How to avoid three hazards in electronic devices: shock, vibration and overheating

Latest update time：2024-11-06

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Shock, vibration and overheating are threats to any electronic system design as they can quickly lead to system failure. In addition, noisy operation will lead to excessive customer complaints and repair calls. In addition, improper cooling will also lead to increased costs.

Vibration and noise can be caused by improperly installed cooling fans. Air leaks around maintenance panels and access ports can cause cold air to leak out, which raises the air temperature and reduces the cooling efficiency of ventilation and air conditioning systems. The chassis may make noises and vibrate due to mechanical resonance.

While noise, vibration and temperature rise are almost unavoidable, they need to be minimized. To do this, designers can use energy-absorbing polyurethane foam pads, cushions and dampers. However, choosing the right materials requires understanding their key characteristics and properties.

This article will use 3M 's ISOLOSS LS polyurethane foam products as an example to explore the key characteristics that designers need to consider when selecting damping materials. The article will show how ISOLOSS LS polyurethane foam products can be used in the most demanding applications to protect critical equipment while saving designers time and costs.

ISOLOSS LS polyurethane foam

3M’s ISOLOSS material is a fine-cell, high-density polyurethane foam. It is durable, energy-absorbing, has low compression set and stable stress strain, and can be used over a wide temperature range. They are available in a variety of densities, thicknesses, and shapes, including spacer strips, round and square shapes, and square and rectangular plates (Figure 1).

Figure 1: 3M ISOLOSS LS polyurethane foam comes in a variety of useful shapes and is ready for use as cushions, pads, isolators and damping sheets. (Image source: 3M)

In addition to a variety of shapes, ISOLOSS LS polyurethane foam is available in four different densities: 10 (160.2), 15 (240.4), 20 (320.4) and 25 (400.5) pounds per cubic foot (lb/ft3) (kilograms per cubic meter (kg/m3)). Density matching is important when matching polyurethane foam to a specific application. All of these foam products have an operating temperature range of -40°C to +107°C (-40 ˚ F to 228 ˚ F).

Polyurethane foams are used in three different application categories: padding, shock absorption and support, and energy control. Padding requires the ability to seal gaps, absorb mechanical shock and vibration, and provide a seal between mating surfaces. Pads between fans and housings provide vibration isolation and provide a seal to prevent pressure loss. Shock absorption and support involve isolating objects from each other, such as a cushion installed on a car door that closes a switch so that the closing of the door can be monitored. The cushion cushions the switch and reduces the shock of the door closing. Small round and square pads, such as the LS-2506-PSA-1-CIRCLE-50PK or LS-2006-PSA-2-X2-50PK , are commonly used in this application. Energy control involves reducing mechanical energy by absorbing shock and reducing vibration.

Main properties of polyurethane foam

All of these applications rely on the foam's ability to maintain its shape and provide a counterforce to the object that compresses it. Polyurethane foam has two specifications that measure these properties: resistance to compression set, commonly referred to as compression set, and compressive stress strain (CFD).

Compression set is a measure of the permanent deformation of a foam after sustained compression. Low compression set values indicate that the foam will return to its original thickness after repeated or continuous compression. According to ASTM D1667, Standard Specification for Flexible Cellular Materials, 3M ISOLOSS LS Foam has a compression set of less than 1% at room temperature.

ASTM D3574 D covers the standard test method for flexible porous materials and specifies the measurement of compression set. The material under test is compressed to 50% of its thickness and exposed to elevated temperature for a long period of time. The compression set is the percentage of original thickness lost after the compression is removed.

A typical application that requires good compression set resistance is the filter holder seal for air conditioners (Figure 2).

Figure 2: An ISOLOSS LS low compression set foam gasket seals the access opening of the air conditioning filter holder, minimizing air leaks while securing the filter in place. (Image source: 3M)

Air filter holders use low compression set polyurethane foam to seal the filter housing and hold the filter in place. When the filter is removed for replacement or cleaning, the foam expands to nearly its full thickness. The low compression set ensures that the seal continues to maintain its performance no matter how long it is compressed. This application would use a cushioning foam like 3M's LS-1025LM/PSA-0.75 " x 180"-1RL. LS-1025LM/PSA is a 0.75" wide, 0.25" thick strip with a density of 10 lb/ft3. This soft foam conforms to the filter and holds it in place while also sealing the access door.

CFD represents how firm a foam is at different levels of compression. When testing CFD per ASTM D3574C, the foam is compressed from 100% to 30% of its original thickness, which is a 10% to 70% compression. As the foam is compressed, the force applied by the compression surface to reduce the foam to a specific thickness is measured. It is important to remember that this is also the force that the foam is applying to the compression surface. Figure 3 shows a graph of the amount of compression versus the applied pressure. CFD tables and/or graphs are provided for each ISOLOSS LS foam density so that the foam selection process can be fine-tuned for each application.

Figure 3: A series of CFD plots for the four available foam densities: 10, 15, 20, or 25 (lb/ft3). Pressure increases can be achieved by using higher density foam or by increasing the amount of compression. (Image source: 3M)

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

Vibration and noise reduction

Structural damping is a means of removing mechanical energy by converting it into heat. Damping materials are attached directly to the surface of the structure using strong adhesives (Figure 4).

Figure 4: ISOLOSS LS foam sheets attached to the surface can provide noise reduction and are compatible with a wide range of 3M pressure-sensitive adhesives. (Image source: 3M EAR Division)

This free layer damping system is the simplest form. The extension and compression of the damping material due to the bending stress of the underlying structure results in the dissipation of energy. Even this simple system can produce significant results if the damping treatment is designed properly, especially for impact noise, where a reduction of 20 decibels (A-weighted) (dBA) or more can be achieved. The damping material is available in square or rectangular sheets, as well as round or square patches. These sheets can be die-cut or laser-cut to facilitate OEM assembly or as a maintenance retrofit kit. Full coverage is not necessary to be effective, as long as 25% surface coverage is achieved, an impact noise reduction of 10 dBA or more can be achieved. Larger sheets, 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 can fit most product designs.

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

Type and thickness of substrate.
The thickness and properties of the damping material at the operating temperature and frequency.
The ratio of the thickness of the damping material to the thickness of the substrate.
The percentage of surface area covered by damping material.

Damping and vibration control technologies utilize the ability of polyurethane foams to convert mechanical motion into low-level heat, thereby reducing noise and vibration levels . ISOLOSS LS polyurethane foams provide a means of energy control for these applications, allowing them to maintain their form, fit and function even in harsh environments.

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

Table 1: Shown are typical properties of the four available densities of ISOLOSS LS polyurethane foam. (Image source: 3M)

Conclusion

Vibration, shock, noise and extreme temperatures are a reality of many system designs, but the right damping materials can greatly mitigate their effects. As mentioned above, 3M's ISOLOSS LS polyurethane foams are available in a variety of shapes, densities and thicknesses to tolerate a variety of environments and provide a long service life. They are suitable for caulking applications to seal openings and reduce vibration. In cushioning and support applications, they can reduce shock and vibration while securing subassemblies. Finally, in shock absorption applications, they can be used to reduce noise.

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