“Vibration, shock, noise, and extreme temperatures are realities that many system designs must face, 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 have a long service life. They are suitable for packing applications for sealing openings and reducing vibration. In cushioning and support applications, they reduce shock and vibration while securing subassemblies. Finally, in shock absorption applications, they can be used to reduce noise.
Author: Art Pini
Shock, vibration, and overheating are a threat to any Electronic system design, as they can quickly lead to system failure. In addition, the high noise during operation will lead to excessive customer complaints and maintenance demands. In addition, improper cooling can also lead to increased costs.
Vibration and noise can be caused by improperly installed cooling fans. Air leaks around maintenance panels and access openings can lead to cold air leaks that can raise air temperatures and reduce the cooling efficiency of ventilation and air conditioning systems. The chassis may rattle and vibrate due to mechanical resonance.
While noise, vibration and temperature increases are almost inevitable, they need to be minimized. To do this, designers can use energy-absorbing polyurethane foam pads, bumpers, and dampers. However, the selection of suitable materials requires an understanding of their key properties and properties.
This article will use 3M’s ISOLOSS LS polyurethane foam product as an example to discuss the key characteristics 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 money.
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, and square and rectangular plates (Figure 1).
Figure 1: 3M ISOLOSS LS polyurethane foam is available in a variety of useful shapes and is readily available as cushions, liners, isolators and dampers. (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) (kg/ 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: packing, shock absorption and support, and energy management. Filling requires the ability to seal gaps, absorb mechanical shock and vibration, and provide a seal between mating surfaces. The gasket between the fan and housing provides vibration isolation and provides a seal to prevent pressure loss. Shock absorption and support involve isolating objects from each other, such as door-mounted bumpers that turn off switches, which can monitor door closing. The bumper cushions the switch, reducing the shock of the door closing. Small round and square pads like LS-2506-PSA-1-CIRCLE-50PK or LS-2006-PSA-2-X2-50PK are typically used for this type of application. Energy management involves reducing mechanical energy by absorbing shock and reducing vibration.
The main characteristics of polyurethane foam
All of these applications rely on the ability of the foam to retain its shape and provide a counterforce to the object that compresses it. Polyurethane foams have 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 the foam after continued compression. A low compression set value indicates 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 standard test methods for flexible cellular materials and specifies the measurement of compression set. The material being tested is compressed to 50% of its thickness and exposed to high temperatures for a long period of time. Compression set refers to the percentage of original thickness that is lost after compression is removed.
A typical application that requires good resistance to compression set is air conditioner filter housing seals (Figure 2).
Figure 2: A piece of ISOLOSS LS low compression set foam gasket seals the access opening of the air conditioner screen frame, minimizing air leaks when securing the screen in place. (Image source: 3M)
The air filter holder uses 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 almost to its full thickness. Low compression set ensures that the seal continues to maintain its performance no matter how long it has been compressed. This application would use a cushion foam like 3M’s LS-1025LM/PSA-0.75″ x 180″-1RL. LS-1025LM/PSA is a 0.75″ wide, 0.25″ thick tape with a density of 10 lb/ft3. This soft foam conforms to the screen and stays in place while sealing the access door.
CFD represents the firmness of the foam under various degrees of compression. ASTM D3574C tests CFD by compressing the foam from 100% of its original thickness to 30%, or 10% to 70% compression. When the foam is compressed, measure the force exerted by the compression face to reduce the foam to a specified thickness. It is important to remember that this is also the force the foam exerts on the compressive surface. Figure 3 shows a graph of compression versus applied pressure. CFD tables and/or charts are provided for each ISOLOSS LS foam density to allow fine-tuning of the foam selection process 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 foams or by increasing the amount of compression. (Image source: 3M)
Considering a shock absorption application, two surfaces must be kept apart with a pressure of 100kPa (14.5 psi). This can be achieved with 25 lb/ft3 foam to about 16%, 20 lb/ft3 to about 28%, 15 lb/ft3 to about 50%, or 10 lb/ft3 to about 70%.
Vibration and noise reduction
Structural damping is a means of removing mechanical energy by converting it into thermal energy. The damping material is attached directly to the surface of the structure using a strong adhesive (Figure 4).
Figure 4: ISOLOSS LS foam board applied to the surface provides noise reduction and is compatible with a wide range of 3M pressure sensitive adhesives. (Image credit: 3M EAR Division)
This free layer damping system is the simplest form. The expansion and compression of the damping material results in a dissipation of energy due to the bending stress of the base structure. Even such a simple system, with properly designed damping, can have a significant effect, especially for impulsive noise, with a reduction of 20 decibels (A-weighted) (dBA) or more. Damping materials are available in square or rectangular sheets, as well as round or square patches. These sheet materials can be die cut or laser cut for easy OEM assembly, or as maintenance retrofit kits. Coverage does not have to be full coverage to be effective, as long as there is 25% surface coverage, 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 into most product designs.
There are four factors that determine the amount of vibration reduction and noise reduction:
1. The type and thickness of the substrate.
2. Thickness and characteristics of damping material at operating temperature and frequency.
3. The ratio of the thickness of the damping material to the thickness of the substrate.
4. The percentage of surface area covered by the damping material.
Damping and vibration control technology utilizes the ability of polyurethane foam to convert mechanical movement into low-level heat, thereby reducing noise and vibration levels. ISOLOSS LS polyurethane foams provide energy control for these applications, allowing them to maintain their shape, fit and function even in harsh environments.
Table 1 summarizes the full specifications for the four available densities of ISOLOSS LS polyurethane foam. In addition to key specifications for resistance to compression set and compressive stress (load) strain, the table lists the test criteria used to qualify foam materials.
Table 1: Shows typical properties of the four available densities of ISOLOSS LS polyurethane foam. (Image source: 3M)
Vibration, shock, noise, and extreme temperatures are realities that many system designs must face, 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 have a long service life. They are suitable for packing applications for sealing openings and reducing vibration. In cushioning and support applications, they reduce shock and vibration while securing subassemblies. Finally, in shock absorption applications, they can be used to reduce noise.