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Cell-phone designers rely on a range of urethane foams to seal and cushion delicate components from consumer abuse.

Poron Soft Seal urethane foam (bottom) conforms around 0.3 mm corners in an electronics enclosure better than does a competitor's foam (top).

It can be tough to reconcile a vision for nextgeneration electronic gadgetry with the realities of consumer abuse. Ever-shrinking and thinner housings on cell phones, media players, digital cameras, and PDAs must be robust enough to protect their internals.

The integration of larger, high-res LCDs (liquid-crystal displays) for video and viewing, for example, now push seal and gasket materials to their limits. LCDs must retain crisp resolution and color even after being bumped or dropped. Any breach in the seal around the LCD that lets dust enter can potentially make a display unreadable, ruin aesthetics, or damage sensitive electronics over time.

Space constraints and environmental regulations put designers under the gun to find new classes of materials for their seals and gaskets. The materials must manage damaging-impact energy yet be highly compressible so that they exert little stress on the thinnest of housings. Seals must also maintain integrity under compression to effectively keep out contaminants over an extended period and in a range of environments.

Sealing and gasketing designers now have another material option — a microcellular (opencell) urethane from Rogers Corp., Rogers, Conn. It is specifically designed for dust sealing and gap filling of sensitive electronic enclosures. The highly compressible Soft Seal grade joins the family of Poron urethane foams that comply with the recent European Union RoHS (Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) Directive.

Soft Seal features a 6-lb/ft3 density which lets designers build housings with a low closing force. This helps minimize the stress on enclosures with thin walls. The 1-mm-thick foam easily conforms around intricate contours. In one case, it handled 0.3-mm-high corners in an electronic housing that defeated a previous seal, which left a significant gap where dust and debris could easily enter.

Designers may question how well a thin, open-cell material such as Poron Soft Seal urethane foam can guard against dust and particulate penetration. The key lies in proper seal and housing design. All materials are permeable to some degree and sealability test results are not absolute.

Most sealing and permeability tests use the same basic technique. A sample is compressed by a controlled amount. Fluid or gas pressure is generated on one side of the material, and the rate-of-flow is measured on the other side. Tests can vary significantly depending on sample size and configuration, measurement accuracy and duration, pressure, chamber volume, and the fluid used. Measurement techniques are generally chosen to match end use application requirements as closely as possible.

Material suppliers, therefore, often devise their own tests to verify product performance based on a specific application. To see how well LCD panel gaskets stand up against dust penetration, Rogers Corp. built LCD panel mock-ups outfitted with gaskets made from Poron Soft Seal urethane and a competitor's foam. Both gaskets were 1-mm thick and pressed between two clear plastic panels at 30% compression.

The mock-ups went inside a bag filled with cornstarch and were tumbled once a week for over three weeks. The Soft Seal foam gasket effectively kept the cornstarch from entering the LCD mock-up panel. In contrast, the competitive foam gasket shows compromised sealing from compression set and rapid stress relaxation. The breach in the seal is seen as "blowouts" of cornstarch inside the LCD mock-up.

Lab tests show panel gaskets made from Poron Soft Seal urethane sealed better than a competitive foam during a three week test in which LCD mock-ups were tumbled once every week in a bag filled with cornstarch. Onemmthick gaskets made from Poron Soft Seal and a competitive foam were compressed 30% between two clear plastic panels. The Soft Seal gasket (on the top) shows no visible sign of cornstarch inside the LCD mock-up, whereas the competitor gasket (on the bottom) shows blowouts where the cornstarch entered due to gasket failure.

GAP-FILLING SPECS (original foam thickness, 1mm)
Final thickness, mm	Grade	Deflection, %	CFD, N/mm 2
0.1	Soft Seal	95	0.3312
0.15	Soft Seal	89	0.1237
0.2	92-09039 P	80	0.2264
0.2	Soft Seal	84	0.0.549
0.3	Soft Seal	74	0.0192
0.3	92-09039 P	74	0.0445
0.3	92-09039 P	74	0.0478
0.3	92-09039 P	74	0.1441
0.4	Soft Seal P	63	0.0101
0.4	92-09039 P	63	0.0191
0.4	92-09039 P	63	0.0168
0.4	92-09039 P	63	0.0423
0.5	Soft Seal	53	0.0066
0.5	92-09039 P	53	0.0111
0.5	92-09039 P	53	0.0094
0.5	92-09039 P	53	0.0194
0.6	Soft Seal	42	0.005
0.6	92-09039 P	42	0.0079
0.6	92-09039 P	42	0.0066
0.6	92-09039 P	42	0.0124
0.7	Soft Seal	32	0.004
0.7	92-09039 P	32	0.0063
0.7	92-09039 P	32	0.0053
0.7	92-09039 P	32	0.0094
0.8	Soft Seal	21	0.0033
0.8	92-09039 P	21	0.0053
0.8	92-09039 P	21	0.0044
0.8	92-09039 P	21	0.0076
For each thickness increment, several microcellular urethane options are listed that can fill the gap, in ascending order of compressive force required.

MAKE CONTACT
Rogers Corp., (860) 928-3622, rogerscorporation.com

Compression force deflection versus Shore hardness

Rogers Corp., Rogers, Conn., reports the hardness characteristics of its Poron family of urethane foams using ASTM D3574. This standard describes procedures for evaluating "Flexible Cellular Materials — Slab, Bonded, and Molded Urethane Foams." In this test method hardness is determined by Compression Force Deflection (CFD), Test C.

CFD is an engineering measurement expressed in pounds-force-per-square inch (psi). It is merely the force required to compress a material a fixed percentage. Rogers Corp.'s High Performance Foams Div. has chosen 25% compression to characterize various grades of Poron microcellular urethanes.

CFD measurements are a useful tool for making

material selections and are also available in compression values other than 25%. The conventional Shore hardness measurements provide only relative information. For example, a material with a Shore "A" 5 rating is softer than one with Shore "A" 10 but says nothing about how much compression force is needed to fill a gap or make a tight seal.

Poron Soft Seal urethanes withstand high strain percentage similar to that of the competitive foams, but require considerably less force to do so. Within typical force conditions for handheld electronic housings, it is the most compressible option.