Insert molding is a type of overmolding where a hard substrate component or “insert” is placed inside a mold cavity in an injection molding machine and then “overshot” with an exterior layer—typically a thermoplastic elastomer (TPE). The chemical reaction between the insert and the TPE must be fully understood to create the strongest possible bond. The surface of the insert should also be free of contamination, including dust or even skin oil—even the slightest contamination can weaken the bond between the TPE and the substrate, leading to premature failure.
Inserts are typically hard plastic or metal. Metal inserts should be pre-heated before placing into an injection mold because:
Pre-heating minimizes the quenching effect when the plastic encounters the very thermally conductive metal, which will draw heat from the plastic, increasing its viscosity and interfering with the flow/attachment to the insert
What is hoop stress?
Imagine a wooden barrel with metal hoops around the outside. These hoops are added to prevent the barrel from bulging outward. As pressure in the barrel increases, the hoops are stretched circumferentially. Likewise, the melted plastic surrounding a metal insert will shrink as it cools, producing a “hoop” stress in the plastic. This hoop stress will be a constant stress throughout the life of the product and possibly result in cracking if the stress is not minimized during the production process.
If plastic is loaded with a constant force/stress it will stretch, or creep, over time. The larger the force/stress, the faster the stretch/creep will occur. Over a prolonged period of time the plastic molecules simply cannot sustain the load and will rupture. Creep rupture charts are created—based on loading and time to rupture—so that a failure points can be predicted. These are also calculated at various temperatures since plastics behave differently at different temperatures. A creep rupture chart should be always consulted to determine the maximum strain/shrink rate, followed by the design of an insert-surrounding plastic hub that will control hoop stress and creep.
The more companies learn about overmolding, the more they want to use this special injection-molding technology to add value to their product lines. Not only does it improve functionality and performance, it lowers total production costs—that’s pretty rare these days.
So, what is overmolding?
Using a one-shot or two-shot process, a thin layer of TPE (thermoplastic elastomer) is overshot onto the hard plastic substrate of the product. This exterior layer is what creates the soft-touch grip for so many products, like toothbrushes and tool handles. But it has lots of other properties that also add value to the final product—it can reduce shock and vibration, dampen sound, provide electrical insulation, boost chemical/UV resistance, be an effective barrier against oxygen and moisture, and perhaps most importantly, create a wide range of colorful, stylish exteriors that catch the consumer’s eye.
More consumers are demanding this look and feel for the personal-care products, tools, electronics, sports equipment, medical devices, etc. they purchase. As a result, overmolding applications continue to expand across a wider range of products. The availability of this technology also gives product engineers more creative freedom in designing attractive and functional products that will keep them on the leading design edge of their markets.
Not every injection-molder can overmold, however; it requires specialized equipment, extensive training, and precise control of multiple variables throughout the production run—even the slightest variations can affect quality, so skilled operators are essential.
Overmolding engineers must have a deep knowledge of material chemistry and know how different types of TPE and substrates interact under varying pressure and temperature conditions. This field of knowledge is also rapidly expanding—suppliers are continuously engineering new blends with expanded characteristics such as chemical resistance, UV resistance, hardness, scratch resistance, clarity, heat resistance, and UL standards. Kaysun Corporation has a vast material science database it has accumulated from hundreds of previous projects and our engineers stay on top of the latest developments in new materials.
It can take some time getting overmolding just right, but it’s worth it—this one- or two-step process delivers products of such high quality that the need for some downstream finishing operations is eliminated, which saves time and money and gets products into the marketplace faster.
Of course, this is common knowledge at Kaysun. We have perfected this process through extensive investments in technology and training; this is why top global companies like Rockwell Collins (defense and aerospace) and Smith’s Medical (health care) rely on Kaysun.
Learn more about Kaysun’s overmolding capabilities by downloading our white paper or contacting us. We’re the perfect place to start—and finish—your injection molding project.