Why is the car light casing made of plastic?
There are multiple reasons for this.
Plastic has stronger elasticity than glass, and the downstream buffer will not easily break under severe impact. Even if it breaks and falls on the road, it can cause strong harm to pedestrians, animals, and the like.
Lightweight, the plastic lamp housing is about 0.5-1.4 kilograms lighter than the glass lamp housing.
Corrosion resistance, good impact resistance, high transparency, good insulation, low thermal conductivity, good mechanical properties, strong plasticity, easy to form and process.
The application of plastic in car lights
1. PC (polycarbonate)
PC is widely used in car headlights. According to relevant data, we know that PC series materials account for about 50% (weight ratio) of plastic materials in headlights. In car lights, PC is mainly used for the lenses and external light distribution mirrors of headlights and fog lights.
Why PC? PC， Polycarbonate has excellent impact resistance and transparency, good mechanical and electrical insulation properties, wide temperature range, high dimensional stability, and high creep resistance.
The internal structure of car lights results in a certain temperature difference in different parts, and the temperature of some reflectors and decorative rings may reach 100-190 ℃. Therefore, PC and PC-HT are generally chosen when selecting materials.
2. PP (polypropylene)
The PP used in car lights is mostly modified PP, accounting for about 30% (by weight) of the plastic materials used in car lights. For example, the rear cover, decorative panel, and wiring harness guide groove (PP-T20) of the taillights, as well as the housing and rear cover of the headlights, are generally made of PP. Currently, PP is the main material for the headlight housing.
PP has strong electrical insulation and chemical corrosion resistance, as well as good fatigue resistance. Among general plastics, it has the highest mechanical properties and heat resistance. In addition, compared to other plastics, it also has a price advantage.
3.PBT/PET
PBT is generally used in the decorative rings of headlights and brackets in adjustment devices. PBT and ASA modified with glass fiber reinforcement can be used to make the shell of fog lamps. The mechanical properties are similar to PA and POM, with low friction coefficient, good self-lubricating properties, good thermal and chemical stability, excellent electrical insulation, high rigidity and hardness, good impact resistance, high dimensional stability, and good heat resistance.
The decorative ring material used to be mainly heat-resistant PC material, but in recent years it has gradually been replaced by PBT/PET. The prominent characteristics of PBT/PET car lamp materials compared to heat-resistant PC are high heat resistance, good processability, and low cost. The material has a high surface gloss; Has excellent processing stability and good hydrolysis resistance; Has excellent fluidity and heat resistance; Compared with heat-resistant PC, material prices can significantly reduce system costs, generally by about 40%. It should be noted that the above decorative ring materials need to be metalized using vacuum evaporation technology.
4. PA (Nylon)
The PA used in car lights is generally PA+GF, while the common screw adjustment or bracket in car lights uses PA6+GF or PA66+GF. Aromatic PA has higher heat resistance and strength than ordinary PA, and some important adjustment structures and internal brackets in car lights also use aromatic PA with fiberglass.
5.PMMA
PMMA， Commonly known as acrylic or organic glass, it is a highly transparent amorphous thermoplastic material. Excellent optical performance, with a transmittance of up to 90% -92%, a refractive index of 1.49, good weather resistance, high surface hardness, and excellent comprehensive performance, mainly used as optical transparent products. Light guiding materials for the exterior mirrors of taillights and interior ambient lights.
Okay, here's the problem! Why choose PC instead of PMMA for the headlight cover of a car?
PC and PMMA both have high transparency and good processing characteristics, but there are some differences in their application in car lights. Firstly, in terms of heat resistance, PC can generally reach up to 130 ℃, while PMMA only has around 100 ℃. Secondly, PC material has better toughness, while PMMA material is very brittle and easily broken by impact PC has a higher unit price than PMMA, and its flowability during forming is also inferior to PMMA. In general, the lampshade of the front headlights is made of PC, mainly due to the high illumination of the headlights, the large amount of heat emitted, and the fact that they are located in front of the vehicle, which requires a material that is both heat-resistant and impact resistant. The lampshade of the rear taillights is usually made of PMMA. Apart from the cheap factor, the rear taillights are usually a combination of single functional signal lights, which require red and transparent dual color alternation. Therefore, PMMA with good fluidity is the best choice.
6. ABS PC/ABS
In the field of car lights, ABS is generally modified with PC material before use, and the proportion of PC content in ABS is determined based on the differences in usage temperature and impact performance. ABS and PC/ABS are generally used for taillight housings and decorative panels.
7. BMC, PEI, PEEK, PPS, LCP and other materials
The headlight reflector is mainly made of materials such as BMC and PET. BMC is a new type of material that combines the advantages of thermosetting plastics and reinforced engineering plastics. During injection molding, it can flow uniformly at the same time and be molded in a very short period of time. It has high rigidity and strength, good molding shrinkage, and dimensional accuracy stability.
Based on the above characteristics of BMC, the reflector of the front headlight generally adopts BMC. The main function of the front headlights is to illuminate the road ahead and ensure the normal driving of the vehicle. The reflector, together with the corresponding light bulb, can emit two types of light: low beam and high beam. The low beam should have a clear boundary between light and dark and high illumination. Therefore, BMC's good molding shrinkage and dimensional accuracy stability can meet this requirement. The rear taillights are generally made of materials such as PET, PC, or directly molded from the lamp hub. The rear taillights are a combination of signal light functions, and their dimensional accuracy is not as high as that required for the front headlights. However, if the reflector can be directly seen from the lampshade, the reflective surface on the reflector must be designed beautifully to ensure a unique light shape after lighting.
3 Development Trends of Automotive Lamp Materials
In the future, car lights will develop towards simplicity, lightweight, high heat resistance, and environmental protection. For material selection, the following three points are the key points that car light materials need to strengthen and break through in the future.
1. Lens material that effectively and uniformly scatters light, with high light dispersibility;
2. Heat dissipation, using infrared transparent materials as the housing for headlights or fog lights;
The weather resistance of external headlights is gradually transitioning from 3 years to 5 years, requiring materials (coatings) to have better weather resistance and temperature resistance.
