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.