Molded polyurethane foam has long been used to package
components. Many design engineers, however, are not aware that properly
formulated foam and carefully designed foamed parts can also be used for thermal
management, acoustical insulation, vibration isolation and shock mitigation. In
addition, these inserts can be used to reduce the number of parts in an
assembly, thereby reducing assembly time.
Electro-mechanical assemblies require designs that take energy
management into account, whether the energy is thermal, acoustical or kinetic. Therefore,
the longevity of electro-mechanical components is closely tied to the
environment in which the equipment they are housed in operates.
An effective approach to efficiently removing heat from an
electronic assembly is by integrating ducted cooling in the assembly enclosure.
Typically colder is better, but the economics of the assembly dictates what is
considered cold enough. Forced convection heat transfer calculations can
quickly become rather complex, but it is intuitive that the cooler the airflow
across a hot surface, the greater the potential for cooling that surface.
Consider the possibility of dissipating the heat generated
in an assembly by directing the cooling airflow specifically across the heat
source. There will always be a need for general airflow in any enclosure
housing electronic components, but instead of over sizing the cooling fan so
that enough cool air will reach the hot components, a properly sized fan can
cool those components by concentrating streams of cooling air specifically on
those components. Problematic in enclosures is the mixing, or recirculation, of
warm air across hot components. A similar problem is the bypass of cooling air
into the exhaust stream. Ducting the cold air across the hot surfaces, then
into the exhaust stream can maximize efficient cooling. Designing channels,
ducts, into engineered molded foam inserts, can make this possible.
Noise caused by a cooling fan, the airflow and the
electro-mechanical components can also be effectively managed with an
engineered molded foam insert set. The default method used by engineers to attenuate
sound is to build an enclosure around the source to block it. Unfortunately,
most noise sources require openings in an enclosure to allow for airflow,
mechanical linkages, electrical wiring, etc. Openings in enclosures present
challenges acoustically since the omni-directional sound waves can easily
escape without being attenuated. Because of this it is crucial to eliminate
line-of-sight access to the noise source. One way to do this is to build a
baffle system that forces the sound waves to travel along a long, tortuous,
acoustically treated path.
A sound spectrum analysis of the components in an assembly,
as well as of the assembly as a whole, can reveal the problematic frequencies
that are propagating to the environment. From this analysis, the wavelengths of
the sound waves can be calculated and an acoustical attenuation package can be
designed by estimating the amount of mass needed to provide a measure of
transmission loss, the amount of absorption material needed to soak up some of
the acoustical energy, and the length of the air path needed to attenuate some
of the noise before it escapes. By designing air path channels in properly
formulated molded foam insert sets, these acoustical principles can be employed
to attenuate the generated sound.
Molded polyurethane foam is open cell foam with a skin of
variable thickness and density at the tool surface, and can be formulated to
provide a measure of transmission loss and acoustical absorption. There are
coatings that can be integrated on the foam surface that enhance the surface
characteristics of the foam as well. Typically, die-cut pieces of open-cell
polyurethane foam insulation, with specific acoustical properties, are
installed in enclosures as patchwork. A considerable reduction of the number of
parts involved can be realized, resulting in cost savings by making the
components easier to assemble and reducing inventory maintenance.
& Shock Dissipation
Engineered, molded open cell polyurethane foam provides
vibration isolation much the same way as it provides acoustical attenuation.
The structure borne energy is dissipated in the foam due to the cellular
interaction of the open cell foam. An engineered, molded foam insert set can be
utilized to isolate sensitive components from the vibrating assembly, or
isolate the vibrating components from the assembly.
Controlling the density of the foam is integral to providing
molded foam inserts that provide the desired damping. Another important
parameter to consider is the durability of the material providing the vibration
isolation. The skinning feature of molded polyurethane foam and the
availability of surface coatings can provide the wear surface required to
encapsulate the equipment while providing durability. Designing the inserts
properly can also eliminate the need for separate vibration mounts, reducing
the number of parts required and making assembly easier.
As with vibration isolation, the cellular structure of
molded open cell polyurethane foam helps dissipate the energy induced by
dropping or bumping an assembly. The longevity of electronic equipment can
often be enhanced by protecting the components from excessive movement and engineered,
molded foam inserts can be used to capture the components in an assembly in
shock absorbing material. Like the materials used for vibration isolation, the
density of the molded foam is crucial to providing the shock absorption