Precise tooling and strict process control are essential to achieving the extremely tight tolerances and repeatability required of miniature and micro-size parts.
By Mark Shortt
Editorial Director, Design-2-Part Magazine
Microtechnology generally refers to the fabrication or manipulation of structures on a micron (one-millionth of a meter, or one thousandth of a millimeter) scale, but there is less than unanimous agreement on what constitutes a micro part. Micromolders may define it as an injection molded part with outside dimensions no larger than 0.5 inch; some say it weighs less than a gram. Still others say a micro part is any part measuring less than 1/8 inch long. It doesn't necessarily help to hear someone say, "I'll know it when I see it," because "seeing it" is no certainty.
Although some are barely visible to the naked eye, micro-miniature parts have been instrumental to the development of innovative products in numerous sectors of the manufacturing industry, including electronics, medical devices, telecommunications, and automotive. Increasingly smaller parts continue to be manufactured to satisfy needs ranging from less-invasive surgical devices to automotive airbag sensors, actuators, fiber optic housings, and microscopic gears, connectors, capacitors, and switches. And while tiny parts have already played key roles in reshaping many areas of our lives, many see miniaturization as part of an evolving trend that will ultimately have an even more dramatic, some say revolutionary, effect well into the future.
"There is a concentrated effort across many industry platforms to miniaturize components for many different reasons and advantages," says Paul Fink, president, Micro Precision Products, Rancho Santa Margarita, California. "There are many international government sponsorships to encourage micro manufacturing [in Western Europe, South America, and Japan]. The United States stands well to position itself for these new technologies and to maintain our clear role in leading the way towards innovative designs that take advantage of these new manufacturing opportunities."
If we could see across the continent right now, we'd see a lot of specialized work being done by job shops in support of product miniaturization. The work represents a virtual cross section of established manufacturing processes, including injection molding, die casting, stamping, machining, and welding. Whether the need is for a micro-stamped antenna with 0.008-inch features, a die-cast part with 0.010-inch wall thickness, or a detailed weld on a 0.003-inch-wide area, it is being met by skilled specialists at a job shop in North America.
In the following pages, JST offers a glimpse of some of the specialty small-part manufacturing processes being used today by job shops that see miniaturization as a niche opportunity. Some of these processes are used to manufacture small, miniature, or micro-size parts; others are used to create micro features on or within the parts, which may or may not be small.
Fourslide Miniature Zinc Die Casting
Fielding Manufacturing (Cranston, Rhode Island) custom manufactures miniature zinc die castings for numerous applications in the telecommunications, fiber optics, architectural hardware, hand tool, appliance, and medical products industries. The ISO 9002-certified company, which also produces plastic injection molded parts, uses proprietary high-speed, fourslide machines to manufacture the precision zinc die castings.
"We consider miniature parts those that will fit within a 1.5-inch cube and smaller," says Steven Fielding, president, Fielding Manufacturing, Cranston, Rhode Island. "It's hard to characterize the lower end of the range because we haven't reached it yet; we're still experimenting. We've done parts as small as 0.020 inch x 0.030 inch, but with zinc die casting, the limiting factors are not overall dimension. Wall thickness and complexity of geometry are the imposing limitations."
The company's four-slide machine technology is a major asset that allows Fielding to do core pulls and side actions. Because these capabilities are integrated into the machine, Fielding can make a core pin instead of having to use a full cam-action mechanism that is very expensive and that would have to be dedicated to a tool. "By using programmable logic controllers, we have full control over any of those slides at any point in time and can modify the sequence," says Fielding. "This gives us tremendous flexibility, not only in making complex miniature parts, but in being able to operate economically."
According to Fielding, the greatest challenge for makers of micro-miniature parts is in adapting to the much smaller scale. As parts become smaller, the degree of difficulty rises exponentially, starting with the tooling. Fielding confirms what those who work with tiny parts know all too well: the smaller the parts, the tighter the tolerances. "If you have a zinc die-cast part that fits in a five-inch cube, tolerance might be 0.005 inch, or 0.003 inch, on most features," he says. "But when you get into micro parts, you might be looking at 0.002 inch, or even 0.001 inch. You've got to be that much tighter because there's less room for error in assembly. Some parts for fiber optic and electro-mechanical applications may have tolerances on critical dimensions that are down to 0.0005 inch."
Other issues include material flow and gating, which become more difficult as parts become smaller. Post-mold material handling can also be difficult. Although companies often handle parts in bulk, tiny parts with extremely fine features require careful handling to avoid damage. Minimizing part-to-part variation is important for establishing Six Sigma quality, but with miniature parts, it often has very practical implications for assembly.
"As the part becomes smaller, the variation from part to part is much more critical and must become smaller for assembly with other parts," he explains. "Basically, your tolerance window just gets smaller. That's why the key is good engineering and good tooling. Make sure you've got a process and the tooling to make the part to the right sizes and control tolerancing so that the parts will fit in assembly.
"I think making small parts is mostly attitude," Fielding continues. "We're willing to look at challenging applications that other companies may pass on. It's more of an attitudeto push the limit to see what can be done. You can't be afraid to try new things. People look at it and say, 'I can't even see it; I don't want to touch that.' But there's a market and a need there, so somebody has to innovate a little bit and push the boundaries."
UltraCast Ltd., of St. Leonard, Quebec (Canada), also uses a four-slide casting process that lends itself well to the production of micro-miniature parts. Reportedly, the manufacturer of precision, miniature zinc die castings is capable of casting to tolerances as tight as 0.0005 inch. For one of the more critical applications that the firm is currently running, it is imperative that UltraCast produce the parts completely flash-free while holding tolerances that are in some cases as close as 0.0002 inch.
"These types of tolerances require superior tooling, a high degree of preventative maintenance, and the involvement of all levels of production, including engineering, QC, and toolroom personnel, as well as machine operators," says Murray Abramovitch, president of UltraCast. The firm was certified to ISO 9001:2000 in June.
The company's multi-slide machines use a heavy-duty rigid clamping system, which allows UltraCast to inject directly along the parting line while employing high metal pressure and high injection velocities. The combination is said to provide superior surface finish and excellent part fill. "The system is ideally suited for the production of miniature parts," says Abramovitch. "It includes a very sophisticated toggle mechanism, pneumatic and hydraulic systems, goose neck, and injection systems that result in superior miniature castings."
The multi-slide process enables UltraCast to build a greater degree of complexity into its tooling than is generally possible with conventional fixed-platen dies. The firm's engineering department strives to design into its molds a high degree of complexity in order to avoid additional secondary operations. Where possible, they can also use the multi-slide features to add more core movements. The use of advanced CAD/CAM software and CNC machining centers further adds to the company's ability to achieve complex angles and dimensions.
Generally, UltraCast runs its single-cavity molds at higher speeds. Abramovitch also notes that technically challenging pieces are sometimes easier to work with using single-cavity, rather than a multi-cavity molds. A case in point involved a complex housing used in an ordnance part. The part drawing called for a small section of the partapproximately the size of a nickelto be porosity-free, a condition that would be verified by x-rays.
"Due to the nature of zinc die casting, whereby the various segments of the mold are held together using very high clamping forces, the part cavity that remains is a natural air trap for the gas that is inside," Abramovitch explains. "Therefore, when injection takes place, the air inside the cavity is compressed and then disbursed throughout the casting that is produced. 'Porosity-free' die castings are, therefore, virtually impossible to achieve on a consistent basis."
Nevertheless, UltraCast was able to eliminate porosity in the area of concern by taking advantage of its multi-slide parting line injection process, by adjusting and shifting the gating as necessary, and by strategically placing overflows. "Essentially by shifting the gate position and creating overflow pockets, we were able to direct a large volume of trapped air into the strategically placed overflows and to less critical areas of the casting," Abramovitch says.
Micro-Miniature Metal Stamping Offers Reduced Cost, Repeatable Dimensional Control
Meier Tool & Engineering Inc., Anoka, Minnesota, produces micro-miniature stampings for a variety of industries. The company manufactures components for single-use biopsy and surgical devices for the medical industry, miniature sensors and lead frames for automotive electronics components; sensor lead frames and wireless antennas for consumer electronics, and switching technology for telecommunications products. For the past 12 years, the company has been successfully producing implantable brachytherapy seeds, measuring 0.028 inch in their largest dimension, from 0.002-inch thick titanium.
According to Rick Meier, president, micro stamping can require intricate and complex tooling, so volumes should be high enough to justify the investment. After the initial tooling investment, however, micro stampings cost less than MIM or micro machining, Meier says. The process offers outstanding dimensional control from part to part and the ability to process parts with extremely thin wall sections down to 0.001 inch, which, Meier says, is impossible to achieve by casting or MIM. Converting projects from other traditional micro manufacturing methods to micro stamping can substantially reduce assembly costs and improve product reliability, he asserts. In fact, he says, Meier Tool & Engineering has reduced its customers' finished assembly costs and, in many cases, improved dimensional control. In one recent example, the firm converted the manufacture of precision biopsy jaw assemblies from micro machining and metal injection molding to precision micro stamping.
The technical challenge involved control of the individual jaws and mating components to ensure precision alignment of the assembly. According to Meier, the biopsy jaw rim had to be coined to define a cutting edge without producing flash on the edges. "Stations were developed in the progressive die to coin the edges to exacting tolerances and ensure that the jaws would extract a tissue sample as well as, or better than, machined jaws would," says Meier.
In another case, the company produced a micro antenna of 0.006-inch thick material and 0.008-inch features, which, it had been thought, could be produced previously only by etching, followed by additional operations.
When asked about technical challenges that are unique to micro-miniature stamping, Meier mentions tooling. "Tool construction is particularly difficult because alignment of the component in a progressive die is proportional to the thinness of the material and the size of the finished part," he explains. "Clearance between the punch and die components must approach zero with materials less than 0.003-inch thick. Collecting, counting, inspecting, and handling micro components can also be challenging."
Because micro stampings are fragile and difficult to handle, Meier Tool & Engineering Inc. uses non-contact video inspection systems on the production floor for in-process inspection and statistical evaluation. Micro features are checked on optical comparators and toolmakers' microscopes. The company has also taken several steps recently to refine its micro-miniature stamping process, according to Meier. "We've standardized the die design to reduce travel and improve die alignment," he says. "We've re-engineered the die build process to reduce lead times on dies by 50%. What had been 12 weeks for delivery of a precision die two years ago is now six weeks or less. Complex micro dies for parts with intricate geometry can take longer. The shortest lead-frame die delivery we have achieved is three weeks and three days, for a carbide 10-station production die."
In Tolerance and in Control
According to Ron Peterson, general manager of Micromold, Inc. (Riverside, Calif.), it's an ongoing challenge to hold the extremely tight tolerances that are regularly demanded for micromolded parts.
"Tolerance is the biggest issue nowadays, not how small you can make it," says Peterson. "But just being in tolerance doesn't cut ityou have to be in tolerance and in control of the process. Most of our customers require a CpK of 1.67, and a Cp of 2.0." The reason, Peterson says, is that customers are buying the quality, not just the part. They want the parts to be on time, in specification, and process-controlled.
A custom injection molder since 1979, Micromold manufactures small, intricate plastic parts for the electronics and medical industries. The ISO 9002-certified company's specialties include custom multi-cavity tooling, insert molding, SPC inspection, and the use of exotic engineering resins. Micromold operates 29 injection presses, ranging in clamping force from 4 to 34 tons; seven of the presses are closed-loop microprocessor-controlled units from Boy Machines. Currently, 10% of the firm's customer base is in the computer data storage industry, 15% in the electronic components sector, and 75% in the medical industry.
One of the firm's strong suits-in addition to its attention to strict process controlis its knowledge of and experience with molding specialized engineering resins. The company is capable of working with a wide range of engineering materials, including liquid crystal polymers (LCP), ABS, polycarbonate, PEEK, DuPont Kevlar, and PTFE. Recently, the firm had success in molding Vectra A430 LCP, from Ticona, for use in a disk-drive load ramp for a computer data storage device. Besides exhibiting favorable lubricity and dimensional stability, the LCP material flows exceptionally well to fill ramp features as small as 0.5mm and as thin as 0.11mm. Micromold chose the material over nylon, which swells and shrinks with changes in relative humidity, and polycarbonate, which lacked sufficient lubricity and strength in thin-walled sections.
Micromold also emphasizes the importance of designing and building precision injection molds. With more than 25 years of moldmaking experience, Peterson is a stickler for detail in tooling, which is built to Class 101 standards. He says that no amount of tweaking will correct the situation if a tool is out of specification. "We hold our moldmakers to extremely tight tolerances, and build on an accurate tool," says Peterson. "We just hold it to a higher standard."
For quality control inspection, the company uses an array of high-end measuring equipment. It includes an OGP SmartScope ZIP 250, a precision non-contact metrology system for measuring micro-size parts, and a Seebrez optical and touch probe measuring machine from Quality Controls Solutions, Inc.
"I've yet to find, within reason, things that can't be molded," says Peterson. "Sometimes, the tolerance is unheard of for holding with a particular part configuration. Cases like those present unique challenges."
Self-Contained Cleanroom Micromolding
Another company pursuing the niche of microinjection molding is Rapidwerks LLC (Chicago, Ill., and Mountain View, Calif.), a subsidiary of StratosLightwave, Incorporated. Rapidwerks sees great potential for success with its fully electric Battenfeld Microsystem 50, a micromolding production cell designed specifically for the "smallest of components in the one-digit milligram range." With the Microsystem 50 machine, Rapidwerks is able to micromold plastic parts weighing 1 gram or less, visually inspect the parts, and package them in blister packs, all in one automated operation that saves time and costs. The 5-ton Microsystem is also equipped with a Class 10,000 clean room module, providing a low-contamination option for machine-bagged, micromolded parts to meet the stringent demands of the medical device industry.
According to Rapidwerks, the Microsystem utilizes advanced clamping technology that permits exact plane-parallel mold guidance. Besides offering high-speed injection (760 mm per second) for consistent melt flow, the micromolding machine is said to significantly reduce sprues and cycle time by accurately controlling injection shot volume (maximum injection volume is 1.1ccm). The injection module of the Microsystem was developed specifically to permit extremely precise processing of the minimum injection volume with "the highest reproducibility and process security." The machine is equipped with a Unilog B2 microprocessor control that provides an optimized regulation system to control machine movements throughout the injection molding cycle. It is said to provide precise process control and complete documentation for quality assurance.
Examples of parts molded by the Microsystem 50 include a 0.01-gram bearing cap for a medical application and a locking wheel, weighing 0.0067g, for a micromechanical device. Latches and rotors for the watch industry, and coaxial plug connections are among the additional applications.
According to Scott Herbert of Rapidwerks, holding close tolerances on tooling can be challenging when working at the micron scale. It is not unusual, he says, for specifications to call for micron tolerances on small-hole position (128 m) and concentricity. "The tolerances are much tighter and closer than industry standards," he says.
Rapidwerks serves manufacturers of medical devices and equipment, automotive equipment and accessories, mobile electronics, and telecommunications products. The company offers a variety of services that include design and engineering, prototyping, precision injection molding, microinjection molding, insert molding, tool building, and contract PCB design, fabrication, and assembly. Rapidwerks also uses stereolithography to quickly fabricate 3-D concept models.
Although it also has expertise in solid state and CO2 laser technology, JPSA Laser (Hollis, N.H.) considers excimer lasers to be its core competency. The company provides laser micromachining and microprocessing services to companies that manufacture LEDs, inkjet nozzles, chips, biosensors (lab-on-a-chip), MEMS, MOEMS, microscreens and particle traps, and microfluidic devices. Its customers also manufacture sensors, diffractive optics, chip-scale packaging, and device-scale packaging.
"Excimer laser micromachining is often the only existing technology that can achieve certain small feature sizes and geometries," says Doug Pulfer, marketing manager for JPSA Laser. The company utilizes the technology to micromachine a variety plastics, ceramics, hard dielectrics, glass, and metals. One of the more noteworthy applications of the firm's services is blue LED (sapphire) wafer scribing at 266nm with DPSS laser, at up to 8 wafers/hour throughput.
In addition to achieving ever-smaller features with increasingly complex shapes, the company must satisfy requirements for increasingly finer feature resolution, as well as tolerances that press the limits of the material composition. Problems associated with materials processing at 157-351-nm wavelengths include material variations from batch-to-batch, refractive/reflective properties of certain materials, and the issue of small feature size and geometry tolerances vs. material variations.
"One client came to us with a desire to drill glass with +/- 2m hole-diameter tolerance," says Pulfer. "They were happy with the work and asked if we could dice the glass pieces to tight tolerances. They wanted the glass to also be grooved, also to very tight tolerances. In the meantime, they encountered upstream problems involving material deposits in the glass due to another manufacturing/bonding step. We developed a laser selective abatement process to remove epoxy from the glass without any damage to the glass itself, or to critical electronic microstructures.
"Finally, they asked us to laser treat small glass substrate areas in order to enhance the devices' performance. The next challenge was to scale up for large-scale production and drive the per-part cost down. We are now running their parts in production at 20% of the original process cost."
CNC Technology Increases Capacity to Produce Smaller Parts
Pacific Swiss & Mfg. Inc. (Clackamas, Oregon), uses CNC Swiss turning centers to manufacture complex, highly accurate parts for a wide range of industries, including the medical, dental, recreation, and semiconductor markets. The company also produces parts for manufacturers of fiber optics, communications, and lock and hardware products.
"Our specialty is precision machined parts, primarily 1 1/8-inch and under," says Greg Cde Baca, president. Within this specialty, much of the firm's work is concentrated on the more complex parts with tight tolerances. Pacific Swiss can produce micro/miniature parts out of most metals and plastics; it is capable of machining parts down to 0.010-inch diameter and holes as small as 0.007 inch diameter.
Cde Baca credits CNC Swiss technology as a significant advancement for the production of smaller parts. Pacific Swiss uses multi-axis CNC Swiss turning centers, manufactured by Citizen, to produce miniature and subminiature parts that hold tight tolerances with high repeatability. Known for their efficiency in producing complex parts, the machines can be programmed to handle both primary and secondary operations. The technology provides current process evaluations on each part, along with full inspection reports and lot-traceable documentation.
"There are not that many parts that can't be made on the equipment we have," says Cde Baca. "Whether the parts are round, hex, square, or odd-shaped, we can, most likely, produce them. Since our Swiss machines have 7-axis machining capacity, we can in most cases make the parts in one operation, saving customers time in costly setups and time to market."
One of the firm's customers wanted to turn a two-part assembly into one part. The main part of the assembly had a hole, measuring 0.027 inch in diameter and drilled through almost 5/8 inch. But consolidating the two parts into one meant that the 0.027-inch hole would become a blind hole. "We couldn't drill from both ends anymore," said Cde Baca. "We needed to get creative on drilling the hole that deep from one side." According to Cde Baca, Pacific Swiss succeeded in drilling the hole and turning the two-part assembly into one part. The result was a cost saving of approximately 40% on the machined part, in addition to eliminating the time needed to assemble the two parts.
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