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Using Molded Foam Makes Assembly a Snap

Today, medical devices are made using a variety of plastic materials and manufacturing processes. Advances in plastic processing make it possible to obtain virtually any shape, form, or function. In addition, the vast assortment of plastics available allows designers to design for the optimal balance of functionality, performance, and cost. Expanded polypropylene (EPP) is a plastic material that is starting to gain traction in the medical device market as product designers become more familiar with the multiple benefits it can provide.

EPP is a closed-cell plastic foam bead that is produced by combining polypropylene resin with heat, pressure, and CO2

EPP is an excellent material choice for medical devices because it can be molded into complex shapes without losing its resiliency. An internal chassis can be molded from EPP that enables other components such as pumps, circuit boards, and fans to be held in place without the need for the additional fasteners, screws, or brackets that are normally required for assembly. (See Figure 1)

This method of assembly can significantly reduce total part count, lower overall product cost, and dramatically improve assembly time. It also makes it easier to disassemble the product for cleaning and service, or for recycling the product at the end of its life. Undercuts can also be easily molded into the foam without the need for core pulls, slides, or lifters. These undercuts can be used to route cables, wiring, and tubing by simply snap fitting them into the channels that have been molded into the chassis.

Using EPP to create the inner chassis of a medical device decouples the functionality of the inner chassis and the outer housing. The EPP chassis provides the structure and support required to house the other internal components, which reduces the strength required from the outer housing and often allows a simpler injection molded exterior part and/or thinner gauge materials to be used. In some cases, it also allows products with different interior configurations to be made without having to change the outer housing.

EPP is a good acoustical and thermal insulator. Depending on the frequency of the sound being generated, noisy components, such as compressors and pumps, can often be isolated in a cavity of EPP to help quiet them. As more medical devices move from the hospital to the home, the need for quieter operation is becoming even more critical. Due to its high R-value per inch, EPP can insulate sensitive components from extreme temperatures. A counterintuitive aspect of designing components with EPP is that even though it is a good insulator, it has the ability to provide more efficient cooling of a medical device through the use of molded-in air channels that direct the airflow to precisely where it is needed. In some cases, this increased cooling efficiency has enabled product designers to use smaller fans, which can provide longer battery life and quieter operation.

When designing parts in EPP, an additional benefit is the ease with which prototypes can be developed without the need for prototype tooling. Once a preliminary design is completed, a prototype can be created very quickly using a CNC machine to mill a billet of EPP. Other internal components can be assembled into place on the EPP prototype and modifications can be made directly to the foam using a razor knife and/or hot wire cutter. The CAD model is then updated with the manual improvements and the next iteration of the prototype can quickly be produced.

The biggest challenge to successfully using EPP foam is often getting medical device designers to think differently about how a product can be designed and manufactured. Capitalizing on the opportunities that EPP provides requires a paradigm shift in the way designers think about how to design products. However, once they become familiar with the material and the benefits it provides, it allows them to innovate in ways they never thought possible!

This article was written by Scott Novak, a Director of Marketing and Business Development at Sonoco Protective Solutions, Arlington Heights, IL, a division of Sonoco Products Company. For more information, visit http://info.hotims.com/49742-166. MD&M West, Booths 510, 5039