Soft Sells (January 01, 2007)

“Going soft” isn’t an expression that most people want to hear, either about themselves, or their businesses. But in consumer goods as diverse as auto interiors and toothbrushes, going soft is big business as the “grippy” feel of thermoplastic elastomer (TPE) gives relatively mundane commodity goods a high-quality feel that adds margins and revitalizes moribund consumer segments.

Hand tools like pliers, for example, haven’t fundamentally changed in decades, but the colour, feel and comfort of TPE-molded grips give consumers a reason to replace a product that normally lasts a lifetime. Can a typical polyolefin molder break into the expanding market of multi-component molding or overmolding with TPE?

WHY TPE?

Typical applications use TPE over a commodity or engineering resin which forms the bulk of the part, like the application most people recognize, the toothbrush.

Why not just shoot the elastomer into the final shape? A primary reason is that the elastomer is often too soft to hold the product shape and last in use. The most obvious reason, of course, is cost. A small weight fraction of elastomer over a cheaper resin provides the necessary soft feel in a cheaper, lighter part. Multi-part molding also gives the marketing department the possibility of different colours and textures in a single part.

Like other forms of multi-part molding, shooting TPEs over rigid resins can be performed as insert molding or on presses with multiple barrels and may include pick and place robotics, rotary or rotating platens and sophisticated molds.

There are major advantages to integrating the whole process into a purpose-built machine. Throughput is an obvious reason, especially with the move of “soft touch” parts down into low cost, high-volume consumer goods.

An equally important reason is the need for a strong, repeatable bond between TPE and resin substrate. Shooting both sequentially in a common mold provides a clean, near-hermetic environment for the bond with the added advantage of integrated temperature control of both the TPE and the substrate during the bonding process.

From a production standpoint, moving into TPE overmolding means different presses, but not necessarily a bigger footprint on the shop floor. The shop’s internal logistics need to be considered, especially if resin is centrally distributed and machines are arranged in a high-density configuration. It’s common for the elastomer mass fraction of a part to be small enough that the TPE barrel can be fed by gaylord and stand-alone hopper loader, making it necessary to allow space to handle raw material at press side.

It is possible to run a parallel central distribution system for the elastomer, which might make sense in high-volume applications. Advantages include better floor space utilization, easier humidity control of the bulk resin and lower contamination risk. Disadvantages are mainly cost and flexibility, especially where frequent colour changeovers are necessary.

MAKING IT STICK

Thermoplastic elastomers are different from commodity and engineering resins and as a result, adhesion to the substrate resin has to be carefully engineered into the part with tooling and the choice of polymers.

“Most TPE materials are shear- responsive, meaning a material’s viscosity is dramatically reduced at higher shear rates,” Paul Killian, technical marketing manager, elastomers, for Winona, Minn.-based RTP Company, said. “The result is easier flow into thinner walls and over longer flow lengths. There are some exceptions to this rule, but in general, more shear means better flow for TPEs, so gating location and size are critical for optimal bonding.”

Bonding is a major issue in two-component or overmolding of dissimilar resins. Adhesives defeat the cost-saving purpose of two-component technology, and mechanical interlocking may require difficult-to-mold undercuts or channels in the base resin substrate. “For many years, processors used mechanical interlocks or chemical agents to bond a soft TPE with a rigid engineering resin substrate in complex parts,” Seth Barron, market segment manager for Houston, Tx.-based ExxonMobil Chemical’s Santoprene TPE, said. “This presented numerous problems such as a non-uniform contact surface, bonding failure and high processing costs. For OEMs, these problems could result in part failure and high system replacement costs”.

ExxonMobil’s guidelines for maximizing bond strength start with three basic principles: part design, tooling and processing conditions. The part must be designed with an adequate thickness of elastomer for repeatable bonding. In the mold, venting is critical, as is the need for proper shut-off, which is partially dependent on the interface between substrate and TPE.

The third consideration in achieving maximum bond strength is processing conditions. Melt temperature, injection speed and packing pressure of the TPE material all affect the bond strength. Processing parameters for the rigid substrate are also important, especially if it’s a filled material. For a filled material, a resin-rich surface is necessary for optimum bonding. This can be achieved using higher temperatures and injection pressures during processing.

Thermoplastic elastomers have several classes and many grades. Styrenic and olefenic elastomers form the majority of applications, with urethanes and copolyesters filling in the high performance and specialty segments. Each has its own properties and reacts differently to the substrate. “It’s not just about filling a cavity, it’s about the interactions between the different materials that you need to put together to achieve a function,” Paul Moruzi, new business development manager/Arnitel for Evansville, Ind.-based DSM Engineering Plastics Inc., said. “For copolyesters, for example, you need polar substrates like styrenics, acrylonitrile-butadiene-styrene copolymer (ABS), vinyls and relatively low glass transition and melting point types. It takes too much surface energy to get good adhesion with polyolefins and nylons. To get best results, preheating the substrate helps.”

WHAT ABOUT THE EQUIPMENT?

According to Mark Hammond, manager, technology, for Engel Canada Inc., in Guelph, Ont., “It’s a specialty; there’s a new level of experience required. Fortunately, TPEs process similar to standard plastics materials. Previous processing knowledge will help.”

Hammond isn’t alone in declaring that good processors can adopt the technology. “It’s not ‘black magic’, but it does require a particular skill set and a good knowledge of molding principles,” Mark Elsass, director of technology at Batavia, Ohio-based Milacron, said. “[TPE’s] are compressible in the melt, so packing the mold is a little different than conventional resins,” Ferromatic Milacron Europe’s general manager Bob Hare added. “It’s important to work with a mold maker experienced in these materials.”

Engel’s Hammond noted that the price premium for two-shot presses isn’t high when compared with conventional machines. “It’s a factor of about 1.3 to 1.5 compared to single shot equipment of similar tonnage. You’re adding a second injection unit, guarding and controls, but not necessarily changing the clamp force. Rotary tables, robotics and rotary molds drive up cost,” he said.

TPE use in consumer goods is exploding, with new resin grades that resist heat, chemical and ultraviolet attack allowing new applications in both the OEM and consumer markets. Equipment is affordable and a molder with good practices and process/quality control systems will find that much of their existing experience translates when shooting elastomers onto commodity or engineering thermoplastics. There’s a lot more to comolding elastomeric materials than any single article can describe, so look for more about this technology in future issues of Canadian Plastics. End users have demonstrated a willingness to pay a premium for soft grip consumer goods, and despite current pricing for raw materials,
multi-component capability lets progressive prossessors bid on higher value jobs molding higher value products. It’s a case where “going soft” is a good thing.

TPE TWO-COMPONENT MOLDING IN ACTION

Canadian Plastics was granted a rare peek at a high-value TPE two-component molding cell at heavyweight automotive OEM Tier 1 supplier Collins & Aikman’s Mississauga, Ont. plant. The application is a soft grip TPE layer on a dashboard control knob for a major volume “Big Three” vehicle.

The press is Florence, Ky.-based Krauss-Maffei Corp.’s dual barrel hydraulic KM 300-700-55 C2 with a rotary table and linear five-axis robot. The base resin substrate is a moderately loaded glass-filled nylon with a shot size under 300 grams, while the TPE overmold shot size is less than 20 grams.

What’s the elastomer? That’s proprietary, but it is about 42 Shore A for a comfortable fingertip feel.

The current cell will be joined by another shortly, and the firm will likely expand overmolding capacity still more in the near future. Collins & Aikman isn’t immune to the financial crisis that’s sweeping big league Tier 1 suppliers like Visteon and Delphi, so why the new investment? “Dual shot will have an increasing demand in the future because it enhances interiors,” Dan Popovici, Collins & Aikman engineering manager, said. “Looks sell and impress customers; the trend today is rich features in low cost cars. The benefit of being an early adopter on a new technology is huge. Once a technology is implemented, the cost lowers dramatically and you can use it in cheaper cars. It’s not about molding parts; it’s a process.”

SUCCESSFUL OVERMOLDING

Guidelines for successful TPE overmolding aren’t complex. Winona, Minn.-based RTP Company recommends the following:

Consistent wall thicknesses contribute greatly to optimizing bond strength. RTP recommends when possible a nominal wall thickness of 0.150″ (3.81 mm) and an absolute minimum wall thickness of 0.028″ (0.711 mm). Long flow lengths can affect the bond strength especially opposite the gate. RTP recommends a L/T (length of flow/wall thickness) ratio of no more than 150:1. If this ratio must be exceeded, consider utilizing multiple gates to achieve optimal fill.

Transitions between wall thicknesses should be gradual to reduce flow problems such as back fills and gas traps. The use of radii, 0.020″ (0.508 mm) minimum, in sharp corners helps reduce localized stress. Deep blind pockets or ribs that are difficult to vent should be avoided. Long draws should have a 3-5 draft to help ejection. Properly designed undercuts, however, are possible with TPE compounds if the part does not have sharp corners and the elastomer is allowed to deflect during ejection.

Successful Overmolding Guidelines:

1. Proper gate size/location

2. Adequate vent location(s)

3. Maximum flow length (L/T)

4. Proper surface texture

5. Applicable de-gating/part ejection

6. Proper shut off technique(s)

7. Use mold filling analysis CAE for appropriate complex geometries

Overmolding Considerations:

* Use 0.150″ (3.81 mm) as a guideline for nominal maximum wall thickness.

* Use of mechanical interlocks on thin wall applications.

* Use 0.028-0.032″ (0.711-0.812 mm) as a guideline for absolute minimum wall thickness.

* Do not vary TPE component wall thickness by greater than 4:1.

* Length/thickness greater than 150:1. Thin walls can pose flow challenges and the ability to pack out the component. The flow length to thickness ratio should be 150 maximum. If higher than 150, consider utilizing multiple gates to achieve optimum fill/minimal knit line issues.

BARREL TEMPERATURE BASICS

Thermoplastic elastomers generally process well, but like any specialty resin, setting up the barrel optimally is a combination of manufacturer’s recommendations and knowledge of how your press behaves under production conditions.

Many manufacturers offer useful guides, often on-line. An example is Pawtucket, R.I.-based Teknor Apex Company, which has a web site — www.teknorapex.com — that contains useful processing hints as well as product specs. Here’s an example regarding barrel set-up, in this case for Teknor Apex Tekbond TPE, but generally useful:

* Lower molecular weight or less viscous grades will require a lower temperature profile set up, while higher molecular weight or more viscous grades will require higher melt temperatures. The feed zone is set 30F lower than the compression zone to prevent premature melting of the compound which often leads to bridging (pellets that stick together at the feed throat and prevent proper feeding).

* Always set the nozzle and end cap heaters to the desired melt temperature. This region of the barrel should neither add nor subtract from the melt temperature. It is simply a heat soak area as the material waits to be injected upon the next molding cycle.

* Use a molding press that utilizes 30-80 per cent of the maximum shot capacity. This will provide good homogenization and adequate heat soak time.

Teknor Apex noted that TPEs with limited heat stability shouldn’t spend excessive time in the barrel; if the process must be stopped for a prolonged time, purge and use fresh resin.

There’s a gold mine of information available on-line. A spec sheet and processing tips printout should be a natural addition to the machine log or set up “book”.