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The term “Overmolding” looks pretty simplistic at first glance, but anyone with experience making Cable Assemblies understands that many factors go into the clean appearance of an overmolded Cable Assembly.
The three most critical items in the design of a proper overmold are the inlet where the overmold material is introduced into the mold, the venting system to allow air to escape from the cavity, and the adhesion of the overmold material to the cable jacket or braided shield.
Let’s take a look at this more casually.
1. Gate Design: How the Material Gets In (Smoothly)
The gate (the part of the mold that melts plastic) is critical to the flow in and out of the mold. The gate design can affect how well a mold fills and how the final part looks.
▪ Smart Placement
The gate should be placed so that it gives you an even material flow. When the flow is balanced, you may have minimal swirl marks, air pockets, and fill issues.
▪ Right Size Matters
Use a slightly larger gate size. It will reduce the pressure drop across the gate, helping you avoid the “jetting” issue – where the material fills the cavity too quickly, creating unsightly lines or deformations.
▪ Better Flow Through Shear
Shear is a great way to help the material flow. Higher shear rates generate heat, helping the material flow through the cavity without requiring excessive force.
2. Venting: Giving Trapped Air an Exit
If the gate is the doorway in, then vents are the little windows that let trapped air out. Without good venting, the material won’t fill appropriately—even if everything else is perfect.
▪ Preventing Defects
Well-designed vents will eliminate defects such as burn marks, bubbles, short shots, and other issues caused by trapped air.
▪ The Challenge with Shell-Type Parts
Air will not easily escape from shell-shaped overmolds due to their design and geometry. This means that if you plan to work with thick or sticky materials like TPE, the venting design must be well thought out and carefully engineered.
3. Bonding to the Cable Jacket or Braided Shield: Keeping Everything Together
The whole point of overmolding is protection—and none of that matters if the molded material doesn’t stick well to the cable jacket. A strong bond keeps moisture out, prevents pullouts, and extends the cable’s lifespan.
▪ Material Compatibility First
Certain materials are naturally more compatible with each other than others, and ideally, the base cable jacket would be chemically compatible with the overmolding material.
▪ Surface Prep Helps a Lot
Cleaning, priming, or otherwise preparing the cable jacket surface, such as by corona treatment, can significantly enhance the adhesion properties of the overmold. While these processes are small steps, they yield drastic improvements in adhesion.
▪ Mechanical Locking If Needed
Where chemical compatibility is insufficient, a design feature can help provide a secure bond. Design features such as grooves, textures, and ribs, when located on the cable jacket, provide areas for the overmolding material to “grasp.”
▪ Process Settings Seal the Deal
The injection molding process conditions determine how well the overmolding material adheres to the cable jacket. Proper injection pressures, melt temperatures, and mold temperatures will enable adequate flow of the injection material to create a mechanical bond with the cable jacket, rather than leaving it unmelted above the cable jacket.
In Short
Good overmolding is all about flowing in the right way, air escaping the right way, and everything sticking together the right way. When gates, vents, and bonding are dialed in, the result is a clean, durable, and reliable cable assembly.
Sam Wu is the Marketing Manager at Romtronic, holding a degree in Mechatronics. With 12 years of experience in sales within the electronic wiring harness industry, he manages marketing efforts across Europe. An expert in cable assembly, wiring harnesses, and advanced connectivity solutions, Sam simplifies complex technologies, offering clear, actionable advice to help you confidently navigate your electrical projects.
