By Jeff Randall

Tip 1: Know Your L/D Ratio

When micromolding medical device parts, the design engineer must be aware of the length/diameter (L/D) ratio. We know that we can micromold very small parts with thin walls. But there is a difference between molding a part that’s 0.005 in. thick and 0.020 in. long and a part that’s 0.005 in. thick and 1 in. long because length of flow versus material thickness comes into play. The appropriate ratio depends on the material a designer selects. While many material suppliers provide this information to customers, they tend to be somewhat guarded about providing it until they are familiar with the part design because they don’t want to commit to a material prematurely and lead the client down the wrong path. Nevertheless, such data from material suppliers can be used as a good starting point for determining the viability of a design.

Tip 2: Beware of Undercuts
Another pitfall is undercuts. Wherever an undercut appears, or wherever a hole exists that’s not aligned with the normal opening and closing of the mold, side actions or secondary operations are required to create the feature. While not a bad thing per se, creating such features tends to increase the cost of the tooling and makes it more challenging to cool or heat the tool uniformly. In addition, performing secondary operations can extend the cycle time and result in higher tooling-maintenance expenses.

Tip 3: Double-Check Your Draft
A word to the wise: When manufacturing micromolded components, keep the design simple. Draft—or the release angle from the mold—is another important consideration. Normal to the direction of the parting line, features need draft so that the part can be ejected from the mold as easily as possible. Although it may be possible to mold a part with a minimal amount of draft, the part may undergo stress when it is ejected from the mold, especially when it has features that tend to shrink onto a core. For example, without sufficient draft, parts with female features that shrink onto a core pin or with two ribs separated by a narrow space can be stressed when they are removed from the mold. Material selection also plays an important role in the draft required. Different grades of material shrink at different rates, and some materials are ‘stickier’ than others.

Tip 4: Choose the Right Material for the Job
There’s an old engineering adage: “When in doubt, build it stout of things you know about.” This advice is great, but it may not be ideal in every situation. Alternative materials may exhibit better processability and produce better parts than well-known materials. Thus, if the engineer is not already locked into a material because of regulatory or business constraints, alternative materials may improve processability while reducing part costs and increasing part quality. A rule of thumb, therefore, is to keep an open mind when selecting materials because a different material may provide the engineer greater freedom in designing a part.

Tip 5: Keep it simple.
Economies can be achieved by trying to design a part with multiple functions. However, engineers can sometimes box themselves in by attempting to do so. Thus, it might be better to design a couple of simple parts and marry them together through a secondary or overmolding operation than to create one multifunctional part. If you’re trying to do too much, you may be trying to do too much.

Jeff Randall is vice present of engineering at MRPC, a manufacturer of silicone, medical rubber, and thermoplastic components and assemblies for the medical device industry. He received degrees in mechanical engineering and business administration and a masters certificate in project management from the University of Wisconsin in Madison. Reach him at [email protected].