Cover Story: Overmolding Over the Top

Use of thermoplastic elastomers, through all of the five major TPE material families, thermoplastic vulcanizates (TPVs), thermoplastic polyurethanes (TPUs), styrene ethylene butylene styrene (SEBS), copolyesters (COPE) and thermoplastic olefins (TPOs), continues to grow. Market studies predict the average worldwide growth rate for TPEs to be about 5% through 2005, compared with an expected growth of about 1% for thermoset rubber. The huge growth in the popularity of overmolding is one of the key factors in this on-going TPE success story.

“Most larger processors provide overmolding as part of their package of molding services,” says Gerry DiBattista, market development specialist at Bayer Plastics. “But shops have to keep in mind that different TPEs require different processing conditions.”

Bayer has just introduced a new line of fully colorable aliphatic thermoplastic polyurethanes (ATPUs) which are being targeted at outdoor and recreation applications. The materials, part of the company’s Desmopan line, are still considered in development, although a number of companies are conducting application studies with them, DiBattista says. They bond readily to a number of substrate materials such as ABS, PC, copolyesters and rigid PVC. Also, by designing in mechanical interlocks, the ATPUs can be overmolded onto other materials.

Colorability gives the ATPUs an extra dimension in comparison to other elastomeric materials used in similar applications.

“The unique thing about these aliphatic urethanes versus aromatic urethane is that they won’t color shift,” says DiBattista. “One of the reasons most elastomeric components of outdoor equipment are black is because that color doesn’t fade.” He says the resins mix well with color concentrates which have either a TPU or EVA as the carrier resin.

The materials are compatible with both insert or two-shot overmolding. When insert molding, DiBattista says it is not necessary to pre-heat the part to obtain acceptable bond strength, although he reports that bond strength is generally higher when the substrate is warm. The ATPUs process in a melt range of 375F to 425F, depending on the grade, and like all TPUs require drying (recommended two hours at 200F to 220F).

Getting a better bond

A recent study conducted by TriMax Division of Prime Alliance, a marketing and design firm based in Boulder, CO, has attempted to provide the definitive answer to one of the key concerns in overmolding: creating a strong bond between the TPE and substrate in the finished, overmolded product. TriMax investigated the bonding characteristics of 29 different TPEs, representing the five common TPE families plus a group of alloys, overmolded onto 14 different substrates, encompassing all the thermoplastic polymer types commonly used in injection molding. The TPEs were insert molded onto the substrates on a standard injection molding machine. To test the effect of temperature on bond strength, the substrate inserts were conditioned at one of three temperatures: room (73 F), 125 F and a third “high” insert temperature intended to mimic a typical two-shot process. Bond adhesion of the materials was measured using ASTM test methods for 90 tensile peel and 180 shear adhesion. Processors must purchase the report to obtain the detailed analysis, although TriMax has released some of the study’s general conclusions about each TPE/substrate (See table below.)

The study also found that warming the substrate above room temperature had some affect on increasing bond strength, although the degree of the affect depends on the precise TPE/plastic combination. Polypropylene showed the most consistent temperature effect, with higher insert temperatures generally resulting in higher adhesive bond strength.

Temperature, design — two keys to success

Advanced Elastomer Systems produces a number of different grades of Santoprene, a TPV. For all Santoprene grades, and elastomers in general, maintaining the recommended processing temperature is critical to obtaining a good overmolded part, says Kim Torti of AES’s marketing and technical services department.

“Some processors may not be familiar with higher processing temperatures required for our TPEs,” says Torti. She reports that Santoprene B100 grades used for overmolding to engineering resins such PC/ABS process in the range of 440F to 485F, while the all-purpose grade for overmolding to PP and PE processes at a slightly lower temperature, depending on the hardness rating of the material. In general, the softer the material, the lower the temperature.

“It’s very important to stay within the parameters of the processing window,” Torti notes. “If you allow the temperature to drop too low or drift too high you lose bond strength.”

Teknor Apex has developed a specific line of TPEs for overmolding on a wide variety of engineering resins such as PC, PC/ABS blends, styrenics, polyesters, nylons and other polymers. Tekbond TPEs are based on SEBS chemistry, and are targeted for overmolding applications involving power tools, cell phones, PDAs, cameras and other devices. The materials are designed to be compatible with either insert or two-shot overmolding. When insert molding onto engineering resins, however, proper care must be taken to design a part that will maximize efficiency of processing and quality, reports Wayne Thornton, marketing manager of consumer products.

“You have to watch for flash and try to eliminate it during the design stage.” Thornton says flash can often be avoided by designing in so-called styling lines, which is an area on the part that can be clamped off to eliminate flash.

When designing a part for insert overmolding, Thornton says it wise to avoid making part sections too thin, which can be distorted when the hot TPE makes contact. Thornton recommends avoiding designing wall sections thinner than 1.5 mm.

Both Thornton and Torti recommend texturing the tool for overmolding applications, but for different reasons. Thornton says texturing will eliminate air entrapment, and as well create turbulent, rather than laminar flow, thereby reducing flow lines. Torti says AES recommends a minimum SPI/SPE no. 3 mold finish to prevent the Santoprene layer from sticking.

Torti also advises against trying to make the TPE flow too great of a distance in too thin of a layer. “The temperature of the TPE needs to stay high in order to chemically bond to the substrate,” she notes. “As a layer gets thinner, it loses heat more rapidly.”

For two-shot overmolding onto polypropylene, when the part is warm, Santoprene’s all-purpose grade may perhaps be molded in a layer as thin as 0.5 mm. For insert molding with grades designed to be used on engineering plastics Torti recommends a minimum thickness of about 1.5 mm.

Nylon presents overmolding challenge

As the TriMax TPE bonding study showed, there are presently few TPEs that exhibit acceptable or superior bonding characteristics with nylon. Researchers at Teknor Apex tested the adhesive strength of a typical pre-colored grade of Tekbond to nylon and obtained bond-strength values that exceeded the adequate level. Thornton says the company is also working on several developmental grades specifically designed for overmolding to nylon.

“A lot of power tools use nylon because the material can pass the drop test,” says Wayne Thornton of Teknor Apex. “There’s a number of things you can do from the design side to improve the bond, as long as you do it early, before production tooling is cut.” Most processors shy away from auxiliary methods to improve the bond, such as adhesives and Corona treatment, says Thornton, because of the time and cost they add.

GLS Corporation’s Versaflex TPE alloys gave the best results for bonding to nylon 6 in the TriMax study. In one particular test the average tensile adhesion of a Versaflex grade overmolded to an Ultramid grade of nylon from BASF exceeded the next best TPE by over 30 percent. For insert molding, GLS says the nylon substrate does not require pre-heating to yield acceptable bonding results.

TPE BOND STRENGTH TO VARIOUS SUBST
RATES

* Strongest average adhesion (both tensile and shear).

Note: TPE manufacturers supplied the study with general material grades.

(Data taken from Evaluation of the bonding capability of TPEs with thermoplastics in an injection molding process, published by TriMax Div. of Prime Alliance, 720-565-8523.)