Design Focus: The name of this game is bond - good bond

Bring together two different materials to take advantage of the best assets of both-that, in a nutshell, is overmolding. One of the most popular material combinations continues to be a thermoplastic with a thermoplastic elastomer (TPE).

The lure is obvious. Parts can have the strength and stiffness of a molded thermoplastic, coupled with a consumer-friendly soft TPE grip. It has been done with toothbrushes, electronic gadgets, automotive interior parts, medical products-practically every market imaginable over the course of the past decade.

And there is a lot more to come, predicts every expert MPW contacted, among them Walt Ripple, VP sales and marketing at TPE compounder GLS Corp. (McHenry, IL). "Soft touch with TPEs...is no fad, it''s exploding more and more," he says. In fact, he says one of the newest compounding techniques is to mix and match TPEs to form alloys that bring relative strengths from multiple TPEs to a single compound for overmolding.

Among parts designers, Ripple says one of the hottest trends is overmolding with TPEs to realize "functionality." Among the many roles that TPE can play are as a noise and vibration dampener, waterproof gasket, or shock absorber, he says.

After years of heady demand growth, one might expect the design world to be full of TPE overmolding experts. In fact, the need for TPE overmolding expertise is higher than ever, says Ripple''s colleague Malar Shetty, technical services manager. "there are many [designers] now who understand injection molding," he says, "but they''re not too familiar with the TPE part of it [when overmolding]." His advice at a project''s outset? "Look for a specialist," both for parts design as well as when sourcing tooling.

Echoing that advice were a number of market insiders at the Molding 2005 conference in New Orleans in February, including John Catalano, business manager at Fairway Molds Inc. (Walnut, CA). For processors considering any overmolding or multimaterial molding work, he says, "It''s vital that you are dealing with [moldmakers] who are experienced. Don''t let a moldmaker get his experience on your dime."

Where did my bond go?

Chris Cooper Sr., processing engineer for injection molding at Bayer Polymers (Pittsburgh, PA), says that even with materials such as TPEs and engineering thermoplastics that typically pose no bonding challenges, "We''ve run into problems where customers, and we, thought materials should bond, but didn''t."

The problem, says Cooper, can stem from any of the host of additives-UV absorbers, impact modifiers, fillers and so on-that a processor adds to his virgin materials. "These can all influence adhesion," he says. Bayer supplies TPUs used in overmolding, and Cooper says the materials bond very well to a range of thermoplastics including ABS, PC, PC/ABS, and rigid TPU. He says bonding performance between PBT or PA and TPU is fair, but can be improved substantially by going to a true two-shot overmolding process.

For TPE overmolding as well as overmolding with other materials, Cooper says designs that maximize surface-area contact between substrate and overmolded materials lead to better bonds between these materials. Adding texturing along the interface between materials can also help, but it may also lead to trapped air bubbles that reduce adhesion. Intense interest in multimaterials molding, including overmolding of TPEs, prompted Bayer to acquire a new mold from MGS Manufacturing Group (Germantown, WI) for testing materials compatibility. Cooper says directly, "I cannot emphasize enough the need for prototyping, or testing, for materials compatibility," he says.

Glass-fiber reinforcement in the substrate material can also have a negative affect on bonding, though at levels below 30% by weight it usually isn''t an issue, Shetty says. He agrees that additives can be problematic. "Lubricants in the substrate material, for instance from a mold release, can hurt the bond," he says.

Andre Oosterlaken, application development manager at supplier DSM Engineering Plastics (Sittard, Netherlands), notes that, "Good adhesion normally comes from polymer-to-polymer interaction," so any materials interfering with this interaction have an affect. Practically, though, he says it has not been an issue in the field, and good adhesion has been obtained between reinforced materials and TPEs.

Know your environment

Do you just need aesthetics? Will your part be in contact with electrical devices? Is the part designed with thick or thin walls? And what about gasoline, grease, or other chemicals-will they be in the neighborhood? These are some of the questions Shetty poses to determine which TPE best suits a molder''s needs.

As a TPE expert, Shetty often assists OEMs and processors with materials selection. "The first thing I ask is, ''what is the part design?''" Although some OEMs do not share the exact product design, he says, "Generally I at least get word on what market the part is in, so I can start asking them about environmental requirements."

Oosterlaken says DSM recommends molders first inject the material that has the lowest melt point. Depending on the chosen hard-soft combination (although, he notes, increasingly soft-soft and hard-hard combinations are seen in overmolding applications) this could mean that sometimes it is better to mold the soft TPE first and then overmold the rigid substrate.

Depending on part design and function, this is not always easy to achieve. However, designers should take this into account, says Oosterlaken, and not always choose the "most obvious" solution of first injecting the hard component followed by the soft TPE overmolding.

At moldmaker Foboha GmbH (Haslach, Germany), a recognized leader in mold manufacture for multimaterial parts, U.S. sales engineer Lisa Mauro agrees, and points to a 16+16-cavity mold Foboha made for medical closures. The mold is designed so that the first shot forms TPE gaskets, and the second shot forms polypropylene closures. "This isn''t the preferred way, but we couldn''t gate on the inside, nor on the outside [as this medical part could not have any gate marks]. So we molded the gasket first, indexed, then overmolded the body," she explains.

Shetty has some tooling recommendations. His experience shows that balanced hot runners are very important in overmolding. He says molders should avoid tools with internally heated hot runner systems or with oversized manifolds. If the tool is off, "You can get a short/short and flash on the same part," an odd combination of problems that can leave a molder bewildered.

Melt-flow shutoff is also critical, Shetty says, adding that shutoff grooves can serve to vent a TPE-overmold part section. He adds that his firm can customize TPE compounds to run on tooling not optimized for overmolding; for instance, a tool first designed for insert molding that a processor now wants to use for overmolding. Shetty also notes that molds should be surface-textured to prevent TPEs sticking to a tool, with heavier texturing at runners, sprues, and gates.

Oosterlaken recommends molders using his firm''s Arnitel TPE choose polar substrate materials-such as amorphous polymers PC, ABS, and styrenics-or semi-crystalline polyesters such as PET or PBT. He says the adhesion of Arnitel to PVC is also very strong. With polyamide, "There is some chemical adhesion, but often not enough" if bond strength is important, says Oosterlaken. But he adds that adding mechanical interlocks (between the nylon and TPE) greatly strengthens this bond.

Oosterlaken says interlocks typically do not affect cycle time, but that interlocks, as well as ridges on an overmolded substrate, can lead to sink marks. Still, he says, proper part design usually can ensure sink marks do not appear on final parts. GLS'' Shetty says that parts designed with overmolded TPU need to account for that material''s shrinkage; unaccounted for, it can lead to sink marks at interlocks.

Typical TPE shrinkage is about 50% to 80% higher than that of substrate materials, says Shetty, which is why GLS recommends substrate material thickness of at least double the thickness of the overmolded TPE layer. Too thin a substrate can lead to warpage caused by the TPE shrinkage.

Most important, Oosterlaken says, is to ensure "good temperature conditions at the interface [between the two materials]." This can be achieved by keeping the substrate as hot as possible or by raising melt temperature to get good polymer interaction.

This advice goes with point two of the three Oosterlaken says are most critical for good overmold adhesion. First is chemical compatibility; second is polymer chain mobility at the interface; and third is to allow sufficient time for that polymer interaction to occur. He adds that, because TPEs often are expected to fill low-wall-thickness areas and travel on flow paths in overmolding applications, melt-flow indices of at least 25 or more are "an absolute prerequisite" to succeed in overmolding.

Colors? No problem. Nylon? At least one solution.

Colored grades of TPE, once blamed for less-than-perfect bonding with substrate materials, now rarely are an issue, says Shetty. Ripple adds that designers now even have their choice of clear TPEs, letting them add special effects such as pearlescence. Also available are thermocromatic colored TPEs that change color when exposed to heat.

Two oft-used thermoplastics, polyethylene and polyamides, have proven tough to successfully overmold with TPEs. However, supplier Zeon Chemicals (Louisville, KY) says it has a solution for nylon with its Zeotherm 100-series of TPVs. The material recently received attention for its use on the all-plastic deck closeout lid for the 2005 Ford Mustang convertible. The part is the first all-plastic solution for this application and replaces a thermoset rubber/metal combination. Firms participating in the project included molder Innatech, moldmaker Dynamic Mold, and Zeon, among others.

Innatech overmolded a glass-fiber-reinforced nylon, supplied by compounder A. Schulman, with Zeon''s Zeotherm 100-series TPV. Brian Cail, business manager, new business development at Zeon, says that in the case of nylon overmold adhesion, the advantage of Zeotherm TPVs (over other TPEs) is that the plastic phase of Zeotherm is nylon (the TPV is comprised of a thermoset elastomer dispersed in PA6). That results in full chemical adhesion between Zeotherm and a nylon substrate, for a bond that stands up to high heat. Zeotherm TPVs bond to filled (glass and mineral) and unfilled nylon, adds Cail.

Cail says that beyond the Ford Mustang application, Zeotherm TPVs are also seeing use in overmolding applications including injection molded cuffs for nylon turbo air ducts, in oil filter components, and in automotive fasteners. Unlike most TPEs, these materials are not limited to continuous-use temperatures below 130_C. Also, he says, the processing window for these TPVs to achieve good nylon overmold adhesion is broader than conventional nylon-bonding TPEs. For processors, the bottom line is that becoming a TPE overmolding expert can only serve to improve a molder''s competitive position. MD

Case study #1: Overmolding with TPE saves time, money for Whirlpool

Appliance maker Whirlpool has switched to overmolding with a TPE from supplier Advanced Elastomer Systems to replace rubber gaskets the OEM had sourced and then manually inserted. Overmolding is onto injection molded polypropylene sumps about 270 mm in diameter; the PP contains 30% talc by weight.

Michael Matson, process engineer at Whirlpool''s Findlay Div. (Findlay, OH), which is devoted to dishwasher manufacturing, presented the OEM''s experience on the material and process transition during the Molding 2005 conference in early February in New Orleans.

Whirlpool went into the project with a number of critical requirements for the overmolded seals. They had to provide a sufficient bond to the PP sumps and to resist detergents and other household chemicals. Resistance to copper, often present in water, was also deemed important. Post-mold shrinkage had to be less than 1%. Plus, "The finished part had to exhibit two-fold savings in labor, material, and production costs," Matson says, with scrap rates of less than 5%. To hit that mark, "We redesigned the seal to make it injection friendly [when we switched to overmolding]," Matson says. Seals also have to be easily removed, in case of after-sales service.

Molding at the plant is nonstop; about 2200 dishwashers are made per eight-hour shift. Whirlpool selected a vertical injection machine (to save floorspace) from Battenfeld (South Elgin, IL), and injection molds from Diamond Tool (Euclid, OH), for the work. Matson describes the tooling as a "mold and a half" with a two-cavity upper half and two interchangeable lower halves, each with two cavities designed to rotate 180_ and then return. Whirlpool opted for molding of the sumps on separate presses.

The transition included extensive molding on a prototype tool; the prototype was a single-cavity tool designed to fit a standard 300-tonne horizontal press. Among lessons Whirlpool noted during testing was that the overmolded TPE need not account for shrinkage of the PP sumps. Previously shrinkage, and how it affected gasket sealing, had been an issue as the sumps had been molded, then stored for two days prior to manual application of the thermoset gaskets.

Matson says the transition has been a success, though one the firm continues to work to improve. Although weld-line strength remains a concern, he says no failures have been observed, nor were any forced during accelerated life testing. MD

Case study #2: This stud (finder) looks good, too

Officials at power tool manufacturer Zircon Corp. (Campbell, CA) thought the technology within their stud finders made them great products, but also recognized that consumers would need to buy the tools to discover this. How to move them from store shelf to handyman''s toolbox?

The OEM worked with TPE compounder GLS Corp. and injection molders Nypro, from its San Diego, CA facility, and EIMO America of Vicksburg, MI, to redesign the tools so that the high-impact polystyrene (HIPS) or acrylonitrile butadiene styrene (ABS) cases could be overmolded. Zircon officials say they also considered painting and gluing of thermoset rubber handles, but the economic and quality benefits of TPE overmolding won them over.

Zircon went with two GLS grades. Dynaflex D-3202 colored grades (based on TPE from Kraton Polymers LLC) gave the first products the standout coloring to catch consumers'' eyes, plus the desired dry, soft-touch grip. Then, as the manufacturer expanded its stud-finder range to include models for professional builders or contractors, it also specified a black Versaflex grade that offered dirt and grease resistance. The new CenterVision product line will use a gray Dynaflex grade.

10 tips for successful overmolding

Heed the top 10 TPE overmolding rules of thumb offered by Malar Shetty, application development manager at GLS:

1. Substrate thickness should be greater than twice the TPE overmold thickness

2. Incorporate air vents at end of fill.

3. Flow ratios (length/thickness) on new designs should be between 80:1 and 120:1.

4. Start with a small gate to maintain low cycle time and ease degating.

5. Incorporate proper flow shutoff of TPE at cavity edge.

6. Add surface texture to molds to prevent sticking.

7. Be certain the runner system is truly balanced.

8. If TPE walls must be very thin, use mechanical interlocks.

9. For tools with 16 or more cavities, consider using a hot sprue or, better yet, a hot runner system to reduce runner scrap and cycle time.

10. For those hot runner systems, use external heating.

Matthew Defosse [email protected]

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