PT_Plastics Today

Plastics Today, September 2015

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InjectIon MoldIng 40 global PlastIcs RePoRt 2015 PlastIcstoday.coM Micro-molding 101: Ensuring manufacturability As parts and features shrink, the micro-molding processing window narrows, creating a host of manufacturability challenges. Jeff Randall, MRPC T he proliferation of rapid prototyp- ing 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 test- ing than previously using prototypes. 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 micro-molded at all; it can be produced only using rapid proto- typing. 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 micro- molding is that as features and parts shrink, the processing window—or the ability to mold parts with thinner and thinner geometries and smaller fea- tures—narrows. Thus, the manufactur- ability of a micro-molded component must be considered even more carefully as parts approach the micro scale. Materials One of the most important parameters that the designer should bear in mind when producing a micro-molded 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 bet- ter flow characteristics than others. Using a certain grade of PEEK may make the part easier to mold, but it may also compromise its physical prop- erties. 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 pre- cision. Associated with the mold and all of its components is the need to ensure proper thermal, or temperature, con- trol—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 compo- nents, materials such as stainless steel are often used to make 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 con- ductivity, which becomes even more chal- lenging to achieve as the part and the mold details shrink. Another consideration for designers of micro-molded 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 ves- tige 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 poten- tially defective part. A micro-molded part measures less than 1 mm in length.

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