The Best Fastener May Be No Fastener
October 23, 2012
Traditionally, the internal components of medical devices, computers and other electronic products are mounted to metal boards or trays using screws, wires or other fasteners. These board or trays are then mounted to the product’s outer housing using an additional set of screws, wires or fasteners. As a result, the assembly of the final product can become a logistical and manufacturing nightmare that is not only labor intensive, but often puts the product designers at odds with the teams responsible for sourcing the components and manufacturing the products. Each one of the screws and fasteners must also be purchased, inventoried and tracked which often places additional stress and demands on an already stressed system.
What if you didn’t need them?
For applications able to use molded expanded polypropylene (EPP), most if not all of the screws and fasteners can be eliminated. EPP is a closed-cell plastic foam bead produced by combining polypropylene resin with heat, pressure and CO2 in an autoclave. The result is a hollow bead with a polypropylene surface that has outstanding cushioning properties and a high strength to weight ratio.
The use of EPP as an interior chassis has been around since the 1980s, based off of the electronic packaging assembly concept (E-PAC) that was developed by Hewlett-Packard and later spun off to the German firm DMT for licensing and further development.
The foam can be molded into complex shapes using steam heat & pressure. Beads are introduced under pressure into an aluminum mold and are heated to their melting point using superheated steam. The heat causes the gas inside each bead to expand which allows the surface of the beads to fuse into one another. The finished parts are then cooled and ejected from the mold. Tooling prices are dependent on both the size and complexity of the part but generally range from $10,000 to $50,000 which makes this process a good option for applications with medium or high production volumes.
The resiliency of the material allows components such as printed circuit boards, fans and power supplies to be held in place using a friction fit, thus fewer screws and fasteners. The components are simply slid or snapped into place and then held in place by the foam.
Current applications utilize a sandwich approach with top and bottom foam pieces that lock the components between them. Instead of securing a cooling fan to a wall or chassis with screws, EPP can be used to sandwich the foam and hold it in place. The resilient nature of the foam will also dampen any vibration or noise created by the fan.
Assembly can also be simplified by press-fitting the components into the internal chassis. Press fitting makes it easier to disassemble for any service or maintenance that is needed or for full recyclability of the product at its end of life.
Design and Prototyping
Using foam to create the inner chassis of a product gives product designers more flexibility in their designs as they can decouple the functionality of the inner chassis and the outer housing. The rigidity and strength provided by the foam chassis reduces the strength required from the outer housing and allows 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 has excellent cushioning properties and will protect the internal components in case of drops and falls. A variety of textures can be molded into the foam and in certain applications such as solar hot water heater control panels; the EPP foam is used as the outer housing as well.
Another benefit of using EPP to create an internal chassis is the ability to mold different types of channels into the foam. Channels can be molded into the foam to provide runs for wires, cables and tubing to be routed. Ribs molded into the sides of these runs allow the wires, cables and tubing to simply snap into place during assembly without any further need for attachments. Channels can be molded into the foam to manage the airflow throughout a product and provide pinpoint cooling where it is needed. By directing the airflow within the chassis to precisely where it is needed, smaller fans can be used, possibly allowing a quieter and more energy efficient design. This feature is especially beneficial for portable devices.
Once a design has been roughed out, an initial prototype can be produced using a CNC milling machine to cut a billet of the EPP material. No special tooling is required to create the prototype. The components can be assembled into place on the prototype and modifications can be made directly to the foam using a razor knife and/or hot wire cutter. The improvements are then added to the CAD model and the next iteration of the prototype can be quickly made. It is not uncommon for product designers, as they become more comfortable with the material, to mock up proofs of concept from foam blocks using razor knives and hot glue guns.
Learning a New Paradigm
While advances in software have made the transition from 2D to 3D much simpler, in many cases, it is the manufacturing and sourcing teams that are the strongest advocates for the process because of the way it improves the manufacturability of the products and shortens the bill of materials.
For designers such a radical shift is like learning to ride a bicycle all over again, or perhaps more like teaching Lance Armstrong to ride a Harley. The biggest challenge in using EPP foam is often getting the product designers to think differently about how a product is designed and manufactured. Most product designers have always taken the use of fasteners as a given and as a result, have a great deal of experience and comfort with them. A foam concept requires designers to think very differently about how to design the product. They must shift their thinking from a 2D view to a 3D view in order to capitalize on the opportunities that the foam provides to reduce the number of parts.
This professional reticence among design engineers has contributed to the relative slowness of the technology to take hold. However recent improvements in the molding process and tooling technologies have allowed even more complicated shapes to be molded and have improved the surface finish to provide additional protection or greater cosmetic value. These sophisticated shapes further enhance the ability of the EPP to hold components in place without the need for additional screws or fasteners and simplify the process to assemble the final product. Perhaps such recent improvements will convince more design engineers to give foam a cut.