David S. Hotter
Take a close look at your cell phone, pager, or laptop computer and you’re likely to see the latest in advanced plastics. Less than five years ago, most of these products were bulky and heavy, as anyone toting a computer can well attest. But, thanks to advanced technology, electronics manufacturers are molding housings with thinner walls that trim a little more weight off these components.
Besides slashing weight, recent developments let manufacturers cut time from production cycles, thereby increasing production rates. They also benefit from flexible manufacturing processes that give them more options for shielding delicate electronics within products from potentially damaging electromagnetic fields. Additional advancements include films used to color and mark electronic housings and phosphorescent plastics to replace battery-powered backlighting.
On a diet
With help from advanced resins that boast improved flow qualities, electronic designers have put portable and wireless components on a weight-loss program. Cellular phones are a good example of the progress made in the last 15 years. In the mid-80’s, cell phones were made of 2.5-mm-thick housings; within two years they were down to 2.0 mm. By 1989, manufacturers had developed models with walls as thin as 1.5 mm.
Today, the bar has been lowered further as engineers opt for enclosures from 0.5 to 1.0 mm thick. This has forced resin suppliers to develop materials with extremely high flow properties, so that electronics manufacturers can mold thin parts using conventional equipment.
Though thinner designs are getting more attention, they aren’t entirely new. Resin manufacturers and compounders such as LNP Engineering Plastics Inc., Exton, Pa., have supplied plastics for thin-wall applications for more than nine years. “Our first experience with thin-wall designs dates back to 1989,” says Bob Findlen, director of marketing at LNP. “For example, Polaroid needed a resin for its Captiva cameras. We worked with them to develop a lightweight resin with enough flow to fill the mold.”
As electronic designs get thinner, the need for high-performance resins increases because components must still withstand the rigors of normal use, which includes bumping and dropping. For resin suppliers, this means developing blends that have high impact strength to help resist shock, vibration, and drop tests.
For Polaroid, the answer came in the form of Thermocomp EP, a high-flow grade of polycarbonate resin. Though polycarbonate is considered a high-cost material, the camera maker still found it more economical because they could reduce the amount of resin used.
“In the past, customers tried to reduce weight and fill thin-wall molds by using polymers with lower molecular weight,” adds Findlen. “But you reach a point when the material becomes so brittle that it loses its properties.” Additives are one way of boosting strength, but they tend to migrate to the surface of plastic parts, degrading surface finish as well as paintability.
Newer formulations improve the flow of inherently strong plastics without using additives. The result is a resin easier to use as drop-in replacements for new designs. In addition, these polymers let processors increase production rates by reducing cycle times because thinner parts require less time to cool.
While processibility and flow are crucial qualities of resins, regulatory requirements and environmental constraints in global markets are also influencing designers’ choices for thin-walled designs. This is particularly important with electronics because manufacturers must use flame-retardant polymers in order to withstand the higher operating temperatures of devices such as laptop computers.
Computer manufacturers meet the requirements of standard codes such as UL 1950 using flame-retardant polycarbonate and polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) blends. In the past, this meant resins were compounded with additives such as antimony, bromine, and chlorine. With global markets becoming more prevalent today, these environmentally unfriendly elements are less desirable.
“Though cost is a major factor in material selection, European environmental regulation TCO’95, for example, prohibits use of halogenated additives such as bromine and chlorine in resins because they produce dangerous dioxins if they burn,” says Tom Hablitzel, industry manager for shielding technology at GE Plastics, Pittsfield, Mass. “In order to obtain an eco-label — a symbol and certification that a product meets recyclability standards after its useful life — a growing number of electronics manufacturers demand plastics with alternate flame-retardant additives.”
To meet these needs, GE Plastics developed Cycoloy-Series resins, based on a PC/ABS blend that derives flame retardance from phosphate additives. In addition, the resin provides a balance of properties such as flow and impact resistance, and it withstands UV exposure.
Advanced molding techniques
As resin suppliers have developed polymers that will completely fill molds, advances in processing techniques also help make the transition to thinner walls easier. This is important because even resins with good flow create high pressures when injected into molds for thin-walled components.
Thin-wall molds create high pressures because resins dissipate heat and solidify rapidly in thin sections, creating a higher skin/core ratio. Therefore, engineers must use multiple gates to deliver resin into the mold. Problem is, more than one gate means dealing with a merging flow front that creates weld or knit lines. As components get thinner, knit lines have a greater effect on degrading impact strength. Weld lines give the appearance of incongruous sections of material melted together, like that of two drops of hot wax dripped next to each other.
GE Plastics works with sequential-valve-gating techniques at its polymer processing development center (PPDC) in Pittsfield, to improve strength and aesthetics. “Surface appearance is critical to consumer products such as portable electronics,” says Kurt Weiss, a program leader focusing on thin-wall injection molding at GE. “Sequential valve gating lets manufacturers produce excellent surface finishes using standard injection-molding equipment.”
Using the technique, gates are timed so that resin injects into the center of the part before the edges. The gates around the outer edges subsequently open as the progressing melt front reaches them, joining it to form a seamless molded part. The process works well for larger parts with flow length to thickness (L/T) ratios greater than 75, which is 3 in. of flow for a 1-mm-thick part.
Besides creating parts that have better appearance, sequential gating also reduces pressure in molding equipment, making existing injection-molding machines adequate to mold thin-wall components. However, specialized molding equipment does provide an advantage to those companies looking to edge out competition by molding components faster and less expensively.
After choosing a resin, engineers must determine how to protect internal electronics from producing or falling prey to electromagnetic interference (EMI). Several methods for achieving such protection include conductive paints, plating, vacuum metallizing, and compounding resins with conductive additives.
“EMI shielding is more important than ever as fundamental frequencies and the resultant harmonics continue to rise in portable-electronics devices,” says Gary Shawhan, market development manager, Enthone-OMI Inc., New Haven, Conn. Experts agree that any one of the coating methods available is as effective as another at shielding electronics from EMI, yet the choice remains which is most cost effective for the particular design.
Conductive paints become an attractive option as designers simplify electronic enclosures. Painting processes work well for moderately complex designs as long as manufacturers can reach all of the surfaces with the spray head. One of the advantages of painting is that manufacturers can use automated equipment to spray parts. Besides speeding up production, automated systems apply a precise, consistent coat in high volumes. This is particularly important when using expensive elements like silver. “You don’t want to apply too much silver coating or lose unnecessary overspray up the ventilation stack,” says Shawhan.
Conductive paints use silver to provide a conductive path to dissipate electromagnetic fields and can be combined with copper or nickel to cut costs. As the composition of silver in the paint increases so does the price. However, the amount of paint necessary to shield a device decreases.
When throughput is important, electroless plating becomes the method of choice. It offers the greatest capacity for high volumes, particularly for double-sided plating. Plating works best for complex designs such as snap fits, bosses, ribs, and standoffs because it coats an entire part.
The process relies on a catalytic reaction to plate parts and therefore is independent of line of sight, which means that as long as plating solution reaches a surface, it will coat the part. In contrast, painting and vacuum metallizing rely on the ability to direct a spray head to coat a surface, making it difficult to cover blind holes and intricately shaped features that are difficult for equipment to reach.
As chips are being packed tighter together in electronics, devices end up with a localized area that needs shielding. Though it’s a two-step process, selective plating works well for such applications by coating a limited area using internal inserts. The process begins by coating the area with a paint followed by plating.
Vacuum metallizing depends on a spray head’s line of sight and therefore isn’t the best choice for complex designs that might not coat completely. Vacuum metallizing also has high fixturing costs, which are partially offset by the capability of coating many parts at a time, particularly in small electronics. However, the process becomes more restrictive as parts get bigger. Engineers are developing new vacuum-metallizing technologies such as vacuum-deposited copper followed by a chrome-nickel layer to improve shielding, though the process is not yet commercially practical.
An alternative to coating plastic electronics housings with conductive coatings is compounding resins with conductive additives, such as carbon and metal fibers. However, the shielding effectiveness (SE) of inherently shielded plastics (ISPs) depends on the concentration of fibers and the thickness of molded parts. This is a concern for thin-walled electronics enclosures because fibers affect the way that polymer flows in molding equipment and the effect it has on surface finish.
“We are studying ways to effectively shield using less carbon, with our Lexan SP Series of polycarbonate resin,” says GE’s Tom Hablitzel. “ISPs can be a cost-effective solution that provides flexibility and design freedom, as well as recyclability which is important for global markets where conductive coatings are prohibited in order to maintain an eco-label.”
Researchers at GE Plastics think the shielding effectiveness of fiber-filled resins has been inaccurately measured using indirect test methods, leading manufacturers to overfill resins with fibers. Indirect methods measure a material’s conductivity by passing direct current across a test piece. Shielding effectiveness is then calculated using a model of shielding.
A more accurate method for measuring shielding effectiveness, according to GE Plastics, uses a direct method that measures the reflection and transmission coefficients of EMI on test pieces. From these measurements, engineers form a transmission line model of shielding effectiveness .
Comparisons between the two methods have shown that dc conductivity measurements underestimate the SE of resins with low fiber filling.
As a result, good shielding materials have often been rejected because of low conductivity. Recent tests, using signal-noise measurements, show that the performance of a 15%-filled resin isn’t as far as once was thought from that of a 35%-filled plastic. These results have proved that as little as 15% fiber filling can provide enough SE to adequately protect the majority of electronic components.
Using a lower percentage of fibers to create an ISP lets electronics manufacturers avoid potential processing concerns. Engineers at GE are also working to prove that it isn’t necessary for polymers to have SE levels of 50 to 80 dB to protect electronics. According to the resin supplier, recent tests show these values can be reduced to a range between 15 and 55 dB, without sacrificing EMI protection.
M.A. Hanna Co., Norcross, Ga., offers an option for shielding plastics that combines attributes from traditional coating methods and compounded resins. The procedure involves insert molding a conductive film into plastic components. Using the same fibers as for ISPs, films form a conductive layer of material inside electronic housings without filling the entire cross section of a part with fibers.
“One of the big challenges with using conductive films is achieving good fiber dispersion,” says Keith Van Kirk, industry manager for business machines, communications, and electronics at M.A. Hanna. “We have focused our efforts on developing a process that assures fibers are evenly distributed during film extrusion.”
Conductive films help manufacturers cut costs by replacing more expensive coating methods that add processing steps. As long as they remain thin, films can provide a continuous conductive path, by matching the detail of the mold tool.
Engineers have a new weapon in their arsenal for competing in the global market: insert-mold decorating (IMD), a new method for adding graphic elements to plastic parts. When designing electronic housings, for example, IMD lets engineers customize logos, labels, and styling effects such as colors and patterns, by molding them along with plastic enclosures. By molding graphics between a protective film and the plastic substrate, known as second-surface printing, images are protected from superficial damage.
Cell phones or pager lenses provide an example by using second-surface printing to shield the black border and other graphics on the display. In the past, acrylics were used with secondary printing. Polycarbonate films now replace acrylics, letting electronics manufacturers produce a more complete finished part, including hard-coat surface, protected graphics, and snap fits in a single component.
IMD outperforms other marking methods because it works well for 3D graphics. Engineers begin the process by reverse engineering the final image shape and screen printing it on a flat piece of film. The film is then formed into shape by vacuum forming or hydroforming onto a substrate.
To form clear images and maintain a precise orientation on the component, films must shrink uniformly when exposed to thermoforming temperatures. It’s necessary to use special inks that can handle the high temperatures found in molding equipment. “We have conducted performance tests on over 150 inks,” says Dave Reis, industry manager for IMD at GE Plastics, Pittsfield, Mass. “Of the 150 tested, about three or four resist washout and have good adhesion, which is critical for maintaining a sharp image.”
Engineers incorporate different gate designs when forming the graphics on film inserts. Gates must be positioned to reduce the amount of shear that the ink surface will see. Therefore, what may be a good gate design for typical molding might not work with IMD. For example, on a typical part the gate would be placed in the middle because this is the best position for flow, but for IMD this may cause shear flow and destroy images.