How Rapid Overmolding Fits Into The Medical Device Puzzle

By Rob Bodor, VP and GM of the Americas, Proto Labs

We’ve discussed thermoplastic, liquid silicone rubber, and metal prototyping in previous posts, and we’ll conclude our Building Better Prototypes series with a look at overmolding. This two-part injection molding process entails the production of a typically rigid substrate part (made of plastic or metal) that is partially or fully molded over by a second, often softer, material to create a finished part. Substrates can be produced by numerous means, including machining, but the most common approach is to use a two-shot molding process: The substrate is molded in the first shot, and the overmold material is molded in the second shot.

In medical applications, overmolding can position a rugged “skeleton” within a softer covering, combining the toughness of one material with the light weight, soft feel, or other characteristics of another material. An outer layer can be used to resist abrasion, to waterproof a substrate, or to improve resistance to chemicals or cleaning materials. A flexible overmolded material can protect a rigid substrate from impact, insulate a user’s hand from vibration, or deaden vibration.

Overmolding can simplify assembly by creating a built-in pneumatic or hydraulic gasket. Overmolded materials can provide a nonslip handgrip over a potentially slippery substrate. Through a range of available colors, materials, and textures, overmolding can enhance aesthetics or even enhance brand identification. And by combining multiple parts into a single piece, overmolding can simplify a product’s bill of materials (BOM).

Applications

Overmolded parts can enhance the performance of many different medical applications, including:

• Surgical instruments — nonslip grips, chemical resistance, biocompatibility
• Instrument housings — impact resistance, noise and vibration control, improved aesthetics
• Handheld devices — soft grips, vibration control, abrasion resistance
• Monitors — impact resistance, noise control, abrasion resistance
• Tubing or Luer fittings — liquid or gas seals
• Electrical connectors — insulation, color identification
• Syringes — chemical resistance, nonslip grips, built-in seals

Bonding and Material Selection

Because overmolding can increase both the functionality and the complexity of a part or assembly, prototyping is especially critical to the development process to validate that both functional and cosmetic objectives have been met. Design has to take into consideration the properties and bonding interactions of multiple resins in the mold. Prototyping, while verifying that the design and materials perform as expected, can also identify unexpected interactions.

Perhaps the most obvious interaction between resins is their bonding. Some resins, if properly overmolded, form a chemical bond which resists any tendency to separate in use. Guidance in the selection of the combination of substrate and overmold material is important to ensure proper chemical bonding occurs. In addition, all materials, whether or not they bond chemically, can form some sort of mechanical bond.

Mechanical bonds can form when the overmolded material wraps around the substrate or locks into undercuts, and such bonds can be enhanced by the texture of the mating surfaces. Designs need to consider which bonding process will take place for the targeted materials. If chemical bonds are not possible, and undercuts are not built into the substrate, the overmolded parts may not perform as desired.

Another critical area of interaction is the respective melt/deflection temperatures of the two resins. If the overmolded resin has a higher processing temperature than the substrate’s heat deflection, the substrate can be damaged. The damage that typically occurs is warping, additional part shrinkage, and melting of the substrate. And while lubricity — the slipperiness of a resin’s finished surface — may be desirable in a bearing or other sliding application, it can interfere with bonding and will require, at minimum, a thorough and well-thought-out mechanical bond.

There are a number of resins, both thermoplastics and thermosets, that are commonly used for overmolding. These include:

• Thermoplastic polyurethane (TPU) is noted for its toughness, flexibility and biocompatibility, and it is widely used in devices that will have external body contact as well as for implants.
• Thermoplastic vulcanate (TPV) offers many of the capabilities of thermosetting resins, but is both lighter and recyclable.
• Styrene-ethylene/butylene-styrene copolymer (SEBS) offers good flow within the mold, high stiffness and reduced tendency to warp, and excellent adhesion to other thermoplastics.
• Thermoplastic elastomer (TPE) materials have the elastic properties of rubber, offer excellent consistency, are easy to color, and are more economical to use than thermosets.
• Liquid silicone rubber (LSR) is a thermoset that offers high lubricity and is highly stable and heat tolerant for ease of sterilization. It is compatible with metal and a variety of plastic substrates, as well as flexible at low temperatures and biologically inert.

Tooling

Material choices — both individual materials and their anticipated interactions — are a major determinant of overmolding success. So is design. A third critical factor is tooling, which is somewhat more complex for overmolded parts than for those comprised of single resins. Here are some considerations when tooling overmolded parts:

• Well-designed shutoffs will prevent overmolded materials from flashing over the substrate and requiring additional processing (pinch off) for cleanup.
• “Two-shot” molds will require separate gates for substrate and overmolded material, complicating their design.
• Pick-n-place molds will have to support the substrate while the second resin is injected.
• Rougher substrate surface texture may be required for good adhesion.
• Part ejection must be considered in designing two-shot molds and, in the case of pick-n-place molding, in both the substrate and overmold tools.
• Presence of the substrate in the mold when the second resin is injected can complicate venting. If gas cannot escape from the mold during injection, the result may be “short shots,” or burning of the resin.

Prototyping

Because it uses multiple materials and complex processes, a successful overmolded prototype should duplicate both the materials and the processes that will be used in production. Medical device designers and engineers need to know that each of the materials will perform its desired functions, that the resins will fit together and adhere properly, that they will withstand use, and that the parts can be effectively molded.

Two-shot molds are cost-effective for mass production, but for runs of dozens, hundreds, or up to 10,000 overmolded parts, pick-n-place molding is generally more affordable. In this process, the substrate is produced in its own mold and then manually placed into an overmold, into which the second resin is injected. The resulting part will duplicate the production part, showing both its benefits and shortcomings. It will also highlight tooling issues and allow designers to correct material selections and design features before investing in mass production tools. And, in the case of medical applications, it will be an aid in qualifying systems that must be certified for use.

Proto Labs will be incorporating overmolding capabilities into its rapid injection molding service in early 2016.