When Intel president Gorden Moore predicted that the number of transistors on an integrated circuit would double approximately every two years, he likely didn’t consider that every supporting aspect would shrink by half within the same time interval. Yet, dramatic progress has been made in miniaturizing electronics. This has been driven by many forces, the chief of which have been increasing integration at the integrated-circuit chip level and competitiveness in the consumer marketplace (according to IEEE Transactions on Approaches to Electronic Miniaturization).
However, manufacturers wishing to cash in on this drive toward the miniaturization of devices in a variety of industries, including electronics and medical, need to know that micro molding is not the same as macro molding (see fig.1) “only smaller". Like crossing the border to a different country, crossing the line into the realm of micro molding entails an abrupt change of laws and rules.
For electronics manufacturers, the successful application of micro molding for micro-sensors, circuit boards, clips, mountings, LED covers and packaging for discrete components has enabled them to satiate the demand for ever-smaller cell phones, digital cameras and other devices. In the medical industry, the demand for micro mold products such as tiny catheter products, surgical instruments, implantable devices, tissue anchors, needle sheathes, and absorbables has opened a new frontier for medical product and device manufacturers to provide solutions to today’s medical problems. Yet, for both industries the miniaturization of products triggers an inverse proportion of challenges.
Once a device component or product shrinks to the point of weighing just fractions of a gram or measuring less than 0.04 in. at the widest point, micro molding techniques are required which adhere to a whole new set of considerations and constraints as compared to macro molding. Electronics manufactures seeking to develop advanced new micro designs (see fig. 2) must familiarize themselves with these new parameters or face roadblocks in making their product a reality.
“On paper you can design the perfect part with five thousandths of an inch (0.005 in.) wall thickness, for example, but actually molding that part in the real world is another story,” says Isaac Ostrovsky, an engineer with Boston Scientific. When entering the cramped quarters of micro molding, product engineers are learning that success hinges on finding a molder that specializes in this quickly growing, but still unique, niche. “I’m already aware of all the restrictions within my field of expertise,” continues Ostrovsky. “But when you get to the specifics of micro injection molding, then I need a partner. For my most recent project - a 0.04- to 0.05-in. diameter element - I turned to micro molding expert Dennis Tully, whose involvement in the creation of the product was extremely helpful.”
As president of Miniature Tool & Die, Tully is a spokesman for the industry. His advice on the differences and important considerations between macro and micro molding can bridge the gap between idea and reality when it comes to helping R & D scientists, design engineers, product managers, and project engineers create micro innovations (see fig. 3).
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Rule 1: Mold tolerances become more critical.
As parts get smaller, any miscalculations have a significantly greater impact. "When requesting a micro mold, aim for 10% part tolerances as opposed to the 25% to 50% percent level commonly seen in macro molds,” says Tully. “For example, when dealing with a 0.006 in. wall thickness, if you’re off by 0.001 in., that represents 16% of the entire size."
Rule 2: Form affects maximum wall thickness.
On a macro design, a 0.03-in. wall thickness allows some flexibility in form. But when the wall thickness falls below 0.005 in., as is often the case for a micro part, then overall size and shape becomes an important factor. “Yes, I can build a mold for a part with a 0.0015 in. wall thickness, but if the design calls for a 3-in. length, then the part will not hold up,” warns Tully. “You have to consider all the geometries and other complexities of the design.”
Rule 3: Gating becomes more critical.
Gating rules are somewhat material-specific. Even still, most materials passing through micro gates sizes of 0.002 to 0.005 in. behave differently than when passing through macro gates of 0.02 or 0.03 in.
“If you are a macro molder and you have trouble getting material to flow, you can just crank up the pressure, temperature, or fill speed,” explains Tully. “But that will not always be an option through a small gate that induces high shear rates, which can change the viscosity of the material. Nor can you just heat the material to a higher temperature to lower viscosity to help it flow. Either case can destroy the properties of the material.” In some cases it is best err on the safe side by running at 75% of the wall thickness for the gate size.
Rule 4: Stand ready to relocate the parting line.
“When dealing with micro molds, the parting line cannot always be placed in the ideal location from a design standpoint,” cautions Tully.
A common mismatch allowance for a macro part can range between 0.003 to 0.005 in., whereas that wide a margin on a micro part might mean missing the whole other side of the mold. The mold must interlock properly to support the critical mismatch requirements of the parting line and improve registration of the two halves.
Rule 5: Material specs cannot always be called out from published data.
While still open to many engineering plastics like resorbable polymers, Ultem, PEEK, PPS, LCP, shape memory polymers, and polycarbonates, micro molding analysis requires special consideration since the lowest published data regarding thickness cuts off at about 0.04 in., which completely ignores the needs of micro-product manufacturers. In such cases, the manufacturer must rely on the molder for empirical data.
“We have a large database that grows with every molding project we complete,” says Tully. “We know how far each material might flow at micro thicknesses and what kind of pressure it takes to move it in the right direction. This experience helps us characterize each material for possible new applications.”
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Tully’s company, Miniature Tool & Die, utilizes test plaque molds that that gauge materials from 0.002- to 0.009-in. thick to give project engineers an appreciation of the relative properties of their first-choice material at a given thickness.
“I needed our part to be flexible enough for the application, yet strong enough so as not to break,” says Ostrovsky. “We collaborated with Dennis Tully and ultimately selected acetal for our product upon his recommendation.”
Rule 6: Micro molds require high fill pressures.
At thicknesses less than 0.01 in., plastic cools extremely quickly so liquid plastic must be shot into the mold cavity at fast speeds and pressures up to 40,000 psi. However, such conditions risk altering the material properties.
“Our part has four holes, and the thickness between these and the outside wall is only 0.005 in., so it takes very high pressures to make the plastic flow into such tight areas,” says Ostrovsky. “MTD had created a mold flow analysis to map changes in the material caused by the intense pressure and shear heat in the micro process to ensure that problems didn’t occur.”
Rule 7: Take into account ejecting
After cooling, the ability to cleanly eject the part from the mold is often affected by its design. “Not only do you have to inject the plastic into the tool, but you have to remove the finished part,” Ostrovsky points out. “But if it has a backward angle, you could easily inject it in, but never be able to get it out.”
“There’s a popular misconception that because the parts are so small, they don’t need draft, but that is definitely a misconception,” notes Tully. “They do because the walls are thin and the relative forces of the very small pins that you use for ejecting still can cause a problem if you don’t have draft that allows the part to release easily.”
In regards to ejecting, and most every aspect of mold design for that matter, accurate tooling is by far the most significant contributing factor to success.
The Future Looks Smaller
Pushing the envelope toward increased miniaturization of electronic components is the integration of electrical and mechanical functions as implemented in 3D- Mould Interconnect Device (MID) technology where a housing can serve as a three-dimensional circuit board. In this and similar applications, micro molding stands to further facilitate the ability of electronics manufacturers to successfully introduce increasingly smaller devices into the market.
“For us, everything turned out well and within budget by working with a micro molder,” says Ostrovsky. “It only took two weeks for MTD to make a working model of our design.”