The soft touch and comfortable, non-slip grip are the first things that come to mind as reasons for overmolding an elastomer around a part or product, either partially or totally. But the process can provide other benefits beyond the tactile ones. Elastomers over a part or housing can improve aethetics, provide a weather seal, dampen noise and vibration, protect against impact, and provide insulation against heat or cold. Elastomers used for overmolding are available in a wide range of colors, textures, and softness grades, offering designers flexibility in how to use the materials.

In spite of those attractive features, designers have often approached the concept warily. In the past, the benefits of overmolding elastomers had to be weighed against the difficulties in overmolding certain substrates. Designers were limited in what could be overmolded because of a lack of compatibility between the elastomer materials and the substrate that is to be overmolded. Compatibility problems can lead to defects, and, potentially, product failures. In response, elastomer suppliers have developed new formulations that address compatibility concerns and provide designers with new levels of functionality.

Overmolding is an injection-molding process in which the material, usually a thermoplastic elastomer (TPE), is molded over a substrate such as a rigid plastic. The overmolding can be partial, on selected places, or completely cover the part. For designers, the choices of materials are growing. This increases options, but also makes matching materials more challenging. The trick is to properly select the elastomer material that will form a strong bond with the plastic without the need for primers or other adhesives.

Overmolding is accomplished in two ways, by insert-molding or multi-shot molding. The most widely used process is insert-molding, where a pre-molded insert is placed into a mold and the TPE is shot directly over it. Multi-shot molding requires a special injection-molding machine that is equipped with multiple rotary molds that allow both of the materials to be shot into the mold during the same molding cycle. This technique requires higher initial costs, but it also reduces cycle times by eliminating some assembly and other secondary manufacturing operations.

The Lifeline AED automated external defibrillator is a handheld device manufactured by Defibtech LEC that is overmolded in the handle and grip areas with Versaflex TPE material from GLS Corp.

The bonding between the substrate and the overmolded elastomer is a critical aspect of the process, and occurs in different ways, depending on the material and the application. One way is through mechanical interlocking, physically holding the molding in place. This approach requires careful thought on the design of the underlying part, since design features such as recesses, textures, or flares must be used to support the interlocking.

The preferred and stronger bond is the one that results from chemical bonding, through the thermally induced interpenetration of molecules between the elastomer material and the substrate.

Another approach involves crosslinking, or covalent bonding, where a chemical reaction takes place between the two polymers that link the polymer chains on the molecular level.

Dan Tolan, engineering manager for Phillips Plastics Corp.’s multi-shot molding facility in Eau Claire, Wis., says his company has been doing both insert-molding and multi-shot overmolding for more than 20 years, and believes that multi-shot production will dominate in the future, especially for parts that require multiple steps. Much of the work that the company performs deals with the soft-touch nature of TPE overmolding.

“The benefits of doing this with the soft touch materials is that you can vary the durometer, the soft touch and feel on a rigid part that can also be very functional,” says Tolan.

The Makita drill is overmolded with Advanced Polymer Alloy materials.

Material selection for both the substrate and the elastomer can be a big factor in overmolding success, he adds. Some of the higher temperature engineering grade resins that might be appropriate for high temperature applications may cause compatibility problems with the elastomer. While the soft-touch, lower-durometer materials may present an adhesion challenge.

Poor adhesion can lead to defects such as peeling, fraying, and delaminating. Also, trying to combine two incompatible materials, such as a substrate that has a high melting point such as some of the nylon materials, could cause elastomer to remelt and lose its shape.

To determine the appropriate composition, designers are encouraged to approach the elastomer suppliers as early in the process as possible. Many of these suppliers have in-house design services as well as sophisticated software that will help them determine the appropriate materials, flow characteristics, knit lines and other molding parameters.

According to Sachin Sakhalkar, new business development manager for the TPE division of Teknor Apex, Pawtucket, R.I., elastomer compounders play a major role in helping the design and production process because they possess a great familiarity with the broad range of substrate types and the options available to the designer.

The range of overmold materials has expanded and now includes at least 40 options including resins such as thermoplastic polyurethanes (TPUs), styrene-ethylene/butylene-styrene copolymer (SEBS), copolyesters, copolyamides, thermoplastic rubber (TPR), thermoplastic vulcanate (TPV). Resin families also include olefins, polyesters, fluoropolymers, silicones, styrenics, polyurethanes and others.

A Motorola cell phone overmolded with Teknor elastomers.

A. Schulman, Akron, Ohio, offers the Invision grade line that comprises seven different TPE chemistries, depending on the particular end use application. Its Invision FX product is specifically designed for nylon applications, for example. In addition, the company is developing a nylon substrate material that would be easier to overmold.

PolyOne, an Ohio-based company with operations around the world, recently teamed up with Kraton Polymers of Belgium, to develop a new range of OnFlex-S KA TPE-S products for multi-component molding to polyamide (PA). (The company is in the process of rebranding all of its products from the Bergaflex brand to the OnFlex moniker.) According to Tim Buckley, TPE Product Manager - North America for PolyOne, the previous brand provided good adhesion with PA 6, but adhesion to more challenging polyamide variants, glass filled moieties and PA 6/6 was only satisfactory. The new range offers better adhesion to PA6, but also offers an improved performance at higher temperatures.

The Santoprene Specialty Products Division of ExxonMobil Chemical, Akron, Ohio, has developed a crosslinking product called Santoprene TPV. A new bonding grade, B150, was recently introduced that will allow a thinner overmold while still maintaining a high bond strength.

In addition to these types of products, designers also have the choice of a self-adhesive silicone product from  Wacker Chemical, Adrian, Mich. The two-part system has a platinum catalyst on the A side and the crosslinker is on the B side.

Plastic touch pad with overmolded self-adhesive silicone rubber from Wacker.

Silicone is used in applications for a number of reasons, but heat is a pivotal one. Silicone can work in temperatures between -40 DegF to 400 DegF without any loss of physical properties. Most elastomer materials cannot handle such high temperatures, according to the Ed Laperriere, marketing manager for elastomer business, for Wacker.

While each company offers compounds geared for certain uses, often the formulations need to be customized to the specific application. These custom blends may simply match the resin blend of the substrate, or they may be customized to meet product specifications, such as coloring, antifungal properties, UV-resistance, or other material attributes.

Some of the properties that must be considered include the hardness level of the elastomer, which is measured on the Shore scale, impact resistance, chemical resistance, UV protection, bonding mechanisms, and the melting point of the material and the substrates.

The substrates that are commonly used, such as nylons, acrylics, polycarbonates, and ABS, all have a polarity to them that helps the bonding process. If the molding material is close in polarity to the substrate, the adhesion is going to be good, says Sakhalkar. If not, then the bond may be weakened, and another choice of substrate material should be made.

High-temperature engineering resins are the most difficult for overmolding and require special chemistry to get their adhesion, says Wayne Thornton, business development manager for GLS Corp., McHenry, Ill. While all of these resins have difficulties because of the high temperature requirements, Nylon 6 is more difficult because of its chemistry, Thornton says.

Wacker silicone used on gaskets.

The melting point of Nylon 6/6, for instance, is 482 DegF, making it one of the more difficult substrates to overmold. Additionally, nylon melting point temperatures can vary by grade by as much as 70 DegF.

Jeff McCoy, business manager for Polyolefins for North America for A. Schulman says that the heat that is required to melt the first layers of microns of the skin to allow a melt bond to take place is often too hot for the elastomer.

One technique is to use the heat of the elastomer to melt the nylon. In an insert-mold application, the nylon substrate would be molded separately and the second step would be to take the nylon part and insert it into the tool at room temperature allowing the heat of the TPE to melt the top skin of that nylon substrate.

In a two-shot molding process, the first shot would be the nylon substrate and while the part is still cooling, shoot the TPE into the mold to take advantage of the substrate’s latent heat.

Another challenging application has been to overmold onto metal. Metal doesn’t melt and so traditional overmolding methods have proved difficult.

Difficult, but not impossible. In 2003, The Tektronix NetTek line of modular testing equipment used a die-cast magnesium housing overmolded with a TPU from Bayer Materials, Pittsburgh. The TPU covering offered electrical insulating properties, and impact and moisture resistance.

A more recent application was Phillips Plastics working with ITT Industries on their Spearhead Tactical Radio. Phillips molded complex geometries with thin walls using the company’s magnesium injection molding system and a formulation of a proprietary elastomeric resin. With this formula, the two companies were able to meet adhesion and durability requirements for soft-touch overmold.

Typically, says McCoy, overmolding metal is not easy to accomplish without treating. The pretreatment will give the elastomer something to adhere to, but it does require separate steps and longer cycle times.

However, this problem is also being overcome. In addition to the applications that have already occurred, Schulman has developed, but has not yet sold, an Ionomer series of elastomers that will adhere to certain types of metals. The drawback is that the metal has to be preheated to as much as to 450 DegF. While on par with the temperature required to melt nylon, the fact that it is a metal substrate may make material handling more difficult.

Wacker also offers self-adhesives silicone grades that will adhere in an insert molding process to stainless steel, aluminum, brass and other metallic materials. In this case, the metal component is made separately, then inserted into the mold prior to the overmolding process.

While a soft-touch surface has its virtues, designers have to be cautious about wading too deep into the soft side of the spectrum, or their customers will be the ones who get stuck.

A softer material may have adhesion problems and may get a tacky feel to it at the lower end of the Shore A range.

According to Thornton, overmolding very soft grades onto engineering resins can be a challenge because they may develop a tacky, almost sticky feel. The softer the TPE the tackier the product can get. GLS has a product that will go down to the mid-20s on the Shore scale and still get great adhesion, without tackiness, when overmolded onto an engineering resin.

Another area of change for the better involves the relationship between elastomer overmold thickness and adhesion. For instance, ExxonMobil Chemical’s Bill Ramsey says that the company’s new bonding grade, Santoprene B150, is an improvement on its B100, which did not have a sufficiently strong bond. In some cases, it required that the overmold be much thicker to maintain bond strength. “After years of research and development, the B150 delivers better bonding performance, and allows the customers to use a thinner overmold and still maintain a high bond strength.”

It’s also important for designers to understand that softness is measured by the Shore scale specification, that one does not achieve it simply by making the overmold thicker.

Thicker material may offer vibration and sound dampening, but it doesn’t necessarily provide a soft touch.

A soft-touch measurement is somewhere in the 30 to 80 Shore A range. For point of reference, skin is in the approximately 60 Shore A range. At zero Shore A, the material turns into a gel. Any measurement at about 30 Shore A, from a perception perspective starts to become too soft, while at about 75 to 80 Shore A, it starts to be perceived as hard.

“The choice of material depends on the hardness,” says Sakhalkar. “TPUs or copolyesters tend to be harder so you won’t often find those materials used on soft-touch tactile applications, those materials tend to be used for the functional characteristics.”

For more information, email:

A. Schulman: info@aschulman.com
Advanced Polymers Alloys: info@APAinfo.com
ExxonMobil Chemical: craig.jensen@exxonmobil.com
GLS Corp: wthornton10201@sprintpcs.com
Phillips Plastics: dan.tolan@phillipsplastics.com
PolyOne Corp: david.honeycutt@polyone.com
Teknor Apex: tpe@teknorapex.com
Wacker Chemical: info.usa@wacker.com