1. The background of anti-fogging for car lights
Anti-fogging of car lights is an old topic. It is a major quality problem that plagues lighting manufacturers, vehicle manufacturers and end customers. Today, with the widespread use of LED light sources and the booming development of interactive control systems, car lights have become more electronic, digital and integrated, their shapes have become slimmer and more complex, and anti-fog requirements have become higher. It has also become a major problem faced by car light design.
The mechanism of fog generation, leaving aside the professional terms, simply put is: the ability of the same volume of air to withstand water vapor is different when the ambient temperature is different. The higher the temperature, the more water vapor it can withstand. big. When the ambient temperature suddenly drops, water vapor that cannot be contained by the air will condense into liquid water.
When the car lights are on, the temperature and pressure differences in different parts of the car light rise at inconsistent rates, causing the gas in the lamp cavity to flow and exchange with the outside world. When the external humidity is high, the air humidity in the entire lamp cavity increases. After the high-temperature and high-humidity gas enters the low-temperature area of the lamp, the ambient temperature drops, and the water vapor condenses and forms fog under the action of condensation nuclei.
2. The mechanism of fog formation in lamps
Through the analysis of the influencing factors of fog in lamps, it can be briefly summarized that the fogging of lamps is mainly related to the temperature field, flow field and humidity field. The relationship between the three and the influencing factors are as shown in the figure:
When the product shape and materials of each component are fixed:
The temperature distribution inside the lamp is related to the power of light sources such as bulbs and LEDs and the external ambient temperature.
The flow field is divided into two types: internal flow and external exchange. Internal flow is caused by factors such as natural convection caused by uneven temperature distribution inside the lamp and gaps between components. The ventilation structure in external exchange is mainly determined by factors such as the number, size and arrangement of ventilation holes, and the type of ventilation components.
The humidity field is mainly related to the air humidity inside the lamp cavity and the humidity of the outside air. The flow of internal air changes the humidity distribution.
3. Dangers of fog
Poor appearance, customer complaints, resulting in returns and exchanges, and increased quality costs
Affecting the optical performance of the lamp, water vapor condenses on the surface of the light distribution mirror, and refraction causes the light to deviate
Affects the life of electronic components. There are more and more electronic components in car lights, and they are becoming more and more complex. Water vapor condenses on the electronic components, causing component failure in the long term.
4. New challenges faced by anti-fog
1. Extensive use of LED light sources
The heating mechanisms of LED light sources and halogen bulbs are fundamentally different. The light efficiency of halogen bulbs is about 5%. 85% of the heat is emitted in the form of infrared radiation, which is beneficial to the balanced distribution of heat within the lamp.
The light efficiency of LED is about 20%. LED is a cold light source. About 75% of the heat is conducted to the substrate through thermal conduction at the solder joints, and then dissipated. The heat is concentrated, which is not conducive to the distribution of heat in the lamp.
2. The shapes of lamps are getting flatter and slimmer, and the space is getting smaller and smaller.
The shape of lamps is becoming more and more streamlined and flat, and the separation of decorative parts such as rings and strips is becoming more and more detailed. The structure of lamps is complex and the space is compact, which is not conducive to the flow of air in the cavity of the lamp.
3. The requirements of OEMs have become higher
The OEM's experimental standards for fogging have been improved. The fog dissipation time has been greatly improved from the previous allowable existence, half an hour to one hour of dissipation, to the current fog that is not allowed to appear.
5. 6 ways to solve lamp fog
It can be seen from the conditions for the formation of lamp fog: lamp fogging is mainly related to the humidity, temperature distribution and condensation nuclei in the lamp. Therefore, to solve the problem of car lamp fogging, we need to start from these three aspects and limit one or more of the conditions. This effectively solves the problem of fogging of car lights.
1. Reduce the humidity inside the lamp
For lamps with good sealing performance, reducing the humidity inside the lamp can be considered from three aspects: the selection of lamp materials, breathable components and the installation of dehumidification devices inside the lamp.
2. Water absorption rate of car light materials
Try not to use materials with high water absorption, such as: avoid using PA series materials for headlight lens brackets, bulb holders and other parts.
3. Selection of breathable components
breathable membrane
vent cap
Function: Isolate liquid water and dust, allowing gas (including water vapor) to pass through. The breathable membrane allows a larger breathable area and is in the form of a stick; the vent cap allows a smaller breathable area and is in the form of a set.
4. Install a dehumidification device inside the lamp
Installing a dehumidification device inside the lamp, such as pasting desiccant, can effectively reduce the humidity inside the lamp, thereby reducing the risk of fog. However, this method also has disadvantages. The physical hygroscopic desiccant used generally tends to be saturated as the amount of moisture absorption increases. After being heated when saturated, it will release the moisture it absorbs into the lamp, aggravating the fogging phenomenon.
5. Balance the temperature field and flow field in the lamp
The temperature field and flow field within the lamp play a major role in fog. The structural design of the lamp will affect the temperature field and flow field inside the lamp. Generally, the following measures can be taken to prevent it in the design:
The utility model avoids the formation of a narrow structure between the car light bezel and the light distribution mirror area, especially the lower part of the lamp. The narrow area is not conducive to the circulation of gas in the lamp, and at the same time meets the heat resistance conditions of the car lamp.
Try to avoid large decorative areas that are not directly illuminated by the light source; or make patterns on the inner surface of the light distribution mirror to reduce the impact of fog on the appearance.
A certain gap should be left between the upper and lower parts of the lamp to facilitate the flow of gas; generally, narrow gaps between the front (frame and mask) and the back (frame and lamp housing) of the lamp bezel should be avoided, as narrow gaps will hinder the Gas flow.
CAE engineers should intervene early in the car lamp design stage and use finite element analysis software to analyze and predict the fogging area and risk level of the car lamp. By analyzing the internal temperature field and flow field of the car lamp, they can optimize the design and avoid The phenomenon of too low temperature and gas flow dead zone occurs in the low temperature area of the light distribution mirror, which effectively prevents or solves the problem of lamp fogging.
The cooling fan is mainly used to dissipate heat from the LED, and greatly improves the air flow and heat distribution inside the lamp, thereby bringing additional benefits such as fog improvement.
6. Reduce condensation nuclei
The method of spraying an anti-fog coating on the inner surface of the lampshade is usually used to reduce condensation nuclei.
Mechanism of action: Make water droplets lie flat on the surface of the substrate (the surface contact angle becomes smaller), forming a very thin water film, preventing condensation into water droplets on the surface of the substrate, reducing the scattering of light on the surface of the water droplets, and increasing the light transmittance .
Contact angle of water droplets on the surface of the substrate before and after using anti-fog coating
Since the anti-fog coating is sprayed on a transparent material, if there are quality defects, it cannot be processed by grinding and polishing and then spraying again. Relatively speaking, the product scrap rate is high. At the same time, the spraying environment of the anti-fog coating is relatively high. , will increase the cost of lamps more than other solutions.
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