Tough times have processors looking at the growing micromolding segment as a road to restore prosperity, but there’s a lot to investigate before starting the journey.

By Stephen Moore 
Published: May 13th, 2009

With the economy the way it is, processors geared towards using larger-tonnage machines are trying to find business in other areas, such as micromolding, and applying the same principles used in standard injection molding,” notes Scott Herbert, president of micromolding company Rapidwerks Inc. (Pleasanton, CA). But beware: The same principles employed in molding of larger parts don’t always apply.

Firstly, processors concur that the right machine choice is essential for successful micromolding. Among a plethora of issues, “It’s just difficult to mold a 7.5-thou part weighing one microgram with a 40-ton press,” says Herbert, whose company operates three Microsystem 50 machines from Wittmann Battenfeld GmbH (Kottingbrunn, Austria) and has two on order to expand its micromolding business. “In simple terms, you don’t get a chance to inject your material and switch over to the holding pressure phase before it’s too late.” Using a machine that’s too large means the screw may move as little as 0.01 inch, and then switch over from injection to holding pressure, Herbert explains. “When this occurs, the next few shots typically are good, but then the tool might flash. This is evidence of an inconsistent process due to using the incorrect machine for the application.”

Many molders attempt to solve this issue by making the runner diameter large enough to allow a standard machine to control the dosing or shot size. Although it does work, it is far from ideal and extremely costly because not only is more resin used, but also cycle times are extended. Further, it does not solve the residence problem that occurs when excess material sits idle in the barrel.
Another Wittmann Battenfeld adopter is Kleiss Gears Inc. (Grantsburg, WI). “We’ve molded prototype parts in small-tonnage standard injection presses but it’s a bit like swatting flies with a sledge hammer,” says company president Rod Kleiss. He notes that in addition to actual processing characteristics, “one of the big advantages of this [Wittmann Battenfeld] system is that it’s fully enclosed. One of the most difficult aspects of micromolding is part handling, but with a system specifically designed for micromolding, parts can be handled under controlled conditions such as a deionized air environment, and inspected with ease.” The rotating mold of the Microsystem 50, for example, presents the parts for takeout and inspection, rather than a robot having to go in and extract them, which would result in real estate issues.

Japan’s Juken Kogyo Co. (Toyohashi) is another machine supplier whose micromolding machines emphasize ease of takeout, typically employing a pneumatic system. Its latest edition is the fully automated JMW-015S-5t custom-built press for insert molding of 1.2-mm-diameter, 0.4-mm-thick micro rotors incorporating helical gears and samarium cobalt magnetic cores.
Kleiss Gears also uses video inspection to check for gross postprocess deformities such as flash or partial fill, as well as a secondary video inspection step of samples that enables part traceability if things go wrong later. With micromolded parts, measurement of dimensions using a probe can present challenges on account of the large size of the probe, typically 0.2 mm, relative to the part. Optical measurement is preferred. “In general, the measurement resolution should be at least an order of magnitude better than the dimension that you are trying to hold,” says Kleiss. In our case, we find 2.5 µm to be an applicable limit to shoot for.”

Donna Bibber, president and CEO of micromolding consultant Micro Engineering Solutions LLC (Charlton City, MA), notes that some processors are very successful at micromolding using standard machines in conjunction with multicavity tooling, “but I’ve seen many more that have tried and failed.” She believes that the absolute enabling component to successful micromolding is the tool, but once that is accurate, “the molding machine gets you the rest of the way there.” With that said, there is value in a good micromolding machine because the injection position accuracy is critical to supplying the correct shot size of material.

Bibber notes that some processors are actually designing their own micromolding machines for specialty materials and specialty applications. “These garage-type machines will soon be in the forefront of micromolding technology, especially if expensive materials in pharmaceutical applications keep on the rise,” she notes. Some materials such as resorbable polylactic acid (PLA) resin or PGA can cost up to $3000/lb, while polyether-etherketone (PEEK) grades can go for $70-$400/lb. The primary drive behind development of such machines is the desire for even smaller shot sizes. “The smallest currently available is around 0.3g, which is still fairly big for expensive materials that are very shear and heat sensitive,” says Bibber. “You can’t afford for the resin to sit in the barrel very long.”

Tooling and hot runners

Yes, tool design is a critical factor in ensuring success in micromolding, and as with the machine side, it brings its own peculiar challenges. Molding shrinkage presents challenges for gears, for example, given that published shrinkage values are based on regular shapes. “It is really one of the fundamental differences that we have to solve,” says Kleiss. Although major elements such as the various diameters of the gear shrink at nearly the same rate, other details such as tooth profiles shrink by widely disparate amounts, leading to unpredictability. The Kleiss Gears solution entails measuring the cavity and gear dimensions from the initial try and then recutting the entire cavity insert to compensate for the observed differences in shrinkage. Wire EDM is the preferred machining method.

Balancing the high injection pressures of up to 48,000 psi typically required to process commonly micromolded resins with tiny, thin, and fragile core pins measuring as small as 100 µm or less is also a common challenge in micromolding, notes Bibber. “You have to be creative about how you are going to hold that pin in place,” she says. Material choice and gating size and location also go hand-in-hand with successful micromolding. “Typically, gate sizes are small so the shear induced lends a helping hand, but many resins used are nevertheless difficult to flow,” says Bibber.

Hot runner systems are also available for micromolders. D-M-E Co. (Madison Heights, MI), for example, offers the Stellar micro hot runner product line that is said to be adapted for running small parts molded from both commodity and engineering resins, and as such is well suited for medical applications. “The Stellar product line is available with our standard off-the-shelf rectangular and round threaded-in manifolds or custom compression-style or threaded-in manifolds,” says Trevor Pruden, a mechanical engineer at the company. Polyshot Corp.(West Henrietta, NY) is another vendor with a track record in hot runner systems for micromolding (see this Product Watch).

Despite this availability, Rapidwerks says such scaled-down hot runner systems are not applicable to its method of injection molding. “Our injection nozzle comes into the parting line and the runner-to-part ratio is typically 5-6:1,” notes Herbert. Bibber adds, “Heat and moisture are usually critical in micromolding and it’s difficult to control both in a hot runner without customizing for each material and application.”

While fabricating tooling using conventional equipment such as machining centers, mills, lathes, grinders, wire EDM, and sinker EDM technology is sufficient for most applications, some require an alternate process to be used, such as when the resolution of EDM is insufficient to capture the fine detail of a cavity feature. Options include laser machining, x-ray lithography, electroplating, silicon etching, and photolithography.

Processors might also consider technology beyond injection molding in order to augment their capabilities. Notes Bibber, “It’s no longer just a molding shop world when it comes to making micro devices. A combination of technologies is needed to best service the medical market.” These might include micro metal injection molding, stamping, and nano imprint lithography. 

Mega micro parts and flow analysis

Not all micromolded parts are micro in size, however. The standard micromolding definition also includes larger parts incorporating micromolded features like the Microwell array from Precision Engineered Products LLC (Attleboro, MA). Used mainly for biomedical assay applications such as DNA sequencing, the microscope slide holds a total of 42 million discrete wells that are 3 µm in diameter and depth. Each well is segregated and precision fabricated to accommodate individual assays in high-speed, automated processes using the latest medical bio-imaging fluorescent technology.

Precision Engineered Products has successfully molded such parts using standard 7- and 15-ton all-electric presses from Nissei Plastic Industrial Co. (Nagano, Japan) under cleanroom conditions. One major challenge faced by the processor is ongoing maintenance of very small and delicate tooling core and cavity and components. “We have to retain repeatability for thousands and thousands of parts,” says PEP’s business development manager Scot MacGillivray.

Flow analysis is commonly used in the macro-molding world, but results appear to be varied in the micro sphere. “From a software point of view, even advanced filling analysis doesn’t recognize many micro parts,” says MacGillivray. “However, we do have experienced staff that can generally tell, based on the material selected and how a part will be molded, whether it will fill properly or not.”

Others have fared better. Rapidwerks claims to be the only Moldflow-certified micromolder in the United States, and notes that a lot of experience comes into play in successfully applying it. “It does give us relevant suggestions regarding gate size and location, and tool heating and cooling, for example.”

“It’s critical to mesh a very high-resolution model for micro molding so you can determine what’s going on in a 0.002-inch gate or a 0.004-inch wall,” says Bibber. “Like any tool, what you put into it is what you get out of it and if the data going in is not acceptable from the start, the error will feed all the way through the process and bad data will be the end result.”

Prototyping, meanwhile, may be a tool absent from the design process. “A part measuring 300 by 380 by 800 µm won’t actually show up using stereolithography,” says Aaron Johnson, marketing manager at Accumold (Ankeny, IA). “Actual hard tooling is required to produce functional parts, which often presents an unfamiliar challenge to the [design] process.” [email protected]

A small Australian processor is performing micro metal injection molding, stamping, and nano imprint lithography. Read about MiniFab.