If you’ve picked up a power tool at the hardware store recently, or used a multimeter to check for power on a kitchen outlet, you’re probably experienced overmolding. The application of soft, durable plastic over a rigid substrate—screwdriver handles, toothbrushes, and cookware, to name a few—is becoming increasingly commonplace, and indeed expected, in a variety of consumer goods, electronics, automobile components, and sporting goods. Yet home repair aficionados aren’t the only ones who appreciate the feel of overmolded products. In the medical industry especially, where surgeons and health professionals perform hours-long procedures, often with lives at stake, overmolded devices and other medical equipment are far more than nice to have—they’re a necessity. (See Figure 1)
Learning the Two-Step
Unlike traditional injection molding, where molten plastic is forced into a mold cavity to create finished products in a single, one-step process, such as a prescription pill container or a plastic forceps, for example, overmolding takes products one-step further. The cap for the pill bottle might be placed into a secondary mold and the outside overlaid with a flexible thermoplastic, making the bottle easier to open. Similarly, a forceps can be overmolded with handles in the gripping area, assuring a non-slip surface. In either case, the original product, called the substrate, is physically enhanced through the addition of material that covers some or all of that substrate. (See Figure 2)
Most commonly, this secondary material is some type of thermoplastic elastomer (TPE), which is typically softer and more flexible than the base material. The result is a product that retains the strength of a rigid substrate such as polycarbonate or nylon, but one that, aside from an ergonomic and comfortable feel, may offer vibration dampening, noise reduction, brand identification, weight reduction, and a host of other desirable attributes not possible with mono-plastic injectionmolded parts.
The possibilities are seemingly endless. Tony Holtz, a Technical Specialist at Minnesota-based rapid manufacturing company Proto Labs, explains that overmolding is ideal for parts requiring a soft touch, multiple colors, or greater functionality than what’s possible through traditional molding, but requires additional planning and sound knowledge of overmolding design principles to be successful. “The key thing with overmolding is understanding how the two materials actually interact with each another,” he says. “You can’t just grab some ABS and a Santoprene (a type of thermoplastic vulcanizate, or TPV) and mold the two. They don’t have the right bonding characteristics, and it’s very likely the materials will peel apart after some time, if not immediately.”
Hold on Tight
There are two ways to prevent that unfortunate situation, Holtz says. The first is through mechanical bonding. Like fitting together the pieces of a jigsaw puzzle, overmolders sometimes rely on ribs, grooves, and undercuts to give the top layer sufficient gripping area such that it won’t move during use. This approach, however, is seldom sufficient on its own, as the material properties that make elastomers pleasant to touch can also make them hard to hang onto. Also, mechanical features such as these tend to drive up the cost of the mold, especially when undercuts are used. These require a sliding sideaction mechanism be installed, one that is pulled out of the way after the mold has cooled, allowing the parts to be ejected.
A better approach—and one that manufacturers and material suppliers alike strive for—is chemical adhesion, also known as cohesive bonding. Functionally, this is little different than the bond between a handful of superglued pottery shards, or using an adhesive to hold together a scale model of the USS Eisenhower. Yet there’s no need for any sticky bonding agents here. All that’s required are the right materials, the right temperature, and enough time in the mold to let the “magic” happen.
Quite often, medical device designers will use both approaches together, especially with less than optimal material pairing, heavy duty cycles and very thin overmolded sections. Aaron Behnke, Southeastern Region Sales Manager for thermoplastic elastomer compounding specialist KRAIBURG TPE Corp., Duluth, GA, says that a properly bonded overmold material will actually tear or break before the bond gives way. “If you get a clean separation between the two materials, they cannot be considered truly bonded.”
This is one reason why material suppliers provide a range of products to not only achieve a predictable bond across dozens of possible substrate materials, but also deliver the desired properties for any given application. Because of this, KRAIBURG TPE’s recipe book contains more than 30,000 types of TPE, spread across a portfolio of 400 or so families. These include thermoplastics based on styrene-block-copolymers (TPS), polyurethane (TPU), olefin (TPO), and vulcanate (TPV), each of which offers its own unique combination of mechanical and thermal properties, as well as resistance to abrasion, chemicals, hydrocarbons, microbes, and bodily fluids. TPEs can also be blended with other polymers to create entirely new materials, or mixed with colorant to produce most any hue, making these versatile materials the pinch hitters of the polymer world.
Wearing a Lot of Hats
Behnke suggests taking the acronym TPE with a grain of salt. “It’s a very generic term for lots of different types of thermoplastics. We’re able to start with a basic compound, say an automotive- or medical-grade material, and tweak it to provide a wide range of characteristics. This could just be a color change, but it could also mean the ability to endure gamma ray sterilization, or become FDA-compliant for food and dairy contact. There’s a lot more to it than bondability in an overmolding situation.”
Material selection is important, but so is the design process itself. Behnke says medical product engineers must constantly adapt to new regulations, a situation that drives a continuous cycle of design and redesign. “Nothing stays the same in this industry. Every day, companies are developing new products, many of which are overmolded. These might call for a special gasket or seal, a new type of closure, a valve, or lock—there are hundreds of uses for this process. In each case, there are challenges that come into play on the design side of it. Designers have to focus on developing moldable part geometries, the correct thickness of overmolded material, properly designed and functional seals, predictable material flow—there’s a lot to it.”
Paul Killian agrees. The Global Business Manager of the TPE division at RTP Company, a polymer supplier in Winona, MN, Killian says each application needs to be evaluated individually, and that biocompatibility, physical and thermal performance is dependent on a number of factors. “RTP offers a set of basic recommendations for success with overmolding, but these must of course be applied by experienced part designers and tool makers.” These recommendations include:
- proper “shut-off” design, the area that stops the flow of heated material during the overmolding process, to prevent peeling at overmolded boundaries;
- gating, which as the name implies, allows material into the mold. It should be kept to a minimum length, and placed in the thickest cross-sections of overmolded areas;
- correct placement of appropriately sized ejector pins, and slowing ejection speeds to avoid “punch through” of soft overmolding materials;
- avoiding sharp corners, deep pockets, and very thick or thin walls, e.g., maintaining uniform material thickness whenever possible; and
- ribbed areas placed parallel to material flow, so as to avoid air entrapment. Likewise, ribs should be proportional to the size of the workpiece. If not, warpage and voids may occur.
Fast, Not Furious
Rapid manufacturing companies and product developers are paying increasingly more attention to the need for quick turnaround molded parts. The value proposition realized by having molded parts within days is particularly relevant in the world of 510(k) submissions to the FDA, where it’s advantageous for manufacturers to get as close to the final part design as possible upfront, thus avoiding delays to market and costly resubmissions after the fact. Quick-turn tooling such as this also allows multiple iterations of the same part design to be tested, often with different materials each offering its own unique mechanical properties, bonding characteristics, and aesthetics.
The Ergo Factor
Most designers and engineers would agree that overmolding offers greater design functionality than traditional “one-shot” injection molding, and produces parts that are both durable and pleasant to the touch. And what better way to obtain those parts than with tooling that also provides some measure of production capability farther down the product life cycle road? At the end of the day, however, engineers and accountants agree—overmolding saves money.
Overmolding has become such an important manufacturing process that jobs are popping up as a result. Human factor engineers are responsible for the design, development, and testing of products that cater to the people that use them, addressing the cognitive and perceptual needs or limitations of those people while creating products that are both robust and inherently manufacturable.
This is an area in which overmolding plays quite well. It reduces costs and, given the right manufacturing partner, speeds time to market. It produces parts that are strong and rigid, yet comfortable to hold. Most importantly, overmolding makes parts that are userfriendly, whether that user is a brain surgeon or a construction worker. All that’s needed to take advantage of overmolding technology is a good working knowledge of the materials that are available, an understanding of the process itself, and a partner that can steer them in the right direction.
This article was written by Kip Hanson, Manufacturing Writer for Proto Labs, Maple Plain, MN. For more information, Click Here .