An ultrasoft seal cushions delicate components and keeps dust from infiltrating displays in handheld electronics.
What is in this article?:
- Can you still see me now?
- GAP-FILLING SPECS (original foam thickness, 1mm)
- Compression force deflection versus Shore hardness
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.