By Jan H. Schut

Reportedly weighing only 0.00000313 grams, an EVA ophthalmic implant molded by MTD Micromolding may be (for now) the world’s smallest injection molded part (it’s the donut-shaped part shown in the magnified inset image). MTD says it molded the part in a one-cavity mold (shown in the large image), which was made using a modified micro-EDM machine from Sarix S.A. (Sant’Antonio, Switzerland). The mold gates’ dimensions are said to be only 0.046 x 0.02 mm (photos courtesy of MTD).

Micromolded parts are getting dramatically smaller and more detailed. For years a PET component weighing 0.00012 grams was claimed to be the smallest micromolded plastic part in the world. It was molded by MTD Micromolding in Charlton, Massachusetts, USA (formerly Miniature Tool & Die) in 1998, and first reported in 2002. Today the “world’s smallest” is arguably an ethylene vinyl acetate (EVA) ophthalmic implant for glaucoma, molded by MTD a few years ago, weighing only 0.00000313 grams. Part geometries and functional surface textures have also gotten infinitesimally fine.

But size isn’t all. “Today, with better molding machines and new tooling technologies, you can make that part a lot better than you could before,” says Neal Elli, president of Empire Precision Parts (Rochester, New York, USA), a molder of small parts and micro-parts. “The cutting edge in micromolding is that more companies are making machines that control very small shots.” Micro-sized hot and cold runners are even being used to control smaller shot sizes, instead of plungers in the molding machine itself.

Micromold-makers are also driving improvements, borrowing technologies from electronics, aerospace, and watch-making—technologies like X-ray, e-beam, and UV nanolithography; laser etching; and micro-EDM (electrical discharge machining)—to put sub-micron features and textures onto inserts for micromolds. By definition, a submicron or “near-nano” feature can only be seen under an atomic force microscope and has at least one dimension (length, width, or height) between one nanometer (nm) and one micron. (By contrast, a micro-scale feature has at least one dimension between one micron and one millimeter.)

“Nanolithography technology has been around for a long time, but there wasn’t much interest in it. Now using it for micromolding is definitely growing in popularity,” says Aaron Johnson, vice president of marketing at Accumold LLC, a micromolder in Ankeny, Iowa, USA. Accumold first molded submicron structures commercially about five years ago, molding a nano-scale diffractive surface for light management onto an optical electronics part.

Radical but Gentle

Two recently developed alternative micromolding machine technologies use entirely new melting mechanisms—ultrasonic vibration and a flighted barrel—to offer gentler melting with little or no residence time, and less heat degradation than in conventional micromolding. Ultrasonic micromolding is fully commercial. It was invented by an alliance of technology centers and industry led by the Ascamm Technology Center in Barcelona, Spain, designed around the concept of saving raw material, energy, and waste while improving melt quality. Ultrasion S.L. in Barcelona, a spin-off of Ascamm, introduced the first commercial machine, called Sonorus 1G, in the USA at NPE2012 for shot sizes from 0.05 to 2 grams.

The patent-applied-for ultrasonic molding melts plastic in an “ultrasonic plastification chamber” with no screw, barrel, or heater bands—just ultrasonic vibration—so it uses less energy. An ultrasonic probe tip or horn comes down into the melt chamber. Then a piezoelectric transducer in the probe fits tightly into the chamber and creates ultrasonic vibrations (at 30 kHz), which melt the plastic in milliseconds. After that an 8-mm plunger fills the cavities. This kHz level is higher than the range of human hearing and is the same for all polymers. 

A dosing device feeds the precise amount of pellets for every shot into a dosing container from which they’re transferred directly to the ultrasonic melting chamber. The process melts only what it doses every cycle. No melt is stored for the next cycle, reportedly giving better polymer properties with less heat degradation and better tensile strength. Sound waves also lower viscosity, which allows the molding of thinner-walled micro-parts and finer surface details. The Sonorus reportedly uses three clamp tons and a much smaller runner system than the cold runners used in conventional micromolding.

“We have found no thermoplastics to date that we can’t mold, but some are easier than others,” Ultrasion general manager Enric Sirera says. “The degree of difficulty is mainly linked to part and mold complexity.” He says Ultrasion has sold some machines to universities and research institutes, but most are for industrial applications, including molding PLA for medical and bio-sorbable components; LCP for electronics; POM for high precision mechanical parts; and PP for medical parts. A second model under development will offer a wider range of shot sizes targeting small parts with micro-features. It’s expected to be available in 2016. 

A flighted barrel micromolding machine, also called the “inverse screw,” was invented by the Institute for Plastic Processing (IKV) at RWTH Aachen University in Germany, and built by Arburg GmbH + Co. KG, Lossburg, Germany, last year. Arburg built two machines for research: one for itself and one for the IKV. The flighted barrel and axially grooved 8-mm shaft hold much less volume than a conventional barrel and 14 mm screw—only 0.4-1.2 cm³ of melt vs. 2.2-6.5 cm³ for a 14-mm screw and barrel. The flighted barrel also reduces residence time because one unit melts and meters, whereas many conventional micromolding machines have two stages. Barrel flights correspond to feeding, compression, and metering zones on a conventional screw.

When the IKV announced the technology in the USA at ANTEC® 2014, the inverse screw could only process POM, not PP or PMMA. The IKV has since made significant modifications, developing a new feed section with a longer opening and more barrel flights, putting deeper grooves in the shaft and shallower flights in the barrel, and adding a chamber before the nozzle. The redesigned shaft and barrel can now process PP as well as POM at screw speeds of 50, 200, and 350 rpm, injecting shots from 0.3 to 2.2 grams with high repeatability, the IKV reports. A 1.4-gram shot of POM, for example, showed 0.5% variation over all three screw speeds, while a 1.4-gram shot of PP showed 0.6% variation.

The IKV concludes that the technology offers good homogenizing and much shorter residence time than conventional micro-injection molding machines, with precise repeatability, so it should be ideal for heat-sensitive materials. 

Ultrasion’s ultrasonic micromolding machine reportedly melts plastic in milliseconds, with no residence time, and uses a much smaller runner system than the cold runners in conventional micromolding (photo courtesy of Ultrasion).

Compression Micromolding

The first machines for compression micromolding, also called injection embossing, were separately developed in Austria and Germany almost simultaneously over the past two years, targeting high-clarity optical parts like tiny Fresnel lenses and overmolded micro-parts for MEMS (micro-electro-mechanical systems).

In 2013-2014, Wittmann Battenfeld GmbH (Kottingbrunn, Austria) developed a new compression micromolding process called HiQ Shaping for optical parts requiring low shrinkage, low internal stress, and low birefringence. (Birefringence is an iridescent effect caused by a material splitting a light beam into two polarized beams at right angles, reducing light transmission and clarity.)

With HiQ Shaping, melt flows into a cavity with a predefined embossing gap (less than 0.65 mm), which closes under gradual pressure when the cavity is full and the mold surface is above the glass transition temperature of the polymer. Software profiles the embossing clamp pressure for 10-12 seconds, controlled by two pressure sensors, one on the mold, one on the toggle clamp. Venting happens through the gap.

Cycle time is reportedly “ten times faster” than so-called “variotherm” dynamic hot/cold cycling to achieve high clarity. HiQ Shaping was developed for Battenfeld’s MicroPower 15 micromolding machine, capable of 0.5 to 4 gram shots. The R&D was funded by the European Union in cooperation with toolmaker Microsystems (U.K.) Ltd. (Warrington, UK) and four technology institutes in Germany, Denmark, and the UK. It was shown for the first time in the USA at NPE2015, molding a 0.007-mg liquid silicone rubber (LSR) tear-duct plug. 

The Kunststoff-Zentrum (KuZ) research institute in Leipzig, Germany, also developed a new compression micromolding machine in which a direct-gated mold closes, leaving a small gap (0.6 mm). The mold is filled using very low injection pressure of only 400 bars, compared to 4000 bars for conventional micro-injection molding. Then the mold closes tightly with 2.4 tons of servo-electric clamp force. KuZ built a vertical micro-compression molding machine with a rotary table and robotics for overmolding and insert molding of MEMS components, where low injection pressure is important so as not to break the micro-features. 

KuZ designed an earlier FormicaPlast horizontal micro-injection molding machine, which Desma Tec in Achim, Germany, has built commercially since 2006. The original horizontal injection FormicaPlast has only 1 ton of clamp force, while the new vertical compression FormicaPlast has 2.4 tons. Both use a tiny 7-mm plasticizing shaft and 3-mm injection plunger. The compression FormicaPlast isn’t commercially available yet, but “when a customer wants it, Desma will build it,” says KuZ R&D engineer for micro-injection molding Gabor Juettner. 

Two Asian universities have also published R&D on compression micromolding. Yonsei University (Seoul, S. Korea) in 2003 reported micro-compression-molding polymer powder to produce micro-lenses 36-96 nm in diameter with radii of curvature of 20-60 nm and a pitch of 250 nm. Meanwhile, South China University of Technology (Guangzhou) in 2012 reported micromolding a part with an aspect ratio of 12:1 by “manipulation [of] compression force.”

KuZ developed a new vertical micro-compression molding machine with reportedly very low injection pressure for overmolding MEMS parts and lenses (photo courtesy of KuZ).

Newer, Smaller Micromolding Machines

Several new micromolding machines are notably tinier than previous models. Sodick Inc. (Kanagawa, Japan), represented in the USA by Plustech Inc. (Schaumburg, Illinois), showed its smallest commercial micromolding machine for the first time in the USA at NPE2015. The 3-ton vertical HC03VRE, introduced last year in Japan, has a 14-mm screw and 8-mm injection plunger, new valve system, and rotary table. It’s said to offer much faster cycle time than Sodick’s previous smallest machine, a horizontal 5-ton model.

Nissei Plastic Industrial Co. Ltd. (Tokyo, Japan) also introduced the new hybrid NPX7 Advance micromolder at NPE2015 (though the machine itself wasn’t shown there). It uses 7.7 tons of hydraulic clamping with only a 12-mm screw that’s fed conventional pellets using Nissei’s Smart Feeder. Typically a 12-mm screw is used for micropellets and 14-mm diameter is needed for regular pellets.

MHS Mold Hotrunner Solutions Inc. (Georgetown, Ontario, Canada) introduced its new “M3 mini” micromolding machine at NPE2015; it has a much smaller footprint than its previous 32-cavity M3 micromolder. The M3 mini is only 150-cm wide x 150-cm high by 50-cm deep, compared with 191-cm wide x 230-cm high x 72-cm deep for the original M3 machine. The mini uses small six-cavity insert plates for up to twelve parts per shot, so it can mold both low-volume prototypes and high-volume production, the company says. It has a clamp force of 2.2 tons, using electromagnets.

Both the M3 and M3 mini use a patented hot runner technology (U.S. Pat. #7,125,246) called Isokor, which was first shown running at K 2013 in Germany. The M3 mini uses a barrel with an extrusion-type screw that delivers melt at lower than processing temperature to the runner manifold, where it is brought up to molding temperature just before the nozzle. Integrated plungers inside the manifold meter and inject the material into the cavities. Shot sizes range from 400 mg down to 1 mg.

Boy Machines Inc. (Exton, Pennsylvania, USA) adapted a new “micro- deck” insert designed and built by Kipe Molds Inc. (Placentia, California, USA) to convert Boy’s XS micromolding machine to LSR. Boy demonstrated the micro-deck for the first time at NPE2015 molding a 0.009-gram LSR part with micro-engraving. The XS micromolding machine is the smallest from Boy, with 10 clamp tons and a 12-mm screw and barrel, capable of 1-4 gram shots of thermoplastics. Kipe’s patented micro-deck (U.S. Pat. #8,641,943) reportedly uses an injection plunger in a cold runner to inject smaller LSR shots for parts as small as 0.003 grams.

New Technologies for Nano-Featured Molds

Dynamic heating and cooling (a.k.a. variotherm molding), raises the mold surface above the glass transition temperature of the polymer during filling to improve surface details and create thin walls, followed by rapid cooling. It has been used for over a decade to mold large parts with glossy surfaces and to micromold one particular medical product: lab-on-a-chip parts for blood testing. These are small, thin, flat parts with submicron channels as tiny as 0.06-0.1 microns deep by 0.3-1 micron wide for blood flow.

Boy Machines adapted this micro-deck insert developed by Kipe Molds for micro-LSR injection of shots down to 0.003 grams. It converts Boy’s XS machine to two-component LSR/thermoplastic micromolding (photo courtesy of Boy).

New technologies for faster dynamic temperature cycling are being tested for micromolding of nano-features. Industrial gas supplier Linde AG in Munich, Germany, recently developed
a new concept for dynamic heating and cooling of micromolds using liquid CO₂, called Plastinum Temp D. Linde developed the technology with gwk Gesellschaft Wärme Kältetechnik mbH (Meinerzhagen, Germany), which builds temperature control equipment (including CO₂ heating, cooling, and pumping equipment), and ISK Iserlohner Kunststoff-Technology GmbH (Iserlohn, Germany), a consulting firm specializing in temperature control of injection molds. 

The CO₂ micromolds have tiny flexible stainless steel capillaries (0.5 mm I.D.) used for both heating and cooling. Thinner fluid channels can be located closer to the mold surface than water lines, so heating and cooling are faster, both at up to 30°C/second. Ceramic heaters heat the CO₂ to 300°C for normal operation, but can go as high as 500°C for PEEK materials. Hot gaseous CO₂ is pumped through the fluid channels in the mold. After the mold is filled, liquid CO₂ at -79°C is injected through the same capillaries, converting from a liquid to “a mix of CO₂ snow and gas,” Linde literature explains (see also the article on this topic in this issue of Plastics Engineering).

Using the same channels for heating and cooling leaves more space in micromolds for gate location. And using CO₂ instead of water means no clogging of water lines. The technology was developed by Linde in Germany and tested at the University of Erlangen-Nuremberg, Germany; however, Linde’s U.S. office in New Providence, New Jersey, is leading the beta site trials.

There’s also what’s believed to be the first use of induction heating to mold a submicron surface texture. Induction heating heats an electrically conductive material like tool steel rapidly by passing alternating high frequency/low voltage current through the metal, creating eddy currents in the metal which generate heat. RocTool S.A. (Le Bourget du Lac, France) licensed its patented induction-heated mold technology to Swedish automaker Volvo to injection mold a near-nano texture on the inside of a radar housing for the 2015 Volvo XC90 SUV. The texture keeps sunlight from entering the device and blinding the radar. Typically RocTool’s induction-heated molds are used to put high gloss surfaces on automotive and electronic parts.

NIL Technology’s first commercial nano-featured mold in 2014 takes the form of this hologram-like package. Optical diffraction creates the colors, with line widths ranging from 350 to 700 nm (photo courtesy of NIL).

Interest in nano-structures on mold surfaces is growing. One of the earliest technologies, LIGA (a German acronym for lithography, electroplating, and molding), was invented by Karlsruhe Institute of Technology (KIT) in Germany in the 1980s for aerospace parts. When LIGA was launched, it made straight-side PMMA structures which were electroplated with nickel and gold. But the process was expensive then, so it wasn’t used for plastics molds. KIT also invented a less expensive and less accurate UV-light lithography to make structures for electroplating.

Now KIT, NIL Technology ApS (Lyngby, Denmark, a spinoff from the Technical University of Denmark), and Cemecon Scandinavia A/S (Aarhus, Denmark) are using nano-imprint lithography and LIGA to apply miniscule structures to mold steel. NIL’s first commercial nano- featured mold was for a hologram-like decorative surface for packaging in 2014. NIL also offers antimicrobial, hydrophobic, and other biomimetic submicron surface textures. Previously, micro-textures were first embossed on film, then applied to a part in a secondary step.

Meanwhile, other companies are developing LIGA. For example, 3-D MID e.V. (Nur­emberg, Germany) and microworks GmbH (Eggenstein-Leopoldshafen, Germany) reportedly make LIGA process nano-featured inserts for injection molding. Mimotec SA (Sion, Switzerland) makes UV LIGA inserts for micromolds, while Micromolding Solutions Inc. (Boucherville, Quebec, Canada) offers laser micromold-making and LIGA nano-featured mold inserts. And temicon GmbH (Dortmund, Germany) uses nanolithography nickel plated on steel for nano-featured inserts.

Note: Jan Schut digs even deeper into micromolding in Plastics Engineering’s blog, at plasticsengineeringblog.com.