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Plastics Today, September 2015

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InjectIon MoldIng PlastIcstoday.coM global PlastIcs RePoRt 2015 41 The design engineer must also ask how the part will be ejected from the mold. In an initial design review, a part may have reasonable wall thickness and features, but the geometry may not allow ejector pins to remove the part from the mold. Thus, the engineer must take this factor into consideration, as well. Rapid prototyping vs. micro-molding One of the dilemmas associated with the rapid prototyping process is that it does not have the same restrictions as molding tooling. Rapid prototyp- ing can create features with oppos- ing undercuts or details that are not within the normal opening and closing direction of the mold. And since parts with such features would not release normally from the mold, a secondary side action—either of a mechanical, hydraulic or pneumatic nature—could be necessary. Alternatively, the mold could require some type of lifter—in essence, an ejector pin or feature that is inserted into the mold at an angle so that it moves away from the part as the part is being ejected. But if the part's features are under- cut, face each other, and are too close together, there is no real chance for the mold components to move to clear the undercut without hitting the oppos- ing mold components, which are also trying to move out of their undercuts. Sometimes, it is necessary to be creative in the tooling approach to work around such restrictions, making it incumbent on the design engineer to be cognizant of the challenges before getting too locked into a design. Know your design As soon as a part design is complet- ed—or even conceived—it should be reviewed immediately to determine whether it can withstand all the manu- facturability challenges it will face. The design engineers should review the mate- rial selection, the part geometry, the ability to fill the cavity with material, the ability of the mold to eject the part from the cavity, and the ability of all the mold components to sustain proper thermal control to achieve optimal cooling or heating of the cavity. One of the tools that can be employed during such reviews is MRPC's MoldFlow simulation, a method by which it is possible to test—from a theoretical or from a simulation standpoint—different gate locations, mold and melt temperatures, and material processing parameters. This method allows engineers to simulate the cavity-filling process, part warping as a result of residual stresses, cooling times, pack times, and pres- sures. Much work has been performed to correlate simulations with actual product results, thus allowing engi- neers to reasonably predict the robust- ness of the molding process. Jeff Randall is Vice President of Engi- neering at MRPC (Butler, WI), a manu- facturer of silicone, medical rubber, and thermoplastic components and assemblies for the medical device industry. This article originally appeared in the June 2014 issue of MD+DI. Five tips For micro-molding medical device components 1. Know Your L/D Ratio When micro-molding medical device parts, the design engineer must be aware of the length/ diameter (L/D) ratio. We know that we can micro- mold 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 prema- turely and lead the client down the wrong path. Nevertheless, such data from material suppliers can be used as a good starting point for deter- mining the viability of a design. 2. Beware of undercuts Another pitfall is undercuts. Wherever an under- cut appears, or wherever a hole exists that's not aligned with the normal opening and closing of the mold, side actions or secondary opera- tions 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. 3. Double-check your draft 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. 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, alterna- tive 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 differ- ent material may provide the engineer greater freedom in designing a part. 5. Keep it simple. Economies can be achieved by trying to design a part with multiple functions. However, engi- neers 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.

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