As medical device parts and features shrink, the micromolding processing window narrows, creating a host of manufacturability challenges.
By Jeff Randall
The proliferation of rapid prototyping and additive manufacturing has made it much easier for designers to produce prototypes or sample parts with material properties that more closely resemble those of production parts. In the past, the range of materials selected for rapid prototyping applications was very narrow and not very functional. But as these materials improve, designers are increasingly able to do much more testing than previously using prototypes.
However, while this capability represents a great technological advancement, it can also produce some false positives during the product development phase. It is possible to produce a rapid prototype, test it, and go too far in the process without realizing that the design is not very manufacturable on a larger scale. In fact, in some cases, a design cannot be micromolded at all; it can be produced only using rapid prototyping. Thus, this article is a cautionary tale directed at designers to ensure that they consider the manufacturability of a design before they go too far down the path and become too locked into a design that may not be reversible or susceptible to a redesign down the road.
The challenge associated with micromolding is that as features and parts shrink, the processing window—or the ability to mold parts with thinner and thinner geometries and smaller features—narrows. Thus, the manufacturability of a micromolded component must be considered even more carefully as parts approach the micro scale.
|A micromolded part measures less than 1 mm in length.|
One of the most important parameters that the designer should bear in mind when producing a micromolded part is material selection: What will the final material be? Today, it isn’t as simple as saying that the part should be made from PEEK, for example, because different grades of PEEK are available, some of which exhibit better flow characteristics than others. Using a certain grade of PEEK may make the part easier to mold, but it may also compromise its physical properties. Thus, when the engineer considers the design of the part, it is also necessary to consider the material’s processability and the design features that relate to this processability.
Designing Molds for Manufacturability
In addition to carefully selecting the right material for the job, the design engineer should consider the features of the mold, or tooling, used to produce the component. As the component shrinks and incorporates finer and finer details, the mold’s features tend to shrink as well, necessitating greater precision. Associated with the mold and all of its components is the need to ensure proper thermal, or temperature, control—regardless of whether the material in question is a plastic, liquid silicone, or other material. Like the part itself, mold components are getting smaller and smaller, making it increasingly difficult to heat or cool them and to provide a consistent molding environment.
With many medical device components, materials such as stainless steel are often used to manufacture the molds. Because such molds do not corrode or rust, they do not require the use of rust-preventive chemicals. This is hugely advantageous because it eliminates the possibility that the chemical can leach into the part. However, a downside to using stainless steel molds is that they do not offer good thermal conductivity, which becomes even more challenging to achieve as the part and the mold details shrink.
Another consideration for designers of micromolded parts is where the part will be gated, or filled. The point at which material enters into the mold cavity is known as the gate. The problem with the gate is that it may leave a small vestige of material on the part, which must either be acceptable in the final product or must be removed through secondary processing. This, of course, adds cost to the product. The location of the gate can also result in localized effects that can change the part’s physical properties. For example, the pressure required to cause the material to flow into the mold cavity through the restricted gate can result in concentrated stress, increasing the shear in the gate area and producing a potentially defective part.
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. Micromolding
One of the dilemmas associated with the rapid prototyping process is that it does not have the same restrictions as molding tooling. Rapid prototyping can create features with opposing 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 undercut, 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 opposing 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 completed—or even conceived—it should be reviewed immediately to determine whether it can withstand all the manufacturability challenges it will face. The design engineers should review the material 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 pressures. Much work has been performed to correlate simulations with actual product results, thus allowing engineers to reasonably predict the robustness of the molding process.
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 firstname.lastname@example.org.