Overmoulded assemblies: sealing, bonding and testing

What an overmoulded assembly is

An overmoulded assembly is a group of parts that a second moulding holds together. The group usually includes a moulded body, one or more metal inserts, a cable or a flex tail, and sometimes a small sensor or a terminal. The overmould is a thermoplastic or thermoplastic elastomer shot over the outside, and it becomes the skin, the seal and the strain relief at once.

The distinction from a simple overmoulded grip matters. A grip has one interface, between the soft material and the substrate. An assembly has several: the overmould to the housing, the overmould to the cable jacket, the insert to the housing, and the conductor to the terminal inside. Each interface has its own failure mode, and the finished part is only as good as the weakest one.

Chemical bond or mechanical grip

The overmould can hold in two ways. It can adhere chemically to the substrate, or it can grip it mechanically through geometry. Many designs use both, and the good ones know which one they are relying on.

Chemical adhesion depends on the material pair and the surface. Some thermoplastic elastomers bond to polypropylene, some bond to polyamide, and many bond to neither without help. The substrate surface must be clean and free of release agent, dust, mould spray and handling oil. A flame or plasma treatment can raise the surface energy of a difficult plastic, and a primer can bridge a pair that will not bond directly.

Mechanical grip is more forgiving. Grooves, holes, undercuts, a knurled insert, a flange that the overmould wraps around, or a through slot that lets the melt form a rivet all give the soft material something to hold. The interlock should sit away from free edges, because peeling starts at an edge and travels along the bond line.

Sealing at three interfaces

A sealed assembly usually has to keep water and dust out at the cable entry, at the housing joint and at the mating face. These are different seals with different mechanics, and one moulding has to serve all three.

At the cable entry, the overmould has to compress the jacket evenly around its circumference. That needs a controlled interference and a jacket material that does not creep away over time. A soft PVC jacket and a stiff overmould can fight each other; a jacket that is too hard may not compress enough to close the path. The overmould should also extend far enough along the cable to spread bending, so the seal is not asked to act as a hinge.

At the housing joint, the overmould usually replaces a gasket. The plastic substrate needs a groove or a step that contains the compressed material, and the tool needs a clean shut-off so flash does not cross the sealing face. A flash line on a seal is a leak path, even when it looks cosmetic.

At the mating face, the seal is often a moulded lip or a face seal that compresses against the mating connector. The lip needs a lead-in, a controlled squeeze and a counterface that is smooth enough not to cut it. If the mating part is a customer supplied item, get its tolerance and surface finish before the tool is cut.

One path is easy to miss: the conductor itself. Stranded cable can pass air or water along the spaces between strands, from a damaged point at the far end to the inside of the assembly. If the product must hold pressure or pass an immersion test, a strand block or a sealed crimp may be needed, not just a tight overmould.

Material pairs and the process window

The common substrate materials are polypropylene, ABS, PC/ABS, polyamide and PBT. The common overmoulds are styrenic block copolymers, thermoplastic vulcanisates, thermoplastic polyurethanes and softer polyamides. The pair decides the bond, the temperature range, the chemical resistance and the feel.

A hard substrate with a soft overmould is the usual combination. Shore hardness is chosen from the function: a cable exit needs a material that flexes without tearing, while a housing edge needs resistance to abrasion and cleaning agents. A harder grade is not automatically more durable, because a hard overmould on a flexible cable creates a stress concentration at its end.

The process window is narrow for some pairs. The overmould must be hot enough to wet the substrate but not so hot that it melts or distorts it. The substrate must be dry, and so must the overmould compound, because moisture in a hygroscopic polymer produces splay, weak weld lines and a poor interface. Pack pressure has to fill the seal and the undercuts without flashing the shut-offs or overpacking the insert.

Where overmoulded assemblies fail

Most production problems fall into a small set. The table links each one to the check that should happen before the tool is cut, because that is when a change is still cheap.

Testing the finished assembly

Testing should follow the function. A sealed assembly needs a leak test at the specified pressure, temperature and duration, using the method the product standard calls up. Air decay, pressure decay, tracer gas and immersion each have different sensitivity, and the acceptance limit should be stated in the drawing or the test specification rather than agreed after a failure.

A cable entry needs a pull test and a bend test. The pull test proves the overmould grips the jacket and that the termination inside is not carrying the load. The bend test proves the relief spreads the bend instead of concentrating it. Both should be run at the temperature extremes the product will see, because a thermoplastic elastomer softens when warm and can become brittle when cold.

Adhesion deserves a direct check as well. A peel test at the overmould edge, or a section through the bond line, shows whether the two materials have actually joined. The section is also the best way to see voids, contamination and whether the melt filled an undercut or bridged over it.

What to send for an overmoulded assembly quote

Send the complete assembly model, not just the overmould. The cable type and jacket material, the insert material and finish, the housing grade, the mating connector and the intended environment all change the process and the price.

State the seal requirement as a standard and a test, for example an ingress rating and the test duration, rather than asking for a waterproof part. State the pull and bend requirements with the method.

• Assembly model with the cable, inserts and mating part
• Substrate and overmould grades, with hardness
• Seal rating and the test method
• Pull, bend and electrical requirements
• Annual volume, variants and release size
• Tooling ownership and maintenance expectation