relief works by spreading load over a length of jacket, so the boot geometry and the bond matter more than the mould material alone.
- Sealing depends on the interface between compound and jacket as much as on the connector seal.
- Validate the assembly with the tests that match its service: pull, bend, twist, thermal cycling and electrical integrity.
What an overmoulded assembly contains
A custom connector assembly joins a cable to a connector and protects the joint with a moulded body. The body may be a simple strain relief boot, a full overmould that forms the connector housing, or a moulded section that seals a splice. The same assembly can include ferrules, a backshell, a shield termination and a label.
The moulded body has four jobs: keep the conductors and terminations in place, transfer cable load away from the termination, seal the entry against dust and water, and survive the environment without cracking or softening. Those jobs pull in different directions. A stiff compound gives good support but poor flexibility. A soft compound flexes well but may not bond to the jacket or hold a seal.
The design starts with the cable and the connector, not the mould. The cable jacket material, the conductor class and the termination method set what the mould has to protect.
The build sequence
Most overmoulded assemblies follow the same route. The details change with the connector, the compound and the volume, but the sequence is a useful checklist for a new programme.
- 01
Prepare the cable
Cut to length, strip the jacket and conductors to the drawing, and inspect for strand damage. A nick in the jacket becomes a leak path later.
- 02
Terminate
Crimp, solder or weld the conductors to the contacts. Check pull strength and position before anything is closed over the joint.
- 03
Assemble the interface
Fit seals, backshell, shield termination and any ferrules. Route the conductors so the bend inside the mould is controlled.
- 04
Load the mould
Place the assembly in the tool, close it and hold the cable in the designed position. Cable position sets the boot wall thickness.
- 05
Mould and cure
Inject the compound, hold pressure, cool or cure, then open the tool. Flash and gate marks are removed at this stage.
- 06
Test and finish
Continuity, insulation resistance, pull and bend tests, then labelling and packing. Keep the test record with the serial or lot.
Strain relief geometry
Strain relief is not a rigid clamp. It is a gradual transition from the flexible cable to the stiff connector. If the transition is too abrupt, the cable bends at one point just outside the mould and the jacket or the conductors fatigue there. A longer boot with a taper spreads the same bend over more length and lowers the strain at any one section.
The bend radius matters more than the boot length alone. A boot that is long but ends in a sharp shoulder still concentrates stress at the shoulder. A generous fillet at the cable exit, a wall that thins toward the cable and a short straight section before the bend all help.
The cable itself sets how much support is needed. A flexible class 5 conductor with a soft jacket needs a longer, softer transition than a stiff class 2 cable, even at the same outer diameter. The number of strands, the jacket wall and any fillers all change the stiffness. A boot designed around one cable construction may be too stiff or too soft for another, which is why a change of cable supplier deserves a new bend test.
Ribs are often added to a boot for handling and to improve flexibility in a controlled way. They should not create thin sections that tear. For a cable that is regularly flexed, the boot and the cable jacket should be treated as one flexing system, and the bend test should reproduce the service motion.
Compound, jacket and adhesion
Overmould compounds are usually thermoplastic elastomers, PVC, polyurethane or a polyamide. The choice depends on flexibility, temperature, chemical exposure and the feel required. A compound that is perfect for a fixed installation may crack in a cable that is flexed daily at low temperature.
Adhesion is the difficult part. The compound has to bond to the cable jacket and to the connector body, or at least form a reliable mechanical seal. Jacket materials differ: PVC, polyethylene, polyurethane and cross-linked elastomers each need a different surface preparation or primer. A release agent left on the jacket will produce a boot that looks correct and pulls off by hand.
Chemical compatibility also matters. Plasticisers in a PVC jacket can migrate into some compounds and soften them. Oil, fuel and cleaning agents can swell the boot. Ask for immersion or wipe test data for the actual fluids, not only for water.
Sealing and the ingress path
Water enters an assembly along the easiest path, which is often the annulus between the cable jacket and the moulded body. A connector face seal does nothing for that path. The overmould has to bond to the jacket, or a separate seal has to close the entry.
Mould shrinkage can help or hurt. A compound that shrinks onto the jacket grips it; one that shrinks away leaves a gap. Tool design, injection pressure and cooling determine which happens. The cable position in the tool also sets the wall thickness on each side; an off-centre cable gives a thin wall that may leak or tear.
If the assembly needs an ingress protection rating, test the finished assembly, not a sample without the cable. The test should include the cable entry, the connector interface and any joint in the boot. Thermal cycling before the ingress test is useful because it opens gaps that a room-temperature test misses.
A sealed assembly also sees pressure changes. Air trapped inside the mould expands when the assembly is moved from a cold store to a warm workshop, or when the equipment heats in service. That pressure can push the jacket away from the compound at a weak bond. A vent, a potted splice or a controlled bond line can relieve it. Sealing is a system property: a single O ring at the connector face does not seal the cable entry.
Tooling and validation
An overmould tool is usually a simple two-part tool with a cavity for the boot and inserts to hold the connector and cable. It costs less than a plastic housing tool, but it still needs a design freeze on the cable and connector before it is cut. A change in jacket diameter or connector shell can scrap the tool.
Validation should cover the failure modes the assembly will meet. A pull test confirms the termination and the boot grip. A bend test, run to a defined radius and cycle count, checks the strain relief. A twist test checks the boot-to-connector joint. Thermal cycling checks differential expansion between the compound, the metal and the jacket. Electrical tests confirm continuity, insulation resistance and, where relevant, high-potential withstand.
Keep the test conditions in the drawing or the specification. A pull test result without the pull rate, the grip method and the acceptance limit is not repeatable.
What to send for a quote
Send the connector part number, the cable specification, the assembly drawing and the length. Add the environment, the expected flexing, the ingress protection target and the test standard. If the cable is not yet fixed, say which properties are open, because the jacket material drives the mould compound and the bond.
| Input | Why it matters | What to state |
|---|---|---|
| Cable and jacket | Sets bend radius, bond and temperature limit | Construction, jacket material, outer diameter |
| Connector and contacts | Sets the interface, tool inserts and termination | Part number, contact type, plating |
| Termination | Controls pull strength and heat | Crimp, solder or weld, with the standard |
| Mould compound | Controls flexibility, sealing and chemical resistance | Material family and hardness |
| Strain relief | Controls flex life at the cable exit | Boot length, exit radius, flex cycles |
| Sealing | Controls the ingress path | IP target and the test method |
| Validation | Proves the assembly, not just the parts | Pull, bend, twist, thermal and electrical tests |
Frequently asked questions
What is the difference between overmoulding and a backshell?
A backshell is a separate part, usually metal or moulded plastic, that clamps the cable and covers the termination. Overmoulding forms the body directly onto the assembly, so it can seal and support the cable with fewer parts. A backshell is easier to rework and better for some shielded or serviceable designs.
How long should a strain relief boot be?
Long enough to spread the service bend over a useful length of jacket, with a smooth taper and no sharp shoulder. The right length depends on the cable diameter, stiffness and bend radius. Prove it with a bend test at the service radius rather than a fixed rule.
Why does an overmoulded assembly leak at the cable entry?
The usual cause is poor adhesion between the compound and the jacket, often from contamination, the wrong compound or a jacket that needs a primer. Mould shrinkage away from the cable and an off-centre cable can also leave a thin or open wall.
Can an overmoulded assembly be repaired?
Usually not without cutting back the cable and rebuilding the end. That is why the termination and the pull test come before the mould. For a serviceable assembly, a mechanical backshell or a connector with a replaceable contact system is often the better choice.
Sources and further reading
- IEC 60352-2, Solderless connections, crimped connections, general requirements and test methods
- IEC 60529, Degrees of protection provided by enclosures (IP code)
- IEC 60068-2-14, Environmental testing, change of temperature
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.
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