Designers under the gun to eliminate assembly and secondary operations are taking a hard look at multicomponent injection molding — a process best known for putting nonslip grips on handheld consumer goods.
Perhaps the best known multicomponent, overmolding technology is the soft-to-rigid combination that puts multicolor, nonslip grips on handheld devices. Here one or more materials (usually thermoplastic elastomers) inject on, around, or over a rigid plastic substrate. With proper material selection, overmolded TPEs form strong, permanent bonds with substrates without primers, adhesives, or mechanical interlocks.
But there are many more uses for multicomponent molding besides decorative, nonslip surfaces on plastic parts. The last decade has seen an expanding array of flexible-to-rigid plastic combinations that give designers a myriad of high-performance components. Injection-molding machinery and moldmakers have also kept pace. Advanced multishot injectionmolding machines and sophisticated rotary and spin stack molds are producing more complicated assemblies growing both in size and robustness.
Ford Motor Co., for example, recently replaced a load-bearing, paintedsteel trim piece on the 2005 Mustang Convertible with a glass-fiber-reinforced nylon (PA66/6) overmolded with an 85 Shore-A TPV (thermoplastic vulcanizate) — two materials historically difficult to bond, says Jack Elder, advanced development manager for Michigan-based molder Innatech LLC. "It is difficult-to bond anything to nylons. They don't even want to stick to themselves in two or three shot situations."
After trying many nylon/TPE combinations Innatech ultimately picked a glass-filled nylon from A. Schulman, Akron, Ohio, for the substrate. High strength and stiffness were the reason, says Elder. "The OEM also specified an ultrahigh flex modulus, a class "A" surface color match, and high heat resistance for both substrate and overmold materials.
The overmolded TPV had to form a durable bond to the nylon so it could take rigorous environmental testing without delaminating. This included an 80°C xenon arc-test to simulate intense Southwest desert UV exposure. Weatherability of the TPV-to-nylon bond was important because the part must retain a tight seal against the rear-deck lid as it tensions the convertible top against the Mustang body," he explains.
To optimize bond strength, Innatech used an automated press/mold system that minimizes dwell time. This let them shoot the TPV quickly following the nylon shot. "We improved surface quality of the high-viscosity, low-flowing TPV by using a hot runner system with sequential valve gates. The TPV injects into short runners and cashew gates. The sequential gates help eliminate knit and hesitation lines, splay, gate blasts, and other visible defects on the class-A surface," says Elder. The switch to the overmolded-TPV/reinforced-PA parts let designers reduce part cost, retain the required structural rigidity, and glean a 24% weight savings.
Soft-to-rigid combinations are also eliminating many assembly operations. For example, complex seal, gasket, and acoustic/vibration-damping components, as well as EMI/RFI shielding elements, can be molded onto or inside plastic housings. This can dramatically reduce total manufacturing costs by eliminating secondary handling and assembly operations.
Such molding also speeds part throughput, improves assembly quality, and may help keep complex injectionmolding jobs on U.S. soil, says Robert Hare, general manager for injectionmolding machine maker Ferromatik Milacron Europe USA, Batavia, Ohio. Changes in manufacturing strategies — particularly Asian competition — and the growing emphasis on reducing in-process time and inventories, says Hare, have made multicomponent molding an economical option for complex assemblies.
THE RIGHT CHOICE?
The technology for multishot, multicomponent molding is well developed, proven, and reliable, says Hare. "It evolved in Europe and is widely used there. But in the U.S. and Canada it has been almost a trade secret mainly confined to large proprietary manufacturers and protectively guarded." Thankfully, however, there is now a maturing group of small privately held injection molders specializing in multicomponent. The number of experienced designers,operators, and proficient moldmakersis also growing. And there are even some capable European moldmakers putting down roots in the U.S. as well, Hare says.
The expertise of specialty thermoplastic compounders is producing precise, finely tuned, and unusual property combinations for soft-to-rigid applications. This expanding array of materials better meets the needs of the more robust, multicomponent assemblies now becoming common in automotive, construction, small appliances, medical components, agricultural, recreational, and industrial applications.
As with any new design, it's important to take the multicomponent molding process into consideration early in design. Ditto for material suppliers that play a pivotal role in the success of multicomponent parts. Homero Endara, spokesperson for GE Advanced Materials, Pittsfield, Mass., advises posing a few key design questions to gage whether or not multicomponent molding makes sense:
What is the target manufacturing cost and annual volume (including product variants)? The cost of complex, multicomponent tools can sometimes be a hard sell to company bean counters. This is especially true when converting an assembled part to multicomponent. Planners must carefully assess the entire manufacturing process to see whether or not it's economical to redesign an existing assembly for multicomponent. Likewise, for evaluating new products.
The "nickels and dimes" lost from piece-part scrap, as well as those spent to track and store part inventories (including colored variants of the same assembly), can quickly add up. Secondary operations, especially manual ones are quite time consuming for injectionmolded assemblies.
Additionally, individually molded parts often first get ejected from the mold into bins where they can shrink, warp, or deform. As they make their way through production they must frequently get reregistered onto assembly fixtures for subsequent secondary operations. Individual, out-of-tolerance parts can lead to costly out-of-spec assemblies that end up as scrap.
Reduced labor, time, and elimination of scrap helps justify multicomponent molding. The process can often make sense for high-quality, complex assemblies produced in quantities as low as 50,000 to 150,000 parts annually.
What is the value to the OEM? Aesthetics have been a driving force behind multicomponent molding in handheld consumer goods. According to Robert Banning, owner of design firm Trimax LLC, St. Louis, multicolor, tactile grips on power tools and handheld electronics have helped differentiate products from their competition. He says custom grips can significantly boost brand recognition.
Multishot TPE overmolding lets designers produce geometries impractical with traditional thermoset rubberbonded grips and handles, says Banning. Improving grip geometry and tailoring of TPE-vibration-damping properties, for example, can significantly improve power-tool ergonomics. This may let OEMs charge a premium for tools, especially when OSHA is looking over an end-user's shoulder. Better material impact, chemical properties, and UV resistance may also field a more valuable product that will see harsh working environments and a longer service life, says Banning.
What material properties are most important for product performance? Performance and environment drive material selection, continues Banning. The top material properties for robust assemblies include impact, chemical, and heat resistance. UV/weather resistance and standards for automotive and smoke/flame ratings may also dictate TPE and substrate options. But it's best to define physical and mechanical properties early in the design process and the standards to which they must comply. This helps streamline material selection. Banning prioritizes top performance needs as follows:
- Maximum/minimum service temperature
- Continuous chemical-resistant environment
- Impact resistance/toughness
- Bondability of the TPE to substrate
- Deflection temperature under load
- Creep requirements
- Colorability and color retention
- Property retention in UV/weather
- Surface properties
Banning says, most multicomponent projects identify the polymeric substrate material first. The range of thermoplastics suitable for substrates in multicomponent assemblies has grown substantially. There are about 40 different options available including various grades of acrylonitrile-butadiene-styrene (ABS), acrylics (PMMA), nylons, polycarbonates (PC), polyethylene terephthalate (PET), polypropylenes (PP), short glass-filled plastics, long-fiberreinforced thermoplastics (LFRT), and blends such as PC/PBT (polybutylene terephthalate) and PC/ABS.
GE's Endara says each family has pros and cons when it comes to evaluations based on material properties, processing ease, and price. Here's a brief overview of each family's strengths and weaknesses that will help pinpoint suitablecandidates for robust designs:
ABS is a copolymerized polystyrene with two rubber modifiers ( polybutadiene and polyacrylonitrile) that boost toughness and give the polymer high impact strength. Varying the three monomers in its makeup gives the polymer a wide range of physical properties. ABS processes easily, has good economics, and bonds to a number of TPEs. On the downside, at elevated temperatures it can creep and has limited chemical resistance.
PMMA has optical clarity as its strongest attribute — the highest (92%) light transmittance of any plastic. It also has low UV and oxygen sensitivity, reportedly loosing only 20% of its light transmittance over 20 years. PMMA also molds with a high gloss and surface hardness. But it has low impact strength, poor aromatic hydrocarbon resistance, absorbs water, and doesn't easily bond to most TPEs.
PC also features good transparency and colors easily. Its toughness and impact resistance is well above those available with PMMA. PC processes easily, is dimensionally stable at elevated temperatures, and bonds well to several specific TPEs. Its main shortcoming is limited chemical resistance.
PBT and PET are thermoplastic polyesters that are good candidates for structural applications. PBT has good injection-molding and mechanical properties similar to nylon. Its service temperature is also above that of nylon and it doesn't have the moisture absorption problems associated with nylon. Glass-reinforced PBTs have stiffness similar to thermosets but retain their toughness.
PET is best known for its use in beverage containers. As an engineering resin, it is generally reinforced with 15 to 50% glass or minerals. In this form its tensile-strength and service-temperature limits can also rival those of nylon and PMMA. PET is slightly stronger and costs less than PBT, but some grades of PBT process easier than PET grades. Both resins have good molded bond strengths to a number of TPEs.
PC/PBT blends have good low-temperature performance and chemical resistance. They also resist stress cracking and impacts and stand up well under long-term UV exposure. They bond to some TPEs. Disadvantages for PC/PBT blends include poor processability in thin-wall designs and shrinkage after molding.
PC/ABS blends offer a good balance of mechanical and physical properties. They process easily and have superior impact resistance. They retain ductility at low temperatures and withstand elevated temperatures and UV exposure. They also bond to several TPE families, but fall short when it comes to resisting chemicals.
LFRT composites are primarily made from PP and nylon matrix materials. They have ultrahigh modulus and stiffness values, outstanding impact resistance, and toughness. Long-fiber reinforcement boosts the deflection temperature under load thus letting the reinforced polymers serve in structural parts at elevated temperatures. However, it's harder to mold complicated designs in LFRTs because of the long fibers. The fibers also slightly degrade surface aesthetics.
THE SOFTER SIDE
Trimax's Banning suggests the material-selection process examine at least two specific plastic/TPE combinations. Both should meet marketing and consumer objectives and have enough bond strength and mechanical/physical properties to perform well in the assembly. Processing ease is also important. "It's important to pick a multishot molder with overmolding experience and a TPE supplier with a significant array of overmolding products for the specific substrates under investigation,"says Banning. "TPE suppliers should also be open to meeting the design and manufacturing needs of the part and not simply push a single TPE technology." Thankfully, says Banning, there is good camaraderie among some of the top TPE suppliers, molders, and OEMs. For example, suppliers that realize their TPEs won't bond with a particular substrate may send a designer to another TPE supplier having a track record for success with the substrate.
Malar Shetty, application development manager of R&D for TPE supplier GLS Corp., McHenry, Ill., says the challenge with multicomponent molding is in getting maximum adhesion between the TPE and substrate.
Several factors affect bond strength. They include the chemical composition of the polymers and processing parameters such as melt temperature, time, and injection-pressure profiles.
Three adhesion mechanisms work together to form TPE/substrate bonds. The first is the interpenetration or diffusion of macromolecules in the boundary layer. Molecular penetration depth determines bond strength. Good adhesion comes from high melt temperatures and sufficiently long contact (cycle) time in the mold.
A second mechanism assumes that bond strength results from interactive electrostatic polar linkages or van der Waals forces at the boundary layer. To ensure durable bonds, surface tension between mating materials is key, i.e., each material must completely "wet" the surface of its mate.
The final mechanism is mechanical interlocking. Used in many designs, its adherence comes from one (or more) components penetrating into the wall cross section of the mating material.
Additives such as colorants, reinforcements, flame retardants, and UV stabilizers affect how well TPEs bond to substrates. It is important to determine early whether additives will change depending on product variations of the same assembly. Researchers at BASF, Florham Park, N.J., recommend that designers determine early if processes and physical properties of the TPE to substrate are compatible.
Processing compatibility concerns the temperature of the melt and of the injection-molding tool. If the melt temperature of the second component, for example, is too high, the fringes of the substrate (preform) will remelt. This can deform the substrate. For this reason, the TPE selected typically processes at lower temperatures and is injected after the rigid thermoplastic shot is complete.
Mechanical and physical properties of the materials also impact processing and bond strength. The modulus of elasticity, coefficient of thermal expansion, and shrinkage properties of all components must be compatible. Materials that have great differences in cooling behavior, for example, give rise to high internal stress. This can cause part warpage and form cracks at the boundary interface of the TPE/substrate.
TPEs are materials in which elastomeric (flexible) phases are incorporated into the rigid thermoplastic phase. There are three basic classes of TPEs. Copolymer styrenic TPE-S versions are hydrogenated styrenic block copolymers (SEBS) and unhydrogenated styrenic block copolymers (SBS). Multiblock polymer crystalline hard segment TPEs include copolyamides (COPA), copolyester elastomers (COPE), polyurethanes (TPU), and polyolefin elastomers (POE). And elastomer blends include uncross-linked thermoplastic polyolefins (TPO), thermoplastic vulcanizates/dynamic vulcanizates (TPV), and melt-processible rubber (MPR). As with the substrates, each TPE comes with its own set of pros and cons. GLS Corp.'s Shetty gives a few tips on narrowing the list for a particular application:
COPAs offer ultrahigh service temperature and a flat modulus curve over an extended temperature range. They have been used in many European sporting goods.
COPEs process easily with fast injection-mold cycle times. They stand up well to UV exposure. But they are one of the highest durometer overmold materials, have a stiff feel not perceived as soft, and have limited range of bondability to plastic substrates.
POEs offer lower cost options and can be incorporated into styrenic and olefinic-based TPE compounds. Their upper temperature range typically lies below 180°F.
SEBS TPEs easily mold with a warm, soft feel. They color easily and have good tear strength. But their chemical and abrasion resistance is limited and they don't stand up well to elevated (over 250°F) continuous-use temperatures. They offer excellent bondability to a broad range of thermoplastics.
TPUs have excellent chemical/oil resistance, ultrahigh tensile strengths, and resist abrasion. They adhere well to ABS, nylon, PBT, PC, and PC blends. They have limited adhesion to PE, PET, and PP and don't adhere to PMMA systems. On the down side, they have high hardness values, are not easily molded in thin sections, and need long injectionmolding cycles.
TPVs have good oil and abrasion resistance. They mold with a rubberlike, matte/dull finish and have good heataging properties up to 275°F continuous-use temperature. They, however, have poor toughness and tear strength, poor adhesion to nonpolyolefin substrates, and lack a soft feel, even at low.
Future trends and development opportunities
The growth of thermoplastic elastomers is expected to continue at an annual rate of 7 to 8%. Besides the current widespread conversion from rubber to TPE in automotive weatherseals, overmolded TPEs will continue to find use in consumer electronics, appliances, power and hand tools, toys/recreational products, disposable medical goods, kitchen and housewares, and automotive tools.
Key growth factors are the ability of TPEs to add value (raise the price) to finished goods; the array of eye-catching design options that are also pleasing to touch; and the broad basis by which to help differentiate new products from the competition.
Enhancements underway at the major established TPE manufacturers will further broaden the range of differentiation available for products:
These TPE improvements will usher in new applications ranging from better performing disposable medical goods and more durable automotive seals, gaskets, bushings, and impact-damping devices; to overmolded pump and valve components less expensive than the iron/thermoset rubber they replace.
TPE overmolding of LFRT composites is also expected to replace vinyl/thermoset-rubber components in watercraft, exercise equipment, and ATVs to reduce costs and overcome previous design limitations. There will also be continued penetration in demanding seal and barrier applications for medical goods, food packaging, sound attenuation, and beverage closure.
All in all, many of these applications see better performance and lower costs thanks to TPE.
— Robert Banning, Trimax LLC, St. Louis
Ferromatik Milacron Europe USA
GE Advanced Materials
Intermec Technologies Corp.
National Detroit Inc.
Royal Philips Electronics
Zeon Chemicals L.P.