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Dow Chemicals Two-Component Polyurethane Foam Improves Noise

Jun 04, 2018

Noise, as the culprit of the destruction of people's working and living environment quality, will not only seriously endanger the human auditory system, make people tired and deaf, but also accelerate the aging of mechanical structures, affecting the accuracy and service life of equipment and instruments. Car noise can be divided into interior noise and exterior noise. Noise outside the vehicle affects the surrounding environment and interferes with people's sleep and work; interior noise affects the physical and mental health of drivers and passengers, driving safety, and ride comfort. Through scientific research and long-term practice, it has been proved that noise can cause the driver's nervous system function to decline, which not only damages the driver's hearing, but also causes the driver to quickly fatigue, posing a great threat to the driving safety of the vehicle. Therefore, the hazard of noise should arouse people's great attention.

In order to improve the comfort of the vehicle, major automobile companies in the world have established strict control standards for the noise level in the car, and control the noise in the car as an important research direction. Especially for cars, the noise status in the car is one of the standards for measuring the grade of cars. The level of motion noise of cars directly reflects the quality level of the car. In recent years, some domestic large-scale automobile plants have continuously strengthened the research and control of noise in the vehicle. Not only has the vibration and noise department been set up specifically, but many high-level talents have been introduced and cultivated, and a large amount of money has also been invested in the introduction of test equipment.

Great Wall Motor mainly produces pickups, SUVs and cars, which are in a period of rapid development. Great Wall Motors attaches great importance to NVH. It participates in NVH work during the development phase, coupled with the effective integration with CAE, and truly creates value-for-money automotive products. This article describes a case in which Great Wall Motor's best-selling car model uses Dow Chemical's two-component polyurethane foam to block the transmission of body noise to improve interior noise.

In-car noise analysis

NVH is an abbreviation for noise, vibration, and harshness (Noise, Vibration, and Harshness). It focuses on the effects of vehicle noise and vibration on vehicle performance and comfort, and related control improvement techniques.

1. The main source of noise in the car

All vibration objects that radiate noise to the surroundings are called noise sources. A car is a complex combined noise source that moves at high speed. The vibration caused by the operation of automobile engines and transmission systems, the rolling of automobile tires on the ground during high-speed driving, and the role of the vehicle body and air are the root causes of car noise. Specifically, the noise sources of automobiles mainly include the following aspects:

(1) Engine noise includes intake noise, exhaust noise, cooling fan noise, and structural noise generated during operation of the engine, and vibration due to engine combustion and inertial moments. The body is induced by the engine suspension and the sub-frame transmission body. Vibration, this type of noise is characterized by low and medium noise.

(2) Road Noise Road surface roughness causes the vehicle body to vibrate through suspension, etc. This type of noise is characterized by low-frequency noise.

(3) Tire noise Tire noise is generated when the tire is rolling on the ground and the air in the groove is caused by the pumping effect generated by the surface extrusion and re-inhalation process and tire vibration. This type of noise is characterized by low and middle frequencies. noise.

(4) The surface pressure pulsation caused by the action of the wind noise, high-speed air flow and the surface of the car, and the air flow passing through the unclosed body and the entire vehicle passage to the cockpit generate aerodynamic noise, which is characterized as high-frequency noise.

2. The way to spread noise inside the car

There are two ways to transmit noise into the car. They are through the body structure and aerodynamic transmission.

3. Reduce interior noise through cavity blocking

According to the mechanism of noise generation and propagation, noise control techniques can be divided into the following three categories: control of noise sources, control of noise propagation paths, and protection of noise receivers. This article focuses on the use of control measures in the transmission of noise, such as sound absorption, sound insulation and other measures.

When the vehicle runs at high speed, the noise caused by the pressure change caused by the separation of the airflow around the vehicle body is what is commonly referred to as wind noise. The vehicle interacts violently with the surrounding air flow field, and the flow field forms a boundary layer on the surface of the car. Generates strong separation, turbulence and turbulence. In addition, the modern mainstream car body design adopts a unitized body. The body side panel is formed by welding inner and outer thin-walled steel plates, and a bypass cavity structure exists so as to generate a high-speed airflow field in the side-cavity channel. Is commonly referred to as cavity resonance noise; At the same time, engine noise, tire noise, road noise and even dust through the side cavity channel to the vehicle. Therefore, cavity blocking and body sealing are particularly important for reducing interior noise.

The study pointed out that airborne noise is proportional to the 6th power of the vehicle speed, that is, the vehicle speed is doubled and the sound pressure level is increased by 18dB. When the car starts, it generates mechanical motion noise, and the engine noise increases with the increase of the vehicle speed. When the vehicle speed is greater than 50km/h, the tire noise gradually appears. When the vehicle speed exceeds 80km/h, the tire noise becomes the noise of the car. The main component; When the car speed exceeds 100km/h, the noise of the high-speed airflow field will rapidly increase; When the car speed reaches 120km/h, it has the same sound pressure level as the tire noise. When the car speed continues to increase, such noise It will exceed other noise as the main source of noise. Therefore, it is very important to block the air flow in the side cavity to suppress the noise from propagating through the air.

Mechanism of Two-Component Polyurethane Foaming Material in Cavity Blocking

Two-component polyurethane foams can be quickly reacted and foamed at room temperature and rapidly formed. When applied to side-by-side bypass cavity seals, it is a very effective technique to block air flow and suppress air passage. While blocking the passage, the polyurethane foam material has an internal structure of a porous sound-absorbing material, that is, has many tiny gaps and continuous air bubbles, due to the internal friction of the material itself and the air and pore walls in the material pores. Friction, so that the sound energy is clearly absorbed and attenuated, this sound absorption material can effectively absorb the sound energy incident on it, which makes it have good high-frequency sound absorption performance. This is because when acoustic waves are incident on the surface of a porous material, there are mainly two mechanisms that cause the attenuation of acoustic waves: First, vibrations caused by acoustic waves cause movement of air within the pores or gaps, causing friction with the pore walls, close to the pores. The air on the walls and fiber surfaces is not easily affected by the pore walls. Due to friction and viscous forces, a considerable part of the sound energy is converted into heat energy, which causes the sound waves to attenuate and the reflected sounds to weaken to achieve the purpose of sound absorption. Second, the small The heat loss caused by the heat exchange between the air in the hole and the wall of the hole and the fiber also attenuates the sound energy.

Therefore, the polyurethane foam material not only has the advantage of three-dimensional expansion and reliable blocking, but also has good sound absorption characteristics that the former does not have, so that it is inhibited. Noise shows excellent performance through the airborne route. Figure 1 shows an anatomical view of the side panels of a partially polyurethane foamed material and a two-dimensional expansive spacer.

The advantages of two-component polyurethane foams in cavity blocking

The two-component polyurethane foam material is applied to the cavity blocking and has the following advantages: three-dimensional reliable sealing; good sound absorption performance; multiple models can share the injection device of polyurethane foam material; no mold investment, the material cost is low; The injection mode can be adjusted at any time; it can be directly extended to future vehicle models without remodeling injection equipment.

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