Manufacturers wishing to cash in on the drive toward the miniaturization of medical products and devices need to know that micromolding is not the same as macro molding “only smaller.” Like crossing the border of a different country, crossing the line into the realm of micromolding entails an abrupt change of laws and rules.
Today's demand for micromold products such as tiny catheter products, cannulas, surgical instruments, implantable devices, tissue anchors, needle sheathes, absorbables, and stent blanks has opened a new frontier for medical product and device manufacturers to provide solutions to today's medical problems. Yet, 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 0.040 in. at the widest point, micromolding techniques are required. These adhere to a whole new set of considerations and constraints as compared to macro molding. Medical product and device manufactures seeking to develop advanced new micro designs 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 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, Natick, MA. (For more on Boston Scientific's approach to micromolding, see box.)
When entering the cramped quarters of micromolding, product engineers are learning that success hinges on finding a molder that is thoroughly familiar with important micromolding rules and thus able to advise customers accordingly. The following seven rules are based on our company's practice.
Rule 1: Mold tolerances become more critical
As parts get smaller, any miscalculations have a significantly greater impact. When requesting a micromold, aim for 10% part tolerances, as opposed to the 25 to 50% level commonly seen in macro molds. 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 0.030-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 become important factors. While it is possible to build a mold for a part with a 0.0015-in. wall thickness, if the design calls for a 3-in. length, then the part will not hold up. All the geometries complexities of the design must be considered.
Rule 3: Gating becomes more critical
Gating rules are somewhat material-specific. Even still, most materials passing through micro gates sizes of 0.002-0.005 in. behave differently than when passing through macro gates of 0.020 or 0.030 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. 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 to error 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 micromolds, the parting line cannot always be placed in the ‘ideal’ location from a design standpoint. A common mismatch allowance for a macro part can range between 0.003 and 0.005 in. However, that wide a margin on a micro part might mean missing the whole other side of the mold. The mold must be interlocked 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
Micromolding analysis requires special consideration since the lowest published data regarding thickness cuts off at about 0.040 in., which completely ignores the needs of micro product manufacturers. In such cases, the manufacturer must rely on the molder for empirical data. The molder should have a large database that grows with every completed molding project. This database should have information on how far each material might flow at micro thicknesses and what kind of pressure it takes to move it in the right direction, all of which helps characterize each material for possible new applications. Our test plaque molds are used to 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.
Rule 6: Micromolds 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 extremely high pressures up to 40,000 psi. However, such conditions risk altering the material properties. Our mold-flow analysis maps 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: Account for eject
After cooling, the ability to cleanly eject the part from the mold is often affected by its design. When a part goes into the human body, the last thing desired is unwanted jagged edges. A popular misconception is that because the parts are so small, they don't need draft; but that is definitely a misconception. Draft is needed 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, accurate tooling is by far the most significant contributing factor to success.
BOSTON SCIENTIFIC'S MICRO PERSPECTIVE
“With my knowledge of the medical industry, I'm already aware of all the biologic restrictions, but when you get to the specifics of micro injection molding, a knowledgeable partner is needed,” opines Isaac Ostrovsky, an engineer with Boston Scientific Corp (BSC), Natick, MA. For Ostrovsky's most recent project, a 0.04-0.05 in.-diam. element that gets inserted through the urethra, BSC teamed with Miniature Tool & Die, Charlton, MA, which claims to be the country's only vertically integrated mold maker and molder that exclusively specializes in micromolds.
“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 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.
“For us, everything turned out well and within budget by working with a micromolder,” says Ostrovsky. “It only took two weeks for MTD to make a working model of our design.