Headlights are indispensable components of automobiles, which are related to lighting, signal and decoration of automobiles. After years of development, degree of plasticization of headlight production has reached more than 96%.
Main components of automobile headlights are composed of reflectors, lenses, shells, light sources, electronic circuits, and installation accessories. At present, main plastics used in headlights in my country are PC, PP, ABS, PC/ABS, PMMA, PBT, PET, PA, BMC and a few other plastic materials. Plastics have advantages of light weight, corrosion resistance, good impact resistance, high transparency, good insulation, low thermal conductivity, good mechanical properties, strong plasticity, and molding processing. They are increasingly widely used in various fields.

Polycarbonate (PC) is a high molecular polymer containing carbonate groups in molecular chain. Polycarbonate is known as bulletproof glue due to its excellent mechanical properties. It has characteristics of high mechanical strength, wide operating temperature range, good electrical insulation performance (but arc protection performance remains unchanged), good dimensional stability, and good transparency. It is widely used in car lights and other fields.
PC accounts for about 50% of plastic materials in headlights, is mainly used in reflectors, aluminum-plated decorative parts and masks of headlights and fog lights. PC has a good bonding strength with aluminum-plated layer, and can be directly aluminum-plated on PC substrate.

 

PC reflector

 

PC aluminum-plated decorative strip

Polypropylene is referred to as PP, and modified polypropylene is mostly used in car lights. Modified PP has strong electrical insulation and chemical corrosion resistance, good fatigue resistance, the highest mechanical properties and heat resistance among general plastics, and has obvious price advantages compared to other plastics.
Modified PP accounts for 30% of plastic materials of car lights, and can be used for rear cover, trim and wire harness guide of taillight, lamp body and rear cover of headlight.

 

Lamp body

PMMA, commonly known as acrylic or organic glass, is a transparent non-crystalline plastic. It has good mechanical properties, is strong and rigid at room temperature, has a light transmittance greater than 92%, and only drops to 89% after 10 years of outdoor use. It remains unchanged after long-term exposure to outdoors. It has excellent properties such as good weather resistance, high mechanical strength, and good machining performance. It can also resist corrosion from a variety of chemicals and is mainly used for optically transparent products.
In the field of automotive lighting, it can be used for taillight masks and light-guiding materials.

ABS is a terpolymer composed of acrylonitrile, butadiene, and styrene. In the field of automotive lighting, ABS is generally used after modification with PC materials, taking into account advantages of both materials. Long-term heat resistance temperature can reach 110~120℃, and PC ratio in ABS can also be adjusted according to use temperature and impact performance. ABS and PC/ABS alloys are generally used for rear light housings and trims.
Front and back of PC/ABS taillight shell

PBT is commonly known as polybutylene terephthalate. Modified PBT has good mechanical properties (glass fiber reinforced PBT accounts for more than 70% of market), similar to PA and POM, with small friction coefficient, good self-lubrication, good thermal stability and chemical stability, excellent electrical insulation, high rigidity and hardness, good impact, high dimensional stability and good heat resistance.
PBT is generally used in headlights for decorative rings and brackets in adjustment devices. PBT and ASA can be used as fog lamp shells after modification and glass fiber reinforcement.

Commonly known as polyester, it is collectively referred to as thermoplastic polyester or saturated polyester with PBT. In the past, decorative ring material of headlights has been mainly heat-resistant PC plastics, which have been gradually replaced by PET in recent years. Compared with heat-resistant PC plastics, outstanding features of PET headlight materials are high heat resistance, good processability and low cost.
Surface gloss of material processed parts is high; it has excellent processing stability and good hydrolysis resistance; it has excellent fluidity and heat resistance; material price can significantly reduce system cost compared with heat-resistant PC.

PA polyamide, commonly known as nylon, is a large type of plastic. Ordinary screw adjustment or brackets in car lights use glass fiber reinforced nylon 6 or glass fiber reinforced nylon 66.

 

Aromatic PA is a special nylon with higher heat resistance and strength than PA6 or PA66. It is used in some important adjustment structures and internal brackets in car lights. PA+50% GF material is used for dimming brackets in headlight bodies. There is also PA66 used for dimming nuts, mounting nuts and ball nuts in headlight internal parts.

 

Aromatic nylon lamp body dimming bracket (+50% GF)

PEI, polyetherimide, referred to as PEI, is an amber transparent solid. It has inherent flame retardancy and low smoke without any additives. Oxygen index is 47%, combustion grade is UL94-V-0, and density is 1.28~1.42g/cm3.
PEI has strong high temperature stability. Even non-reinforced PEI still has good toughness and strength. Therefore, PEI's superior thermal stability can be used to make high-temperature heat-resistant devices. It has excellent mechanical properties, electrical insulation properties, radiation resistance, high and low temperature resistance and wear resistance, can transmit microwaves. PEI also has good flame retardancy, chemical reaction resistance and electrical insulation properties.
Some newly designed car headlights and headlight reflectors have begun to use PEI.
Headlight reflector

BMC material is a thermosetting material, mainly used for production of headlight reflectors (also called reflective bowls, etc.). BMC is a lump prepreg made of chopped glass fiber and unsaturated polyester slurry. It is suitable for molding by molding, transfer molding, injection molding and other processes. Products have good mechanical properties, high dimensional stability, good surface finish, water resistance, oil resistance, corrosion resistance, heat resistance, and excellent electrical properties, especially arc resistance can reach about 190s. Resin slurry is prepared according to formula. Main components are same as sheet molding compound, filler is increased, and thickener is generally not required. Resin slurry is fully mixed with chopped glass fiber (about 3 to 25mm in length).
(1) General properties
BMC (DMC) has a high specific gravity, between 1.3 and 2.1; Product has a bright appearance, good hand feel, and a hard and heavy feeling; Heating with fire will produce a lot of oil smoke and a styrene smell; Some varieties of BMC (DMC) are difficult to burn, but some varieties are very easy to burn, leaving inorganic substances after burning.
(2) Dimensional stability
Linear expansion coefficient of BMC (DMC) is (1.3~3.5)*10-5K-1, which is smaller than that of general thermoplastics, so BMC (DMC) has high dimensional stability and dimensional accuracy. Temperature has little effect on dimensional stability of BMC (DMC), but humidity has a more serious effect. BMC will expand after absorbing moisture. Linear expansion coefficient of BMC (DMC) is very close to that of steel and aluminum, so it can be compounded with them.
(3) Mechanical strength
Tensile, bending, impact strength and other properties of BMC (DMC) are higher than those of thermoplastics, and its creep resistance is also better than that of thermoplastics.
(4) Water and solvent resistance
BMC (DMC) has good corrosion resistance to water, ethanol, aliphatic hydrocarbons, grease, and oil, but is not resistant to ketones, chlorohydrocarbons, aromatic hydrocarbons, acids and alkalis. BMC (DMC) has low water absorption and its insulation performance is still very good after soaking for one day.
(5) Heat resistance
BMC (DMC) has better heat resistance than general engineering plastics. Its heat deformation temperature HDT is 200~280℃ and it can be used at 130℃ for a long time.
(6) Aging resistance
BMC (DMC) has good aging resistance and can be used indoors for 15~20 years. After 10 years of outdoor exposure, its strength retention rate is above 60%.
(7) Electrical properties
BMC (DMC) has the most outstanding arc resistance, which can reach about 190 seconds.
(8) Low odor
Styrene crosslinker used in BMC (DMC) will still have 0.1% residue after curing, and it will emit an odor when heated. Therefore, BMC (DMC) used for food utensils (such as microwave tableware) should use UP resin without residual styrene monomer. In short, automobile industry is one of standards for measuring a country's manufacturing industry. Continuous development of engineering plastics has brought infinite vitality to automobile industry. There are so many applications for car lights alone.