The Microtechnology Revolution
Analysts predict that microsystems technology will have a far-reaching influence on device manufacture within the next couple of years. A new generation of injection moulding machines is central to this technology.
Norbert Sparrow
There's a big future in small things, according to Helmut Detter, head of the department of precision mechanics at the Technical University of Vienna. As one of the coordinators of the Micro Moulding Machinery project (Wiener Neustadt, Austria), a consortium of companies and organizations developing complete microsystems production solutions, Detter is helping to bring miniaturized device components from the drawing board to full-scale manufacture. One-mm-diam micromotors performing arterial functions, implantable hearing aids fitted with a 0.002-g sensor, and micropumps for use with implantable drug delivery systems are among the existing and potential applications that he cites. Detter's confidence in the rapid emergence of microsystems technology (MST) and microelectromechanical systems (MEMS) as driving forces in device design and fabrication is buttressed by a recent market analysis.
A micromotor for device applications was developed and manufactured at the Institut für Microtechnik in Mainz, Germany, in collaboration with Dr. Fritz Faulhaber GmbH & Co. KG. Shown with the component are the individual moulded parts of the motor's epicyclical gear.
The total world market for MEMS and MST is expected to surge from the current US$13 billion to US$38.5 billion by 2002, according to a report commissioned by the Network of Excellence in Multifunctional Microsystems, a body of the European Commission that promotes MST within European industry. While the automotive sector has absorbed most MEMS- and MST-derived products thus far, "the biomedical branch shows the greatest growth and largest potential for further increases," the report states. "It is expected to be the most important segment for MST and MEMS products as we enter the millennium." By way of example, the report's authors predict that drug delivery systems, which currently generate US$10 million in revenue, will become a US$1-billion business by 2002.
The social and economic impact that microsystems technology will have in the next few years could equal and even surpass that of microelectronics, stresses Detter. This is due largely to the breadth of potential applications. "Microsystems technology is multidisciplinary, with micromechanics, microoptics, and microbiology playing an integral role," says Detter. "This creates even greater industrywide potential [than microelectronics]." If the technology is to fulfill its promise, however, it must first bridge the gap from technical feasibility to cost-effective production. Detter again draws a parallel with microelectronics.
"The first integrated circuits were designed at the beginning of the 1960s, but it wasn't until the 1980s that they emerged on a large scale in the marketplace," he says. That was brought about by the development of manufacturing technologies that could produce and assemble these components in an economically viable manner. Similarly, the emergence of microtechnology on a vast scale is dependent on manufacturing systems that can reliably, rapidly, and economically produce the components. For many applications, microinjection moulding has developed into a key enabling technology.
Breaking the Mould
"Today, we have a firm understanding of the problems associated with microinjection moulding," says Holger Moritz, an engineer at the Institut für Microtechnik (IMM) in Mainz, Germany. Established in 1990 to pursue applied research and development and to form partnerships with industry, the IMM's micromoulding department has successfully developed and manufactured products such as micropumps, miniature brackets to hold endoscopic lenses, and flow sensors.
"We initially encountered several challenges in moulding miniature components," says Moritz, the most critical of which involved producing a mould insert. IMM has developed several processes for creating moulds, from milling and electric discharge machining to laser ablation and LIGA technology, which combines x-ray lithography with microelectroforming and micromoulding.
As for the moulding process itself, IMM has resolved such critical issues as positioning accuracy, melt degradation, and a phenomenon known as the "diesel" effect. The latter describes what happens when air, trapped in the mould prior to injection, compresses and heats up. "It can reach 600°C and destroy your polymer," says Moritz. The diesel effect is neutralized in conventional injection moulding machines by means of a 10-µm-deep slit in the tool. "But a slit of this size would fill up with polymer in our microinjection moulding process, so we needed to develop an [alternative] evacuation system."
IMM has also devoted considerable effort to reducing cycle times. "When research into micromoulding started about 15 years ago, the cycle time was roughly 30 minutes," says Moritz. Currently, one to two minutes is standard, he adds. "But I am very confident that we will be down to 30 seconds this year."
A precision bracket designed to hold endoscopic lenses was moulded at the Insutut für Microtechnik in Mainz, Germany.
A difficulty that IMM is currently wrestling with is developing a handling mechanism for these miniature parts. "We have developed a new handling and assembly method, but it is really suitable for only a couple of structures," says Moritz, primarily a micromotor that was developed at IMM in collaboration with Dr. Fritz Faulhaber GmbH & Co. KG (Schönaich, Germany).
Measuring 5.5 mm long and 1.9 mm diam, the motor attains speeds greater than 100,000 rpm. For most applications, a gear is necessary to reduce the motor's rotational speed and to increase the effective torque. IMM developed a multistage epicyclical gear with an external diameter identical to that of the motor that worked well, but it had to be manufactured in tolerances below 2 µm. This could only be achieved by means of microtechnological fabrication processes, explains Moritz. "Thus far, we have produced about 1500 motors and gearboxes and we plan to install a full production line this year," he says. The project goal is to reach an output of 10,000 units per year. "To achieve that, we must develop moreadvanced assembly automation," says Moritz, "and that is currently the focus of our research."
Machine Makers Start Thinking Small
To capitalize on anticipated demand for microinjection moulding machines, some machine builders have developed novel systems designed to consistently, accurately, and rapidly manufacture moulded parts that weigh less than 0.1 g.
Battenfeld (Melnerzhagen, Germany) developed its Microsystem 50, an electrically driven injection moulding machine for miniature parts, within the Micro Moulding Machinery project. The machine was launched at K 98 in Düsseldorf, Germany, where it was shown moulding a polycarbonate part weighing 0.002 g that is designed to house a sensor and to be implanted in a human ear. According to Martin Ganz, manager of the company's microtechnology division, other potential medical applications for the system include micropumps and minimally invasive surgical instruments.
Because of process parameters that are unique to micromoulding, Battenfeld chose to develop a new system rather than modify an existing machine. Conventional equipment uses large sprues and runner volume to minimize material dwell times and prevent melt degradation, according to Ganz. Consequently, the required heating and cooling stages lead to higher energy consumption and longer cycle times. In addition, long flow paths in relation to the part size require the use of more-powerful injection units. Battenfeld circumvented these limitations by incorporating a 5-mm-diam plunger injection unit and a rotary table into the system.
This POM microsensor housing weighing 0.0028 g was moulded on Battenfeld's Microsystem 50.
"The injection device, which works like a plunger, reaches the splitting line of the mould," says Ganz. This design enables injection of the material in close proximity to the cavity, resulting in a very small sprue. Because there is less volume to cool down, shorter cooling times are achieved. In addition, a rotary table system enables cavity filling to take place at one station while the parts are ejected at the other. According to Ganz, these innovations have helped to reduce cycle times for the production of microparts by as much as 50% compared to conventional systems.
The parts are removed by means of a two-axis system fitted with suction cups that also orient the parts for quality control and packaging operations. The entire system is contained within a cleanroom enclosure.
Dr. Boy GmbH (Neustadt, Germany), which introduced a 12-t injection moulder with a 0.3-g shot weight in 1998, has announced the launch of a new model that can produce parts weighing as little as 0.0015 g.
"The Boy 12 A micro will mould a minimum shot weight of 0.15 g," says the company's UK sales manager Ian Crawford. Because the moulds can contain as many as 100 cavities, particular attention has been paid to quality control processes. The machine includes a CCD camera with a vision system to monitor cavity filling and parts-ejection processes along with other quality control parameters.
The primary feature of the Boy 12 A micro, according to Crawford, is the screw plasticizing unit with needle injection. "The needle in the screw ensures that even the smallest shot weights are accurately metered, and it prevents problems associated with bypasses and dead corners that may arise with downstream plunger injection," he says.
The relatively large screw ensures optimal homogenization and constant plasticizing time; the screw geometry eliminates feed problems that may be encountered with the use of smaller screws. In addition, extended dwell time, which can adversely affect heat-sensitive materials, is reduced by use of the first-in, first-out (FIFO) principle.
Ferromatik Milacron S.A. (Maurepas, France) has designed a microinjection unit that can be adapted to fit the company's K-series machines with clamping pressures from 40 to 110 t. "The points we wanted to address in developing this unit were moulding accuracy, feed problems, and dwell times in relation to thermally sensitive materials," explains sales engineer Jean-Pierre Petel.
Fitting the unit with a specially designed 18-mm screw solved many of these problems, according to Petel. "This screw diameter lets you use standard-sized pellets, but it does pose a problem regarding dwell time." Some companies have dealt with this by using a 14-mm screw, says Petel, "but then you have feeding problems because the screw is too narrow to process standard pellets." Ferromatik Milacron chose to reduce the screw's length/diameter ratio from the customary 20 or 22 to 15. "Consequently, the material will not stagnate in the screw and you reduce the risk of thermal degradation," says Petel. The unit's needle piston works according to the FIFO principle to further prevent extended dwell times.
Battenfeld recently introduced its Microsystem 50, which is designed to mould parts weighing less than 0.1 g.
The microinjection unit is currently being used at IMM, Petel adds, for the production of moulded medical and electronic parts.
Microstructures are destined to play an enormous role in medical technology, says Detter, and microinjection moulding is clearly the process of choice for the reliable and cost-effective fabrication of many of these plastic components. While purpose-built machines have eliminated many of the difficulties involved in moulding micron-sized parts, obstacles remain in the automation of parts handling and assembly operations. New materials adapted to the demands of microinjection moulding and the intended use of the final product will also become a key issue as MEMS- and MST-derived devices enter the mainstream in the new millennium.
Wanted: Micromaterials"Microtechnology uses micro amounts of materials," says Holger Moritz of the Institut für Microtechnik in Mainz, Germany, and that is developing into a problem for companies moulding miniature components. "We are unable to find suppliers who are working on materials with properties suited to this technology," says Moritz. The cause is primarily a matter of economics. "Polymer suppliers think in hundreds of tonnes, but it only takes one tonne of material to produce about 100 million of the gearboxes we are currently making for a micromotor," explains Moritz. Engineers have learned to work with commodity resins, stresses Moritz. "We have successfully processed about 40 different types of polymers so far at the institute. We can build microstructures in PEEK, polycarbonate, POM, to name a few," he says, adding that the best results are achieved with low-viscosity grades. PTFE, however, has proven resistant to micromoulding and Moritz regrets that compounds are not suitable. "The compound component is as big as some of the parts we mould, so you see the problem," says Moritz. "It's unfortunate that we cannot yet use reinforced materials with enhanced properties in some of these projects." IMM is participating in research to develop nanofilled polymers, but the endeavour is faced with a formidable obstacle: the tiny asbestoslike fibres may be carcinogenic. "It's not because of the material per se, but because of the fibres' tiny dimensions," says Moritz. Nevertheless, future breakthroughs in microtechnology will rely partly on developments in materials science, according to Helmut Detter, head of the department of precision mechanics at the Technical University of Vienna. "Mass-produced polymers will not be sufficient for the fabrication of parts in a dimension of micrometers," says Detter. "We will need a new type of high-value, small-quantity material," he says, adding that his department is currently investigating a material used in the manufacture of stents that has promising properties for micromoulding applications. |
