How to Protect Sensitive Electronic Equipment from Shock, Vibration, and Temperature

Shock, vibration, and excessive heating are a threat to any Electronic system design as they can quickly result in system failure. Also, excessive noise while operating will cause customer complaints and an inordinate number of repair calls. In addition, poor cooling can rack up costs.

Vibration and noise can come from an improperly mounted cooling fan. Air leaks around service panels and access ports can leak cold air, raising air temperature and lowering cooling efficiency in ventilation and air conditioning systems. Enclosures may rattle and vibrate due to mechanical resonances.

While noise, vibration, and temperature increases are almost inevitable, they need to be minimized. For this, designers can turn to energy-absorbing polyurethane foam gaskets, bumpers, and dampers. However, choosing the right material requires an understanding of their primary characteristics and performance capabilities.

This article will look at the key characteristics designers need to consider when choosing a dampening material, using 3M’s ISOLOSS LS polyurethane foam products as real-world examples. The article will show how ISOLOSS LS polyurethane foam products can be applied to protect critical equipment in the most demanding applications to save designers time and cost.

ISOLOSS LS polyurethane foam

3M’s ISOLOSS materials are fine-celled, high-density polyurethane foams. They are durable and energy absorbing with low compression set and consistent force deflection, and are useable over a broad temperature range. They are available in a variety of densities, thicknesses, and forms, including gasket strips, circles and squares, and square and rectangular sheets (Figure 1).

How to Protect Sensitive Electronic Equipment from Shock, Vibration, and TemperatureFigure 1: 3M ISOLOSS LS polyurethane foam is available in a variety of useful shapes ready for use as bumpers, gaskets, vibration isolators, and damping sheets. (Image source: 3M)

In addition to being available in a variety of shapes, ISOLOSS LS polyurethane foam is available in four different densities: 10 (160.2), 15 (80.1), 20 (320.4), and 25 (400.5) pounds per cubic foot (lb/ft3) (kilograms per cubic meter (kg/m3)). The densities are important in matching polyurethane foam to specific applications. All of these foam products operate over a temperature range of -40°C to +107°C (-40˚F to 228°F).

Polyurethane foam is used in three different classes of applications: gasketing, cushioning and support, and energy control. Gasketing requires the ability to seal gaps, absorb mechanical shock and vibration, and to provide a seal between mating surfaces. A gasket between a fan and an enclosure provides vibration isolation and provides a seal to prevent the loss of pressure. Cushioning and support involve isolating objects from each other, such as a bumper on a door closing a switch to monitor the door closure. The bumper cushions the switch, reducing the shock of the door closure. The smaller circular and square pads, like the LS-2506-PSA-1-CIRCLE-50PK or LS-2006-PSA-2-X2-50PK, are commonly used for such applications. Energy control involves reducing mechanical energy by impact or shock absorption and vibration damping.

Key characteristics of polyurethane foam

All of these applications depend on the ability of the foam to retain its shape and to provide a force against an object that is compressing it. The two specifications of polyurethane foam which measure these characteristics are compression set resistance, usually referred to as compression set, and compression force deflection (CFD).

Compression set is a measure of the permanent deformation of the foam after sustained compression. A low value of compression set indicates that the foam will return to its original thickness after recurring or continuous compression. 3M ISOLOSS LS foams experience less than 1% compression set at room temperature, per ASTM D1667, the standard specification for Flexible Cellular Materials.

ASTM D3574 D, which covers the standard test methods for flexible cellular materials, specifies the measurement of compression set. The material being tested is compressed to 50% thickness and exposed to high temperature for an extended length of time. Compression set is the percentage of the original thickness lost after compression is removed.

A typical application requiring good compression set resistance is in an air conditioner’s filter rack seal (Figure 2).

How to Protect Sensitive Electronic Equipment from Shock, Vibration, and TemperatureFigure 2: An ISOLOSS LS foam gasket with a low compression set seals the access door of an air conditioning filter rack, minimizing air leaks while holding the filter in place. (Image source: 3M)

Air filter racks use polyurethane foam with a low compression set to seal the filter housing and hold the filter in place. When the filter is removed for replacement or cleaning, the foam expands back to nearly its full thickness. The low compression set guarantees that the seal continues to maintain its performance, regardless of how long it was compressed. This application would use a gasket foam like 3M’s LS-1025LM/PSA-0.75 inch (in.) x 180 in.-1RL. The LS-1025LM/PSA is a 0.75-inch wide, 0.25-inch thick strip, with a 10 lb/ft3 density. This soft foam conforms to the filter and keeps it in place, while still sealing the door opening.

CFD represents the firmness of the foam at various degrees of compression. ASTM D3574C tests CFD by compressing the foam from 100% to 30% of its original thickness, that is 10% to 70% compression. As the foam is compressed, the force that the compressing surface applies to decrease the foam to a specific thickness is measured. It’s important to remember that this is also the force that the foam is exerting on the compressing surface. A plot of compression as a function of applied force is shown plotted in Figure 3. CFD tables and/or charts are provided for each of the ISOLOSS LS foam densities in order to fine-tune the foam selection process for each application.

How to Protect Sensitive Electronic Equipment from Shock, Vibration, and TemperatureFigure 3: A series of CFD plots for the four available foam densities (10, 15, 20, or 25 (lb/ft3). Increased force can be achieved by using a higher density foam or by using increased compression. (Image source: 3M)

Consider a cushioning application where two surfaces must be held apart by a 100 kPa (14.5 psi) pressure. This can be achieved using a 25 lb/ft3 foam compressed to about 16%, a 20 lb/ft3 compressed to about 28%, a 15 lb/ft3 compressed to about 50%, or a 10 lb/ft3 compressed to about 70%.

Vibration and noise damping

Structural damping is a means of eliminating mechanical energy by converting it to heat. Damping materials are applied directly to the surface of a structure using a strong adhesive (Figure 4).

How to Protect Sensitive Electronic Equipment from Shock, Vibration, and TemperatureFigure 4: ISOLOSS LS foam sheets attached to surfaces can provide damping of noise; they are compatible with a broad range of 3M pressure sensitive adhesives. (Image source: E.A.R. Division of 3M)

This free-layer damping system is the simplest form. Energy is dissipated as a result of extension and compression of the damping material due to the flexural stress from the base structure. Even with this simple system, properly designed damping treatments can produce remarkable results, especially for impact noise where reductions of 20 decibels (A-weighted) (dBA) or more are possible. Damping materials are available in square or rectangular sheets, as well as circular or square patches. These sheet materials can be die or laser cut for easy OEM assembly, or as service retro-fit kits. Coverage need not be total in order to be effective, and impact noise reductions of 10 dBA or more can be achieved with as little as 25% surface coverage. Larger sheet forms, such as 3M’s LS-1506/PSA-5″x7″-10PK and LS-1006LM-PSA-12″x12″-6PK, are useful in damping applications. Due to their flexibility, these foams are conformable to most product designs.

There are four factors that determine the amount of damping and noise reduction:

  1. The type and thickness of the base material.
  2. The thickness and characteristics of the damping material at the temperature and frequency of the operation.
  3. The ratio of the damping material thickness to that of the base material.
  4. Percentage of the surface area covered by the damping material.

Damping and vibration control techniques use the ability of the polyurethane foam to convert mechanical motion into low-level heat, thereby reducing noise and vibration levels. ISOLOSS LS polyurethane foams provide energy control for these applications, and they hold their shape and maintain form, fit, and function, even in harsh environments.

The full specifications of ISOLOSS LS polyurethane foam for the four available densities are summarized in Table 1. In addition to the key specifications of compression set resistance and compression force (load) deflection, the table lists the test standards used to qualify the foam material.

How to Protect Sensitive Electronic Equipment from Shock, Vibration, and TemperatureTable 1: Shown are the typical properties of ISOLOSS LS polyurethane foam for the four available densities. (Image source: 3M)

Conclusion

Vibration, shock, noise, and temperature extremes are a reality for many system designs, but the right damping material can greatly mitigate their effects. As shown, 3M’s ISOLOSS LS polyurethane foams come in a variety of shapes, densities, and thicknesses, and they tolerate a wide range of environments and offer long service lifetimes. They fit gasketing applications where they serve to seal openings and reduce vibration. In cushioning and support applications, they reduce shock and vibration while securing subassemblies. Finally, in damping operations, they are used to reduce noise.