Overmolding Defined

Overmolding is a process that begins as one part or set of parts assembled using two or more different materials to form one ‘body’. Typically the first material, known as the substrate, is partially or fully covered by subsequent materials (overmold materials) during the overmold manufacturing process.

START WITH THE SUBSTRATE

The substrate can be almost anything; a machined metal part, a molded plastic part, a lens, or even an off the shelf product like brackets, threaded hardware, connectors, or electrical components. It is the first piece in what will eventually become a single continuous part composed of chemically bonded and often mechanically interlocked materials of separate compositions.

CHOOSING THE BEST PROCESS: PRINTING, CASTING & MOLDING

POLYJET (3D PRINTING)

PolyJet 3D printing uses a UV cured jetted liquid photopolymer with a hardness that can be modified throughout the build’s geometry and in the same print, making it a great choice for prototype parts that will at some point be overmolded in production environments—a soft, grip handle for a power tool, for example, or a weatherproof, gasket cover for an electronics housing. Digital photopolymers in white, black, and clear/translucent are readily available on many machines that are calibrated for high volume prints by service providers (like PrintForm), and shore hardness levels range from a soft 30A durometer to a fully rigid 80D.

_{3D Printed Shoe Sole Prototype 3D Printed on a Connex (PolyJet) Objet Printer by Stratasys.}

To validate an overmold design, this is a good place to start, just be sure to verify with your prototype service partner (or in house engineering team) that the 3D Printed part can also be molded, or mass manufactured in some cost effective manner—some product engineers find themselves painted into an additive manufacturing corner, only to learn that their design isn’t feasible for cost-effective mass manufacturing.

INJECTION MOLDING (FORMING)

Overmold materials (typically plastic) start off in pellet form, no different than a typical injection molding set up. These pellets are mixed with additives like colorants, agents, and other fillers (Glass fiber, Carbon, etc.) The material is heated to a specified melting point and injected into the mold cavity as a viscous liquid. There are some limiting factors for what materials are suitable for overmolding.

If overmolding a metal part with plastic, most plastics are suitable. If overmolding one plastic part with another plastic (or a rubber or urethane elastomer), then confirming compatibility is key. Material manufacturers/distributors will generally publish a compatibility chart for overmolding. If one is not readily available, simply ask. In most cases, it’s been done before, so you might as well take advantage of the experience that is already documented to save your team time and budget during the product development process.

CAST URETHANE (SILICONE MOLDING)

As with production insert molding (or injection overmolding), the RTV molding (Silicone Molding, or Cast Urethane) process uses a primary mold to form the base (or substrate) component and another similar secondary tool for the overmolded area. The secondary mold houses the substrate as well as a cavity to pour the overmold portion adhering it to the main body. Each mold requires a pattern into which the liquid urethane or plastic mixture is poured. For parts that fit roughly in a shoe box, typical lead time for patterns and molds is usually two to three weeks for completion of the first cast parts. Most experienced silicone molders will then await an engineering review to confirm the part meets specs before continuing the production quantities. At PrintForm we call it awaiting ‘First Article approval’ before proceeding to production.

Applications & Uses

^{Range of Common Products with Multiple Materials permanently adhered to one another used in their design: (L to R) Bicycle handlebar grip, Screw driver, Garden sheers, Hex wrenches, water cup}

The overmolding process is utilized for a number of applications and will vary according to the specifics of the particular project. Some common, more recognizable examples include toothbrushes, power tool hand grips (e.g. cordless drills and screw drivers), and personal care products (e.g. shampoo bottles, baskets and shaving razors). Some additional examples of typical overmolding applications follow.

REAL LIFE EXAMPLES

		Plastic On Plastic: First a rigid plastic substrate is molded. Then another rigid plastic is molded onto or around the substrate. The plastics may differ in color, chemical properties, resin family or hardness.
		Rubber On Plastic: Begins with a molded rigid plastic substrate. Then a soft silicone, rubber or TPE is molded onto or surrounding the more rigid substrate/base “core”. This is often used to give a soft grip area to a rigid part and is the most common engineering application, particularly in the electronics, machinery, and consumer product categories.
		Plastic On Metal: First the metal substrate is machined, cast or formed (or purchased from a stock supplier). Then the substrate is placed into an injection molding (or casting) tool/cavity and the plastic is molded onto or around the metal. This is often used to enshrine metal components in a plastic part.
		Rubber Over Metal: First, the metal substrate is machined, formed or cast. Then the substrate is placed into a tool cavity and the rubber, elastomer or TPE is molded onto and  around the metal. This is often used to provide a soft handheld grip and designing in specific, intentional holes or grooves for the soft material to mechanically “grip” the metal substrate.

Considerations when planning your next overmolding program:

Material compatibility.
Research the limitations and potential compatibility issues between different materials.

No need to stop at two.
The process doesn’t have to be limited to only two materials. Sometimes three or four different materials on a single part can be designed in order to achieve color or durometer breaks and textured grip surfaces.

Make the connection.
When possible, it is best to design a substrate and an overmold with interlocking connections from a mechanical capacity standpoint, rather than relying solely on a chemical adhesion between the materials.

Be a Draft King.
Sometimes this interlocking strategy is referred to as “Reverse Draft”, and in this way, the two materials will not only be bonded together chemically, they will also be held together physically. Link two fingers together and pull – this is effectively what the mechanical connection accomplishes under the surface of the parts when properly designed.

^{Interlocking features within overmolded designs can add needed strength in addition to adhesive or chemical bond.} 

PROTOTYPING VS PRODUCTION

More often than not, overmolding projects involve a soft rubber attached to a plastic or metal substrate. One offs, prototypes, and varying geometry tests rely on PolyJet, a 3D Printing process that can print in multiple durometers on the same build. Typically, the next step after initial prototypes, the mid range option is silicone molding, which involves creating plastic or rubber molds to hold the substrate in place and then mixing polyurethane elastomers to be injected into a void or cavity. Once cured, the elastomer and substrate are removed as one part. While the geometry is fixed once the molds are created, hardness and color can be varied from one ‘batch’ to another.  In mass production, with thermoplastic elastomers, overmolding can be accomplished in cycle times ranging in seconds. However, in prototyping or casting, it can take several hours per part because the two-part casting rubber needs time to cure.

WHY OVERMOLD?

• As a means to mix or break up color (cosmetic reasons).
• To provide a soft grip surface around a part of the separate rigid material.
• To add flexible areas to a rigid product for fall or drop protection
• To reduce assembly time.
• Skip the adhesive and 2x manufacturing time involved in separately joining the two parts manually.
• To capture one part inside of another without having to use fasteners or adhesives.

About the Author: Bill Artley is the Vice President of Operations at PrintForm and has written many articles & white papers on product development, sourcing, additive manufacturing, and materials. He is a contributing author and peer reviewer of The 3D Handbook (http://a.co/h9fL1b5), a top 10 Amazon Best Seller in its category.