It’s a world of handheld devices we live in today: smartphones, tablets, GPS assistants and specialized power tools for the shop, lab, kitchen and garden. Medical facilities depend on portable digital devices to care for patients, contractors align studs with pocket-sized levels, troops perform safely in the field with reliable, high-performance equipment and gear, and households are stocked with all kinds of gadgets that make life easier.
And what do these devices have in common? They’re all made, more or less, from plastic — and many from two kinds of plastic: Inside, a molded plastic frame provides strength and rigidity. On the outside, another type of plastic gives the device a precisely-engineered look and feel, bright colors, a distinctive shape or contour, and — perhaps most appealing — a slightly soft, tactile surface that’s pleasant to the touch and just tacky enough to prevent slipping and sliding.
Plastics, especially as used in handheld products, have come a long way in recent years. They've shed their, well, "plasticky" feel while advancing in durability, impact resistance and ergonomics. Plastic cases can now be produced that are much less vulnerable to dropping than their predecessors. Screens are more resistant to scratches. And overall, devices can be shielded much better against extreme heat, water, vapor and dust.
Making all this possible is a technique called overmolding. It involves molding an outside layer of one kind of plastic over or around an inner structure molded from a different plastic, thus creating a single item that combines the best qualities of each material. Using this two-stage process, handheld products can be produced that are highly resilient and also pleasing to the touch and to the eye. Or, the outer layer of plastic may be chosen because of its special functionalities like electrical insulation, sound or vibration absorption, or perhaps water resistance.
Overmolding also offers a means of producing highly intricate pieces that would be too difficult to make in one injection molding process.
The Overmolding Process
Overmolding takes special skill and experience to be executed at high volume and with acceptably high manufacturing yields while overcoming one major challenge: the two materials involved may have radically different physical properties such as the temperatures at which they melt or how much they expand when heated. If there’s a mismatch, the inner and outer layers of plastic may not fit together properly, or they will adversely affect each other, mechanically or even chemically.
Thus, great care is required at every step of the design and manufacturing processes. First, materials must be chosen for maximum compatibility. Then, great precision is required in designing the pieces to be molded, constructing the two molds, and controlling the two-stage molding process. The goal is to get the two molded plastics to bond together in such a way so as to produce a single item that has complete and long-lasting structural integrity. No primers or adhesives are used, it’s all a matter of fit.
Practical Uses for Overmolding
Kaysun prides itself in its deep experience in overmolding and we're dedicated to putting seasoned design, manufacturing, and assembly methods to work for makers of products in a wide range of industries.
Rockwell Collins, for example, chose Kaysun to build the case for its Defense Advanced GPS Receiver (DAGR), a lightweight, handheld device designed for use by soldiers in the field. Kaysun was able to build all of the tight-tolerance molds and tools involved in just 6 weeks. (Read the case study.)
For Smiths Medical, Kaysun helped to design and ultimately built the case for a line of handheld pulse oximetry devices used to non-invasively measure certain qualities of blood. After analyzing more than two dozen components, Kaysun offered a number of design suggestions, developed the manufacturing and assembly processes, and leveraged its deep expertise in polymers to select the right materials for the oximetry product. The one-platform manufacturability cut costs while providing an end user product with more options and greater flexibility, strengthening Smith Medical's positioning in the market.
For more information on improving the design, appearance and performance of injection molded applications, download Overmolding Fundamentals. Click the button below for your free copy!