In cable assembly, overmolding is the addition of various plastic coatings to seal and protect the connector and the harness. An injection die molding procedure is used in single or multiple steps. The injected material is usually a thermoplastic elastomer; rigid first, to seal and lock in the connector, then if a second overmolding is required, it’s usually a flexible material to provide strain relief and or locking tabs. Additionally, insert molding is used to add different colors as well as identification marking.
After the wiring company receives the overmold drawing, an injection die is designed. A two-part mold is machined to create steel interlocking dies. The designer will select the injection points, as well as designing slight inward tapers so the part is easily removed. The die will have runners to allow the plastic material to properly flow into the entire cavity. The extra material from the runners and injection points will have to be trimmed off after molding.
Extensive testing after the first mold test involves high voltage dielectric, salt water or water spray tests, as well as bend cycling of the strain relief.
At assembly, the wiring is completed to the connector, soldering the joints. The assembly is inserted into the die. The die is closed as the hot liquid plastic is injected under pressure into the die with various inlet holes. A few air exhaust holes allow the hot air and a little of the plastic to run along the internal flow tubes to ensure proper filling of the entire mold. After filling and cooling, the mold opens. The operator removes the molded part, setting another for the next molding cycle. To perform the dual overmolding operation for multicolor or flexible strain relief molding requires a second set of different dies and a second molding operation. The engineers determine the most effective materials to be used in the overmolding of the cable assembly. Different materials have different benefits, like increasing the rigidity of the assembly and flexibility. Usually a thermoplastic polyurethane (TPU) or thermoset plastic is used to be injected into the mold. It resists cracking and has a high degree of elasticity. TPU is particularly suited for colder regions. Additionally, polyvinyl chloride (PVC) is a very common material with excellent durability, ability to resist moisture, and ability to withstand high strain.
During the injectable molding process, the mold resin flows from the hopper into the molding machine. An injection screw pumps it into the die mold cavity. As the material is heated in the barrel of the screw, it will begin flowing into the mold. To compensate for known shrinkage of the plastic resin during cooling, a small amount is overshot into the die as it cools to help compensate for the shrinkage. Typical injection molding rules apply; avoid deep or blind pockets or deep ribs in your design. Use gradual transitions between wall thickness to reduce or avoid problems with the flow such as backfills, gas traps and excessive shrinkage.
Overmolded cable connectors provide many major benefits compared to hand mechanically assembled connectors:
- Lower high-volume costs
- Improved reliability
- Improved environmental performance
As the plastic encapsulates the wires, it naturally secures the soldered wire to the pin. This provides insulation, strain relief, and improved moisture and water protection compared to a mechanically assembled connector.
A two-mold system creates a support structure for the outer mold. Omitting an inner mold can cause cosmetic defects such as pits, sinks, and other injection molding problems. When a PCB is included with the connector, different molding techniques are used to ensure full encapsulation. A generic cable assembly with an inner mold can then have multiple outer mold geometries, colors, or logos applied to it later.
Cost vs. Benefits
Overmolding is an ideal solution for providing a seal for cable assemblies and their connectors. This is especially useful in situations where the assemblies will face rugged or extreme conditions. As well, they offer medical cables that can withstand sterilization procedures with built-in strain relief. The benefit of using a cable overmolding means the cable assembly will last longer and perform better than it would without the added protection. A significant benefit of overmolding is creating a seal for the cable assembly connection that can be completely water-tight, resist moisture and oil, as well as harsh environments.
The benefits of overmolded cable assemblies are many, but there is an increased cost associated with adding these extra layers of protection. Careful considerations are best decided with the help of an experienced custom cable assembly manufacturer. Overmolding allows the designer to change the angle of the exit point from straight to a right angle, or just about any other customization required. Molds can be made with long life hardened steel for longevity in use, or be made out of aluminum for lower use prototype applications. The die itself is routinely customized with logos or part numbers.
Overmolding cable assemblies can be the correct addition to your next design project. It will increase the level of physical and environmental protection of the cable assembly while providing the required flexible strain relief. The overmolded connector will improve the aesthetics of the assembly while giving visual cues that aid with installation of the assembly into its designated connector.
Cooler Cao is an engineering manager at Cabletree Industrial Company. Cables, cable assemblies and wire harness are, more than ever, an integral part of today's electronics. With the development of automotive and medical diagnostics, electric automobiles, solar and wind power has come the design and develop of newer and more sophisticated connection devices.