Our world is one of handheld devices. Medical facilities depend on portable digital devices to care for patients. Contractors align studs with pocket-sized levels. Troops rely on high-performance equipment and field gear. Households are stocked with all kinds of gadgets to make life easier.
The devices serve different purposes, but they are all made, more or less, from durable plastic — and many from two kinds of plastic: Inside, a molded plastic frame provides strength and rigidity. Outside, another type of plastic provides a precisely-engineered look, distinctive shape or contour, bright colors, and — perhaps most appealing — a slightly soft, pleasantly tactile surface that's just tacky enough to prevent slipping and sliding.
Making all of it possible is plastic overmolding, the ultimate in multi-material injection molding. Overmolding seamlessly combines multiple materials into one part or product to improve product design, performance, and aesthetics.
THE PLASTIC OVERMOLDING PROCESS
Most commonly, the plastic injection overmolding process involves using multi-barrel injection molding machines to inject several materials into the same tool during a single, automated operation. There is also an approach that uses multiple machines, one for molding the base component (or substrate) and another for applying a thin layer of thermoplastic elastomer (TPE) to the substrate. The result? A hardwearing plastic product with a comfortable, non-slip grip — and a host of versatile application-specific solutions that provide enhanced durability, impact resistance, and ergonomics with no "plasticky" feel.
Overmolding also offers a means of producing highly intricate pieces that would be too difficult to make in one injection molding process.
The EXPERTISE BEHIND PLASTIC Overmolding
Special skill and experience are required to execute a multi-material injection molding process at high volume and with acceptably high manufacturing yields, particularly as it relates to material compatibility in overmolding. For example, the two plastics involved may have radically different physical properties, such as melt temperatures 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 throughout an overmolded product's design and manufacturing, ideally with upfront and ongoing involvement of an experienced custom injection molder.
First, materials must be selected based on the molder's in-depth knowledge of the materials' properties and proven history of compatibility. Then, great precision is required in designing the pieces to be molded, constructing the two tools, 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. With overmolding no primers or adhesives are used, it’s all a matter of fit.
Practical Uses for PLASTIC Overmolding
Kaysun has an impressive depth and breadth of experience in overmolding, and we're dedicated to putting seasoned design, manufacturing, and assembly methods to work for our manufacturer partners.
For Smiths Medical, as an example, Kaysun helped design and ultimately build the case for a line of handheld pulse oximetry devices used to non-invasively measure certain characteristics of blood. After analyzing more than two dozen components, the Kaysun engineering team offered a number of design suggestions, guided materials selection, and developed the manufacturing and assembly processes for the oximetry product. The one-platform plastic overmolding manufacturability cut costs while providing an end user product with more options and greater flexibility, strengthening Smiths Medical's positioning in the market.
For more information on using overmolding to improve the design, performance, and aesthetics of injection molded applications, download Overmolding Fundamentals for OEMs. Click the button below for your copy of this insightful white paper.